Home » Public Forums » GMCnet » [GMCnet] vibration on hard acceleration
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| Re: [GMCnet] vibration on hard acceleration [message #309215 is a reply to message #309187] |
Sun, 23 October 2016 11:49   |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Senior Member |
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Torque steer is not a vibration or even a thumping, it is a pull from uneven torque distribution that is not being cancelled out by the mass of the vehicle and is instead transmitted to the steering system which is not an inflexible system. Uneven torque will exacerbate ANY looseness in the mounting of the driving wheel system, front or back. You just don't notice it much in a rear wheel drive system because the lever arm is the length of the wheelbase and unless you have a lot of power and traction you will have much greater control of the resultant torque offset. As others have said, lengthening the axles will amplify the effect, but that is not what I would call a vibration. The wheel slippage could cause a vibration if the tire is not round and is a possibility, but most drivers know when they are burning rubber or losing traction. I know it when I spin the right front tire on takeoff in my GMC and in the FWD minivan, and it does not cause a vibration, just smoke and smell and a thumbs up from the spectators riding in the back of the pickup truck at the corner store! 
Going uphill, weight load is taken away from the front end and ANY play in that system is going to have more opportunity to show itself. You need to check those 4 ball joints thoroughly and with less than full weight load on the wheels and do the same for the tie rod and other linkage joints. Perform Rob Mueller's front end check carefully and then do it again with the system loaded at different amounts. Also check the front wheel bearings for ANY play at different load heights and make sure the bearing assemblies are not loose in the knuckles. Make sure ALL of the engine mounts are secure and the axles are tight to the final drive. Jack both front wheels off the ground by the bottom of the control arms, put stands under the frame, and spin the wheels with the engine while turning the steering lock to lock. Use the service brake to apply some load to them and repeat the test while observing the wheels from outside. Watch for any wobble or bounce in the wheels. This WILL require a qualified operator at the wheel, someone that does not panic first and faint later. DO NOT get under the vehicle during this type of testing and DO NOT stand in the travel path in case a jackstand breaks, and make sure it will not drive into a building or other obstruction if something gives way unexpectedly.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: [GMCnet] vibration on hard acceleration [message #309216 is a reply to message #309215] |
Sun, 23 October 2016 11:56 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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I believe what Lorne described IS torque steer. Terry, your concise,
thourough, response contains several very good points about checking the
front end of perhaps any front drive vehicle. I am amazed by the expertise
on this net.
Jim Hupy
On Oct 23, 2016 9:50 AM, "Terry" wrote:
> Torque steer is not a vibration or even a thumping, it is a pull from
> uneven torque distribution that is not being cancelled out by the mass of
> the
> vehicle and is instead transmitted to the steering system which is not an
> inflexible system. Uneven torque will exacerbate ANY looseness in the
> mounting of the driving wheel system, front or back. You just don't notice
> it much in a rear wheel drive system because the lever arm is the length of
> the wheelbase and unless you have a lot of power and traction you will
> have much greater control of the resultant torque offset. As others have
> said,
> lengthening the axles will amplify the effect, but that is not what I
> would call a vibration. The wheel slippage could cause a vibration if the
> tire
> is not round and is a possibility, but most drivers know when they are
> burning rubber or losing traction. I know it when I spin the right front
> tire
> on takeoff in my GMC and in the FWD minivan, and it does not cause a
> vibration, just smoke and smell and a thumbs up from the spectators riding
> in the
> back of the pickup truck at the corner store! :lol:
>
> Going uphill, weight load is taken away from the front end and ANY play in
> that system is going to have more opportunity to show itself. You need to
> check those 4 ball joints thoroughly and with less than full weight load
> on the wheels and do the same for the tie rod and other linkage joints.
