Bump steer

[gpaterson]

I am working through the compliance checklist so as to register my 818SR for use on New Zealand roads. There are a couple of issues that can be fixed relatively easily like bolts being too short (i.e. the must have 2 threads exposed) and inadequate seat belt mountings for the inertia reel belts, but the biggest issue is with bump steer.

My donor is a JDM 2004 WRX and has the standard steering rack. I have mounted it as per the manual. I have leveled it off with an additional shim (about 10mm) under the driver-side of the rack but it still has a major issue with bump steer - it has about 30mm toe movement over the 100mm suspension travel. I have spoken to Junty who advised on the need to shim the standard mounting rack mounting position.

Is this a problem specific to my car (as Justin's seems ok) or is it a problem with the RHD version of the kit. Is it a general problem with all kits? What have others done to solve this issue if it is a design problem or don't the US compliance authorities worry themselves with such things?

Open to all suggestions.

Cheers
George

[Junty]

Hey George,
Although I initially thought I didn't have any bump steer, my RHD also has the same toe movement. My bump steer is happening at maximum suspension droop, so when the car is airborne or near to it - with very little weight on the front wheels. I guess and suspect that many will be also having similar bump steer, however due to the issue occurring at this end of travel, it causes the least concern or safety? When bump steer happens upon suspension compression, you have maximum weight and traction on the wheels and therefore the bump steer is a huge safety concern.
The purpose of me clarifying these points - is primarily to demonstrate where the problem is happening, and to clarify that it is necessary to 100% resolve for compliance within New Zealand. I'd be very keen to understand if anyone else with LHD 818S's have measured, noticed and hopefully resolved any bump steer?

[Hindsight]

There was a post about bump steer a while back. I could be remembering wrong but I recall that bump steer was not an issue at the S-height, but was at the R-height.

[gpaterson]

I think I have solved my bump steer issue.

It has involved the following adjustments:
1. Leveling off the steering rack. The standard mounting points are not quite right so it needs to be adjusted so the tie rods are symmetrical both sides. It involved putting some spacers on the drivers side.
2. Relocate the shock to the R ride height ie use the top hole.
3. Relocate the top A arm above it standard mounting.

Adjusting the ride height flattens the bottom arm and removes the aggressive bump steer that occurs are the bottom of the suspension range - this is Justin issue.

Relocating the top A arm means that there is not so radical camber change at the top of the suspension travel. My car had a major issue whereas Justin's didn't. I know have only 5mm toe change whereas when I started the exercise I had 37 mm!

The bonus of shifting the top A arm is that I can now have a little more camber at normal ride height. I have now 2.5 degrees whereas the maximum attainable before was 1.5 mm. The camber also doesn't change so much upon compression.

May not work for everyone or every RHD car but worked for me.

[gpaterson]

Pic of the top A-arm.

2015-03-29 19.20.03 (Small).jpg