Bump Steer, How to tell...? Options

[jim_hoyland]

Is the test for bump steer driving over a speed bump straight on or at an angle. I don't get any noticable pulling when taking the speed bump straight on. Just want to get the test situation straight, any input will be appreciated.

Car is lowered about 50%

[ConeDodger]

Jim,
I have never heard of a test for bump steer but I have experienced it. Hit a bump in a turn and have the car pitch. My 911 is too low and I experience it all the time.

Your A-arms should be no greater than parallel with the ground with a settled suspension. If the ball joint is higher than the spindle you will probably experience bump steer.

[underthetire]

If you get a "that feels kinda funny" making a turn at speed after hitting a bump you have it. The spacer kit is cheap. 50% lower than stock you need one IMO.

[r_towle]

Jim,
Take a pic of your car, on the ground.
Lay down and shoot the pic of the lower front a-arm from the front of the car. Post that so we can see the whole a-arm.

I know...but if you roll over on your back and stick the camera under, you can do it.

RIch

[jim_hoyland]

Jim,
Take a pic of your car, on the ground.
Lay down and shoot the pic of the lower front a-arm from the front of the car. Post that so we can see the whole a-arm.

I know...but if you roll over on your back and stick the camera under, you can do it.

RIch

Will do.

[orange914]

Is the test for bump steer driving over a speed bump straight on or at an angle. I don't get any noticable pulling when taking the speed bump straight on. Just want to get the test situation straight, any input will be appreciated.

you won't feel bump steer when both wheels are evenly going up and down at the same time. hiting the bump with both wheels straight on won't be felt. you will only really feel "bump steer" when one wheel is effected. when the car darts around is a good indication. sooo... i guess a good test would be to drive around on our pot hole'd california streets where you'd have ample opertunity to feel your car "dart" (IMG:style_emoticons/default/smile.gif) (one wheel bump at a time).
the bump steer kits attempt to locate the tie rods level (not up or down). that way it's "travel ark" is least likely to pull or push the toe as it travels. thus the effected wheel has minimal toe movement.

Car is lowered about 50%

sorry but someone has to ask. is 50% mean half of lowered to the ground (100%)? (IMG:style_emoticons/default/beerchug.gif)

mike

[orange914]

Post that so we can see the whole a-arm.
RIch

like rich said the original a-arm geometry is altered and the rack bumpsteer kit helps to correct this, but doesn't re engineer the new altered a-arm angle characteristics(sp???)

[jim_hoyland]

Pics:

[r_towle]

your fine.

I will never suggest to anyone to put in the bump steer washer....
I put it in and then took it out.
It binds things in other places...namely the upper u-joints.

You are not low enough to have bump steer.
You are damn close...
If you are not autoxing the car I would suggest you raise the whole car 1/4 of an inch measured from the fender lips.
It will ride better.

Rich

[jim_hoyland]

your fine.

I will never suggest to anyone to put in the bump steer washer....
I put it in and then took it out.
It binds things in other places...namely the upper u-joints.

You are not low enough to have bump steer.
You are damn close...
If you are not autoxing the car I would suggest you raise the whole car 1/4 of an inch measured from the fender lips.
It will ride better.

Rich

Thanks; 1/4 should be doable. Just a DD, no AX

[stewteral]

Is the test for bump steer driving over a speed bump straight on or at an angle. I don't get any noticable pulling when taking the speed bump straight on. Just want to get the test situation straight, any input will be appreciated.

Car is lowered about 50%

Hi Jim:

Let's start with a definition: Bump Steer is the deflection of the Front AND rear wheels from their original path when the suspension encounters a BUMP or as a result of ROLLing in a corner (the latter is technically called Roll Steer).

I have a 914 street/track that I have been trying to get to handle correctly for quite a while. I have learned a number of things:
-The 914 was cleverly designed to be use AS DESIGNED. After lowering my car and making a HASH of the handling, I have raised back to almost STOCK ride height.
-When the car is lowered in the front, you CHANGE the range of suspension travel to a area of movement NOT intended. In this situation, you will be CREATING bump-steer issues as the Radius of travel of the suspension arm and that of the Tie-Rod become increasingly different. Thus and the suspension travels up (or down) the difference with the tie-rod will cause the wheel to DEFLECT from its intended path.
-In my car, I have 911 struts and have cut the shock tube to RAISE the spindles and lower the car. I took DAYS bump-steering and adjusting with a bump-steer tool that measures the deflection with a DIAL INDICATOR. After I heated and BENT the steering arms on the struts down, something like 3/4"--different for each car) I was able to get the front bump steer defection DOWN to about 0.010" at 2" of deflection. I made my gage, but one can be bought from STOCK CAR RACING outfits for something above $200.

