Harmonic Resonance/vibration question
 

Hi guys, this car is brand new to me, so never driven it on street, only as fully built race car. So on my first race weekend, I noticed around 5500 RPM's or so, there is a big vibration in the car, you notice it more in higher gears since it takes longer to rev through that range.

I have stiffer motor, trans mounts, along with the fluidamper crank pulley, and the ACT HD clutch with Street-lite flywheel, etc. so obviously everything will be amplified, but I'm just wondering is this a normal vibration? Do others with similar race cars know about or feel this vibration/harmonic? I thought I noticed on the Fluidampr test info about their pulley that there is a big natural harmonic around 5500.

Just trying to deduce quickly if it's a vibration from another component (like my AL driveshaft), or just a natural harmonic of the engine at those RPM's. Will save me a lot of trial and error hunting for it. Thanks for the input peeps!

From personal experience, I don't have any more particular vibrations at 5500 RPM or any RPMS around that on a mostly stock car. I would believe the trans and motor mounts to be the main cause of your vibrations as this has been stated by many to increase cabin vibration and give the car a harsher feel.

I got a lot of NVH in that range after I went to an aluminum 1 piece drive shaft.

Big vibration is usually something big that has excessive movement.

Start with the driveshaft.

I am no expert on this subject, so my thought process may be misleading. But I would think that there is more excessive movement in an engine and transmission than a driveshaft. The driveshaft SHOULD be balanced properly and not cause any vibration issues with its balanced rotation. Sound may increase from an aluminum driveshaft, but it should not cause vibrations unless you have a defective driveshaft. Again, I could be very wrong, and am ready to be corrected if so, but just taking a good guess at it.

Driveshaft vibration related issues:
https://www.machineservice.com/techn...tional-issues/

Think it has more to do with the fact that most aftermarket drive shafts are one piece and not two with a carrier barring like the OEM one, then the properties of AL exaggerating the issue more. I went from not having much vibration, to having a lot in similar RPM range the OP is describing. Only change was the drive shaft.

Yeah, that is interesting, and comforting at the same time. It is also worth noting that I have SPL solid subframe bushings, and we "raised" the subframe about 0.5" toward the body (to allow for better axle angles after lowering the car), and this also slightly affects the drive angles. Nothing extreme, but it seems it may be more related simply to the AL construction than anything else, given others also note the same vibration.

The easy way to find out is to slap my OE drive shaft back in, and if it goes away, then we have our answer. Perhaps CF driveshaft would be better.

I bought my AL one locally for only $200 so couldn't resist it, so it's not a huge deal if I move away from it. It definitely felt like more of a "whole car" vibration, and while I've felt engine harmonics that bad, I can see how it might be the AL driveshaft as well.

Anybody with CF driveshafts notice anything similar?

I have a CF driveshaft and have vibration between 105-115kmph. I turned it ten different ways to Sunday and it didn’t go away. I’m going to try and fit it better in the winter.

I have flipped mine every way I could and only saw marginal improvement. Of course the first way I installed it was the worse. I don't think driveshaft shop is very careful with balancing.

Yeah, was thinking to re-index and see if it gets to an acceptable level.

Powertrain NVH engineer here...

I'm going to have two answers here, and feel free to ask me anything on the subject.

Short answer:

Without high-resolution noise/vibration data with a high resolution encoder or optical tachometer, it is effectively impossible to pin-point the source of a resonant vibration. However, if you can give me a relatively high quality video (with good in-car audio NO WIND NOISE) with CANbus RPM reading I can point you in the right direction.

Long answer:

There's multiple techniques we use to pin-point the root cause of a noise/vibration. Most notably, we use high resolution data to analyze the amplitude of the order content. An "order" is something that occur "X" times during a rotation event. In ICE applications, there's quite a few things rotating but the most important tends to be the crankshaft. Thus, we can figure out whether a noise/vibe is being caused by the combustion cycle of an engine by measuring the amplitude of source vs. engine RPM. In a four stroke, four cylinder a firing event will occur twice every revolution, meaning the primary order content of a four cylinder engine will be 2nd order.

