TXT-1 gearbox designed for 4274 brushless motor

[MadInventor 3886]

After having a reasonable success with fitting the 4274 Ezrun motor in my TXT-1, I've decided to try and improve the fitment by designing and building my own gearbox for it, designed around the 4274 sized motor.

I set myself the following design goals:

1. Centre the motor above the drive shaft.

2. Lower the motor to improve the centre of gravity.

3. Allow the fitment of Traxxas Revo UJs with threaded securing pins, as well as standard Tamiya drive shafts.

4. Re-use the existing Tamiya TXT-1 gears

5. get rid of the need to use 1160 sized bearings on the main transmission shaft.

6. Allow the use of a 12tooth to 20tooth pinion.

7. Lower the output shaft to help straighten the UJs

This is what I've got so far:

This is the main rear plate of the gearbox. It holds 2 idler shafts, and the main transmission shaft. The bearing housing for the main shaft holds 2 1260 bearings.

Rear bolts are recessed into the body. Idler shafts are press fit so do not drop out or rotate.

Comparison of size with a normal transmission case.

Supporting rods for the other main plate are 5mm steel

Home made steel components. The hex drive started life as round bar. It will be loctited onto the main shaft with gear locker eventually.

Assembly so far. It is interesting to note that the TXT-1 gears have a perfect tooth count for 2 of the idler gears to run on one shaft. Perhaps it was originally the intention for the gears to be fitted in this manner, but some other design requirement resulted in the actual TXT-1 layout that uses 3 idler shafts instead of 2.

The next plate will have the motor mount incorporated into it, so that will show the design off better. The motor sticks out in front of the transmission as it does in the king hauler gearbox. The 4274 will hopefully slot between the top links of the suspension. With the idea that I currently have for the motor mount plate and the method for attaching the transmission to the chassis, the motor will sit 20mm lower in the chassis. I have also decided with this project, to build 2 gearboxes, one for myself, and one to sell on.

[SteveU30 1011]

Dibs on the one you're selling! Can't wait to see this develop matey.

[MadInventor 3886]

Done a bit more on this today. Got the other half of the gearbox plate roughed out, so I could see what it looks like in the truck. Assembled it and the gears run smoothly so it looks like everything is lining up ok. Still got lots more to do but it is possible to see the potential in it now. The bottom drive shaft now runs in 1260 bearings instead of the hard to find 1160s.

The other half is 5mm alloy plate:


I've worked out I can fit anything from a 12tooth to a 24tooth pinion in it. The only problem is that the 20tooth+ pinions will probably rub on the socket caps holding the motor in, so I will have to recess the bolts if I'm going to use the larger pinions. I think 20tooth should be plenty large enough.


The gearbox will be about the same depth front to back as the bottom of the TXT chassis, but is a lot narrower. I left a gap down the side in case I wanted to fit the gearbox in a Juggernaut as well. Clearance on the suspension arms will not be a problem with the Jugg.




With the gearbox mounted at the bottom of the chassis the fat drive shafts still clear the anti roll bars. The motor is significantly lower than standard and sits between the front upper links. The bigger the pinion the higher the motor will go. If it looks like the motor will foul the front links I can easily make some upper links from 5 or 6mm steel and put a bend in the end of them close to the axle so to provide more clearance.


Don't forget that's not a 540 in there. A 540 is 36mm diameter * 56mm long, this motor is 42mm diameter*74mm long, so a 540 sized brushless is going to slot in there no problem.

Edit 26/06/2014:
Just looking back at the thread I originally did when I fitted this motor and found the pic below:



This highlights how much closer to the middle of the truck, and how much lower the motor is, with the new gearbox compared to the standard one. I think it's come downwards and back towards the centre of the truck by about 20-25mm in each direction.

[skip665 169]

Great work! Following with interest!

[MadInventor 3886]

Done a little bit more this evening. Loctited main 2 drive shafts, one for each gearbox, and had a further look at fitment options. There is a lot scope for altering where the gearbox is mounted, I had it a little too far forward in the pics above, the 4274 will clear the upper suspension rods without an issue. I also made a misake with my calculation of the diameter of the pinions, and when I corrected it found that the 24tooth pinions will fit without having to recess the motor retaining bolts. I've also figured out that it will be quite easy to fit steel skid bars to the bottom of the gearbox, to prevent it getting scraped. I'm torn now between making the 2 bulkheads for gearbox 2, or pursuing making the side plates for this one so that I can finalise the positioning. Trouble is if I finish one, it's then being bothered to complete the second one ....

