The Stovebolt Site Forums The Shop Area General Truck Talk Bumpsteer and scrubing woes 79 to 66 front

I just got done converting my 66 C10 over to disc using a 79 donor. I used the spindles, outer/inner tie rods, centerlink, and idler arm from the 79 and used the remaining parts of the 66 (arms,springs, pitman). Now the problem I am facing is bad bumpsteer and scrubing when turning (Akerman effect). Now I done a lot of reading and question asking to lead to this project but no one ever mentioned this. Did I miss a step or are the rest of yall that did this conversion suffering the same side effects. What is the fix if any.

I suggest that you see an alignment specialist to get the toe, camber, and caster angles set. Sounds to me like the toe is way off.

This is after the alignment shop. Thanks though

Hmm, shouldn't do that if it was set properly. What kind of backspace are you running? Sometimes if the wheels are set out too far on the hub center, they'll do that, but it really sounds like the toe isn't right.

They are the stock steel wheels off of the 79. The toe is at 0.05 on the left and 0.10 on the right. The specs are min 0.05 and max 0.10 on thier computer so its reading just in the min on the right and max on the left.

Is your center link horizontal and not higher at either the pitman arm or idler arm and is it parallel to the centerline of the front crossmember? Are your guide rods parallel to the ground or just slightly lower at the outer ends? Are the ends of your steering arms the same distance from the spindle centerline and the same height from the floor on both sides? If yes to all this you may have to pull the springs and then raise and lower through the entire suspension travel by a jack under the front crossmember and watch what is happening. Something at some point is pulling things out of line. These trucks did scrub the tires to a degree when cranked against the lock, usually more one way than the other.

The center link is 3/16 lower on the idler arm side. The center link is also closer to drivers side alowing 14in. on tie rods as opposed to 16in. on pass side. The outers are lower then inner. Also there was 40 psi in the tires instead of 36. I dobt that would have caused anything. The 3/16ths is probally the culprit and the only way I can think it got out that much was I had to use the worn pitman on mockup. Thanks for everyones input. I will give it a try and see.

by the way the shop used the specs for a 79 model istead of a 66. Would this cause any problems.

Since you're dealing with some pretty sophisticated 3-axis trigonometry on that steering system, some alignment shops don't have enough theoretical knowledge to spot a potential problem, especially with a modified system. With the wheels pointed straight ahead, both the Pitman arm and the idler arm should be parallel with the frame rails. This makes a perfect rectangle with the Pitman, the idler, and the relay rod. The relay rod also must be parallel with the shop floor. The distance from the inner tie rod pivot on the relay rod to the steering arms should be virtually identical side to side. The inner tie rod pivot points should be almost even with the lower control arm pivots, or at least the same distance away from each side.

When the steering is at full travel both directions, the toe-out should be equal. The "Ackerman" angle is a theoretical figure which establishes the turning radius for each wheel. That's why the inner wheel on a turn needs to toe out, to keep both front wheels turning on the correct radius to prevent them from fighting each other in a full-lock turn and scrubbing the tread off. On the race cars, where we would often bend steering arms from rubbing wheels with each other on the dirt track, we had a set of Ackerman lines drawn across the floor of the shop, and we'd bend the steering arms back into place until the Ackerman angles were back into specification.

Most of that kind of geometry is beyond the capability of anything but a race car alignment specialist. Do you have a pro race team in your area? (NASCAR, not drag race!)
Jerry

Alright i tried to move the idler arm up but it will not clear the frame. Do I have to use the pitman off the 79 model, bend mine, or is the way I have it the only way and it's made up in the tie rods. There has to be somebody thats done this swap before, I hope the info I read wasn't just speculation. I think it's mainly the pass side tire thats bumpsteering and it is the link thats 16 inches as opposed to the 14 on the driver side.

Bumpsteer generally occurs when the pivot point on the inner tierod end doesn't line up with the pivot point of the lower control arm.
So that 2in different from side to side maybe the problem.

What you have listed for the toe readings tells me that they did not set it correctly. The toe should be the same on both sides, not at min spec on one and max spec on the other.
Now everything that HRL told you is correct, and that would be the place to start. If you have access to the other idler, you might try it to see if there's any improvement. You might see if you can find an alignment shop that's more welcoming to modified/older vehicles to have it re-set. It seems that the shop you took it to did a "toe and go" job.