Rotec Cycles

Sponsor of BRISCA F1 Stockcar # 92

F1 Stockcar Structural Design

10th October 2009

 

Thought I would take the opportunity of using the new car build photos to further explain triangulation and framework.

 

If you look at a side view of the car, you can see how the triangulation is used to tie the down bars and sump guard to the main chassis rail. You will see the same sort of arrangement in a crane boom and the result is a very strong framework structure - in this case vertically to prevent the chassis bowing.

As explained previously, due to the plated roll cage the rear of the chassis is strong from withstanding vertical loads but without additional bracing, chassis often bowed from the roll cage forward. Down bars help this but are limited unless they are structurally tied into the chassis.

I have done this triangulation with 25x25x2.5 box section and the strength increase this gives for the added weight is substantial. If I did not have to consider the additional loads we get from Stock Car racing, ie hits from the side, then I could reduce the size of the main rail such that the frame would weigh less than a chassis rail but would be stonger. However, because of side loading, I have to keep a reasonable size main rail to prevent damage - I have used 80x40 box with the strength sideways as the framing looks after the vertical strength.

 

 

 

 

 

 

 

 

30th September 2008

 

In the structural info that’s given in this section there plenty about basic structural design and relevant to F1 chassis design but based on some recent forum discussions, I thought the following summary may interest a few people.

 

Just thinking about main chassis rails (but relevant to all parts of the chassis), I thought it would be of interest to look at the different options ie box section, circular section + box section at 45 degrees. I will start with the first two and see how it goes…….

 

Chassis main rails are subjected various types of loading:

 

Compression Loads

 

Tensile Loads

 

Bending Loads

 

Most common are compression(ie head on car impact) & bending loads (ie also from front/rear/side hits + loadings from bumper bracings, etc) and initially I will consider that we are talking about comparable materials ( ie similar grade carbon steel). The main influence of what happens to the chassis rail is due to the dimensions and the shape of the rail. So long as we are comparing similar sizes and weights we are considering similar cross sectional areas and up to the limit of what I will go into here that means that the rails I am considering will handle compression loads equally.

 

However, the way a rail will handle bending loads is more a function of it’s shape. Whilst I beams are great at dealing with bending loads they can only handle this in one direction but we need to handle loads both vertically and horizontally. The general structural solution is to box section steel where loading in all directions is required.

 

If you draw a circle of say 70mm superimposed with a square with 70mm sides, the parts of the square which fall outside the circle are the reason why there is a difference in resistance to bending even allowing for differences in weight.  Because we are looking at similar materials, the shape comparison is used to determine the effectiveness of resisting bending.

I won’t go into the details here but to summarise, for the shape being considered you need to determine:

 

-the centre of gravity (obviously easy for a circle or square! but can be more involved for say 2 different sizes of box welded together).

 

-the second moment of area is then determined based on the C of G – this is based on the length of sides of the box or diameter of circular section.

 

-from the above, the section modulus is calculated. Effectively this takes into account the amount and position of material of the section.

 

From this and the strength of the material, the maximum bending load can be calculated so the way to go is to compare the various section moduli.

 

So for a starter consider 70mm x70m x 5mm thick box and 70mm dia x 5mm circular section (note not a standard size!):

                              

                                Section Modulus (cm3)       Area(cm2)      Weight(kg/m)

70 x 70 x 5 box                26.3                                13                   10.1

 

70dia x 5 circular             15.5                                 9.5                    7.4

 

So OK its not a direct comparison as the box weighs 37% more but the box resistance to bending is 69% greater. So the section modulus takes into account the area of the section and also how far it is from the C of G so how thick would the 70dia circular section have to be to have a similar section modulus?

 

This is the result:

                               Section Modulus (cm3)           Area(cm2)     Weight(kg/m)

70 x 70 x 5 box                 26.3                                13                    10.1

 

70dia x 11 circular            26.3                                 20.7                 16.1

 

I calculated that the circular section would need to have a wall thickness of 11mm to have an equivalent section modulus as the box but at a cost of 59%

more weight than the box. Even if you could buy that circular section it’s obviously not the way to go!!

The only way to go to have comparable section modulus and weight is to increase the diameter of circular section. Now the 70 dia section is not a standard so I will make the comparison with the closest circular section I can get from my steel stock holder:

                      

                              Section Modulus (cm3)             Area(cm2)    Weight (kg/m)

70 x 70 x 5 box                 26.3                                 13                  10.1

 

88.9 x5 circular                  26.1                                 13                  10.3

 

Thats as close as I can get using available material. It’s a personal choice of what section to use but I really can’t see the point in having the additional fabrication complications for no gain. I can get both the box and circular sections in high tensile steel if that’s your preference.

