Posted on June 4th, 2015 in Opinion,Suspension by Julian Edgar

I think that suspension design is one of the most complex areas of automotive engineering. I am not thinking here about just spring and damper rates – but those are of course hugely complex in themselves – but of changes that occur through suspension movement.

For example, a wheel experiencing bump and rebound is likely to have a designed-in change in its camber – going into negative camber when in bump. This helps keep the wheel more upright when the body rolls in cornering (or at least the outside, most important, wheel anyway).

But what about something like anti-dive geometry? This set of suspension angles causes the car’s front springs to resist compression when the car is being braked.

So how does this work? Imagine a very simple front suspension system with a leading arm per wheel – an arm that runs longitudinally to support the wheel. (A Citroen 2CV is the only car I know of with this approach, but that’s OK.) The brake is mounted on the wheel, so the torque loads of the brake are fed back through the suspension arm.

The car drives along, and then the brakes are applied. The brake load causes the longitudinal suspension arm to try to rotate around the wheel, so applying an upward force on the suspension pivot. This prevents that end of the car from compressing its springs as much – more of the car’s weight is being dynamically taken via the suspension pivots than through the springs.

The result is that the car dives less under brakes.

Now as mentioned, not many cars have longitudinal suspension arms, so this arm is often actually virtual, being in effect created by (for example) the front wishbone mounts not being parallel when viewed from the side. If they form an angle that converges towards the rear of the car, anti-dive will occur.

So anti-dive sounds great… but there are also downsides.

Under brakes, the car does not behave as many expect it to – it doesn’t dip its nose as much.

Under brakes, the suspension is less springy – more of the weight (and so vertical acceleration of bumps) is fed through the suspension pivots and not the springs.

Under brakes, in many cases the geometry is such that the wheel moves forward, so meeting bumps more harshly. (And there’s already a higher effective spring rate, remember.)

And it goes on: under brakes and when hitting a bump, the steering geometry (eg castor) may not adopt the same angles as would normally occur if anti-dive were not present… so the car may react to bumps differently when being braked.

So should anti-dive not be used? No – in cars with soft suspension and a high centre of gravity, it can be very effective.

But then again, the amount of brake dive also depends in part on the stiffness of the slow-speed bump damping provided by the shocks.

You see? Every aspect relates to another aspect: not one suspension design criterion can be viewed in isolation.

It’s all very complex….

2 Responses to 'Suspension'

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  1. Mike Clarke said,

    on June 4th, 2015 at 2:51 pm

    How about a little info on scrub radius. I’ve always believed it to be used to reduce kingpin inclination, reducing or eliminating positive camber on the outside wheel when turning

  2. Jono said,

    on June 16th, 2015 at 10:27 pm

    The GU Nissan Patrol is another car with a similar leading-arm setup to the 2CV you described. In practice it actually has an anti-squat characteristic too, but I have never taken the time to find out how that works. From memory it is a 5-link rear end.

    The anti-squat is very noticeable under heavy acceleration or even when ‘stalling’ it up against the brakes. The back of the car feels like it raises about 50mm.