rear vibration - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Locutus · 03-01-2003, 07:25 PM

I have a severe vibration that begins about 70 mph, I suspect tires but it seems to severe for that, could the fact that I put heavy duty springs in the rear be intensifying the effect of an out of balance tire. I have not had the truck very long and have not had many oppurtunities to drive it at that speed so I don't know if this is new or not. Recent work done involved putting in new springs, shocks, and I had the rear gears professionally changed from 4.11 to 3.08(fuel mileage and this is to be my daily drive plus I want to haul my race car to and from the track with this truck hence the heavy duty springs. ) I am running a trubo 350 that is not orginal. It was orginally a 3 on the tree( I wish it was still a manual) Any ideas on locating this problem.

Mike

PS I hope its not the rear end cause they had my truck for 2 months installing those damn gears.

jjg · 03-01-2003, 08:48 PM

Does it just shake at 70mph or does it shake alittle when you let off the gas???
Did you check the driveshaft. go underneath and give it a good wiggle. if there is quite a bit of play then i would say it is your U joints.

Purs · 03-01-2003, 09:09 PM

just had the same problem. It turned out that the guy before me had put the wrong yoke on it.....? Fixed it right up though....

72CustomCamper · 03-02-2003, 01:43 AM

I second the drive shaft notion. The harmonic frequency for most driveshaft/rear end/wheel combos occurs at 60-70mph and smooths out at higher speeds (until the next harmonic at 120mph)

Locutus · 03-02-2003, 09:32 AM

Iwill check the U-joints I have considered just having them replaced since I do know how old or the quality of the ones that are there. As far as the yoke how do you tell if you have the wrong one ore not? I do know that the yoke is a very tight fit but I chalked that up to a new seal on the the back of the tranny. It slides on fine till the seal. What about drive line angles? Could the new springs have changed the geometry to much? I also don't know how well they did the tranny swap. Is it normally a straight forward deal or is there alot of custom work involved? Does anyone have measurements for crossmember location, maybe even pics of the crossmember and tranny mounts?

BTW my truck is a short wheel base 69 with 350/TH350, it is also a one piece driveshaft.

Mike

Locutus · 03-02-2003, 04:30 PM

Its been raining off and on all day. Haven't had a chance to check it out yet. Any other things to check out maybe?

TTT

Locutus · 03-02-2003, 08:25 PM

TTT

Locutus · 03-03-2003, 12:06 PM

one last time for any new ideas.

TTT

Tynee · 03-03-2003, 12:45 PM

Sorry, I'm no help, I'm just curious if changing down to the 3:08 helped the MPG, if so how much? I'm thinking about swapping from 3:73's. THanks, Tynee

Locutus · 03-03-2003, 02:24 PM

I haven't had a good judge of MPG yet I haven't driven it enough. The PO had put in 4.11's so it is a significant difference in what my RPM's are at hiway speeds. It was almost 4000 at 70, now I am about 2800 I am still thinking of overdrive to get it down even more. I do alot of city driving and even though I can afford the gas I prefer to save as much as possible. I do know that I didn't buy this truck to save gas, I bought it cause I wanted this model truck, but I am going to save as much as I can and still have the truck I want. I will let yall know how it goes. I am thinking of installing TPI as well. TPI with an overdrive ought to net me close 18-20 MPG.

Locutus · 03-05-2003, 10:26 AM

This is the stance of the truck as it sits today. I am wondering with it being dropped in the front could add to the vibration in the back. The PO dropped the front by heating the springs. I am planning to replace springs and ball joints but that is still a couple weeks away. It is supposed to stop drizziling today so maybe I can get under there and drop my driveshaft and get the Ujoints replaced and the drive shaft balanced. Just to eliminate that possibility.

wnc mountain · 03-05-2003, 11:58 AM

I would say either wheel out of balance of the drive shaft/univesal joint will be the problem. If it is only while accelerating or decelerating then it could be something else.

Long Knight · 03-05-2003, 04:21 PM

I had a vibraiton, or more of a moaning noise that would pulse, that is come and go avery few seconds at speed. I thought it was my rear end. However, the problem stopped after I put in a new tranny and changed all the u-joints. I think it was the tranny because the old joints weren't that bad, just thought as longs as it was off...

Locutus · 03-05-2003, 04:28 PM

I wouldn't describe this as a moan. There is a definate shake involved. It makes everything rattle even my head.

If it would ever stop raining I could pull the driveshaft and eliminate that problem at least. It is the least expensive thing it could be.

MandarinSixSeven · 03-05-2003, 08:26 PM

The vibration could very likely be because of the pinion angle, with the front dropped like that
You can shim the rear end to adjust it...im no expert, but i think the correct angle is somewhere around 19 degrees

I have a bad vibration, i checked and replaced everything, thinking since its an old truck, it must be drivetrain parts...turns out the rear wheels are BOTH bent haha, i overlooked the most obvious thing doh!

