Scrub radius, how important?

[pigeondave]

Hi,

I'm looking at changing my wheels and hubs on my kit car. The new ally hubs are 10mm wider than standard heavy steel ones.
As I am in a position to slightly change the scrub radius with the wheel off set, what should I be looking at. The problem is if I move the wheels too far in they might catch the side pod or chassis.

OR

Is the scrub radius fairly irrelevant to a road car(that I do tour in) with occasional track use.

Anyone?

[Sam_68]

It's pretty important to steering feel, weight and stability.

Do you have any opportunity to try it out by replicating the offset with your current wheels, using spacers, before going to the expense of buying new alloys only to find that it tramlines like a bd?

[pigeondave]

Good idea I never thought of that.
Would 10mm make a massive difference?

[Sam_68]

I'd expect 10mm to make a noticeable difference, particularly on a light car, yes.

It's difficult to offer any definitive advice on just one aspect of steering geometry, because the overall result is a balance between a large number of different factors (KPI, Ackerman, scrub radius, caster, toe, tyre pressure, tyre sidewall stiffness, etc., etc...), but some general pointers/observations:

• The main effect of scrub radius is to introduce a turning moment between the centre of the tyre contact patch and the point where the steering axis hits the ground. Obviously, the bigger the scrub radius, the bigger this moment will be. Since scrub radii tend to have quite small measurements, 10mm is proportionately quite a large change, so will almost certainly have an appreciable influence.

• On a RWD car, even if the tyre loads are equal, tyre drag will tend to load the wheels toward toe-out with a positive scrub radius (usual) or toe-in with a negative scrub radius.

• If there is more drag on one side of the car than the other (whether from braking or just as result of variations in the road surface), the effect will be to pull the steering around. This means that bigger scrub radii tend to lead to more tramlining and kickback.

• When a car goes round a bend, it transfers weight laterally, with the outside tyre becoming more heavily loaded. With positive offset, this means that the greater 'drag' from the scrub radius on that side will be fighting te driver trying to turn the steering into the bend... hence bigger scrub radius = heavier steering. Not always a bad thing, because since the resultant steering weight builds up in proportion to the amount the outside front tyre is being 'worked', it gives feedback on what is happening at the contact patch. Too much, though, and the steering can load up excessively in bends and/or start causing those problems with tramlining and kickback (if you want a masterclass in how not to do it, drive a non-PAS TVR Griffith, then drive an Elise to see how the experts do it. ). The disclaimer here is that caster and KPI have a big influence, too.

• Negative scrub radius tends to give a very stable, but dead feel (ask Audi), so for that reason we can rule it out for a lightweight RWD kit car, 'cos no doubt you'll be wanting something that feels nimble and lively.

• Zero (or too little) scrub radius can cause the car to feel vague and 'wandery', because the slightest variation in load from one side to another can reverse the direction in which the steering is being pulled (often just by taking up the slack in the joints and bushes). Since positive scrub radius tends to cause toe-out, you have to think about this in conjunction with the static toe settings (it may be acting to cancel out some toe-in, for example, or to add to toe-out).

So... the bottom line is to think about your steering behaviour as it is at present.

• If it's just right, you're probably best off maintaining the same scrub radius as you have at present.

• If its a bit vague and wandery and doesn't load up as much as you'd like to give you feedback in corners, increasing the scrub radius might help.

• If it's heavy in bends, tends to kick the steering wheel about a bit in your hands and/or tends to tramlineon ruts and cambers, you might want to try decreasing the scrub radius a bit.

...but since you can balance changes in the scrub radius against other factors (some as simple as varying the tyre pressures by a couple of PSI), you can juggle other settings so that they tend towards cancelling out (or magnifying, of course) the effects of the scrub radius: there's always more than one way to skin a cat.

Sorry if that's all a bit woolly and undefinitive... best advice I can give is to start from how the car steers at the moment and think 'where do I want to go from here and how do I use the physics behind the scrub radius to nudge things in the right direction'.

[pigeondave]

Thanks for the above. Its more or less what I was looking for. I suppose it comes down to a personal feel too as long as its kept on the heavier side of things.

It does appear that as you say there are many ways of skinning this cat.
The more I read, the more variables there are and the more they affect each other.

[RichardD]

Sam_68 said:

* When a car goes round a bend, it transfers weight laterally, with the outside tyre becoming more heavily loaded. With positive offset, this means that the greater 'drag' from the scrub radius on that side will be fighting te driver trying to turn the steering into the bend... hence bigger scrub radius = heavier steering. Not always a bad thing, because since the resultant steering weight builds up in proportion to the amount the outside front tyre is being 'worked', it gives feedback on what is happening at the contact patch. Too much, though, and the steering can load up excessively in bends and/or start causing those problems with tramlining and kickback (if you want a masterclass in how not to do it, drive a non-PAS TVR Griffith, then drive an Elise to see how the experts do it. ). The disclaimer here is that caster and KPI have a big influence, too.

I didn't think you'd be able to resist referring to a particular TVR example when mentioning how not to do certain suspension / steering things !!

To the O.P., here's a link showing some Elise data, maybe you could compare to where your current car is now, but assuming it is a Fury I think the best thing to do regarding modifying the steering is absolutely nothing

http://www.grabercars.com/phpbb2/viewtopic.php?p=1...

[Sam_68]

RichardD said:

I didn't think you'd be able to resist referring to a particular TVR example when mentioning how not to do certain suspension / steering things !!

Sorry - soft target!!