ABQ Slim

I saw some info about suspension geometry in the Evo upright post, but did not want to hijack.

I'm new to high-speed (OK - medium speed) Porsche driving.

OK. That's out of the way. When I read about "bump steer," I thought it referred to going around a turn, hitting a "bump," then having the steering thrown out of whack. Doesn't seem like a big deal. At slow speed.

Well, awhile back, I'm following some fellow PCAers down a winding two-lane in the mountains, going up and down, and side to side. We're going about 70 mph. There's a sweeping right hander going up the hill, then suddenly down the hill. As I reach the crest of the hill, my steering input suddenly is not causing the car to go to the right anymore. Instead, it starts leaning into the oncoming lane. So I adjust with more right turn on the wheel. No problem - the car adjusts and I'm back in the lane.

Then, as the weight of the front of the car settled past the crest of the hill, on the downhill side, my (formerly correct) steering wheel input caused the car to abruptly shift to the right, making me turn the wheel to the left to prevent going into the weeds. I didn't brake or accelerate during this adventure, and experienced this phenomenon several times on this drive.

Is this "bump-steer"? Is this normal? Is this improper suspension geometry? Do I simply need to get used to it and learn to adjust? It was quite scary at the time. My 1990 964 Cab is entirely stock except for drilled rotors.

Thanks a lot, and Happy Holidays!

james944

It sounds like you understand what happened just fine (at least if I understand what you mean). You were near the limit of grip for the front tires. As they became unloaded at the top of the hill they could not turn the car as well as they could under load. This is normal behavior near the limit. On a flat surface this is the point where you can steer the car with the throttle: decreasing throttle to feed the front tires grip and turn more sharply or increasing throttle to shift weight to the rear and make your turn open up.

If your local PCA has a DE or car control clinic, instructors there will be able to teach you a lot about this in a safe and controlled environment. If you get the chance these events are a blast.

"Scary at the time" for sure, save this sort of learning experience for the track!

Bull

What happened to you is natural for a front end that is unloaded of weight or loaded with more weight, and has to do with decreasing and increasing the grip of your tires. It isn't bump steer based on what you decribed.

Bump Steer is the term for the tendency of a wheel to steer as it moves upwards into jounce. It is typically measured in degrees per metre or degrees per foot. In other words, the tendency of a vehicle to suddenly veer or swerve to one side when hitting a bump or dip in the road.

Geoffrey

Bump steer is perhaps not a precise term for what is occuring because it can happen in roll, in suspension compression, and suspension rebound. At ride height the toe of the car is set to the desired specification. Lets say 0 degrees. Now, as the ride height raises or lowers (or the suspension rebounds or compresses), the toe may change. In the Mcpherson strut design we have, the arc the lower control arm travels is different than the arc the tie rod end travels. This is best measured in thousandths of an inch (.001"). In the case of the 964, the tie rod is attached at the back of the upright and it travels in a shorter arc than the lower control arm. When the suspension compresses, the tie rod arc shortens and the wheel gains a toe out attitude compared with the static toe setting (in this example 0 degrees). If only one wheel compresses, then the car will begin to steer towards that wheel. If both wheels compress (under braking for instance) then the car will dart back and forth. This is why I never put slight toe out in the race cars, unlike a torsion bar chassis where we run toe out.

To correct this issue, you need to minimize the arc differences in the range where the suspension operates. The GT2 Evo upright helps this issue, but does not come close to eliminating it. Depending on your ride height the amount of adjustment to the tie rod end will be different which is why a generic kit like ERP may improve, but not eliminate the toe change without modification.

ABQ Slim

Thanks a lot for your input, guys. I appreciate it.

boxsey911

Your post hi-lights what I love about these cars...the need to respond to what they are telling you. Pure driving pleasure, unlike some of the modern day electronic boxes that you just get in to at one end of the journey and out of at the other, ignoring everything in between. I even love the little bump steer moments that send a tingle up my spine to remind me of what I'm driving and to be alert

stv951

so there is a bump steer kit for these cars ?
I havent found one