stevieturbo

I have a 3" chromoly shaft in my car, 1350 yokes front and rear. It was supplied by a well known maker, although I did have to shorten it slightly ( my bad ), which was done locally by a suitable machine shop.
They did tell me it was perfectly balanced afterwards.

Query is, that supplied with the shaft was info relating to its critical speed, which was to be avoided. Funny thing is, that when I get to about 125-130mph, a nasty vibrations happens. I havent tried driving through it, as its not nice.
That speed occurs having just gone from 4th to 5th gear flat out. Which should mean prop is at about 6500-7000rpm, which according to the spec with the shaft would be around the critical speed for a 3" 4130 shaft 52.5" long

Surely this cant be right ? I have relatively tall gearing ( 3.27 gears ), so staying away from that shaft speed would be impossible on many other cars with shorter diffs.

Should it be expected to vibrate as they declare with their "critical speeds", or has the machine shop not balanced it properly ??
Ive tried flipping the shaft in the yoke by 180deg, and it makes no difference.

Thanks, Stevie

wrencher

iTrader: (2)

Are your pinion angles adjustable? Sure the shop that re-did it had the ability to balance the shaft at those type speeds? Maybe they balanced it at a 1/4 of the rev's?

stevieturbo

Their machine spins up to 3000rpm, which they claim balances accurate to 14000rpm.

Pinion angle is adjustable, 4 link rose jointed rear with pinion set almost straight.

stevieturbo

I have had the shaft re-balanced locally again, they did add some small weights, but in reality very little.
I re-fitted the shaft and while it was still on axle stands, I drove up to same speed in the air ( stands under axle ). Same horrible vibration enters car.

The shaft runs virtually straight, with gearbox end at very very slight angle.
Rear end is 4-linked rose jointed Ford axle.
There was info with the shaft suggesting it could vibrate at its "critical speed" which would be about 7000rpm for my shaft length of a bit under 53", and you should stay below this speed. I see that as an impossible scenario to reach.

Im using 3.27 gears, if virtually any other car had this shaft it would vibrate a lot lot sooner on the road, and be totally useless, as many use much shorter gearing..

Same axle and old locally made 2 piece shaft had been fitted and working fine with old engine/trans combo. Only change to the axle is the fitment of the U-bolt pinion flange to suit the new shaft.

WTF is going on ?? Is the shaft still at fault ? Is this critical speed thing nonsense ? ( I do understand materials have natural frequencies where they tend to vibrate etc )
What do I need to fix it ? Can anyone actually test spin the shaft on a rig to 6500rpm ?