70 Convertable vibrates/shutter at 55mph
Thread Starter
5th Gear
Joined: Jun 2008
Posts: 5
Likes: 0
70 Convertable vibrates/shutter at 55mph
1970 350 Convertable (TH400). Feels smooth until about 55mph and smoothes out again at around 60mph. Seems like the body is shuttering.
I had this same issue with my 66 427 Convertable (M21) and never figured it out!!! I can feel it in the seat and stearing wheel but I think mostly through the seat. Tires are ballanced, shocks new, rear pig replaced and rear shaft u-joints replaced/greased. I checked the frame/body mounting bolts in the rear tire wells and the two up front and they 'seem' tight . Most suspension bushings have been replaced.
Weird that this is the 2nd car doing this... Could be 100 things I guess.
Any ideas?
I had this same issue with my 66 427 Convertable (M21) and never figured it out!!! I can feel it in the seat and stearing wheel but I think mostly through the seat. Tires are ballanced, shocks new, rear pig replaced and rear shaft u-joints replaced/greased. I checked the frame/body mounting bolts in the rear tire wells and the two up front and they 'seem' tight . Most suspension bushings have been replaced.
Weird that this is the 2nd car doing this... Could be 100 things I guess.
Any ideas?
Burning Brakes
Joined: May 2008
Posts: 860
Likes: 0
From: Stratford Ontario
My guess is it is a balance issue, finding it will be a major PITA. I think it will be in the back by Your discription. I would look for the obvious first as You have been doing. Check Your U joints and make sure they are seated properly. Were the shafts banged at all while they were out? They may need to be balanced. Im sure its not the front but check Your harmonic balancer and see if the rubber looks intact. Just a few spots to eliminate.
Le Mans Master
Joined: Jan 2007
Posts: 5,485
Likes: 7
From: Peekskill NY
Can you take a short video & post it? It might help if there's a particular sound someone might hear where they had the same issue.
You mentioned the half shafts were good, but the drive shaft could be out of balance, or have shot u-joints.
Also might want to recheck the tire balance.
go here, print this out and go down the list.
http://www.tracyvette.com/vibrationdiagnosis.html
I had a vibration issue that came on around 3K rpm, took a year to figure out the problem. Ends up when I replaced my water pump, the fan hub has elongated bolt holes and had been mounted off center. Now it's centered and no vibration.
Glenn
You mentioned the half shafts were good, but the drive shaft could be out of balance, or have shot u-joints.
Also might want to recheck the tire balance.
go here, print this out and go down the list.
http://www.tracyvette.com/vibrationdiagnosis.html
I had a vibration issue that came on around 3K rpm, took a year to figure out the problem. Ends up when I replaced my water pump, the fan hub has elongated bolt holes and had been mounted off center. Now it's centered and no vibration.
Glenn
Thread Starter
5th Gear
Joined: Jun 2008
Posts: 5
Likes: 0
I cant take a video of it. The shudder is not engine RPM related because if I throw it in neutral at 55mph I still have the issue. Applying light brake makes no diff. I have pulled the whole rear end and half shafts out trying to correct this (needed done anyway to replace leaf spring).
Safety Car
Joined: Sep 1999
Posts: 4,930
Likes: 11
From: AnyTown NJ
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
First I would have my tires balanced by a shop that uses a Hunter GSP9700 which not only balances a wheel but also checks for "out of round". This is important because even a square wheel can be balanced. 
http://www.gsp9700.com/pub/features/intro.cfm
If that checks out I would look to have the driveshaft balanced with the yoke assembly attached.
Good luck!!
http://www.gsp9700.com/pub/features/intro.cfmIf that checks out I would look to have the driveshaft balanced with the yoke assembly attached.
Good luck!!
Drifting
Joined: Dec 2002
Posts: 1,887
Likes: 1
From: Hill AFB area Utah
yea yea been balanced yea hey I have had this exact issue take them to discount tire have the ROAD FORCE balanced that will tell you if you have a tire or wheel issue. Mine balanced fine on the regular machine and i still had vibs. had the road force balance them they said “OH you don’t need that” . I insisted hey guess what one tire would not road force verdict tire was defective, thrown a belt. It’s all better now do it, it will at least eliminate that as a source
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
Racer
Joined: Oct 2000
Posts: 405
Likes: 31
From: Houston, Texas
I have the same issue on my 72. I had all new tires rebalanced at a reputable shop, and found a replacement rim out of round. Still waiting to replace the rim and hope it goes away. I will check the drive shafts and half shafts if it does not clear up after the new wheel.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Out-of-round tire, out-of-balance brake rotor, loose front wheel bearing, improper wheel alignment...lots of possibilities. To assess tire issues, drive the car and make your best judgement as to where the vibration is occurring (which wheel). Then, rotate the 4 tires in an "X" pattern and drive it again. If the vibration seems to have moved, you have a defective tire or poor balance on one [or more] tires. If the vibration does NOT move or change, it is in the other hardware somewhere.
Burning Brakes
Joined: Sep 2005
Posts: 1,139
Likes: 109
From: B'ham Alabama
It's still going to be a wheel or tire balance issue. Have you had it "re-balanced" or tried roatting the tires? The symptoms you describe are definitely wheel related. Often times a wheel balanceing can be done poorly.
I had this on my 81 Z28. It ran smooth with the old worn out tires. I purchased new tires and sudenly had a vibration at 60mph. Slower or faster it was smooth.
It took it back in and had it balanced again,...same problem, (Big 10 Tires).
At a car show I was complaining about the problem. A guy was there from Firestone who asked me to bring it to his shop. He balanced them again,...and this time it is perfectly smooth.
I had this on my 81 Z28. It ran smooth with the old worn out tires. I purchased new tires and sudenly had a vibration at 60mph. Slower or faster it was smooth.
It took it back in and had it balanced again,...same problem, (Big 10 Tires).
At a car show I was complaining about the problem. A guy was there from Firestone who asked me to bring it to his shop. He balanced them again,...and this time it is perfectly smooth.
