Electric 914 has resonant frequency at 5400 RPM, Could the motor/transmission have a bent shaft? Options

[TimK]

Here's a real corker of a problem that I'm completely stumped on. Any input or experiments to try would be most welcome. Just a warning: it's a rather long post...

I recently converted a 914 to an all-electric vehicle and everything works beautifully except for the whole car shakes nastily at 5400 RPM. I've been trying to figure this one out for months now.

You can see the whole electric vehicle conversion process (including a transmisison rebuild) at 914ev.blogspot.com. Most of my more recent posts, especially those in September, document all the experiments I went through to isolate the issue.

My last experiment went like this: I removed the friction disk out of the clutch and attached the electric motor to the transmission so that the only moving parts that touched the input shaft of the tranny was the pilot bearing in the flywheel. I put the transmission in gear so that the input shaft wouldn't turn (or at least I'd see it on the CV joint couplings). After spinning the electric motor (attached to flywheel) up to 5400RPM, the whole assembly rattled nastily and skittered across the floor like a hockey puck in slow motion.

Here's the confusing part. I acquired another "broken" transmission that had no oil in it and put that on instead. Again, the input shaft won't spin and the only thing it touches is the pilot bearing in the flywheel. I can spin this assembly up to 8000 RPM and get no rattling whatsoever.

The only measurable difference between the two transmissions is that the first one (that I rebuilt at home) has a runout on the input shaft of about 10 mils (10 thousandths of an inch). The "broken" transmission has only 2 mils of runout on the input shaft. The Porsche specification for the runout is 3.9 mils.

This leads me to the possible conclusion that (1) the rebuilt transmission has a bent input shaft or (2) there are loose parts that hit a resonant frequency of 5400 RPM inside the rebuilt transmission.

Since the non-functional "broken" transmission has no resonance, I'm planning on buying a transmission rebuilt by a real mechanic (not me) and see if that resonates. Still, until that arrives, I'm looking for other experiments I could try. Perhaps I could take the input shaft out of the "broken" transmission and put it in the home-rebuilt transmission and see if the runout goes away and the vibration with it.

Any suggestions or experiments to try are welcome. Thanks!

Cheers,
Tim

p.s. The photo below shows the 914 in the garage with the electric motor (replaced the engine) attached to the home-rebuilt tranny on the left with the "broken" tranny on the right.

[Dr Evil]

Hey Tim,
thanks for getting back to me with those links and explaining what problems you were having. I would suspect a bent input shaft. The foremost bearing on it may have let loose due to the bend. The good news is that the input shafts are interchangeable with no critical tolerances to worry about regarding the case and such. I noticed in your blog you did not mention that you replaced any sets of synchro teeth (dog teeth), did you? If you replace the bands only then the bands will soon be scrap and they will not do a very good job. You need new teeth. The original quote I said for a full rebuild was if you bought a new 1st gear slider and related 1st gear parts ($525). If you have these you will be set, but I didnt see that you replaced these. If you did not, your rebuild is not done my friend (IMG:style_emoticons/default/smile.gif) We can get you set up at the clinic for $300 and you should need no parts other than 1st gear should you want to do that (total of $850 + travel expenses) Bring both boxes and we will harvest the best from each (IMG:style_emoticons/default/thumb3d.gif)
If you can make it to SJ on the 4, 5th of Jan you should rebuild your tranny there. With your back ground you would benefit a lot from the experience.

[TimK]

Hi Dr. Evil,

Thanks a bunch for the input. Since I was an amateur when rebuilding this, I skipped several critical things that you mention:

- I didn't replace any synchro teeth

- I aligned the sliders and shift rods by mostly eyeballing them or following marks I made on the rods. I didn't use the proper jig.

- The old intermediate plate gasket was complete goo, so I didn't know how thick to make the new gaskets to align the gears with the differential

I would thoroughly enjoy being at the SJ clinic, but I have another vacation planned for that weekend. (IMG:style_emoticons/default/mad.gif)

Thanks again for the great info. I'm going to start looking around for first gear parts...

Cheers,
Tim