In addition, it has to be remembered that the main reason for using circular section is where there is uniformally distributed loading (ie pipe, pressure vessel or submarine hull) or straight tension or compression, whereas box section was developed for normal structural fabrication. Circular section can have uniformally distributed loading when fabrication is with all the same size material (ie in our roll cages) as welding is on the full circumference – if small diameter or say box is welded to it then more elaborate plating needs to be used when compared to plating box. ie at  the bumper bracing point for the front bumper, the box chassis rail might be plated with a piece of say 70mm x 100mm x 5 plate – for the circular section I would recommend plating half the circumference which would need a larger piece of plate or adapt to flat pads (similar to Ivan uses on his 45 degree box chassis) – these again should take in half the circumference, all of which adds more weight.

 

OK what about the comparison with “Ivan’s” 45 degree box approach. Well assuming we are talking about using the same box then area and weight are identical. However, the section modulus works out lower:

 

                                 Section Modulus (cm3)    Area(cm2)     Weight(kg/m)

70 x 70 x 5 box                    26.3                        13                    10.1

 

above at 45degrees             18.6                        13                     10.1

 

The reason the lower section modulus is that this is calculated by dividing the second moment of area (identical in both cases) by the distance from the

C of G to the farthest point of the section – in the case of the box at

45 degrees this is a greater distance than using box normally and results in a lower section modulus and reduced resistance to bending compared box used normally.

With regard to general fabrication, it’s again preferable to use either the same size box (also at 45 degrees) or plate in a similar manner to circular section ie plate 2 sides of the triangle or convert to flat pads. Ivan looks to have done the latter and made an exceptional job of it.

 

Personally, it’s not something I would choose to do based on weight restrictions when using our big block/LD axle arrangement but I assume Ivans using a small block. Whatever, it’s the “dogs b*******” nuff said!   

 

 

 

18th August 06

 

I had described the front bumper bracing that we are now using and have a couple of photos so you can see in more detail:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The cross bracing is 3 pieces of 40x40x3 box which is built around the main rails then an 8 mm plate which connects it all together and makes the mounting for the bumper bracing. This allows us to use 2 pieces of 40x40x3 box welded together for front bumper bracing - this is particularly useful for the long front outside bracings. 

 

                          Structural Design for Brisca F1 Stockcars

 

I initially started this topic on the Opposite Lock Forum and this was where I left it. When I get some free time I will start on the detailed design for a new car and will keep you updated on progress.

 

Sorry but a few basics required to understand the stuff later

 

Section 1, Steel properties:

 

Up to what is called the Yield Point, steel in tension will stretch proportionately to applied load, up to the Yield Point the steel is an elastic material and if the load is removed, the piece of steel will regain its original length.

After the Yield point is reached, the steel ceases to behave like an elastic material and if the load is released, the steel will not revert to it's original length and if load is continued to be applied it will eventually break and this is called the Ultimate Tensile Strength (UTS).

 

In general structural design, the engineer must ensure that steel is loaded below the Yield Point and generally, a factor of safety of typically 1.6 is used. So the Yield stress is divided by 1.6 and that is the maximum stress the steel is subjected to.

 

OK nuff of the theory because without sticking strain gauges all over the chassis and repeatedly attempting all possible ways to muller the car (hit fence posts and walls at max speed in as many different ways possible – something Si has been trying in any case!) it is not easy work out all the different stresses that the car will see.

 

For example if you have a look at the video of my boy testing a Wimbledon fence post in 2000 with the chassis pictured, what actually happens is the car hits the post with the outside front bumper, this takes out the bumper bracing completely, ie bracing compressed into an S shape, sheared (think you could say the Ultimate Tensile Strength has been reached for the bit of 60 x 40 x3 box!), bumper end bent flat onto chassis, front  axle the bent 90 degrees, side nerf bar then half ripped off before the fence wires are tensioned enough to throw the car spinning across the track onto the centre green all the space of a few milli seconds.

 

As the main chassis survived all this ( as cars have been surviving similar incidents for years) it’s fair to say that with a few exceptions, F1 car designs are in general strong enough for the type of tracks we race on and the incidents encounted.

 

Things can be slightly different in steel sections in compression (during the crash) than it would be in pure tension because the load that squashed the bracing will also depend on the length of the bracing. Theres not a lot of difference with a short length of bumper bracing (about 18 ins long) but if that was longer, the compression load required to make it fail will be less as it will tend to bow. This brings into the equation the slenderness ratio.

 

Now I’m not suggesting you try this cos bits fly everywhere (but of course everyone has done it!) grab your cheap 12” plastic ruler by the ends and pull it (in tension) – will take a bit to break it in pure tension. Now push both end together (in compression) and unless you have a strong ruler (or you are more puny than even I am!) then it will easily bow and should you wish to be destructive you can break it. Obviously different material but same applies with steel. When it’s broken (and remember I told you not to do that) and you have 2 or more likely 3 pieces on ruler – try the same with a short length (preferably without cutting you hand or wrist) and the shorter it is the harder it is to make it bend and break. In addition, this is also affected by how the ends are restrained – a piece of bumper bracing is effectively restrained at both ends which is the best situation, whereas that’s not always the case with the front end of an older type chassis but we will come onto that later.

