MOPED ARMY

Chapter 2 - Moving Away, Planning, and Starting

After chopping it all up, it did get a nice layer of phosphoric acid (Prep-n-Etch), because I knew I would be moving soon, and probably wouldn't work on it for a while. I was right about that.

I moved to Colorado in August 2015 to live with my now-wife. This meant the "mopeds are job one" life would be ending, and compromise would be introduced (bye No-nos! Thanks Kim and Sam and Brad and everyone else for helping me get prepped and moved).

Basically, things sat for almost another year, but once settled in I came up with a sketch for the build, literally:

I then moved on with the initial fab work. First step was to build some new motor mounts. By this point I had ordered wheels, sprocket, axles, brakes, spacers, etc., but it was kinda tricky to confidently get everything placed and spaced, when I had neither a mounted engine for spacing the sprocket on the axle, nor an assembled wheel/axle/brake/sprocket to space the engine. It was highly iterative, with lots of dimensional stack-up to calculate the correct spacings on both wheel-side and engine-side (I wanted better than just eyeballing it).

If you notice in the before/after photos of the chopped up frame, the big vertical motor mount plates were cut back quite a bit - this was in an effort to get the output sprocket as close to the swingarm pivot as possible (vertically and longitudinally) to ensure the best chain geometry.

Anyway, the motor mounts ended up being sorta simple, 1/8" bar stock and like 1/2" x 1" rectangular tubing from Home Depot, some doubled up to get the right spacing:

Chapter 4 - The Engine

So, my searching for fast 50cc motors, at least those readily available in the US, led me to kids' dirt bikes. KTM 50 SX Pro Sr, circa 2001-2006 or so, seemed to be the best compromise. Parts are plentiful enough, and their clutches were frustrating enough to keep prices for motors somewhat low. The newer generation(s) of the KTM 50 have better cylinders and clutches, but they cost more, at least at the time. And the version I ended up with certainly ain't bad! Hard numbers are hard to come by, but seems to be something like 14,000rpm, 10hp, 9000rpm clutch out of the box. Not bad for a single-speed!

Of course, I am trying to nudge the limits of 50cc single-speed, so I dove into the beginnings of a plan to extract more power. More power would necessitate a better clutch, so I ended up with a Tomar Hole Shot. Apparently, much less frustrating than the stock 3-shoe, and a quality item. Of course, the real tough part of single-speed, well, speed, is balancing clutch meltdown/explosion with acceleration with top speed with durability. Pick two, maybe three of those. I'm sure many, many of you know too well.

But to start, I just wanted to see what kind of power could be extracted from the stock cylinder. It has pretty good ports, better than pretty much all moped kits, with a 3-port exhaust and 7 transfer windows (4 main, with the boost port broken into three interconnected windows). The main downside is the rather sharp bend in the main transfers, and less-than-ideal reentry of the auxiliary exhaust ports back into the main:

I used these port molds to start modeling the engine for simulation in EngMod2T, a fantastic software. Some of the world's most accomplished 2-stroke tuners use it and it helps them push the envelope in their classes - think 50hp from a 125cc kart engine - they have found good correlation between the simulation and the dyno. Like any mathematical modeling, there are significant assumptions baked in (especially because EngMod is a 1-D simulation), but if you're in the ballpark, one of the most powerful aspects of simulation isn't the raw numbers (50hp from a computer doesn't mean anything), but the ability to compare design/tuning options back-to-back. That said, as with most things, garbage in = garbage out.

So at the time (May 2016) I explored some options - given a need for high clutch stall, should I go for a crazy high-RPM setup, or closer to the stock motor? If I'm going for raw performance, durability be damned, high rpm setup (exhaust duration increased about 20deg, transfers about 10, for a total timing of something like 205/130, with pipe to match) makes much more power under the curve:

As for progress, that was about as far as I got back then. For the next 3+ years, everything sat on shelves.

