Nagakura San

TB-03RA drive problem...

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Hello, glad to be a part of the community. I’ve almost finished my first Tamiya project in years and I’m having a problem with drive issues. I bought a super cheap but very clean used TB-03D to do a rally conversion (rally block tires and wheels, EVO III WRC body).

 

I’m running a stock gearing of 64t Spur and 23t .06M pinion. I believe the previous owner installed a one way spool in the front because the car basically has FWD only and zero reverse. I get no wheel spin in the rear. I assume I have to tighten something in the diff or the drive shaft/spur, but no idea as this is my first Tamiya build in over 10 years. Any help/tips? 

Link to a short YouTube video of the problem: 

 

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Edited by Nagakura San
Forgot Video link

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Can't remember if these are gear diffs or ball diffs. If it is a ball diff it looks like it needs adjusting. If it's a gear diff... You've got other issues. I'd be stripping both front and rear diff cases just to give them a once over anyway.

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The rear diff looks loose to me because the rear wheels do move, but they are barely coupled.  The good news is your video doesn't have any bad crunching sounds, so the ring/pinion gears for the ball diff are likely ok.

If you don't have it already get a copy of the TB03D manual here: https://www.tamiya.com/english/rc/manuals.htm

Take a look at steps 1, 2, and 3.  BA4 is the diff adjusting screw.  You'll want to tighten this *slowly* with a 1.5 mm hex driver until you start to feel the screw's resistance increasing.  Then loosen the diff screw about 1/8 to 1/4 turn.  The rear wheels should now move properly.

If not, you'll need to remove the diff from the rear gearbox and do some troubleshooting by taking it apart.  Another possibility is the diff loosened over time and then the slipping caused the ring gear to melt where the balls are installed.  But I would start with trying to adjust the diff adjusting screw first.

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I just opened the rear diff and it’s a ball diff, everything looked fine, no wear in the bevel gears, all the balls inside the diff were there and fine. I did tighten down the diff screw through the center and put it back in and that seems to have solved the forward drive problem. I’m still getting little to no reverse drive. There is a blue aluminum one-way diff up front from the previous owner...

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If the pinion and ring gears are intact, and the balls are properly clamped with the diff adjusting screw, then it sounds like a problem somewhere else.  I was going to suggest inspecting your motor pinion/spur gear mesh, but the front wheels drive as expected.  Might be worth double-checking anyhow.

When you run the motor in both forward and reverse, do the rear diff outdrives spin properly even if the rear wheels don't move?  That might suggest a dog bone problem or missing cross pins from the wheel hexes.  

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I just rewatched your video and the outdrives and wheels do seem in sync.  Is there a chance that there is a missing cross pin from the rear pilot shaft for the pinion bevel gear?

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I didn’t see any missing pins. The wheels spin in reverse when it’s in the air but as soon as they meet the slightest resistance, they stop. Spur keeps spinning. 

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Check parts BA8, BA19, and BG1 in step 2 of the manual.  BG1 should turn in unison with the spur gear; make sure you cannot move BG1 on its own by hand.

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I’ve been trying to find the part number for the stock ball diff to replace the one-way in the front. Can’t find anything on searching. Parts of it, gear diff, one-way, coupling etc etc but nothing for the full Ball Diff set. That seems unlikely. Any ideas?

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To the best of my knowledge I have never seen a part number for a complete ball diff set that is equivalent to the stock kit parts; most people just need a few replacement parts or they upgrade to something else (gear diff, one-way, spool, etc.).  So you have a few options:

- Run the car exactly as you have it today with the rear ball diff and the front one-way.  You might actually like it that way as the front one-way reduces the understeer normally experienced with a front diff, and braking will be similar to pulling the handbrake in a real car allowing you to pivot the car quickly.  It might be fun for a rally car model!

- Buy the individual parts to build the kit diff from scratch, including the S parts, diff rings, ball bearings, ring gear, balls, thrust bearing, screw, spring, and nut.  The back of the manual should list all part numbers.  No idea what that would cost, but if it gets expensive then another related option is to find a used TB03 that can be a cheap donor car.  Then you have more spare parts in case other things break.

- Spec-R used to make a gear diff for the TB03 that has the correct ring gear tooth count (39T).  If you can find one of those that would be a drop-in replacement for the front one-way.  A gear diff functions the same as a ball diff, and is a little more robust as well.

- If you start looking at TB04 parts then understand you'll have to swap both the front and the rear one-way/diff at the same time so both axles have the same gear ratio.  The TB03 runs a 39T ring gear; the TB04 runs a 40T ring gear.  In theory you can run TB04 gear diffs, ball diffs, spools, and one-ways in a TB03 because they use the same gearbox casings, however I have read in the past you may need to modify the TB03 pilot shafts slightly so they don't rub on the TB04 gear diff cases.  The change over to TB04 internals should be possible, but it's not a simple drop-in replacement based on what I have read before.  I haven't tried it personally yet.

 

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Thank you for such a detailed response. I’m gonna look at the gear diff option. Would it be a direct drop in and still work fine with the rear ball diff? I wouldn’t need to get gear diffs in back and front?

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I would not start buying parts as you dont know what is wrong yet. If i bought someone's used rc car i would take it all apart completely and clean it with dish soap. Once taken apart you will see if you have any broken or worn parts that need to be replaced. Then buy and replace the broken or missing parts and put it all back together. 

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