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Annoying suspension geometry questions - part 2

Annoying suspension geometry questions - part 2

(OP)
Almost a year ago I asked some questions on this board regarding rear suspension geometry and the answers were helpful, thank you. Had some time off the project (real job got in the way) but now I'm finalising the rear and deciding on the front. Anyway, before I go fabricating I just wanted to ask advice / get a sanity check on my numbers.

So far I haven't positioned the steering rack or spent any time calculating spring rates, so some of the numbers are still missing. I'll be going back to look at anti squat & dive once I have rates.

I found the target figures in the table below in a presentation by multimatic on the Caterham CSR independent suspension which they were subcontracted to design. It's freely available on t'internet.

Initially I had option A front and rear. Then I started to think I'd been too aggressive on camber gain so I did option B. Option B has less camber gain, less scrub, lower roll centers.

So my questions / worries are:
Are these numbers sensible?
Is option B an improvement over A? Should I go further and do option C?
In real world terms would I be able to tell the difference?
Is there something else I need to consider before fixing the inboard wishbone points and moving on?

Thanks in advance for any help, I know I'm being a bit of a pain dumping a load of numbers in a thread..

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RE: Annoying suspension geometry questions - part 2

(OP)
Forgot to add. Those are initial rates away from ride condition, not average over the whole suspension travel..

RE: Annoying suspension geometry questions - part 2

Looks like the output(s) from a kinematics (only) analysis program. As such, its not much value except to start a synthesis with. No steering or chassis compliance metrics listed, so its a napkin (or back of a napkin actually) hand waving exercise. I would NEVER build up a car based on just this alone. Its probably just enough tease for a technical looking presentation that mag writers can spew over.

Clarification of signs would help. Front toe-out?? Roll oversteer in the rear?? 4% to 7.5% roll understeer in the front (that's an awful lot)??

What's the weight distribution (with fluids and occupants)? Are you aware of the Yokohama lateral force, aligning moment and overturning moment properties at your expected usage pressure and rim width? Split sizes ?

You don't set spec bands for a suspension around some unknown tire. It's a recipe for dis-azzter as well as false claims, IMHO...

In God we Trust. All others, bring Data.

RE: Annoying suspension geometry questions - part 2

(OP)
Hi Cibachrome. Thanks for your response.

There are some details which I deliberately didn't include in my post, because I didn't want them to affect the response of others. I am one (structural) engineer building this car as a hobby and learning exercise. I don't have access to tire data (for that tire), or to a license of anything more sophisticated than a kinematics package. So by manufacturers standards that does mean I'm working somewhat blind, but does that really mean I can't proceed with the project? Also, currently I am designing around off the shelf uprights so I don't have control over some parameters. I guess I was hoping there might be people on this forum with some inside knowledge (perhaps regarding performance tires) who could give me some pointers.

I agree about the signs, I'm not sure how I can work this out? I am sure it is static toe out at the front and in at the rear though, what's wrong with this approach? Comments like "4% to 7.5% roll understeer in the front (that's an awful lot)?" are what I was looking for! How much is an acceptable/desirable level of roll steer? I'm designing to 3-4 degrees of roll at 1g lateral. So top band would be 0.3 degrees of steer at full roll, that does sound like quite a bit.

Mass split was 0.53 front 0.47 rear, with the old suspension. Do you have any guide for pitch angle due to 1g braking for a sporty car?

RE: Annoying suspension geometry questions - part 2

(OP)
Hi Greg, thanks for your input.

My upright design is fixed, so I can only reduce caster trail by sacrificing caster angle. 5 degrees caster gives 28.5mm of trail, do you think this is a better option?

Thanks, tom.

RE: Annoying suspension geometry questions - part 2

5 deg castor is fine. 28.5 trail is getting a bit much (18-25 would be better as a target) but is in the range for normal cars.

Unless you know a great deal about your tires and steering then the best bet is to set your car up initially somewhere in the middle and hope that any changes you need to make once it is built are containable.



Cheers

Greg Locock


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