Pinion angle

[cudarob]

Have 1970 Cuda balkhalfed with 31x18.5 x 15 mt sportsman pro's on rear, new 205-75x15 on front. Have vibration over 60mph. Transimission is 3 degrees up, drive shaft is 1 degree up and rear housing is at 0 degrees measured off os universal cups at 90 degrees to floor. Some sites advise 5 degrees for leaf spring cars, my car is lowered with 2 one inch blocks on leafs and, have coil over springs on schocks. Car is 383, running a 727 and 430 gear, will be for street and strip mostly street, any advise on how many degrees to shim down pinion and housing?

 

[moparleo]

Check the balance on those rear tires first. Very large tires are notorious for high speed vibration. Take it to your local trusted tire shop and watch for excessive runout of the tire/wheel or both when mounted to an off the car balancer . If the
runout is good, Check the dynamic balance on the tire ,wheel combo.
Put weights on both sides of the wheel to balance properly. Don't believe any one telling you that they can balance using weights on one side. In order to accomplish this,they set up the machine for a static balance. This ignores the reading on the outside of the wheel, giving you a false balance. When a tire hops up and down, it is telling you that it has a heavy spot on the tire/wheel but it doesn't tell you how much it is out on inside or out side. Dynamic balance fixes the wobble induced from the weight being off on the inside and outside of the tire/wheel combo. It is easy to make the machine say what ever you want, but once the tires are on the ground is the true test. You can also put your rear axle on jack stands and have someone in side the car to put it in gear, gradually increasing speed while you watch the tires from the side NOT the rear of the car for safety reasons.

Tell us what you find.

 

[moparleo]

Check the balance on those rear tires first. Very large tires are notorious for high speed vibration. Take it to your local trusted tire shop and watch for excessive runout of the tire/wheel or both when mounted to an off the car balancer . If the
runout is good, Check the dynamic balance on the tire ,wheel combo.
Put weights on both sides of the wheel to balance properly. Don't believe any one telling you that they can balance using weights on one side. In order to accomplish this,they set up the machine for a static balance. This ignores the reading on the outside of the wheel, giving you a false balance. When a tire hops up and down, it is telling you that it has a heavy spot on the tire/wheel but it doesn't tell you how much it is out on inside or out side. Dynamic balance fixes the wobble induced from the weight being off on the inside and outside of the tire/wheel combo. It is easy to make the machine say what ever you want, but once the tires are on the ground is the true test. You can also put your rear axle on jack stands and have someone in side the car to put it in gear, gradually increasing speed while you watch the tires from the side NOT the rear of the car for safety reasons.

Tell us what you find.

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Sorry about the duplicate post !

 

[cudarob]

Will try the balance, however what about pinion angle.?

 

[Chryco Psycho]

we used to set the pinion 5-7* down , the pinion will climb the ring gear & straighten the angle under power , the softer the springs the more down angle you need , with a 4 link we set 2* down , ladder bars more like 4* , clamping the front leaf spring section with u bolt will help keep the pinion down & increase traction also .
I wuld just use some 2-3* angled shims from a 4x4 shop & slide them in between the spring & sprng pad & retighten the U bolts to try it

 

[moparleo]

If you are going to just use a leaf spring set-up, You will also want to get an adjustable pinion snubber. You can adjust the distance from the snout of the pumpkin to the floor, to compensate for axle wind up as Chryco was refering to.

 

[Chryco Psycho]

pinion snubbers work great but are awful on the street beating the floor to death which is why I try to stiffen the front spring section more

 

[cudarob]

found the issue cannot balance tires, they needed 11 ounces on one side then machine goes to error. Then went to different shop for road force balancing advised they could not as well tire bouncing up and down too much. Mickey Thompson advised bubble balancing is all i need, nothing wrong with tires as i am looking at a deal from them on a new set of Sportsman sr radials? Did put 4 degree shims in pinion down can't road test till i get a set of tires.

 

[moparleo]

That will be a normal problem with very large street tires. Actual racing tires have very thin casings and tread rubber. A plus when dealing with a lot of mass spinning. High quality radial tires usually have no casing splice , which will induce a thump when driving. Forget the Mickey Thompson expert, that is the standard line on tire/wheels that are hard to balance. Go to the track and you will see weights every where and usually taped over with duct tape to help them stay on..As I was saying on post #3 , there is a process to do this but it is time consuming. Big ones look great but there is a price to be paid for good looks. You don't need that large of tire to check driveline vibration. It will look funny but borrow a pair of tires from someone you know with a Mopar or older Ford. Just bolt them on and make your test. Then take back off.

 

[Chryco Psycho]

sometimes rotating the tire 180* on the rim will help but I doubt it in this case

 

[moparleo]

Chryco is right. But you need to have someone who really knows how to balance a tire/wheel assembly. New tires usually have yellow stamp on the sidewall indicating the heavy spot on the tire. When mounting, the tech installing the tire should align the valve stem hole with the stamp. It is called match mounting and will use the least amount of weight to balance.