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Roll Center - Torsion Bars vs. Coil overs?

I am, trying to understand this somewhat confusing topic. Also, it is a topic that has different thoughts in regards to unsprung weight.

So...what is better for "roll center"? Coil overs or the new heavy torsion bars that 24 and 36mm? This is assuming that both are heavily sprung.
Old 05-29-2017, 04:42 PM
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Quote:
Originally Posted by jamesjedi View Post
I am, trying to understand this somewhat confusing topic. Also, it is a topic that has different thoughts in regards to unsprung weight.

So...what is better for "roll center"? Coil overs or the new heavy torsion bars that 24 and 36mm? This is assuming that both are heavily sprung.
Roll center has nothing to do with springs versus coilover. Look here:

https://balancemotorsport.co.uk/suspension-geometry
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Old 05-29-2017, 04:49 PM
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Thanks for the reply. This is not the easiest to understand.

If the weight of the coils (perhaps minimal, I am not sure) is mounted higher on the car than the weight of the torsion bars, which are very low on the chassis, the distance between the center of gravity and the roll center increases.

If the weight of the torsion bar is mounted very low on the chassis the distance between the roll center and the center of gravity decreases. Which is desirable, but at the expense of heavy torsion bars.
Old 05-30-2017, 08:02 AM
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I don't think the amount of weight we're talking about will move the CG enough to change the roll center noticeably.

I could be wrong, but I sincerely doubt there's enough of a change on a street-car-based 911 to notice.

--DD
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Old 05-30-2017, 09:51 AM
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Quote:
Originally Posted by jamesjedi View Post
Thanks for the reply. This is not the easiest to understand.

If the weight of the coils (perhaps minimal, I am not sure) is mounted higher on the car than the weight of the torsion bars, which are very low on the chassis, the distance between the center of gravity and the roll center increases.

If the weight of the torsion bar is mounted very low on the chassis the distance between the roll center and the center of gravity decreases. Which is desirable, but at the expense of heavy torsion bars.
all things being equal the roll center does not change. but the CG will move higher with with the coils so in effect it will increase leverage up on the rollcenter. this will make it softer or easier to induce roll.
Old 05-30-2017, 11:17 AM
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I would think that torsions weigh more than coil overs? I would think they are considerably more heavy.

Last edited by jamesjedi; 05-30-2017 at 12:07 PM.. Reason: I wrote it backwards
Old 05-30-2017, 11:29 AM
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Originally Posted by Dave at Pelican Parts View Post
I don't think the amount of weight we're talking about will move the CG enough to change the roll center noticeably.
Roll center has nothing to do with the mass of the vehicle.
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Old 05-30-2017, 12:02 PM
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Quote:
Originally Posted by jamesjedi View Post
I would think that coil overs weigh more than torsion bars? I would think they are considerably more heavy.
A torsion bar weighs as much as a coil spring....maybe more.
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Old 05-30-2017, 12:03 PM
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Originally Posted by panzerfaust View Post
all things being equal the roll center does not change. but the CG will move higher with with the coils so in effect it will increase leverage up on the rollcenter. this will make it softer or easier to induce roll.
The roll center has nothing to do with weight, torsion bars, or coil springs!!!

Torsion bars are difficult to deal with as they are difficult to change and make it difficult to adjust ride height. The very slight increase in CG height is well worth the trade offs.
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Old 05-30-2017, 12:05 PM
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Woops - I wrote that backwards. Yes, I would think that the torsions are considerably more heavy. That said I have not weighted them.

I will edit.
Old 05-30-2017, 12:06 PM
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Quote:
Originally Posted by jamesjedi View Post
I am, trying to understand this somewhat confusing topic. Also, it is a topic that has different thoughts in regards to unsprung weight.

So...what is better for "roll center"? Coil overs or the new heavy torsion bars that 24 and 36mm? This is assuming that both are heavily sprung.
Roll center is a geometric construct that helps to describe/predict the dynamic behavior of the car.

The roll center is on the line determined by the lateral Instant center and center of the tire and statically is directly beneath the CG. The lateral instant center is determined by a line extending from the lower control arm and a line perpendicular to the the strut upper mount. All 3 of these imaginary points move around dynamically. The closer the roll center is to the CG the less roll force is exhibited


CG or CoM is determined by he distribution on all the masses in the car, yes a t-bar suspension will have a somewhat lower CG than a coil over, but it is such a small % of the total weight as to be not particularly significant.

Lowering a car lowers the CG, but besides increased bump steer, it also lowers the lateral IC and thus the roll center, this increase the distance between the CG and RC, which gives roll forces a longer lever arm to operate on.

You can see the geometry here,


Raising the spindle on RSR struts is a band aid for bump steer but does nothing to improve the IC/RC geometry.

Basic good practice is to keep the lower control arm inboard end no lower than the outer end, ie control arm is horizontal at most and hopefully points up on the inboard end.

