2025 - Traxxas Jato
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The Jato always appealed to me, ever since I got into this hobby back in 2013. It took me long enough to get one.
For some reason I convinced myself I would have nowhere to drive it. Based on my previous experiences with Traxxas RCs, I anticipated it would be loud as hell, overpowered, and not particularly graceful to control.
I was right about the loud and overpowered part, anyway. In stock form the Jato runs the TRX 3.3 engine, which is...frankly, deafening. The porting on the TRX3.3 prioritizes exhaust blowdown duration rather than exhaust blowdown rate, and while that is an effective strategy for generating a stupid amount of power in a small space, it's also an effective strategy for generating a stupid amount of heat and noise in a small space. I suppose that doesn't bother the boys down in McKinney, but I actually have neighbors. So I put a different engine in. The obvious choice, a nearly direct swap, would've been the OS 21TM, or for a slightly saner driving experience, the older and smaller 18TM, which is what I put in my Traxxas Slayer a few years ago. Naturally I had to make my life harder than it needed to be, so I chose something different.
(Plot twist: This isn't actually a story about a RC truck, it's a story about a RC engine. The truck was the easy part.)
At first glance the OS 12TG doesn't seem like it would make my life harder. It's a noticeably smaller engine than the TRX 3.3 or the OS 21TM and 18TM, yes, but it should fit the same, right? Nope.
You see, when I first bought a 12TG, I didn't buy it for the Jato, I bought it because I needed spare pullstart parts for the 18TM that I put in my Slayer. The 18TM and 21TM have a special pullstart backplate that's a direct fit for Traxxas EZ-Start systems, which is nice, but I prefer pullstarts because they're lighter than starter boxes and they're more reliable than electric starters. (it sucks to get to your driving spot and only then realize your starter battery is weak.) I know lots of people have had the opposite experience with pullstarts; all I can say is they work fine for me and I don't know what other people are doing wrong. Pulling from the shoulder instead of the wrist, perhaps? It's a tiny engine, if you put your back into it like you're starting a lawnmower you're going to rip the pullstart clean off while not even turning-over the engine fast enough for it to catch.
ANYWAY, tangent aside...
I didn't want to use the Traxxas-compatible backplate and the official Traxxas pullstart on my OS 18TM. It feels cheap and I don't like it. After some careful examination, I realized the OS pullstart for the 12TG ought to be able to fit. I couldn't find those parts separately, so I just bought the whole damn 12TG engine. Sure enough, the OS pullstart for the 12TG bolted right onto the 18TM and worked perfectly. That took care of my Slayer build, but now I had most of a 12TG engine kicking around. I could part it out on eBay, but I decided to hang onto it and look for something else I could use it for. As luck would have it, I came across some replacement OS pullstart parts so I could put the 12TG completely back together, and now I just needed a project to use it in. A touring car, perhaps? Eh...I could, but I already have a bunch of nitro touring cars, and building another one just to use a specific engine didn't seem compelling enough.
But wait...what about a summer companion for my HPI Nitro MT2 that I built a decade ago to run in the winter? I always liked the form factor of stadium trucks, but somehow I only owned a single stadium truck, and it was specially tuned for running in below-freezing weather. I didn't want to sacrifice that carefully-dialed-in engine tune. Why not build a summer stadium truck? But are there any good nitro stadium trucks left? I realized this was the justification I needed to finally give a Jato a try -- with a smaller, more polite engine that wouldn't scream like a banshee and echo off every building within a quarter-mile radius. Again, I have neighbors.
So that's that, then, right? Buy a Jato, stuff my freeloading 12TG engine into it, maybe tweak the wheel alignment and drive gears, and be done with it. Right? Right?
Yeah, no, of course not, There's always some kind of snag. See, I had forgotten that Traxxas specs their engines with nonstandard crankshafts, so the first thing I had to do was carefully trim down the 12TG's standard-length crankshaft to fit the restricted mounting space and tiny clutch that Traxxas nitros use.
Okay, done. Got the clutch installed, got the engine mounted, and got on with the build.
The Jato is a really well built truck overall, but there are a couple weak spots. First and foremost, the brakes suck. I don't know why this is a recurring problem with Traxxas RCs that use fiber brake discs and steel "pads" instead of vice-versa, but it is. It's not difficult to make a fiber brake disc and steel "pads" work just as well as a steel disc and fiber pads -- they're literally the same two materials pressing against each other -- but somehow Traxxas RCs with fiber brake discs and steel "pads" always seem to treat braking as a polite request instead of a safety-critical feature that keeps people from getting injured. So that was the first thing I had to fix.
The first thing I did was to drill a new linkage mounting hole closer to the pivot point on the 90-degree brake linkage adapter, to increase the 90-degree adapter's leverage so it would amplify the clamping force applied to the brake disc a bit more. That by itself helped a lot, and to be honest I probably could've called that good enough, especially if I didn't have a nice soft spring on the other end of the brake linage to make brake actuation as smooth as possible. I know there are some people who are content to have a piece of rubber tubing as a brake linkage spring, or just have no spring at all and let the servo yank on the brake linkage directly. For them, the little leverage tweak I made to the 90-degree adapter would be good enough, plus doubling-up the steel brake "pads" to reduce flex. But I like my brakes strong and progressive, so I continued on and installed a second brake disc.