Easy enough right? A high amplitude 2nd order indicates that the actual combustion of the engine is driving that vibration. The issue, is that there's obviously a LOT of things moving and spinning in an engine. You have alternators, A/C Compressors, Water Pumps all ran directly off of the crankshaft (all rotate at different rates depending on the pulley diameter) and you also have the things connected I.E the driveline. In this case, there will be torsional vibration transmitted through the crankshaft to the harmonic damper and the flywheel. Like you mentioned, harmonic dampers are tuned to damp a specific frequency spectrum. In the case of viscous fluid dampers, they target a broad frequency spectrum as opposed to traditional dampers which target quite a small frequency band. Past that, you have moving parts in the transmission (input/output shaft, ring/sun gears), driveshafts (you mentioned you have an aluminum driveshaft), and the differential. Order content can be looked at based on the # events per rotation. Additionally, every material has a natural resonant frequency (steel is different than aluminum etc.) which can create horrible noises, shakes etc.

So in the end, the real answer is extremely convoluted and complex. But if you want to give it a try, send me a video mentioned above (preferably a 3rd gear WOT up to max RPM) and I'll see what I can do.

That's awesome but google "brz one piece driveshaft vibration". It is well documented and frequent that an aftermarket one piece causes a bunch of extra NVH even on a stock car. A recommendation I got from a guy at the track that was retired doing the same thing for Boeing, said to put a wrap of tape a few layers thick about a 1/3 of the way up the driveshaft. I just haven't tried it yet.

Oh no doubt, a lightweight driveshaft isn't great for NVH. I guess I'm more offering my services if we want to literally find the exact origin of the noise/vibration and offer a solution.

What is the tape supposed to accomplish? I’ve seen a video of a guy balancing a driveshaft with hoseclamps.

It appears that the shaft has been balanced. There’s a few weights glued on it. FWIW, I think the one piece shaft delivers all kinds of vibrations from the rear. It makes sense not having the carrier bushing.

I did notice when I switched track tires that it lessened. It’s still there but not as profound. That or I’m getting used to it lol

Something about dampening a certain frequency range. I don't exactly remember.

Interesting. Vibrations are a waste of energy and destroy shit.

Damping*

Dampening is when you're getting something wet :)

I've seen lots of people on here with AL shafts that complain. Not as many with CF. I have a carbon one from driveshaft shops and I have a tiny increase in whine, but no added vibrations. I've had it in for about 20,000 miles so far. Note at this point my car is mostly stripped.

So if it is the driveshaft, then it certainly sounds like an imbalance. I don't know where you can take it to get it balanced, but that would certainly solve your problem if it IS in fact the driveshaft and you would like to hold on to it.

Some interesting instructions to finding if you have an imbalanced driveshaft I found here. I would be far too lazy to do this though :) I guess in your situation it's basically impossible anyway, as you only experience it at higher RPM's.
http://www.demandaam.com/technical-s...haft-balancing

Since it's always around 5500 RPM in any gear, wouldn't it make sense for it to be something on the engine side of the transmission rather than the driveshaft?

Brain fart :bonk:

Can't speak for the OP but mine does it in other areas too, it is the worst at 5500. Similar to the OP mine is worse in upper gears.

I have read in other threads that balancing didn't help.

How did you adjust the subframe?

SPL's kit has shims, so you can install at OE height, or raise/lower it depending on what you want to do. Super easy to adjust.

How many out there with CF driveshafts have vibrations ?

1

I’m not done tweaking it yet. I want to check the pinion angles. My rear is dropped slightly with bushing inserts and diff inserts. I don’t know if that threw it off.

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.

How bad are these vibrations? I have vibrations and/or resonance from either one, or a combination of, subframe and diff bushings, shifter bushing and trans insert and I really don't mind them. It makes the car sound raw and visceral. So how would a carbon driveshaft worsen the vibrations? Any insight on this? I wonder if anyone's done it in the same order

What you said there is probably very true. As the motor and trans have natural vibrations, having mounts on them sends it to the rest of the chassis and parts. In this case, your driveshaft would certainly gain some substantial vibration, and not due to an imbalance (which makes more sense, not sure why I didn't think of that before).

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?

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.

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.

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!

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

DSS

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.

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.

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 :bellyroll: