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Steering Geometry
Ackerman Angle

How Mr Ackerman Got the Wheels to point the right way all the time!

Car, going round a corner, the centre of it's radius shown.
Now, outside wheels have to go round a longer curve than the inside wheels, and the back wheels always follow a tighter radius than the fronts.

So, we have four wheel tracks, with the front outside wheel following widest arc, inside rear wheel following the smallest.

And the front wheels turn on the ends of the axle, rather than the axle pivoting on its centre, so the inside front wheel is effectively further round the corner than the front outside wheel.

Which is another reason we have some curious geometry effects; but basically we need the front wheels to point tangentially along the curve we want them to follow.

And if you look at the angles the wheels are pointing, those tangents aren't parallel to each other, they are skew, each wheel needs to point in a slightly different direction.

So, simple parallelogram steering arrangement.........
 
Like this, isn't going to work!

If you make the steering via a parallelogram linkage, whereby the steering swivel housings are turned about the swivel pivot by parallel radius arms connected by a track rod, of equal length to the distance between the pivots, everything will stay parallel, and the wheels will always point at the same angle as each other.

This means that they cant both point along their respective radius', so they are going to pull against each other and 'scrub'

So, Mr Ackerman...... looked at this problem, and realised that if you wanted the wheels to point at different angles, you couldn't use a parallelogram linkage, you'd have to use a trapezoid linkage.

 

Like this:-

Which is pretty helpful, but the trick is not JUST getting the wheels to point at different angles, but getting them to point at the CORRECT different angles!

And, cutting a long story short, Mr Ackerman is to steering Geometry what Pythagoras is to ordinary geometry, and after a lot of number crunching, Mr Ackerman derived his 'Perfect' angle.........

Which depends on the wheel-base of the vehicle in question, because basically the 'ideal' Ackerman angle is derived by drawing a pair of straight lines through the steering axis and the centre of the back axle.

Wider the track width, in relation to the wheel base, the more Ackerman angle you need.

As the track gets narrower and the wheel-base, longer, so the shallower the angle.....

Which all makes very little of what was an awful lot of work for Mr Ackerman!

But, that, put simply is Ackerman angle, and why it is important, because it's what points the wheels in the direction we want them to, throughout the arc of the steering, and accounts for the wheels needing to point at slightly different angles to each other, depending on how tight we want the car to turn.

And having explained it, and said how important it is........

I'm going to tell you that there aren't MANY vehicles that use 'ideal' Ackerman's in their steering geometry.

Reasons are many and varied, and Other bits of this section will elaborate on them, but one that you MIGHT have pondered, is that in the pictures, the steering pivot is shown just inboard of the stub axle, and so in most cases, the contact patch of the tyre on the road, doesn't pivot on the steering axis, but turns around it.

This brings us to other bits of steering geometry, like the wheel 'off-set' (which is part of what that is), camber angles, castor angles, rake, and trail, which all have thier influence over how a car turns, BEFORE we look at how any of that may change as the suspension bounces up & down!

Thing is; the steering wheels NEED to point in different directions when you turn becouse of the different radius they need to follow, and the Ackerman 'Principle' is the starting point for making them do that.

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