Scrub Radius, effect on steering centralising ?
Scrub Radius, effect on steering centralising ?
(OP)
I am currently running zero 'Scrub Radius'on the front wheels of my Cobra. My question is, what effect (if any) will a positive or a negative scrub radius have on the centralising of the steering, whilst moving in a forward direction?
Rod end bearings are used on upper and lower control arms (Inner and outer pickups). Castor inclination - 3.5 Degrees positive, Kingpin inclination - 6 Degrees.
Any comments or assitance on this topic would be greatly appreciated.
Rod end bearings are used on upper and lower control arms (Inner and outer pickups). Castor inclination - 3.5 Degrees positive, Kingpin inclination - 6 Degrees.
Any comments or assitance on this topic would be greatly appreciated.





RE: Scrub Radius, effect on steering centralising ?
What mechanical trail are you running? I'd say you were a bit light on for castor, and if you combine that with too little trail you'll get poor returnability.
Are you still running MacP struts?
Cheers
Greg Locock
RE: Scrub Radius, effect on steering centralising ?
Are you talking about a AC Cobra or kit car? This must be the case, because the late model mustang cobra has a KPI of about 16 deg, not 6 as you stated. With that assumption, it seems hard to believe that you have 0 scrub radius with only 6 deg. of KPI, although I'm not saying it's impossible.
I support Greg's advice. Calculate your mechanical trail for starters, then you will know more.
joest
RE: Scrub Radius, effect on steering centralising ?
General trend is to reduce scrub to near zero, but not zero. If it is dead on zero then the point of application of longitudinal loads will wander from one side of the kingpin to the other, generating an unstable feeling under braking, in particular.
Cheers
Greg Locock
RE: Scrub Radius, effect on steering centralising ?
Thanks Guys for the response and apologies for for not including more information on the car, which if I may, do so now.
It is a project (Prototype) which we have been busy on for four and a half years (full time)& yes it is a replica of an AC Cobra. We decided from the onset to create a car, unmistakebly 'Cobra' in appearance, but designed from the ground, up, not utilising donor parts. That being said, the car is now running and we are very pleased with every aspect of such, but for this returnability problem.
Spec's as follows:
Camber: 1.5 degrees negative
Castor: 3.5 degrees
Mechanical Trail: Zero
Ride Height: 125mm
Roll Centre: Front 21 mm, Rear 22mm
Suspension Travel: 50mm up and 50mm down
Tires: Front 245 x 40 x 17, Rear 335 x 35 x 17
The lower rod end bearing supporting the Kingpin is inside of the rim whilst the upper is close to the inside of the tyre, hence our 6 deg. angle on the KPI & zero Scrub Radius.
That being said, what would be better, induce Scrub, if so how much, or increase Castor?. Bearing in mind that we are looking, essentially for a 'Road' car, which would be
reasonably competetive on a race track. A difficult one I know, but as the saying goes, "Low aim, not failure, is a crime"
Thanks again,
Warren.
RE: Scrub Radius, effect on steering centralising ?
RE: Scrub Radius, effect on steering centralising ?
Wide low profile tyres have such short contact patches that they need all the help they can get with returnability.
Also check out your ball joint friction etc.
Scrub won't help.
Cheers
Greg Locock
RE: Scrub Radius, effect on steering centralising ?
RE: Scrub Radius, effect on steering centralising ?
In practical terms I can't see much problem in 30 mm, but like I say, you'll lose a bit of sensitivity at the extremes.
Bear in mind that you need to think about the shape of the contact patch - a low profile tyre will have a shorter CP than a high profile tyre, so you have to use less mechanical trail to avoid masking the changes in pneumatic trail.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.