TheSamba.com :: View topic - Steering Geometry Rack & pinion 2.25 over 100 mph bug

Mon Aug 25, 2014 6:45 pm

Hi all, So I finally got the bug running great and able to bump up to 100 mph on freeway with ease (not that I do it much or for any length of time).

Its time to tackle the steering once and for all. Its not my daily driver but its driven nearly every day on the street.

I have a 6" over beam with 2.25 x 1 over front arms with standard height combo spindles. Center rack and pinion steering - mounted at about 21 degrees.

Currently, if I set ride height where the trailing arms are just about level and the tie rods are also very level, I have about 7" of wheel travel up and 7" of wheel travel down (at least once I clearance the frame for the tie rods). The problem is my tires TOE OUT about 1.5" or more at these extremes. I have a decent grasp of geometry and understand the arc theory of the tie rods and have rotated the rack considerably.

The tie rods are clearly pulling at the combo spindles, swinging the front of the tires out.

It is baffling me and am ready to drop the car off at whoever can do it for any amount of money if I can't get it figured out soon.

Any ideas? The beam tubing is 4.75" center to center and the combo spindle pins are 5.25" center to center. I assume this is not the issue but just mentioning it.

Thanks

Mon Aug 25, 2014 6:53 pm

you will have to cut the rack mount off, get 2 big hose claps and clamp it all together and cylce suspansion till bumpsteer is minimal
.
possibly need to move rack mount further back

Mon Aug 25, 2014 6:59 pm

Thank you, but I have rotated this rack from 15 to 25 degrees from vertical with very little or noticeable change.

When you say back - do you mean towards the front of the car as I think for the arc theory of rotation of the tie rod the tie rod should start from the centerline of the beam from front to back?

Mon Aug 25, 2014 8:00 pm

KrAzY-BaJa is correct. You will likely have to relocate the rack and pinion so that the arc of travel of the trailing arms are as close as possible to being in sync with the tie rods. This may mean moving it forward, back, up, or down on the chassis. The devil is in the details. It is near impossible to get zero bump steer due to the difference in the arc of travel, so you just need to find the best compromise.

The end load rack you are using makes the tie rods short. If you had A-Arms, then the arc of travel of the arm and tie rod could be nearly perfectly matched with minimal bump steer since they pivot the same direction together. Since the arc of travel of the tie rod in your setup is in - out - in, and the arc of your trailing arm is forward - back - forward respectively, you will need really long tie rods to help minimize the toe in / out effect. What you should be using is a center load rack and pinion, this way your tie rod arms are as long as they can get, and at 8" up and down your arc of travel would be the closest to the trailing arm you are going to get.

Tue Aug 26, 2014 3:32 pm

At a glance, moving your rack back will assist. A centre-pull rack, like that fitted to a 75-79 Super (or Passat, or some Opel/Daewoo-designed GM cars) will provide longer tie-rods and less bump steer... how much work are you prepared to do? But you will never NOT have some bump-steer with the trailing arm front suspension design

Tue Aug 26, 2014 5:38 pm

Hi Thank you for the replies. Just for clarification the rack is a typical "center load" rack as far as I know - The tie rods are fastened at the center of the rack just under the center point where the steering column connects.

I have beeen adjusting all day and am within 3/8" toe out at both top and bottom extremes. I did this by basically lowering the rack mounting position. I guess my first question would be how will 3/8" perform on the road? Typically not more than a few inches of travel??

Can you clarify when you say move the rack back - Is this towards the front of the car - it is difficult to move it more than 1/8" more as the tie rod connectors interest with the beam.

Thanks

Tue Aug 26, 2014 10:23 pm

My mistake, from the perspective of your pictures it looked like an end load rack. If you only have 3/8" toe out at the far extremes, that might be about as good as you are going to get. That also suggests that your up/down location of the rack is pretty well centered, leaving only forward to back as potential for improvement.

If you want to be sure, try adding 1" spacers(washers) between the rack and the rack mount with longer bolts. This will temporarily move the rack back laterally and test it at the extremes again. Make sure to set the toe again before measuring. If its worse then you have no room for improvement, put it back to the way it was. If its better, then consider permanently moving the rack mount.

Tue Aug 26, 2014 11:42 pm

You are way off where you need it to be if you are getting those sorts of measurements on bump.

There is a video of a 6" wider BJ Beam below with Daewoo R&P we setup.

You have two options to set it up.

Option 1 - Keep doing what you are doing But make us some adjustable tie rods that slid in and out so you can change the position of the rack and keep going.

Option 2 - Get it draw up in CAD and mathematically calculate the position it needs to be.

You need to understand what is going on and what you pivot points are on everything. You have aftermarket spindles, arms etc so chances are they were never designed anyway and just made meaning the angle of your steering arm might not even be correct and its location relative to the rest of your spindle and pivot points is going to make life hard.

A lot of the aftermarket crap is designed for racing and thus use on dirt where dump steer is less noticeable as the driving surface gives way so to speak.

The more travel you have the more bump steer you should expect.

Factory VW BJ Beams with steering box have ZERO Bump steer, those German Dudes where very smart.

Wed Aug 27, 2014 3:12 am

Hi Bajalobster, sorry, I thought it was an end-loader rack, too... You must be getting close

Good vid and info Iguana :) You are right about the standard front end being good with bump steer.. the challenge for us all is to keep it that way with all the other 'tricks' we need/want such as camber, caster :shock: