Scrub radius in gravel rally cars

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
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Kovacevic
Posts: 13
Joined: December 2nd, 2011, 3:18 am
Location: Melbourne, Australia

Scrub radius in gravel rally cars

Post by Kovacevic » April 6th, 2012, 11:51 am

I'm currently building a 131 Abarth replica gravel rally car, and was wondering if anyone has experience with running substantially wider wheels on a 131 in the gravel, or wheels with a wider offset that would give more positive scrub radius (I'm using the wide flares).
I know some thinking says that with Macpherson struts you are better with negative scrub, and the double wishbone set-up is better with positive scrub radius. I've never dwelt on it too much before, but I'd like to get the geometry as good as I can from the start.
Any comment or advice is welcome.

Thanks,
Alex

fingers99
Posts: 99
Joined: December 9th, 2009, 6:24 pm
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Re: Scrub radius in gravel rally cars

Post by fingers99 » April 8th, 2012, 12:50 am

I've found very little of any use on this except for Troy Robinson's attempts to explain the strange behaviour of Lee Butcher's Red Top engined Mini in PPC magazine. Even Staniforth hardly discussed it in anything I've seen.

Conventional wisdom is that excessive positive scrub can lead to steering so sensitive that hitting a bump can literally tear the steering from you hands or have the car heading towards the nearest ditch.

So the conservative option is always to keep ET to within a few milimeters of the stock figure while increasing the rim width.

Doesn't really seem to matter whether you're running struts or double wishbones, although an adjustable double wishbone set up might allow you to tune out some of the effects.

Of course, the more "sticky-out" the front wheels are, the better turn in is likely to be.

You might like to see if you can borrow some rims (or maybe use spacers as an approximation) and see if you can find something that suits. On a closed road, please!
GC_06

Kovacevic
Posts: 13
Joined: December 2nd, 2011, 3:18 am
Location: Melbourne, Australia

Re: Scrub radius in gravel rally cars

Post by Kovacevic » April 8th, 2012, 10:13 pm

Thanks for that, I like that idea of having a play with some spacers/wheels and seeing the effects before I order any wheels. As for the sensitive steering tearing the wheel from your hands, that would explain why my current car is a handful on rough terrain but turns in well. At least I know the pitfall I'm looking for now.
Thanks,
Alex

PS - PPC and Staniforth are new to me, but I'll Google them and have a look.

Daveyboy
Posts: 38
Joined: March 24th, 2010, 9:21 pm
Location: Southampton

Re: Scrub radius in gravel rally cars

Post by Daveyboy » May 24th, 2012, 8:14 am

In case you didn't know, PPC stands for Practical Performance Car magazine.
GC Book # 103

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