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Demonic harmonic vibrations

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Ratnest

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Now that Ratnest is basically a daily driver, I'm focused on working out a few minor issues that I can prioritize and get to one by one.
I've had some pretty bothersome harmonic vibrations and since the OEM rally wheels are straight and balanced, the front-end is aligned, and the tires are new. I decided to check out the driveshaft first. Sure enough the shaft was slightly bent. I found another at a wrecking yard and immediately took it to a shop that balances them. Wouldn't you know, it was bent too! I decided to have one custom made and balanced. The cost took my breath away, but at least I can now eliminate the driveshaft. After a couple test runs with the new driveshaft, I noted that the vibrations are a lot less...but still noticeable from about 45 thru 65 MPH. The vibrations are at their worse at 60 and totally gone at 70.
The motor mounts and trans mount are new and solid. I've checked for broken welds and loose shocks or springs and all looks fine.
Anyone have any suggestions on what I may have missed?:confused:
 
Have you tried getting it up to speed and then putting it in neutral to see if the vibration continues? Might help you to zero in on the culprit.

I'd be suspicious of the tire / wheel balance. My Tacoma did the same thing you are talking about when I had BFG TKO's on it. I had lifetime balance on the tires, kept having the tires re-balanced every 5,000 miles, and it would be better, but then get bad again. When I bought some new tires, from a different tire place, the issue went away.

If the tires or wheels are out of round they will still balance just fine.

 
Thanks, John for the suggestion. I have put it in neutral at different speeds and the vibrations stop. Of course, when out of gear I let the rpms go back to idle. Also, the tires are Goodyear Wranglers with sort of an all weather tread. Whatchathink?
 
Do you have a tachometer in your truck? Does the vibration seem to happen in each gear at the same rpm? Maybe hold it in gear and see if it replicates at a given rpm.
 
Yes on the tach. The trans is a freshly built TH350. Rear end is a 2.73 posi. At 60mph on flat surface I'm turning 2100 rpm. I'll try your suggestions.
 
Any up and down play on your trans. tail yoke? or rear pinion yoke?
Any noise to go along with vibs.?
These things can sometimes be tough.
I'm not familiar with your truck, you haven't put in a long driveshaft and removed a carrier bearing have you?
When you had the driveshaft out, did you turn the pinion over by hand to check it out?
 
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With input from you guys and some research on the internet, I'm better informed about driveshaft angles and problems that surface when those angles are not right.
I removed my driveshaft today and used an inclinometer to determine the angles of the output shaft and the rear yoke. I was shocked to see that the trans is 3 degrees down and the rear end is 6 degrees up! I imagine that's the source of my harmonic vibrations. When putting Ratnest's frame and suspension together, I was careful to make sure I installed the rear end at 3 degrees up. Obviously I failed to account for something as that angle ultimately grew to 6 degrees!:eek:
Rather than cut off the axle housing perches, I think I'll look into getting some shims and adjust the rear end angle downward to 3 degrees. That should eliminate the vibrations. Any suggestions are welcome.;)
 
I think you found your problem. The angles need to be within 1/2 of 1 degree of each other. The only other thing I could think of (other than what was already mentioned in this thread) is driveshafts (u-joints) need to be "in sync." I figured with all your driveshaft attention, that was surely ok.
 
Thanks Steve. The u-joints are new and in phase. I just ordered a pair of shims. Looking forward to a test drive after I install them.
 
Cheap tires

Thanx for the memory John ;

Back in the mid 1970's there was one of those ex Racer Boy crappo tire shops diagonally across from my old Shop , I remember some guy in there with his brand new BMW an the Manager jerking him off with both hands because their top tier ' sport ! ' tires were *so* poorly made they couldn't be balanced :rolleyes: .
 
Today I installed the billet aluminum 4 degree shims to lessen the angle of the rear end. After installation I determined the rear end now sits at 1 degree up (I guess the shims were a tad over 4 degrees) and, of course, the trans is still 3 degrees downward. During a test drive I noted most of the harmonic vibrations are gone. However, some harmonic vibrations are still present and begin at 58 mph and continue thru 72 mph. When coasting in neutral they stop and they also stop if I leave it in gear and just get off the accelerator. While coasting at rpm's between 2100 and 2500 the vibrations do not recur. It seems the vibrations occur when in gear and pulling the weight of the truck. I think I will shim the trans tail shaft up a degree or two and take another test drive. I'm not optimistic though.:confused:
 
Thanks for the update. Your plan to shim the trans. up makes sense. Let us know how it turns out.

I've never checked this on my truck, I'll have to throw the angle finder on it next time I'm out in the shop just for my own edification.

Seems like with the trailing arm rear axle design the more you load the truck the more the rear axle nose dives. I have a model of our trailing arms type suspension on CAD.. I'll look at that too. Just curious how much vertical movement of the axle affects the pinion angle?

