Driving speeds?

[investig8]

Hello everyone,
Over the past couple of months, I have had Fiona out for a drive and find that she seems to do best in the 35-40 m.p.h. range. I checked the specs which say that 45 is cruising speed... At 45, the noise and vibration feels like she could come apart. Has anyone else experienced this and if so, is it normal?
Thanks,
Dennis

[John Mc]

THere is no two ways about it, these are some noisy trucks.

Before my engine was rebuilt I was always a bit nervous about going much over 40 or 45. The noise levels increased dramatically above those speeds. Part of this was due to a crack in the exhaust manifold, but there were other noise sources as well. There was also some sort of harmonic vibration that seemed to to hit a resonant frequency right around 42 MPH. I'd get a very noticeable wub-wub-wub sound. At first I thought it might have been tire balance, but then I noticed the same thing at similar RPMs in lower gears.

After swapping in a rebuilt 251 engine (with a good exhaust manifold), and having the transmission and transfer case rebuilt, things are a good bit quieter. This engine is smooth. I no longer feel uncomfortable driving at 50 MPH. However, things are still noticeably quieter under 40 MPH than they are much above that speed..

I've also noticed that the floor boards make excellent sounding boards. Things are significantly quieter if I have some things piled on the floor of the passenger side. I've been debating putting in some heavy rubber mats as sound dampers.

[Cal_Gary]

Mine is most comfortable around 43 MPH. It gets pretty wound up any higher than that, and since I know my timing chain needs to be replaced I don't want to push too hard.
Gary

[PoW]

wub-wub-wub is usually caused by tires being out of balance, or a drive shaft with the same problem.

45-47 is usually a happy speed.

PoW

[John Mc]

wub-wub-wub is usually caused by tires being out of balance, or a drive shaft with the same problem.

45-47 is usually a happy speed.

PoW

Tire balance is exactly what I initially thought, though it was not the case in this instance. it was definitely an engine balance/harmonic vibration thing. It did sound a bit like an out of balance tire, but not exactly the same. My spelling of the sound is not the greatest, but I did not know how else to describe it (maybe a whum-whum-whum?) It was definitely coming from the front of the truck, but no vibration at all in the steering wheel. I drove it for about 1500 miles after the engine rebuild, but with no changes to the tires or driveshaft, and that harmonic vibration was gone. (I have since replaced the tires, but that was much more recent.)

At any rate, these are generally not quiet trucks inside the cab.

[investig8]

Thanks for the input gentlemen... My thoughts were leaning toward tire balance or u-joint vibration. However, the u-joints seem tight and don't show the typical deceleration vibration associated with that problem. Maybe just tires and road noise... just didn't want to push her too hard. I don't mind the slower speed, just didn't want to get run over so I bought a "Caution - Slow Moving Vehicle" magnetic sign I put on the tailgate if I have to drive on the highway...
Enjoy your weekend!
Dennis

[John Mc]

My alternative to the slow moving vehicle sign (given to me by Tim Holloway):

Just go around.jpg (59.42 KiB) Viewed 1024 times

[Cliff]

My M-37 had bad vibrations in it when I first got it. It did not feel comfortable driving it over 35mph. It turned out both the front and rear drive lines were way out of balance and the rear one was not straight. So I had the front one balanced and I replaced the rear one and the short drive line between the transmission and the transfer case. That solved the problem. However my truck still sounds and feels the best between 40 and 43 mph. When I say sounds the best I mean it is in it’s Sweet spot.

[investig8]

Thanks Cliff, I hadn't thought of the out of balance issue with the driveline. The vibration certainly feels like that... How did you go about having those checked? Who would I take them to? I was planning on replacing the u-joints in the spring and that would be a good time to have the drive shafts checked...
Dennis

[PoW]

Beware buying U-joints of recent manufacture, they don't always fit right and wear prematurely.

Shop the MV parts guys for originals in the little brown boxes. Especially on the smaller ones in the front & rear shafts.

250K miles on M37's taught me a few things.

Prince o' Weird

[Cliff]

Dennis to be honest I am told that any reputable driveline repair shop can do it. Frankly what I did was take my truck to the local dodge dealer who I deal with all the time for my many other mopar vehicles and they took the drive lines off and sent them out to a driveline repair shop. When they were ready I went to the driveline repair shop and picked them up so I could talk to the guys about them and also have them show them to me and explain the issues with each one. They were great to talk to. In any event I reused the one they could balance and I replaced the others with refurbished ones from Vintage Power Wagons. I got new u-joints from Vintage as well even the guys at the shop told me there wasn’t anything wrong with my old ones. What a big difference. Good luck, Cliff

[NAM VET]

My engine is balanced, and once the flat spots of my NDT's become round again, my cruise is about 40-43 or so mph. My gearing is stock, and this speed is right in the torque of my engine, climbs small hills easily, accelerates nicely. My truck will certainly go faster, and there have been times on short runs on an interstate, I have had to run 50 mph.

But there on the rural SC back roads, no need to go faster. If I have a hill coming up, I will gain speed on the down slope, and carry as much speed as I can up the next hill. Works pretty well. Gearbox shifts like a dream.

Hal

[PoW]

Yep, the last nice small tactical is a pleasure to drive when it is working correctly.

No apologies to M715ers or HUMMV guys either.

PoW

[investig8]

I decided to replace the u-joints to see if it will remove any of the vibration. I took the shafts to a drive shaft company locally, who told me the u-joints were bad. I have changed out the intermediate and rear shafts, but need to move the truck to access another vehicle behind it. Driving it with the front shaft off the truck will not create any balance or drive issues will it?
Thanks, Dennis

[Cal_Gary]

Nope, no problem being short one shaft. The M880s had issues moving w/o the front shaft; the HETT as well.
Gary