Coach Vibration

[edroelle]

OK.  I would like to tap the expertise of the board.

A bus buddy, Don Clark, has a GM 4104, 6V92T, V730, with Centramatic wheel balancers.   After many years and miles with this coach, he has developed a vibration that starts at 58-60 MPH. 

Vibration is usually caused by a rotating member that is out-of-balance, or a slight imbalance that excites a weak structure.   (For instance, I have never seen a case where a bad wheel bearing has caused a vibration.)

Don has had the tires trued and balanced, new u-joints, and the driveshaft balanced.  He does have some cracking on the skin, indicating flexing in the body.

What kind of experiences do you guys have, that might lead us to a correction.  We are going to take a serious look at his bulkhead also.

Ed Roelle
Flint, MI

[gmbusguy1]

Hi Ed, is the 4104 of Don's capped and have a very nice white and purple paint job?

Chris

[Melbo]

Ed

My 8 has a vibration that starts at about 50 and ends before 60 mph.

I took the bus in and had the balancing powder put in the tires -- no help.

I took the drive shaft ( slip yoke) out and had it balanced -- no help.

I really do not know what the cause is but I have driven over 12,000 miles and it has not gotten worse or better.  I has not increased or decreased in intensity.

It is alway present at 55 -- not overpowering but present. It is more annoying than anything.

I just try to avoid driving in Illinois where the speed limit for motorhomes is 55 even on the interstate.

SOOOOO Ed if you figure out something to look for let me know.

Thanks

HTH

Melbo

[buswarrior]

Stand by, I'm mining the depths of Big Transit here, lots of folks on vacation, so it might take a little while.

happy coaching!
buswarrior

[cody]

My iggle is self balancing but takes time to achieve the proper weight to power ratio.  I'll go sit in the corner now lol.

[edroelle]

Chris (GMbusguy),

You have a good memory.  Yes, Don's bus does have a cap, nice white and purple paint, and stainless lower panels.

Ed

[expressbus]

Ed,

Have you torqued the lug nuts to the correct spec? Have you inspected the stud holes in the wheel rims for any "egging" of the hole? How about tie rod end bushings and other steering linkage points?

Just some random thoughts on things that would make a wiggle that begins at higher speeds. My old Harley had a vibration between 55-60 MPH slower or higher and it was smooth.

Hope you get the culprit identified and banished from your coach.

[skihor]

On my '67 Mc 5A I had a vibration on the hiway. Replaced the u-joints, balanced the drive shaft, wheels etc... The PO had defeated the auto level adjusters and put in 4 adjusters on the dash to use for camping leveling system. Anyway it turns out the driveline is VERY sensitive to ride height. PO had put in a 6V92 w/ a 4 speed Allison auto and the driveline is a bit shorter than stock. I just kept playing with pressures until The vibration went away. Fortunately My airbags don't leak down.

Don & Sheila

[gus]

Where is the skin cracking?

The rear upper corners just aft and above the side windows are part of the engine support structure.

[RJ]

Ed -

I, too, suggest checking the ride height, making sure it's set to spec.

Had a Flx "New Look" coach at the transit property I worked for that the driver complained almost daily about a front end shimmy at 38 - 42 mph.  Shop put hours into new kingpins, shocks, dogbone bushings, new brakes & drums, wheels and tires - practically a whole new front end over a six-week period.  Thought the driver was crazy, but complaint never materialized when "complainerr" was assigned another coach.

Problem was solved when the lease tire company's technical service rep was in town, and they ran the problem by him, again thinking it was tires.  First thing he did was check the actual ride height vs. "DA BOOK" - coach was set 1" (25mm) too high.  Shop reset the ride height, complaint went away.  Spent a lot of tax money to make repairs when all that was needed was a simple adjustment. . .

The other possibility, perhaps, has also been mentioned, and that is the radius rod (dogbone) bushings.  Might just be enough slop in one to cause the back end to wiggle at certain road speeds.

Keep us posted on what the cause/cure is - enquiring minds want to know!

FWIW & HTH. . .

[gus]

I've had two loose radius rods.

The front one I saw when I was greasing the front end, never felt anything while driving.

The rear one was spotted by a tire shop guy who noticed my rear axle moving while backing up to his jacking station. Never felt that one either?