Sunday, September 30, 2007

More Resonant Experiments

My friend Rick dropped by tonight to run the accelerator cable and monitor the RPM on the laptop while I put my hands on the motor/tranny to get a better feel for what's going on. We ran the motor with the transmission attached for weight without the clutch friction plate. I also put the transmission in first gear to guarantee that the input shaft was not turning and contributing to any vibration.

I short, we experienced a classic resonance in the system as described well in the following link:

The motor/transmission behaved just like the amplitude graph in the link above. We got a resonant frequency at 5450 RPM and the vibrations remained but at a much lower amplitude above that, all the way up to 9000 RPM.

One funny thing to note is that without the clutch friction disk in the system, there is very little play in the clutch fork. If I pull it towards the rear of the vehicle, it tries to depress the clutch, but I don't have the strength with my hand to move it. Just for yucks (DON'T TRY THIS AT HOME KIDS!), I lightly pushed it towards the front of the vehicle and it hit the face of the clutch pressure plate while it was still spinning! Yikes! Against my better judgement (engineers try these things...) I used the clutch fork to monitor the status of the flywheel while the system hit resonance by verrrry lightly touching the spinning clutch. During normal operation (say 3000 RPM), I felt a uniform depth all the way around the clutch. During resonance, the face of the clutch seemed to oscillate rather significantly with the "beats" of the resonance. See the following link:

Just to get a good comparison, I removed the transmission again and got no vibration from 0-8000 RPM (just motor, adapter, flywheel and clutch pressure plate).

As before, I'm a bit confused about what to do at this point. I'm imagining that Azure Dynamics didn't take into account that the system would have the cantilever of a transmission attached to it, especially a Porsche 914 transmission. I believe that Electro Automotive didn't prototype this motor/tranny configuration, so they probably didn't bump into this issue.

Craig out at Camp914 said he had a spare 914 transmission sitting in his garage that I could borrow to see if a similar mass causes the same issue. I'll probably borrow his transmission and see if I get the same result. If so, there's probably not much I can do except limit the RPM of the system to 5300 RPM and take the performance hit. If his transmission works fine, I might talk to him about swapping it. My doesn't leak a drop of fluid and shifts quite well after the rebuild.

Even with lots of damping devices such as new motor mounts, running the system through its natural resonant frequency is a really bad idea since the system will probably fail quickly. I'll be posting a question to the 914ev discussion list to see if anyone else has spun up their system.

I still might borrow an accelerometer from work tomorrow and log a bunch of data with the audio input on my laptop. Using the audio inputs on the laptop to capture a .wav file and then running an FFT with audio software on the file would definitely show some interesting stuff.

Just another side note: I believe the resonant frequency was actually at a lower RPM with the original flywheel before I had the teeth and backside ground off, so I would think others would hit this too.


pjorg said...


Maybe you could glue a couple hundred pounds of lead to the transmission to change its natural frequency?


TimK said...

Wouldn't it be great if I could glue the lead-acid batteries to the transmission?

Assuming you're not joking (the car is already 3000 pounds), I'm concerned that adding weight would only lower the resonant frequency into the 2-3000 RPM range which would be worse from my perspective.

Cheers, Tim

onei57 said...

My thinking has been that there is a mechanical cause for this. Is the pilot bearing ok? When you had the disc out and it still danced aroud the floor was the input shaft on the trans spinning indicating a bind on the shaft?

Maybe the next time you can remove the clutch and the disc. Attach the trans and thru the holes in bellhousing see if you can freely turn the input shaft thru the starter hole. Also is the input shaft in the bearing past the lead angle? When the motor runs is there any axial thrust in the motor or trans input shaft that binds it?

Lastly the redline on the tacometer is 5,750 rpm. While the electric motor can run higher, why would the Engineer @ Porsche build the rest of the drivetrain for 10,000 rpm? I just wonder if driving that much past the redline would damage trans, cv joint etc.

TimK said...

I put the transmission in first gear and monitored the CV joints to make sure that the input shaft wasn't spinning or binding. I suspected the pilot bearing too, but it seems to be okay. I'll probably replace it since it does squeak just a bit.

I agree that the tranny may not have been designed to run past redline. My gut tells me that the redline is mostly for the gas engine, though. I still don't like the 5400 RPM limit...