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%

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.

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.

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.

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" (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.


sorry but someone has to ask. is 50% mean half of lowered to the ground (100%)?

mike

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???)

Pics:

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

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

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.

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.?

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?
Click to view attachment

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

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

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.

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.

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.

Elephant racing makes the tie rod end solution. Like everything they sell , it is high quality, and pricy: $335

Click to view attachment Click to view attachment

We've been using a similar setup on the green 911 race car for years without any trouble.

Those work great and are easy to adjust.
Andy

I really thought my car had this and just needed some adjustment. Guess I got to buy it sooner rather then later.

Does raising the spindles or using the ER bump steer kit end up doing the same thing ?

No. Raising the spindles allows you to lower the car past what is possible otherwise.
Andy

No one has given a definitive answer and even elephant racing doesn't give one. I would really assume the goal is to get the tie rod close to parallel to the lower a arm.

In the pictures shown you can see it makes a trianlge. A arm is pointing down and tierod is pointing up. Correct me if I am wrong but that seems to me it would cause the tires to toe out over a bump. All the bump steer kits bring the tie rod down making this trianlge less severe.

I think moving the spindle up makes the problem worse.


Again no experience with this just looking at the pictures.

An old fat guy thinks your understanding of the 914s suspensions is pretty good.
Your conclusions are....well, *your* conclusions & thankfully, not mine.

Lowring the car is very worthwhile.....as long as it is done right & with consideration given to bump.

Angles..we gots em and it works. I'm at 4-3/4 inches at the doughnuts. I was lower but had wheel to A arm interference at a lot of steering lock & suspension compression..so back up we went.....who'da thunk it.....

Jim:
With raised spindles it takes either bending the steering arm.....which buggers up the Ackerman or tie rod extensions....these are by ERP. It can run into money to get both.

Yeah, but your overall suspension travel is what, a micrometer?


On a mostly stock suspension, with lots of travel, the more unparallel the arm is in regards to the tie rod the worser the bumpness steering gets ...
Andy

I have 2 1/2 inches of front shock travel & use it all. 21 mm T bars ain't all that stiff.
What you want to avoid is going over center with the tie rod....and the A arm for that matter.

You need to account for the pivot points of the tie rod are constantly changing in relation to the a-arm, which is static.

Oh, and what about camber and caster.....

Ya gotta account for a lot of things. You won't find them all till you try something.....and, of course, it is possible to make a hash of it.....then go to plan B.

you mean the steering shaft U-joints?

They're splined, and the shaft can be adjusted for length.

put in the spacers, loosen the clamp nuts, let the joint shafts retract into the outer shaft, retighten.

What type of steering shafts you using ?? Mine do have the splined ends as most 914's I 've seen , but some also have a half moon cutout on them for the bolt that binds them to slide through and locate the u-joint on the shaft , adjustment , all I can do to mine is install a thinner hockey puck on the rack to allow for raising the rack to run spacers. The rack shaft to inside is covered up by a plastic cover and it would have to be trimed back as it butts up against the upper shaft bearing that's under the clamp behind the fuel tank. There maybe some adjustment there , but not much other than a thinner hockey puck.

Jaxdream

This is what is on clubrsx from my searches while modifying my suspension on my rsx




From what I understand, camber can and will affect toe, so the common results of bumpsteer from after lowering a car is bad alignment, creating what the quote said. Is this true, and could be why Jim hasn't noticed massive bumpsteer, because the alignment probably isn't that far out of wack? I don't really know, but alot of their stickies makes sense for the rsx applications, so I assumed this one was good also.

The alignment is pretty close; forgot the numbers. Tire wear is even as well

Then that's probably why...

when you drop a car, alignment gets off, and suspension parts get out of wack, like thetie rods work differently because of the angle, etc.

This is from my understanding though, I could very well be wrong

HP Books - "How to Make Your Car Handle" - Fred Puhn

1981 - ISBN 0-912656-46-8

All the formulas, theories and headaches you could want!

Steve Smith Autosports Publications - "Advanced Race Car Suspension Development" - Steve Smith

1974 - ISBN 0-936834-05-6

More of the same

I like "Tune to Win" and "Engineer to Win." "Engineer to Win" gets a little more in depth as far as bump steer is concerned. "Tune to Win" might be a bit more applicable because your working with something already built.

Wrote this long thing and then just made a diagram real quick. I need to install the 3d software already.

Great Info here ! Thanks. Based on the diagram and discussion, I would say my suspension looks more stock than lowered...gotta use the floor as a guide, camera is at a slight angle.

Thanks to everyone who chimed in, learned a lot.

Well keep in mind I haven't got a clue what I am talking about. Only what I see happening by looking at the suspension myself. When I go absolutely nuts this winter with my suspension I am still gonna take it to Chris Foley since he has 20 or more years experience with these cars.

I still think this kind of setup is the way to go. Gives a huge range of adjustability




Since we are talking about toe movement I was intersted if someone could explain what this does. And how would you adjust the rear to control toe movement under load if you wanted to?

http://www.patrickmotorsports.com/part/1096/

That part makes it easier to adjust the toe on the rear suspension. Normally with the slots on the stock suspension, you have to loosen the 3 screws and tap it and then tighten it up an see where you ended up. With that little adjuster link, you can loosen the 3 screws and turn the adjuster to achieve more or less toe and then tighten it up. It would be nice to have if you do wheel alignments often. I don't have it. That's why once I get the rear toe set, I tighten those 3 screws and refuse to loosen them for anybody!!!