By on February 11, 2016


1980s cadillac. shutterstock user Fortgens Photography

Timothy writes:

This isn’t a question about any car that I current own, or even a car that was ever mine.

While watching one of my all time favorite shows, “The Rockford Files,” our hero Jim was stuffed into the back of a mid-’70s Cadillac Fleetwood. As the driver dropped it into drive and the Cadillac moved out, I noticed a very peculiar rear-end wiggle. This isn’t the first time that I’ve noticed this in a General Motors vehicle.

Growing up, my parents had a ’81 Chevrolet Caprice coupe and a ’77 Pontiac Catalina coupe, and an ex-girlfriend of mine had an ’81 Pontiac Catalina sedan that did the same thing. (While I love my parents dearly, I’ve never understood why they bought Ford trucks repeatedly, but never ventured into a Ford car.) They all had that exact same low-speed rear-end shimmy.

What the heck is that?

Keep the esoteric Ford fanaticism alive, please!

Sajeev replies:


Any chance you can tell me which episode had the GM wiggle, or show me it elsewhere? I am not entirely sure what you might be referencing, and I have these episodes on my DVR.

Timothy answers:

I knew you were going to ask to see some evidence! “The Trouble with Houston” – season 2, episode 20, at 45:40.

Sajeev concludes:

I thought I had that episode on my DVR, but it was erased along with scores of other unwatched, retro-TV shows while recovering from the worst of Stevens Johnson Syndrome (mentioned previously here). Not that I like to make excuses, but now I’m gonna assume you saw wheel hop, probably of the axle tramp variety.

I think the GM wiggle you’re referring to is the same as the Ford and Chrysler wiggle. Let’s not single out GM; aside from the leaf-sprung Chryslers, they all made soft, oversprung sedans with 4-link, stick axles struggling to put down the diesel-like torque of a low-compression big block V8 on whitewall tires.

Four links can control unnecessary axle wiggle with a Panhard rod, torque arm or a Watt’s link configuration. Even the last Panthers performed somewhat OK with the Watt’s link. I doubt any of this was in the bad guy’s obligatory, black Cadillac Fleetwood 75 (“The Rockford Files”) or Lincoln Continental Town Car (“Starsky and Hutch”). Add the sloppiness of non-gas charged shocks and every 1970s bad guy makes a wikkid-bad escape from the crime scene with ease.

It was acceptable to have a dancing rear suspension back then: Mercedes still couldn’t penetrate our country’s demand for Panther Love, for lack of a better phrase, and nobody woulda bought a Toyota with some funny “L” emblem back then.

It was a simpler and stupider time when Cadillacs were the only way to rule the highways!

[Image Source: Shutterstock user Fortgens Photography]

Send your queries to [email protected] Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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25 Comments on “Piston Slap: You (Axle) Tramp!...”

  • avatar

    It’s not the links, springs or shocks. They do what they’re designed for just fine. It’s the soft bushing (mounts) at the points where everything is held together. The bigger and softer the bushings are, the better the “ride”. But as the bushing age, they get “too soft”. They get damaged by exhaust heat and oil too.

    Bushings collapse, rebound, collapse, rebound, giving you the the axle tramp or “hop”.

    After market “prothane” bushings to the rescue, right?

    • 0 avatar

      I replaced all the rubber bushings on my MY95 Cobra Convertible to prothane, that took what was formerly a boulevard cruiser to a much more hardened stance. This was exactly what she needed as I lowered her suspension a good inch and a quarter, replacing the springs, struts and lower control arms with Maximum Motorsports adjustables. Tightened that arse right up and finally was able to put the limited power from that last 4.9L HO into action.

  • avatar

    It especially noticeable on cars like GM B bodies be the bushings are not preloaded by the springs. The springs act directly on the axle.

    Adding the factory-style sway bar helps a lot because the bar is rigidly bolted to both lower control arms, keeping them parallel in plan view.

    I built a Caprice with all the suspensuion bushings replaced with Heim joints. No wiggle there!

    A really great suspension engineer once told me, “no car is better than its rear suspension”, and I’m convinced he was right.


    • 0 avatar
      Felis Concolor

      Yow, nice fix!

      I remember decades ago the performance magazines would mention the use of aluminum(!) bushings to replace the weak rubber units as an extreme race-only means of eliminating wheel hop or unwanted axle movement, but always with the caveat “don’t do this on your street car; you’ll regret it.”

    • 0 avatar

      I would never consider any B-body, Panther or M-body without the suspension upgrade option.

  • avatar

    Wow, thanks for trying to answer my weird question. I even went back and watched some footage of my father’s ’77 Catalina on grainy VHS tape driving down the driveway. I remember distinctly this car doing this move, but oddly never my mother’s ’81 Caprice. It’s just one of those weird memories that flashed back to me while watching the Rockford Files, and now I have an answer. Awesome.

    • 0 avatar

      The ’81 Caprice likely had less power and less aggressive gears in the rear end, but the same axle/bushing setup.

      • 0 avatar

        My mother once took off after me when I had left the house forgetting something she thought was important. I was about 18 years old and still driving my Celebrity, when she left 5 min after me she was driving the Oldsmobile Cutlass Supreme Brougham sedan that was later passed down to me.

        When I got home to our gravel driveway I could see the telltale signs of the V8 and positrac axle tramping on the passenger side halfway down the drive.

  • avatar

    I upgraded a Mustang’s 8.8 axle with the a T-Bird’s T-Coupe 8.8 and went from 3.08s to 3.73s and drums to disks. 1st the wider axle made the tires rub the wheel lips going around corners and of course insane axle tramp from the gear ratio upgrade.

