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IRS Bumpsteer |
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The KB rear steer adjustment kit pictured below was installed to enable bumpsteer adjustment in the IRS. The product replaces the inner tie-rod
ends and tie-rods, and provides more than enough adjustability to accommodate the required adjustments. However, due to vehicle production tolerances, the sleeve needed to be machined to provide proper fitment. A rear steer adjustment kit will be
offered by MM in the near future that will address vehicle production tolerance issues and provide some other performance-enhancing features. |
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The data below shows how much of an improvment can be made in bumpsteer with a rear steer adjustment kit.
The factory toe-curve of the IRS
provides 'roll-understeer' at the rear of the car. In roll-understeer, body roll induced by cornering results in toe-in at the outside rear wheel and toe-out at the inside rear wheel - effectively steering the rear of the car out of the turn rather
than around the turn. At 2" of bump travel, the factory roll-understeer and bumpsteer were considerable - roughly 0.35 degrees of toe change at each wheel (i.e., toe-in at the inside wheel and toe-out at the outside rear wheel). This has a significant
adverse affect on handling, but is what Ford wanted to make the car 'safer'.
The new toe-curve provides 'roll-oversteer' at the rear of the car. In roll-oversteer, body roll induced by cornering results in toe-out at the outside rear wheel and
toe-in at the inside rear wheel - effectively steering the rear of the car around the turn. |
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| Use of a bumpsteer gauge (or an alignment rack) is mandatory for adjusting bumpsteer. In my car, the driver side and passenger side of the IRS
required completely different shim stack-ups (as depicted in the data above) to provide uniform and desired results. Approximating/eyeballing the tie-rod orientation could result in worse handling than with the stock tie-rod ends or an unsafe condition.
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Copyright 1999-2003 David O. Simmons
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