High speed shimmy.

[Chadwick717]

Hi all, my 79 k10 with a np203 and a part time kit has a shimmy starting around 55 mph.  It also has a 350 with a th-350.  The previous owner told me about the shimmy and he said he had the tires balanced and the truck aligned and it didn't help. It wasn't to bad when I first got the truck, but now I feel like it is getting worse.  It seems to come and go, and it gets worse the longer you drive the truck.

I understand that these aren't 75 mph trucks, but I would like to cruise along between 55-60 without it shaking. I'm going to get the wheels and tires balanced again, and then my next step was going to be unbolting the front drive shaft and drive without it and see if that was the problem. Is this a good first step or am I going in the wrong direction?  I know it could be any number of things, I just want to narrow it down to the most common problems.  Oh it has the stock suspension setup as well.

[zieg85]

I've had the drive shaft slip yoke be the cause of a shimmy

[bake74]

I've had the drive shaft slip yoke be the cause of a shimmy

     The front rotors can also cause a shimmy.  When you have your tires done, have them check for any out of round issues with the tire itself.

[Chadwick717]

As far as the rotors go there is no shimmy while braking, so I was thinking they were not warped.  Interesting about the slip yoke though.

[bd]

Although tire pressure and runout should have been checked at the time the wheels were aligned, don't take it for granted that they were.  Check tire pressures then inspect the tires for radial runout and separations.  While you're there you can just as easily check axial runout of the rims.  In other words, make sure there are no bubbles or distortions in the tires and the rims aren't bent.

You can check radial runout by raising the wheel off the ground and positioning the saddle of a jack stand against the tire tread so that it just barely touches the tread.  Spin the wheel as you watch the gap between the stand and the tire.  The gap should remain consistent the entire circumference of the tire. 

Check axial runout by placing the stand near the bead wall of the rim so that it almost touches the bead wall without interfering with wheel weights.  Spin the wheel and watch the gap for consistency. 

[Chadwick717]

Thanks bd, this is good advice.