 

Section 2 Section properties

 

Along with material properties for steel which I have briefly covered, of equal importance are section properties which brings on board the shape/dimensions  and thickness of the material. In the equations for bending and deflection of a beam under loading, having decided the material properties of the steel and the length being used, section properties are extremely important in determining overall strength.

The term used in the equations which makes correction for this is Moment of Inertia or sometimes known as Second Moment of area – this defines the efficiency of a section (shape) in its resistance to bending. This is often different depending if the beam is loaded from above or from the side but we can start off nice and simple and consider square and circular sections of the same nominal size. Units are a bit strange but if we keep to comparisons we can keep out of this.

The Moment of Inertia is available for many sections or has to be calculated for more unusual shapes and where we are dealing a frame type construction.

In both the equation for bending and the equation for deflection, the larger the value of moment of inertia, the stronger the beam is in bending and lower the beam deflection will be. When simple shaped single beams of the same nominal size are considered, it is reasonable to compare the 2nd moments of area but later on with more complex shapes, the Section Modulus has to be used where both the 2nd moment of area and a dimension based on the depth of the shape have to be used.

 

A summary comparing the main values for similar box and circular tube (nom 50mm) of varying thickness is as follows:

 

Section

2nd Mom of Area (x1000mm4)

Area (mm2)

Weight

( kg/m)

50x50x5

box

308

900

6.99

48.3diax5

tube

165

680

5.33

50x50x4 box

262

736

5.72

48.3diax4 tube

138

557

4.37

50x50x3

box

209

564

4.39

48.3diax3

tube

110

427

3.35

 

If box and circular sections of the same thickness are considered, it can be seen that the extra steel at the extremity of the box lead to a larger 2nd Moment of area but at the cost of an increased area and hence weight. The above comparisons are dependant only on shape not on material so as long as equivalent materials are used (same yield stress) this applies.

 

I will not repeat my earlier comments when asked why I used box instead of circular tube on the roll cage of Si’s first car in keeping with the rules, I used 4mm thick box so effectively took on extra 30% weight – if you compare 3mm thk box to 4 mm thk circular section, you will note that the 2nd moment of area for the box (resistance to bending) is still substancially larger than the circular (about 50%) for approximately the same weight.

 

This obviously only touches on the topic and I will devote much more time later to so this aspect to show how to use different section steel in car construction.

 

Probably as good a time as any to explain a bit more about bending and deflection in beams. The type of fixing of the beam is extremely important in determining the loads it can take – beams can be simply supported with free supports at each end (like laying out a length of box with an axle stand at each end) or cantilever (like RAM tail lift – fixed one end only) or fixed both ends, etc.

There are also considerations to the type of loading which tend to be a point load (assumed to act at one point) or uniformly  distributed load (ie load /metre – in effect our piece of box laid out on axle stands will have a uniformly distributed load purely from it’s own weight), or a combination of loading.

Permissible bending stress is same as looked at previously in material properties, yield stress divided by a factor of safety. In design, deflection is usually limited by typically beam span (distance between supports) divided by 360 (or less), ie for a 2m span beam, maximum allowed deflection in design would be 5.5mm.

 

Continuing with section properties, if we now look at a piece of say

60mm x 40mm x 3mm thk box, if you consider the section it should be apparent that a beam in this material will act differently depending on which side the load will be applied – so the larger vertical load can be taken if the 60mm side is vertical due to the amount of metal at the extremity. The 2nd moment of area is therefore different for loads in vertical and horizontal for a fixed position of the beam. If you turn it on it’s side (with 40mm vertical), the larger load can be taken from the side.

 

You may also be familiar with RSJ’s (I section beams) which are designed for maximum vertical loading but can only take minimal loading from the side – their principle is to get steel to the vertical extremities but keep these tied together. This gives a large 2nd moment of area from a vertical load – great for general constructional, not much use to us due to weight.

In general, the I section beams are used for main floor supports in buildings, etc, some used in offshore modules I have worked on have 2m deep beams, pretty heavy stuff.

But for general fabrication, box section structures are often used. Now just to make a few things clear, I’m not saying that circular sections are not used, it’s just that in general, due to their lower resistance to bending for similar weight to box section, they will generally only be used were there is pure tension or compression. For example, circular sections are frequently used in bridge construction in place of wire rope, in pure tension.

 

It is not too strange that circular section steel has ended up in roll cages for most forms of motor sport. Many forms of motor sport came from racing road cars ie Rally, Track racing, NASCAR and our Stockcars and it’s far easier to bend a roll cage from circular section tubing to form a cage in a saloon car. In the likes of NASCAR, frames are now built first but the general design basis was sorted a long time ago and their frames are designed to absorb 150mph impacts which will distort the frame but still protect the driver, so they use multiple thinner tubes such that in a big impact loads are absorbed by the deforming frame which is thrown away afterwards.

