Rag Top Shimmy

Discussion in 'The ragtop shop' started by DeeVeeEight, Sep 24, 2017.

  1. DeeVeeEight

    DeeVeeEight Well-Known Member

    With the top down and on a relatively smooth black top road, my '71 will act like it has egg shaped wheels at speeds between 25 and 35 mph. I have gone through it all, wheel and tire run out, balancing, brakes, drive shaft, rebuilt front end, frame stiffeners, new shocks, steering box, re balance the tires again etc...
    The fenders shake, the column wobbles and it all feels like it (the suspension and body) is going in different directions at the same time. You have to be driving to notice it from the feed back in the steering. The passenger is usually completely unaware.

    Is this just something I have to accept with an older convertible or can this low speed shimmy shake wobble nonsense be eliminated? At highway speeds or speeds above 35/40 there is no issue. On concrete roads it is hardly noticeable. I am tempted to install a steering stabilizer (horizontal shock absorber on the drag link) to see if it will help.
     
  2. buick64203

    buick64203 Buick Hunter Staff Member

    Without balancing your wheels on a Hunter road force balancer, you can not rule out the wheels or tires as the culprits.
     
  3. Ziggy

    Ziggy Well-Known Member

    Balance is likely not an issue at that speed unless it worsens with speed. If it does, you would have to have a severe imbalance in one or more wheels.
    What tires are on it and what specs was it aligned to?
     
  4. DeeVeeEight

    DeeVeeEight Well-Known Member

    The alignment is within factory specs, that has been checked 2 or 3 times. The front tires are admittedly old, about 7 year old Firestone Firehawks and the rears are about a year old Cooper Cobra's.
     
  5. Ziggy

    Ziggy Well-Known Member

    I am assuming that the tires are worn fairly evenly, no lumps in the tread, no scalloping etc. Firehawks are radials and it's likely that your car was produced with bias ply tires along with the caster specs for them, i.e. negative caster. Negative caster and radials can produce steering shimmy at certain speeds. Tire pressure, wheel offset, and toe in are all factors as well. To some extent, extremely loose wheel bearings can cause thouble as well.
    In my experience, if you set the caster equally and as high as you can, the car will have a much more positive and stable straight ahead feel
     
  6. DeeVeeEight

    DeeVeeEight Well-Known Member

    Yes, the tire wear is exceptionally even and no scalloping, I couldn't be happier with that. There is a shop not too far from me that specializes in alignment work, they have been there for decades. I may have to bite the bullet and take it to them. You have confirmed some of my suspicions. The car came with 14" bias ply tires, it now has 15" off set (staggered) radials. I am also questioning the quality of the front end parts that I selected. I purchased all McQuay-Norris parts (ball joints, tie rods, center link, etc.) and wonder if they are inferior.
    You also mention the caster. I have an issue where the steering does not completely "return to center" if you let go of the wheel after making a turn. I believe It has been suggested that I try to achieve a higher negative caster value, do I remember correctly?
     
  7. buick64203

    buick64203 Buick Hunter Staff Member

    If you set up the car with equal caster, wont the crown of the road pull the car to the right? Ideally you want the right wheel slightly ahead of the left to compensate for that.
     
  8. Ziggy

    Ziggy Well-Known Member

    All other things being equal, yes. However, camber can be offset slightly more positive on the left side, and more negative on the right side to accomplish the same crown compensation. The problem I've run across over the years is that crown varies significantly from road to road, making a given setting bias correct only in a small window of conditions. Also, loads in the car vary, affecting front end geometry. This is especially true if the car is only occupied by the driver, and a big one such as myself. It's easy enough to explain to a customer that the car will tend to want to run off a crowned road and this effect will vary depending on crown, but a little more difficult to explain that it's normal to pull to the left on a flat pancake interstate.

    I generally like to see about .25 degree more positive caster on the right side than the left.

    DeeVeeEight, your parts are probably just as good as anything else available out there. I learned when I restored my 67 that most if not all the available parts for these cars are some flavor of Chinesium. I think as long as everything is tight, without binding throughout the range of travel, that you can safely look elsewhere for your issues.
    You want to increase your caster(more positive) to increase the self centering effect and steering center stability. The original reason for minimal positive or negative caster was to make turning away from center easier for the driver. All cars had bias tires and many were manual steering in the early 60's when these cars were designed. Our power steering is over boosted so it is more than capable of overcoming the increased effort that the alignment changes will produce. Power steering was somewhat of a novelty when it came out and I remember my parents being thrilled that they could steer lock to lock with one finger. Road feel and feedback was gone and everybody loved that....it wasn't until years later that some amount of feedback and effort increase became important.
     
  9. cjp69

    cjp69 Platinum Level Contributor

    Lee,

    Any update on this? Mine does the same thing, before and after a front end rebuild and before and after new non egg shaped tires......
     
  10. DeeVeeEight

    DeeVeeEight Well-Known Member

    Nothing new yet. Funds are a little tight right now and I have to put the $ towards other things. I am planning to take the car to an alignment shop at some time in the future. The alignment shop has been around since I was a teenager, hopefully there are a few old timers there who still know their stuff.
     
  11. azzie

    azzie New Member

    Hi all...new to the forum. Just breezing through some posts and notice your wobble concern. I also have a 71 Skylark convertible and are experiencing the same shake. I recently had it up on a lift for inspection as I though the driveshaft may have been bent but running it in the air everything looks good. It feels just like a tire with a shifted belt but they all spin true while on the lift. Next step is to spin them on a road force balancer but it's not a tire balance issue ( too slow of speed to be tire balance). I'll keep you all posted if I find anything....Art
     
  12. wildcatsrule

    wildcatsrule Well-Known Member

    I had a similar problem on my convertible and it turned out to be a broken transmission mount-might be worth a look.
     
  13. gsjo

    gsjo Platinum Level Contributor

    You may be describing "cowl shake" or shimmy with the top down most all ragtops will have some to certain degrees.With the top down the center to /front of car suffers from a lack of structural rigidity.If you have new body mounts hopefully they are good quality-retighten each to spec.If they are original they may be tired.There is something you may try if you end up replacing your mounts.If you do PM me I can give you a tip I do on all ragtops.thanks Joe
     
  14. telriv

    telriv Well-Known Member

    IF you are large/heavy as you say customers of mine that were the same I would do an alignment with them sitting in the car. Since most of the time they are driving by themselves it helps to compensate for the added "Load".
     

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