just wondering scrub radius question... 2

[bikesrfast]

If rear wheel drive cars generaly have positive scrub and some front wheel drive cars have negative scrub, what would be the effect of giving a rwd car negative?


(apologies if I got the pos and neg around the wrong way)

 

[patprimmer]

The steering and kickback would be lighter and reduced.

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[patprimmer]

Not really my area of expertise, but I have stuck my neck out with a wild stab at it.

Also steering knuckles would need to be more complex or king pin inclination would need to be higher to overcome space constraints. This is also the case with FWD, but the need is greater, so the cost is more easily justified.

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[cibachrome]

Some rwd cars actually can get negative scrub even though they may have positive scrub radius with geometry. Reason is that tire brand specific properties introduce a dynamic scrub radius effect. It can go both ways. Its aspect, pressure and rim width sensitive, but clearly measureable on a tire test machine. When the 'favorite' tire math models are fit to the data, the effect is generally lost because the math models presume a certain degree of symmetry. Three brands come to mind which deliver +, ~0 and - dynamic scrub radius effect in many of their tires, especially performance constructions. BTW, this characteristic affects the rear axle's traction (launch) stability of the vehicle which some of you may have heard referred to as "forward bite".

 

[bikesrfast]

Just building a, um well dont really know what - track day, hillclimb,clubber etc. and made some uprights and now looking at wheels and A-arm lengths to determine my prefered track. Uprights have about 5 degrees kpi (25mm offset over 250mm pivot points)and will easily fit into a 13" alloy wheel. Wheel offset choice will of course determine scrub and I find a few that can give me pos or neg easily - why I am asking, to decide if advantage either way?

 

[GregLocock]

Some RWD cars have neg scrub. I can't get very excited about the difference between small amounts of neg, vs small amounts of postive, gut feeling is that it is better to have a small, stable, amount of either.





Cheers

Greg Locock

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[mjmghdm]

Excuseme, But...
If it is a large amount (around 20mm or more), Is there a rule to make a negative scrub radius for FWD and a positive one for RWD?
Or, Always the positive one is recomended for the both?

 

[GregLocock]

20mm isn't much

I don't think that rule works, neg scrub always seems better for me for traction and braking stability.



Cheers

Greg Locock

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[mjmghdm]

...Oops,I had made a mistake ( +&- ) in writing the last sentence. And you answered before I can correct it.

Actually I knew somethings about that, but, we have two pretty similar cars with a difference on their driving axle (FWD and RWD), and their scrub radiuses (that are according to the post of "bikesrfast"; - and +).

I used to hear about the necessity of negative scrub radius for preservation of stability in unequal braking conditions. As you said, it can be useful for traction stability too.

So, I think, We'd better change our RWD's scrub radius to a negative value instead of making it more positive.

Am I Right?
.

 

[GregLocock]

If you can, yes. Trouble is that usually means your KPI increases.

KPI is bad. It has one good effect (returnability from high lock angles), I don't think it makes anything else better.

To be honest for a performance car I don't think neg scrub is at all important - if the steering wheel pulls when braking then you've actually told the driver something useful.



Cheers

Greg Locock

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[mjmghdm]

Wow!In 3minutes?!

But I think negative scrub radius has a more important effect in an elastokinematic manner, directly on the tire and its linkages, whitout any need to turn the steering wheel!

& high lock angles?: Do you mean large steering angles which make returnability act reversely?

 

[GregLocock]

Once you start talking about compliance effects then concepts like scrub radius become far less easy to generalise about. For instance I'm working on a suspension where the 'scrub radius' changes markedly with steer angle.

" Do you mean large steering angles which make returnability act reversely? "

Yes

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.