    Of course the factory bushing were the size of glazed donuts. Had to go with aftermarket upper and lower control arms with prothane bushings. Whole other car at that point.

  • avatar

    Sanjeev, the wobble was most noticeable while slowly going over a speed bump in the road. Had nothing to do with torque.

  • avatar
    GS 455

    Still one of my favorite TV show music themes. I remember the only times Rockford wore his seat belt was when he was about to do a J-turn and chase after a bad guy. The free wheeling seventies…

    • 0 avatar

      The “Rockford” or J-turn is what we were dying to try and master when we 1st got behind the wheel. Once you learn not to touch the brakes, it’s a snap. Snap turn, snap correct and you’re there.

  • avatar

    My, that Fleetwood is a correct size.

  • avatar

    Glad to see someone else who appreciates The Rockford Files, the best private detective show of all time.

  • avatar

    Old leaf-spring cars would use traction bars or slappers to avoid the axle winding up. Hideous of course, hanging down.

  • avatar

    Using urethane bushings forces the control arms to twist as the suspension cycles. Eventually the arms fatigue and break. If you strengthen the upper arms, then the brackets will break off the frame.

    A better way to solve the problem is to remove the 2 upper arms, replace them with 1 arm mounted longitudinally, and add a Watts linkage. All doable on a GM B-body. A well laid out Panhard rod works almost as well, and that’s easily done on a B-body.

    Axle hop under high torque conditions is usually caused by the suspension allowing the axle to rotate excessively (side view). This is a common Fox platform problem.

    Axle tramp, when the axle rotates in rear view, is often caused by inadequate damping (worn shocks) or unequal tires.

    None of these conditions seem to be what the initial question was about.

    • 0 avatar

      Yes, couldn’t agree more. Way back when I was a young lad, I’d stare up at the rear axle of a ’62 Chev, when it was parked up over the edge of a mild grass bank. Could not see how one wheel could go through a pothole with big suspension motion, without twisting the trailing arms, and that big center arm mounted on top of the banjo looked vulnerable as well.

      Well, since nothing did break, it had to be those big fluffy bushings which allowed articulation, when none was designed in if the joints had been “fixed” just by bolts or polyurethane bushings.

      Then there was the Panhard rod giving lateral location. No attempt had been made to make it as long as possible, so that when the suspension moved a lot, the axle moved sideways relative to the car. Always caused a wiggle (like the original poster talks about) when you goosed the car and the right rear wheel unloaded due to torque reaction, and the car “rolled” or “swayed”.

      Then the ’64 Chevelle came out with the four-bar linkage (angled upper arms) which prevented sideways axle movement, but since lower and upper arms were different length, again the bushings had to give to allow vertical movement. Subject to roll-bind, they used to call it.

      The original Lotus Cortina had a hard mounted triangular linkage bolted to the banjo, in addition to two trailing arms. Guess Colin hadn’t considered the car would lean in corners, so that banjo connection used to fail. All later Lotus Cortinas after the first batch came with leaf springs and tramp bars.

      Have no idea what the Caddy in Rockford had for rear suspension, but if a Panhard rod was part of it there’s your wiggle over a speed bump, magnified visually as seen by an observer due to the 8 foot long trunk.

      When BMW ditched that sad trailing arm rear suspension and went to 5 links, again, it was only rubber bushings (squeaking in protest, no doubt) that allowed actual movement.

      All hail the rubber bushing!

  • avatar

    Next I hope somebody can explain why the Nova and its siblings often seemed to crab down the road (my guess is there were a lot of improper axle to leaf spring adjustments on those cars. If so, what was the deficiency that allowed this, as it rarely seemed to happen to other cars.)

    Ps. Rockford rocks!

    • 0 avatar

      For one thing, Novas of that vintage had a wider front wheel spacing than the rear. From behind, it looked like they were not tracking straight.

    • 0 avatar

      The Panhard bar (aka track bar) that Sajeev refers to is what tends to keep the rear axle laterally aligned with the chassis.

      I don’t have any specific knowledge of that platform/those cars, but it could be related to a lack of or a poorly designed track bar/Panhard bar.

    • 0 avatar

      It is called a stub frame bolted to the body with more of those big soft and squishy rubber bushings, and only 4 of them. Makes the car bend in the middle when cornering.

  • avatar

    Novas and their ilk had the axle isolated from the leaf springs by large, soft rubber isolaters. They would fail, and the axle would shift on the spring. Aftermarket urethane isolaters fixed this nicely. The wide front rack and narrow rear track made to misalignment more noticeable.

    Replacing these isolaters, and using urethane bushings in the shackles, dramatically improved the handling of my Seville, a Nova clone chassis-wise. Retaining the rubber spring eye bushings maintained the good ride and almost all of the isolation.

    Once again, no car is better than its rear suspension.


    • 0 avatar

      I saw a lot of those crab-walking Novas.

      I heard a different version about why — don’t know whether it’s true. Supposedly that generation of Novas, Omegas, Phoeni, etc. had the axle located laterally by one bolt that wasn’t up to the job. The way I heard it, sometimes the bolt would shear before the car even left the factory. Afterward the axle was held in place by nothing but the leaf springs, leaving the car to crab-walk for the rest of its days.

      Frankly, relton’s theory sounds more believable to me.

  • avatar

    My family owned a ’78 Catalina sedan. I do not remember the rear end wiggle but I remember that it had such a smooth ride that passengers would comment on it.

  • avatar
    Big Al From 'Murica

    I think Rockford is still on Netflix if anyone wants to check this out first hand.

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