 

Section 3 Framed Structures

 

Probably best to start with the basic theory before getting to the facts of life but it gives a basic understanding that I think will help.

A frame is a structure built up of three or more members which are normally considered as being pinned or hinged at the various joints. Any loads which are applied to the frame are usually transmitted to it at the joints, so that individual members (beams) are in pure tension or compression. A member in  compression is called a strut, a member in tension is called a tie.

 A perfect frame is one which has sufficient members to prevent the frame from being unstable, An imperfect frame is one which contains to few members to prevent collapse, A redundant frame is one which contains more than the number of members which would constitute a perfect frame.

 

The above theory is used for simple frame analysis and is too simplistic for real life but it does show that the simplest frame is a triangle which really is the meaning of life!

 

Step into the real world and if you have a reasonable new house with a conventional roof, designed roof trusses have probably been used. OK in home these are made of wood but at B& Q or similar they will be steel (but maybe no ceiling to support).

The main outside triangle supports the roof tiles + loading from snow, etc and the bottom member supports a ceiling and takes the weight of all the rubbish in the attic – other than you scrambling around in the attic, and at the location of the water tank, most loading is uniformally distributed and not point loads. Members are connected between the two to allow for the lightest possible roof construction – as each angled roof member is supported along its length, the roof member can be reduces in size – as its taking vertical load, it should be obvious by now that the larger dimension of the beam section is vertical. Same applies for the horizontal beam which supports the ceiling – it too is supported.

Obviously all joints are fixed and the loading of the interconnecting beams will depend on the ceiling weight, tile weight, etc and will change as you get a bit of snow on the roof. The trusses are supported normally by the outside walls of the house with take overall loading. I ordered roof trusses when I turned our bungalow into a house and they just input data on span (between house walls) tile type/weight, etc into a programme to design the truss – this includes factors for safety, snow loading, wind loading, etc. Any what is the basic shape of the internal bracing – triangular of course.

 

What happens though if I spurious lump of plane drops from the sky and hits your roof (point load) – one hole would appear. These type of point loads are not considered in the truss design – if they were, the trusses and roof covering would have to be strengthened up and larger supporting walls would be required…..etc.

 

Suppose if you are the worrying kind I should have given a warning not to read this before going to bed but the probability of that lump of plane hitting your house is small. 

 

 

Section 4 Main Problem Areas in chassis design

 

Theory helps to give an understanding of the subject but in practice we all know things are a “bit “different! The idea of “point loads” is fine for calculations but that’s where it stops.

Just have a look at your wife’s high heels (or yours!) and see what damage they can do – I’m referring to wooden floors of course! For a 5 x 5mm heel with @ 30kg load (being generous) that’s a loading of 120kg /sq cm whereas in flat shoes with a heel of 50 x 50mm for the same load the pressure is 12 kg/sq cm.

One can seriously impact your wood floor and the other at 10 times less would not be a problem.

Back to steel (unless you wear high heels in the stockcar), to completely annihilate a short piece of 25 x 25 x 3 box in pure compression will take about

12 tonnes f loading – if that was applied to a piece of 70 x 70 x 5 box in anything resembling a point load it would punch a clean hole. Using the 25 box directly onto the centre of the 70 box would cause a bit of damage as well. Not only are we talking about pressure but also where it is on the box. If you look at a section of the box it’s almost like a beam situation with the beam supported at both ends - the closer the load is applied to a support, the less chance of damage, ie the 25 box is welded to the top of the 70 box instead of centre.

General principle is to spread the load over a wider area by locally plating the larger main rail.

 

You don’t need me to tell you that the largest loading and cause for damage comes from front impacts in a F1 car – can be rear as well if car is in a fence post for example and takes a rear hit. The type of loading can be very different between a wall and a fence post – many hits on walls even square on result in the car being turned sideways on impact whereas with a fence post it can be straight in and stop! If the fence post hit is directly in line with the main chassis rail then there a bit of bumper to absorb the hit and the rest is a big compressive load on the rail (of rails if there is a frame type structure). A mid bumper hit on a fence post can leave a lasting impression on the bumper and with the deformation of the bumper, the loading will try to pull the main rails inwards. As there are more walls than fence posts around these days, most loadings are on bumper ends. Bumper bracing therefore is extremely important in preventing damage to the chassis – whatever happens to the bumper or bracing, the idea is get keep and undamaged chassis. Many cases of chassis damage result from over bracing of bumper ends such that the main chassis rails and sometimes even internal chassis bracing are deformed.

