Rear vibration 65-70mph+

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Rear vibration 65-70mph+

July 04, 2014, 03:31:16 pm

I've done a search and read through the threads and Barry's archive regarding rear end vibration. My coach is 1998 with 74,000 miles on it. I just bought it and put about 3,000 on it in the last couple weeks. I cruise around 63-64mph. At 65mph I can feel a vibration in the rear...at 70mph is when it peaks. It's not a tire or axle as the frequency is too high; it's drive-shaft or something spinning at that speed.

My coach is of the year, from the threads I read, that Allison didn't install the p3 carrier bearing or something correctly. Also, Barry said foretravel did some modifications on his old coach to adjust driveline angle. And he said he had a loose balancer.

I greased u-joints 2000mile ago and old grease flowed out of them with one pump indicating they were not dry.

I crawled under the rear end with a bar and pulled, turned, pried, and jiggled everything I could. Everything seems tight and there is no movement in the output/balancer on the transmission. There is a tiny clink in the rear when I turn the drive shaft back and forth with brakes on, but this seems like normal gear mesh.

At ride height (which is correct) the drive shaft angles down from the transmission to the rear slightly. The angles all appear to be small and not a problem. It does look like the transmission is "sagging" slightly and is not at a 90 degree angle to the ground, but this could be how the coach is sitting or as designed. I'm surprised there are no mounts under the transmission; instead it is only held by the bell housing.

I will try to contact the mechanic listed at Foretravel and another transmission mechanic recommended on this site. If my 1997-1998 md3060's are known for this vibration issue, I figure it is a matter of time before a repair is required. But at 63mph and below, I don't even notice it and the ride is smooth as can be. I'm wondering if it is something else.....or how long I can run it without doing any harm.

Any input? One thing I can try is going down the road and adjusting the ride height slightly to see if vibration changes.

Re: Rear vibration 65-70mph+

Reply #1 – July 04, 2014, 03:54:14 pm

Krush,

Check forums for retarder dampener failure. Check sooner than later. Are you sure you don't have an inside dual that is flat?

Pierce

Re: Rear vibration 65-70mph+

Reply #2 – July 04, 2014, 03:55:34 pm

in that speed range, I am always suspicious of tire balance or out of round tires - 55 - 70 MPH is where those issues tend to be most obvious.

Re: Rear vibration 65-70mph+

Reply #3 – July 04, 2014, 04:09:12 pm

We do not have any vibrations, and I am not aware of this being a known problem. This has to be difficult to diagnos. Even though you say it cannot be a tire, rotating tires may be an easy way to see if something changes. It seems that you cannot eliminate anything.

We did put a wired camera under our coach and on top of it while driving to try to find a noise. Cameras are inexpensive and can be a fun diagnostic tool. Pointing one at the U-joints, tires, etc may help. Our camera came with a long coax with a video RCA plug, which we put into our old VCR and recorded while we drove and later looked at the tape. It was interesting to see the brake mechanism move on tape.

Re: Rear vibration 65-70mph+

Reply #4 – July 04, 2014, 04:32:48 pm

I've read through all the forum threads which are also summarized here (thank you Barry!). The second paragraph says P3 bearing was a problem on these years. U295 Driveline vibration issues

I do not have flat or low tires, the axle is full of oil, no brakes are dragging etc. A 22.5 inch tire spins pretty slow and an out of balance or tire problem would be a much lower frequency.

This is much higher pitch, and though I don't have vibration equipment installed, I feel that it would be caused by something spinning the speed of the drive shaft. This is just from experience working on big fast rotating machines.

I checked the retarder damper, as stated in my first post. I went under and pulled and pried on everything. Nothing is loose.

Re: Rear vibration 65-70mph+

Reply #5 – July 04, 2014, 06:15:02 pm

Gary Omel has extensive experience with the carrier bearing issue. Send him a Pm and ask him to give you a call.

Mark

Re: Rear vibration 65-70mph+

Reply #6 – July 04, 2014, 08:08:50 pm

We have a vibration that generally peaks around 55 mph, but sometimes peaks at around 51 mph. I've had things checked twice at FOT. I asked them specifically to check drive shaft, universal joints, and transmission components for problems. I've also inspected the drive shaft and connected pieces for any signs of problems. Wayne at FOT claims the vibration is from the tires. We run Michelin XZA-1+. I've the have about 45K miles on them. I've had them balanced and added Centramatic balancers since the vibration started. Tires all look good, although at about 4 1/2 years on the coach, they are showing some weather checking on the sidewalls.

