Hank Cohn

I've searched a read a lot of interesting posts about this subject. I'm not sure which toe links I have, but they look exactly like the ones in the attached picture:

As there are no spacers on the outboard end, I don't think they can be adjusted to correct for bump.

Looking under the car sitting on its wheels in the garage, the links already slope up from their chassis attachment point to the upright. It looks like as the rear is loaded further the wheels will toe out. If I were to replace these with the ERP or Cup parts with spacers should they be parallel to the ground at rest?

Thanks in advance,
Hank

DavidNR

They look a lot like the ones I got from TRG. Not adjustable for bump. But you can leave the eccentric bolts inbound and rotate them to adjust for bump.

NJ-GT

Hank,

They should be in parallel to the ground, with your weight in the car and the fuel load you use at the track. This way, the effect of the pivot point won't be too drastic on toe changes.

The links in your picture don't have adjustment for bump steer. Rotating the eccentric bolt all the way in, will increase just 3 mm on the inner connection height. It's not enough to dial out the bump steer that comes on the stock car.

Be aware that Porsche re-designed the rear uprights on the 996 GT3 RS, to move the outside attachment 10mm lower. This is an expensive solution. You can achieve the same geometry change with the right set of toe links and spacers.

I just recently got installed a set of ERP Racing toe links at the front, to dial out the excessive bump steer from that axle. These links are adjustable for bump steer as well. I run the front at 106mm with the street tires and 95mm with the competition tires. Using stock 996 GT3 tires sizes, it would read 107 mm.

At the rear, I have the eccentric bolts locked towards the inside, and 10mm spacers. At 1.5" of compression, it has 0.04" of toe change per side. To compress my car currently by 1.5", it needs an extra 3,000 lbs on the rear axle, something it would be hard to see. This is due to the 1000# springs I'm using.

I have my rear ride height set at 130mm for the street tires, and 125mm for the competition tires. Using stock 996 GT3 tires sizes, it would read 117 mm.

During acceleration the rear suspension design tends to toe-in. Unfortunately the suspension compression forces the toe-link towards toe-out, and the result from both is still more toe-out compared to the static load.

During braking, the rear suspension design tends to toe-out. The unloaded springs will change the toe links towards toe-out also, unless they're set as the stock ones, in which case they tend towards toe-in, because their static angle still has some reduction in total length.

Rickard 993 Turbo

I use ERP toe links at the rear with a 15mm spacer this has reduce all bumpsteer at the rear

Im also soon is going to install a ERP kit at the front to eliminate it in the front, i run 265/35/18 and MPCS in the front and i run 108mm in ride height


In the rear i run 315/30/18 in MPCS and im running 122mm in the rear

Dave S

Is there a place I can read and educate myself about how the suspension creates this bump steer? It sounds fascinating but I do not understand it.

Z06

Rad,

You have the RS uprights on the front and you steel feel the need for the front ERP toe link..with spacer?
I guess the RS upright was not enough of a fix?

You went with a 1000lb spring for the rear, what are you using for the front?

Where do you buy the ERP parts from?
thanks

NJ-GT

George,

I did not install my RS uprights per recommendations of Jon Groom Racing. Jon was right, I could only run the car at the stock front height, any lower than stock was going to bring back the bump steer. Just 3 weeks ago I got the front bump steer problem fixed by 911 Design.

Loren at 911 Design has a good relationship with ERP Racing. Same thing goes to ViperBob. The last time I did my own shopping for ERP parts, I used StableEnergies.com in NJ.

Springs: F:150/750 R:250/1000. Tender/Main. These are the same springs I was using 2 years ago. The car is still easy on the streets, the wife-o-meter didn't notice the change.

Hank Cohn

Rad and Rickard,

Thank you for the information. It is great to have your experience to draw from. You guys are tremendous assets to this board. I am looking forward to the upgrade and the results.

Thanks again!
Hank

Hank Cohn

Dave,
Most street cars, even GT3s are designed more for the street than for the track. The extreme loads caused by weight transfer in turns or actual irregularities in the track surface may result in significantly greater suspension travel than would normally be experienced on the street. Bump steer is caused by a significant enough deflection of a wheel or set of wheels to cause one or more of the suspension’s locating arms to cause an adverse change in toe. The result is instability at that wheel or end of the car.

