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5" Scrub Radius ?!!!

Discussion in 'The Hokey Ass Message Board' started by PewterRat, Jul 12, 2010.

  1. PewterRat
    Joined: Dec 10, 2009
    Posts: 9

    PewterRat
    Member
    from Texas

    Is a 5" scrub radius going to really suck? Am I totally screwed?

    If i want to run original 1934 Chevy spoke wheels I have to add 2" wheel spacers to clear the brake caliper. Running different rims (2-1/4" backspace), and no spacers, the lowest scrub radius is about 3".This is freaking me out because I'm going to use manual steering.

    Does anyone else have an SR that high on a car that drives well?
     
  2. 61bone
    Joined: Feb 12, 2005
    Posts: 890

    61bone
    Member

    A 5" scrub is going to mega suck and yes you are. Even power steering will not handle that much well. It's going to jerk the steering wheel with every bump. Steering will be very hard at slow speed and unpredictable at high speed. Try to get it under 3' and the less the better. Not to mention the fact it will be hell on wheel bearings.
     
  3. PewterRat
    Joined: Dec 10, 2009
    Posts: 9

    PewterRat
    Member
    from Texas

    My current thought is to run a steering damper to compensate for the wheel jerk.

    In terms of low speed, its my understanding that the older cars had 3-4" of scrub because it makes turning at low speeds easier since the wheels are tracking around a larger radius circle rather than just spinning in place.
     
  4. Scrub radius ?? I thought scrub was the line from bottom of rim to bottom of rim Nothing was to hang lower, so when you had a flat nothing would catch the ground and cause a problem.
     

  5. R Frederick
    Joined: Mar 30, 2009
    Posts: 2,658

    R Frederick
    Member
    from illinois

    You're not talking about the same thing I'm thinking about either.:confused:
     
  6. Antny
    Joined: Aug 19, 2009
    Posts: 1,071

    Antny
    BANNED
    from Noo Yawk

    The scrub he's talking about is the distance between the center of the wheel rim and the king pin projected axis, where both points meet the road. The greater the distance, the greater the steering scrub/harder steering.
     
  7. scootermcrad
    Joined: Sep 20, 2005
    Posts: 12,375

    scootermcrad
    ALLIANCE MEMBER

  8. F&J
    Joined: Apr 5, 2007
    Posts: 13,221

    F&J
    Member


    If that's what he means, and his center of tire contact is 5" away from the king pin line....then forget that whole idea. It won't work.

    See you on the next death wobble post begging for help :rolleyes: :D

    Some or most disc brake rotors move the line out too much as it is, then add 2" and it's just not going to work.

    Maybe put up some info on what axle, spindle, wheel info, pics etc...someone might have an idea to help.
     
  9. learn something new every day! So the bigger the scrub radius the more the wheel is being pushed or pulled around the arc of the pivot axis. I can see where bump will be greatly amplified the bigger the S.R. gets, and harder on parts.
     
  10. F&J
    Joined: Apr 5, 2007
    Posts: 13,221

    F&J
    Member

    That's an understatement. Any mechanical engineer from hamb could list all the things that would be affected.. but just imagine the compounded stresses on king pins as well as the strain on those old wire wheels. Tie rods, steering arms, steering box, etc

    It's real hard to get a rod correctly done even when using correct parts :)
     
  11. I have had "suspension gurus" tell me worrying about scrub radius is much ado about nothing. One theory is the larger the radius the better the on center feel and better tracking with more neutral caster/camber settings. Of course the larger the scrub radius the more pressure on the steering components, namely the tie rod. But that's about it, so long as the rest of your geometry is correct.

    If this were true you have to wonder why modern suspension geometry is placing the KPI incident point as close to the center of the tire contact patch as possible.

    That said, 5" seems like a ton but in reality it is about an inch more than most of the Ford beam axle setups I have around here.
     
  12. hotrodjeep
    Joined: Feb 3, 2009
    Posts: 867

    hotrodjeep
    Member
    from Tama, Iowa

    SO just for fun I worked out you problem.. I think

    I assumed the King pin angle was 7 degrees and you were running a 30" dia. tire.
    I figured that a taller tire would change the scrub radius.. In therory a taller tire would decrease the SR and smaller tire would increase the SR (according to Scooters drawing)

    In order to get a 3" SR with only a tire change, you would need a tire with a diameter of 65.5"

    Better Scrub Radius.... Yes
    Higher stresses... Yes
    Impracticle....Yes

    Thats what you get when you think outside the box.

    Jeff
     
  13. F&J
    Joined: Apr 5, 2007
    Posts: 13,221

    F&J
    Member

    EP, can you put up some more words on that? It just does not sound possible with a wheel like a 40-48 etc. You are always way ahead of most of us backyarders, I know :), but I am lost again. :D
     
  14. flatheadpete
    Joined: Oct 29, 2003
    Posts: 10,231

    flatheadpete
    Member
    from Burton, MI

    Don't be so hard on yourself....I've never even met you and I'd never call you a wheel jerk! :)
     
  15. DD COOPMAN
    Joined: Jul 25, 2009
    Posts: 1,121

    DD COOPMAN
    Member

    I'm gettin' the popcorn ready, now. Then I'm just gonna put my feet up and......DD
     
  16. Cshabang
    Joined: Mar 30, 2004
    Posts: 2,458

    Cshabang
    Member

    i would do everything in my power to remove the spacers.
     
  17. F&J
    Joined: Apr 5, 2007
    Posts: 13,221

    F&J
    Member

    A picture might help to see what 5" away from the tire center contact would look like. It looks worse that I expected with a skinny tire like most of us use. Sorry the pic angle is off a bit, but I was trying to show most of the king pin bosses..