I am considering replacing the XZA-1+ with XZA-3 tire per Wayne's recommendation. Next spring will mark five years since we installed the tires. Some were almost one year old when they were installed.

This may be a very different vibration from your issue. I share it because we each experience a vibration dependent and the speed of the coach, and we've not found a solution.

Re: Rear vibration 65-70mph+

Reply #7 – July 04, 2014, 08:45:38 pm

With rear tires balanced and/or Centramatics, the next thing to check on the rear tires is RUN-OUT. You will have to chock the front wheels, raise the rear axle off the ground and disengage parking brake. Then check tire run-out.

Even a perfectly balanced "egg" shaped tire will-- roll like an egg.

Brett

Re: Rear vibration 65-70mph+

Reply #8 – July 05, 2014, 03:15:12 am

Quote from: Dave M – July 04, 2014, 10:11:18 pm

If your really into tracking down vibration, you will need a vibrating reed type device with a range from 450 - 6000 hertz. with that you can track down any & all vibrations.

That sounds like a good way to track the vibration. It's a new concept to me, but sounds very useful and productive. What would be the source for such a device? May I borrow yours?

Perhaps I should plan a trip to Virginia, a trip to NAC on your schedule, or somewhere else on your schedule.

Re: Rear vibration 65-70mph+

Reply #9 – July 05, 2014, 03:19:29 am

Quote from: wolfe10 – July 04, 2014, 08:45:38 pm

With rear tires balanced and/or Centramatics, the next thing to check on the rear tires is RUN-OUT.

That sounds like a good plan. I think Herman Power checked for proper seating of the the beads, and balanced the tires. I don't think they check for run-out.

All the ideas presented as responses to the problem are likely to applicable to krush's issue, as well as mine.

Re: Rear vibration 65-70mph+

Reply #10 – July 05, 2014, 10:13:53 am

One way you might try to run it down using available hardware is to download a vibration sensing and analysis app for your smartphone. These typically will log the results into memory or a storage card. Then take the phone to an area where vibration seems most noticeable, duct tape it to a surface, and drive it at 5 mph increases in speed in intervals. Should give you the dominant frequency which will help narrow down vibration source.

I've used the accellerometer in these devices for many applications, they are amazingly sensitive and accurate.

Re: Rear vibration 65-70mph+

Reply #11 – July 05, 2014, 11:30:33 am

Using this: Tire Diameter And Circumference Calculator (I know Michelin sizes a little unique), tire circumference is 125.108 inches= 10.43 feet/rotation. 70mph = 102.67 ft/sec. -------> 9.843 rotations per second (which would be 9HZ...very low)= 590.62 RPM.

MD3060: I found 6th gear is .65 gear ratio. My tach does not work (on list to get repaired, but lets say 1900rpm) 1900/0.65 = 2923 rpm ----> 48 rotations per sec...48 HZ

A tire would be a very low frequency vibration...which is not what I'm feeling. I appreciate the input, but I'm wondering if anybody did a quick check of the link I posted from Barry's site. My description of my vibration seems to match those on that link exactly.

Oil is full in rear end, u-joints are greased. Alignment is great. Brakes are not dragging (new right rear rotor and pads). Does it at 110psi and 100psi. I have 18,000lbs on rear and 9k on front axle with full fuel.

I can jack the tires off the ground and run it to 70 mph and see if I can find more facts. I can also pull both axles and run it up to 70mph going nowhere...and if it vibrates, well then we eliminated lots of things!

Quote from: MAZ – July 04, 2014, 06:15:02 pm

Gary Omel has extensive experience with the carrier bearing issue. Send him a Pm and ask him to give you a call.

Thanks Mark, I'll do exactly as you suggest.

