Rear scrub raduis

[johnny99]

Folks, I am in the design phase of a single seater very similar to a D Sports Racer. I have most stuff covered, but I cannot find much info on rear scrub radius. I believe it's not too important but I would like to hear the inputs of people on this forum.

John

[Tim.Wright]

It's not really critical in terms of any steering effects but it does affect the split of the longitudinal forces between the wishbones and toe link.

Do you have a picture of the proposed design?

[johnny99]

No not yet Tim, just geo I want to meet without too much compromise. I hope to collect 2 sets of Formula Renault or F3 wheels when I get time in the next week or 2 and design around. Picking up a GSXR 1000 engine this weekend.

John

[Greg Locock]

You could use it to modify your toe in and out under braking and accelerating, but if everything is rigid (it isn't) then it has very little effect on anything. I don't think I've ever seen it discussed and I know I've never been asked to calculate it.

Having said that, I have been asked to look at its sideview counterpart, the mechanical trail. This obviously interacts with the lateral compliance of the suspension to give toe effects under lateral loads, and is quite important in your understeer budget, although subjectively using too much rear compliance steer results in nasty steering feel.

[johnny99]

Ah Greg, now I have more thinking to do, mech trail on the rear, never thought of that. With my budget maybe I should go with zero of each and when it's working well, do some changes then.

John

[Jersey Tom]

I would indeed start with zero of both unless you're really clever with compliance tuning.

[Greg Locock]

Definitely the place to start, and once you've done that you'll probably never need to change it.