The Stovebolt Site Forums The Shop Area The HiPo Shop Vibration at 60mph

I searched the threads for an answer to this but no luck, so I'm starting this one.

A brief history. I bought this truck (1949 GMC FC101) as a stalled project out of NM about 3 years ago. It was a frame off and completely disassembled when I got it. They had swapped out the rear axle with a early 80's Blazer and that's as far as they got. The truck then sat for 20 years (I found a parts receipt and some parts wrapped in newspaper from 1998 when I was inventorying the parts that came with the truck).

It is now on the road with a SBC 350, a 700r4 and the original 3.73 gearing. I did replace the straight axle front suspension with a IFS unit and coil overs. And took out every other leaf spring in the rear suspension. The rear shocks are rather light duty.

Now the problem...I get a fairly serious vibration at 60mph that feels like it originates from the rear. I feel it in the seat, not much in the steering. It starts at approx 55mph and then starts to go away at 65mph and pretty much stops at 70mph. I have had the wheel balance checked and the driveshaft balance checked. No improvement.

I'm starting to think that it starts a harmonic vibration in the rear suspension, possible caused by the undersized shocks.

Looking for suggestion in how to isolate the issue and correct it. Any help would be greatly appreciated.

Tommy Boy

PS if your interested in seeing the truck you can check out my Facebook page, I have a few pictures there and will be updating it with a more soon.

Is the rear driveshaft phased properly? Is the pinion angle compatible to trans/engine?

Ed

A couple things I can think of to check-- run out on rear wheels while on the truck, which could be caused by bent wheel, wheel not seating square on hub, bent axle or flange. Only other thing I could suggest would be to have the wheel balance checked on the vehicle if you can find someone that has the equipment. My first thought as I started reading your post was a bent drive shaft because I had a car once with the same problem you discribe but if the balance has been checked then I guess you are good there.
Good luck
Dave

Going out on a limb here...
Do you actually feel the rear end bouncing, or is is purely an audible thing?
If it's just audible, then I'd suspect wheel bearings or other driveline bearings... can you feel the vibration any better with a hand on the transmission?... then maybe driveshaft phase/angle

If it's the whole truck bouncing, then I'd suspect the wheel/tire balance. Many tires, especially mud or all terrains can't be fully balanced with wheel weights.

One could put the rear on jack stands and see if you notice anything while the wheels turn.

Hi Tommy Boy. You might check to see if your rear drums are true. If they aren't, this can set up some really odd and puzzling vibrations. Not from balance issues...from grabbing the shoes (usually after you adjust the brakes). Good luck.

Like Ott3r said, put it on jackstands. See if you duplicate the problem that way. Get some one to hold the truck at the vibration speed and crawl around looking for the culprit. Should jump out if its ovious. Is there a carrier bearing on this setup?

Put the truck on jack stands and run it with the wheels on and off. Remember to put a couple of lug nuts back on when you remove the wheels so the brake drums stay put! If the vibration isn't there when running in the shop, you've got a suspension bounce problem like a harmonic vibration or bad shocks.
Jerry

Put 100pounds of cargo just ahead of the tailgate and test drive. That's easy and free, and will let you know if it was the best idea to take out leaf springs. If there is no change in the vibration, then removing the leaf springs was not a bad idea.

On rides we build the chassis and suspension from scratch it can run into serious dollars so they have to handle/drive good and be vibration free. The suspension and steering geometry is drilled into our heads pretty deep so usually not a problem. For tracking down hard to find vibration issues we use a variable speed strobe light and an infrared heat gun. With the vehicle on the drive on rack and stands we set the strobe to "stop action" speed so any moving component seems to be standing still (think timing light). The strobe is then barely slowed down till the the spinning part seems to be just creeping in movement. When viewed at this speed one can spot any wobble or run out. Vibration creates heat so running in the vibration range will form a hot spot that the heat gun will pick up. With all the fancy equipment I still had a mid sixties Lincoln that drove me crazy; turned out the lads at Ford had too great a distance between pulleys/idlers for the a/c compressor belt(s) which would go into harmonics that one could feel in a 5000# tank so who knows?

Thank you all for the great suggestions.
I have a scissors lift so I will be checking out the wheel for run out. I will also be loading up the back with some weight and checking it that way. As far as the drive shaft angle i checked it at 5 1/2 degrees. It was 6 1/2 and I lowered the tail end of the trans to see if that would make a difference ( a suggestion from a local trans shop that checked the balance of the drive shaft) and it did not.

Anyway, thanks again for the suggestions. I'll keep you posted on my progress, it may be a while, I have my daughters car on the lift rignt now waiting for wheel bearings.

Oh, and I mentioned my Facebook page if you wanted to see a few photos and forgot to put my full name in the post so you could find it ..Tom Strojinc

Checking the driveshaft angle is not the best way. The center line of the pinion should be lower than the center line of the crankshaft by about 2 degrees for street use and up to 4 degrees for drag racing. An angle finder on the front balancer or even pulley, a machined spot on the bell housing, or even on the extension housing seal (if it's driven in flush and is smooth) in a pinch will give crank angle and the pinion yoke turned with the bearing cups vertical will give the pinion angle. Do not use the top of the differential housing or engine valve cover.

What is the u-joint angle? Tailshaft to driveshaft and driveshaft to pinion ?

When the truck starts vibrating, is it in overdrive or direct drive?
If you slip it in neutral, does it continue to vibrate?
If you go faster, does it go away or increase?

Here is a guy that's been chasing a vibration for years, https://www.jalopyjournal.com/forum/threads/so-im-learning-a-little-about-tires.1192479/

When I was 16, my dad's '54 3100 had a governor in it. Once you hit 50 mph, it shook so bad beyond that which made it impossible to exceed the highway speed limit. I think he left it that way on purpose!

For sure for a 16 year old driver.

Ed

Another thing to have done is a rotate and balance (assuming you have the same size tires and wheels all around). A least do the balancing.

Balancing tires won't correct an out-of-round condition. Tires can flat-spot from sitting too long; did you let it sit in one spot for months while building the truck?

Check u-joints. And do it unloaded. Jack up the rear, jack stands under rear axle. Neutral and make sure there is no play in the joints.

Have not resolved the vibration yet. Followed all suggestions with nothing pointing to a fix. I had the truck out for a 30 mile drive yesterday and the vibration seemed to dissipate as the road got smoother and would come back on a rough stretch. I'm starting to focus on the rear shocks. I think they may be undersized and allow the harmonic vibration. They were extras I had laying around the shop. Does anyone have suggestions for shocks that would be appropriate for this application?

You could probably go two ways on this. Keep driving until the problem gets worse/you have a part failure or, tear all rotating assemblies down and look for failed bearings, races, bushings, etc.

This is a real thinker, could you keep us posted once you resolve the problem. "I've got nothing". Doc

I doubt the problem is the rear shocks. An easy test would be to remove them from the truck and drive it. It would not surprise me if removing the rear shocks would be noticed much at all. I had one come of at the bottom and just hung there for who knows how long, but did not notice any difference in the ride.

Doc.Hall, will do.

I'm still inclined to think the tires are out of round. Balancing will not fix it. I've seen tires flat-spot from sitting in one position too long (very common on classic vehicles), even sitting upright in the tire store racks. In the old days tire shops had shavers that would true tires by taking rubber off the high side, but the only place I know that still shaves tires are some truck/motorhome alignment shops.