Castor on the rear of a single seater

[Majormallock]

Hi all I have restored an 83 ff1600, a Lola642e, setting up the car need some guidance as to rear cast. My question fundamentally is what to set the rear caster to and WHY? What does caster at the rear do? Spec sheet say 1.25 degrees top back

[St Eval]

Normally 0 degrees on the rear. The main thing is to make sure you have 0 bump steer on the rear. All of the cars that I have had and set up if you have 0 caster you should have 0 bump steer.


Roy

[ydd]

Originally Posted by Majormallock

Hi all I have restored an 83 ff1600, a Lola642e, setting up the car need some guidance as to rear cast. My question fundamentally is what to set the rear caster to and WHY? What does caster at the rear do? Spec sheet say 1.25 degrees top back

So positive Caster helps wheel alignment stability. At the rear it will help give your driver rear-end stability.

Attempt at an explanation below - sorry too new here to link you to anything useful!

If you think of a shopping trolley. Their wheels are attached by casters.

If you push the trolley the wheels swing round and line up in the direction of travel.

If you then try to push the trolley in the opposite direction, the wheels have a moment of instability, swing round and line up in the new direction of travel.

With a shopping trolley this caster effect is achieved by offsetting the pivot of the wheel at the top where they are attached.

In a (racing) car the same effect is achieved by applying caster angle to the wheel hub.

How? - Imagine projecting a line through your wheel hub along the caster angle and to the ground.

With positive caster that line will intersect the ground ahead of your tyre.

The distance from this intersection point to your tyre contact patch is known as your Caster Offset.

Why is it important?

You want some caster offset to give your racing car wheel directional stability.

If you don't have Caster offset your wheel will be much easier to turn (potentially good i.e. at the front!) but much less directionally stable. Think of a shopping trolley wheel having a wabble just before it settles into its new direction of travel.

The more Caster offset you have the more stable it will be - to a point.

As a side note, it is the caster at the front that determines much of your steering feel as a driver and setting efforts (when moving).

There is a bit more too consider as well. This is to do with the structural stiffness (or compliance) of your suspension.

Nothing is infinitely stiff so you need to therefore manage the situation of what happens to your suspension geometry under various loads - such as cornering or braking loads.

Whilst you might feel their is some theoretical benefit for it, the reason you don't want 0 degrees is that this can easily turn into a negative caster offset when forces are applied - especially under braking.

Think back. If you have a negative caster offset at any time, your wheels are going to want to switch direction exactly like the shopping trolley being pushed in a new direction.

That is going to cause the driver to have an uncomfortable feeling in the car, at a time (say trail braking into a corner) when they most want / need to have maximum confidence.

Does that help a bit?

There is a bit more too it tbh but hopefully that gives you some introduction.

Apologises if you already knew this!

[Majormallock]

Originally Posted by ydd

So positive Caster helps wheel alignment stability. At the rear it will help give your driver rear-end stability.

Attempt at an explanation below - sorry too new here to link you to anything useful!

If you think of a shopping trolley. Their wheels are attached by casters.

If you push the trolley the wheels swing round and line up in the direction of travel.

If you then try to push the trolley in the opposite direction, the wheels have a moment of instability, swing round and line up in the new direction of travel.

With a shopping trolley this caster effect is achieved by offsetting the pivot of the wheel at the top where they are attached.

In a (racing) car the same effect is achieved by applying caster angle to the wheel hub.

How? - Imagine projecting a line through your wheel hub along the caster angle and to the ground.

With positive caster that line will intersect the ground ahead of your tyre.

The distance from this intersection point to your tyre contact patch is known as your Caster Offset.

Why is it important?

You want some caster offset to give your racing car wheel directional stability.

If you don't have Caster offset your wheel will be much easier to turn (potentially good i.e. at the front!) but much less directionally stable. Think of a shopping trolley wheel having a wabble just before it settles into its new direction of travel.

The more Caster offset you have the more stable it will be - to a point.

As a side note, it is the caster at the front that determines much of your steering feel as a driver and setting efforts (when moving).

There is a bit more too consider as well. This is to do with the structural stiffness (or compliance) of your suspension.

Nothing is infinitely stiff so you need to therefore manage the situation of what happens to your suspension geometry under various loads - such as cornering or braking loads.

Whilst you might feel their is some theoretical benefit for it, the reason you don't want 0 degrees is that this can easily turn into a negative caster offset when forces are applied - especially under braking.

Think back. If you have a negative caster offset at any time, your wheels are going to want to switch direction exactly like the shopping trolley being pushed in a new direction.

That is going to cause the driver to have an uncomfortable feeling in the car, at a time (say trail braking into a corner) when they most want / need to have maximum confidence.

Does that help a bit?

There is a bit more too it tbh but hopefully that gives you some introduction.

Apologises if you already knew this!

Thought I replied to this, sorry if repeat. I understand front caster, it is the rear that gets me the spec sheet say 1.25 degrees rear pointing caster
Like to know why? Cheers Phil

[ydd]

Originally Posted by Majormallock

Thought I replied to this, sorry if repeat. I understand front caster, it is the rear that gets me the spec sheet say 1.25 degrees rear pointing caster

Like to know why? Cheers Phil

All good. Sorry I didn’t make it clearer. The issue is trying to avoid a situation where you get toe out on the rear.

Think rear steering dumper truck. You want to avoid this.

When the rear wheel toes (steers) out it is very uncomfortable for the driver. In a straight line and corner tbh.

The caster on the rear gives the wheel some stability in toe. It does this by using the caster offset

In addition, when you brake, the lower wishbone moves back and the top forward, this reduces the caster and caster offset. Therefore if you start at zero caster, when you brake you’ll get negative caster, negative caster offset. This will make the rear wheel very unstable in toe. Like the shopping trolley wheel wanting to swap direction.

In summary it’s about making sure the rear wheel toe is controlled and stable under all circumstances.

Does that help a bit?




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[S griffin]

Hope all that info helps you, be nice to see an old single seater like that out on the track. Good luck with it

[Majormallock]

Hi, yes that helps a lot. Many thanks

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