GreenMystChevy · 03-05-2003, 08:47 PM

After checking all of the previous stuff you might look into the flywheel. Sometimes those get out of whack from them hitting something or if they are externally balanced they could lose a weight.
Then there is the bushing in tail of the tranny where the driveshaft goes in. Check for slop there.
I personally wouldn't think that a pinion angle that small would cause such a vibration but you never know.

Good luck, it can get very frustrating so be patient.

Locutus · 03-05-2003, 08:48 PM

Raising or lowering the front won't affect the driveline angles. I was wondering about the springs in the rear affecting the angles. My concern about the rake is with it being so foward it makes the rear to light and it bounces adding to the vibration.

72CustomCamper · 03-06-2003, 09:31 PM

Lowering the front doesn't appreciably affect your weight distribution either. To sum it all up, it is one of these things:

Tires
Wheels
Rear End
U-Joints
Driveshaft
Transmission
Flywheel

Just replace them all!

Actually...when was the last time you had the wheels balanced. I had a belt break in a Michelin once and it was diagnosed on a balance machine. It shook like an 8.0 earthquake!

03-01-2003, 07:25 PM	#1
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	rear vibration I have a severe vibration that begins about 70 mph, I suspect tires but it seems to severe for that, could the fact that I put heavy duty springs in the rear be intensifying the effect of an out of balance tire. I have not had the truck very long and have not had many oppurtunities to drive it at that speed so I don't know if this is new or not. Recent work done involved putting in new springs, shocks, and I had the rear gears professionally changed from 4.11 to 3.08(fuel mileage and this is to be my daily drive plus I want to haul my race car to and from the track with this truck hence the heavy duty springs. ) I am running a trubo 350 that is not orginal. It was orginally a 3 on the tree( I wish it was still a manual) Any ideas on locating this problem. Mike PS I hope its not the rear end cause they had my truck for 2 months installing those damn gears.

03-01-2003, 09:09 PM	#3
Purs Got Big Block? Join Date: Apr 2000 Location: West Texas Posts: 1,510	just had the same problem. It turned out that the guy before me had put the wrong yoke on it.....? Fixed it right up though.... __________________ "Experience is what you get when you didn't get what you wanted."

03-02-2003, 01:43 AM	#4
72CustomCamper Registered User Join Date: Feb 2003 Location: Bremerton, WA Posts: 86	I second the drive shaft notion. The harmonic frequency for most driveshaft/rear end/wheel combos occurs at 60-70mph and smooths out at higher speeds (until the next harmonic at 120mph) __________________ '72 GMC C20, "2500 Super Custom", Custom Camper, New 454 '93 K2500 Suburban, Old 454 (157K)

03-02-2003, 09:32 AM	#5
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	Iwill check the U-joints I have considered just having them replaced since I do know how old or the quality of the ones that are there. As far as the yoke how do you tell if you have the wrong one ore not? I do know that the yoke is a very tight fit but I chalked that up to a new seal on the the back of the tranny. It slides on fine till the seal. What about drive line angles? Could the new springs have changed the geometry to much? I also don't know how well they did the tranny swap. Is it normally a straight forward deal or is there alot of custom work involved? Does anyone have measurements for crossmember location, maybe even pics of the crossmember and tranny mounts? BTW my truck is a short wheel base 69 with 350/TH350, it is also a one piece driveshaft. Mike Last edited by Locutus; 03-02-2003 at 09:34 AM.

03-03-2003, 12:45 PM	#9
Tynee I miss this truck. Join Date: Dec 2001 Location: Lexington, KY U.S.A. Posts: 2,863	Sorry, I'm no help, I'm just curious if changing down to the 3:08 helped the MPG, if so how much? I'm thinking about swapping from 3:73's. THanks, Tynee __________________ '07 GMC Acadia SLT. Sweet ride. '08 Crew Cab Z-71 short bed. Really like this truck A LOT.

03-01-2003, 08:48 PM	#2
jjg Registered User Join Date: Mar 2000 Location: elk horn, ia, USA Posts: 554	Does it just shake at 70mph or does it shake alittle when you let off the gas??? Did you check the driveshaft. go underneath and give it a good wiggle. if there is quite a bit of play then i would say it is your U joints.

