2017 - Tamiya DF-03Ra #2 2-Speed

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Months ago I built a HPI Nitro RS4 2 from several used parts-cars I bought for the purpose. One of the parts-cars I bought was an Electric RS4, which I think I was dimly aware of previously, but I'd never bothered to look into them because I'm happy with my Tamiya electric cars. However, in the process of researching the Electric RS4 I'd bought, I found out there was an optional 2-speed transmission for it -- this surprised me, because there was no place to mount a mechanical brake disc on the Electric RS4. I was very curious how it could work without requiring the car to coast to a stop, which would be disastrous for driving anywhere but on a track.

At the time there happened to be a new-old-stock 2-speed transmission for the RS4 MT on eBay, so I bought it purely out of curiosity. Once I got it in my hands, I saw the secret of how it worked -- BOTH gears are mounted on one-way bearings, with the second-gear OWB flipped around so it can apply reverse torque to the transmission input shaft. So it had a normal centrifugal clutch for shifting from 1st to 2nd when accelerating, but when braking the 2nd gear carrier would lock onto the transmission input shaft using its reverse-OWB, to provide braking force from the motor. Mystery solved!

So...now what? I had a cool vintage 2-speed electric transmission, but I didn't want to invest in spare parts for yet another vehicle platform, since the Electric RS4 is different in many ways from the Nitro RS4s I already own. So I scanned over my RC shelf for inspiration. Fitting it to a truck wasn't an option, because even if it was originally specced for the RS4 MT, it was built in an earlier era of RC and modern electric power systems would destroy it. There was no sense trying to install it on any of my Nitro RS4s, because they already have 2-speed transmissions and mechanical brakes. I couldn't put it on my Tamiya TB-04R, because the pinion and spur gear are right next to the center driveshaft and there would be no room for a larger transmission. What about an XV-01? It has a transverse motor that's mounted along the midline of the chassis, so the transmission could hang off the side of the motor-mount plate. No, that wouldn't work, because the transmission fits tightly between the front shocks, so there's no extra room on the sides.

Then my eyes fell on my DF-03Ra rally car. I took it off the shelf to look it over, and it looked like it might possibly have enough room for the 2-speed transmission...unless that weird forward-swept rear axle got in the way. I liked my DF-03Ra the way it was, so I figured I'd have to buy another DF-03Ra kit to experiment with.

...no, wait! I had a second one, already assembled, sitting in a box after many failed experiments to build a mid-rear-motor RWD touring car. I could just use that one! As a bonus, that would save me a bunch of money and actually make this project possible, because I'm still paying-off my cat's $3000 emergency-room visit and bladder surgery from earlier this year. So I dug out my mothballed DF-03Ra #2, installed the front half of the drivetrain, and then began a months-long process of cutting, grinding, gluing, shimming, shopping on eBay for upgrade parts, and sending things to a machinist for modifications more precise than I could achieve at home.

It worked.

Okay, so the body isn't the most exciting part of this build, but I like to show off the bodies on my cars anyway. This one is a Toyota Corolla AE86 Levin, which has a pleasantly-boxy 80's look to it, and it sits quite nicely with the slightly-shortened suspension I installed. I didn't want this car to be a rally car like my other DF-03Ra, so I added a thin down-travel limiter in each shock to lower the car's ride height, and the 6mm-offset wheels and tires sit perfectly in the wheelwells at that height. This is actually one of the most visually-satisfying stances I've managed to achieve with my on-road cars, because most of them are raised a little for driving on poorly-maintained pavement. You might wonder why there's an extra set of body-post holes, and the reason for that is because I had to change the position of the front body-posts to accommodate other upgrades after I initially built the car and painted the body. I thought about filling them with little circles of Lexan, but then I decided I just don't care that much; they're clean and round, not jagged and torn, so they don't look messy, and frankly I was ready for this project to be done.

Aside from giving this car a fun "90's ricer" appearance, the wing was definitely necessary because this car goes faster than a rally car, but it was a challenge to fit because of the "factory-option" trunklid spoiler glued to the body. I ended up having to pry the "factory option" spoiler off and cut slots in it for the wing mounts, then touch-up the paint on the underside of the spoiler and glue it back on after attaching the wing.

Right, now about that 2-speed:

As you can see, I was in fact able to fit the 2-speed to the DF-03Ra gearbox without having to chop the car to bits. In fact, I didn't have to modify a single thing on the DF-03Ra chassis; all of the modifications were made to the 2-speed itself. Getting it to fit required lots of careful measurement and cutting, in particular the cylindrical shims needed to size-up the HPI input shaft from 4mm to 5mm so it could fit the DF-03Ra's transmission gears.

