Costin and Phipps Appendix I question

[Pcar928]

I have a question regarding the methods used to analyse a frame in the appendix. You are suppose to take a safety factor of 1.5 and then multiply it by 3 to simulate a 3g bump at the wheel. I thought 3g's was very low for the given example of hitting a curb. No damper or spring rates were given to make sense of the effects of the forces at the wheel on the frame. I must be understanding this incorrectly because this is just not right. I was hoping someone had a copy of this classic and could provide some guidance.

 

[GregLocock]

Good question. Firstly, many successful cars have been designed using 3-2-1, so despite the low loading it is not a terrible place to start. But bear in mind they are talking about cars with relatively tall sidewalls, at fairly low speeds, and using sensible (high ductility) materials in the frame. And of course no-one is going to sue them if it fails.

In your thinking you need to separate out the instantaneous shock acceleration on the spindle, from that distributed into the body. The latter is a lot smaller than the former.



Cheers

Greg Locock


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[Pcar928]

So a force that accelerates the body at 3g should be calculated, not the wheel at 3g. This makes much more sense now.

Do you know how the 3-2-1 method compares to modern or even 80's/90's methods? What about road cars?

 

[GregLocock]

Yes I do. For modern road cars 321 would not give acceptable durability. However as a starting point for a proto it is not too bad, so long as you inspect for cracks frequently.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Pcar928]

By proto do you mean race vehicle prototypes?

 

[GregLocock]

No I mean road vehicle protos. I think with the amount of scrutiny a race vehicle gets you should be OK, although 2-2-2 seems more sensible to me for a non aero circuit car.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?