load factors for design

Finally I am nearly ready to embark on the design and build of a racecar. My preferred class is DSR, but I have been thinking about Formula Ford to simplify construction of the bodywork. One bit of info that I have not found in all the design literature is what load factors to use for stress allowables. If it was a fighter plane, it might be 9 Gs, but that probably doesn't apply here. Any suggestions for info sources?

One practical question is whether your car will be spaceframe or monocoque.

If it is the former then , if you can get it , I would suggest "racing and sports car design" by Costin and Phipps. Yes it is very old but in the back it has most of the key stress calcualtuions for the mk 9 ( I think) Lotus done by a qualified aero structures guy.

Whilst there are many more modern , smarter tools the advantage of this is the simplicity and many load factors are given. You could prabably mofify the calcualtions by substituting your own frame layout.

If youa re going monocoque then much more sophisticated FEA etc but the first thing is to figure out where inside sheets of whatever you use the load paths will actually lie. Then I think you can decide the load factors.

Historically many circuit cars have been designed using 321

That is 3g vertical 2g longitudinal and 1 g lateral at the CP

That is a bit low for the lateral, in my opinion, especially if you kerb it.

So I'd be inclined to use 3 2 2

For a road car I'd use 5 4 3

Thumbs up for Costin, I bought it 3 months ago. In practice I'd use FEA, but then I've been using FEA for 26 years.

I strongly recommend CroMo for your members. Aluminium is OK for machined parts but has to be re heat treated after welding and I don't trust it (OK, I know modern bike frames don't bend and they are OK). The steel parts of cars designed using 3 2 1 and welded cro mo tubes will last for many thousands of km. Even with my welding.

Costin uses 3-4.5g up and 1 g in the other two directions in his sample calcs.

Bof,

May I offer a few suggestions as you plan your project race car:

1. Don’t even think of F Ford; a one off builder starting at the beginning as you are will be so far off the pace that you will lose interest as soon as you put it on the track, assuming you get that far. FFs are so finely tuned through thousands of design iterations that it is hopeless for a newbie to go there. Even if you were extremely lucky, about a 1% chance, you would spend a year or so fine tuning the suspension before you even got up to the back of the pack. Stick with DSR; it is much friendlier to the low volume builder. You are likely to be able to find the pack.

2. Unless you are a lot further along in your knowledge and experience base than I think you are build your first one with a tube frame.

3. Mariner pointing you to Costin and Phipps is a good direction. Also read Len Terry and Alan Barker’s Racing Car Design and Development and all of Carroll Smith’s stuff.

4A. With due respect to Greg Locock’s suggestion of using CroMo, don’t even start to go there. Stick with ERM round mild steel tubing of SAE 1025 or less. Lotus, Brabham, Lola, all of the early FF builders in the UK went this route as well as such as Australia’s Nota, Elfin and Renmax. I haven’t approached a new Van Dieman or Mygale or Ray with magnet in hand but a strongly suspect that they still use mild steel. CroMo is a strangely American affectation probably going back at least to the mid-1900s where Indy regulations required CroMo construction.

4B. CroMo is stronger in UTS than low carbon mild steels, which means that once you have hit the wall it will take slightly longer to break, which is probably why Indy required it. But by the time you hit the wall it is too late anyway! Nobody designs for strength so don’t worry about G-loads. Everybody designs for torsional deflection which once you are in an acceptable range is overly strong so high UTS numbers are meaningless until you hit the wall noted above. Interestingly, CroMo and mild steels all share the same value for Young’s Modules, E, so deflections under load are the same. Mild steels are much easier to weld, need lower skill levels and don’t need stress relief as is required when you use things like 4130 CroMo.

5. Use 1 inch 18 SWG for main frame members. Make all long diagonals of 7/8 inch 18 SWG with the same material in ¾ inch for short diagonals. Bracket using 16 SWG mild steel. Make sure every bay has a diagonal and every bracket is in a corner and you will be both plenty strong and have decent torsional deflection. You should end up with a fully bracketed frame weighing in the neighbourhood of 65 pounds/30 kg. Any weight savings from that level aren’t worth talking about for your first effort.

