Drive train vibration at 55 rpm

[Irish_Alley]

yeah physics can be confusing. like a balanced crank, a friend got one but it was hanging up on the piston skirts (forget the brand). but called them up and told them about the problem they said to grind off some of the crank so it clears the skirts. we then asked about throwing off the balance and they said it would be ok if you took of "X" about as long as you dont go above 8600 rpms. but think of it like a ceiling fan, if you dont put the weights on it will be ok on low but put it on high and it will be all over the place

[Greybeard]

Hmmm, my first thought would have been to clearance the skirts and the crank, but mostly the skirt. It's easier to re-balance the piston than it is to re-balance the crank. Or just buy short skirt pistons (stroker pistons) or pistons with relocated wrist pin holes and lower decks. 

But it makes sense that some out of balance is so subtle that it takes speed to cause it show up to the point that it is noticeable.

[Irish_Alley]

yeah i would of thought to grind on the pistons since they have no rotating mass just reciprocating motion. oh yeah it was a scat crank.

[philo_beddoe]

I figure i would add to this, play in rear yoke, is this causing my driveline vibration? Is this not supposed to have play?
https://vimeo.com/162083848

[VileZambonie]

What you see is excessive backlash and it won't necessarily cause a vibration at 55 mph.

Keep it simple, check the u-joints, check driveshaft phase and for any missing weights. Rotate the driveshaft position to see if it changes and or try adding a hose clamp with a small amount of weight and see if it changes the speed at which you hear or feel it.

[philo_beddoe]

Ok, i'll try the hose clamp and let you know. Dont have the big hoseclamp on hand, but i do have a big piece of gum. Lol

[philo_beddoe]

I tried the hose clamp, put one on and drove all they way home from work. Absolutely a difference, less prominent and the pattern was different. When i got home i added a 2nd clamp and i think it was even less. I had to really pay attention and try to pick up a pattern and feel it. I need to try them in a different position, experiment a little.

[philo_beddoe]

After the second clamp, and trying a few other positions, the one clamp at the rear weight was best. So,,,,i guess i should have the dshaft balanced?

[VileZambonie]

Mark the shaft and keep playing with it until it's gone.

[philo_beddoe]

Well, i was going to take it to a good shop and have them inspect from the slip yoke to the rear end. I hate not knowing.

Unless....you have a few minutes next sat.?

[philo_beddoe]

Taking drive shaft to specialty shop for balancing, possible new u joints and what-not. Will post results.

[philo_beddoe]

What you see is excessive backlash and it won't necessarily cause a vibration at 55 mph.

Keep it simple, check the u-joints, check driveshaft phase and for any missing weights. Rotate the driveshaft position to see if it changes and or try adding a hose clamp with a small amount of weight and see if it changes the speed at which you hear or feel it.

Hey, i should have read this more carefully, i never added a small amount of weight. I will now, then re post results.

[philo_beddoe]

Results: vibration much less with added weights, but still there. Taking DS to a drivetrain specialist, getting balanced and new u joints. Should do it. If not, then we are isolated to the rear end. Then, its up to VZ.