Single Seater chassis stifness

[GTRdesign]

I'm busy designing a Formula Vee in pro-engineer and would like to bounce a few ideas off you guys, starting with the chassis. I'm using Mechanica to do a FEA analysis on the frame, specifically to optimise rigidity.

Does anyone have a good idea of what a good target number is to aim at? Maybe comparable to a good Formula Ford, or other tubular space frame chassis?

[Carlos]

I'm busy designing a Formula Vee in pro-engineer and would like to bounce a few ideas off you guys, starting with the chassis. I'm using Mechanica to do a FEA analysis on the frame, specifically to optimise rigidity.

Does anyone have a good idea of what a good target number is to aim at? Maybe comparable to a good Formula Ford, or other tubular space frame chassis?

Racing & Sports Car Chassis Design by M Costin/D Phipps is the standard reference for space frame design & construction - check Appendix 1 Chassis Stress Calculations. 1500ftlbs/deg. Welcome to the forum GTRdesign.

[Jersey Tom]

I would aim for an order of magnitude higher than either the front or rear roll stiffness (ideally the higher of the two) as a starting point. Keep in mind the important thing here is really HUB TO HUB stiffness, not necessarily chassis. But for now let's call it frame stiffness...

The reason you want a rigid frame.. when the car goes into roll from inertial forces that angular deflection is transmitted to the front and rear suspension. Combination of angular deflection and roll stiffness is going to be a major contributor to chassis balance. Compliant chassis is effectively another spring paired in series with the front or rear suspension.. so you would have a generally lazy car and VERY hard to tune. Balance won't be what you expect out of the box, and it will take big spring or bar changes to do anything.

On the other hand, you can go way overboard with stiffness with say a carbon tub on a FV or FF or what have you, in which case you're really not gaining anything but cost. Cars with stiffer suspensions and more sensitive drivers (a la F1) require a stiffer structure.

I'll stop rambling now...

[GTRdesign]

Thanx guys, usefull info, but Tom's thread leads me to my next question;

The formula vee uses a single axle rear suspension, with a zero-roll installation, ie, all the rear roll stifness is as a result of the rear roll centre location, as there are no springs betwean chassis and suspension, and only sometimes, a very small anti roll bar attached at the rear.

Another beautiful thing about the Formula vee rear is that as a result of the beetle swing axle suspension, the rear roll centre height is very close, if not above, the vehicles centre of gravity.

Thus..., do you guys think chassis stiffness would have a big influence on weight transfer with a "zero roll" rear end?

[Jersey Tom]

Why would having the RC at the CG height be a good thing? In any event the roll axis inclination is probably more important than it's actual height at the CG. High angle is going to give one axle a lot of geometric stiffness.

Do you have any pictures of a FV rear end? Keep in mind that the chassis doesn't necessarily roll about the roll center. Just a way of expressing either rolling vs non-rolling moments (and even then that's a bit dicey) or relative effects of jacking of the sprung mass to tire lateral force. If there's a small roll bar on the rear.. there must be some spring connection of axle to chassis.

The front is still conventionally sprung, no? That would be reason for a rigid frame so you can make spring and bar changes there which will make an effect on handling. Only time you'd want a floppy frame might be in a kart, or at least have the option to adjust frame stiffness since you have solid connections front and rear.

No matter how you slice it, a well-designed rigid chassis is a good thing, and not terribly difficult to design.

[GTRdesign]

sorry about the sarcasm you're right, high RC is not a good thing at all!

The roll axis would be rather steep, as the front is a trailing arm setup with RC at ground level. I want to post some pics, but just need to figure out how.

But i suppose that even without any "spring" stifness in roll at the rear, the geometrical roll stifness still needs to be transfered to the front somehow. Just weird to visualise, cause when you jack the car up at the back, you can rock the back wheels up and down with your hands.

[RH1300S]

It might be worth you while buying Carroll Smith's "xxxxx to Win" Series. Tune to Win & Engineer to Win. In one of those volumes he actually deals with the unique problems of a Formula Vee