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Build Complete: Tamiya FF-03R

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  • 3 Post By fyrstormer
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Build Complete: Tamiya FF-03R

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Old 03-13-2018, 10:52 PM
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Default Build Complete: Tamiya FF-03R

So continues my long line of Tamiya electric touring cars.



I'm not entirely sure why, to be honest; I think it's just that Tamiya likes to experiment with off-the-wall ideas, and that, combined with their highly-interchangeable parts, appeals to my inner Lego maniac. (I don't actually play with Legos anymore, they're not mechanically capable enough for me, but I still enjoy "Lego-ing" parts to see what I can build.)

So anyway, here's my latest car, a Tamiya FF-03R in short-wheelbase configuration.



My first touring car was FWD, a FF-04 in fact. I didn't like it, and I ended up cannibalizing it to upgrade a XV-01 I bought later on. But the FF-03, despite being older, has some features I like better: The motor is mounted lower and further forward for lower center-of-gravity and more traction on the front tires when accelerating, the battery is center-mounted instead of side-mounted so the chassis doesn't need a bunch of ballast added to one side to balance it, and the chassis has walls around the battery compartment so I don't have to use fiberglass tape to strap the battery in-place before each run. The adjustable wheelbase made it a little more interesting, because I'd never owned a SWB touring car before. As you can see in the pictures above, I ended up putting a HPI Peugeot 106 SWB body shell on it, which wasn't a perfect match for the FF-03's shortest wheelbase, but after enlarging the wheelwells a bit you can't really tell. (it was a good idea anyway, because it sucks when the tires eat the corners of the body shell.)



I put this together with my standard complement of electronics: A brushed ESC, a Hitec low-profile digital servo, and a Spektrum receiver. The ESC is different than my usual choice, though. I tried my normal Dynamite DYNS2210 ESC, but I discovered the braking on that ESC is preset at 50% when it's operating in F/B or F/B/R mode, and that wouldn't work with this car because I'm running a really high drive ratio. So I decided to try the HobbyWing WP1080 that someone had recommended to me on a different forum about a year ago, because this ESC has a much wider range of adjustments. It was preconfigured for rock-crawler use, but after I disabled the drag-brake and freewheeling options (I'm still not sure what freewheeling is supposed to do, honestly, it seems like just a weaker version of drag-brake), and after I cranked up the braking force to 100%, it proved it could get the job done. I also got instructions from HobbyWing for how I can replace the pushbutton switch with a mechanical switch, so I think I'll be using this ESC for all my brushed-motor needs from now on. Good recommendation, Kingy, wherever you are.



My FF-04 used a 12-turn brushed motor, and that had way too much power for a FWD touring car. I wasn't as good at selecting tires for touring cars back then, but even with the stickiest foam tires I could find it was still spinning the wheels like crazy. This time around I used a 27-turn rock crawler motor from Holmes Hobbies. The endbell fan was an unexpected addition after-the-fact, because the motor got a LOT hotter than I was expecting -- well above 180F when I peeked into the comm chamber using my IR thermometer. That can be attributed to the high drive ratio I mentioned previously, and I simply failed to appreciate that a LiPo pack could shove quite so many amps through even a 27-turn motor -- but that's why we love LiPo packs, right? A stiffer set of brush springs and an endbell fan work together to keep the comm well below 140F, even with a quiet little fan that provides a light breeze. (air is bad at absorbing heat, so there isn't much extra cooling to be gained by blowing tons of air across a hot surface, because the air doesn't have time to absorb any heat before it's blown away by the fan.)

I suppose it's also worth mentioning that, since this is the "R" version of the FF-03 chassis, it came with a gear diff, and I fitted that with steel internal gears, lightweight outdrives, and an aluminum side cover, all from the TA-06, and better-sealing orange O-rings from the TRF419. I filled it with a mix of 30k and 100k oil, achieving somewhere in the vicinity of 65k viscosity, which seems to be just-right -- very stiff, but still allows enough outdrive movement to ensure the car doesn't pull to one side or the other as the outdrives wear-down over time. (I had that problem with my XV-01FF FWD rally car back when I ran a spool in it, and I eventually switched to a gear diff in that car for the same reason.)

The suspension is set-up with 60wt oil in the front and 30wt oil in the rear, and yellow-stripe medium springs all around. I started with blue-stripe firm springs in the front and red-stripe soft springs in the rear, because that's what my XV-01FF needs to drive properly, but I quickly discovered that setup causes massive understeer in the FF-03. I had forgotten to account for the significantly stiffer on-road springs that the FF-03 uses, plus its significantly shorter suspension travel, and its reduced need to handle large bumps without bottoming-out. The cardinal rule of front-wheel drive is "Keep the front wheels firmly anchored to the ground at all costs", and the softer springs in front do that. The stiffer springs in the rear make it possible to swing the rear-end around more easily, and oversteering a FWD car is a kind of fun that never gets old.



