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Tune With Bump Steer

If you asked the average RC racer to list the common suspension and steering adjustments, they’d quickly rattle off the obvious such as camber, shock angles, shock fluid and maybe roll centers, but there’s no doubt they’d most likely fail to include bump steer. In fact, most would describe bump steer as an ill–a symptom of a bad design or poor geometry. Bump steer, however, can be extremely useful in lowering lap times or even taming a truck that’s a bit too tough to handle.

This Losi XXX-SCT is set up with a slight amount of bump steer with toe out. There is zero toe out when the suspension isn't compressed.

WHAT IS BUMP STEER?
Bump steer describes how the front tires either turn in or out as the suspension compresses. More accurately put, bump steer is the increase of either toe in or toe out as the suspension compresses. Bump steer is influenced mostly by steering link length and caster.

WHAT BUMP STEER DOES
Despite its name, bump steer doesn’t come into play only when going over bumps. Before a truck enters a corner and brakes are applied (you brake straight and turn on-power, right?), weight is transferred forward. This increases front traction, but it also compresses the suspension. So, if a truck has bump steer dialed in, it will come into play durning cornering.

On smooth tracks, bump steer has little noticeable impact when landing from jumps unless there is an excessive amount of bump steer. Bump steer, however, can have a profound impact on handling when racing on uneven, rough tracks. The reason for this is that when a truck lands from a jump on smooth ground, the toe may quickly change, but just as quickly returns to normal as the truck travels straight. On rough, rutted section where the wheels are rapidly traveling up and down independently, the front tires simultaneously exhibiting different degrees of toe in and/or toe out. Essentially each bump causes erratic steering.

Bump steer can also be used as a tuning aid. It is actually quite useful because toe out provides aggressive turn in going into corners. The down side is that toe out can make a truck hard to control down the straights as the steering will react to every steering input. The truck will simply not want to just drive straight. Thus, having toe out only when the suspension compresses can be useful to help steering and yet keep the truck drivable. Keep in mind this setup usually only works on smoother tracks.

Bump steer is adjusted by adding or removing washers under the steering link's outer ball stud.

ADJUSTING BUMP STEER
Bump steer is adjusted by adding or removing washers under the steering link’s outer ball stud. Adding washers will increase bump steer with toe out. Removing washers will reduce bump steer. Start with zero bump steer. Test your truck by getting it race ready and compressing the suspension while watching it from above. On some trucks, if you remove too many washers, you will get bump steer with toe in. This depends on the truck, of course, as some vehicles have geometry designs that provide no amount of bump steer with zero washers under the ball stud.

Bump steer should always be checked after making caster or “kick up” adjustments. When adjust static toe, bump steer should also be checked.

 

Photo by Patricia M. Strickland

 

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