Harmonic Resonance/vibration question

[prandelia]

OK I actually just went and drove my race car on some back-roads where I could really focus on the vibration, instead of on a racetrack in a race where I don't have time to pay attention. So the exact vibration can really only be felt in 5th gear, around 6500-7000 RPM, from ~105mph - 112mph. It disappears after that, and is perfectly fine in 6th gear and beyond.

You can't really feel it in any other gears. I think some of the vibration was exacerbated by perhaps nipping some track-out curbing before the kink at Road America, and the uphill front straight where it takes longer to accelerate. I'm guessing it's some sort of frequency thing, with the driveshaft. What I'm wondering is that if I re-index it, the vibration will just be sent to some other part of the powerband/gear....potentially in a more annoying spot. Or could the re-index move it higher, so I don't see it at all?

[new2subaru]

Originally Posted by prandelia

Which one do you have? PST, DSS, or Verus ?

I'm beginning to think if you have stiffer motor, trans, and diff mounts, typical of a race car, then virtually any 1 piece drive-shaft will vibrate badly. Maybe I'm wrong.

That’s quite possible and gives me more reason to yank them out and test it again. I just want to measure the pinion angles first.

[prandelia]

Originally Posted by new2subaru

That’s quite possible and gives me more reason to yank them out and test it again. I just want to measure the pinion angles first.

My research seems to point to a potential design flaw, and potentially why almost EVERY person with an AL driveshaft has vibration, which is based on critical speed and pinion angles. Any 1 piece driveshaft has a lower critical speed than a 2 piece of the same total length, plane and simple.

Change the pinion angles slightly with a 1 piece, and a 3" Aluminum driveshaft that is ~50" long doesn't seem to have a critical speed of more than the redline of our car, 7400 RPM, anymore. So no matter what you do, you will likely always exceed the critical speed (frequency) of the driveshaft, unless I'm missing something about the DSS construction, or you somehow balance it or change the frequency.

As I approach 6500-7000 RPM in 5th gear (1:1 ratio) that it vibrates. I believe this may be the critical speed of the DSS AL driveshaft. Others who feel the same vibration at the same speed also then makes sense. You just blast through it faster in lower gears, but in 5th, where you more slowly approach it, the vibration is felt. If you have stock cars, or interior, or rubber, you dampen it and don't feel it, but it's still there.

Carbon Fiber has a higher critical speed, but DSS used a smaller diameter version again, which lowers the critical speed. Case and point, the Verus CF driveshaft actually uses a 3.25" diameter. Now why do they use a .25" thicker diameter than DSS AL 3" version, even though CF already has a much higher critical speed for the same length/diameter?? It's already known that larger diameter also raises critical speed of a shaft.

I am pretty certain the answer is to ensure the critical speed for the length is HIGHER than the shaft speed it will see on this car. This is exactly why the OE unit is 2 piece. I think this is what they mean in their marketing about "frequency balanced". You can balance any driveshaft to any RPM, but EVERY driveshaft still has a critical speed (based on length, diameter, and construction material), in which vibration is induced and unavoidable, which is based on it's natural frequency. So they can also balance it at that frequency to further avoid vibration.

So unless the wall thickness of the 2.75" DSS CF driveshaft is crazy thick, I'm not even sure if it's critical speed is high enough. At this point, I'm heavily leaning toward the Verus one. Not to mention, the Verus is dry laid, which is also known to be more rigid. So they use a much bigger diameter, of the same CF, but also use a method which makes it more rigid. Very interesting. They question is, why?

I specifically asked Verus today if that was the reasoning for the larger diameter of their unit? I'm waiting for their email back, but I'm almost certain that is why.

[new2subaru]

That’s interesting. Let us know what Verus days.

Mine starts to vibrate at about 110kph and stops at around 122ish. I have a friend who has an aluminum driveshaft and IIRC his was starting to vibrate at around 140kph (Before anyone starts up we were on track)

There are many who claim they have zero issues with both aluminum and CF. It’s written that aluminum is more prone to vibration.

[prandelia]

I've ordered the Verus DS, and will test it. Will get installed Monday/Tuesday of next week, and will take it on the same backroads to confirm no vibration prior to my race next weekend.

I'm confident this design (diameter/material) will provide the higher critical speed necessary to let the car operate fully throughout it's rev range, in all gears.

Once you get a LW driveshaft, it's hard to go back to OE steel.....

[new2subaru]

Originally Posted by prandelia

I've ordered the Verus DS, and will test it. Will get installed Monday/Tuesday of next week, and will take it on the same backroads to confirm no vibration prior to my race next weekend.

I'm confident this design (diameter/material) will provide the higher critical speed necessary to let the car operate fully throughout it's rev range, in all gears.

Once you get a LW driveshaft, it's hard to go back to OE steel.....

Cool. I wasn’t expecting such a difference with the CF shaft but was pleasantly surprised!

[prandelia]

Originally Posted by new2subaru

Cool. I wasn’t expecting such a difference with the CF shaft but was pleasantly surprised!

Which one do you have? PST, DSS, Verus?

[new2subaru]

Originally Posted by prandelia

Which one do you have? PST, DSS, Verus?

DSS

[prandelia]

Ran the Verus CF driveshaft this weekend, and it was perfect. Shifting is WAY smoother, and zero vibrations. So based on it's performance so far, I don't expect any vibrations as it's critical speed is properly designed to handle the RPM of this car. It's like driving a different car. The other manufacturers really need to evaluate the design of their offerings, especially the AL DSS one, it's clear that it's critical speed is well below the RPM/Speed this car sees.