 

Based on the above, I have typically used 60 x 40x 3 box as bumper bracing and for the cross member with strengthening plate on the main chassis rail – I have found that the bracing can squashed flat without damaging the main rail or cross bracing. Because its angled bracing, the actual maximum load perpendicular (at 90 degrees) to the main rail is about 20 tonnes f – there is then resilience to bending provided by the main rail backed up by the cross member. Increase the bumper bracing to say 2 pieces of 40x40x3 box and the load transmitted on failure is increased by approximately 60% assuming the load is taken on both pieces of box (welded together or just equally loaded). This is about 32 tonnes f and there is likely to be localized damage to the 70 box main rail and failure of the 60x40x3 cross member – not good!

 

While we are on the subject, there needs to be similar consideration for bracing of side nerf bars particularly as there are not normally cross members to rely on. It’s not too bad as bracing is provided by engine mounts but always best to keep angled bracing and not get too large – I prefer to keep to 40x40x3 internal angled bracing between nerf rail and chassis where possible.

 

 

                                    Section Comparison Table

 

Section

(all in mm)

Section Modulus

 - Vertical (cm3)

Section Modulus

- Side (cm3)

Steel Area

(mm2)

Weight

(kg/metre)

25 x 25 x 3 thk

 

         1.76

         1.76

     264

   2.04

25 x 25 x 4 thk

 

         2.04

         2.04

     336

   2.6

40 x 40 x 3 thk

 

         5.1

         5.1

     444

   3.45

40 x 40 x 4 thk

 

         6.6

         6.3

     576

   4.46

50 x 50 x 3 thk

 

         8.3

         8.3

     564

   4.39

50 x 50 x 4 thk

 

         10.5

        10.5

     736

   5.72

60 x 40 x 3 thk

(40 side vertical)

         7.2

        9.1

     564

   4.39

60 x 40 x 4 thk

(40 side vertical)

         8.9

        11.5

     736

   5.72

60 x 60 x 4 thk

 

        15.6

        15.6

     896

   6.97

60 x 60 x 5 thk

 

        18.7

       18.7                   

   1100

   8.54

80 x 40 x 4 thk

(40 side vertical)

        11.5

        17.8

     896

   6.97

80 x 40 x 5 thk

(40 side vertical)

        13.5

        21.2

   1100

   8.54

70 x 70 x 4 thk

 

        21.9

        21.9

   1056

   8.20

70 x 70 x 5 thk

 

        26.3

        26.3

   1300

  10.10

2 off 50 x 50 x 3 thk

 

        22.3

        16.7

   1128

   8.78

2 off 50 x 50 x 4 thk

 

        28.8

        20.8

   1472

  11.44

2 off 60 x 40 x 3 thk

 

        18.4

        18.4

   1128

   8.78

2 off 60 x 40 x 4 thk

 

        23.5

        23.0

   1472

  11.44

 

 

As an all round performer, you can see that the old basis of 70x70x5 box is a pretty good choice for main rails but you can also see that the older deal of using fabricated main rails with two pieces of box were not that dumb either - remember that the section modulus from side loading is used for calculating strength in bending so the web formed by the joining the 2 pieces of box does not show as an advantage (ie if I did the calcs to compare say 100x50x4 box with the 2 off 50x50x4 boxes, there would be only a small difference because the web is not located on the outside of the box). But practically, that web really strengthens up the box profile from side loadings without necessarily needing any plating.

However, having told you all the benefits of using frame type structures, we need to look a bit further before deciding on main rails.

 

In theory, structural needs are best handled by a fully framed structure ie space frame chassis. However, we have to race in the real world and it’s not so simple…………….

Remember the roof truss, designed for all the max uniformly distributed loads – great for general roof requirements until your next door neighbours tree drops on it…… In stockcar racing we frequently get impacts that cannot be predicted so a space frame chassis has to be carefully designed to ensure it is as strong as a conventional chassis. Remember that our engine has to be close to the centre of the chassis and there is a minimum chassis width – this means that engine will always be within the chassis “rails”. Many space frame type chassis (not to our specs) are built to allow the engine to be able to sit partly over where our main rails would be – not a lot of use to us! But if you do build one for our racing, bracing is even more essential than on a conventional chassis and you can end up with no real advantages for all your efforts. For example, it would not be easy to build a space frame chassis for a big block, keep to minimum weight and be as strong as even a conventional chassis – if you used two rails one up and one down using 50x50x4 box (going thinner would make the rails susceptible to local damage at say bracing points) then those rails would collectively weigh more than using 70x70x5 box plus you need a load more internal bracing ( from table, 2off 50x50x4 boxes weigh 11.44 kg/m, 70x70x5 box weighs 10.1 kg/m). If you consider it for a small block, do you think it would be any stronger than say the FWJ tar car construction which has all the frame but also still has 70x70 main rails.

 

It’s cool to be unconventional but I would not go that route unless there were advantages and I cannot see any for our type of racing. In a while, I will show you additional resilience in using a frame structure and it’s reasonably easy to calculate this – it gets a lot more difficult for a full space frame chassis and it would be easier to use a framework analysis programme on the old computer (still waiting for a freebie!).

