"Bump Steer Made Easy" shows you how to measure bump steer accurately, using only a single point laser you can buy from the hardware store. The laser is pointed at a mirror fixed to the hub, with the laser beam reflected back to a target. The laser point moving horizontally on the target shows toe change, as you jack the suspension up and down.
The end result we want from the bump steer measurement and correction process is clear. Whether road or race car, we simply want zero change in toe in - toe out, as the wheel moves in it's normal range of bump and rebound travel. (In some modern vehicles such as the Lotus Elise, bumpsteer is used to modify handling. We would not try this on our race car without specific knowledge because the results would be uncertain.)
This course is a complete, practical guide to what you need to know about bump steer.
Suspension workshops do not measure it because car manufacturers do not make any provision for the adjustment of bump steer. With un-modified late model cars, it should not change from factory specs. Older vehicles and race cars are a completely different matter - we do need to check it, and fix it if it's out.
For modified race and road cars, checking the bump steer should be a high priority in the initial set-up of your car. The regular changes you make to suspension geometry, such as more caster, will more than likely put the bump steer out. Fitting steering rack kits or roll centre height adjusters to strut cars are just two examples where bump steer correction will most certainly be required. Of course, all purpose built race cars and other vehicles with fully adjustable suspensions must be bump steered regularly.
Now you can check bump steer accurately with out an expensive and hard-to-get bump steer gauge. See Longacre Bump Steer Gauge demo on youtube. These gauges are a pain to use. With the laser, as you will learn here, once you have worked out how to attach a mirror to your hub or disc, it's plain sailing and you'll just be reading off the bump steer on a simple target.
To measure bump steer, we put the car on jack stands or a two post hoist, remove a spring from one corner of the suspension and re-assemble the suspension. We can now move the wheel up and down without restriction from the spring. With the suspension position set at ride height, we then move the suspension in the bump and rebound directions, over the normal travel of the suspension, and measure the toe change.