Re: Rear vibration 65-70mph+

Reply #12 – July 05, 2014, 02:10:14 pm

Perhaps our 99' has different rear axle, but the brochure showed as having a Rear Axle Ratio of 4.63:1
Don

Quote from: Dave M – July 05, 2014, 02:01:56 pm

Oh my, My Michelin data pages show the 275/80X22.5 tire at 520 rev per mile and with your 5.13 rear axle my calculator sez the drive shaft is turning more than I expected at 2667 rpm @ 60 MPH. I am not up on such crazy rear gear ratios, so needed to get up todate.
So, drive shaft frequency at 60 mph would be 2667 hz, yes you can figure the engine rpm in 2nd OD at about 1733 rpm AT 60 mph, you can do the math for any other speed/rpm your interested in.
Tire pressure or wheel loading has nothing to do with frequency.
Surely some bright bulb will chime in and clear me up, Being uneducated I guess alot more than engineers, so need facts.
Dave M

Re: Rear vibration 65-70mph+

Reply #13 – July 05, 2014, 02:21:53 pm

Don,
For sure, but the data from the Beam Alarm page for the 98 270 clearly shows the rear axle as 5.13, that is what I was going by, if that is not the case, of course all numbers would be incorrect.
Having played the gear ratio's for many years vs tire size/rev per mile transmission gearing etc, I feel comfy in this arena, maybe wrong but comfy.

Re: Rear vibration 65-70mph+

Reply #14 – July 05, 2014, 06:08:36 pm

Thanks for the info everybody. I spoke on the phone with somebody that chased vibration problems at 70mph in the past on a u270. I learned a lot of info.

My initial gut feeling/hunch was that the final drive on this coach is probably low, which causes the driveshaft to spin pretty fast at highway speeds. Looking at the specs: 1998 and older 8.3 equipped coaches come with mechanical 8.3 the md3060 transmission with .65 6th gear ratio and 5.13 rear end ratio. 1999 units come with electronic engines, md3066 with .67 6th gear and 4.63 ratio. U320 with M11 has 3.91 final drive and .67 OD (6th gear)

Assuming a 590 rpm for the tires at 70mph, here's what theoretical RPM of components would be:

tire   rear   OD   drive shaft   % dif   engine   % dif
590   5.13   0.65   3026.70      1967.36
590   5.13   0.67   3026.70   0.00   2027.89   -0.03
590   4.63   0.65   2731.70   0.10   1775.61   0.12
590   4.63   0.67   2731.70   0.00   1830.24   -0.03
590   3.91   0.67   2306.90   0.16   1545.62   0.16

From these quick calcs, we see that a 1999 U270 has a driveshaft (and transmission output bearing) spinning ~300 rpm slower. Also, engine cruise RPM is lower which rests better on the torque curve.

These facts probably won't be spoken about much, but design changes hint to what really happened. The MD3060 likely was designed to just work in school busses and trash trucks...all operate at lower speeds and don't have HP to go 70mph.

As for me, I'm going to check out a few things and see if I have some run-out on the output shaft/retarder damper. I'm going to try to find an allison distributor that knows their stuff and have them tighten up the carrier bearing....or see if I can tackle it myself.

And I may start looking for a 4.63 rear end out of a wrecked coach. It would help 60mph cruise as well. In my opinion, the final drive is too low and it shifts into 6th gear at 50-55mph.

Some issues are often unspoken.

Re: Rear vibration 65-70mph+

Reply #15 – July 05, 2014, 06:39:01 pm

Most coaches from FT are geared for. 65mph at rpm which produces max engine torque. (Usually the most efficient rpm in a diesel

Re: Rear vibration 65-70mph+

Reply #16 – July 05, 2014, 07:09:29 pm

Krush

I had p3 bearing replaced at inland truck repair Dallas TX, Option for me was bearing only with 6 month warranty or complete overhaul with 2 year warranty. I went complete overhaul which jumped it from 3 coach bucks to 6 coach bucks. It has been awhile but I think my vibration was from 50 mph up.

I tried new tires with balance new u joints, neither solved my vibration problem. Overhaul corrected vibration.

I would not go back to inland, Transmission, I a had leak they fixed it but gave me a bill
but finally agreed it should be warranty.

Call Stewart Stevenson (Allison Rep) Longview TX they were familiar with this problem and may give you some direction to go on.

Good Luck.

Larry
1998 U 270

Re: Rear vibration 65-70mph+

Reply #17 – July 05, 2014, 07:15:34 pm

You need to contact foretravel (James) about this problem, they can tell you how to properly check and correction this vibration problem, like adjusting ride height, checking the transmission bushing for proper bearing clearance and how to adjust the drive line angle at the differance and the tranmission.

Re: Rear vibration 65-70mph+

Reply #18 – July 05, 2014, 07:57:00 pm

Well now, thanks to this forum, I learn yet another tidbit about our coach and the differences between the different years. It can be very dangerous to make assumptions! But then, I knew that much already...

