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Scrub Radius, effect on steering centralising ?Helpful Member!(2) 

Spimage (Automotive) (OP)
16 Feb 05 14:04
   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.  
Helpful Member!  GregLocock (Automotive)
16 Feb 05 16:49
Not much.

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?


Greg Locock

Helpful Member!  Joest (Mechanical)
16 Feb 05 17:53

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.

GregLocock (Automotive)
16 Feb 05 18:28
Good catch Joest, I thought he meant a Mustang Cobra and was scratching my head. Henc eteh MacP question.

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.


Greg Locock

Spimage (Automotive) (OP)
17 Feb 05 13:44
    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,
Joest (Mechanical)
17 Feb 05 17:23
Double the caster (+7 deg), re-adjust bumpsteer and give us a report on how it feels afterwards.
GregLocock (Automotive)
17 Feb 05 18:30
And give it some mechanical trail  20 mm would be a good place to start.

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.


Greg Locock

wickedmotorhead (Automotive)
26 Apr 05 3:35
Say I wanted to make a spindle that would utilize stock control arms and with shims would only allow 4.5 to 5 degrees of caster.  Would it be recommended to add some mechanical trail in a car that is used primarily for the steet with about 5-10% road race duty?  How about for a manual steer car vs. power since this would add a higher steering force.  If so still about 20mm?  Comments?
GregLocock (Automotive)
26 Apr 05 7:04
The more mechanical trail you get, the stronger the self centring, but, the more difficult it will be to feel the change in 'weight' as the tyre saturates.

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.


Greg Locock

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