Here's what the modified 2-speed looks like fully-assembled:

...and here's what it looks like fully disassembled:

Like I said, lots of measuring and cutting and shimming.

Unlike most RC 2-speed transmissions, this one actually had bushings to support the gear carriers on the input shaft so the one-way bearings only had to handle the transmission of torque, instead of also supporting the shear-load of the gears pushing against the side of the input shaft. For a little extra durability, I decided to replace the bushings with ball-bearings, and of course the bushings were a special size (4x8x2.3mm instead of 4x8x3mm), but after a few days of searching I managed to find some 4x8x2.5mm bearings that were close enough to work.

If you've owned a DF-03 or DF-03Ra, you know the input gear is press-fit onto the input shaft. Fortunately it isn't press-fit very hard, so I was able to press it off and use it on the 2-speed instead of needing to hunt down aftermarket Mod 0.6 gears to make a new input gear. I carefully drilled, tapped, and de-burred a set-screw hole to anchor the input gear to a matching flat-spot on the HPI input shaft.

The 2-speed transmission wasn't the only thing I did differently on this build. I installed the optional carbon-fiber-and-aluminum shock towers, which give the camber links, shocks, and body posts something stiffer to mount to. I also installed a rear hinge-pin brace from 3Racing, to keep the rear A-arms from flexing too much when accelerating hard.

I normally use Tamiya axles on my Tamiya cars, but the DF-03Ra is a special case, because the forward-swept rear axles chattered like crazy with Tamiya CVDs installed. I tried a set of GPM CVDs instead, which have the dogbone end rotated 90° compared to the CVD end, and they run much more smoothly as a result. (I'm sure there's some math that explains why, but I haven't quite figured it out yet.)

Up front I did more interesting things. I already had aluminum front hubs from Yeah Racing, because the stock hubs were made of VERY flexible plastic, and when I found out Yeah Racing's caster blocks used ball-bearing pivots, I opted to get those too. More stiffness + Less friction = Better steering, after all, which was also the rationale for getting the aluminum steering assembly from RC-Square on both of my DF-03Ra's.

Then, I found myself wishing I could install Tamiya's double-cardan axles on the front, to eliminate chattering when cornering. On a whim I decided to test-fit one, and I found out it was actually a lot closer to fitting than I thought. I needed 2mm of extra stub-axle length sticking out of the hub for it to fit correctly, so I tracked-down some 5x11x3mm bearings for the inboard sides of the front hubs (which was harder than you might think -- it's another very uncommon size), and then sent the hubs to a machinist to have the inboard bearing sockets milled 1mm deeper. That gave me the clearance I needed, and now my DF-03Ra has TRF-spec double-cardan front axles.

The last handling-related modification I made, which you can kinda-sorta see here, was to install titanium turnbuckles on the camber and steering links. The turnbuckles for the steering links needed to be cut shorter, because the next shorter size was too short and the size I had was too long. So that was tedious, but at least it came out well.

Now, what about making the car move? Well, what better way to power a vintage transmission than a vintage motor?

This is another one of those vintage parts I discovered while searching for information about older vehicles. The Tamiya Dyna-Tech 01R motor was originally released in 1989, and it was one of the first rebuildable brushed motors for RC use. In particular, I was intrigued by the brushes, which are attached to pivoting carriers similar to the brushes in disposable motors, instead of the now-universal sliding brushes with braided shunts. I saw one on eBay for a not-too-insane price, so I decided to give it a shot and see how it worked.

Verdict: Not very well. :D At least, not in stock form. It came with a 19-turn armature, which was WAY too slow, and there is an optional 17-turn armature available, which is also WAY too slow. I ended up fitting it with a 12x2-turn armature from Team Orion, and then as you might expect, it ran insanely hot. Clearly I was pushing this motor beyond its limits, but to be fair, part of the reason I got it was because I figured an old outdated motor could benefit from the 2-speed transmission I spent so much time and effort installing; a new motor wouldn't need any help moving something as light as a touring car.

I sent the motor to Holmes Hobbies, because I suspected that the magnets had weakened during the motor's long time in storage, and it turns out...they were just fine. No weakening whatsoever. Okay then, so why *was* it running slower and hotter than the same armature installed in a different motor?

After I got the motor back from Holmes, I gave the commutator a nice mirror-smooth lathe job and took the car out for a drive on a nearby tennis court, which is great for testing car handling because it's smooth, flat, clean, and incredibly grippy. After a single run, the commutator was burned black. Well, there was my answer; the motor was running slower and hotter because the brushes were arcing under high load. But this motor uses a specially-designed leaf-spring to hold the brushed against the commutator, and there are no tuning options available. What could I do?

I could improvise, that's what I could do.

