Bump Steer - Part 2 - Theory Into Practice

Right then - lets see if it works!

First job was to fix a mirror to the brake disc (used a piece cut off a cheapo mirror from B&Q):
Then the laser was set up and the piece of graph paper was fixed conveniently to the garage door about 8 feet away:
The results of the first test can be seen in the video below:

There did appear to be some bump steer effect remaining - remember the effect is magnified - firstly the mirror doubles the angle and secondly the plot is being made some 8 feet way from the mirror. You can see that there is a marked increase in bump steer during the last 1/2 inch of droop. I noticed this during the visual check with the two straight edges.

So I spent a bit of time mucking around with the rack position - easy to do - just watch the dot on the wall as you move the rack. I found I could reduce the bump steer effect over the majority of the suspension range but with the exception of the last 1/2 inch of droop. The following video shows the end results of my best efforts:

Still some bump steer effect - but how much?

So taking the points I made on the graph paper I could determine the movement of the laser and with a bit of basic trigonometry relate these measurements back to angular movements at the hub. The following graph shows the results in terms of change in toe-in measured in arc seconds and camber change measured in degrees. Camber change and toe-in change are taken to be zero at the notional ride height of 11.5 inch damper length.

CLICK ON GRPAH FOR BIGGER IMAGE

You can see the marked increase in bump steer effect (green line) over the last 1/2 inch of droop - over the majority of the range there doesn't appear to be a great deal. But how do arc seconds compare to toe in? Lets plot another graph - this time of toe in change of the wheel with relation to damper length:

CLICK ON GRAPH FOR BIGGER IMAGE

So if we forget the last 1/2 inch of bump the effect on toe in is +/- 0.5mm - toe in on bump and toe out on droop. Over the middle range (10.5-12.5 inches on the shock) it's bugger all.

The only thing that has just crossed my mind is that during all this the steering rack was free to move - I wonder if this has affected the results?

Right - back to the garage - lock the rack and see if it makes any difference? - I'll be back shortly.