> Perform Rob Mueller's front end check carefully and then do it again with
> the system loaded at different amounts. Also check the front wheel bearings
> for ANY play at different load heights and make sure the bearing
> assemblies are not loose in the knuckles. Make sure ALL of the engine
> mounts are
> secure and the axles are tight to the final drive. Jack both front wheels
> off the ground by the bottom of the control arms, put stands under the
> frame, and spin the wheels with the engine while turning the steering lock
> to lock. Use the service brake to apply some load to them and repeat the
> test while observing the wheels from outside. Watch for any wobble or
> bounce in the wheels. This WILL require a qualified operator at the wheel,
> someone that does not panic first and faint later. DO NOT get under the
> vehicle during this type of testing and DO NOT stand in the travel path in
> case a jackstand breaks, and make sure it will not drive into a building
> or other obstruction if something gives way unexpectedly.
> --
> Terry Kelpien
>
> ASE Master Technician
>
> 73 Glacier 260
>
> Smithfield, Va.
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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| Re: [GMCnet] vibration on hard acceleration [message #309222 is a reply to message #309187] |
Sun, 23 October 2016 13:48 |
Handyman
Messages: 229
Registered: April 2016
Location: The Netherlands
Karma: 2
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Senior Member |
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Just another possibility that could ad to your problem.
Not on a GMC, but on a Fiat Ducato, frontwheeldrive, turbodiesel, 150 HP, I encountered same thing, last week, after I got the RV back from the workshop, after big maintenance, greasing, oilchance, breakfluids etc ....
Already a half mile drive and in the first curve the outer frontwheel and the steeringwheel started to vibrate intens ...
With no accelleration and going straight almost nothing to sense.
If they had forgotten to tighten up the wheelnuts!
I directly went back, they checked and torqued both front wheels. No problem there.
Then lifted the RV again and looked at the steeringshaft etc .... "Ohh you need a new steeringshaft "
What the ..... @&&€',!,!.
I just went back from a long drive from the south of France and drove without problems through little villages, curved roads and highways at a speed of 75 mph ...
Pfffff... Silence on the mechanics-side ...
Then I saw that the front tires were very high inflated ... Those Continental Vanco 2 FourSeasons 225/75 R16, on my Ducato Maxi chassis, need on the front axle and a load of aprox. 1750 kg only 3 to 3.5 bar and they put more then 5.5 bar in them ...
That was the solution, after deflating and driving the same route, there was no problem what soever ...
Daniel
Daniel Jacobs, NL-USA 1977 GMC Eleganza II, Rebuild 455 (2019) 3.55 FD. FiTech and (Modified) FCC, Electric Pump, insulated GasTanks, 100A Alternator, APC, McDash, Schräder Valves + extern Fills, Ceramic Film, TPMS, FlexSteel Seats
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| Re: [GMCnet] vibration on hard acceleration [message #309244 is a reply to message #309216] |
Sun, 23 October 2016 21:16 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Senior Member |
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It's definitely a possibility James, as I have had many, many customers describe a problem and my interpretation of said description is way off base, and I get a total surprise when I test drive or try to find out what the problem is. Hopefully Lorne will let us know what the culprit is when it is discovered.
In a similar situation, I have a noise that sounds EXACTLY like the fan scraping the shroud when accelerating, but cannot find evidence of such action. I was sure it was the goopy front motor mount, but replacing that made no change in the sound at all. Still no evidence of the noisemaker seen and I can't duplicate the condition in the driveway, and couldn't get the sound recorded on the phone to post it for experts on the net to hear. Was hoping it was the carb stud scraping the engine hatch, or the extra lid I had on the air cleaner (which was scraping the hatch), but neither of those was the offender. So now I need to find that "qualified driver" I mentioned to drive the coach down the road while I hang my head down in the engine compartment and find that noise before it stops. 'Cause when it stops, most likely the coach will too  The noise is repeating at less than engine or wheel RPM, but is definitely a repeating squeak, squawk, or scraping sound. Engine noise drowns it out when trying to record it on the cell phone with the hatch open, so I'm gonna have to use regular ol' ears to find it 
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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