If you look closely at the springs, you can see something that generally isn't used on electric RCs: fuel tubing. I cut four little donuts of fuel tubing and stuffed them between the two sides of each leaf-spring to increase their spring-rate in the most consistent and predictable way I could think of on short notice. It looks kinda cheesy, but it works really well; the car's top speed jumped from 35mph to 39mph, the motor's temperature dropped by 20°F, and the brushes stopped burning the commutator. I call that a win.

So, how does it drive? Quite well, I think. The things I did to stiffen the chassis and suspension helped a lot, the vintage Tamiya motor works well with the vintage HPI 2-speed, and the vintage HPI 2-speed isn't showing any signs of premature wear as a result of being chopped to bits and put back together Frankenstein-style. Hopefully it continues to work well, because I plan to keep it for a long time.

I realized about a month ago that I could improve the way the 2-speed transmission worked if I could just find a way to fit it with clutch shoes instead of the centrifugal-ratchet mechanism it was originally designed to use. As it turns out, the OFNA clutch shoes that I use with my HPI RS4s were a fairly close fit, but slightly too large for the original clutch bell. Still, it was too close for me to ignore the possibility of finding a way to make it work.

So I contacted a machinist who has modified parts for me in the past, and I asked if he could make me a new 2nd-gear clutch bell from scratch. I sent him a photo, he sent me a sketch and asked me to fill-in the critical measurements, and then he turned it into a blueprint. After a couple rounds of revision, the design was set, and he bought the bar-stock to make the clutch bells for me. About a week and a half later I received the clutch bells, and about about a week after that I received some cylindrical shims to make the clutch hub fit on the narrow transmission shaft.

I test-fit all the pieces, resized some cylindrical spacers I had made for the original incarnation of this 2-speed transmission to account for the wider clutch assembly, and figured out how to install the revised transmission in the car. The order of assembly had to be changed because the clutch hub can no longer be removed from the end of the transmission shaft without first disassembling the entire clutch mechanism, but I got it sorted out after a few minutes of experimenting.

It shifts much more smoothly now. I'm contemplating coating the clutch shoes with a dusting of molybdenum di-sulfide powder to smooth the engagement even more, but I'm going to try that on a different vehicle first, with a transmission that is easier to disassemble and clean in case the moly powder makes the clutch shoes slip like crazy. But even if I decide to skip the moly powder, having a smoother-shifting clutch-type transmission in this car means I no longer need to worry about the plastic transmission gears stripping from repeated driveline shock, and that is a worthwhile upgrade in itself.

One last change I made, and this one may be temporary, was to try a different vintage Tamiya motor -- a Dyna-Tech 02H in place of the Dyna-Tech 01R I was using previously.

The Dyna-Tech 02H was built by Johnson instead of Mabuchi, and has several notable design improvements vs. the older Dyna-Tech 01R:

- The brush springs press directly against the brushes, instead of pressing against the carriers and having less leverage to hold the brushes against the comm as a result;

- The brush springs have multiple mounting positions to adjust the pressure on the brushes without needing to replace the springs;

- The ends of the motor are keyed so they are always installed in perfect alignment;

- Commutator timing is adjusted completely independently of the screws that hold the motor together;

- The armature is fitted with a cooling turbine much like a disposable motor.

Even modern rebuildable brushed motors don't have most of these features. It does have some disadvantages, such as the brushes being hard-to-find and requiring complete replacement of the carrier they're built-into, and the bearings being a different size than every other rebuildable brushed motor I've ever seen, but these aren't huge problems. This motor may be better-suited to this car, because the cooling turbine helps the comm stay cool even though the motor is tucked into a little cubbyhole with almost no natural airflow, and this motor was designed from the outset to work well with the optional 10-turn armature I have installed. I had to do a bunch of work to the Dyna-Tech 01R to get it to handle the 12-turn armature I'm running in it without incinerating the comm. This new motor is 1mph slower than the previous one, but the brushes aren't fully broken-in yet, so we'll see if it manages to break 40mph after a few more runs. If not, I can just reinstall the previous motor and find something else to use this one for.

Minor update to this vehicle: I installed an aluminum rear diff to save a few grams, because I'm currently on a weight-saving kick on my touring cars. There isn't an aluminum diff available for this vehicle, but a little trial-and-error and a lucky guess resulted in discovering the Yeah Racing aluminum diff for the TB Evo 3 is a very close match. I had to buy thrust washers for the long-discontinued F201 4WD F1 car, and I had to shave 2mm off one side of the ring gear to keep it from rubbing against the side of the diff housing, but there's still plenty of contacting area to keep the ring gear from stripping under load. (actually I could've shaved less material, but I wanted to maintain the original clearance between the side of the ring gear and the side of the diff housing, just because.)