My 996 GT3 has been lowered. This already compromises the optimum geometry of the toe link as it is no longer parallel. The link that controls rear toe is connected to the sub frame at one end and the upright (hub carrier) at the other. The attachment point to the upright is located behind the centerline of the stub axle. When the wheel position due to suspension loading or unloading causes the toe link to be at an angle that is either greater than or less than parallel the effect is a shortening of the toe link. As this link is connected to a point behind that of the rear stub axle you get undesirable toe out. As you might imagine, turning into a high-g right turn, for example, with a lot of left side suspension compression resulting in a left rear wheel tending toward toe out doesn’t make for a very stable or pleasant experience.

The best way to fix this problem is to install uprights and subframe pieces that are corrected for a lowered car and stiff enough springs to minimize suspension travel thereby minimizing toe change among other suspension geometry changes. As this is extremely expensive, there are toe links that can be adjusted for bump steer buy changing the attachment point with shims and long bolts. I think there are some unintended consequences of this kind of fix, but the primary problem is significantly reduced if not eliminated.

There are guys on this board who know far more than I do who may modify or correct some or all of these statements.

Hank

Dave S

Thank you, thank you Hank.

mjb

Hank, thanks for the explanation. I've been mysitfied by the term "bump steer" as well.
Still not sure I completey understand tho. When you say attachment point is changed, do you mean that the angle is changed from parallel (static setting) such that under dynamic load the link then is close to parallel and thus maintains the desired toe in setting? What does this imply for the toe setting when the car is not loaded?

Z06

How is the balance with this sping rate?

I was thinking of trying a slightly stiffer spring, it still moves around at high speed with the 600/800, was also thinking about a stiffer roll-bar, what do you think?

What did you notice most with the bump steer corrected?
do you find that the front end bounces?

NJ-GT

Mike, in the 996 GT3 the toe link attachment to the subframe is lower than the attachment to the upright. The link has a positive angle compared to the control arm, and much more to the ground.

As the suspension compresses, the body will sink, while the wheel/tire remains at the same height (almost, due to tire deflection). In this case like in a heavy turn, the toe link attachment to the subframe will be even closer to the ground, while the upright attachment will be at practically the same height, so now the toe link has a higher angle.

This higher angle is like reducing the length of the toe link, but because it is attached to the upright, it will pull the upright towards the center. If the toe link was located in front of the control arm, this would create toe-in, but in the 911 it's located behind the control arm and it creates toe-out.

It is desired to have this toe link in between the angle of the lower and upper control arms. The lower and upper control arms run at practically the same angle, so having the toe link set in parallel to them is ideal.

If your 996 GT3 is using the stock 128mm rear height on stock 295/30R18 MPS2 Nx tires, your control arms will be in parallel to the ground.

If your car is lowered, now the suspension geometry has been compromised, as the upper/lower control arms and the toe-links have different angles.

A lowered 911 GT3 will gain negative camber. But it will tend towards toe-out with less suspension travel. I have experienced this behavior with my car when I lowered it 20mm not knowing any better back in 2004.

As Hank says, the spacers or adjustable bolts for the toe links are a work around for the whole problem. Ideally the solution is on the uprights, because they fix the toe links angle, as well as the control arms angles.

Porsche has been evolving this suspension through many years. The 997 GT3 got it right at the front, not so much at the rear. Our benefit is that the GT3 RSR is at least 4 years in the future compared to what we get on the street car, and we can learn and apply the RSR technology to our street cars.

blake

Awesome posts Hank & NJ-GT! I wish Rennlist had a "definitions" section so that this thread could be posted there... I have not fully understood this issue for a few years, and now I completely get it.

Thanks,
-Blake

mjb

Thanks for the additional clarification. I think I can visualize whats going on now.

So then can I assume that with stock ride height and a toe link (I have TRG) that will hold the toe setting in the rear, plus proper static toe-in (say between 35-50min total), bump steer shouldn't be an issue in most cases?

Also, if the suspension is modified for less deflection (stiffer springs, less rubber) are you able to use a less aggressive toe-in setting in the rear since the angles will not change as much under load, reducing toe-out?

Sorry for all the questions but this is interesting stuff and I figure good to know in order to have a more informed discussion with your favorite suspension setup guy. Really appreciate all the knowledge people like you and Hank are willing to take the time and share here on RL.