Cloud9...68

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.

Droops83

Much of what you are asking will be answered by these books:



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.

divil

You might find this interesting too, from the website of rennlister Harvey: https://newhillgarage.com/2012/06/11...ension-camber/

MAGK944

Droops83

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:

FrenchToast

They only did that on the 924, and specifically only on the 933 and GTR+ cars. The latter has an entirely reworked front axle - new uprights, hubs, struts, sway bar, sway bar mount, etc.

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.

Cloud9...68

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).

squid42

Suspensions always work best when the horizontal elements are actually horizontal. Even if you have double wishbone. You can avoid the camber gain on compression with double wishbones, but now your wheel is changing position, and that can change toe.

Ideally you would lower a car not by holding it in a different place with shorter or softer springs but by moving suspension elements up the body. Which is a bit of trouble. Or smaller diameter tires.

What kind of camber you actually get relative to the road also depends on the anti-roll bars.

So if you are lowering a car you always suffer from the car not having been placed "correctly" in the first place. If you are lucky you can at least get the base camber back to where you want it. But compression behavior still changes.

Droops83

These are all valid points, which emphasize the can of worms opened when you start making changes to your car's suspension. There are a number of software programs available (like the one in the YouTube video above) to help model suspension geometry changes and their effects. To be honest, I have not tried any of these programs in conjunction with changes on my own 951; my own experience has been trial and error over the last 12 or so years, coupled with learning about proven suspension geometry principles as time has gone on. Suspension setup is quite subjective, however, and even an "improper" setup might feel good to some drivers, so it can be difficult to give good advice without deep knowledge of a given setup, and/or extensive telemetry data.

Even with all of the data in the world and powerful software modeling, nothing is proven until the rubber hits the road. Look at Formula One! Yes, a lot of the issues are with complex aerodynamics versus simple suspension geometry, but each team has scores of the smartest engineers in the world, and even then only a few teams get it right at any given time.

Make one change to your suspension at a time, figure out a way to quantify the results, rinse and repeat. Do what you can within your budget and timeframe, and have fun.

All of that said, the notion of toe changing with compression is definitely an issue, regardless of suspension design (this also happens on a multilink rear suspension, think 993 rear kinematic toe adjustment, 996 GT2 factory alignment woes, etc) . However, there are numerous bump steer correcting tie rod kits on the market for the 944, and for many other chassis. Once bump steer is properly measured, shims can be used to minimize this effect.

Dubai944

It's just basic geometry. When the angle between the strut and the control arm is greater than 90 degrees compression causes an increase in positive camber.

Cloud9...68

Thanks for the responses to my questions, and sorry for the slow response - been pretty busy. The video Droops83 linked was very good; interesting that it was from the RC world, but I guess those guys need their "cars" to handle optimally as well.

I found this very simple video showing (albeit in a pretty exaggerated fashion) the mechanism of camber change in a McPherson Strut suspension. Now it makes sense to me; I guess it is just "simple" geometry, lol.


I fully appreciate the complexities and interactions involved with suspension setup. Here's how I see it: When Porsche designed these cars, they did so with a set of parameters (ride height, weight distribution, suspension geometry, front/rear wheel/tire widths, etc.), all designed to perform optimally together. Then people like us come along and do things like lower the cars pretty drastically, take a bunch of weight out of them, fit wheels and tires of significantly different widths and grip levels (a square set-up in my case), all to make them go faster around the track. This improves a lot of things, but also creates some adverse side-effects that can make the cars handle unpredictably. My goal is simply to negate the side-effects as much as possible in order to allow my car to achieve its full potential, while I simultaneously continue to work on my driving (trying left-foot braking is next on the agenda...). And I totally agree with Droops83 to only change one thing at a time. I just got a set of 275/35-17 Maxxis RC1's to go on my 17 x 10.5" Signature SV503 wheels. I've only done one brief session on them, and they felt amazing, so I will spend the next few months (weather permitting) acclimating to the higher grip levels of these tires, before taking on the next level of changes, which will include raising the front and rear roll centers, and optimizing the spring rates and front/rear roll stiffness. But my real focus is on my driving - the car mods are just for fun and learning.

951and944S

Never really mentioned it here as far as I remember, but, years ago, pffft, he must have been 12 years old, but my son and I both used to race Nitro Sedans.

There are just as many, probably more aftermarket parts for those than any full sized race car, aluminum and carbon chassis strengthening parts, stiffer uprights, real miniature CVs (stock parts are "dogbones") low friction bearing kits, etc., etc,. The tunability and adjustment precision teaches you a whole lot as to what makes a car do what. Even the tires are customizable with different tread choices, foam firmness fillers and methods of keeping the tire bead on the wheel.

And yep, we dominated at that racing too....

T

Cloud9...68

Interesting, but it makes sense. For some reason, I never got into RC cars (or karting - I wish I could go back in time to when I was five years old and convince my parents to let me get into karting), but I can see how it's highly competitive, and that optimizing how they handle and respond is critical to success.