Flat Top

I have noticed that there are varying attitudes when it comes to bump steer and its measurement.

Which one is you?

a) I just leave it as it is because it is too tricky;
b) I just add the spacers and leave it at that;
c) I trust the repairshop to come up with the correct answer;
d) I follow Carrol Smiths recomendations or
e) I have my own system (explain).

Bill L Seifert

I'm a) Sure would love to have someone explain how to do it to a 944.

Bill Seifert

1987 944S Race Car

Professor Helmüt Tester

f.) I measure it and I make appropriate adjustments. Too many people think it's some strange voodoo...it's not.

Most repair shops have no idea what 'bump steer' is, or how to measure it or correct for it. Be prepared for a lot of vacant stares when you say 'bump steer'.

Measure it yourself:

1.) Measure from a known point (like a spot on the fender) to the center of the wheel hub with the car at static height. Write that measurement down.

2.) Jack up car, remove wheel & spring. Fasten metal plate to the hub (if you have a drilled plate, great, or you can just vise-grip it to the rotor).

3.) Place a jack under the front knuckle and jack the hub back up to the static height previously measured (see #1).

4.) Fix two dial indicators to something solid and adjust them so that they contact the plate approximately 20-25" apart. Record their readings, then jack the suspension corner up and down 1/2" at a time and record all readings, until you've gone thru most the suspension travel range (which you should have determined long ago with the old 'zip tie on the shock shaft' trick).

5.) Plot the #'s you measure on a sheet of graph paper. Voila ! Bump steer data, and no computer or DAS required !

Now...what to do with that data ?

Chapter 2 to follow...

Cory M

Picture of the standard bumpsteer gauge from another site:

Bill L Seifert

Axiously awaiting Chapter 2. Hey, when I got my Smart Camber guage it came with instructions on how to do bump stear with it. Has anyone tired that? Does it work?

Bill

Professor Helmüt Tester

Onward...

Cory - great picture. Note the lack of spring on that strut, and the floor jack and 'organic composite' spacer on top, used to move the suspension thru it's vertical range. I prefer hemlock fir, but you can use spruce. The purists will use a rare hardwood, like bubinga.

The pix shows a plate, appropriately drilled for the hub center. Nice, but not essential. All you need is a piece of aluminum (smooth surface for the dial indicators) of the right size. 'The right size' = approximate (~+/- a bit) diameter of the tire in length, approximate suspension travel distance in height. You can 'Vise-grip' it to the rotor, above or below the hat, whichever works best.

The pix shows a commercially made frame for holding the two dial indicators, but you don't have to buy the expensive stuff. Buy two cheap dial indicators and magnetic bases (2 x $16.95 from Enco - http://www.use-enco.com/CGI/INPDFF?PMPAGE=189), and stick them on your steel tool box (load it with tools so it won't slide around) at the right height to contact the 'plate' on the wheel hub.

Longacre has a good write-up on measuring and correcting bump steer on their site - http://www.longacreracing.com/articles/art.asp?ARTID=13

So...by now you should know how much bump steer you have thru your normal suspension travel range, and have a clue about why it's there (excessive lowering, weird-*** components, etc.). Now you've gotta fix it. You have two choices - 'move the inner parts' or 'move the outer parts'.

'Move the inner parts' - You can correct the problem by moving the steering rack (or steering box) up or down, to return it to its 'design geometry range'. For you Porker guys, you can use something like this: http://www.paragon-products.com/product_p/rs-050.htm

Now...I've only moved 'steering boxes' up and down in a chassis, but not racks, because I generally detest German cars, and the German cars I've raced were in classes where we weren't allowed to diddle the racks. Some Porker expert could probably comment here with actual knowledge of the ease/lack of ease of this project (imagine that...actual 'knowlege' on the iNtARwEb...!!!).

"Move the outer parts" - It's actually much easier to modify the connection between the tie-rod end and the steering knuckle, and lots of kits are available to do this. Generally, since you've lowered the **** out of the suspension, you need to lower the connection to the knuckle, restoring the original geometry. Thus: http://www.gprparts.com/brands/rsrProducts/index.asp

Basically, this is a kit of 'sleeve spacers' on an extended stud that fits into the taper where the tie-rod end connects to the knuckle. Add spacers, measure bump steer. Add or subtrack spacers until it comes into range. The kit shown only has spacers in 1/4" increments...and I don't think that's 'good enough'. The Baer kits I've used supplied a variety of spacer thicknesses, from 1/32" to 3/8". Be nice to your local machinist and he can make you a handful of spacers in 30 minutes. Or...buy a lathe and learn how to do it yourself.

