Bumping into Bump Steer

Front view of a race car (partial) suspension system to illustrate with a steering arm at an incongruous angle – this will cause bump steer.

I had a car accident back in the 1970’s when “racing” a bloke on a fast motorbike on the Yarra Boulevard in Melbourne.  Motorbikes are fast in a straight line but have little chance against cars in corners and the boulevard is all corners.   This guy insisted he was much faster.....    The car I had at the time, an Aussie built V8, had an automatic transmission and the link between the throttle and the transmission popped out (due to my aggressive driving) and jammed the throttle part open.  The delay between lift-off and braking, combined with engine push was enough that I ran out of road and went up an embankment (that was lucky because the alternative on the other side of the road is a long way down to the river!).   The poor motorbike guy, coming along 10 seconds later, was laughing so much at seeing me up the bank that he crashed into the back of me and broke his ankle.  He recovered, by the way.  The whole thing was a great life lesson for me because I had genuinely been on the limit so anything at all going wrong was bound to cause an accident.  You can’t be on the edge on public roads and I was lucky to have such a lesson without more serious damage being done to others or myself.

 My car was repaired which is when I really felt “bump steer”.   I realise now that one of the steering pickup arms (either on the steering rack or on the car’s upright/hub) had been bent and wasn’t replaced (by deduction only because I sold the car before realising where to look).   I took the car back several times, demonstrated the problem to about a dozen separate wheel alignment specialists and tyre places, all to no avail.  The problem was extremely disconcerting because, if the right front wheel hit a bump, the car would violently turn right and then turn left as it recovered.  That incident got me interested because none of the “experts” I went to see had any idea where to look to find the cause of the problem.  All other parameters seemed perfect to them (of course they weren’t car designers).  Eventually the violence of the forces involved caused that wheel to fall off the car (happily at low speed).   Problem still wasn’t identified.

 When I started working on racing cars full time, not long after this, setting up the bump steer on a racing car was part of the race by race setup routine that was done.  This gave me a name for the phenomenon but it wasn’t until I’d done some more digging that I realised what the exact cause was.  If you don’t have experience of adjusting bump steer on a race car, using shims to adjust the height of (usually) the outboard end of the track rod (steering arm) is the normal way it is done. 

 Still in Australia, mentor and teacher Frank Gardner (famous Aussie race driver and engineer) walked me around a few race car paddocks looking at cars and pointing things out.  “So, what you do boy,” he told me, “is walk around looking sleepy, but take note of what you see.”   “See that Formula Ford over there.  The guy that’s designed that clearly doesn’t know anything about suspension kinematics.”   Hell, I didn’t have any idea what he was talking about at the time, but I was sure going to figure it out!  Over time you can and I did.

The Aston Martin Cheetah Group C car.

Then in 1984, I worked for Ray Mallock in the UK.  Ray had been requested to race a Swiss built, Aston Martin engined Cheetah group C car.  He wanted to be confident it was safe so I went over to Switzerland (my first Swiss adventure) to assist with preparations for the test.  The owner and car designer Chuck Graeminger took me into the workshop where his boys happened to be doing the bump steer.  I’d had a few seconds to have a look at the car and being impulsive (having been a bit horrified at the look of the kinematics of the car) I noted that “surely that’s impossible with that track rod and rack!”  Poor Chuck was a bit taken aback.  He’d checked the bump steer at the design stage, he said to me, a bit indignant, and it should be fine!  You can imagine, some young upstart from Australia telling a Swiss engineer he doesn’t know what he’s doing didn’t go down that well.  So I asked the boys – and they said they could get it close on bump but in droop it was way out.

So Chuck and I had a look at his drawings – which to his credit were perfect – but not like the car.  The rack was clearly much wider on the car than in the drawings but nobody had spotted it.   To make the system fit the boys had simply cut down the track rods (steering) so that they could get the wheels to point straight ahead on zero steering lock.  That’s easier than cutting down the pinion of the rack.   It seems that the steering rack manufacturer had a long standard rack design which was then cut down to suit the particular application.   This one hadn’t been cut and the mechanics hadn’t known what to do.

Simplified front view of (part of a) race car suspension system.  Steering rack longer than reality and steering arm shorter to emphasise the illustration.

 Why was it obvious to me?  When viewed from the front (easier with a racing car when bodywork and wings are removed – and even easier if the car is up on stands so you can look at it at the same height) the suspension can be thought of as simple links, each of which move in arcs from a fixed point on the chassis.  That’s an over simplification of course.  If all the outboard ends are a similar distance from car centreline then the inboard ends should start almost as if they’re on a line (the blue dashed line in my rough sketch).  If your steering arm is low it should be of a similar effective length to the lower wishbone, if it is high it should be a similar effective length to the upper wishbone.  Why – so the wishbones and the track rod pull the wheel in or push it out about the same amount at each ride height so steering angle is not changed.

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