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Explanation of camber change under load on a lowered car

Old 11-08-2018, 07:43 PM
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Cloud9...68
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Default Explanation of camber change under load on a lowered car

I've seen it stated many places, including on this forum, that one of the downsides of lowering a car with a McPherson strut suspension is that it promotes positive camber change under cornering load, but I haven't found an explanation as to why this happens. It must have something to do with the fact that the control arms tilt upward at the outer attachment points (the ball joints) on a lowered car, but I'm not seeing how that promotes positive camber under load. Could one of the suspension dynamics experts on this forum explain the mechanism by which this happens?

On a related note, I've seen conflicting statements about whether s double wishbone type front suspension is less susceptible the a drastic lowering of the roll center on a lowered car than is a car with a strut type front suspension. I'm not seeing why that would be, because the control arms tilt on a lowered car, apparently to the same degree, with both suspension designs. And is a double wishbone front suspension less susceptible to positive camber change under cornering load? If so, why? Thanks.
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Old 11-09-2018, 03:05 AM
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Droops83
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Much of what you are asking will be answered by these books:

https://www.amazon.com/How-Make-Your-Car-Handle/dp/0912656468/ref=sr_1_1?s=books&ie=UTF8&qid=1541741610&sr=1-1&keywords=handling+fred+puhn https://www.amazon.com/How-Make-Your-Car-Handle/dp/0912656468/ref=sr_1_1?s=books&ie=UTF8&qid=1541741610&sr=1-1&keywords=handling+fred+puhn

https://www.amazon.com/Performance-Handling-Handle-Techniques-1990s/dp/0879384182/ref=sr_1_2?s=books&ie=UTF8&qid=1541741781&sr=1-2&keywords=handling+don+alexander&dpID=5103WJK3WHL&preST=_SX218_BO1,204,203,200_QL40_&dpSrc=srch https://www.amazon.com/Performance-Handling-Handle-Techniques-1990s/dp/0879384182/ref=sr_1_2?s=books&ie=UTF8&qid=1541741781&sr=1-2&keywords=handling+don+alexander&dpID=5103WJK3WHL&preST=_SX218_BO1,204,203,200_QL40_&dpSrc=srch

Both are old books (first is from the early 1980s, second is from the early 1990s) but the basic info is relevant today and both are well-written and informative. Both have good diagrams and explanations of roll center (especially the Puhn book) with different suspension designs with helpful diagrams.

As I'm not in much of a mood for typing, you could also read a Cliff's Notes version of the above as applied to the Porsche 911 suspension by checking out the attached PDF of an article I wrote for Excellence Magazine last year (complete with a horrible, not-to-scale rendering of a MacPherson strut roll center diagram done by yours truly).

But, in a nutshell, a MacPherson strut design tends to have more negative camber loss during cornering when compared to a double wishbone design, no matter what the ride height is. In a double wishbone suspension, the upper arm length/angle is the main determinant of any camber gain characteristics. With both designs, any excessive ride height alterations can change the relationship between the roll center height and center of gravity height and cause issues. This effect varies greatly with the length/angle of the control arms, especially in the case of a double wishbone design.

Hope this helps, and I hope anyone else with more knowledge of the subject than myself will chime in.
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Old 11-09-2018, 10:44 AM
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Originally Posted by Cloud9...68 View Post
I've seen it stated many places, including on this forum, that one of the downsides of lowering a car with a McPherson strut suspension is that it promotes positive camber change under cornering load, but I haven't found an explanation as to why this happens. It must have something to do with the fact that the control arms tilt upward at the outer attachment points (the ball joints) on a lowered car, but I'm not seeing how that promotes positive camber under load. Could one of the suspension dynamics experts on this forum explain the mechanism by which this happens?

