American Car - November 2015

Don’t get a bum steer on bump steer

Words: Dave Smith

as the suspension compresses,

the fixed length of the suspension

arms and track rods are causing

steering input – bump steer.

Still with us? Probably not, as

that above paragraph is a long-

winded and confusing method

of describing a simple and basic

premise. Perhaps this diagram

(1) will help. Most of us can get

along in our street cars in blissful

ignorance of bump steer, unless

we happen to be hammering

around a circuit. The problem will

become more noticeable when

we lower the car’s suspension. All

of a sudden, the track rods aren’t

horizontal at ride height, and

when the suspension compresses

further, the arc described by the

track rods is more pronounced,

making that steering input much

more noticeable. If you’re just

cruising the streets in your lead-

sled, again, you’ll probably never

notice. It’s only if you’re pressing

on that it might become an issue.

There are ways around it. One

way of keeping the track rods

horizontal on a lowered car is

extended tapers on the track

rod ends, or spacer brackets.

This can work, but it puts a lot of

extra stress on the track rod ends

and, on many cars, the track rod

ends sit inside the offset of the

wheel so extending them may

cause clearance issues. Some use

extended tapers on the ball joints

(2) to correct geometry issues on

lowered cars, but this again can

cause stress relocation. There are

even ways of raising the steering

rack on its mounts, but the

benefits of this have often been

questioned and it’ll likely cause

more clearance problems than

solve geometry issues. Possibly

the best way of lowering an

older car whilst keeping most of

the suspension geometry stock

is the dropped spindle (3). This

uses a spindle with standard

locating points, but raises the

stub axle – the stub which the

hub is fixed to – and brake

calliper mounting points (if

applicable) upwards on the

spindle, lowering the car without

altering the steering angles.

roughly vertical, perpendicular to

the road, and possibly toeing-in

a fraction at the front edge. Now

imagine you’re heading into a

right-hand bend, or going around

a roundabout, at speed. The

weight of the car will shift to the

left, meaning the front left-hand

spring will compress. The steering

rack is fixed, but the track rod end

on the front spindle has moved

upwards, meaning that the track

rod is no longer horizontal; the

outer end has moved upwards,

and, because the front suspension

arms are fixed too, the spindle is

describing an arc when viewed

from front or rear. This means

that the lateral distance between

the rack and the spindle has

decreased, although the length

of the track rod hasn’t. Effectively,

Hell’s Angles II

Bump steer is a very difficult

concept to explain, but – and this

may be controversial – to many

of us, it won’t matter a great deal.

However, it’s worth considering.

Firstly, bump steer has nothing

to do with a bump mid-corner

affecting the car’s steering or

direction. That will affect any

car. Bump steer is to do with the

difference between the car’s

suspension and steering angles at

ride height, and the angles when

the suspension on that corner of

the car is compressed.

Consider a car with rack and

pinion steering. At normal ride

height, the track rods – the rods

between the steering rack itself

and the knuckle joints of the

track rod end – will be about

horizontal. The wheels will be

1

ACM

jargon

buster

86

UPPER LINK

STEERING TRACK ROD

UPPER INBOARD PIVOT

LOWER LINK

STEERING RACK END JOINT

UPPER BALL JOINT CTR

OUTER TRACK ROD JOINT CTR

LOWER BALL JOINT CTR

GROUND PLANE

STEERING OFFSET OR SCRUB RADIUS

LOWER INBOARD PIVOT

C/L

UPPER INBOARD PIVOT

GROUND PLANE

BUMP HEIGHT

STEERING ROD LENGTH CHANGE

LOWER INBOARD PIVOT

C/L

KINGPIN INCLINATION

jargonbuster

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jargonbuster.indd 86 21/09/2015 14:16:05