Tires and shimmy

[tombarb33]

I'm sure this question has been discussed many times, but, does the type tire ( radial, bias, belted) effect shimmy on beam axle Fords?

[FlatheadTed]

Make sure every thing is tight , Perch bolts , tie rod joints bearings ,king pins .Wheels are balanced , Aliment Steering box play .My car wondered ,so I checked the aliment it was 3/8 towed out , Now the car drives great , if they shimmy then there's a problem not related to tyres .

[George/Maine]

My 52 f1 pu had a bad shimmie and had a bad king pin,i did put new radials,works good now.

[Vergil]

I have radials on '31 coupe and bias on '30 pickup, neither shimmy, the radials give a better ride but the bias look better (to me).

Vergil

[41ford1]

As long as every thing is tight, tie rod, drag/cross link, steering box, king pins, and wheel bearings there should be no shimmy. I'm running 15" radials on a 41 pickup. It goes where it's pointed. No shimmy or wander.

[flatjack9]

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Ditto on my 39.

[tombarb33]

Thanks for the input. The car is a 33 coupe. The tires seemed to be all that's left I've tried all the normal things ie. new king pins/bushings (reamed to tight fit), tie rod ends are tight, same on drag link, F1 box adjusted to no slack, rebuilt shocks adjusted "stiff", balanced wheels, balanced hubs, new perches, bushings and shackles, new Posies reversed eye spring, axle caster set at 7 degrees. The car still shimmys with any toe in. It doesn't get better until about 3/8" toe out or more. Then it still feels like it wants to shimmy.

[TJ]

Looks like you may have too much caster.

[tombarb33]

I hadn't tackled the caster yet as I wasn't sure 2-3 degrees would cause a shimmy problem. That might be it though, as there is not much left to change.

[John R]

7 degrees caster is close to the factory spec. Must be something else. Have you checked camber or steering offset?

[tombarb33]

Thanks for the input. I haven't checked the camber, but looking headon from a distance, the camber looks the same at both wheels. I'll measure it. Is steering offset related to the center of travel of the box?

[Bums F-1]

My truck had bias tires, king pins etc all good. It would start the shimmie after going over a railroad track. Installed radial tires never did it again.

[John R]

Steering offset is the distance between the point where the king pin axis contacts the road and the point where the center of the tire contacts the road. This can sometimes be way off if you're using different hubs, like for a Buick brake drums setup. Pretty obvious if its way off. If your setup is 100% stock 33 I wouldn't worry about it.

[Frank The Plumber]

Take your tires to the tire shop and have the guy put them on the balancing machine, then you know that will be good.

[tombarb33]

Thanks for the input on the steering offset. Should the contact intersect point (kingpin axis and tire center) be the same point or inboard/outboard relative to each other? Re balancing the tires, sorry I wasn't clear in my list of things I've done. The shop balanced the wheels with the tires mounted on them. Then the brake hubs were balanced by themselves.

[Flathead Fever]

I worked as a fleet mechanic working on trucks with straight axles and have 30 years experience dealing with shimmies. Certain vehicles are prone to getting shimmies. We had particular problems with 1991 1 ton Ford trucks with aerial booms mounted on them. These trucks had I-beam front axles with parallel leaf front springs and a panard bar that controlled the side movement of the axle. Just the slightest play in the panard bar bushings would result in a violent shimmy on some of the trucks but not all of them would do it. There was some minute difference between the trucks that made one truck do it and others not to. It could have been how much weight they were carrying in the back? I had one truck that if the tires were worn down about halfway it would start having violent shimmies. The tires appeared round and checkout find on the tire balancer. I would put new tires on this truck and the problem would go away for a few months. It could have been just the couple of pounds difference in the weight between the worn tires and the new ones that made the difference on this truck, that is how touchy this stuff can be. Ford added a steering damper to the tie rods on these trucks a couple years later. I started installing them on the older trucks and it seemed to cut down on the problems. I definitely noticed that the Ford I-beam trucks tended to have more problems with front-end shimmies than the ones with the twin I beam independent front suspensions. It must have something to do with tying the two wheels together with an axle. The up and down movement of one wheel effects the other side. If one side goes up it changes the camber on the other wheel.
　
　
A shimmy is different than a vibration. A vibration such as a slightly out of balance front tire will result in a little shake in the steering wheel that will increase with speed. A shimmy can be a violent event, the larger the tires the more violent the shaking will be. On large commercial trucks it is a truly frightening, it's a I’m about to meet Jesus moment. There is usually no way to stop the violent shaking unless you can bring the vehicle to a complete stop.

Shimmies are usually caused by a combination of play in the suspension or steering components and out of balance tires or out of round tires that setup a rhythmic movement in the suspension system. It’s like a bouncing basket ball. The out of weight tire or loose components acts like the hand that keeps the ball bouncing. It takes more force to initially get the ball bouncing but only a small force to keep it going. Going over a pothole or railroad tracks will set off the event and an out of balance tire can act as the small force to keep it going. You might drive the vehicle for a month and not be able to duplicate the problem. You need to be at just the right speed with a certain load on the vehicle and hit a bump just right way and the shaking will begin.

Check for play in the king pins, wheel bearings, spring shackles, spring perch bushings, loose spring clamps, broken leaf springs, tie rod ends, loose drag links, loose steering box mounting bolts, excessive play in the steering box, bent steering components and a worn radius rod ball. Have someone turn the steering wheel back and forth with the tires on the ground while you watch for play in all of these components. You will see play in components and axle’s moving from side to side that you cannot see without the weight of the car on the ground. Lift the wheels off the ground and spin them to check for bent rims, tires separations or tires being out of round, also check for flat spots on biases ply tires that have set for a long time. If the tires are really out of balance, the heavy spot will rotate to the bottom and you will immediately know you need to balance the tires. . Look at the wear patterns on the tires for signs of alignment problems. Try rotating the tires to the rear. Disconnect the shocks and check the resistance in them. They are there to stop the tire from bouncing up and down. if they are bad they will contribute to the shimmy problem. These are the things you can try for free before spending money on an alignment and tire balance. If everything is in specifications you might try adding a steering damper. So-Cal Speed Shop sells them for street rods.

Before changing the design of any vehicle such as adding a steering damper you have to ask the question, did the vehicle have this problem when it was new. Is this a common problem with this model vehicle. If the answer is no, than modifying the system is not the answer. You need to discover what has changed on your vehicle. It could be something as simple as the wrong size front tire or wrong offset rims. Even the weight of the wheels could cause a problem The spring and shocks are not only designed to hold the car up but to control the forces of the un-sprung weight, which is everything connected to the spring eyes.

The very first thing I look for is has anybody changed anything from how the vehicle left the factory? If they have, nines times out of ten that is where the problem will be. Example: if someone installed a dropped axle it could contribute to bump steer. That is where the steering wheel wiil move everytime the tire goes up and down. There are so many droped axles under old Ford's that if you didn't know better you would think they came that way.

Jerry

[John R]

Originally Posted by tombarb33

Thanks for the input on the steering offset. Should the contact intersect point (kingpin axis and tire center) be the same point or inboard/outboard relative to each other?

The points where the two lines intersect the ground should be close together. The closer the better. A slight positive offset (say 1/2") where the wheel center line is outboard of the kingpin axis is OK. But it's hard to measure directly. Flathead fever has posted an excellent discussion of shimmy. You might also see if reducing the tire pressure has any effect.