On a related note, I've seen conflicting statements about whether s double wishbone type front suspension is less susceptible the a drastic lowering of the roll center on a lowered car than is a car with a strut type front suspension. I'm not seeing why that would be, because the control arms tilt on a lowered car, apparently to the same degree, with both suspension designs. And is a double wishbone front suspension less susceptible to positive camber change under cornering load? If so, why? Thanks.
You might find this interesting too, from the website of rennlister Harvey: https://newhillgarage.com/2012/06/11...ension-camber/
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Old 11-09-2018, 03:31 PM
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This might also be of interest. When the factory lowered their 924/944 race cars they went to great lengths to keep those a-arms parallel, not only did they extend the outer extremities of the steering and suspension, they also mounted the inner a-arm mounts and the steering rack higher up on the crossmember.



Source: 9technik


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Old 11-09-2018, 03:32 PM
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Originally Posted by divil View Post
You might find this interesting too, from the website of rennlister Harvey: https://newhillgarage.com/2012/06/11...ension-camber/
Thanks for sharing. I've come across that site before, and that guy does some cool experiments with his car. However, while the camber loss of the 944 MacPherson strut suspension is less than I thought, it is still camber loss, and this is even more with the front wheels turned, as they would be while negotiating a turn. He also seems to consider this experiment as validation to lower the front of his 944 to the point where it might affect the roll center, which is not ideal as explained in my article above. Also, when the body/chassis rolls, the outer pickup point moves lower, which changes the angle of the lower control arm even more . . . .

With a properly-designed double wishbone suspension, the outside front wheel will actually GAIN negative camber with suspension compression induced by body roll, which is why it is preferred in a sports or racing car. Therefore not as much static negative camber is needed, which affects straight-line braking traction, etc.

A quick Google search revealed this video, which does a good job of illustrating this effect, along with changing the roll center:


Last edited by Droops83; 11-13-2018 at 11:36 AM.
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Old 11-13-2018, 11:25 PM
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Originally Posted by MAGK944 View Post
This might also be of interest. When the factory lowered their 924/944 race cars they went to great lengths to keep those a-arms parallel, not only did they extend the outer extremities of the steering and suspension, they also mounted the inner a-arm mounts and the steering rack higher up on the crossmember.
They only did that on the 924, and specifically only on the 933 and GTR+ cars. The latter had an entirely reworked front axle - the only thing from a normal car is the core steering rack and engine cradle (with non-stock mounting holes). Everything else is unique, down to the sketchy AL tie rods.

The 944 and 968 received no such treatment for motorsport; barring the 944 GTP (which essentially ran a 924 GTR driveline), and of course the Fabcar-built tube-frame 944 GTR. Even the 968 Turbo RS, from what I can tell, ran a stock body shell (seam welded) and stock A-arms with normal mounting. The only non-stock component besides the bushings was the billet hub (of quite questionable design) if the car was equipped with centerlocks.

Consider upgrading the actual suspension components before worrying about optimizing geometry, camber and toe gain, etc. The basic suspension design and components were already over ten years old when the 924 debuted. Additionally, you can run statics calculations all day long, and still not take into account distortion and flex of all the relevant components: tires, wheels, arms, knuckle, frame rail, strut tower, etc.

Last edited by FrenchToast; 11-14-2018 at 05:37 PM.
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Old 11-14-2018, 12:44 PM
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Originally Posted by FrenchToast View Post

Consider upgrading the actual suspension components before worrying about optimizing geometry, camber and toe gain, etc. The basic suspension design and components were already over ten years old when the 924 debuted. Additionally, you can run statics calculations all day long, and still not take into account distortion and flex of all the relevant components: tires, wheels, arms, knuckle, frame rail, strut tower, etc.
lol - I'm definitely covered on that front: heavy duty Racers Edge control arms, Racers Edge billet hubs, spherical/solid bushings everywhere, RE camber plates, strut tower brace, caster block brace, and a half cage. Nothing flexes on my car but the tires, and even those a relatively stretched (275/35-17's on 17 x 10.5" wheels).
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