Scrub Radius

Discussion in 'Technical' started by ztoy, Sep 4, 2014.

  1. ztoy

    ztoy Autospark Evolution

    Does anyone know what the scrub radius is for a zed with stock rims and suspension?
     
  2. Instamatic

    Instamatic Active Member

    Nope, but you can check it for yourself quite easily.

    With the car stationary and sitting on something like coarse asphalt or concrete, turn the wheel from lock to lock. Remove the wheel and look for the circular marks where the tread face was scrubbed against the ground. Measure the distance between the centre of this circle and the centre of the tread, and that's your scrub radius.

    The smaller the better, but I bet you already knew that :cool:
     
  3. East Coast Z

    East Coast Z Well-Known Member

    Scrub radius is also known as steering offset, and scrub geometry.
    It is the distance between 2 imaginary points on the road surface - the point of center contact between the road surface and the tire, and the point where the steering-axis center-line contacts the road surface.

    If these two points intersect at the center of the tire, at the road surface, then the scrub radius is zero.

    If they intersect below the road surface, scrub radius is positive.

    If they intersect above the road surface, scrub radius is negative.

    The effect of scrub radius - positive or negative - is to provide a turning moment which attempts to turn the wheel away from the central position, when the vehicle is in motion.

    On a rear-wheel-drive vehicle with positive scrub radius, the vehicle's forward motion and the friction between the tire and the road causes a force which tends to move the front wheels back. This would cause the wheels to toe-out.

    If it has negative scrub radius, the front wheels again tend to move back, but this time, they toe-in.

    On front-wheel-drive vehicles, the opposite occurs. Positive scrub radius causes toe-in, and negative causes toe-out.

    During braking, on any type of drive, if braking effort is greater on one side of the vehicle than the other, positive scrub radius will cause the vehicle to veer towards the side with the greater effort.

    Negative scrub radius will cause the vehicle to veer away from the side of greatest effort. How much it veers depends on the size of the scrub radius.

    This is why, vehicles with a diagonal-split brake system have negative scrub radius built into the steering geometry. If one half of the brake system fails, then the vehicle will tend to pull up in a straight line.

    Since the offset of the wheel rim determines where the center-line of the tire meets the road surface, it is important that the offset is not changed if wheels are being replaced.

    Changing the rim offset changes the scrub radius, and also the predictability of the vehicle handling, if brakes should fail.
     
    Last edited: Sep 7, 2014

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