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