03-02-2003, 04:30 PM	#6
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	Its been raining off and on all day. Haven't had a chance to check it out yet. Any other things to check out maybe? TTT

03-02-2003, 08:25 PM	#7
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	TTT

03-03-2003, 12:06 PM	#8
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	one last time for any new ideas. TTT

03-03-2003, 02:24 PM	#10
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	I haven't had a good judge of MPG yet I haven't driven it enough. The PO had put in 4.11's so it is a significant difference in what my RPM's are at hiway speeds. It was almost 4000 at 70, now I am about 2800 I am still thinking of overdrive to get it down even more. I do alot of city driving and even though I can afford the gas I prefer to save as much as possible. I do know that I didn't buy this truck to save gas, I bought it cause I wanted this model truck, but I am going to save as much as I can and still have the truck I want. I will let yall know how it goes. I am thinking of installing TPI as well. TPI with an overdrive ought to net me close 18-20 MPG.

03-05-2003, 10:26 AM	#11
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	This is the stance of the truck as it sits today. I am wondering with it being dropped in the front could add to the vibration in the back. The PO dropped the front by heating the springs. I am planning to replace springs and ball joints but that is still a couple weeks away. It is supposed to stop drizziling today so maybe I can get under there and drop my driveshaft and get the Ujoints replaced and the drive shaft balanced. Just to eliminate that possibility. Attached Images

03-05-2003, 11:58 AM	#12
wnc mountain More hobbies than money! Join Date: Jan 2002 Location: Raleigh, nc, usa Posts: 655	I would say either wheel out of balance of the drive shaft/univesal joint will be the problem. If it is only while accelerating or decelerating then it could be something else. __________________ '70 cst10/swb 350, 3 on the tree, factory buckets. '70 GMC lwb 350/350 ps,pb '72 c-10 lwb 350/350 a/c, ps, pb '72 Blazer project

03-05-2003, 04:21 PM	#13
Long Knight Spank 'em if you got 'em! Join Date: Nov 2002 Location: Kansas City Posts: 628	I had a vibraiton, or more of a moaning noise that would pulse, that is come and go avery few seconds at speed. I thought it was my rear end. However, the problem stopped after I put in a new tranny and changed all the u-joints. I think it was the tranny because the old joints weren't that bad, just thought as longs as it was off... __________________ Long Knight '71 C-20, 350ci, TH350, Edl. 1406 Carb w/elec.choke, Ignitor ignition, Viper Tires, Orange Dipstick

03-05-2003, 04:28 PM	#14
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	I wouldn't describe this as a moan. There is a definate shake involved. It makes everything rattle even my head. If it would ever stop raining I could pull the driveshaft and eliminate that problem at least. It is the least expensive thing it could be.

03-05-2003, 08:26 PM	#15
MandarinSixSeven Seasonal Poster Join Date: Apr 2002 Location: Kelowna BC Canada Posts: 896	The vibration could very likely be because of the pinion angle, with the front dropped like that You can shim the rear end to adjust it...im no expert, but i think the correct angle is somewhere around 19 degrees I have a bad vibration, i checked and replaced everything, thinking since its an old truck, it must be drivetrain parts...turns out the rear wheels are BOTH bent haha, i overlooked the most obvious thing doh! __________________ 67 GMC Short Step 350/4-Speed/3.08 Posi Front/Rear Sway Bars Kelowna BC, Canada Member of DTPW



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03-05-2003, 08:47 PM	#16
GreenMystChevy Registered User Join Date: Jan 2002 Location: Woodinville, WA Posts: 513	After checking all of the previous stuff you might look into the flywheel. Sometimes those get out of whack from them hitting something or if they are externally balanced they could lose a weight. Then there is the bushing in tail of the tranny where the driveshaft goes in. Check for slop there. I personally wouldn't think that a pinion angle that small would cause such a vibration but you never know. Good luck, it can get very frustrating so be patient. __________________ Jeremy '71 C-10 '72 GMC K2500 '72 Nova (Dad's) '70 Chevelle SS '55 Sedan Delivery Attempting to build up this here 4x4...make her real tall and purty...well, maybe just tall Last edited by GreenMystChevy; 03-05-2003 at 08:49 PM.

03-05-2003, 08:48 PM	#17
Locutus Registered User Join Date: Jan 2003 Location: Santa Fe, Texas Posts: 250	Raising or lowering the front won't affect the driveline angles. I was wondering about the springs in the rear affecting the angles. My concern about the rake is with it being so foward it makes the rear to light and it bounces adding to the vibration.

03-06-2003, 09:31 PM	#18
72CustomCamper Registered User Join Date: Feb 2003 Location: Bremerton, WA Posts: 86	Lowering the front doesn't appreciably affect your weight distribution either. To sum it all up, it is one of these things: Tires Wheels Rear End U-Joints Driveshaft Transmission Flywheel Just replace them all! Actually...when was the last time you had the wheels balanced. I had a belt break in a Michelin once and it was diagnosed on a balance machine. It shook like an 8.0 earthquake! __________________ '72 GMC C20, "2500 Super Custom", Custom Camper, New 454 '93 K2500 Suburban, Old 454 (157K)

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