5-link independent rear susp design questions

[byndbad914]

I am building a full tube chassis Porsche 914 (mid-engine/oversteers/525HP V8 engine) and my chassis builder wants to go 5-link independent rear and I know a whole-lotta-nothing about them, so here I am.

General design configuration is (looking at one upright)
2 links pointing straight forward all the way to the firewall (one top/one bottom)
2 links pointing 90deg (inboard) from the upright (one top/one bottom)
1 link pointing ? to set/hold the toe.

Questions (and there are many)
1. The two links pointing straight-forward
a. should they be parallel, or should one be tipped relative to the other for anti-dive or anti-squat?
b. should they be parallel to the ground at static ride height?

2. The two links pointing inboard 90deg
a. The top one will be shorter for camber gain, but is there any suggestion for how much camber gain I would want on an 11" wide rear tire, 58" track width, at 2" of suspension compression (would be 'considerable' motion)?
b. should these be parallel to each other?
c. should these be parallel to ground at static?

I would suspect I want the roll center to be about 5"-7" off the ground and centered as I would suspect that is roughly where the cg height of the rear is, but I have no idea how to compute the rc with this setup...

3. The toe link - what angle does that run at for least amount of toe change? Should I run it parallel and inline with one of the inboard arms? This link basically takes all the load in holding the rear wheel straight, so I am thinking maybe a 45deg angle or something like that to help combat wheel motion?

any help in understanding this arrangement is greatly appreciated. I am sure my chassis guy knows what he is doing, but I want to have an understanding that isn't just his understanding.

thanks,
Tim

 

[GregLocock]

That is not a terrible suspension, but, essentially, it is a double wishbone with semi trailing arms - not exactly flavour of the month. The toe control link then adds a bit of interest to proceedings.

Given that you only have 2" of jounce travel then architectural philosophy is probably is not too important, as it obeys Chapman's dictum - any suspension will work if you don't let it move. And since the arms are relatively long any evil effects will be minimised. I think the drag links are a good move, in context.

1. The two links pointing straight-forward
a. should they be parallel, or should one be tipped relative to the other for anti-dive or anti-squat?

How high is your CG? if it is between the arms then I wouldn't worry about antisquat. In fact, in general I wouldn't deliberately put it in so long as the pitch under acceleration is acceptable. I would never use pro-dive on the IRS of a high performance car.


b. should they be parallel to the ground at static ride height?

Yes


2. The two links pointing inboard 90deg
a. The top one will be shorter for camber gain, but is there any suggestion for how much camber gain I would want on an 11" wide rear tire, 58" track width, at 2" of suspension compression (would be 'considerable' motion)?

I'd suggest enough to give the outer wheel a little negative camber at 2" of roll


b. should these be parallel to each other?

probably not, use this to set RCH and camber gain

c. should these be parallel to ground at static?

The long one should be parallel to ground, as a first guess.

I would suspect I want the roll center to be about 5"-7" off the ground and centered as I would suspect that is roughly where the cg height of the rear is, but I have no idea how to compute the rc with this setup...

See Milliken. it is the standard double wishbone setup, as the longitudinals cannot react lateral forces.

3. The toe link - what angle does that run at for least amount of toe change?

That depends on everything else, unfortunately.

Should I run it parallel and inline with one of the inboard arms? This link basically takes all the load in holding the rear wheel straight, so I am thinking maybe a 45deg angle or something like that to help combat wheel motion?

I'd say it is too complex and too important to rely on a rule of thumb, I'd draw it out at -2, 0 and +2 inches of jounce to see how much toe it adds.

Note that the toe link will affect the RCH /in practice/, so you need to be rather careful.

If you look in the FAQ you'll see some recommendations for programs. Susprog is probably your best bet, but WISHBONE.BAS would handle it.









Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[byndbad914]

awesome...awesome...awsome thanks for the responses Greg! A lot of the answers were "what I suspected" which is reassuring to me. I read these suspension books until my head spins, then on comes the nausea and so forth ) The cg will be between the arms "most likely" (won't really know until we start fitting everything in there) so I won't worry about the anti-squat as much.

And I had to laugh at the Chapman dictum! It is what I said when everyone was pestering me about running the stock semi-trailing arms. I am going to this 5-link setup really because it "fits" in the car so much better than the arms were - the arms were getting in the way of everything.

 

[byndbad914]

Couple more questions after overnight thought....

1. Is there any advantage to running "positive caster" in the back, or tipping the upright rearward? Since the wheels don't turn left/right, it seems the self-righting characteristic wouldn't matter, but then I see some Indy car pics or other similar suspensions and it looks like the upright is tipped back. Sometimes rather agressively (I would guess 5-10deg). What purpose might this serve?

2. In terms of the camber gain, I am running stock front struts, so is it wise to just set the rear to the same camber gain? It definitely was an oversteer car (though hard to say how the tube chassis car will really turn out), but if it were to continue to oversteer, is there an advantage to having more or less camber gain in the rear?

Got the Milliken Bros. book this morning... head is spinning...

 

[GregLocock]

1) I don't understand castor in rear suspension, sorry can't help. If we define castor as 'the geometrical effect that gives a change in camber in response to a change in toe' and we keep our toe under control, then it is unlikely to be a big factor.

2) More camber gain at the rear will slightly reduce steady state oversteer, but racing cars tend to spend bvery little time in steady state. Rule of thumb (bearing in mind what Milliken says about rules of thumb) is that you want a bit more at the back than you do at the front.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.