The brake disc carrier is from Hot Racing, and it's sold separately as well as part of a kit that includes two steel brake discs and 3 carbon-fiber brake "pads". I put "pads" in quotes there because they're not really brake pads, they're clamping plates that slowly wear away instead of having dedicated friction surfaces. And CF doesn't make particularly good brake pads anyway. So I opted to just use a second stock fiber brake disc and an extra set of steel clamping plates, because that's actually a legitimate combination of materials for a brake system to be made of. That took care of the brake strength issue. In fact, with the second brake disc installed, I had to dial down the maximum braking force to about 70% just to keep from skidding the rear tires every time I stabbed the brakes. That should soften up a bit as the brakes continue to wear-in.
The other big weak point with the Jato is its wheels. There's nothing wrong with the stock wheels, per se, but for some reason Traxxas put the front wheel bearings inside the front wheels instead of inside the hub, with stub axles for the wheels to bolt onto, like the year is 2005 instead of 2025. And in 2025, finding stadium truck wheels with bearing slots instead of hexes is damn near impossible. I spent weeks trying to figure out a solution to this, until eventually my mind drifted in the direction of the HPI Firestorm stadium truck that passed through my hands a few years ago. On a whim I decided to buy a set of front hubs for the HPI Firestorm and see if I could find a way to make them fit the Jato.
The RC gods smiled on me, and it worked. All I had to do was drill out the lower pivot hole slightly to fit the Jato's wider pivot pin, insert shims as needed, and find a set of narrow wheel hexes to preserve the correct wheel spacing. Hot Racing provided the hexes, and Pro-Line provided the tires, and now my Jato has proper modern wheels and tires.
While I was at it, I decided to take apart the transmission, inspect all the moving parts (what's the joke? "RTR means 'Ready To Rebuild'" or something like that?), and tune the rear diff. I also wanted sealed CVDs since Traxxas is nice enough to offer those for some of their RCs, instead of the plastic sliding axles the Jato came with.
Hot Racing provided the aluminum diff housing, which is a nice thing to have, and of course Traxxas provided the sealed CVDs. That was easy. Okay, all the major problems taken care of. Good work team, pat yourselves on the back and have a good weekend.
- - -
...except...(sigh)..,The end is never really the end, is it?
I wasn't happy with the 12TG's performance. It wasn't bad, but its power limitations relative to the deafening TRX 3.3 were pretty apparent. With the original gearing, the Jato couldn't even hit max RPM in second gear. And its max RPM seemed strangely underwhelming for a little .12 engine. Tiny engines are supposed to be able to rev higher, right? That's what I was banking on originally, being able to run a smaller engine at higher RPM with lower gearing to get comparable performance without the noise and fuel consumption of the bigger engine.
I looked up the specs on the 12TG and saw that its peak RPM was about 32,000. Back when I started this hobby, that kind of number was mind blowing, but after more than a decade, well, its pretty pedestrian. I would expect that kind of speed from a .18 or .21 engine, not a .12. But what I hadn't yet taken into account was that the 12TG is old. It's closing in on 20 years old now. So I took it back out of the Jato, took it apart, and started fiddling with it. The first thing I looked at was the crankshaft. I knew it had a round webbing connecting the crankshaft to the crankpin, but I hadn't fully processed before that the crankshaft was only "balanced" to account for the crankpin on one side, not to account for any of the reciprocating weight of the piston and conrod. No doubt all that extra vibration was wasting a ton of energy at high RPM. So that was the first thing to get fixed. The picture below shows a stock 12TG crankshaft on the left, a fancy racing crankshaft from a 12TZ on the right, and my modified 12TG crankshaft in the middle. I didn't get the balance perfect, but it's a lot better than it used to be.
I also cleaned up the edges of the intake port, because there's no way such sharp edges weren't screwing up breathing at high RPM.
And I did my usual thing of cutting a second notch in the pullstart shaft to balance the notch cut into it at the factory.
And replacing part of the pullstart bushing with a ball bearing to reduce friction and keep the OWB better aligned as the engine ages.
And I replaced the crankshaft bearings with stainless/ceramic bearings with rubber seals, as I usually do, for reduced drag, reduced gumming and corrosion, and better sealing against oil leakage compared to the original steel-shielded non-sealed bearings. I don't have a picture of that, but you know what ball bearings look like.
So that was all fine and good, but did it improve performance any? Yes, it did, but not as much as my next modification:
I raised the upper inner edge of the exhaust port about 1.5mm to increase exhaust blowdown duration and improve breathing at all RPMs, I matched the outer edges of the exhaust port to the shape of the exhaust header on the back of the crankcase to give rebounding intake-air charge a cleaner path back into the cylinder before the exhaust port closes, and I cut Awesome Flanges™ into the upper corners of the exhaust port to increase exhaust blowdown rate more without needing to increase blowdown duration more. That improves breathing at high RPM without killing power at low RPM by shortening the power stroke. None of this is rocket science, but it's worth mentioning.
So that's all very nice in theory, but did it actually do anything?
Yes, yes it does. 😁 The engine now free-revs 13,000rpm higher than factory spec, and revs 10,000rpm higher than factory-spec when it's actually moving the truck. That gives it a top speed of 55mph even with the lower gearing and smaller displacement. It also still produces usable low-RPM power -- more than it used to in fact, enough to spin the rear tires from a stop now -- and does this while remaining pleasant to drive and not requiring a knife-edge tune. I can't take all the credit for this, as the 12TG is basically a very de-tuned 12TZ, and all I did was tune it back up, but...the 12TZ never came with a pullstart, and it cost a lot more too. Just goes to show engine performance depends on design and tuning, not on magical fairy dust.
(I told you this was actually a story about a RC engine, not a RC truck.)
So there, that's my first RC build in about 3 years. It's been a tumultuous time, but I didn't forget what I enjoy in life.
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