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Motorsport Wheel Alignment: Bump Steer & Roll Steer

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Bump Steer & Roll Steer


00:00 - 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.
00:10 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.
00:18 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.
00:29 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.
00:43 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.
00:53 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.
01:01 First let's assume that our steering tie rod and our lower control arm share a common pivot point.
01:08 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.
01:19 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.
01:28 The difference in arcs results in a change in toe as the suspension compresses, which then results in bump steer.
01:35 Generally we find that the bump steer effect becomes more pronounced as we move the suspension further away from the normal factory ride height.
01:43 In order to eliminate bump steer on a MacPherson strut suspension design, the steering arms should be designed to point at the instantaneous centre.
01:51 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.
02:01 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.
02:14 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.
02:21 Alternatively the inner steering tie rod must be in line with the lower contol arm inner pivot point.
02:28 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.
02:39 If we draw a line here, through the steering arm pivot points, this should also intersect with the instant centre.
02:46 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.
03:02 Often this is made a lot worse when we lower a car which is a common modification when building racecars.
03:09 So far we've been considering bump steer at the front of the car, however this can also occur at the rear.
03:15 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.
03:27 There's a wide variety of rear suspension designs and some are more susceptible to bump steer than others.
03:33 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.
03:46 In stock form, the beam axle is usually designed so that the trailing arms are parallel to the ground.
03:52 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.
04:03 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.
04:15 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.
04:27 the result can be a significant toe change due to body roll.
04:30 This situation is often referred to as roll steer.
04:34 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.
04:45 This suspension design uses links at the front of the arm that angle inwards towards the vehicle centreline.
04:51 Due to this angle, as the suspension moves into compression travel, the wheels will move into toe out.
04:57 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.