 

 

 

 

section ‘a’

section ‘b’

section ‘c’

section ‘d’

Conventional

      26.3

     26.3

     26.3

    326

Si Mk1

      72.7

    149

     174

    406

Strength factor

      x 2.7

     x 5.6

        x 6.7

      X1.3

Just while we are on subject of main rails, I think it’s good time to give you an idea of how a frame design can increase strength on a chassis. Now you must understand that the following is a very simplistic 2D view of things but as internal chassis bracing, engines/mountings, are very similar, it’s not too far from reality particularly as this is a comparison. I did this for Si’s first chassis and made the comparison to a “conventional chassis – for both this is based on using 70x70x5 main rails and for Si’s chassis includes additional 50x50x3 box from cab height at roll cage to front bumper, fully braced to main rail with 25x25 box. I also only included the top part of the chassis and did not include the full sump guard.

 

Again the table is a comparison based on section properties for the same quality steel. Again, this only works when sensible bracing is used because the frame adds substantially to the overall resistance to bending but it does not prevent a main rail being bent through having too heavy bumper bracing.

 

For section ‘d’ only the cab side of the roll cage is used not the top part of the cage which is not really fully supported to be included in the frame – the difference between the conventional and Si’s chassis here is only due to the 50 box cage + slightly deeper cab sides used. As advised previously, the back half of the chassis with roll cage is very strong – the area of the chassis that is most at risk is just in front of the cage which is what I was trying to protect.

 

 

 

So how can this be used to our advantage – well because the framed structure gives so much additional strength in resistance to bending, the main rail section could be changed to say 80x40 box ( again refer to the FWJ chassis from 2000 which Alan England is/was using) where the lighter main rail was used. Or perhaps 60x60 main rails can be used – only problem is if you want to race car in Holland but technically you can have a car that’s appreciably stronger providing bracing issues are covered correctly (in both options, resistance to bending from side loading is slightly less compared to 70x70 box). I have to admit that I chickened out with Si’s car as I thought I had better stay with 70x70 main rails due to other bits I was trying. Had I known that FWJ was upto, I may have tried something less conventional!!!! (both Si’s car and the FWJ car debuted at the 2000 Euro meeting!).

You just have to remember that the framework bracing needs to be well designed (and preferably triangulated) to take full advantage from this.

 

I was having a read back over the topic just to check on some of the previous comments so that I can ensure they get fully covered as we’re heading into the region of front bumper bracing and front end strength. I will just quote these in full then hopefully cover them in enough detail as we have a bit more background now to do then justice.

 

Quote from David M

“One thing I have often wondered especially with more solid (armco) fences appearing is the merit of having a bumper that would compress under load (heavy impact with fence) and return to its start position after to give some can kind of controlled deformation. “

 

Quote from Glen

“As the car speeds have increased and the car construction quality has risen to cope with the demands of modern racing. Are the cars becoming too strong? Is there going to become a point when the weak point in the chassis becomes the driver. As posted earlier, perhaps some form of crumple zone within the front of the chassis may well help in a big impact with the wall. After a long chat last night with a current F1 driver discussing this subject, he was thinking of shortening his main rails and mounting his front bumper more F2 style. Not bolted on but triangulated away from the main rails to add some "give" to the front of the car in a heavy impact. In thoery it could also lessen main rail damage in a heavy front end shunt.”

 

Quote from Dragon

“Having read a couple of posts I get the impression that the main thing about building the green monster was strength? A lot of the braces, brackets, bumpers etc all work together to increase the overall strength in the car. I understand why you are doing this is to have less damage on impact with the wall or other cars. But isn't there a turning point? You can build an almost rock solid car but there must be a point that an impact can't be taken by the chassis anymore. This means that something else has to absorb the forces that are taking place. For example the chassis will keep it's place but the engine mounts/bolts just snap due to the forces that can't be absorbed by the chassis, resulting in the engine being moved. Or other things that have been welded/bolted on that let go because the chassis is too strong. Don't you want a certain part of the car build stronger then other parts of the car and in such a way absorb the forces some more? It could also safe you some weight?”

 

 

I will start off with David M comments:

 

Whilst it did pass through my brain some years ago about the possibility of using say hard rubber between chassis and bumper/ bracings, it would mean that a method of guiding and securing would also be required and for F1’s I decided that unless this was a mega design you could still end up with loose bumpers flying around. If you can appreciate the loadings that are involved with a heavy F1 impact, as I’ve tried to describe, then I hope you will see what the problems are.

I really must try and put on a picky from Si’s first Wimbledon wipeout – I previously mentioned bumper back to chassis, squashed bumper bracing, 90 degree bend in front axle and part damage to side nerf bar. Well I pulled out the piccies again to remind me that the side nerf bar was basically chopped off (all chassis mountings sheared off through box) + back axle bent by about 20 degrees and even damaged the rear bumper bracing.