[MadInventor 3886]

Made a bit more progress on this on the weekend. I've now made the brackets to hold the underguard in place, which the gearbox will bolt down onto.

Here they are fitted to the chassis:

I'm hoping to get the underguard cut out and drilled this afternoon. I'm making it from 1.5mm thick steel plate, which should resist any twisting action caused by the gearbox, as well as making a tough underguard and adding weight low down in the chassis. (I'm not too concerned about the extra weight of the underguard as the 4274 has 'ample' torque )

These are parts I've made so far for the 2nd gearbox:

The flat plate is a side plate for the gearbox. I've made 3 so far, but don't have any pictures as yet of them fitted to the gearbox.

Edit: Got some more done this afternoon, and managed to get a 1st fit done

Gearbox now bolted to the undertray:

Motor sits neatly between top upper links. This is with the motor in the lowest position for a 12tooth pinion. Using bigger pinions will raise the motor further away.

This is clearance on the motor with the suspension at full deflection:

Steel underguard. 1.5mm thick. I still need to flare up the front and rear edges, but I'm going to use this one a template to the make the next one first.

Cut down the front drive shaft and fitted it with yoke ends that accept a screw in retainer pin at both ends.

Drive shaft adequately clears the motor at full suspension deflection.

Another view of the front drive shaft at full suspension deflection.

So far I'm quite pleased with this. It's doing everything I wanted it to, and the only bit left to do to complete the first gearbox now is the top cover.

[MadInventor 3886]

Nearly finished both of these gearboxes now, had a blitz on it on the weekend, and just need to make the pinion covers and find some shims to pack out about 0.5-1mm on each end of the idler shafts.

[MadInventor 3886]

Another quick update, finished one gearbox and the other one just needs a pinion cover. I ended up making my own shims for the gearbox, very time consuming for a little sliver of steel .....

Both underguards with flare edges front and rear:

Machined a pinion cover from a block of aluminium:

Gearbox with top cover fitted:

and the pinion cover:

Number 2 is nearly ready to roll off the production line

Just got the pinion cover to make and this one is done

The finished gearbox fitted in the truck. I've run it this afternoon and it's nice and quiet. Very pleased with it, hopefully I'll be able to do a video of it shortly

I had to move the ESC and RX around slightly in order to get the wires from the motor out through the radio tray in a convenient place.

Very pleased with how this has turned out.

[MadInventor 3886]

Finished the 2nd gearbox and it's now ready for sale SOLD

Here's the components laid out:

Got a few too many M3x10 screws on the pic, but otherwise that's all of it.

Here's a few pics showing the overall condition, with the blemishes that got added during manufacture:

Pic showing other sides of plates:

The following pics show step by step assembly:

And that's it

Just a quick reminder of the plus points of this gearbox design:

1. All metal. No heat dissipation issues

2. Uses 1260 roller bearings (3) to support the final output shaft, rather than 1160 bearings.

3. hex drive fitted to final output shaft is steel, not aluminium, and no splines to wear.

4. Gearbox uses existing Tamiya gears.

5. Motor is mounted centrally and is lower than the standard item. This means you can also lower the battery tray if you don't mind losing a bit of rear suspension travel.

6. 1.5mm thick steel underguard integral into the design and provides excellent protection for the gearbox from scrapes.

7. Final output shaft is lower than standard, so the drive shafts are straighter in the vertical plane.

8. Traxxas revo drive shafts fit straight on, they just need cutting to length.

9. Designed to fit 4274 brushless motors, but 540 sized (3656) motors will fit as well with the supplied adaptor plate.

10. Being basically a box, it should be easy to modify to fit in other vehicles if required.

Thanks for looking. Now, back to the grapple skidder

[Wild Willy Speed Shop 73]

Thats tough!

[ukfox 347]

Wow that's a lot of engineering

Well done, I take my hat of f to you sir

[MadInventor 3886]

Just a quick update, I've painted the underguard, and machined a couple of slots out to make it easy to extract the UJ securing pins:

[Sayer 742]

Dude, if I had your tools and more importantly, your skills, my wife would never see me.

[MadInventor 3886]

Dude, if I had your tools and more importantly, your skills, my wife would never see me.

I think that's why there's a picture of me on the front of the fridge, so my other half doesn't forget what I look like.