Wheel travel for track car

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
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chrislandy
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Wheel travel for track car

Post by chrislandy » Fri Apr 16, 2010 7:10 am

Hi,
I have just started planning my next project which will be classified as a "sports libre: sports racing car" under the MSA sprint/hillclimb regs i.e. two seater race only car.

I have been reading up on suspension (Staniforth, Milliken&Milliken, Carroll Smith and a few others!) and as this is the first independant suspension I have designed from scratch I just wanted to run a few things past others who may have done it before.

The car will be mid engined, rear wheel drive and purely used for sprints, hillclimbs and the odd trackday.

Typically, do you have more travel on the front, rear or equal?
I'm swaying towards slightly more travel on the rear and limiting total wheel travel to 2-3"

Does this sound like a good place to start?

Thanks
Chris

Cousin Cleotis
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Re: Wheel travel for track car

Post by Cousin Cleotis » Thu Apr 22, 2010 10:03 pm

Chris, all I know about suspension(or all I want to remember!) is what I have read in the Alan Staniforth and Carroll Smith books so no more qualified than you.

By total wheel travel, do you mean from full droop to full bump? If so 2-3" doesn't sound like much. How do you plan on limiting the travel?

When I come to redesigning the suspension on my car I am going to try to make it so I can change things easily because I'm sure it wont be perfect. So if i think 3" bump travel will be enough I will make it so the suspension can travel more because I can always limit the travel by a rising wheel rate or properly used bump stops if it needs to be.

Paul

Guy Croft
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Re: Wheel travel for track car

Post by Guy Croft » Fri Apr 23, 2010 7:48 am

I haven't thought this thru qualitatively but it interests me. Here's a start?

Travel in bump is governed by the spring rate and installed length same as a valve spring. The wheel travel in bump for given surface conditions is more a function of unsprung weight and spring rate than the physical space available for that travel. For the unsprung weight to cause the vehicle to lift the compressed spring force must exceed the car's corner weight or else the spring must be fully boxed.

Anyone take it up, get this thread moving? Got to get on with my book.

GC

Christoph Thuerey
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Re: Wheel travel for track car

Post by Christoph Thuerey » Sat Apr 24, 2010 6:25 pm

If you have an aerodynamic car, with diffusor and flat undercarriage you will have to reduce bump and rebound as much as possible. I would suggest not more then 1" rebound and slightly more bump. There you will use very hard springs and try to get the car in a good aerobalance. Springrates up to 250 N/mm are not uncommon because you have to prevent the car suck to the ground. Once there you`re in real troubles, the diffusor produce more downforce and you will lose dramatically verticalforces at the tyres.

If you have a non aerodynamic car and perhaps more bumpy tracks you*re good with 2" bump and 1" rebound for race only. If you have high curbs a little bit more of both. If you have a track like the green inappropriate (Nordschleife) you would like to have nearly the same amount of bump and rebound.

No need for more travel at the rear. But aware of the damped frequency! Avoid to have the same at the front and at the rear, otherwise your starts to bounce.

Do you build your suspension from scratch? Then I would recommend you to read a lot about anti-dive, anti-squad, instantaneous center and so on. Then measure your possible hardpoints at you chassis and download a test version of OptimumG or the SuspensionAnalyzer.

It`s a lot of work but you will learn a lot and it will help to setup your car.

Regards Christoph

chrislandy
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Re: Wheel travel for track car

Post by chrislandy » Sun Apr 25, 2010 5:53 pm

The idea is to have 40mm static ground clearance and closed wheel so would have a portion of aero in it from a rear spoiler, front damm/splitter and some well thought out bodywork sculpting. I was thinking along the lines of 1.5" bump and 1.5" droop as 1.5" is 37-38mm but given your comments Christoph I will design it around to 1" bump and 1.25" droop. Most of the hillclimbs and sprints around me are reasonably well surfaced tracks/roads so limited travel is not a problem.

Yes, I am building the suspension and chassis from scratch. I'm recovering from an operation at the moment so have ooodles of time on my hands to read, sketch and design before I can go back to work. I was planning on designing in anti-dive and anti-squat and have a few ideas on that already.

Mats
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Re: Wheel travel for track car

Post by Mats » Wed Oct 13, 2010 2:04 pm

Be sure you design in the possibility to run larger bump/droop, you will not be a happy man when you realize that you need more because you bottom out in the turns. You do not want to hit any limiters during racing because that is a sure way to upset the car/tyres.
If you plan to do tracks you will need to think about curbs because you will run over them sooner or later and it should never result in anything binding and therefore changing the handling, the graveltraps and the armco is hard enough to miss even if you don't have a snap-oversteer tendency every time you miss an exit ever so slightly. Also true for slipping a wheel inside/outside the track on a hillclimb.

Are there any regulations regarding ground clearance? Something like "no part of the bodywork should touch the ground with two punctured tyres on the same side"?
/Mats Strandberg

RollingCamel
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Re: Wheel travel for track car

Post by RollingCamel » Thu Oct 14, 2010 3:14 am

I think enough said already but what are you doing about your other suspension aspects?

Tyres, track, roll center, etc...? Did you weigh all the components?

WhizzMan
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Re: Wheel travel for track car

Post by WhizzMan » Thu Oct 14, 2010 8:09 am

RollingCamel wrote:I think enough said already but what are you doing about your other suspension aspects?

Tyres, track, roll center, etc...? Did you weigh all the components?
I think he is still in the planning phase and as such, does not have any weight figures.

Some inspiration: http://blogs.insideline.com/straightlin ... round.html

Suspension is a science of it's own and there are a few forums about it. If I were you, I would subscribe to those as well.
Book #348

RollingCamel
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Re: Wheel travel for track car

Post by RollingCamel » Thu Oct 14, 2010 12:14 pm

In order to design a proper suspension and chassis you need the tyre data. What tyres you are going to use?

If you are going with 13" tyres then you might want to check the Tire Test Consortium data. Other than that you will need to test it yourself.

chrislandy
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Location: Cheltenham, UK

Re: Wheel travel for track car

Post by chrislandy » Sat Oct 23, 2010 6:35 pm

Bit of an update. A few things have happened recently to divert my attention from this and the other half has decreed that I need to build her a road car first before I get to build the track only one so this one is still simmering away in the background. Thanks for all your advice so far and I shall return to this project in a year or so once I've finished my MSc and got my new workshop built :)

Chris

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