So my recommendation would be to carry on having fun squashing box section and keep everything fully welded….

 

……….which leads me to the comments from Glen and Dragon:

 

Just touching on Dragon’s comment first, I think I covered weight aspects about the green machine and I hope you understand I am trying to cover things in a general way to give info to all concerned – I have just used this to help demonstrate the basics.

 

I have referred to the FWJ type (think Carlos built his own) tar car with regard to strength of the front end and overall car, utilizing all available weight in the car strength. Now I have not crash tested these machines and I cannot give you a pain vs gain comparison but there is substantially less give in the front end than on many cars. I made the point a few times that these guys don’t seem to be doing too badly on the UK tar tracks so you pays your money and takes your choice. So in answer to Dragons other points about this strength on the front end exposing other problems like weak engine mounts, FWJ’s performance have not been too shabby and there have been few DNF’s.

Remember that these cars are out on track at most tar meetings in the UK and from experience we know that they can better survive the hits to complete a race better than “conventional “cars. We have only experienced one final last bender with FWJ for a real place and we ended up with the bent bumper and axle not him! (still got 4th though!).

 

We decided to strengthen things up a bit at both ends (talking chassis of course!) and what we did was strengthen up the front bumper cross bracing using a sandwich of 2 off 40x40x3 and 1 off 60x40x3 box to make a  140x40 cross member which goes above and below the main rails to the outside of the chassis all tied in with a decent plate for welding on the bumper bracing. Front bumper bracing is now 2 off 40x40x3 box welded together to make 80x40 – can still be bent but survives many more hits and yes Si’s teeth rattle more!

To specifically answer Glens point, even disregarding the point about whether you want the front end to have more give or not, as driver suggested, I too am looking towards stopping the main rails before the front bumper cross bracing then welding the cross member full width (to outside of main rails) onto end of rails. If a sandwich arrangement is used, you can take decent bumper bracing and you can then weld on the front section of rail(s) onto the cross bracing which has the sandwich internal strengthening. From this point you can soften things as you want and depending on what is allowed for Holland decide on final design – certainly for UK if main rails were 70x70 box, I would consider using say 2 pieces of 60x40 box in a V configuration to give better spread of bumper loading and also giving more options for varying the strength. And yes you F2 guys this is used a lot already but not that much on F1’s.

I will make a few sketches to illustrate this better. We have also beefed up the back bumper cross bracing but not gone as elaborate – there is a bit more steel in all this but for a new car we can look at the best use of HT steel. 

 

With regard to where to use HT steel, I think it’s worth just going over info so a decision can be made.

Now I said previously that HT steel could be purchased that was approximately 40% stronger than standard carbon steel, so the limits of what HT steel you can get hold of (size and thickness and UTS) will finally determine where and how we can use it.

Whilst there are obvious dangers in mixing HT and standard steels if you do not fully understand things, it doesn’t mean you can’t do it, you just have to be careful. In fact there is no reason (other than availability and cost) why you cannot use it for all the car – from my observations, it appears some may be doing that.  

 

Remember also that there are many different ways to approach all this but you have to start off by understanding the basics whichever route you go, keeping things well controlled.

 

For example are you trying to save weight or increase / reduce strength in certain areas? Looking at say bumpers, they have to be minimum 140mm deep and most tend to go the route these days of a sandwich of 2 off 40x40x3 and 1 off 60x40x3 to make 140 x40 bumpers – in “the old days” most bumpers were made from 3 off 50x50x3 box when the regs required 150mm deep bumpers.  In a comparison of weight per metre thats 11.3 kg/m (now) vs 13.2 kg/m (using 50x50x3 box) – if you want to save weight, you could go for 1off 80x40x3 and 1 off 60x40x3 at 9.7 kg/m. If you went this route with standard steel you would probably find that with the one less box join (hence 6mm internal web), these bumpers would get chewed up fairly quickly unless you used HT steel. The difference between 11.3 and 9.7 kg/ m for a pair of bumpers is about 5 kg and more money – not my choice. If you could get hold of HT 60x40 and 40x40 box in 2.5mm thickness, the weight for conventional bumpers would reduce from 11.3 to 9.4 kg/m with an overall weight saving of 5 kg for the pair whilst strength would be increased.

 

If weight is not the issue then you could use the conventional arrangement but with HT steel and your bumpers would be 40% stronger. This possibly may not cost you too much additional money providing the rest of the construction is adjusted to suit – the bumpers will be more resilient to damage. Again it’s a case of less force being absorbed by the bumper and more compression loading being put on the main rails and to bracings and ultimately more load on the driver.