So, about that high drive ratio. Since I was planning to use a rock-crawler motor in this car because it doesn't need a powerful motor, I decided to set myself a little challenge to see how fast I could make the car go with that rock-crawler motor. I said before that I enjoy Lego-ing parts to see what I can build, and for this car I installed a slipper clutch from a Tamiya DN-01 to let me fine-tune the maximum torque delivered to the wheels. (believe it or not, it still spins the wheels like crazy without a slipper clutch.) Then, to get the highest gear ratio I could, I sent the slipper plates to a machinist to be trimmed-down to fit the spur gear from a Traxxas Mini E-Revo. (don't laugh.) The result is a 1:1 ratio between the pinion and spur gears, for a 2.6:1 FDR. I had to experiment with different slipper-pad materials to find one that would work well; I initially tried the stock Traxxas slipper pad, but it has metal bits in it and it's too abrasive for a hard-anodized aluminum slipper plate. Next I tried Teflon, but that wore-down too fast with the constant slipping it had to deal with. Finally I settled on fiber-sheet gasket material from the auto parts store, which provides just enough friction without damaging the slipper plate or wearing-down too fast.

I also had to experiment with the thrust bearing installed in the center of the MERV spur gear; I always thought Traxxas' use of a normal radial bearing was a bad idea, and that proved to be correct when the radial bearing failed after a couple runs of near-constant operation with the huge lateral load applied by the pressure spring. I couldn't find a proper thrust bearing in the correct size, so I went old-school and bought some Oilite bushings to do the job. To make matters slightly more complicated, the slipper shaft has an English diameter of 3/16", while the bearing socket on the spur gear has a Metric diameter of 8mm. Luckily, an English bushing with a 3/16" inner diameter also has an outer diameter of 5/16", which equals 7.94mm, and the remaining .06mm is easily accounted-for with a drop of superglue to hold the bushing in the socket.

So, why did I use a Mini E-Revo spur gear? Surely there are other gears available in a 50-tooth size. Well yes, there are -- ServoCity.com sells one, for example, and I could've fitted the screw-holes in that spur gear with Teflon friction pegs from the Traxxas T-Maxx as I did for my XV-01FF FWD rally car -- but there are a couple reasons why that wouldn't work here. First of all, in my XV-01FF FWD rally car, I was able to fit 16 Teflon friction pegs to carry the load, whereas a tiny 50t slipper gear would only have room for 6 friction pegs, so the slipper clutch would be constantly losing its adjustment as the pegs wore-down quickly. And even if I decided to skip the friction pegs and just use the plastic material of the gear itself as a friction material, I'd still have to contend with the fragile nature of Delrin. See, the 50t pinion gear I'm running is way too big to fit inside the FF-03's gear cover, so that meant the pinion and spur would have to remain exposed to grit kicked up off the pavement. Delrin is a very stiff plastic, which mean you can machine it the same as metal for very precise tolerances, but it's also very brittle, and every exposed Delrin gear I've ever run has suffered from broken teeth in a short period of time. The MERV's spur gear is made of nylon, which is much more resistant to damage caused by grit getting sucked through the gear mesh, so it was a better choice for this project.



So what did all that tinkering get me? The last time I checked, on 2S LiPo, with a rock-crawler motor clocked to 24 degrees of endbell timing, and rolling on tiny 1.9" touring-car tires, this car will go 40mph. It tickles me to see such a weird combination of parts going that fast. It drives better than my FF-04 did, too.
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Old 03-14-2018, 12:26 AM
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That's a really clean build.

Why not do current limiter instead of the slipper?
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Old 03-14-2018, 03:05 AM
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Because wheelspin is caused by excess torque, not excess current flow. Limiting current flow is only a partial solution to limiting torque. A fast-spinning electric motor is less efficient at producing magnetic flux than a slow-spinning electric motor, so as an electric motor accelerates, it requires more wattage to maintain the same rate of acceleration. If I set a current limit that prevents wheelspin on takeoff, the car's top speed will drop. On the other hand, if I set a current limit that doesn't affect the car's top speed, it will still be able to spin the wheels on takeoff, thus defeating the point of having a current limiter at all.

At present, I have the slipper clutch tuned so it allows a little wheelspin on takeoff, but not a lot. I can hear both the whistling sound of the clutch slipping as well as the scuffing sound of the tires spinning against the pavement. I haven't seen any electronic limiter that can achieve such a fine balance.
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Old 03-14-2018, 05:22 AM
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Originally Posted by fyrstormer View Post
Because wheelspin is caused by excess torque, not excess current flow.
But in electric motor, torque is equal to current, isn't it? So limiting current will limit torque.

About max speed issue: Years ago, I've seen lightbulb used as current limiter in some device. It was neat idea, because bulb wire has low resistance at low current, but high resistance when it heats up by higher current. Since electric motor draw less curent when it's getting to max RPM, it could work well.
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Old 03-14-2018, 12:08 PM
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Torque is *roughly* equivalent to current, but in the real world there are inefficiencies that increase as the motor accelerates. My concern is that an electronic current limiter will not be as precise as a slipper clutch. Granted, slipper clutches sometimes seize, but I've had pretty good luck selecting friction pad materials that don't seize. Also, I can adjust the slipper clutch using a wrench in the middle of a run, instead of having to connect the ESC to a computer.

Using a lightbulb is a pretty neat trick though, I have to admit.
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Old 03-14-2018, 12:19 PM
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Brilliant build, love the slipper idea.. I've been pondering similar.
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