    Here is a 33-36 Ford axle with the commonly used 37-41 spindles and 40-48 16" steelies. It appears that this setup is about 1" to 1-1/4" from the true center contact on this tire diameter.

    But you can see how wrong it would look to have the line run at 5" true inches off of center.

    My opinion is that all the steering parts will be max stressed when a wheel drops into a big pothole. I was even thinking about "off shore" cheaply made steering u-joints and weak tie rods ??
     

    Attached Files:

  18. I will elaborate in a few, got way too much going on at the moment. From your picture there, looks like 2.5" of scrub. The cars I measured have Lincoln style or Buick brakes and aftermarket wheels.
     
  19. F&J
    Joined: Apr 5, 2007
    Posts: 13,221

    F&J
    Member


    It's misleading at that angle,but honestly it can't be more than 1.5...anyways...... I'm glad you brought up the finned Buick drums. I ended up not looking for those because it sure looks like it moves the line quite a bit.
     
  20. fordf1trucknut
    Joined: Feb 13, 2007
    Posts: 1,147

    fordf1trucknut
    Member

    While not optimal bearing strength wise it still may be able to be comfortably driven if you add a stabilizer.

    If you look at just about any older solid axle 4x4 truck the center line of the kingpin or ball joint is quite a distance away from the centerline of the rim and many of these trucks had manual steering prior to the mid/late 70's or so.

    On my daily driven 4x4 F-1 I have close to 6 inches and it still drives well. I did add a steering stabilizer (I initially did get death wobble ever 5,000 miles or so if I hit a series of pot holes just right...no DW since I put the stabilizer on) AND I did replace the steering box since I have been driving it (the box wasn't the greatest when I put the truck together).

    I will say that I am a big guy (6'-8" and 245 pounds) and a smaller guy may have a tough time steering my truck BUT the last 40,000 miles have been trouble free.
     

  21. That's where my mindset is. Then consider a 10" wide wheel with a 2 1/2" back spacing and 44" tires...long story...

    Personally if there is a way to not run spacers I would do that but if you can't then run it and make it work. You may run into some issues but if determined I feel you could make it drive just fine.




    BloodyKnuckles
     
  22. 117harv
    Joined: Nov 12, 2009
    Posts: 6,590

    117harv
    Member

    A model t as well as other early auto manufacturers had front spindles with with 0 inclination and approx. 4" or more scrub radius. They wern't going that fast and mostly on dirt roads, they finally saw the engineering light and changed the design.

    I see many early rides with wire and steel wheels that have buick drums and or spacers with as much, and they see many miles of driving. It is harder on a lot of parts in the steering areas for sure, but it is in the area of form over function, as with alot of hot rod styling cues.

    Can you redrill the bolt pattern? get thinner adapters?
     
  23. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,281

    fiftyv8
    Member
    from CO & WA

    Yes, the line made by the kingpin down to where the tire rubber contact is with the ground.
    That line is call the angle of inclination.

    The line should be hitting the ground inside the tire ground contact zone.
    I guess the ultimate is to have that contact in the center of that zone, but would almost never happen with solid axles on old cars.
    Take a look at the front wheels on big trucks, you will note how the tire sits way in and exposes the rim center and brake drum in an attempt to account for this theory.

    If the contact is out side the zone you should be taking a 2nd look at your set up, but if it is that far out I would be very concerned.

    How about considering a larger diameter rim to clear stuff.
    I hate to mention it, but safety 1st, especially since you are aware of the problem, you are 1/2 way there to solving it.
     
  24. Heo
    Joined: Jan 8, 2010
    Posts: 524

    Heo
    Member

    split a steering box in half
    when i hit a pothole with to
    big scrubradius and crashed
    in a tree when i was young
    and smarter than my dad
    that told me thats whats
    going to happen
     
  25. Antny
    Joined: Aug 19, 2009
    Posts: 1,071

    Antny
    BANNED
    from Noo Yawk

    Big trucks are a different animal than lightweight hot rods. The rods we're talking about generally don't run power steering, which would offset the steering effort on a large scrub radius situation.
     
  26. PewterRat
    Joined: Dec 10, 2009
    Posts: 9

    PewterRat
    Member
    from Texas

    Contacted Rally America and bought some new Buffalo style wheels for the car. They will clear the calipers and I can avoid the wheel spacers. Not to mention, the Buffalo style from Rally America has a 3" backspace, so the Scrub Radius is down to 3" or a little less.

    I knew full well that the 5" was no good. When things go bad you just need someone to confirm your concerns to make you do what you know you should do anyway.

    I don't replace rusty parts with more rusty parts, so in terms of stresses on components, they can all take it. I machine a lot of my own components and do engineering analysis on most of it. I think it will all work out well now.

    The front end is 400# lighter than the stock 1934 Chevy Sedan. I'm running 1953 Pontiac Chieftain spindles and 6 bolt C1500 rotors. I built the front axle from 2"x2"x1/4" steel tubing. Turned axle bosses and cut the axle tube ends at the KP angle of 6° before welding them on with an alignment jig.

    Steering box is a Saginaw 525. Tie rod is 3/4" with a custom drag link connector in line to run cross steer.

    Now you have the specs and my final solution. Thank for all the posts.
     
  27. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,281

    fiftyv8
    Member
    from CO & WA


    Nice to see he followed the big truck idea and pulled his wheels in and remember a steering principle follows whether it be for a truck or a car.
     
  28. PewterRat
    Joined: Dec 10, 2009
    Posts: 9

    PewterRat
    Member
    from Texas

  29. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    Try a taller tire, the kingpin inclination will end up being closer to the wheel centerline at the tire footprint.
    The problem you've got is also very common on macpherson strut front suspension [ with widened or smaller diameter wheels ]
     

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