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Front end geometry help / wobble / bump steer

Discussion in 'The Hokey Ass Message Board' started by choppedsled, May 14, 2012.

  1. choppedsled
    Joined: Jun 2, 2007
    Posts: 301

    choppedsled
    Member
    from Spokane WA

    Here's my setup. Any advise would be appreciated in sorting this out. I just got this thing on the road after a 2 year build. Driven about 30 mi including 15 miles of freeway driving @ 70+ mph. Ah, life was good until..…I had a situation over the weekend that about made me piss my pants. I encountered a low speed death wobble while crossing over a badly rutted asphalt intersection. I was able to duplicate it again with some effort, same situation, crusing over a bumpy rutted intersection. Hasn’t happened in any other situation, hitting potholes etc.
    I’ve got 7* of caster, all rod ends are tight, steering box adjusted to the nuts, and my best effort at 1/8” toe in. Tires are balanced, but the tread on one of them wonders from centered about ¼” when spun.
    Have a look at my pics. I believe I need to correct my drag link angle. I didn’t want to heat and bend my pitman arm to bring the drag link end up and out, not a big fan of how that looks, but if that irons things out so be it. I ran this setup by some guys and they said it would be fine, but it really doesn’t intersect with the end of my bones.
    So does this look like a glaring case of bad geometry, or ? Advise before I dig out the torch.
    Thanks
     

    Attached Files:

  2. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 2,979

    ROADSTER1927
    Member

    I believe that your bump steer is because of your severe angle from your steering box arm to the spindel. See pics Gary:(
     

    Attached Files:

  3. Those shocks look way over 45* . Is that just the picture?

    You want the drag link to be in the same plane as the radius rod or bones. Ideal the pitman ARM and the rod pivot as close to the same location as possible, with them being the same length as possible.

    They actually intersect mid way but swing on two very different arcs.
     
    Last edited: May 14, 2012
  4. choppedsled
    Joined: Jun 2, 2007
    Posts: 301

    choppedsled
    Member
    from Spokane WA

    Here's a pic of the shock angle, should be a ok.
     

    Attached Files:


  5. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    Yeah, those shocks were the first thing I saw. Are they really laying over like that or is it just the picture distorting them ?

    Ok, I see your second picture and that angle is ok. Do you have a SoCal stabilizer on there ? It will help beyond belief.

    I don't think your steering arm position is a big issue. Look at the picture below of my 27. It was taken when I was having my front tires spin balanced, and as you can see the angle is very similar to yours, but my car has always handled as it should. I think that thing about the steering draglink angle having to exactly match the radius rod angle might be not as important as we think. In the ideal world it would be that way, but in the real world our steering arms sometimes end up in a different spot and have to be there because of the way the car is built.

    [​IMG]

    Don
     
    Last edited: May 14, 2012
  6. choppedsled
    Joined: Jun 2, 2007
    Posts: 301

    choppedsled
    Member
    from Spokane WA

    Haven't tried a stabilizer yet. I want to rule out the rest first. When I was mocking up the setup I really questioned the drag link angle, and should have modified my pitman arm from the get go. I've just had a few people tell me it should be fine, but it's the glaring issue as far as I can tell. I think the only option at this point is to heat and bend the arm up and out, thus shortening the arm, bringing the end of the drag link up, and more in an imaginary line with the end of the bone, correct?
     
  7. It will bump steer set up like that, that by itself huge problem as long as it doesn't start the oscillations and death wobble. Dons recommended steering stabilizer would help, maybe into the level of acceptable.
    Get a couple sticks and mock up your set up and see how far it actually steers in a bump.
    Coulpe nails, pencil and a board.
     
  8. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 2,979

    ROADSTER1927
    Member

    Bending the arm up and making it shorter will mess with your leverage big time! and a stabilizer is a band aid for something that is not right. Wish I had better news for you. Gary
     
  9. Are those the 450 500 15 or 16's firestones from Coker?
     
  10. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,252

    Hackerbilt
    Member

    The draglink angle looks fine to me.
    Not supposed to be parallel in a wishbone setup...unless its a cowl steer and then it works because the links are so long that no real arcing takes place within normal suspension movement.
    I'd be looking for loose components and maybe shackle angle etc before I started shortening pitman arms!
    That can cause trouble in itself!
     
  11. jimvette59
    Joined: Apr 28, 2008
    Posts: 949

    jimvette59
    Member

    I think you should lower the Hairpin to correct the angle also have the tires shaved and balance the wheels on the car. I had a low speed wobble on my 34 and found it was a caliper hung up I installed a steering damper after I found the problem oh I was using a reversed cast body Corvair box the damper made the steering feel much better. It steered to easy with out the damper. That was my experience with the ( shimmy ).
     
