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Steering rack and FARB shimming - Suspension/Brakes/Wheels/Hubs/Steering/Geo - The Lotus Forums Jump to content
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Steering rack and FARB shimming

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I have changed the chassis and am bolting a new rack onto the new chassis. Does anyone know how to shim the rack?

Also same question for the FARB.



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I assume the ARB is castor adjustment (on a trunnion model),   so I'd just use previous setting then get alignment checked 'on a machine'    Not sure regarding rack....

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The rack is adjustable in terms of mounting position for the purpose of minimizing bump-steer. This is a phenomenon which you should familiarize  yourself with in detail unless you turn over assembly of the front end to a capable professional.

Bump-steer, in brief, is the measure of how toe changes as the wheel travels through its arc of articulation as dictated by the suspension and steering links. It is generally thought desirable that the toe change only minimally through suspension travel so setting bump-steer is a matter of trial and error positioning of the rack while checking toe settings through wheel travel. A rough outline of how to proceed:

1) - Equip yourself with a means to measure toe - it can be as simple as a trammel bar or as clever as using a laser reflected off a mirror applied to the wheel;

2 - Assemble front end minus springs/dampers, leaving pivot fasteners just short of nipped so as to afford free travel up and down. Set static toe-in to spec in the manual;

3 - Support front of car with a jack under box section ( plate between to distribute load and not damage the chassis ) to facilitate raising/lowering to emulate wheel travel;

4) - Placed sheets of newsprint or similar between floor and tyres to negate resistance to travel owing to scrub;

5) - Measure toe at various points through the full travel of suspension and graph for interpretation. Toe out is less desirable than toe in, generally, but no great measure of either is to be tolerated. Repeat measurements with rack in alternative positions until changes to toe are minimized, then lock the rack in that position.

No Lotus should be sent forth after re-work of the steering components without the bump-steer exercise, they're a sublime chassis above all and should be enjoyed accordingly. Bear in mind the aforementioned exercise assumes all suspension and steering links to be in good nick, otherwise there may be confounding inconsistencies in measurements. End of sermon.


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