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Q: why does my van shake from the rear violently? on 2003 GMC Savana 3500

My van shakes from the rear violenltly and the faster I go the harder it shakes. I've had all tires balanced an rotated and replaced the shocks.
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So without confirming the symptom you are sure that its the U-joint and it can't be a tire out of balance, bent wheel, broken belt in a tire, (bent axle, brake drum balance etc....)
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With "The faster you go the harder it shakes" it has to be a balance or run-out condition. One of those conditions I recently had to figure out had brake drums that were out of balance. Depending on how fast the vibration is it could be a driveshaft or U-joint problem. By paying close attention to the frequency of the vibration you can tell the difference because the driveshaft will be spinning about four times faster than the wheels are turning. The numbers work out to approx. ten pulses per second for a wheel/tire vibration at 60 mph and about forty times a second for a drive-shaft problem.
The van has disc brakes. I had the tires balanced and rotated about a month ago. I replaced the u-joint by the differential today
and did a very basic wheel bearing check, took the wheel and brakes off and made sure there wasn't a wobble, Hub spun excellent
without any play. With the wheels on there wasn't any play either. Thank's for the help, but, it's probably off to the shop and have a real mechanic look at it.
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Sounds like common cense has an easier answer than all the scientific mumbojumbow. Nice I could under stand that, but I think the questioner could understand the #2 answer. Just keep it simple, ATTA BOY.
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Isn't this what has been wrong in the trade for just about as long as there have been cars? Instead of coming up with a repeatable routine to prove what is wrong why don't we just jump to the conclusion of the day? When I teach techs how to discern vibrations understanding the frequency of it is the first most critical stage in the identification. There is a lot of science to auto repair today, in fact there was a lot of science to it back in the seventies when I first became a tech but it was rarely looked at that way. FWIW I won't let an employee or a student jump to a conclusion like this, they must have proof or the ticket doesn't get sold.
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You're not teaching Tech's on this web site. Some of these people do not have a clue or mechanical ability to understand what your talking about. Keep it simple; nobody is impressed with answers they don't even understand. Keep the classroom at the classroom. PS; I was working on Jets and Helicopters in the '70's as well.
now now men. no need to get nasty. I appreciate all the input. I learned something from all of your responses. Thank-you all.
I finally decided to check the tires even though they have plenty of tread and they were rotated in early October. I have several of these vans and always take the spare off as there is no way the tire is going to be changed out on the road. Amazingly it seems to have corrected the problem.
Changing the shocks was necessary as the right upper had a missing grommet. If the u joint was good, oh well ,at least I learned how to change one and that feels pretty good.
So, once again, thank's to all of you as this weekend was not a total loss because I learned how to do something I'd never done before .
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