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Old 10-08-2013, 05:54 PM   #1
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1977 28' Argosy 28
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P32 low speed bounce...

Hiya Gang... are there any chassis and driveline specialists out there?

Our '77 Argosy 28' motorhome on the P32 chassis is giving me a fit! Well, not really the coach, but (and I hate to say this) the "know-it-all" mechanics around my area are giving me a fit. Mind you, most of these are guys have NEVER SEEN a carbed engine. "Where do I plug in the diagnostic computer?", and the "old timers" around here are so expensive it ain't funny!

The problem with the coach is this- at low speeds, especially below 15 MPH, the coach literally bobs like it was rolling over an undulating surface. It goes up and down with every rotation of the driveshaft.... It does it as fast as you're driving....In other words, it feels as if the wheels are oval-shaped, and in PERFECT unison....EVERY expert has told me that one of the tires "COULD" have a ply separation or belt shift.
I KNOW THAT THIS IS NOT THE CASE, because as the truck bobs along, there's an audible squeak when the coach is on the uppermost part of the bob. (Does that make sense?)That's what makes me believe it has something to do with the driveshaft or axle pinion... causing a sort of "kink", maybe?

I'm also dead certain that all six of the wheel rims are true and round, the tires are NEW and are balanced perfectly.... So they couldn't be the problem.

I wonder if it could possibly be an issue with the pinion angle? You know, causing the rear u-joint to "bind"?
It's much less noticeable at higher speeds, say 40-60 MPH, but the shimmy and squeaking noise are still there, albeit quieter and less bouncy, but MUCH FASTER.... and STILL NOT ENJOYABLE!!!

Since I'm not a pro mechanic (my dad and brother were for YEARS, but are in Houston, Tx), and haven't got the tools to figure this out... what would your suggestions be?
Anyone now of an honest mechanic near Columbus?

I'm truly hoping that it iS the pinion angle... I had the u-joints and carrier bearing replaced last fall... It may be a possibility that the mechanic put the rear joint together "backwards", no? Maybe misaligned the splines at the transmission end?

I don't mind spending a few bucks on any service, especially those that I'm incapable of doing myself for lack of tools and equipment... but the whole "we'll have t go through it until we find the problem" bit just doesn't sit well with me, and seems dishonest... My pop used to be able to diagnose MOST things within 5 minutes of driving a vehicle.... I guess that must be one of those "lost art" things.

Anyhoo...Any ideas would be greatly appreciated... It's just aggravating when I have a pretty good idea as to what's wrong, but don't have the diploma saying "this guy knows a little bit about this"! It seems like all of the honest mechanics have gone the way of the Dodo, and it looks like everyone else is on a money grab these days.

THANKS GANG!
I sure do appreciate havng Air Forums to turn to!

CHEERS!
Slick in Columbus
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Old 10-08-2013, 06:03 PM   #2
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Interesting problem.
Did it begin after any repairs? I always look were the last person had his or her hands.
My next thought is that perhaps you should remove the driveshaft and inspect the u joints. I would be especially look at the rearmost joint as a failure here would be especially apparent at low speeds.
Hope you figure it out!
Let us know....
Bruce
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Old 10-08-2013, 07:56 PM   #3
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I notice you emphasise that the tires are NEW.... and that red flags this for me. I agree with Bruce B about the last person to do work on a vehicle with sudden-onset problems. Your symptoms strike me as classic out-of-round tire, and I'd ALMOST bet that this is the problem. Why should any component of your driveline suddenly kink, unless you've been off-roading in a big way? Have someone drive either side of you on a divided highway to look at the road wheels, because probably you've got one wheel hopping around like a watusi dancer. When you've eliminated the tires FOR SURE, start to think driveline.
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Old 10-08-2013, 11:05 PM   #4
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The key may be determining the exact time, location and circumstance when you first noticed this symptom. Then, try to remember what happened at that specific instant, or just prior to it (e.g., installed new tires, replaced carrier bearing, replaced U-joints, drove over big rock or curb, high-centered on back road, etc.).

Other possibilities:
  • Mud, dirt or rock trapped in wheel(s)

  • Broken motor mount on engine

  • Flat spot on nylon cord tires. -- If you have nylon tires that sit in storage for long periods, they can develop flat spots that are noticeable when first driven. This usually goes away after a mile or two of driving.

  • Bent driveshaft

If all else fails, look for a shop that has a dynamometer; some speedometer shops have these. This will allow you to run the drivetrain up to highway speeds and actually see what is vibrating or oscillating.
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Old 10-09-2013, 11:36 AM   #5
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Since this is happening at very low speeds, my first step would be to find a big parking lot or empty road and get the wife to drive slowly while I walked along outside to listen and watch.
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Old 10-10-2013, 07:32 AM   #6
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Just an FYI, low speed vibrations tend not to be tire related. Any tire related thing that would cause a low speed vibration should also show up as a high speed vibration.

I would look at u joints and the pinion angle. Please note that many people are not familiar with how to measure the pinion angle, because this is only rarely a problem and they just don't understand the effect pinion angles have on vibrations.
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Old 10-10-2013, 09:06 AM   #7
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Quote:
Originally Posted by CapriRacer View Post
Just an FYI, low speed vibrations tend not to be tire related. Any tire related thing that would cause a low speed vibration should also show up as a high speed vibration.

I would look at u joints and the pinion angle. Please note that many people are not familiar with how to measure the pinion angle, because this is only rarely a problem and they just don't understand the effect pinion angles have on vibrations.
But if you read the post, OP said: "the coach literally bobs like it was rolling over an undulating surface." and that IS typical of a tire related issue, and does become reduced with higher speeds. How do I know? Because I have experienced it, this summer. Just one bad tire.
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Old 10-10-2013, 09:40 AM   #8
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Will jump in and vote for the out of round tire , have never experienced any thing other than vibration with a driveline problem .
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Old 10-10-2013, 09:47 AM   #9
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An out-of-round tire certainly could be the source. If the tire RPMs just happen to match the shock absorber spring rate, you would get a harmonic bounce that would disappear at higher tire RPMs where tire revolutions and shock spring rate are out-of-sync.
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Old 10-10-2013, 10:33 AM   #10
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I'm voting for a tire too. I overloaded a pickup until a bubble popped up on one tire and had similar symptoms.
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Old 10-10-2013, 10:37 AM   #11
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The P32 Chassis was not available until the early 2000's The chassis you have is the P30.
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Old 10-10-2013, 10:59 AM   #12
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Check to see if the driveshaft is "out of phase". Worn/rusty/missing needle bearings can cause the shaft to bind.

Inspect the center carrier bearing between frt and rear shafts.

Bob
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Old 10-10-2013, 01:15 PM   #13
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Quote:
Originally Posted by slickandrews View Post
The problem with the coach is this- at low speeds, especially below 15 MPH, the coach literally bobs like it was rolling over an undulating surface. It goes up and down with every rotation of the driveshaft.... It does it as fast as you're driving....In other words, it feels as if the wheels are oval-shaped, and in PERFECT unison....
One thing to remember is that the driveshaft rotates faster than the wheels. For example, with a 4.10 ratio rearend the driveshaft rotates 4.10 times for every time the wheels do. This would tend to indicate that if it were the driveshaft, it should happen (in this case) roughly four times for every revolution of the wheels rather than in perfect unison.

While I wouldn't say that it is definitive, these things can be difficult to judge from the driver's seat after all, it is something to consider while looking for the problem.

Good luck! Be sure to let us know what you find.
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Old 10-10-2013, 04:00 PM   #14
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Just realised I wrote nonsense, so erased it.
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