T-Rex 44 with open diff

[58_Willys]

Not sure if this is the right place for this but thought others might be interested...mod please move if it's wrong.

I am currently building a car for my Dad. The drive train is a 4x2 RWD wth leaf springs and the TR44 seemed like the perfect axle to use for this. Only problem, it comes with a locked diff, which is great for dirt and any other loose driving surface, but for my onroad RWD, it would make steering difficult and wear out quickly so I needed an open diff. After what seemed like days of research, I found what I think would be the perfect candidate.

I ordered the 1/16 E-Revo diff(Traxxas #7078 ) from ebay for all of $16.
First thing I did was, started comparing the Revo diff to the Trex diff. Sorry for the poor pics, my eyes were fried and I misjudged the focal point on my camera and didn't notice until I got the thing all back together.



As you can see, the axles for the Trex are flat drives not splined. Looks like they would fit a Tamiya TT01(I think that's the one)diff, which is also a favorable upgrade for the CC01 front diff. I should have tried them to see if they'd fit, but I was so excited to get this fitted to my Trex, I forgot to check.
Next thing I needed to do was to tear down the diff to find out if it was really going to work.
The bearings in TR are the same for Revo diff, 8x12mm, so no need for new bearings. Since the ends of the diff were plastic, it was easy to drill them to accept the larger, 5mm, TR axles. I used a #8(.199") bit in my drill press. Be sure to use a new and sharp bit, otherwise it can melt the plastic. The Revo outer diff gears have the same slotted shape as the TR axles just a little smaller (for 4mm axle stubs). At first, I tried to grind the Revo gears to fit the axles, but after destroying a dremel bit and 2 files,(these things are HARD), I opted to grind the axles instead. You don't have to take much off the flats, I just removed the black and that was enough. Then I put the axle into my drill press and used a flat file to get the round sides down to fit the diff gears. You only need to grind about a 4-5mm area from the end of the axle.

Now the pinion...this was a little tricky. At first I was going to use the pinion that I ordered with diff, Traxxas #7079, and sub in some 4mm bearings. Turns out, the shaft is too short, it only comes out of the axle about 1/8". So, plan B...the pinions are both 15T and the ring gears are TR=38, Revo=39. You can see here the difference between the pinions.

So I put the TR pinion in my drill press and while it was spinning, used my dremel with stone grinder bit to take the end down by about 2mm. You'll want to do this slowly, so not to create heat in the gear. Then wire brush in the dremel to clean up the edges. You can see, in pic 5 above, I didn't grind it down exactly as the Revo pinion because the dia of the Revo ring gear is slightly smaller than the TR but the diff area is larger. So I only removed what was necessary so it didn't hit the diff housing.
And after many adjustments with shims I got this.

It's not perfect, there is a small amount of noise from the gears, kinda like a TR60 unshimmed. I can live with that...it's not getting mega power, or running locked on pavement, so it should be fine.

If any one has a different idea for the pinion let me know. Overall time involved, is probably less than an hour. Took me awhile because I was trying different things and the fitting with the shims took longer than anticipated.

And here is the total build thread

http://www.scale4x4rc.org/forums/showthread.php?t=67699