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Old 11-07-2021, 09:42 AM   #1
gpvecchi
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Theory behind single piece driveshaft vibrations and fix

As I'm planning to fit a single piece carbon driveshaft, I made a lot of research, as many users seam to be affected by vibrations, especially when equipped with stiffer bushings/inserts (that as we'll see later, are not the cause, at least because of the stiffness).
Let's analize the issues: noises, intended as hums, hiss, even whistling and relative vibrations are normal, they are dued to the structure of the driveshaft (because rececar).
But many users are reporting wobbling or even shaking of the chassis, like an unbalanced wheel; somebody reported mirrors become useless because of vibrations; this is dued essentially to a couple of problems.
1) poorly balanced driveshaft (and I saw even known brands with this problem): nothing to do than rebalance it.
2) wrong pinion angle; this is what in my opinion causes most of the problems.
What is it? Stock driveshaft is in 2 pieces with 4 U-joints, while the single pieces have just 2 U-joints.
This changes the angle these joints work; unluckily U-joints have a very limited correct working angle, between 0.5 and 3 degrees.
Why not 0, perfectly straight? Because of lubrifications issues, they need preload.
First point: any angle over 3 degrees will probably cause wobbling (and U-joint premature wear).
Second point: a not perfectly straight rotating U-joint will cause an unbalanced force; but an equal opposite force will balance it: this is the joint on the other hand. But to balance it, it will have to create the same force, so it will have to rotate with the same opposite angle (or at least he closest).
So, we found the way to prevent wobbling: no more than 3 degrees at each U-joint, and a difference of angles of no more than 1 degree.
Luckily, to help this happens, the engine/gearbox is tilted down, while the differential is tilted up.
Now, we have to measure the angles between gearbox/driveshaft and driveshaft/differential.
In this case it can be helpful an app, the Tremec Toolbox, available for free on the store, or a digital level and this website https://spicerparts.com/calculators/...gle-calculator
This one even lets you select multiple pieces driveshaft.
To find the engine angle, you can put the level on the engine pulley (the crank shaft will be in line with the gearbox, if it's not, the driveshaft NVH is the latest of your problems); then you have to do the same with the driveshaft, then with your differential.
For this one you need an absolutely flat location, like the machined flange for the driveshaft, or the seal of the differential.
As I said, no more than 3 degrees each joint, and no more that 1 degree difference.
Now, how to adjust the angles; as the mounting points are fixed, and the space is limited, you can correct the gearbox angle using shims (and longer bolts) on the gearbox mount (depending on where you put them, you can raise/lower the angle). On the differential, you can just make it point down using shims under the front bushings or spacers between the rear subframe and the chassis.
This will raise your back a little, so you'll need coilovers to correct it; for the same reason, watch out if you use inserts for the subframe bushings, as this will change the pinion and transmission angle.
This is all; so stiffer bushings will amplify the effect of a pinion angle, stock bushings will probably be enough to make it disappear, but if the angles are not correct, the issue is there, and even if you can't notice it, it could cause premature wear. I really suggest to check the angles, if you go with a one piece driveshaft.

Last edited by gpvecchi; 11-08-2021 at 03:41 PM.
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Old 11-07-2021, 09:53 AM   #2
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These seems like problems that would be solved more easily by sticking to the stock geometry. So why does nobody make a 2piece graphite drive shaft? Even cars as old as the 350z had one stock, so Iím not buying the cost argument.
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Old 11-08-2021, 12:47 AM   #3
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A bit off topic but. I never realized a tailshaft changes rotational speed constantly even if the cars speed remains exactly constant.
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Old 11-08-2021, 09:08 AM   #4
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A bit off topic but. I never realized a tailshaft changes rotational speed constantly even if the cars speed remains exactly constant.
I'm not sure how you arrive at that conclusion. Could you elaborate?
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Old 11-08-2021, 01:43 PM   #5
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Good research and something I suspected as well. I agree that most of the issues are actually caused by the pinion angle. I noticed this when driving uphill or downhill. Sometimes the vibration would begin sooner or later depending on the incline, which made me think it's a pinion angle issue. I suppose removing the diff bushings helps not to transfer the vibration to the body but the shaft still vibrates.
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Old 11-08-2021, 03:36 PM   #6
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I would really like to see a 2 pieces carbon driveshaft, probably the benefit will be less than single piece ones, but without many annoying isssues.
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Old 11-08-2021, 08:50 PM   #7
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Critical speed is also potentially an issue. I believe prandelia went to a Verus CF shaft and never had an issue afterwards.

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Old 11-09-2021, 09:02 AM   #8
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In the 50's -80's Nascar ran single piece steel drive shafts for hours with out any vibration problems. I bet the thickness and diameter of the shaft was very uniform through the length of the shaft. I wonder how consistent these dimensions are in CF and AL shafts. Do the manufacturers verify the uniformity of the entire shaft?
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Old 11-12-2021, 03:39 PM   #9
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May someone with carbon driveshaft take the measures and post them?
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Old 11-12-2021, 10:06 PM   #10
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Quote:
Originally Posted by StraightOuttaCanadaEh View Post
I agree that most of the issues are actually caused by the pinion angle.
As long as you are balanced...that's it. Pinion angle... aka geometry.
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Old 11-13-2021, 11:25 AM   #11
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The latest Verus release used CV type joints. Someone believed this reduced or eliminated the pinion angle influence. But yet I had fairly severe issues with 2 of them. Anyone with further information regarding this concept?

Last edited by billstock67; 11-14-2021 at 09:57 AM.
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Old 11-14-2021, 03:08 AM   #12
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The latest Verus release used CV type joints. Someone believe this reduced or eliminated the pinion angle influence. But yet I had fairly severe issues with 2 of them. Anyone with further information regarding this concept?
Can you please measure the angles in your car?

Last edited by gpvecchi; 11-14-2021 at 09:50 AM.
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Old 11-14-2021, 03:16 AM   #13
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What's the point to go with a carbon driveshaft? I went in the past with a lightweight flywheel and although there was a very small gain the NVH was horrible. It wasn't worth it at all and it makes sense only for a track focused car. The gains of a carbon driveshaft will be even less than a flywheel cause of the very small diameter of the shaft (i.e. small moment of inertia).
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Old 11-14-2021, 09:56 AM   #14
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Can you please measure the angles in your car?

I will, but cannot immediately.
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