Castor/Heavy Steering

[Russ Noble]

4.5" would seem to figure.

I imagine original design would have been about 2" with about 6" rims. As wheels got wider, using the same suspension, scrub radius would increase as nearly all of the extra width would be on the outside of the rim. It's not usually possible to have any increase on the inside due to fouling the wishbones on lock. At that rate an increase in rim width to 10" would add an extra 2" scrub. Wider discs and bigger calipers would probably also add a little.

I have never heard of anyone complaining about the handling or feel of any of the JWA cars or the Mk4/J cars. On the contrary.

I think Randy may be right about different settings being required for radials. Did any of the later GT40's or J cars use radial slicks?

 

[Ross Nicol]

I agree with Russ on the wheel width.For it makes sense that as wheels got wider scrub radius increased.Of course if you have wheels on the front that you can see the centre dished back a long way, they no doubt will have a negative offset and that offset adds to the scrub radius just like if you fitted spacers between the hub and the wheel.Howard I'll check my current scrub radius and post it here.
Most of the guys I race against have switched to radials now, due to availability of cross plys drying up.I started racing on radials back in '99' so I have my backside tuned to their characteristics,and I agree with the difficulty in sensing when they are about to let go.I've never known though whether the car contributes to this effect, but it stays gripped up and lets go suddenly.Recently while coming into a corner in the wet I was surprised to find the front wheels were locked just before I turned in. I can only surmise they locked on a wet shiny piece of tarmac, but in the wet sensing takes on a new meaning.

Ross

 

[Trevor Booth]

Russ, if memory serves me correctly -) it was ~90mm with 8" rim.

however, dont be fooled by scrub radius as the only cause for heavy steering.