Sunday, February 8, 2009

Pumpkin Interference, V

Man, this motor mounting project has turned into a pain... This week, I finished the motor mount I had designed last week. I cut it, welded it, bolted it, painted it, and installed it on the motor. When I went to install the motor, the trouble started... but let's step back in time first. Here are the original parts, all primed pretty:

The bolts are torqued down to 25 foot-pounds per typical specs on Grade 8 hardened steel bolts. The primer is just rattle-can metal primer. The welds are there to keep everything aligned more than for structural purposes - the bolts themselves are plenty strong to hold everything together. The two longer parts I call the "cradle" since it cradles the motor feet. The two shorter parts are called the "forks" for obvious reasons (they go around the motor mounting bushing).

I finished painting the parts (rattle-can black) and mounted the cradle on the motor. The 3/8" bolts holding the cradle on were torqued to 45 foot-pounds, along with some blue threadlocker. Those guys won't come off unless I want them to. The forks are temporarily mounted for photogenic purposes. The handy thing about the cradle is that it provides a good place to loop chain or cable around for lifting with a hoist:

And here is where the trouble began. I'm not sure how, but I measured the distance between the two motor mount bushings incorrectly. I measured it at 19" - but it is really closer to 18.5". This means it is impossible to mount both forks at the same time. In this picture, you can see that I have mounted the passenger's side fork, but that the whole motor is pushed over toward the driver's side about one quarter to one half an inch:

Because of this skew, I could not install the driver's side fork. So, I put a temporary bolt on the driver's side just to support the motor while I went back to the drawing board (this is obviously only a temporary measure):

The good news is that there is sufficient clearance between the highest point on the motor mounting system and the bottom of the front battery rack (so at least I measured *something* correctly):

After much thinking, I came up with a new design. The issues were:

  • The current design is too wide
  • But I can not make the fork tines shorter while still using box tube because the main 1/2" bolt head will interfere with the motor mount bushing retainer
  • Additionally, the box tubing on the fork "tines" was too thick to easily get a nut onto the bolt that goes through the bushing
  • And, I'm not happy using threaded rod - I'd rather use a grade 8 hardened steel bolt thru the motor mount bushings, and those typically only come in 6" or less lengths (unless you special-order them).
I finally realized that I could do three main modifications to the fork design:
  • Replace the long box tubing with an angle iron
  • Shorten the tines by 3/4"
  • Cut the tines in half at the motor bushing bolt end to make more clearance for bolt heads etc.
So, I did it. Here is a rendering of what the completed motor mount will look like with the modified forks:

Here is an exploded view of both the driver's side cradle as well as the new driver's side fork:

And, I built and painted the new forks. Installation will have to happen some time this week or next weekend. I'm out of daylight...

You may find a PDF of the blueprints for the driver's side motor mount (both the cradle and the fork) here. Note that the passenger's side is a mirror image of the driver's side.

All this for a stupid front differential... if only I could have found a two-wheel-driver front axle and transmission...

1 comment:

Anonymous said...

Me again, I would suggest a cross member of some sort to stabilize the torsional stresses on the mounts. It will give some support to those single bolts on each side.