Mave
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| posted on 1/10/07 at 10:26 AM |
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How important is constant roll-centre height?
I'm analysing my Indy's suspension, to see if it would make sense (other than looks) to switch to Cortina uprights instead of the Sierra
ones.
Right now I don't really think the suspension would improve (by the way; I'm not saying it is bad as it is! I'm considering the
change because I feel the Cortina uprights look so much better; I just don't want to make the suspension work less, as that would not justify
the improved outlook). When switching to Cortina uprights, and maintaining the chassis pickup points, the rollcentre will get higher considerably.
Doesn't seem a good idea. I could solve that by making the top wishbone pivots higher on the chassis.
But one thing I noticed (both with Sierra uprights, as with Cortina's); the rollcentre moves a lot relative to the chassis, during suspension
travel. Somewhere in the order of 70 mm. In Kurt's book "Kimini" he repeatedly says that he's aiming to keep the RC as
constant as possible to the chassis, to make for predictable handling. All the suspension design books I have don't say anything about how
constant it should be. Can anyone elaborate?
Oh, and by the way; no, the Indy is not the only Seven that has this "bouncing RC". I think it has to do with the lengths of the top and
bottom wishbone; the difference should be larger. (as Kurt writes; top wishbone length 2/3 of bottom wishbone seems to give a very constant RC height
relative to chassis). But I don't know any Seven that has this 2/3's relationship between wishbones.
Marcel
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designer
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| posted on 1/10/07 at 10:59 AM |
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Due to the dynamics of a car, the Roll-centre position always moves, no matter what. It's the amount that matters.
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MikeRJ
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| posted on 1/10/07 at 11:19 AM |
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quote:
Originally posted by designer
Due to the dynamics of a car, the Roll-centre position always moves, no matter what. It's the amount that matters.
Unless you use a Watts linkage?
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Ivan
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| posted on 1/10/07 at 11:30 AM |
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Although not an expert in any way I do not believe that the average driver will be affected by the roll centre moving about as little as 70mm.
I think it realy only matters when it moves by hundreds of millimeters.
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Syd Bridge
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| posted on 1/10/07 at 11:45 AM |
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Roll centres aren't. They have no basis in reality.
It's what the things do that form the geometry that counts.
Have a good look at the geometry at the back of any topend racecar, that has very wide back tyres, and then you'll pick up a few clues.
Syd. 
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britishtrident
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| posted on 1/10/07 at 01:25 PM |
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Roll centres are useful but only think of them as a design tool the aim is to get the rc height and roll axis inclination approximately right for the
application.
Beware copying the suspension of modern racing cars as the massive aero forces tend to dictate the suspension stiffness mean they don't roll as
much as a Seven type car. Also because of the massive aero down load the static ride height is usually well above the ride height once the car is
rolling at anything above 40mph.
In my view the Cortina upright isn't much good for a light sports car. AQlthough ugly and heavy the Sierra upright with an offset mushroom at
least allows you to play about with the relationship between king pin inclination and camber and castor.
A Triumph GT6 upright is more suitable or if you want modern bits look at the MX5
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Mave
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| posted on 1/10/07 at 03:11 PM |
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I do see them as a design tool (amongst many other things). But when I design the front suspension to have the RC at 35 mm, and during travel it
floats around; what kind of effect does that have?
I know a suspension system is always a compromise, but what are the factors that really make a suspension "good" or "bad"? Is
roll centre height variation one of them?
In normal bump I would say that the height variation doesn't matter at all. Camber control is more important. But does that change during
cornering? Of course camber is still very important, but should the rollcentre remain at the same height (well, actually it more or less does; it just
shifts from left to right).
Hmm, and what if you encounter a bump mid corner........
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MustangSix
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| posted on 1/10/07 at 04:28 PM |
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suspensions are dynamic and everything changes when in motion - roll centers, camber, instant centers......
Unless the changes are radical, I doubt you would ever be able to sense the difference in this type of car. It's a Locost, not F1.
[Edited on 1/10/07 by MustangSix]
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t.j.
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| posted on 1/10/07 at 06:33 PM |
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I gave the moving around up to the ones with a higher level off graduation.
IMO after reading some stuff;
- Place your static RC good.
- try to get control off your camber-gain in the right way.
- take enough caster.
- No bump-steer
Btw: if it doesn't drive very well I can put some peat in it and some nice flowers
[Edited on 1/10/07 by t.j.]
Please feel free to correct my bad English, i'm still learning. Your Dutch is awfull! :-)
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