Rear Castor Angle

[dtype38]

Done a search and can't find anything on this so here goes with more of my general ignorance of all things motorsport...

While contemplating the advice from another thread on propshaft angles, I've just noticed that the input flange on my diff points up in the air. This seems to be because the rear wishbones, which bolt to the diff, are also angled and give a castor to the rear uprights of approx 4.5 deg (I am told different versions of my car may use different angles).

Can anyone explain why a car would need rear castor and what decides the amount. Thanks.

[Chris Y]

Anti-squat ?

[zefarelly]

The Cortina ones do also, somepeople started using wedged to dip the nose down. no idea whether it worked or was just a passing fad . . .

[GORDON STREETER]

We used to do this on the Hot Rods (diff nose down) it helped to stop axle tramp on acceleration.
The diff pinion tries to wind itself round the crown wheel, obviously this forces the whole diff housing and axle as well and the leaf springs assume an S shape.
When the spring fights back this causes axle tramp.
Starting the diff position nose further down certainly used to help as the propshaft would assume a straighter line on maximum tramp.
But as for solid "dead" diffs with IRS I don't think it makes any difference ?

[Tim Falce]

Does your input flange point in the air because you may have modified the chassis? On mine, which I thought was the same as yours, the pinion shaft and lower wishbones appear to be more or less on the same plane as the chassis rails but definitely nowhere near as much caster as you describe.

[JohnD]

If that were front suspension, with a kingpin or equivalent, that amount of castor would induce a considerable self centering moment. In a solid (?) rear axle, would it tend to make the rear axle go straight ahead, and induce more oversteer?
I'm asking - I don't know!
John

[dtype38]

Just to clarify.... on my front suspension, the wishbones are horizontal and the castor is produced but the upper wishbones being set back from the lower so that the upright is at an angle. With the rear, however, the whole diff and wishbone system is angled up at the front so that the nose of the diff is in the air and the whole IRS suspension system is angled at 4.5 deg to the horizontal.

The only possible reason I can see for it is maybe that it helps absorb ripples and bumps using the rear dampers. But the main question is... is it hurting my handling or can I rebuild the car using it as is?

[Tim Falce]

Why not contact John Arnold and ask him. The later Revival models have the diff set a a shallower angle to supposedly improve handling.

[phoenix]

The angle of the wishbones will provide some anti-squat, as mentioned previously, which may be used to optimize the contact patch under acceleration, if squat induces a camber change in the rear tyres.

Rear Caster can be used to tune rear bump steer also, so it would be a good idea to measure what is happening to rear toe as the suspension moves up and down before setting the caster to anything different.

In your case it may be for neither of these reasons, but for the prop shaft alignment also mentioned above.

[Al Weyman]

I can adust the pinion angle on my later model coil sprung car, it has lower trailing arms, a panard and a long torque tube. This car does not hook up well coming out of bends so my question is can I gain anything by adjusting the angle of the diff, at the moment I have it set as per Steve Spohn Racing's start off settings.

[dtype38]

Thanks for the advice. It's really difficult to change anything, so I would need to be sure before doing any changes, so I think for the time being I'll just leave it as it was originally set up. At least I'm used to it.

[zefarelly]

I rarely get tramp ( the idea being, once your away, don't slow down!) but occaisionally on a bumpy or rippled surface with slow Chicanes (Castle COmbe) I do . . . . .my car doesn't even have radius arms, just single leaf springs

I might get wedged up over the winter and see if I notice the difference

[dtype38]

Originally Posted by zefarelly

. . . . .my car doesn't even have radius arms, just single leaf springs.

I have radius arms but they are quite short and run straight forward. They have rubber bushes to take up the length change as the wheel moves up and down. Never been sure if they are actually doing much for me, but I'm loath to take them off just in case.

[GORDON STREETER]

You could always try it if they come off easily.
Years ago I made up some long top link bars and used them in a couple of races , although it wasn't too bad one of them broke in qualifying at the next meeting so I took them both off for the race. It transformed the handling and I won with a new lap record, so you never know !

[dtype38]

Hmmm.... well they do come off easily!

[zefarelly]

the Cortina ones aren't perfect and break apparently . . .or rip bushes etc, the'yre also very straight and short. I've rarely seen a race car that has the genuine original Ford lugs and mounting positions

thinking about it |I don't think I've ever seen a Cortina with std axle position