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

[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).

[Dr Evil]

The best deal by far on first gear parts is the $525 kit from Auto Atlanta. It has the slider, synchro teeth, synchro band, and a seal kit.
Without the jig your alignment is most definitely off. Without the seal measurements the pinion depth is most likely off. Without new teeth, your shifting would be poo very soon. No worries, though, it is an inexpensive learning curve, all things considered. (IMG:style_emoticons/default/smile.gif)

Dont forget to harvest those new bearings from the old tranny and ask lots of questions (IMG:style_emoticons/default/thumb3d.gif)

[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

[TimK]

Without the seal measurements the pinion depth is most likely off.

Dr. Evil,

Thanks for the info on the parts sourcing! Since the original intermediate plate gaskets were heavily deteriorated, how could I determine the proper gasket depth in the future to set the pinion depth? Is there a special tool for this?

Cheers,
Tim

[Dr Evil]

If it is complete mush then I guess you cant. That is the least of your concerns given the other things (IMG:style_emoticons/default/wink.gif) That depth is actually less important than people think and if it is off a bit then 99.9% of the time it causes no ailment.

[Chris Pincetich]

I'm totally psyched on your project - I want to do one some day!
My advice - since you only use 2-3 gears just change the shaft or go with the other transaxle and be done. That rebuild alignment was your original problem and this is a tough one to get right w/o expert help+tools. Get that cool car back on the road and perhaps work on the perfect rebuild WITH NEW RATIOS after you have a good feel for the way the AC drives.
(IMG:style_emoticons/default/beerchug.gif)

[racunniff]

I'm totally psyched on your project - I want to do one some day!
My advice - since you only use 2-3 gears just change the shaft or go with the other transaxle and be done. That rebuild alignment was your original problem and this is a tough one to get right w/o expert help+tools. Get that cool car back on the road and perhaps work on the perfect rebuild WITH NEW RATIOS after you have a good feel for the way the AC drives.
(IMG:style_emoticons/default/beerchug.gif)

Actually, for higher speeds, it is still convenient to use 4th and 5th gear - the torque curve of the motor falls off after 4500-ish RPM. Which is why I took the time to adjust my shift rod... http://volt914.blogspot.com I don't seem to have the resonance that Tim does, but the car is definitely noisier at higher RPM. Also, I wonder if it is good for the transmission to run it consistently above designed redline speeds...

[Twystd1]

You aren't running your tranny over the designed limits... Not even close.

There are many guys that run those tranny's at 7K plus all the time.....

No probs.

The F production guys run em past 8K all the time.

Me thinks you don't have enough torque with an AC motor to really hurt the 901.

Now a DC motor, running 150 volts + is another matter entirely.

As torque is what kills these trannys...

RPM is much less likely to wear bearings and gears.

Cheers,

Clayton

[JRust]

Hey Tim, I don't now if it would help but... I have a tranny that is torn apart in my stash of parts. I got it from a guy who had taken it apart to do a rebuild. Was over his head. Supposedly everything is there. You are welcome to it for parts. I can't guarantee it's all there but you can have it for free. This is Jamie down in Corvallis. I bought your engine last year. Just let me know if you want it? Good luck either way

[TimK]

Hi Jamie,

Thanks a bunch for the offer. I have the second tranny for parts already, so I think I'm good to go.

How is that engine running? I hope it's working out for you.

Cheers,
Tim

This is Jamie down in Corvallis. I bought your engine last year.

[boxstr]

Tim, Contact me in reference to ordering trans parts, and if you are heading to SJ or need a trans to go down.
Craig