The B6.4 platform has a very wide range of caster and front axle height adjustment, and now includes an adjustable rack height to allow bumpsteer to be kept in the correct range. Washers are used on the outer end of the steering to fine tune bumpsteer but with so many options I have put together a quick reference guide of which bumpsteer setup to start with for various caster and axle height setups.
Bumpsteer describes how the front steer angle changes as the suspension compresses, and is a critical setup parameter. Bumpsteer changes with the angle of the steering link from the rack to the hub and must be adjusted to compensate when changing caster angle and axle height. Generally 2.5deg of caster requires a 1mm change in washer. With the 6.4 having a massive range of caster and axle height adjustments, the adjustment of the outer ballstud washer is not enough so the Associated design team allowed for a range of steering rack height adjustment as well.
With our outdoor setups we often run high caster / low axle heights and we found we were using 3-4mm of bumpsteer washers which is not ideal, so for these setups we have also found it useful to lower the rack a further 2mm.
Typically we are aiming for minimal bumpsteer on 2wd. If you reduce the washers the steering will toe in under compression, which will give more mid corner steer but can be more edgy in bumpy corners. Make any changes in 0.5mm increments as it is very sensitive.