Don

Quote from: Dave M – July 05, 2014, 02:21:53 pm

Don,
For sure, but the data from the Beam Alarm page for the 98 270 clearly shows the rear axle as 5.13, that is what I was going by, if that is not the case, of course all numbers would be incorrect.
Having played the gear ratio's for many years vs tire size/rev per mile transmission gearing etc, I feel comfy in this arena, maybe wrong but comfy.

Re: Rear vibration 65-70mph+

Reply #19 – July 05, 2014, 08:39:55 pm

The rear axle will have a tag at one of the 3rd member fasteners with the ratio on it and possibly the number of teeth on the ring and pinion gear. Good way to tell what transmission is in it as the four speed in ours has a 1:1 fourth gear and a 3.07 rear axle ratio. If it had a six speed with a .65 overdrive sixth gear, the rear axle ratio would have to be a much higher number to come out with the same top speed. A diesel with the same horsepower but at a higher RPM would need a rear axle ratio with a higher ratio (number). A diesel with the same RPM but less HP would also need a higher ratio as it would not have the power to reach the same top speed (unless you went downhill). Tire diameter also makes a difference.

Yes, fun to play with calculations.

Pierce

Re: Rear vibration 65-70mph+

Reply #20 – July 05, 2014, 11:16:52 pm

You might check as to the cost for a shop to replace the gears only? I regeared many motorhomes

Re: Rear vibration 65-70mph+

Reply #21 – July 05, 2014, 11:32:10 pm

Quote from: Caflashbob – July 05, 2014, 11:16:52 pm

You might check as to the cost for a shop to replace the gears only? I regeared many motorhomes

This seems to be the long term solution after ensuring the output bearing in the transmission is in good health.

I could buy a gearset and regear my rear with existing components. Or, I could find a rear that is 4.63 and just pull the axles and bolt it right in. This way, if I screw up, I can also put the original rear back in!

I find it interesting that in 1999 the axle was changed to 4.63 from 5.13. Sure, there is 25 hp more on the eletronic engines and a MD3066 with .67 instead of .65 6th gear. Design changes happen to fix problems--even if "there is no problem."

Re: Rear vibration 65-70mph+

Reply #22 – July 06, 2014, 10:43:07 am

Yes, a wrecked RV with your 4.63 third member should be $500 or under and a couple of good mechanics (heavy third member) should be able to R&R in no more than an hour if all the gaskets release cleanly. At only 34 feet, you are much lighter than a 40 footer so it should be a good match. That ratio should increase fuel mileage as well. Check with your Allison dealer as you may be able to have them change your shift points.

Buying a gear set is always problematic. Takes a talented mechanic to do it right. Different torque for new vs used bearings, getting the pattern just right on the face of the ring gear, lots of stuff that many don't get right. More of an art than a tech school can teach. Bad setup can catch up way down the road.

Sounds like a good move to me. The only thing better would be a two speed rear end. HR or CC had two speeds in their top coaches over certain years. I would love to have one.

Pierce

Re: Rear vibration 65-70mph+

Reply #23 – July 06, 2014, 11:24:29 am

I have to make some phone calls this week and check out a few things. I'm going to do some data gathering and data crunching as well to figure out what rear ratio would be the best. It may make sense to drop down a bit more (4.40, 4.30, etc whatever is available). This could make 6th gear as a true OD 65+mph and use 5th for around 55-60. Gears 1-4 are pretty spread out and low, so I don't think this would hurt acceleration much.

With full fuel and half water,half loaded with junk, and my motorcycle on back, I scaled right around 27k.

I'll have to analyze it a bit, though.

Re: Rear vibration 65-70mph+

Reply #24 – July 06, 2014, 11:26:38 am

Quote from: Dave M – July 06, 2014, 11:20:47 am

Would strongly consider the 4.3 rear, with the 6 speed w/2 OD gears and with the light weight 34' no slide would appear near perfect for a gear head. Am not a fan of the clunky 2 speed rear.
The single biggie is the 2 O.D. trany. Using torque instead of hp, would add to the overall economy.
You have much more gear choice, meaning using thought, get much better mpg.
For my 3.91 gearing is about ideal as mpg is not the big issue, but have enuff torque $ hp to pull a 3.5 the 3 3 is little steep.

We were typing at the same time. As said, I'm thinking going down around 4.3-4.4 may make sense. I have no idea why they geared it so low with a 6sp transmission!

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