Vibrations at 80+

RAU03MACH · 2026-07-07T14:33:05-0400

think of it as your spine

RAU03MACH · 2026-07-07T14:34:16-0400

it's reinforced internally

dsrtjeeper · 2026-07-07T17:29:14-0400

RAU03MACH said:
keep in mind the ford racing aluminum shaft
is spiraled inside no flex

It still flexes at it's critical speed. It starts to whip like a jump rope. I used to have a video of mine doing so. I placed a camera under my chassis to record it.

Also, the driveshaft is not spiraled. The cardboard tube inside is. It's used for dampening.

https://www.dieselplace.com/attachments/dscn0329-jpg.77046/

OldZeuski · 2026-07-07T17:43:21-0400

dsrtjeeper said:
It still flexes at its critical speed. It starts to whip like a jump rope. I used to have a video of mine doing so. I placed a camera under my chassis to record it.

I just don’t understand how everyone seems to say when they switched to an aluminum shaft it fixed their vibration issues. I can’t be the only person with a ford performance aluminum shaft that gets the car over 80 mph. And I’m not sure I believe 4200 rpm is the critical speed of an aluminum tube.

The equation for critical speed is easy to solve. I seriously doubt the critical speed of this shaft is anywhere below 7500 rpm.

RAU03MACH · 2026-07-07T17:47:38-0400

The one i have is
But mine was built in the 90s shit now not worth a shit
Better quality

dsrtjeeper · 2026-07-07T17:55:11-0400

Not everyone. Look into the new Mustang crowd. One driveshaft manufacturer discontinued their aluminum shafts for them due to so many issues with vibrations. The ideal aluminum shaft for our cars would be 4" in diameter and thicker than the Ford Racing shaft. Unfortunately, we can't fit a 4" shaft under our cars.

I tried the calculations for critical speed. You have to know the thickness of the aluminum to do so. I'm baffled by the numbers too. Carbon fiber shafts aren't just for weight savings. They often fix vibrations.

Something to consider is not all driveshaft balancing machines are alike. If your shop can only balance the shaft with the yokes removed, they're worthless. It's important that the shop has the capability to balance with yoke installed and at high speeds. Many shops balance shafts at under 1000rpm.

dsrtjeeper · 2026-07-07T17:58:16-0400

RAU03MACH said:
The one i have is
But mine was built in the 90s shit now not worth a shit
Better quality

This has been stated many, many times. The Ford shafts went to crap. I've had two new ones with bent tubing.
Mark Williams straightens the tubing before building a shaft. Their shafts don't need weights attached. They are a piece of art.
I'm currently switching to the new LMR shaft that's high speed balanced and has no weights.

RAU03MACH · 2026-07-07T18:00:40-0400

I have had mine since I bought my mustang in 95
And that shaft holds up pretty good
And im not a light foot by all means

RAU03MACH · 2026-07-07T18:12:58-0400

Hang tight i will put a little more horses on that shaft
I will be starting the 351 build soon
Many of the mustang guys run these shafts with no problems
I would be looking at the whole drive line for vibrations
Including all control arm bushings
Clutch flywheel

RAU03MACH · 2026-07-07T18:13:59-0400

Down to a simple tire balance

OldZeuski · 2026-07-07T20:23:08-0400

For what it’s worth I was wrong. It’s an LMR house brand aluminum drive shaft.

I’m about to find an OEM shaft and throw it back on tho.

dsrtjeeper · 2026-07-07T20:28:05-0400

Oh great. I hope mine is okay.
By the way, adjusting your pinion angle with the lca's isn't the way to go. Adjusting the lowers changes your wheelbase and if not adjusted evenly; will throw off your thrust angle.

You should measure the drive line angle at the harmonic balancer. How are you measuring the angle at the pinion?

What is the angle of your crankshaft currently?

OldZeuski · 2026-07-07T20:45:22-0400

dsrtjeeper said:
Oh great. I hope mine is okay.
By the way, adjusting your pinion angle with the lca's isn't the way to go. Adjusting the lowers changes your wheelbase and if not adjusted evenly; will throw off your thrust angle.

You should measure the drive line angle at the harmonic balancer. How are you measuring the angle at the pinion?

What is the angle of your crankshaft currently?

I know it isn’t ideal. But I wanted to maintain OEM uppers to avoid binding the rear suspension.

I laser’d the pinion angle horizontally and it’s still pointing dead straight. I may have drawn the rear wheels 1/4” at most toward the front, it isn’t noticeable nor has it interfered with anything else.

I do measure from the balancer for my trans angle, and I use a machinist rule when the shaft is still installed to measure the pinion angle. Although I just measured with the shaft out, using the pinion face.

They are currently within .25 degrees of one another with the pinion being slightly down. I could bring the pinion back down a degree to account for suspension wind up, but these new lowers are solid and this vibration isn’t dependent on acceleration or deceleration, so im ruling pinion angle out.

dsrtjeeper · 2026-07-07T20:48:03-0400

It sounds like you've done everything correctly. What is the current down angle at the crankshaft?

dsrtjeeper · 2026-07-08T13:56:41-0400

With the vehicle weight on the tires, mark the slip yoke where the tail housing seal lip sits on the yoke. Now unbolt the shaft from the pinion and push the slip yoke as far as it will go into the tail housing. Mark where the seal lip sits on the yoke. What's the distance between the two marks?

OldZeuski · 2026-07-08T15:01:36-0400

dsrtjeeper said:
With the vehicle weight on the tires, mark the slip yoke where the tail housing seal lip sits on the yoke. Now unbolt the shaft from the pinion and push the slip yoke as far as it will go into the tail housing. Mark where the seal lip sits on the yoke. What's the distance between the two marks?

what’s that tell me?

My seal is longer than the OEM so my yoke almost touches the seal when installed and on the ground and if I shove it all the way in I’m compressing the seal. But I can measure from the yoke face to something on the trans for a difference.

dsrtjeeper · 2026-07-08T15:15:34-0400

That should still be fine with your current seal. If the yoke sits too hard out of the tail housing, it will wobble and whip. My Ford driveshaft is a different length than the LMR one. You want 1" of movement. 1.5" is too much.

dsrtjeeper · 2026-07-08T15:17:34-0400

I take that back. I don't believe the measurement will be accurate with the longer seal.

OldZeuski · 2026-07-08T16:38:26-0400

I’d say it’s about an inch if not less.

But I did measure yoke radial play when inserted and I’m getting between .010 and .015.

After a quick google this is not out of spec.

dsrtjeeper · 2026-07-08T17:20:55-0400

OldZeuski said:
I’d say it’s about an inch if not less.

But I did measure yoke radial play when inserted and I’m getting between .010 and .015.

After a quick google this is not out of spec.

It's definitely within spec. I need to actually measure my Ford and LMR shafts to see the difference. I had them side by side and it was obvious. I'm currently nursing a total knee replacement and plan on installing the TKX on Aug.1st. I'll measure the shafts then.