- In order for the driver to be able to accurately position the car on the track
consistently, it's obviously important that the steering inputs result in a predictable
movement of the front wheels.
While we'd like to assume that when we turn the steering wheel a certain amount
that it will result in a consistent movement of the front wheels, this isn't always the case.
Bump steer is a condition where the toe angle of the wheel or wheels change
without any steering input from the driver and it occurs as the suspension moves
through its bump and rebound travel.
This could be the result of body roll compressing the suspension as you
negotiate a corner, suspension movement under heavy braking, or it simply
could be the result of the suspension movement as the wheels travel
over bumps on the racetrack.
It's caused by the suspension linkages travelling through different arcs compared
to the steering linkages or toe control arms as the suspension moves through its travel.
This is a tricky concept to understand without some help so let's look at
the common MacPherson strut suspension design to illustrate the problem.
First let's assume that our steering tie rod and our lower control arm share a common
pivot point.
We can see that as the suspension compresses, both the lower control arm
and the steering tie rod travel through the same arc and hence there is no change in
toe or in other words no bump steer.
If on the other hand the steering tie rod and the lower control arm share
different pivot points, we can see that these arms would now travel through different arcs.
The difference in arcs results in a change in toe as the suspension compresses,
which then results in bump steer.
Generally we find that the bump steer effect becomes more pronounced as
we move the suspension further away from the normal factory ride height.
In order to eliminate bump steer on a MacPherson strut suspension design,
the steering arms should be designed to point at the instantaneous centre.
That instantaneous centre is an imaginary point that is easiest to understand
if we draw another simple diagram of the suspension viewed from head on.
If we plot a line through the lower control arm pivot points and another line
perpendicular to the strut top, the intersection of these two imaginary lines is referred to
as the instant centre or instantaneous centre.
If we now add in the steering arm, we can see that if we plot a line through
the arm, this also intersects with the instant centre.
Alternatively the inner steering tie rod must be in line with the lower contol arm
inner pivot point.
The situation is much the same with double wishbone suspension systems
where we can draw an imaginary line through the upper and lower wishbones and the point
where these lines intersect is again the instant centre.
If we draw a line here, through the steering arm pivot points, this should also intersect
with the instant centre.
You may rightly expect that a stock production car would have no measurable
bump steer but often packaging compromises mean that the steering rack and hence the
steering tie rods can't be located in an ideal position which results in some
amount of bump steer being present.
Often this is made a lot worse when we lower a car which is a common modification
when building racecars.
So far we've been considering bump steer at the front of the car, however this can
also occur at the rear.
This is the result of the suspension linkages of different lengths at the rear
of the car moving through different arcs and causing toe in or toe out as the
suspension moves through bump and rebound travel.
There's a wide variety of rear suspension designs and some are more susceptible
to bump steer than others.
To give you an idea of how this works and how modifications to the suspension design
can affect bump steer, let's consider a very simple rear beam axle with trailing arms
which is common in the rear of many front wheel drive cars.
In stock form, the beam axle is usually designed so that the trailing arms are
parallel to the ground.
In this case when the car rolls in a corner, one arm moves up and the other moves
down, however while both arms are moving in opposite directions,
their arcs are the same so the net toe change is zero.
Now if we substantially lower the same beam axle rear suspension, we end up with both
trailing arms pointing downwards towards the pivot points on the chassis from that
beam axle.
This time as the car rolls in a corner, the trailing arm on the inside of the corner
will move down, pushing the axle rearwards while the trailing arm on the outside of the
corner will move up, pulling the axle forwards.
the result can be a significant toe change due to body roll.
This situation is often referred to as roll steer.
Another example of a rear suspension design that offers significant bump steer
and roll steer problems is the semi trailing arm design that was popular
in many cars around the 1980s.
This suspension design uses links at the front of the arm that angle inwards
towards the vehicle centreline.
Due to this angle, as the suspension moves into compression travel,
the wheels will move into toe out.
Bump steer and roll steer can make it hard to position the car accurately and
they can make the car unpredictable and difficult to control,
particularly through bumpy corners.
In later modules we'll discuss how to measure bump steer and what you
can do to help reduce it.
That was just one of the lessons from our Motorsport Wheel Alignment Fundamentals course.
If you want to learn how to optimise the alignment of your race car then this course is going to be
perfect for you.
You'll learn about the different suspension designs and how they operate.
You'll learn about concepts such as roll centre height, scrub radius, bump steer and ackerman
steering as well as how these aspects affect the handling of your car.
You'll then see how you can perform fast and accurate alignments in your home shop
or at the racetrack with products that won't break the bank.
Finally you'll learn a simple 7 step process that you can apply to performing an alignment
on any car.
This will ensure that you get the right results without overlooking any critical steps.
To learn more, click the link in the description.