6. Don’t worry about a jig for a one or two off. You will spend more time designing and constructing the jig than you will building the first couple of frames. If you weld one corner a time and wait until welds are not hot to touch you will be OK. Have a big rubber mallet at hand and keep everything square and un-twisted after every weld by applying mallet. Keeping progressively flat and square you will end up with a frame that is true to within about 1/16 inch/1.5 mm. It will also be free of in-built stresses as long as you follow the mild steel recommendation noted above.

Many more suggestions can follow but this is enough to absorb for a first effort.

Regards

I agree about designing for stiffness, now I'm puzzled why (or even if) we used cromo.

Originally posted by Joe Bosworth
Bof,

May I offer a few suggestions as you plan your project race car:

1. Don’t even think of F Ford; a one off builder starting at the beginning as you are will be so far off the pace that you will lose interest as soon as you put it on the track, assuming you get that far. FFs are so finely tuned through thousands of design iterations that it is hopeless for a newbie to go there. Even if you were extremely lucky, about a 1% chance, you would spend a year or so fine tuning the suspension before you even got up to the back of the pack. Stick with DSR; it is much friendlier to the low volume builder. You are likely to be able to find the pack.

I totally agree... and I believe that applies to DSR as well. The only place the homebuilder has a shot with a first effort is in the Production and GT classes.

Originally posted by Joe Bosworth
Bof,

4A. With due respect to Greg Locock’s suggestion of using CroMo, don’t even start to go there. Stick with ERM round mild steel tubing of SAE 1025 or less.

I totally agree about chrome moly. The only reason to use it in the small bore categories is to save a little weight, and it is not worth the bother. The subject of mild steel vs. chrome moly has come up before in this forum... but people continue to be attracted to it. The mystique of chrome moly continues.

However, ERM is not permitted in most racing associations, including NASCAR, NHRA, USAC, and SCCA. They allow DOM only. The full SCCA CGR is now available for downloading (in pdf form) at SCCA.org.

I would just like to add one thing about CrMo / Mild steel debate...
I was in the same position and wanted to go for mild steel DOM, but I just could not source it locally in the wall thickness I wanted... so in the end I had to import tubing directly from abroad, and in the end went for 4130... now, that will not apply to most of you USA guys, but is sometimes a consideration...

I am also modeling my car by using Greg's 5 4 3 rule, as I want to use my car on the road too, but in the FEA I am applying all three loads at once (I am suspecting a worst case scenario of hitting a pothole right in a middle of a turn under braking ) and trying to keep the loads under the max yield strength of the material... I sometimes run the calculations for mild steel although I am using Crmo, just for added safety factor... does that sound like good practice..?

best regards
vlado

Be careful applying 5 4 3 loads simultaneously. Counterintuitively it may not be the worst case stress for a particular location.

for a start the 3 could be +3 or -3

The classic way to handle this is to analyse each of the 3 directions separately, then since you have a linear system check the vulnerable areas for valid combinations. Linear superposition applies.

ie

5 0 0
0 4 0
0 0 3
5 4 3
5 4 -3
5 4 0
5 0 3
5 0 -3
0 4 3
0 4 -3

Is the full matrix, I think

Admittedly for a one-off I suspect many people apply all 3 together, or just one at a time, and leave it at that. I occasionally have 'discussions' with 'experts' all over the world about this at work.

We don't actually do it this way, we have combinations of loads that we've seen /in practice/ at the proving ground.

Thank you all.
This is all excellent information and I very much appreciate it.

boffin5

Originally posted by Greg Locock
[B]Be careful applying 5 4 3 loads simultaneously. Counterintuitively it may not be the worst case stress for a particular location.

ok, I understand, a valid point.. when I use all three togeather, I always take into account the directions and try to simulate a real world worst case scenario... for example, a pothole in a right hand bend, for me, will see longitudinal forces towards the back of the vehicle, side load to the right and vertical force upward... There is also a question of corner weight, and for an added safety margin I tend to presume full weight transfer to the outside wheel...

also, I think that when you model, for example, an upright, you should simulate the loadings as they come in in real world, i.e. trough the wheel contact patch.... so I usually put in an additional member that goes from the spindle down to the ground...


if my budget was only a little bigger, I'd just put togeather a trailer togeather , with my suspension and uprights, a concrete block for test weight, and just drag it around some potholes..