I then bump-steered the REAR SUSPENSION and learned something about the design that really surprised me: As the LOADED rear suspension moves UP, it TOES-IN at about 0.200" in 2 inches of travel!!! At the same time the UNLOADED wheel moves DOWN and goes TOE-OUT about 0.200" in 2" of travel. The design goal became clear: Porsche did NOT want the 914 to oversteer. When the car loads into a corner, BOTH wheels deflect INTO the corner!
-I finally understood why I could NEVER stop the understeer on my first street 914 which ran on just 185 radials.
-I was at the Monterey Historic races a week ago and talked to a sharp Porsche specialist who prepared the 914-6 for one the of fat , old, rich guys who get to drive. He concurred on what I found with the rear suspension and told me that he usually sets up 914s at ZERO static toe-in and will go TOE-OUT on some cars, depending on the driver. The caution here is that TOE-OUT in the rear can make the car "tail-happy" and the driver must be VERY vigilant when driving the car. Oversteer can be a lot of fun in slow corners, SCARY in high-speed corners and TREACHEROUS in the wet!!!!! If you don't know now, due to the LOW POLAR MOMENT OF INERTIAL (that is to say all the WEIGHT in the middle of the car), 914s like to spin like a top when they get loose!!

I hope all my work can help you understand what is happening with the 914 suspension: I have to keep reminding myselft that the car was DESIGNED around 100 HP and 165 radials....ONLY. So the best tip I can give is to run the car at STOCK suspension ride-height and setting......and if you PLAN to be "Up on the Wheel" maybe 1/6- 1/8" TOE-OUT in the rear.

Best of luck,
Terry

[turboman808]

Wouldn't you idealy like the tie rod to be parralel to the A-arm. I can see how the suspension moving up can cause the toe to change over bumps. I know I have some wiggle over bumps with my car right now. Might be a bit low in front.


Anyways I am really interested in this setup.
(IMG:http://www.vonshocks.com/product/911/front-coilover-doubleshear600.jpg)

[orange914]

i really don't have real world experiance with modifing the 914 geometry like many of our a/x racers here. but much of long arm/short arm theory applies.

from the pictures the lower arm being level creates a good swing ark. as long as the tie rod is parallel to it, via bumpsteer kit. the theory is if the two move up and down parallel to each other there won't be "bump steer" or one pull more than the other. there are many factors that will effect this though, like unequal lengths. a cool study of this is how carrol shelby relocated the mustang upper arm 1.5" lower. this only changed the upper arm angle from downward angle to level. it also altered the ride height, altered the charicterists of the camber swing. ever see the inner wheel on a turn change to negative camber as it lifted on an old mustang before the mod.?

[orange914]

looking at the picture and reading rich's comment on it binding somewhere on him made me wander why there is a rack shim kit but i've never seen a tie rod end relocating kit. i would think that would be better. my mustang bump steer mod. was at the tie rod end and not only did it correct bump steer but repositioned the tie rod slightly to correct o.e. ackerman angle. is there a tie rod bump steer for the 914?

[r_towle]

looking at the picture and reading rich's comment on it binding somewhere on him made me wander why there is a rack shim kit but i've never seen a tie rod end relocating kit. i would think that would be better. my mustang bump steer mod. was at the tie rod end and not only did it correct bump steer but repositioned the tie rod slightly to correct o.e. ackerman angle. is there a tie rod bump steer for the 914?

yup
Elephant racing or tarretts, I cant remember who...but yes there is a tie rod offset solution.

Tie rods always need to be at an angle

Rich

[SirAndy]

Tie rods always need to be at an angle

(IMG:style_emoticons/default/confused24.gif)

[orange914]

Tie rods always need to be at an angle

(IMG:style_emoticons/default/confused24.gif)

(IMG:style_emoticons/default/agree.gif) as "stewteral" put it "you will be CREATING bump-steer issues as the Radius of travel of the suspension arm and that of the Tie-Rod become increasingly different." this may not be spacifically refering to the tie rod only, but i think it shows how the tie rod radius of travel is effected. it is less extreem or exagerated when its parallel. it kind of splits the differance, if that makes any sense

[jmill]

There is a bump steer test. It's alot easier to do on cars you can pull the spring out with upper and lower A-arms. You put the car on jackstands and place a jack under the A-arm. Jack the A- arm up to ride height. Place a large plate in front of the rim making sure that the distance between the front and rear of the rim to the plate is the same at ride height. You then jack the A-arm higher and measure toe. Then lower it to measure your toe in full droop. Now adjust to to get minimal deflection in bump and droop. Longacre(sp?) sells special bump steer plates.

[jim_hoyland]

Thanks Terry-That is a good explanation. Am I correct that a lowered 014 should use the raised spindles to reduce ( or eliminate ) bump steer.

Does my picture indicate a bump steer conditin ? I do not get the feeling I have it.

[Cevan]

From my own experience, my car had pretty significant bump steer when it was lowered probably 1 1/2 to 2 inches from stock. I put bumpsteer spacers in which helped. Then I got a proper alignment at Chris Foley's shop and it was like night and day. Granted, my alignment was pretty out of whack to begin with. Now I have zero bumpsteer.