Done this on several cars, including the 'Red Sled'. It had a really bad bump steer problem which got worse when I went from '65 knuckle/spindles to the bigger & stronger '70+ knuckle/spindles. How bad ? Try 3/4" bump steer over a 5" travel range. Yikes. You can't imagine how bad the car felt on-track before the fix. After the fix, it was still a big wallowing overpowered sled, but the bump steer was solved and some of the 'terrifying' part was gone.

Next season, I'm going to play with major changes in 'Ackerman' on the Red Sled. Not quite sure how I'm going to do it...but I think I have a plan. The circle track guys pay a lot of attention to it...and on 'suspension set-up', they're a lot smarter than road race guys. I think there are some major gains to be found there....

Bill L Seifert

Holy cow, you are about 10 feet over my head. One question though. You say you do not like German cars. Do you know if the kit you speak of is available for 944's? Or could I make one?

Is what you are saying is that you lower the end of the tie rod (The end that attaches to the hub) so that the tie rod is level with the rack?

Bill

kurt M

That is it in a nut shell for the most part. The idea is to restore the original tie rod angles the car maker intended.
Check the Porsche peformance parts sellers you can get kits to do that or you can lower the rack as well.

Cory M

Bill-

I think the first step for you is to determine if you have a bump steer problem, then figure out the best course of action. I don't know of any 944 bump steer kits but maybe they're out there, you can't just use rack spacers on a 944 though because the rack attaches to the crossmember differently than the 911. Bump steer probably won't be a problem unless you've lowered your car a lot. Most 944 guys who have excessively lowered their cars and are still using the stock control arms run into ball joint problems.

Geo

Bill, I don't recall any allowance for altering the bump steer in IT.

However, VERY stiff springs will help combat several ills: bump steer and camber change for starters just by reducing the range the suspension will sweep through. They will of course help keep the ball joints from binding assuming you haven't lowered it so much that any travel will bind them.

I had a short discussion with Don Istook on Friday about the suspension conversation Vaughan and I have been having. Don was a bit surprised at the rates we want to run, but I explained we basically have been observing what some of the pro teams have been doing and trying figure out why. The bump steer and camber change are the two reasons I think WC teams are running ungodly rates. I could be wrong. It's a bit of monkey see, monkey do, but with some rational behind it. If anyone has any other ideas why they are running these rates I'd love to hear it. Of course I should have asked Don what they are running on the Grand Am S4s they run, but the conversation quickly moved a different direction.

Anyway, I don't think we can fix bump steer issues other than high spring rates.

Bill L Seifert

Kurt, thanks for the input.

Geo,glad you chimed in, I kinda figured lowering the rack would be against IT rules. Besides going to 600 pound springs, I don't think I am going to lower the new car as much as I did the old 83. (The 83 was lowered 1 1/2 inches, I think I will just lower the new car 3/4 to 1 inch.) I know I have asked this before, but I can't remember what you told me. What size torsion bars do you have on your 944 race car?

Thanks everyone,

Bill

KPV

Is anyone aware of a bump steer kit available for a 2002 996TT similar to the Professor's post above?

If so, please post a link.

Larry Herman

Hey Perfessor, when I was trying to solve a bump steer issue on a car with clandestinely raised spindles, I just went neanderthal on the spindle tie rod arms and BENT THEM! Put a 4 foot length of pipe over the tie rod arm, and with a 4 foot bar bolted throught the brake caliper ears, v-e-r-y s-l-o-w-l-y bent them down 3/4". Worked like a charm. You could probably massage them in or out this way to play with your Ackerman, but I wouldn't move them too much.

Professor Helmüt Tester

I have 'un-bent' tie-rod arms on knuckles before using the 'red wrench' (oxy-acetelene), but I never like bending castings...gives me the *******. I've seen some knuckles that were really 'pretzeled' that were straigtened back out, but there is a fine line between 'enough' and 'too much' when dealing with a casting, and I don't know where that line is. I always figure I have 'one shot' at getting a casting back straight, and for my Ackerman experiment I want something that I can change at the track, so I can do a back-to-back test.

As it's an old 'steering box' system with a center drag link and opposite side idle-arm, I'm thinking that Crazed Machinist friend and I can cook up an adjustable 'slotted & stepped' center link for experimental purposes.

Had a lot of talks with the Goodyear and Hoosier tire guys about getting more bite in the front end without cooking one tire vs. the other, and it all pointed towards optimizing Ackerman.

Larry Herman

I remember my old British sports cars that had anti-ackerman. The outside front turned greater than the inside, the theory being that the outside runs at a greater slip angle than the inside. This is probably very true with those old skinny tires, but I don't know how that would affect today's mini steam roller tires. This is one area (of many) that I know little about.