Bump-steer question

[oldgingerbeer]

I am at present assisting with the build of a custom front end on a scratch build and working on the question of bump-steer. As reference documents I am using Information sheets #04-2010 Bump-steer Measurement Background Information and #05-2010 Bump-steer Swing-check Procedure.

I am working with swing-check bars built to the same pattern as the LVVTA bars, though there may be some dimensional differences. I am able to measure the angular change as the suspension articulates, using the 575mm separation between marks as prescribed.

It is clear that the goal is to minimize bump steer and to engineer the solution so that the steering toes-in under compression from the nominal ride height.

Because of real world limitations, it is not going to be possible to achieve zero bump-steer, so the question then arises: What is the maximum acceptable angular bump steer from nominal ride height to full compression and nominal ride height to full droop? This information does not appear in either Information Sheet.

I want to work with the angle rather than toe-in/out in mm to avoid any ambiguity which may arise from any dimensional differences from the hub pivot point to positions “A” and “B” on LVVTA bars and the ones I am using.

On Page 6 of #04-2010 Bump-steer Measurement Background there is a heading which reads: “What maximum figure to put on toe?” Within that paragraph it says: “There is little data readily available so LVVTA will be relying on data from LVV Certifiers to build up a picture of common figures for standard and modified vehicles over time.”

What picture has in fact been built? What is an acceptable maximum target?

[Chris]

Jan 18, 2021 10:59:16 GMT 12 oldgingerbeer said:

I am at present assisting with the build of a custom front end on a scratch build and working on the question of bump-steer. As reference documents I am using Information sheets #04-2010 Bump-steer Measurement Background Information and #05-2010 Bump-steer Swing-check Procedure.

I am working with swing-check bars built to the same pattern as the LVVTA bars, though there may be some dimensional differences. I am able to measure the angular change as the suspension articulates, using the 575mm separation between marks as prescribed.

It is clear that the goal is to minimize bump steer and to engineer the solution so that the steering toes-in under compression from the nominal ride height.

Because of real world limitations, it is not going to be possible to achieve zero bump-steer, so the question then arises: What is the maximum acceptable angular bump steer from nominal ride height to full compression and nominal ride height to full droop? This information does not appear in either Information Sheet.

I want to work with the angle rather than toe-in/out in mm to avoid any ambiguity which may arise from any dimensional differences from the hub pivot point to positions “A” and “B” on LVVTA bars and the ones I am using.

On Page 6 of #04-2010 Bump-steer Measurement Background there is a heading which reads: “What maximum figure to put on toe?” Within that paragraph it says: “There is little data readily available so LVVTA will be relying on data from LVV Certifiers to build up a picture of common figures for standard and modified vehicles over time.”

What picture has in fact been built? What is an acceptable maximum target?

Gidday OldGinger

If you are checking a custom IFS with an LVVTA Component Type Approval, the bump-steer target data will be provided and must be met.

A one-off custom IFS that has been approved by TAC or an IFS from a recognised manufacturer is expected to have less than 10 mm total bump-steer and in all other cases, less is best.

Total bump-steer is measured over the full travel of the suspension from full droop to full compression.

Cheers

Chris