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.

[Demick]

10mils of runout is quite a bit. I would suspect this is probably a problem.

As far as a replacement transmission, your 914 is electric. So how often do you change gears? Don't you just leave it in one gear all the time? What gear is that?

Since you shouldn't have the need to change gears much - except into reverse and back, during which time the electric motor is probably stopped - the condition of the synchros and stuff isn't important. So there should be almost no reason to rebuild a used transmission that you get (unless it has major problems, in which case you don't want it anyway).

[TimK]

As far as a replacement transmission, your 914 is electric. So how often do you change gears? Don't you just leave it in one gear all the time? What gear is that?

Hi Demick,

I thought I would just be able to leave things in one gear all the time too. In reality, the electric motor was undersized for this application and only puts out about 30 horsepower. AC motors are also not great at torque off-the-line from a dead stop, so I rev the motor up to 3000 RPM and let out the clutch to start at stoplights.

I also thought that I could just leave things in second gear for highway driving; however, the torque on the AC motor starts dropping after 5000 RPM, even though it can spin up to 12000 RPM. In short, I drive in 2nd around town and keep it in 3rd on the freeway. Given the low horsepower of the motor, I suspect that the transmission is well oversized.

Cheers,
Tim