Again nothing to do w/ coil over or torsion bar other than the very minor difference in CG
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Old 05-30-2017, 12:50 PM
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Excellent answers. Thanks to all who contributed.
Old 05-30-2017, 01:22 PM
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Quote:
Originally Posted by Bill Verburg View Post
Raising the spindle on RSR struts is a band aid for bump steer but does nothing to improve the IC/RC geometry.
Not correct!! Raising the spindle indeed affects roll center since it changes the angle of the A-arm for any given chassis height.
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Old 05-30-2017, 02:16 PM
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In fact, on a 911 lowered enough, raising the spindles is the only way to keep the roll center from getting too low in relation to the rear and to keep it from going under ground.
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Old 05-30-2017, 04:12 PM
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Quote:
Originally Posted by winders View Post
The roll center has nothing to do with weight, torsion bars, or coil springs!!!

Torsion bars are difficult to deal with as they are difficult to change and make it difficult to adjust ride height. The very slight increase in CG height is well worth the trade offs.
Chill... please point out what is inaccurate with what I've posted? when did I ever said the roll center had something to do with weight, torsion bars or coils? When did I ever say it's changes the roll center?

But the locations of the weight or CG dose if you replace the tb's with coils. So if u remove tb's (the weight down low) by adding coils which are located higher than tb location doesn't that move the CG up? If the CG moved up with the SAME roll center doesn't that create move leverage? It's simple suspension 101

CG change may be minimal given slight jungling of weight location but then again folks including porsche have relocated oil tanks or removed clocks, pass visor glovebox lids etc....

Now please stop yelling at me :-)
Old 05-30-2017, 07:11 PM
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The weight location might have a small impact on roll-rate, but not necessarily roll-center as this is related to geometry.

The only reason to change from T_bars to Springs is for higher spring rate or tunability. Not really much help on the weight and geometry front.
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Old 05-30-2017, 08:02 PM
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Quote:
Originally Posted by panzerfaust View Post
Chill... please point out what is inaccurate with what I've posted? when did I ever said the roll center had something to do with weight, torsion bars or coils? When did I ever say it's changes the roll center?

But the locations of the weight or CG dose if you replace the tb's with coils. So if u remove tb's (the weight down low) by adding coils which are located higher than tb location doesn't that move the CG up? If the CG moved up with the SAME roll center doesn't that create move leverage? It's simple suspension 101

CG change may be minimal given slight jungling of weight location but then again folks including porsche have relocated oil tanks or removed clocks, pass visor glovebox lids etc....

Now please stop yelling at me :-)
Chill? I was emphasizing that roll center does not change because you have torsion bars or coilovers or because the mass changes. Where did I suggest CG didn't change when mass is moved around?

If I were yelling, my text would be in all caps....
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Old 05-30-2017, 09:02 PM
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Quote:
Originally Posted by Bill Verburg View Post
Roll center is a geometric construct that helps to describe/predict the dynamic behavior of the car.

The roll center is on the line determined by the lateral Instant center and center of the tire and statically is directly beneath the CG. The lateral instant center is determined by a line extending from the lower control arm and a line perpendicular to the the strut upper mount. All 3 of these imaginary points move around dynamically. The closer the roll center is to the CG the less roll force is exhibited


CG or CoM is determined by he distribution on all the masses in the car, yes a t-bar suspension will have a somewhat lower CG than a coil over, but it is such a small % of the total weight as to be not particularly significant.

Lowering a car lowers the CG, but besides increased bump steer, it also lowers the lateral IC and thus the roll center, this increase the distance between the CG and RC, which gives roll forces a longer lever arm to operate on.

You can see the geometry here,


Raising the spindle on RSR struts is a band aid for bump steer but does nothing to improve the IC/RC geometry.

Basic good practice is to keep the lower control arm inboard end no lower than the outer end, ie control arm is horizontal at most and hopefully points up on the inboard end.

Again nothing to do w/ coil over or torsion bar other than the very minor difference in CG


Quote:
Originally Posted by winders View Post
Not correct!! Raising the spindle indeed affects roll center since it changes the angle of the A-arm for any given chassis height.
Yes, raising the spindle changes the angle of the control arm, when the outer end is raised(or the inner end lowered) the IC is lowered which also lowers the roll center. The CG also is lowered. Due to the leverage inherent in the geometry the IC drop exceeds the CG drop, thus lengthening the roll moment arm. Most would agree that this is not an improvement. But of course you are entitled to you own opinion.
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Old 05-31-2017, 03:38 AM
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I just got the August Excellnce magazine. Concerning raised spindles on the 73 RSR:

"Because the roll center height decrease was greater than that of the center of gravity, the potential body roll actually increased! "

I had to read that sentence about 5 times!
Old 05-31-2017, 06:37 AM
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I think I am understanding this a little more - the distance between the C of G and the RC is called the roll moment. With 73 RSR or any other car that has its spindles raised, the distance of the roll moment is actually increased. This happens even though the C of G has been lowered, as has the RC. It sounds like a confusing compromise.

This guy helped me in understanding a LITTLE bit;

https://www.youtube.com/watch?v=yU2c03KkF6k
Old 05-31-2017, 07:19 AM
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