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Discussion Starter · #1 ·
I had someone ask me what bumpsteer was, so I thought I would post this.




Bump Steer is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the track interacting with improper length or angle of your suspension and steering linkages.

Most car builders design their cars so that the effects of bump steer are minimal. However, you must still take care to bolt on your suspension carefully so as not to create unwanted bump steer. Make sure that you are always using the correct components for a particular car. Bump steer must be designed into the car and cannot be adjusted out if improper parts are used or if pivot points are moved without considering bump steer design principles.

In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm (See diagram below).

The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center.

Sounds complicated? Really it is very simple. To achieve zero bump the front end must be designed correctly. The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires.

To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc. In this scenario the tie rod would shorten much quicker through travel than the suspension and the car would toe in drastically over bumps. The shorter arc of the tie rod would pull on the spindle and toe it in through travel.
 

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Discussion Starter · #3 ·
No problem. I had someone ask me what it was and couldn't really explain it too well so I found that info and thought maybe other people may be not 100% sure what it was and could use the info.
 

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Basically as the suspension cycles through its travel bump steer will cause the front tires to toe in. The stock KFX has almost ZERO bump steer, BUT be very careful buying a-arms and shocks for this bike. Those companys that have not done their homework will run into MAJOR issues with bump steer. My front end kit will fix this issue and will be designed correctly.

FUELATV
 

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It's toe-out.

I don't know how much though.

This front end has a lot of "tire scrub"... difference in how narrow it is extended vs how wide it gets maximum in compression... so we are pretty fortunate we don't have a ton of bump steer.
 
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