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scrub radius
davef - 23/1/03 at 09:33 AM

Hi Guys this a query to all you suspension boffins. assume i have a car originally fitted with 13 inch rims. i presume the kingpin -ball joint inclination is set to inter sect with the tyre center at road contact, or zero scrub radius. now i fit 16 inch rims with the same off set. now the line through the ball joints hits the road outside of the tyre center, result negative scrub radius. my question is how dangerous is this. also i notice from photos on this site that most if not all the locost,s have large positive scrub radius. cheers davef.

Spyderman - 23/1/03 at 04:58 PM

First off I would measure to make sure you were getting zero scrub with the 13 inch rims.

How does the width of the rims compare?
If the 16" rims are wider then you have moved your scrub radius already, even assuming that the overall rolling radius is the same.
You can move the scrub radius just by putting different profile tyres onto the same rims as the scrub is relative to overall radius as well as offset.

Yours is most likely due to increased rolling radius.
By itself it is not dangerous unless it is a great amount.
It will put more strain onto the wheel bearings, but again depends on degree.

Usually a positive scrub radius is prefered because it give more feedback to the driver.
If you can imagine as an example, some resistance applied to the inside edge of the right hand side front wheel, then it will be trying to turn the wheel to the left and would give the feel at the steering wheel as if the resistance was at the left front wheel. This is a very basic analogy!

In some cases it could cause tramlining on severe cambers.

At least negative scrub can be remedied with spacers, but this will affect your mudguard positions.

Hope I haven't confused you more?

I'm sure somebody else will be able to explain it more clearly!

Terry

johnston - 23/1/03 at 10:32 PM

me thinks rorty is ur man for this 1

johnston - 23/1/03 at 10:35 PM

me thinks rorty is ur man for this 1

Rorty - 24/1/03 at 12:55 AM

davef, is this a general query, or are you mulling over the idea of fitting 16" wheels?
If you're after the 16"wheel look, tell us your specifics, and we'll try and sort it out safely for you.

quote:

...result negative scrub radius. my question is how dangerous is this

It could be anywhere from plain anoying to downright dangerous, depending on the severity.
Bull has beaten me to it with his description of the effects, but I would just add (and I'm sure you're tired of hearing me rant on), with the light weight of a sevenesque Locost, you would be well advised to not go on the negative side of +10mm scrub. You could go as far as +25mm max. In a light car like this, zero/negative scrub, could be disasterous. What's more, it would feel like driving on ball bearings or patchy ice.
Again, as I suggested before, just try and keep to the original scrub specs (relevant to your uprights), because if something doesn't feel right when you finally get to test your car, you'll know at least the scrub is correct, and you can then start examining castor settings.
A nicely handling car is a real joy (very few people have had the experience), but an ill manered car will frustrate you, and spoil the thrill of having built your own.

Liam - 25/1/03 at 01:51 AM

Rorty (or any other gurus) - can I pick your brain to slightly increase my scrub radius trivia (very important down the pub)?

I understand the mechanics of scrub raduis well enough and many of its affects on the behaviour of cars - providing feel, steering weight, forcing toe changes under braking etc etc, but I'm a bit miffed in the area of positive vs negative scrub.

Lets assume we have a RWD car - why not a locost. I can well understand why you want some scrub radius to put some feel into the steering - but why positive? In terms of providing feel, surely a certain amount of positive scrub is as good as the same amount of negative scrub - because forces on the road wheel will put the same magnitude of torque on the steering wheel? I can see that if you have negative scrub instead of the usual positive, the feedback due to scrub will be in the opposite direction, but wont this feedback be stuff like bumps and vibration that arent really directional anyway? Directional feedback from castoring force will still be the same as with positive scrub wouldn't it?

And if negative scrub is annoying/dangerous in a car like a seven, how come it's used in FWD cars? If negative scrub increases stability under braking by toeing the front wheels in, surely its good? I guess whether or not the steered wheels are driven has an effect - in a RWD all you feel is what the road does to the front wheels, wheras in a FWD you feel what the wheels are doing to the road - kind of the other way around - maybe that's it?

I dunno, but I'll stop going on about it.

Cheers,

Liam

P.S. If you look at an escort upright it has quite a vertical kingpin inclination (compared to a sierra upright anyway). Most escort based Locosts that I've seen have a pretty large positive scrub due to this - much more than 25mm at any rate. My mates bike engined locost had spacers (around 12-15mm I think) on the front (to match the rear that he spaced out to fill the arches). With the escort uprights the kingpin projection actually hit the ground almost completely inside the tyre! He would have been around 90mm positive scrub. But the car always handled excellently, didn't have heavy steering even with the quickrack. Where do your suggested figures come from? How should this large scrub have manifested itself? Or rather what would have improved if he didn't have such a large scrub - or any escort based locost for that matter? Cheers again.

[Edited on 25/1/03 by Liam]