 

I have already said quite a lot about front bumper bracing and if you are going to stick with conventional front bumper chassis bracing (ie inside main rails), be very careful. If you have a mix of HT and standard steel around in the workshop, make sure the HT steel carries a “health warning” by painting a white stripe or similar. It would be fairly close to use a piece of 60x40x3 HT steel in place of 2 off 40x40x3  standard steel bumper bracing (as we have now) – without the more specialized chassis cross bracing if you used 60x40x3 HT instead standard steel you could seriously damage your chassis. We could use the 60x40x3 HT as we have things now – it’s down slightly in strength for a saving of 2.5kg/m (per length of bracing) compared to our 2 off 40x40x3 standard steel bracing.

 

Just to make sure that if you are going to “invest” in HT steel you get what you pay for, you must ask for the inspection certificate for the steel you are being supplied – this will only be a copy as it will be for a batch of the same size box. Earlier on I quoted some steel specs but there are a few different types you could get – my request was for HT steel equivalent to BS 4360 -50Cor D. The certificate supplied stated actual spec as EN10210: 1994 Grade S355J2H but regardless of this, the actual yield

stress and UTS are stated and these are actual test results taken from a sample of the same box. It must also be noted that minimum requirements for the strength are required so whilst your steel will be guaranteed to be the minimum valve, it could be higher.

 

Typically mild steel(normal strength) BS4360 grade 43C has a minimum yield stress of 255 N/mm2 and a UTS of minimum 430N/mm2.

 

The designation of the HT steel (S355) I was given certs for, refers to the minimum yield stress – actual yield stress was stated in certificate as 363 N/mm2 and UTS as 557 N/mm2.

 

For an idea, last years price worked out as follows (based on buying only 2 lengths of each):

 

70x70x4 S355:  ? +vat per length (7.5m)

 

60x40x3 S355: �� + vat per length (7.5m)

 

40x40x3 S355: �� + vat per length (7.5m)

 

In real terms, for this  HT steel, yield stress is up 42% on typical standard steel and UTS up 30% so you actually have to see the certificate to work out what strength advantage you get – our stockist provided a quote with certificates attached.

 

So decision time – need to decide on the master plan for a car. I think the real place to start has to be the most “budget” conscious build which has to involve a big block. Now whilst it’s tempting to go the way of a light rear axle (tranny), I think we have to consider this as a dual surface car so I will use an LD axle – bit more money to outlay but it should prove to be beneficial in the long term. Also for the purpose of this build it also gives the worst case scenario with regard to weight control. Next problem is what we do about main chassis rails – if you want to try and race in Holland or not. I think what I will do is to give options and let you guys decide – no one came back with any info on the actual rules in Holland but as Andy confirmed, 70x70 main rails have to be used but I still don’t know if this is bumper to bumper so as I discussed earlier, I will give the option of the front end where the “main rail” stops at the front bumper bracing cross member – also don’t know if I can make the same build to the cross member then continue with 70x70 box (ie 70x70 box has to be continuous between bumpers?) – it probably makes no difference cos you can’t tell the difference unless you cut it all up, so I will stick with the same idea.

 

I will give you an idea of some options,

all with Big Block and LD back axle:

 

1.  70x70 main rails stopping at the front bumper chassis bracing

 

2. Full length (well almost!) 70x70 main rails

 

3. Something a bit more unconventional (and banned in Holland!!!!!!!!!! And theres not much that is!)

 

I have produces a couple of suggestions for chassis but there are obviously many options and this is just how my view on things and may give a few different ideas on things. Incidentally, I have only really been looking at the front end of things and the rear arrangement is really basic and so long as you leave room for the diff and have sufficient bracing, it’s your call.

 

Both chassis 1 & 2 follows my description of stopping the main rail at the front bumper bracing cross member – in this case I would definitely look at using 2 off 60x40x3 HT steel to give 60x80 (vertical) which gives a web for strength in compression – this would go to outside of chassis to “cap”the main rails .

For chassis 1, I was thinking of 70x70x4 HT for main rails up to front cross member then instead of 70x70 box to front bumper use say 60x40x3 standard steel which is equivalent weight wise but will give better bumper support. My take would be to use 2off 40x40x3 standard steel for front bumper

bracing. There needs to be a decent thickness plate (min 8mm) to cap the cross bracing to allow good stress distribution for the bumper bracing. I will detail my suggestions and add weights so we can get some comparison.

 

For Chassis 2, I continued with 70x70x4 box after the cross bracing could be HT or “soften” things up

using standard steel. Similar comments to above.

 

As in suggestion 3 above, if you are not wanting to race in Holland then there are options to using 70x70 main rails  - say use 80x40x4 HT flat or 60x60x4.

 

Which ever you go for, it’s really worth putting in either a piece of 40x40x3 box or 2” tube from roll cage cab height to at least the front bumper cross bracing with as much bracing to main rails you can manage with the available weight – there is more available weight using the 80x40 or 60x60 approach.      

 

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