steering axis

[abhishek]

steering axis actually passes through the center of the wheel but i've read somewhere that it is shifted a little to the left or right of the centre. can someone please offer an explanation as to why this is done???

[steve]

The offset of the point where the steering axis (kingpin axis) meets the ground is shifted to one side resulting in kingpin offset ( scrub radius)in order to give there driver an indication of wheel locking whilst braking. When one of the front wheels locks up the amount of braking force it makes reduces, meaning that the wheel on the other side of the car is producing more force. As both wheels are connected via the steering, a pull to one side will be felt at the steering wheel.

Another reason its done in production cars is to aid stability if a tyre blows out

Further, in FSAE cars which require very large amounts of steering in order to get round the very tight corners, it may not be possible to place the wishbone to upright pickup points inside the wheel to much depth, otherwise the wheel rim will contact the wishbones when large amounts of steering are applied

[Mark Ortiz]

One other effect of scrub radius, especially in tight turns, is that when combined with caster, a large scrub radius de-wedges the car as lock is applied. That is, the percentage of normal force borne by the inside front and outside rear tires increases as we steer into the turn, and the car rolls out of the turn, as steer into the turn increases.

The greater the scrub radius, and the greater the caster, the greater this effect.

This effect is highly significant in autocross, as it reduces understeer in tight turns.

[Mark Ortiz]

I publish a free e-mail chassis newsletter, available upon request from me at markortiz@vnet.net. The August 2002 issue contains a fairly lengthy discussion of steering geometry variables and their effects on car behavior.