Bump steer?

tlarocque · 10-09-2012, 12:29 PM

I apologize in advance for being a complete moron when it comes to how my cars work but.... I have an '88 Carrera that my dad purchased new. I bought it from him several years ago and one of the first things I did was get the car lowered (its pretty low but I like the way it sits). I didn't drive the car too much until I moved back out to CA a few months ago. It is now my daily driver and I love it!

My question.... I drive the (crappy) freeways quite a bit. Yesterday I hit a couple big bumps while driving at around 75 mph and the car nearly got away from me! Is this "bump steer" and would a bump steer kit be the way to go to try to remedy this characteristic?

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by tlarocque

I apologize in advance for being a complete moron when it comes to how my cars work but.... I have an '88 Carrera that my dad purchased new. I bought it from him several years ago and one of the first things I did was get the car lowered (its pretty low but I like the way it sits). I didn't drive the car too much until I moved back out to CA a few months ago. It is now my daily driver and I love it!

My question.... I drive the (crappy) freeways quite a bit. Yesterday I hit a couple big bumps while driving at around 75 mph and the car nearly got away from me! Is this "bump steer" and would a bump steer kit be the way to go to try to remedy this characteristic?

Nice car! and very low

all 911's have bump steer, the amount varies depending on how the car is set up, bump steer is a result of the different arcs that the steering and A-arms travel through, as the wheel goes up it pulls the steering one way and as it goes down it pulls the steering the other way, this is felt as a liveliness in the steering wheel. Excess bump steer causes car control to suffer

you get excess bump steer when the car is lowered a lot w/o some sort of compensation in the steering mechanism, the steering rack can be lowered, the ball joint can be extended, the strut steering link can be raised or some combination of the 3 can be used.

Other things that could also cause car control to be difficult is if the shocks bottom out

megerian · megerian's Garage

FYI - my car had pretty rough handling and a bad case of bump steer. after new tires and an alignment, the bump steer was immensely improved, to the point of being barely noticeable.

So - a bump steer kit is not the only remedy.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by megerian

FYI - my car had pretty rough handling and a bad case of bump steer. after new tires and an alignment, the bump steer was immensely improved, to the point of being barely noticeable.

So - a bump steer kit is not the only remedy.

On a very low car it is, alignment of course is important but it does nothing to ameliorate bump steer which was increased by lowering the car

here's a 911/930 front suspension

If this was installed w/ the geometry pictured the car would be extremely low

look at the angle of the steering arms and A-arms, any bump at a wheel the causes ipward wheel travel will cause excess pull on the steering arm of that wheel

for minimal bump steer the A-arm will be pointed slightly down and the A-arm will be horizontal
like this

tlarocque · 10-09-2012, 01:45 PM

Very helpful, thank you!

Would a simple bump steer kit (that utilizes spacer to raise steering rack) suffice for a street driven car or do I need to consider something like the Elephant Racing or RSR products?

1QuickS · 10-09-2012, 02:04 PM

The rear suspension has bump steer also which if the car is very low is worse than when set nearer normal design ride height. There isn't a fix to minimize bump steer for the rear without significant suspension surgery.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by 1QuickS

The rear suspension has bump steer also which if the car is very low is worse than when set nearer normal design ride height. There isn't a fix to minimize bump steer for the rear without significant suspension surgery.

Yes that's true but isn't of so much concern because there is no steering arm involved in back,

rear bump steer is caused by toe changes induced by wheel travel.

77TARGAV8 · 10-09-2012, 02:54 PM

Our sponser here sells a bump steer kit. It helped me, took about 1/2 hr total to install. It raises the rack a 1/2 inch.

esonefour · esonefour's Garage

Quote:

Originally Posted by tlarocque

Love the wheels itching to find a set for sale in NZ.

I'm just about to lower mine this weekend. In the front it has a threaded adjuster is yours wound all the way down?
Haven't looked into the guide for doing the rear yet, don't think it's going to be as easy as the front

Walt Fricke · 10-09-2012, 04:32 PM

Kiwi - you can't judge how low you are by how much or little adjustment is left. The adjuster can be in several positions on the splined end of the torsion bar, and that determines how far out the adjuster sticks at any given ride height. So if you back the adjuster off all the way, and your car is still too high for your tastes, you can just reindex the adjuster on the splines, and start again with many more turns of the screw to go.