I am sure you've already seen a diagram like this, but will post it here just for Schlitz and grins :D

 
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Thanks John for the diagram. I checked my carb to see if was level and it was. So, I decided to put a thin shim ( 1/16th ) under the trans mount and see what difference it made. The carb is still within the level marks with that shim and during the test drive I noticed minor improvement. Now the trans is 2 1/2 degrees down and the rear end is 1 degree up. I'm going to tinker with rear shims till I get it to 1 1/2 degrees up. I figure a one degree difference will make things even better.
 
Rear Pinion Angle Change Due To Ride Height

Jim,

I pulled up the CAD model of a 1965 C10 rear suspension and articulated the suspension from ride height to 4" lower than ride height (4" frame to axle height change).

4" of articulation = a decrease of pinion angle of 5.14 degrees

Which begs the question, since we are told that drive line angles are critical in minimizing vibration issues, what happens when a truck is loaded and the pinion angles change?


^ Rear Pinion Angle Change Due To Ride Height ^


^ Top View ^


^ Bottom View ^


^ At Ride Height ^ (gas tank & rear frame crossmembers removed for clarity)


^ 4" Below Ride Height^ (gas tank & rear frame crossmembers removed for clarity)
 
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GM design...

My question would be, what would these angles be if loaded only to GM design vehicle weight rating and on new springs? Also, loaded evenly in cargo box, not concentrated. The trailing arm design is not the best design for trucks that haul heavy loads every day.
The vehicle weight rating for pickups is low, and frequently ignored.
The first semi-tractor I owned had a four spring suspension with tracking rods. The forward driveshaft would "shallow out" a little when fully loaded but the forward rear axle angle would remain relatively unchanged. The truck would transmit alot more road vibrations through steel springs when loaded. (It was also a back-slapping cab-over, but, that's another subject.)
All the other trucks I've owned were air ride, and MUCH better in so many ways.
 
Back in '72 my Dad bought a new C20 LWB with a 350 and granny 4-speed. It was a beast.

We owned a sawmill and pulled a 22 foot dual axle trailer with an Allis Chalmers tractor w/ front end loader for pulling trees out of the woods and loading the trailers.

Needless to say that C20 saw a lot of heavy loads.

I never once recall it ever having any drive line vibrations.

I do remember the 8 ply tires flat spotting and vibrating the truck until the tires warmed up, but not drive line issues.

As for pinion angles... leaf springs would be much better than the trailing arms, maybe that's why they switched?
 
Those drawings and diagrams are great ...more food for thought for sure. I increased the trans shim from 1/16th to 1/8th and did a test drive an hour ago. I swear, the vibrations got worse... as the angles got better! Go figure. I'm pulling the driveshaft in a few minutes and taking it back to the "custom driveshaft maker" in the morning and have him check it out. This is frustrating the crap out of me.:eek: John, I recall your comments about bad tires and after reinstalling the driveshaft if things don't improve, I will have the tires and rims checked-out.
 
Vibration

I have gone through all the things you have. My '58 has a 350/350 in it. It used to vibrate badly. A new driveshaft with the best u-joints removed most of the problem. I still get a small amount of vibration from 40 to 45 MPH. Only the driver feels it in the steering wheel. Rear end is a 12 bolt out of a 1967 1/2 ton Chevy truck and in great condition. Ratio is 4.11. After trying everything you have, I have come to believe the remaining vibration is probably the torque convertor. A 350 turbo flex plate has 6 bolt holes in it, the torque convertor has 3 places to bolt it. I rotated the torque convertor one set of holes at a time and test drove it. Every set of holes produced a vibration different than the last one. Turned out the set of holes I started with had the least vibration. Since the vibration is not noticeable to any one but me ( the driver ), I have decided to live with it. My convertor has not lost any balancing weights, the flex plate is not cracked. Before I installed the tranny I had it rebuilt and bought a new convertor at that time. Also tried different harmonic balancers, no help there, vibration was the same.
You might fix yours with angle plates or you might have the same problem I do. Oh yeah, I have angle plates too.
Good luck
Gerald in Florida
 
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Thanks 58Apache....sounds like you can emphasize with my frustrations. I think the torque converter and harmonic balancer are not the culprits as they are new. You know that story "new ain't necessarily right nowadays". Those parts are still on my list to check if necessary.
 
Just had the driveshaft checked out by the fella who made it. When I took it to him the trans yoke was still attached and he pointed out some issues with the yoke (which I got new from the transmission shop that built my TH350). It appears that it was not precisely machined. With the u-joint and yoke in place, the u-joint cup(s) extend out one side of the yoke .029 and .060 on the other side! I had not noticed that before. He said that situation will cause a mass weight shift when the driveshaft is turning. So...maybe Chinese manufacturing strikes again.:mad: I'll talk with the trans shop Monday and see who manufactured the yoke and determine what they want to do about it. Since Ratnest goes to the paint shop on the 13th, I may not resolve the yoke issue till I get the truck back from the painter.
 
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