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Old 10-30-2009, 08:06 AM   #21 (permalink)
Engineering first
bwilson4web's Avatar
Join Date: Mar 2009
Location: Huntsville, AL
Posts: 843

17 i3-REx - '14 BMW i3-REx
Last 3: 45.67 mpg (US)

Blue Bob's - '19 Tesla Std Rng Plus
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Have you given any more thought about swap vs rebuild of transaxle?

  • Potentially gets car 'on the road' soonest
  • Risk of swapping in a trannie on last legs
  • Leaves broken trannie for rebuild (known failed parts)

  • Puts replacement trannie at "0 miles" condition
  • Leaves car immobile during rebuild (may lead to domestic issues)
  • One trannie replacement

  • Could be pricy, offset by reselling the first swap trannie
  • Allows unpressured blueprint of final trannie
  • Inspection of failed and the blueprint trannie gives insights to enhancements
  • Need a winter project that can be done in a warm place

One of the differences between the NHW11 and NHW20 transaxle is the use of a bushing on one of the gears, NHW11, versus a bearing in the NHW20. Now there may not be enough casing metal to support a bearing replacement in the NHW11 but for sure, bushings wear. At a minimum, I would recommend measurement of the bushing but I would probably order the replacement and have a machine shop replace it anyway along with all of the seals.

Oil analysis has shown no molybdenum suggesting there may be a materials issue with moly greases that are often used in engine and transmission rebuilds. I would recommend using a non-moly based assembly grease, which may mean use of a petroleum-only grease. However, there is boron suggesting a hex-version of boron nitride impregnated grease might be an excellent option. For sure, I would work hard to make sure no moly is used without doing a through investigation ... especially around the magnets!

I have seen enough transaxle oil samples to suspect the area around the differential may accumulate 'debris.' I would recommend that before final assembly, adopt the cleanest standards possible and throughly clean every nook and cranny that might harbor any debris.

There is good data from the Dept. of Energy studies and my own that warming the transaxle to operating temperature improves performance. I have tried two, JC Whitney oil pan heaters and both have failed due to vibration between the power cord and the flat, resistance heater. Worse, half of the flat resistance heater is on the outside of the pan. My next version will be an immersion heater element built into the pan with a very secure, insulated, oil-tight, connector. This will reduce the electrical load and solve the vibration problem.

I am also intrigued by the use of a by-pass, micro-filter to get particles smaller than 10 micron out of the oil. But this is a very advanced technique and not cheap. Still, my early, hill roll down tests suggest this will significantly reduce rolling drag on cold startup.

BTW, would you elaborate on the 'pulsing drag' (I think you called it cogging) felt when pushing the car about?

Bob Wilson

2019 Tesla Model 3 Std. Range Plus - 215 mi EV
2017 BMW i3-REx - 106 mi EV, 88 mi mid-grade
Retired engineer, Huntsville, AL

Last edited by bwilson4web; 10-30-2009 at 03:08 PM..
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