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scrub radius and steering feel...again

Discussion in 'Technical Q&A' started by snj5, Sep 1, 2004.

  1. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Does anyone know what the stock scrub radius is typically for a 246 or 308 Dino? How does it compare to say a 911 or other sports car?

    I now understand that with ABS cars the scrub radius is very small to avoid the steering wheel being jerked from your hand with differential ABS application, but was wondering how the #s actually compared from ABS to non-ABS cars.

    Ferraris have also often described as having a dead on center feel - would that be because of carrying minimal scrub radii? I have also read that steering feel (and especially kickback) are directly proportional to scrub radius. So, if I wanted to increase steering feel, could I do that by changing the wheel offset with spacers? I haven't read where this would change the steering response however.

    If camber is controlled by lower A-arm mounting shims, wouldn't changing this also change scrub radii (although not much) ?

    My many thanks as I learn about front suspensions. I hope a couple of others (e.g. Auraraptor the Offset King :) ) might be interested in this as well.

    best to all
    rt
     
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  3. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
    2,149
    way north california
    Full Name:
    chris morse
    My meager understanding of scrub radius is: the linear distance, on the ground, from the contact patch centerline to the "King pin" intersection point on the ground. Since no sane manufacturer uses actual king pins but rather ball joints, we may use the line drawn through the ball joints to give us the intersection point.

    I have a drawing of the 308 front suspension from the shop manual and have drawn a line through the balljoints, actually extended the castor measurement line on to the ground. Lacking a scale or the time to scan and blow up the drawing so that a acale reads true, i am guseeling that the scrub radius is about 3 inches.

    My guess is that the scrub radius and the castor angle contribute to the self centering effect, giving a dead center feel. Turning the wheel causes the front end to raise, so the natural effect of the weight of the front end is to push down and thus center the wheels.

    Looking at the front end drawing, i can see that installing more camber shims would only move the whole arm and tire out the same amount, thus no or very minimal change in scrub.

    Ading spacers, on the other hand will increase the distance from the suspension and the centerline of the wheel, thus increasing the scrub.

    Antilock brakes are kind of like the voodo of automatic transmissions, so i am clueless to advise in this regard.

    Having owned a 911, i could probably scare up a drawing enlarge it to scale and give you a comparison of the difference in scrub.
    advise if this would help.

    best,
    chris
     
  4. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Chris -
    The 911 data would be great. Am learning about suspensions and the comparative data is very helpful; also had a 911 SC that I really liked the steering feel and response. I did dig through the internet and did come up with stock settings to compare:

    Dino 246
    scrub radius ~2"
    front camber 0 - 0'15"
    front toe-in 1.5 - 3.1mm
    front caster 4
    rear camber -0'50" to -1'15"
    rear toe 2 - 4 mm

    308GT/4
    scrub radius ~ 3"
    front camber 0'10" - 0'30"
    front toe-in 1 - 3 mm
    front caster 4
    rear camber -1'20" to -1'40"
    rear toe 2 - 4 mm

    Mondial 3.2
    Scrub radius: near zero (+/- couple mm)
    front camber -0'20" - 0'40"
    front toe-in 1.5 - 2.5mm
    front caster 4
    rear camber - 1'20" to -1'40"
    rear toe 2 - 3 mm

    Originally what started me thinking about all this was seeing a set of 16 inch qv wheels on a Mondial. They actually really look great with the concave vs the covex stock wheel, and add 1" of track and scrub radius per side without protruding from the wheelwells, but definitely filling them up (I can't post my photos, but will email them to anyone that is interested). This would put the Mondial scrub radius, near zero stock so an assymetric ABS activation will not jerk the wheel from your hands, at about 1", well within the 246/308 numbers with near the same suspension settings. The benefits (besides looking way better) would be a bit more road feel and track width. The downside would be if the 1" placed the wheel centerline too far out of the wheel bearing load parameters (I'm measuring this this weekend). Could actually tune the road feel with the suspension settings, but not sure of the steering kickback (I've actually never had my ABS go off).

    Many thanks
    rt
     

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