Tamiya TRF201 Project

Page 3: Converting to XM

Here is the XM conversion kit which I purchased separately. Although it doesn't come with very many parts, what it does come with is expensive. You can see machined aluminum transmission housings, a heat sink motor mount, an aluminum motor guard, and a carbon fiber shock tower and battery clip. There is also a new body included which is a bit odd since the XM can only be made from the XR which uses the same body. They can, however, be trimmed a bit differently at the back to help hide the motor.

The original TRF201 transmission uses 3 gears. Because the XM flips the transmission around to put it ahead of the rear axle, it also needs to rotate in reverse. To accomplish this an additional idler gear is added. The XM upgrade kit has this gear in black which is supposedly an upgrade in some way, and also includes a new black ring gear for the differential. I didn't want to rebuild the differential so it stayed white. The left hand picture shows the original transmission and the right hand picture shows the XM. It wasn't strictly necessary for the XM to use aluminum instead of carbon reinforced plastic, but I'm glad it does. It looks cool, and it leaves me with an extra stock transmission.

Here is the updated and completed transmission which also includes a new machined motor mount plate.

The original rear shock tower sat exactly where the new transmission needs to go, so it had to be replaced. The new shock tower is an incredibly thick (5mm) carbon fiber plate which bolts right to the transmission housing. Part of the reason for the aluminum housing was probably to make this connection nice and strong. The old diagonal brace that spanned the gearbox and shock tower is gone. The wing mounts also connect to the shock tower.

Now the old transmission and rear suspension can be removed to make room for the new. Note that I've added a new curved aluminum motor shroud. The carbon plate ahead of that holds down the back of the battery. The suspension arms connect using the same parts as before, but there is one subtle difference. If you read my build of the original chassis you may recall that the rear uprights were switched left to right (left labeled part installed on right, right labeled part installed on left). Part of the XM conversion is to switch them back the right way around. This effectively moves the ball joint for the upper link from ahead of the upright to behind it. This move will be needed for the new shock tower position.

Now the new transmission assembly can be installed. Obviously the output drive cups need to be in the same position as before to link up with the axles, but the rest of the transmission has been moved well forward. The big bore shocks now sit behind the rear arms instead of ahead of them which moves them much closer to the wing mounts.

Here are some closer pictures of the new rear assembly with the motor installed. With space on the chassis now taken by the motor, the same long racing pack can no longer be used. The manual suggests using saddle packs but I found that the square pack from the Short Course Truck fits perfectly. This shape battery pack has become quite rare so I may have to change to a different profile in the future.

Here I compare the XR and XM chassis variants from above. The front half is exactly the same before and after. In the rear you can see how far back the shock tower has moved. You can also see that the rear upper link and shocks have moved behind the rear suspension. The motor, obviously, has been moved forward. And the new version has a lot more blue.

Here I compare the XR and XM chassis variants from below. The wheelbase is exactly the same but you can see the blue transmission housing through the slot in the rear, and the position of many of the countersunk screws has changed. The XM version was obviously already planned at the time the XR was released because the chassis plate had all the hole options drilled for it.

Here I compare the XR and XM chassis variants from a 3/4 view. The XM has the illusion of being longer because the shock tower has moved back.

I ended up with two bodies because one came with the 42167XR kit and one came with the XM upgrade kit. I sent one off to the painter and used the other for a test fit as shown. It leaves very little room for the ESC but it looks like everything will work. One hangup (literally) is that the thumbscrews used to hold down the battery touch the sides of the window which may cause a problem scratching the paint. The wing you see in this photo is not correct, I borrowed it from a DT-02 MS just to see how it looks.

Here are the body and wing as they came from the painter. I failed to heed my own advice and scratched the paint at the front corner of the window when test fitting the body. I was most chagrined. I could black out the windows and put protective tape over them, but the battery screws would still interfere and prevent the body from fitting properly. I will probably just remove the thumbscrews and use standard nuts. I decorated the body with a handful of TRF stickers as shown.

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