    Last edited: May 14, 2012
  12. Zombie Hot Rod
    Joined: Oct 22, 2006
    Posts: 2,453

    Zombie Hot Rod
    Member
    from New York

    Everyone else brought up a number of good points. Not enough toe in can cause a good bump steer. . . I'd also check your tire pressure. I've found that if it's too low, the tires become "springy" and can bounce a solid axle back and forth. Once it gets into a rhythm things get crazy.

    Try adding five more pounds and take it for a ride.

    Not saying that this is a sure thing that will cure all of your headaches, but it requires nothing more than a pressure gauge and a few quarters (depending on where you live).
     
  13. manyolcars
    Joined: Mar 30, 2001
    Posts: 8,707

    manyolcars

    you do not bend the pitman arm!
    bend the steering arm down at the spindle
     
  14. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida


    That is an old wives tale. The stabilizer is NOT a bandaid, but a great addtion to any straight axle car. My 27 never had any issues but is a totally different, better car since adding one. All 3 of our cars have one and RR tracks and uneven pavement is never a concern any more. I just received a new one yesterday from SoCal and will be putting it on my rpu project, and will never build another car without one. Evan if you have a great handling hot rod now you will see the difference if you add it.

    Lots of cars come from the factory with them installed as standard equipment (Jeeps for example).

    Don
     
  15. choppedsled
    Joined: Jun 2, 2007
    Posts: 301

    choppedsled
    Member
    from Spokane WA

    All good advise, keep it coming.
     
  16. choppedsled
    Joined: Jun 2, 2007
    Posts: 301

    choppedsled
    Member
    from Spokane WA

    450 - 16's, small I know. A taller tire is in order down the road.
     
  17. Hello Chopped

    What you want to do is have the arc of the axle and the arc of the draglink (at the slingshot) producing a parallel path within the suspension travel...do you have room to step down your steering arm (slingshot) down toward the axle?
    Try not to shorten up that pitman...as you will loose leverage and slow down your steering...hope this helps
     
  18. Moving the rear of the radius rod down at the back will help...you will have to adjust for caster changes...
     
  19. Butch11443
    Joined: Mar 26, 2003
    Posts: 353

    Butch11443
    Member

    I have an offset bent in my draglink to make it parallel with the split wishbones. Also have the damper on mine. Have never had a problem.
    Butch
     
  20. choppedsled
    Joined: Jun 2, 2007
    Posts: 301

    choppedsled
    Member
    from Spokane WA


    Yes, as Manyolcars suggested too. I do have enough room to drop it maybe an 1.0" - 1.5" or so. Great idea, and I'd feel safer doing that than messing with the pitman arm.
     
  21. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    The additional caster you pickup beyond the 7 degrees you have now might help also. Every car is different, my 27 is at almost 10 degrees and loves it there.

    Don
     
  22. MATACONCEPTS
    Joined: Aug 7, 2009
    Posts: 2,069

    MATACONCEPTS
    BANNED

    If you have bolt on steering arms, check if they are sitting flush on the inside against the backing plate, that there is NO lip edge from the spindle. You might have to grind the spindle in 4 places after you take off the backing plate, if this is an issue or allen wrench head bolts.
     
  23. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 2,979

    ROADSTER1927
    Member

    Sorry Don, I dont agree with you on the damper. If you look at all the original ackerman principals and others, most cars are some what different than that spec and the damper is to help with that problem. When we set up my roadster we checked it on the degree plates from a front end machine and it turned out to near perfect specs. When you cramp the wheels to one side on my car and push it it rolls away and straightens the wheels out, like it should. Of course I then put a spool in the rear end the roll test doesnt work any more but the spec did not change. It is serious fun in the twisties. Have a great day Gary
     

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  24. jimvette59
    Joined: Apr 28, 2008
    Posts: 949

    jimvette59
    Member

    I agree I installed one on my 34 Hot Rod and it greatly improved the feel of the car. They were on all the old VWs in fact that's what we used 40 years ago. Go Figure !!!!
     
  25. In the Jeep application the stabilizer was installed to combat possible rough terrain...ruts, holes.Bumping a rock etc...might wack a couple of digits of in the steering wheel spokes... ouch!
     
  26. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 2,979

    ROADSTER1927
    Member

    OK, you do not like my BAND AID reference soooo let me put it another way-----It is a factory fix for a problem they built. I doubt they originally put an extra cost part on a car that did not need it. Have a great day Gary
     

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