cs3tcr
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| posted on 29/11/06 at 04:21 AM |
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Roll Centers
Can anyone shed any light on roll centers? I know it a fairly vague question, but is it desirable to have the front and rear roll centers at
different heights? IE the rear slightly higher than the front?
I'm currently toying with redesigning one of my projects, so that it would use a Mumford link at the back, but i dont want to upset the handling
with a roll center at the rear lower than the front roll center.
Thanks,
Rod
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designer
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| posted on 29/11/06 at 07:55 AM |
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The rear roll centre must always be higher than the front.
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JoelP
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| posted on 29/11/06 at 08:41 AM |
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i think the theory is more, 'look at what works and copy its roll centres'. Cars with a reputation of good handling often have the rear
roll centre about twice as high as the front, ie maybe 60mm front and 120mm back.
With normal road driving you wouldnt notice the difference anyway.
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britishtrident
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| posted on 29/11/06 at 02:15 PM |
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Depends on a lot of things, istr clubmans racers with the munford link used a front RC quite close to ground level, but also tyre/wheel width and
roll stiffness comes into it.
The original Lotus Seven and Caterinvan also have a much lower rear RC than the Locost/live axle Westie take a look at pictures of the front those
and work out the front RC height.
In any case within reason any imbalance can be taken out with springs/anti-roll bar stiffness changes.
[Edited on 29/11/06 by britishtrident]
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JB
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| posted on 29/11/06 at 06:37 PM |
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Roll Centres
Complicated business, and every suspension man has his own views. I am not an expert and do not profess to understand everything about RC`s, so
hopefully I will be corrected where I am wrong and also learn. It is also sometime since I have been heavily involved in suspension design and my
books are not currently with me so I am working from memory.
A lot of my knowledge has come from reading Carroll Smith and Milliken.
They state the front roll centre should be lower than the rear even with front engine cars so the thinking of the roll centre matching the mass
centroid (c of g along the car length) doesnt hold.
My thoughts on the front roll centre being lower than the rear comes down to my experience in designing suspension. The lower the RC the less the
camber gain. Now I never run a rear ARB because it makes power on oversteer worse, so I need as much camber compensation as possible under roll to
keep the tyres upright. (Independent suspension) I am not sure how this effects a soild axle and Mumford link.
The RC is also important in relation to the tyre contact patch in the way forces are transmitted (maybe the wrong word). My understanding is that
there is a force acting along the line from the tyre to the RC. The higher the roll centre the larger the vertical component of this force, which
leads to a jacking effect. This was observed in Shitfires, Beetles etc. This is also one of the reasons that having the RC match the C of G doesnt
result in utopia.
So I have my roll centres low on my Minor, 38.1mm front and 63.5mm rear. These figures were guessed at by looking at other cars (very difficult info
to get) and by reading the above books. I have also tried to keep the RC`s static for roll or at least get them to move in a similar fashion.
I am sure there is more to RC`s than the above.
Race Tec did an excellent article on the Mumford link and is worth reading. I have a copy but not with me.
John
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Syd Bridge
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| posted on 29/11/06 at 07:00 PM |
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ROLL CENTRES
LMFAO infact ROTFLMAO 
Snake oil, fairys at the bottom of the garden, moons made of green cheese, the moon landings were faked. leprechauns in Eire, yeti's, man
monsters in the western usa forests.......................All true of you believe in rollcentres!!!
Just look at the physics of what is really happening, and you'll see how false 'roll centre theory' really is.
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cs3tcr
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| posted on 29/11/06 at 07:37 PM |
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Care to explain the physics? I'm curious as to why the roll center theory is bunk.
I understand the swing arm theory (instantanious centers), and i can see how the roll of the car affects this. But why is the whole theory a load of
rubbish?
Cheers,
Rod
Oh, and i have seen one of those Samsquanch thingees (your man monster of the North West)
quote:
Originally posted by Syd Bridge
ROLL CENTRES
LMFAO infact ROTFLMAO
Snake oil, fairys at the bottom of the garden, moons made of green cheese, the moon landings were faked. leprechauns in Eire, yeti's, man
monsters in the western usa forests.......................All true of you believe in rollcentres!!!
Just look at the physics of what is really happening, and you'll see how false 'roll centre theory' really is.
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Fred W B
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| posted on 30/11/06 at 11:10 AM |
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Syd has never yet condescended to explain to all us mere mortals how it does actually work. Even if a car does not behave completely as people think
they do in the roll centre theory, surely it makes a convenient graphical way of comparing one suspension to another?
Fred W B
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Doug68
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| posted on 30/11/06 at 01:55 PM |
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If you really want to know why and what of the subject, then you need to read some books on the subject.
I would personally recommend this one:
http://www.sme.org/cgi-bin/get-item.pl?BK05PUB74&2&SME&
ISBN: 1899870318
What I would personally add to the subject is:
Roll center heights.
Swing arm lengths.
Weight distribution.
CofG location.
Wheel frequencies.
Wheel Track.
etc.
Are all tied together and discussing one with out the others is largely pointless exercise.
Doug.
[Edited on 30/11/06 by Doug68]
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