Multi Link Suspension

[TRK110]

We are designing and fabricating a circle track race car to be run on dirt. It is going to be mid/rear engine and rear wheel drive, therefore requiring independent rear suspension. We've normally dealt with a live axle rear suspension with a 4 link design, so we don't know exactly how an independent rear suspension works. The basics aren't hard to understand, and we believe that it has several advantages over a live axle, since each wheel acts on its own, and not affecting the opposite wheel as it travels over bumps and such. What we need to know, is what creates traction on a multi link suspension as far as forward/side bite in the turns while on the throttle? Are there better designs than others? We are open to any and all ideas.

Thank you very much!

 

[GregLocock]

Antisquat geometry generates bite, so in side view for a double wishbone look at the intersection of the hinge lines of the upper and lower arms, for example. That is your virtual swing arm.

BUT I wouldn't say double wishbone is necessarily what you want. The most important thing with your car is to react the traction forces reliably, and a typical double wishbone struggles a bit there. You might be better off using a long trailing arm, and then whatever choice of lateral links take your fancy. Look at a Lotus Esprit for a simple, robust if not especially tunable example.

Does Mark Ortiz discuss IRS for circle racing at all?

Once you go to an IRS you'll probably want to soften off your wheel rates to take advantage of it.

Cheers

Greg Locock


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[TRK110]

I appreciate the reply! Is there any way to simplify that down just a little to make it easier to grasp?

 

[TRK110]

I have been researching antisquat geometry today, and I have learned some neat stuff. I found a good program to calculate anitsquat percentage, but so far have only got the demo version which limits you quite a bit. The thing I see though, is that it is designed for a live axle, but I didn't know if it would change anything for independent. When you say antisquat bites, does that mean that it bites whether its independent or live axle? Also, is this a case of the higher antisquat percentage the better?

What I'm most confused about, is that in a dirt late model, the rear end is actually rotating, which makes it bite. On an independent suspension with a rear/mid engine, it doesn't have that. The only thing that rotates is the CV joints. That's why I was wondering if anitsquat geometry bites no matter what type of suspension it was.

 

[GregLocock]

Ah, can you explain what /you/ mean by bite?

Cheers

Greg Locock


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

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[TRK110]

I am talking about getting power to the ground, getting the tires to "bite" the surface. Since the differential torque reaction is not transmitted through the suspension members directly like with a rear end, I am trying to find a way to get as much power to the ground as possible.

 

[GregLocock]

OK, that's fine. For instantaneous longitudinal grip as you hit the throttle a large side view swing arm is recommended, in practice that means that if you have a drag link or swing arm then the body side pivot needs to be higher than wheel centre. If you have a conventional double wishbone the front body side pivots need to be higher than the rear ones, particularly for the lower arm. But as I said, double wishbone is probably not the right way to go.

What class are you racing?




Cheers

Greg Locock


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

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[TRK110]

It is called PRO 4. Basically, the only rule is that it has to be a 4 cylinder. Ours has 354hp, and will be in a tube chassis. Suspension is completely open, so that's why were looking for something a little differnent.

 

[DeanD3W]

Have you thought about using a De Dion rear suspension? Solid rear axle control arm approaches remain possible, while accommodating the fixed inner ends of your axles. It may be a better approach, especially if an IRS would need to have short control arms due to space constraints. If a De Dion is good enough for the Caterham R500, then it may be a good choice for your car.

See

http://www.google.com/imgres?q=de+d...&ndsp=45&ved=1t:429,r:0,s:0,i:90&tx=114&ty=47

and

http://www.locostbuilders.co.uk/gallery/a110323-SSeven11.jpg

I hope this helps.

Dean

 

[GregLocock]

While the Poms have historically loved a De Dion, it seems to me it needs most of the complexity of an IRS while completely failing to offer its main benefit. For a track car its other virtues may dominate, but for traction in corners on rough roads even the crudest IRS will be superior to any beam axle.

Cheers

Greg Locock


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

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

 

[RossABQ]

Take a look at the torsion bar IRS used on late-'70's VW Vanagons and Porsche 944. Very rugged yet compliant. They can be set up to use coil-overs too.