You want to judge ride height the way Porsche does - by the method shown in Bill V's diagram. What is the difference between the axle centerline and the torsion bar centerline. This is pretty easy to measure approximately (eyeballing where the center of a circle is). I don't know where the 125mm in the diagram comes from, but the Euro spec for an SC is, I believe, 108mm plus/minus 5mm. But the cars will take rather more than that. In which case, those inexpensive rack spacers and longer bolts are a good idea. Got to reset the toe after doing this.

I don't like the method of measuring fender height. Depends on tire diameter/radius, and also on how evenly the fenders were attached - they are often off by an inch or more. Measuring Porsche's way is much better. It also gives you a pretty good indication as to whether your chassis is parallel with the ground.

The bolts on my rack have 16mm heads. Luckily, I have 16mm sockets, but I don't recall any other fastener on a 911 using this size of bolt head. Do I have some kind of oddball fasteners (which came 20+ years ago with the kit)?

I know this because I removed the rack so I could thread the turbo tie rod ends deeper in, and cut 5mm off their ends, as I was running out of threads and adjustment to keep the toe at zero. Somehow I'd put the turbo tie rods on too tight to be unscrewed - even with the rack held in a vice and a huge pipe wrench on the ball joint part. But this complication need only be dealt with if you lower to that extent.

tlarocque · 10-09-2012, 06:27 PM

I think I will order the spacer and hope it helps. I've never worked on a car but seems like I might be able to handle this??? I know I will have to re-align after.

911pcars · 10-10-2012, 10:06 AM

The "bump steer" kit is basically a couple of spacers and longer bolts. This raises the steering rack higher in the chassis to compensate (somewhat) for a lower ride height. When raising the steering rack, also adjust the steering shaft length in the U-joint. An adjustment compensates for the difference in the now-closer steering rack.

Make sure you can still rotate the steering wheel lock-to-lock. If the steering shaft intrudes too far into the U-joint, it can bind. Grind the end to provide needed clearance.

Finish by adjusting front toe to specs.

Sherwood

RWebb · RWebb's Garage

do you really want the car as low as it is?

does it hit on driveways, entrances?

Walt Fricke · 10-10-2012, 12:41 PM

Getting a car as low as possible (shock and strut travel impose absolute limitations, and suspension geometry can become less than optimal) makes sense only for track cars. It is a compromise for dual purpose cars. I just lowered mine more, and have to get out the boards for loading/unloading on the trailer. Just slide by on the driveway curb cut if I take it at an angle. But I've pretty much retired the SC from the street (we now have a 997 Turbo for that).

And when I can figure out how to attach a lip spoiler below the current front valance, I'll add that, which will really complicate loading/unloading but help on track some. Some kind of quick release would be ideal. Maybe I'll succumb to buying a composite bumper/valance/splitter held in place with four or so Dzuses.

Can't have it all. When I drove the SC to work as well as running it on the track, Ihad the front (with taller street tires) up high enough that it would just sneak over parking lot curb blocks if I pulled a bit too far ahead. That seems a reasonable limit to lowering for the street.

Installing bump steer spacers is about as easy a thing as a DIY guy could expect to do himself.

I haven't run into a need to shorten the lowest splined shaft when doing this, but that's certainly a good thing to look for. You have to bet in under the trunk to loosen things up, or I don't think you can pry the rack up enough to get the spacers in.

The spacers in these kits are about as tall as you can use. You can only add a couple of washers worth more before the rack hits the chassis, and doing that will press, if not exceed, the ability of the U jointed plus intermediate bearing system in the steering column to function without binding or needing significant modification.. Which is why, for ultimate lowering, racrers favor, if rules allow, either bending the trailing arms, or using a special tie rod end attachment system. Those also allow fine tuning, though measuring toe change with suspension movement and adjusting to optimize on a torsion bar car is pretty tedious, as you have to disconnect the torsion bar as a practical matter in order to do the measuring needed.

But to each his own on how he wants his special 911 to look on the street. Practical disadvantages will quickly become apparent.