- Home
- About Us
- Join/Renew
- Member Benefits
- Member Pages
- Log In
- Help
- Museum Store
The latest PASB reviews some of the comments on trying to prevent shimmy. As I have mentioned before there is never any discussion of the kick shackle, perhaps since there seems to be no information for adjustment. The kick shackle was introduced in the late 20’s specifically to eliminate shimmy, which was a serious problem for pretty much all cars with solid front axles to that point. Given the efforts to eliminate it is safe to say that even brand new cars adjusted to spec were not immune. On the Pierce it is located on the front left hanger and has two springs on a pivot using an additional ball bearing assembly as used on the other shackles. The weaker spring has some adjustment to it – mine broke from corrosion when removing and had to replace it and guess at the preload adjustment. Perhaps the adjustment and operation is completely insensitive, but does anyone know that for sure?
I would think that some mention of the kick shackle should be made when trying to diagnose shimmy.
One of the issues that contributes to shimmy on the Pierce is that they continued to have the fixed hanger on the front spring pivot instead of the back which tends to have more of a couple between the steering drag link and the motion of the spring as it flexes.
In Packard world (horrors!) the story goes that the kick shackle was discovered by accident on the proving grounds test track when it was found a car with an unintentional loose shackle was much less prone to shimmy, and they then did a purpose design and patented it. It is a very different non-adjustable design than Pierce and is at the back
hanger not the front. Perhaps Pierce was avoiding Packard’s patent?
Jim
I can add little, only the following: Pierce commenced the kick shackle midway through the 1930 model year “to reduce wheel fight.” The Parts Catalog actually shows different front fenders, BOTH of them, for cars so equipped. Perhaps an additional bulge to clear the shackle was required on the left side, and a new right front fender was required to mirror the appearance of the LF fender.
My 1930 roadster is about 30 chassis numbers (on the 134″ wb) BEFORE addition of the kick shackle. When I got the car, restored, it would almost lunge when leaving a highway for an exit ramp. A toe-in adjustment cured 80% of that (1/4″ total toe-in as a minimum), and experiments with tire pressure solved most of the rest. I now run 42 psi cold on the 700 x 18 Bedford Famous Coach tires.
I have not experienced the “death wobble” that so many others have. I did diagnose the death wobble in a 1930 Model A 7p touring as due to the replacement front wheel bearing seals. On that car, because OE design seals have not been available for many years, often a metal-cased seal is paced in a soft red plastic “carrier” (as I’ll call it) whose OD lip is deeper than the recess in the hub. That results in the drum and hub not being fully seated against the spindle backing–and there will always be wobble after the nut is set and cotter-keyed, because of the rebound effect of the soft plastic. The solution is to use a different carrier piece, OR to machine a deeper recess in the hub to accept the too-long outer lip of the plastic carrier.
I’ll appreciate any correction of nomenclature.
Considering that you could buy a 1904 Pope Toledo luxury car that
would do a mile a minute off the show room floor and race cars were
hitting in the mid-nineties during the 1911 Indy 500,it amazes me that
they waited until the thirties to engineer a cure for such a serious
inherent safety problem. I’m going to assert that the death wobble was
not a problem for new early cars with high pressure tires (55 to 75
lbs.). Wore out or out of specs is another issue. When balloon tires
were introduced in ’25, with low pressure, perhaps the dynamics
changed. I’ve never seen an early car with a shock absorber mounted
to relieve the effects of the “Wobble of Death””.”
I can imagine that lower pressure tires are likely a big contributor. The lower pressure creates a wider footprint which increases the steering moments from side to side as the tire turns. My ’36 Packard with 700-17 tires tracks nicely if I keep the pressure above ~33 psi, doesn’t shimmy but does follow the ruts if I let the front pressure drop much below that. It is enough pressure that I tend to wear the tread in the middle before the sides.
Jim
I have a collection of the SAE technical journals and the ’20s journals were FULL of articles on shimmy or wheel “wabble”” as they called it plus axle tramp. The conclusion was that the balloon tires that were designed to run at 20 to 25 lbs were the primary culprit. The consensus seemed to be that medium pressure tires running at 35 to 45 psi were the best compromise and that’s mostly what we have now.
Bill”
I really appreciate being in a club that is willing to share such knowledge. Thanks to you all.
Otto Klausmeyer explained the function of the kick shackle during the August 2, 1968 Technical Session at the 11th Annual Meet in Huntington, Pa (as reported in PASB 68-4). The device is intended to reduce telegraphing a jolt to the steering wheel from the left tire hitting a bump. It allows the left spring to float slightly fore and aft, which it could not do if the front end was fixed like the right side spring. Removing that “kick” to the steering wheel is unrelated to front-end “shimmy”,”wobble” or “wabble”.
The following is from the abstract of the SAE report describing the development of the kick shackle in 1929 which I think highlights how complex the problem is and that low pressure tires were a big part of the issue.
“BEGINNING with a review of the effects of the almost simultaneous adoption of balloon tires and front-wheel brakes, the authors outline the dynamic conditions of the front-axle system of the conventional car. They show that two types of vibration, otherwise independent of each other, are coupled together by gyroscopic forces when the wheels are rotating. The effect is greatly to lower the frequency, so it can come into synchronism within the speeds at which the car is driven. Shackling the front springs at the front end reduces the error in steering geometry, but cannot always entirely eliminate shimmy and wheel kick. A solution was found by adding a cushioned bracket at the rear end of the left front spring. This introduces damping, because of a phase difference between the gyroscopic forces and the elastic and friction forces, thus eliminating shimmy and at the same time reducing the reaction at the steering-gear to an amount so small that no kick is felt at the steering-wheel rim. Further analysis of shimmy is given in the discussion, and two speakers state that independent springing of the front wheels seems to be the only sure cure. Another speaker reports that independent springing has proved unpopular in Europe, where it has been most extensively tried.”
I suspect the potential advantage of the independent suspension was being able to avoid the fore and aft plus lateral tilt of a fixed axle as it deflects a leaf spring that in turn gets fed back into the steering gear.
Bill, of the SAE papers you have, can you recommend any others that describe the issues well? The quote from the abstract above is from SAE 290005 that I will order now that my interest has been retweaked.
Daydreaming on how the geometry dynamics interplay suggests many things that could potentially effect shimmy problems, including the arch of the spring. I don’t know that this will lead to anything practically useful.
Thanks, Jim