Two big thumbs up thus far.

[NoHaveMSG]

Props for the research.

I'm just going to live with it. The only reason I bought the DSS AL driveshaft is it was cheaper then a stock one and I needed a new one.

[FTEIGHTSIX]

Originally Posted by prandelia

Which one do you have? PST, DSS, or Verus ?

I'm beginning to think if you have stiffer motor, trans, and diff mounts, typical of a race car, then virtually any 1 piece drive-shaft will vibrate badly. Maybe I'm wrong.

Why would you have any idea of the opposite?

The OEM mounts are engineered with flex to absorb majority of the vibration coming from the powertrain...IN ALL CARS.

Replacing with stiffer anything removes said flex.

Same could be said for a one piece driveshaft. You lose the flex in the joint as well as the rubber bushing in exchange for more responsiveness. You get a more direct feel because no power is lost to loose mount (more apparent in old mounts).

[NoHaveMSG]

Originally Posted by FTEIGHTSIX

Why would you have any idea of the opposite?

The OEM mounts are engineered with flex to absorb majority of the vibration coming from the powertrain...IN ALL CARS.

Replacing with stiffer anything removes said flex.

Same could be said for a one piece driveshaft. You lose the flex in the joint as well as the rubber bushing in exchange for more responsiveness. You get a more direct feel because no power is lost to loose mount (more apparent in old mounts).

The vibration from a one piece is nothing like a little bit of NVH increase from swapping out the bushings and mounts. I had stiffer bushings and mounts on the engine, diff, and trans before going to a DSS drive shaft and they made a bit of a difference. When I went to a DSS AL one piece it was like, "wtf is wrong with my car?!?!"

If you bother to read the whole thread you would see it has to do with the critical speed of the shaft in relation to it's wall thickness and diameter. On the DSS drive shafts it is insufficiently thick, and too small of diameter.

I can relate

[FTEIGHTSIX]

Originally Posted by NoHaveMSG

The vibration from a one piece is nothing like a little bit of NVH increase from swapping out the bushings and mounts. I had stiffer bushings and mounts on the engine, diff, and trans before going to a DSS drive shaft and they made a bit of a difference. When I went to a DSS AL one piece it was like, "wtf is wrong with my car?!?!"

If you bother to read the whole thread you would see it has to do with the critical speed of the shaft in relation to it's wall thickness and diameter. On the DSS drive shafts it is insufficiently thick, and too small of diameter.

I can relate

I think you missed the point.

The point was not to compare the amount of NVH increase from the bushings and mounts to the increase felt with the driveshaft. The point was that anytime you replace any part with flex designed into it with a solid one piece replacement, you will gain vibration.


Everyone is acting mystified and trying to engineer a reason why when it should be expected.


See formula below:

Remove stock part + add racing part = lose comfort


Now, I understand that there is more vibration in comparison to the verus driveshaft, but everyone is forgetting the difference in materials and how this affects resonance.

Carbon fiber will dissipate the vibration and resonance a lot faster than aluminum.

Imagine having a cylindrical pole of CF and AL and hitting each pole with a something metal and solid like a wrench. The Aluminum pole would continue to resonate like a chime or xylophone, while the carbon fiber pole would absorb and dissipate the vibration.

Of course, thickness and diameter play a role, but not as much as the choice of material.


To NoHaveMSG:
If you bother understanding the obvious overlooked variables, instead of believing everything that is read on the internet, you wouldn't sound like such a condescending prick.

My whole point is that WHY IS ANYONE SURPRISED? This should be expected.

[NoHaveMSG]

Originally Posted by FTEIGHTSIX

I think you missed the point.

The point was not to compare the amount of NVH increase from the bushings and mounts to the increase felt with the driveshaft. The point was that anytime you replace any part with flex designed into it with a solid one piece replacement, you will gain vibration.


Everyone is acting mystified and trying to engineer a reason why when it should be expected.


See formula below:

Remove stock part + add racing part = lose comfort


Now, I understand that there is more vibration in comparison to the verus driveshaft, but everyone is forgetting the difference in materials and how this affects resonance.

Carbon fiber will dissipate the vibration and resonance a lot faster than aluminum.

Imagine having a cylindrical pole of CF and AL and hitting each pole with a something metal and solid like a wrench. The Aluminum pole would continue to resonate like a chime or xylophone, while the carbon fiber pole would absorb and dissipate the vibration.

Of course, thickness and diameter play a role, but not as much as the choice of material.


To NoHaveMSG:
If you bother understanding the obvious overlooked variables, instead of believing everything that is read on the internet, you wouldn't sound like such a condescending prick.

My whole point is that WHY IS ANYONE SURPRISED? This should be expected.

I put a joke at the end of my post about having a small dong and look who got heated

I wasn't surprised, I read quite a few posts with peoples experience before going one piece. It was just worse then what I thought it would be. And what is wrong with some of us having a thought exercise about it? What prandelia dug up was right on par with what I was told by a retired Boeing engineer at the track. His back ground, NVH on planes. So, "the internet" is not my only source here. I haven't tried his suggestion yet but his solution to help it a bit was to add some wraps of tape about a third of the way up from one end. The goal is to dampen in a specific frequency range because the problem is not consistent vibration. I have mine down to just vibrating above 105mph. Before it would do it in multiple ranges.