Owners of performance Spridgets will frequently mention the "dreaded" Bumpsteer but what is it and what can we do to improve it?
Fortunately, despite the hype, bumpsteer is not too difficult to understand with a couple of pictures. As the wheel moves vertically with respect to the chassis, the top and bottom suspension links proscribe arcs which mean that the camber changes as the wheel moves up (see next month's article!). The same thing happens with the track rod which pivots around the rack end and the track-rod end (TRE). In an ideal world, these 3 links would make a perfect set of curves as the wheel moves. But back here in the real world it they don't.
So as the wheel moves up the track rod end changes position with respect to the upright and the car steers itself!
Bump - Wheel moving up, Steer - Car changing direction.
And the situation gets worse when going round a corner because the kingpin angle and the caster angle increase the difference between the link lengths.
AND it gets worse when you lower the car as the track rod is at more of an angle so for every mm of vertical movement you get more change in effective length.
How does it feel?
Really bad bumpsteer can cause the car to steer every time you go over a bump even in a straight line. But unless something awful has happened to a Spridget you shouldn't get that as the basic geometry isn't too bad.
However, when the cars are being driven hard the roll of the body combined with undulations in the road act to shorten the trackrod and cause toe-in which makes the car feel unsettled. On the road with normal tyres it is manageable but on circuit with a bit more grip...
the car will feel 'darty' at the front and unsettled at the back especially on turn-in with brake transition.
On a lowered car using shorter springs this actually gets worse. And if you have a mismatch between anti-roll bar stiffness and spring stiffness it can be worse still.
The first thing to do is make sure that the links, dampers and springs are working correctly and have the correct values (see basics below).
Bumpsteer occurs when the arcs of the suspension and trackrod are different so the length changes affect the steering. There are 2 things we can change: 1 - the amount of suspension (wheel) travel per unit force applied; 2 - the amount of length change per mm of wheel vertical travel.
Wheel travel per force applied is simply down to spring rate - running stiffer springs means the wheel will move less vertically which means the length changes are minimised - you don't really get bumpsteer on a gokart because there isn't any suspension!
Length change per vertical travel is a function of where on a circle the link is positioned. Imagine the links are all at 9 o'clock, when the wheel goes up the links are at 10 o'clock and the horizontal change between 9 and 10 is fairly small. But if the car is lowered and the links are at 10 when we start and the wheel moves vertically up as far as 11 then the amount of horizontal change is much larger. To reduce the effect of bumpsteer under these circumstances means moving the pivot points to make the links closer to parallel. Depending on the series and class you run in there are different options available including fabricated wishbones. In our shop we have adjustable slider plates and anti-ackermann kits to move up the inner mounting points of the top link and move down the TRE which have the effect of getting the links closer to the ideal 9 o'clock position for a more stable turn in.
Note: Rear leaf springs do not last well on race cars and the inversion of a rear spring feels very similar to bumpsteer as it causes uncontrolled geometry changes over bumps.
Make sure there is no undue play in the suspension - pay particular attention to panhard rods and wheel bearings as they are frequently the first to suffer.
The front damper units are both top link and damper so do a lot of work on a performance Spridget, it is common to have excess play in the bearing or the damper itself - they must be replaced / reconditioned if this is the case.
And make sure the tracking is spot on (with driver and fuel in car), Spridgets can be very sensitive to tracking changes, the front should have only the smallest amount of toe-in such that under braking the force of the wheels acting against the motion of the car gives 0 toe.