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Technical Question about Scrub Radius?

Discussion in 'The Hokey Ass Message Board' started by jaw22w, Aug 16, 2019.

  1. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    What is the standard for scrub radius minimum and maximum on our beloved straight axle cars? I have a Speedway forged axle with the Speedway spindle and GM brake kit. I have read that this setup moves the WMS outboard. What distance I don't know. I very carefully measured the scrub radius on my setup. Front tires are only 22" tall. The scrub radius with this setup is 2". It seems to me that you wouldn't want zero scrub radius for the same reasons you don't want zero toe. You need all the slack pulled one way, so it is predictable. So I think you need more than zero scrub. So the question is how much more than zero scrub radius is desirable and acceptable? I have never heard or read the answer to this question. Anyone have any insight? (or maybe incite?) IMG_0518.JPG
     
  2. Blues4U
    Joined: Oct 1, 2015
    Posts: 3,811

    Blues4U
    Member
    from So Cal

    I can't give you an answer in inches, maybe someone else can, but I've never seen it either.

    What we know is that positive scrub radius, as you have on your car, will tend to induce toe out. This actually helps keep the car running straight, but this isn't a case of "if a little is good, a whole lot is better". The more positive the scrub radius the more leverage the wheels input into the steering, which increases steering effort, and on cars with a tendency toward steering oscillations, or death wobble, can accentuate that tendency. Under braking conditions it can emphasize unequal braking, which is more common with drum brakes than discs. This can make the car pull to one side when braking. In emergency conditions, or hard braking while cornering this can be disastrous. Add in a sloppy steering box, and you're in for Mr Toad's Wild Ride!

    Large positive scrub radius can make turning the wheels when parking easier, as the wheel will rotate around an axis, and with zero scrub radius the tires have to slide as they pivot instead of roll.

    I think 2" positive scrub radius is fine. I don't have data to back this up, just my gut feel, but if the diagonal line from the king pin inclination angle falls within the footprint of the tire, unless you have some really wide tires, the scrub radius is acceptable. If the same diagonal line falls inside of the tire footprint, the scrub radius is excessive. You should work with wheel offset to try to correct it.
     
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  3. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    Anybody know what the scrub radius of a stock model A is? Or any other beam axle car?
     
  4. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 5,648

    scrap metal 48
    Member

    I don't know but you could make your tires flat and see if it scrubs....
     
    Last edited: Aug 16, 2019
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  5. 1946caddy
    Joined: Dec 18, 2013
    Posts: 1,489

    1946caddy
    Member
    from washington

    Off subject a little but it appears that your scrub line is too low by the picture but it's hard to tell from the front.
    You can reduce your positive scrub arc by going to a taller tire if needed
     
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  6. 1946caddy
    Joined: Dec 18, 2013
    Posts: 1,489

    1946caddy
    Member
    from washington

    Good question but the scrub radius would change as Ford went from 21" wheels to 16" wheels through out the years assuming he used the same camber in his axles. I'm guessing someone like Sid who drops axles would know if the camber changed from year to year.
     
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  7. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    That would be scrub line, not scrub radius.
     
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  8. Hnstray
    Joined: Aug 23, 2009
    Posts: 10,672

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    I was with you all the way until paragraph 3.In my understanding of what scrub radius is, and the geometry changes it induces, this statement is exactly in reverse of the effects.

    Zero scrub radius is what I believe was called ‘center point steering’, meaning the imaginary line of the king pin inclination (not caster) intersects the center of the wheel tread. THAT minimizes tire scrub when the wheel is turned by the steering linkage.

    I tend to agree with your views expressed in paragraph 4.

    Ray
     
    Last edited: Aug 16, 2019
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  9. Hnstray
    Joined: Aug 23, 2009
    Posts: 10,672

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    What you are referring to as ‘camber’ is actually ‘king pin inclination’, which is a component of camber and caster depending on the geometry of the spindle shaft in relation to king pin inclination.

    I do agree that that changing wheel diameter, assuming the same wheel offset, tire width and spindle dimensions would change the scrub radius. But if the smaller diameter wheels had a different offset and/or width that could easily keep the scrub radius constant....or not. It depends on the wheels design dimensions.

    Ray
     
    Last edited: Aug 16, 2019
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  10. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    Really I don't want to change my car. It drives great. I was asking more for the educational value. I could help the scrub radius with a taller tire, but then I start to run into tire rub on the fender braces. I was asking more for the educational value.
    Scrub line is a whole 'nother subject.
    When did the change from 21" to 16" occur?
     
  11. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    X2
     
  12. town sedan
    Joined: Aug 18, 2011
    Posts: 1,072

    town sedan
    ALLIANCE MEMBER

    [QUOTE="jaw22w, post: 13206008, member: 213900"

    When did the change from 21" to 16" occur?[/QUOTE]

    If I remember right..., 21" to 19" in '30
    19" to 18" in '32
    18" to 17" in '33
    17" to 16" in 1935
    -Dave
     
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  13. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    Thanks. I don't think there was a KPI change for any of those axles, was there? Do the spindles from all those year interchange geometry- wise? If they do then a 5" tire difference would account for a pretty good change in scrub radius, leading me to believe that Henry wasn't too worried about scrub radius. I'm sure his engineers must have been familiar with the concept. They had everything else right.
     
  14. Hnstray
    Joined: Aug 23, 2009
    Posts: 10,672

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    Again, wheel dimensions/geometry (width and offset) could easily accommodate the scrub radius change that smaller diameter wheels could induce if not taken into account during wheel design....but, as you say, “his engineers must have been familiar with the concept”.

    Ray
     
  15. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    Yes, wheel changes could accommodate the changes in height. I just wonder if they did. It would be very interesting to know the design scrub radius of the cars of the beam axle era.
     
  16. Gasolinefed
    Joined: Apr 17, 2018
    Posts: 105

    Gasolinefed
    Member
    from OR

    My Herb Adams chassis engineering book says zero scrub is ideal for handling, control and steering effort..
     
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  17. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 5,648

    scrap metal 48
    Member

  18. jaw22w
    Joined: Mar 2, 2013
    Posts: 834

    jaw22w
    Member
    from Indiana

    :cool:
     
  19. X-cpe
    Joined: Mar 9, 2018
    Posts: 683

    X-cpe

    If I remember right..., 21" to 19" in '30
    19" to 18" in '32
    18" to 17" in '33
    17" to 16" in 1935
    -Dave[/QUOTE]
    As they were decreasing rim diameter what were the doing with rim width and offset? Also were they going to taller sidewalls on the tires?
     
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  20. Mimilan
    Joined: Jun 13, 2019
    Posts: 187

    Mimilan
    Member

    Zero scrub radius can cause too much tyre tread scuff on wider tyres [or resistance to turning effort]

    Positive scrub radius creates tyre drag on the outer edge ,causing the wheels to want to Toe-out.
    This is actually desirable to help counteract Toe-in forces caused by positive caster. [FWD cars usually have Negative scrub radius on the driving wheels for the same reason]

    Too much positive scrub radius can cause "tram-lining" which feels similar to bump steer on uneven surfaces.
    On a beam axle, too much positive scrub radius combined with too much positive caster can result in "death wobble" [especially when the "vertical forces" from caster exceed the front weight of the vehicle]
     
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