Bump Steer

Why a post about bump steer? Well, because we’ve built two trucks and both suffered from horrendous bump steer problems.  One was brought on by the installation of a drop axle, and the other by upgrading to later GM power steering setup.

How to make it livable? You need to get the drag link parallel with the ground. A short drag link that is steeply inclined could see you bouncing into a ditch or into the oncoming lane. Not knowing how significant the effect might be, I built a simple model.

If you start with a steeply inclined drag link,
and hit a bump,
you get a lot of steering effect.
If you start with a more level drag link,
and you hit the same bump,
you get negligible bump steer. Compare to the original bump steer line.

The ’48 Chevy had a CPP kit which facilitates an upgrade to a 1967-89 Chevy truck power steering setup . This entails  mounting the steering box to the outside of the frame rail. CPP sells a Pitman arm for lowered trucks that allows the drag link to be dropped in from above the Pitman arm. In addition, we fired up the oxy-acetylene torch and bent the both the Pitman arm (yellow arrow) and the steering arm (white arrow) to achieve a horizontal drag link when the car was sitting on the ground. This picture was taken with the ’48 on the lift so the drag link is at a slight angle in this photo. It took a couple tries, but we finally made the steering manageable.

The twist in the Pitman arm is to prevent binding at extreme bump or extension angles.

Since this truck has significant body roll, especially at the front end, at some point we will source an anti-roll bar. (We hope) that will further decrease the bump steer effect and make the truck a little more sporty in the corners.

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