Originally posted by Greg Locock
Be careful applying 5 4 3 loads simultaneously. Counterintuitively it may not be the worst case stress for a particular location.

for a start the 3 could be +3 or -3

The classic way to handle this is to analyse each of the 3 directions separately, then since you have a linear system check the vulnerable areas for valid combinations. Linear superposition applies.

ie

5 0 0
0 4 0
0 0 3
5 4 3
5 4 -3
5 4 0
5 0 3
5 0 -3
0 4 3
0 4 -3

Is the full matrix, I think

Admittedly for a one-off I suspect many people apply all 3 together, or just one at a time, and leave it at that. I occasionally have 'discussions' with 'experts' all over the world about this at work.

We don't actually do it this way, we have combinations of loads that we've seen /in practice/ at the proving ground.

Certainly for a 'classic' double wishbone motorsport suspension you will probably find that each link has a maximum stress at some intermediate combination of forces. E.g. combined braking / cornering with say 3g braking 2g lateral (given the 5 4 3 example above). Also don't forget to include braking backwards in a spin situation, I have seen this be the worst-case for certain links in a non-aero car.

If you are really trying hard to save weight, you can determine the likely contact patch force distributions of your vehicle given it's weight distribution, load transfer (both lat and long), aero forces etc. Then formulate a 'grip circle' and develop a matrix similar to Greg's but without exploring the 'corners' of the g/g diagram that your tyres can't reach.

Then go drive the car and measure the true x,y,z accelerations to check your design safety factors again. You might also want to specify different safety factors for tension and bucking in the various links.

Regards, Ian

Could I add a couple of things

1) the hard part of a spaceframe design for a single seater espeically is getting triangulation acroos the cockpit entry and the engine bay. The steering colunm bulkhead is also open for the drivers legs but usualy a sheet hoop will work.

2) For the cockpit you have these basic options

- build a hoop inside if your chassis width is above the drivers shoulder width.
- Triangulate outside with stringers to the front and rear bulkheads
- double up the side tubes and hope
- design the areas from the front bulkhead to the rollbar bulkhead as two wedges so the area under the seat is the "hinge" then the top side rails are for bending only

Also watch the engine mounts, with all the enthusiasm for end to end torsional strength it is easy to overlook that the heaviest part of the car in vertical and side loading is the engine.

Finally if you want to see a "perfect " spaceframe go beyond the racing stuff and look at the frame of the original GM Sunraycer (?)solar car. It was a very elegant set of interlinked tetrahedrons.

I meant to ask this before- but wouldn't make more sense to (theoretically) keep the loads under fatugue limit (or specified number of cycles- say 100, 1000 or whatever).

In Kitkiturbo's example- dimensioning a part to exactly 5-4-3 rule just under yield strength and hitting the bump at that load would IMHO result in almost instant component failure, wouldn't it?

You need to apply an appropriate safety factor to them. For instance, if I had an aluminium weld in there that was mission critical I might use a factor of safety (on yield) of 10.

For a circuit car I'd use a safety factor of 1 so long as it was steel or billet and I was confident of the design details.

Originally posted by murpia
Certainly for a 'classic' double wishbone motorsport suspension you will probably find that each link has a maximum stress at some intermediate combination of forces. E.g. combined braking / cornering with say 3g braking 2g lateral (given the 5 4 3 example above). Also don't forget to include braking backwards in a spin situation, I have seen this be the worst-case for certain links in a non-aero car.

If you are really trying hard to save weight, you can determine the likely contact patch force distributions of your vehicle given it's weight distribution, load transfer (both lat and long), aero forces etc. Then formulate a 'grip circle' and develop a matrix similar to Greg's but without exploring the 'corners' of the g/g diagram that your tyres can't reach.

Then go drive the car and measure the true x,y,z accelerations to check your design safety factors again. You might also want to specify different safety factors for tension and bucking in the various links.

Regards, Ian

Perhaps I didn't make the need for appropriate safety factors clear enough...

They need to be applied according to the detail design (manufacturing technique) and load case, as Greg points out for a couple of examples.

Analysing suspension links for example, in tension you can pretty much go as close to 1.0 safety factor (to yield stress) as you dare, as long as you can get hold of accurate material properties. Yielding a link in tension slightly won't do you a lot of harm in a track car.

But, in compression you are dealing with a non-linear behaviour: buckling. This is dependent on the detailed geometry of the link cross section, the details of the ends of the links and the material stiffness. And buckling a link when driving pretty much guarantees a trip into the scenery...

So, different safety factors for the different load cases in tension and buckling would seem sensible.

Regards, Ian