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  1. #1
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    Default Scrub radius and camber

    When I first thought of this the answer seemed simple, but like everything else it looks not to be so.

    This is in general and not chassis specific. When you increase static negative camber does scrub radius get reduced due to the relative increase in KIA. In theory is should not. In practice it may due to that even though the tire is cambered to the negative the tires contact patch remains flat. With the contact patch remaining flat the tire center line should remain the same. If that is true than the relative increased KIA should reduce scrub radius. Is that pratical or am I missing something.

    Brian


    I have an unlimited budget. That bad part is I have already used it up.


  2. #2
    dennis68 Guest

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    Yes Brian. As negative camber is increased there will be a reduction in scrub radius. A minimal amount of change but some none the less. Any change in the physical location of the upper or lower ball joints will change scrub radius.

    Size or shape of contact patch makes no difference as the measure of scrub radius is the centerline of the tread regardless of itís angle.

  3. #3
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    Brian, Here goes some thinking out loud, don't have a clue if it's right.

    STATIC/STRAIGHT AHEAD RUNNING
    I've never read anything on your example, but also consider in a static (running straight) condition, with a deg or two neg camber, the tire is now more heavily loaded on the inboard edge, and so it deflects more on the inboard side. The deflection makes the tire loaded radius shorter (inboard side only) so there might be more distance between the KIA line and tire centerline because they are converging lines. It all depends on how much extra loaded height loss there is at the tire centerline (probably none).

    BUT the inboard tread is now more heavily loaded (straightline) and that SHOULD move the effective contact patch inboard, thus the new tire centerline is not really the center of load anymore, and I think load center is more important than static tire centerline.

    DYNAMIC (CORNERING)
    Another thing that happens is, under cornering conditions, the main point of contact moves inward and forward as the car corners, so you have a dynamic loss of scrub radius and caster. This may be why some positive scrub radius is used on most cars. Cars with FWD have different tire loads since they are pulling, and seem to require less scrub radius than RWD cars.
    As the tire begins to "give up" due to overload in a VERY hard cornering situation, the rear of the contact patch shrinks from the rear to front, giving a lighter feel to the steering. The front of the tread is being slammed down into the pavement by the inertia of the rotating tire, so it is part of the reason the front of the contact patch hangs in there.

    When I wake up tomorrow, I may have a different take on this, thanks for the headache!
    Last edited by David Pozzi; 04-05-2005 at 09:41 PM.
    67 Camaro RS that will be faster than anything Mary owns.

  4. #4
    dennis68 Guest

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    Owwwwww. No, thank you David.

  5. #5
    Join Date
    Mar 2005
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    Manhattan Beach CA
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    Default

    on sort of the same subject, what are your oppenions on the best method of changing cambur. do you guys feel that shims or double adjusters are better. the shims seem like a better idea because they dont change the arm length, but the double adjusters seem good because they dont change the mounting point. can anyone think of other pros/cons to both of the methods?

  6. #6
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    Default

    I forgot to mention pneumatic trail, but on a low profile tire it may not be much of a factor.

    stanglou,
    If the arms are made right they probably don't need to be adjustable. But it doesn' hurt anything to have the adjustability. One concern is to try and keep both sides adjusted the same or you will have different geometry from side to side. Keeping the upper arm as short as possible should increase neg camber gain but only a slight amount.
    67 Camaro RS that will be faster than anything Mary owns.

  7. #7
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    Default

    that is true, although i wasn't really worried about cambur gain as the curve i hae already is good. i was more worried about static settings. i guess like everything else in suspension design, there is compromise on everything.

  8. #8
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    Yes, the more you can do to the geometry, the more you wonder what the tradeoffs are. You can have lots of static neg camber, and/or lots of camber gain in the geometry, but how much is enough? Or how much can you have before it hurts braking or tire wear?

    I've been asked a zillion times, "why not do the Guldstrand mod but lower the holes even more?"

    I talked to one guy who did the Guldstrand mod PLUS installed the Pro-Motorsports balljoint spacers! That's an additional 1 15/16" higher, PLUS the GMod!

    If Guldstrand wouldn't do more on a race car, I hesitate to do more.
    BUT, today's tires are very different from the bias ply race tires the GMod was created to work with.
    No production Corvette or Viper has a camber curve nearly as agressive as the Guldstrand mod on a first gen. They do use high amounts of caster though.
    67 Camaro RS that will be faster than anything Mary owns.

  9. #9
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    I`m not going to touch the camber vs. scrub issue,partly because I don`t want a headache right before bed and partly because I`ll bet that for our purposes it makes no difference whatsoever. ;) I`ll take a stab at the adj. arm vs. shims one though. IMO the adj. arms make it possible to get arms the *exact* right length and offset the first time. There are well informed folks on both sides of the shim vs. adj. fence but if you prefer shims just adj. the UCAs to the right length and offset,make them the same then shim them to taste. You`ll need a minimum of shims this way. If you like using the adj. UCAs to set the alignment too (as I do,because the extremely small length changes make almost no difference in the curves at all) then just do so. Basically if you have adj. arms you have your choice of doing it either way. Marcus
    ----Savitske Classic & Custom----
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