LITTLE TANKER

Are there any of you's that can explain what bump steer is? This ia kinda embarrassing................... LOL!

Edward

When you bump into a tree or guardrail, it tends to steer you into a different direction.
(badum ...tshhh!)

Ok ok ...when you encounter bumps/dips in the road and the suspension compresses, that change in steering geometry "steers" your car in a direction different from where it was pointed. So imagine you're driving, hit a dip/bump in the road, and the car wants to sidestep for a nanosecond ...yeah, disconcerting, especially if you're mid-turn, and/or at speed. This is an unintended consequence of cars (any, not just 993s by a longshot) that are lowered too much, or simply a car that is not properly aligned. Moderate lowering + good alignment yields excellent results in our 993s, but there is definitely a point of diminishing returns.

And good to have you here, tanker!


Edward

trophy

In respect to our 993'a on compression (Bump) the front alignment toe's out, the bigger the bump the more compression, the more toe out.

If this happens on one side of the car you have one wheel pointing ahead, the other will toe out when it hits the bump. The wheel that hits the bump will toe out (turning away from the other wheel) and will cause the car to temporarily go in the direction of the toed out wheel. as soon as the suspension levels out it will return to straight (or what ever your alignment is)

ehanauer

Thanks for the explanations. I was wondering about it too, as probably many others.

LITTLE TANKER

And as for you Eric, thanks for making feel like I’m not the only one! LOL

Eric! Steven! I definitely have a problem then! My 993TT does that often, and gets worse as speed is increased. Was on an on ramp merging on to a hwy/freeway.....big swooping left........ With big expansion joints every 100ft or so........ Doing 100kph! Nothing for the car…….And almost lost control of my beast! I assure you that I can drive quite well, and did not take the turn too aggressively............ scared the hell out of me and even more so, made me slip into a minor depression the rest of my drive, due to the fact that I knew this is not the way she should be handling the road......... for s*** sake!...... my 07'Toyota Tundra does a better job on that ramp! So....... here's the deal....... My TT is pretty dame low and it currently has a coil over susp. From what I was told, with ibach springs (which I heard have the tendency to sag). It was installed roughly 10 yrs ago. As far as alignment, I had it done this past summer by a well respected local wrench. I personally think the susp. has seen its day a while ago......was thinking, and pretty much set on installing the PSS10's (maybe even doing myself) then bring it to wrench for alignment. One thing I know for sure is that my animal is not showing me what it is supposed to do, or what it can do! and that is sad and exciting at the same time, cause when I do get it up to it’s par at least, I’m going to have to contain my hormones! LOL!!!!!!! What do you boys think!? The sagging springs would be a culprit!? And old shocks!? Am I on the right track!? Thoughts…………..

P.S Thanks for the warm welcome!
Vince

jdistefa

^ re. bump steer on 964/993s

If you lower the car too much (generally less than RS + 5mm ride height with exacting alignment and fresh a-arm bushings and tie-rod ends) then you simply need RS/RSR front uprights and RS/RSR tie rods.

Do a search, much written about this

LITTLE TANKER

Thanks Matt, read a bit on this........ just for your info, my ride height is too low for my liking. I purchased the car with this set-up and simply dont even like the looks of it anyways. Also another pro for me to go to the PSS10's.

Vandit

Yea, simply write a check for a couple thousand dollars to acquire those parts, NBD. lol

trophy

As per Matt, anything below RS +5mm or even RS +10mm ride height really needs to have the RS/RSR/GT2 uprights and tie rods. This is not an insubstantial spend. If you are looking at upgrading the coilovers, this is the best time to do the uprights.

jdistefa

There are sets around

And yes, expensive, but it's the only way to solve the problem. Depends on your expectations.

race911

Explained a different way, and as simple as I can come up with, would be to say that the tie rod ends up traveling on a different arc than the suspension as it goes through its travel. Thus, toe changes.

We can further complicate by introducing the difference between bump steer and roll steer..........

(Just got back from a day at a data acquisition seminar where this was part of a discussion on the REAR of MX-5 Cup racers toeing out under extreme compression. Reminded me of my completely crude attempt to measure 911 torsion bar chassis toe changes through the pivot of the trailing arms back in the Olden Days.)

Essexmetal

The reason lowering the car creates bumpsteer is the steering rack bolted to the chassis, as the car is lowered the geometry between the steering rack inner pivot and outer pivot changes. Since the tierod is not in a neutral plane ( as design intent) it becomes longer / shorter as the suspension goes through bump / rebound pulling on the steering arm, steering the wheel. It can get pretty extreme when both wheels go through a dip together. Buying the RS hubs and tierods gets you closer but won't guarantee to eliminate the bumpsteer. That takes a bit more finite adjustment for that. On the other hand 911's have all had bumpsteer since inception, many refer to it as great steering feedback.

One early cars you adjusted for bumpsteer by shimming the rack. Not as easy on the 993 with it's racks saddle style mount. Adjustments are better made at the outboard end by raising the outer tierod end. There are kits available to do this which can get you the bumpsteer improvements the RS hubs offer. They usually create a condition where they put the outer balljoint into a greater single shear position, that is probably where the Pro's come in to make sure nothing is to extreme.

Rick

Essexmetal

What Ken said, that was not there when I started writing mine. He makes the simple point perfectly.

mgianzero

I believe the RS height is 30mm lower than our US stock height and ROW is 10 or 20 (can't remember which.) So you mean to say that we cannot lower our cars more than 10 mm without having to consider the increase in bump steer?

I was interest in getting a set of Bilstein PSS10's. I believe they can adjust down to 1.75" which brings them to roughly 45mm below US stock. So what's the recommendations when using PSS10's? When (how low) can I drop my height before I have to consider RS or GT2 uprights, which would certainly make this improvement a very slippery slope.

Marc. G.

jimbo3

^^^RS +10mm may be a good starting point to consider. It also happens to be equal in height to ROW Sport -10mm. No scraping on most roads and driveways, no significant bump steer issues. Some have gone as low as RS+5, but that's probably pushing the envelope with the standard suspension and allows no room for error. Remember that you're not likely to hit a planned height exactly- plan on being as much as 2 mm over or under after alignment and corner balance.