food for thought

[blumule]

ive had a speed sensitive shake ive been dealing with for a good while now. does it at exactly 43 mph.engine rpm is 2000.i can shut the engine off down hill and coast up to 43 mph and shake starts. Its not violent but there nonetheless.Put brand new tires on, still there. balanced driveshaft ,still there.tried another set of rims,still there.Heres the deal, I bought a cheap set of wheels and tires on craigslist that were 16 inch as opposed to the 15 inch that have been on there. sure enough ,shake starts. but now at 39 or 40 mph. I don't really understand engine,driveshaft, wheel speed as they relate to each other but I feel this new discovery is pointing me in the right direction. What do yall think is causing this shake?

[fordman]

The larger tires made ur speedometer read slower is all that changed. I wonder about balancing driveshaft means that u phased it properly. (Lined up the u joints). U could have a worn suppension or steering component. What more about the truck can u tell us.

[blumule]

sorry,its a short bed one piece ds.new spicer universals,shocks and all new steering and suspension bushings.Cab mount bushings also replaced. shake feels more like its in the cab area more than steering wheel although I can feel it in steering wheel also. Its not a violent shake and I can drive out of it by slowing down or speeding up.its just annoying at cruising speed . I swear it feels like unbalanced tires but theve been road forced at two different locations.Axles maybe? I put a dial indicator on rear hub and runout was about 7 grand.front discs were perfect.the guy that balanced my shaft said it was way out of balance. he high speed balanced to 120 mph.i just wonder if he even checked it at the lower 43mph speed.when I say 43 mph that's exactly when it does it. Sorry for long post. im just venting.

[Ranchero50]

Sounds more like the trans mount is loose or washed out. Could be any mount really. It sounds like you have a sympathetic harmonic vibration. You could get someone to drive your truck while you ride beside them and see if you see anything shaking or wobbling.

.007" right? Not 7 grand.

[blumule]

yeah .0007, . im confident its not wheels or tires.the only other things rotating are driveshaft and axles. ill discover it by accident one day.

[rbeck204]

Did you lower the truck? If so pinion angle could be causing that vibration.
Rich

[AZf100]

I Had the Rubber Break down on a harmonic balancer once, It Would run rough at idle and once up to speed smooth out, I tried every thing, until it gave away one day. replaced it and sure enough, that was it . May not be your problem but worth shining a light on.

[crazyhorse]

1- You might try while truck is sitting still with out moving , put in park or neutral, slowly raise the rpm to the same rpm it vibrates at while moving. This should tell if the vibrating is in the motor.

2- Jack both rear wheels off the ground, make sure the truck is safe, have some one get in the truck, put in drive or high gear, raise the speed to the vibrating speed and look, listen as you observe the spinning rear wheels that is raised off the ground.
Hold the speed at the vibrating speed and look, listen , there should be something to give you a clue where it is coming from.
Notice the wheels/tires while they are spinning slowly, they should be smooth and straight with no bumps or wobble.
If you want to be more precise, use a piece of yellow tire chalk and hold it just touching the tire while the tire is spinning slowly and to the wheel lip as it is slowly spinning.
You can do the same with the front tires if you have some one spin them while you mark them with the yellow chalk.
This procedure if done correctly will tell if the tire/wheels are straight & in round.
I use to do race car tire & wheels like this.
This is very dangerous , make sure no one is injured while doing this, remember safety first.

If you do not find the vibrating with above procedure, it is probably in the front wheels, transmission area.

I hope you find the vibrating because I know how irritating it can be.

Good luck, keep us posted.