More updates coming, will just take some time to get them written up :)

Chapter 6 - Exhaust Possibilities - Modified Stock-ish Pipe

So the longer-term plan is to build a pipe from scratch, using EngMod to get a good initial design. As you may have seen in other threads recently, I'm thinking of a couple accessories to add to it, but starting out, a good old fashioned expansion chamber. But, I don't feel like tin-snipping the pieces for it, having done that before. I plan to get it laser cut to save time and hassle, and end up with a better result, but that means I need to be pretty certain of the dimensions first - not quite there yet. Thankfully it will be pretty cheap, like $30 or so, for the laser cut pieces. So no biggie if the first design isn't perfect.

In the meantime, I wanted to throw a stock pipe on there from the Pro Sr. You can get new pipes from FMF or Pro Circuit I think, but I was pretty sure I'd be hacking into it to change the shape regardless, so didn't want to drop $200+ for that. Stock replacement was about the same, but then a used aftermarket pipe came up that I'd never heard of - from "Doma." Decent price for it, so I got it, and spent some time reconfiguring it to fit. Here's how it came to me, looking super ready for TIG welding on the header...

...and after hacking and rotating in three spots. Kinda sucked to get it clean enough for TIG:

And here it is mounted up, with some other stuff done in the meantime. Experimented with some TIG brazing here, where the stinger attaches (another cut and rotate), and where the muffler goes on. For now, muffler is not removeable, because I can't repack the thing anyway, refuses to separate. There are some other updates shown in this photo, but more on them later:

Chapter 7 - Seat Mounts

As you might have noticed, I got a fiberglass seat from Treats. These have no mounting provisions, so I added some aluminum bar stock to span across the bottom of the seat in the rear and fiberglassed it into place. Also added a couple c-shaped guys in the front to engage into some posts that live on the frame. No good pictures of those, sorry. The front things didn't end up being needed, but they at least align things for assembly.

As for the frame, I used some angle to make the front mount for the seat:

The rear used bar stock, but the section of frame it mounted to was actually a little out of alignment. Had to shim it up on the right side, which is why the weld looks like it was wandering - it actually just had to span a gap on the right side:

Chapter 8 Pt. 1 - Footrests - Upper Mount Pt. 1

So the frame had zero provisions for footrests, since the stock Sachs motor had built-in pedals. Or if I'm wrong about that, they got chopped off in 2015. So I used some bar stock to start building a sword, then turned it into the upper mount for the footrests. Started with this empty space:

Made the sword:

All the pieces for the upper mount:

Chapter 8 Pt. 3 - Footrests - Footrest Itself

Rather than moving on to the bottom mounts on the frame, I did the footrest itself. It's nice when parts jig themselves up, everything ends up fitting better. So the idea here was to cut and assemble the pieces for the footrest using the completed upper mount as a guide. Then once the footrest was finished, use it to build the lower mounts in exactly the right spots on the frame.

The other thing about this design is that I wanted something removable and interchangeable. I'm not entirely sure what kind of riding this bike will be used for, and thus the ideal riding position isn't easy to define. So I can pop this piece out and build something different later on if needed.

Chapter 9 - Small Things and First Test Ride

With the footrests done, there were plenty of other little things that got worked on to get it ready for a first ride. Stuff like adding an anti-rotation tab for the rear brake plate, as well as a cable mount/stop onto the brake plate itself. Nothing fancy, don't think I got any pictures of that.

Got the throttle mounted to the bars, realized I needed better spaces for the cheapo clip-ons from Treats. The General fork tubes are 26mm, and even though the clip-ons were advertised for that size, they kinda bottomed out, even with the spacers included in the kit. I cut up some thicker round tube stock to copy the spacers, much better. Might still need some sort of anti-slip compound, but they're okay for the time being.

Got the petcock, fuel lines, etc., all hooked up, air filter mounted, rear brake cable installed and adjusted, all that last little stuff. It fired up on the second kick!

Here's a shot from the first test run:

No good video from that day, but the stock Pro Sr clutch was slipping at what sounded/felt like 8000rpm. Engine simulations showed a huge dead spot there, and the acceleration matched the sims. The road I was testing gets zero traffic, but it's short, so for a while never got out of the clutch slip while I messed with the needle.

I finally managed to get it above the 25mph or so for the clutch to engage by going around the block a bit, and when it finally climbed out of that dead spot and came onto the pipe, holy cow! It pulls HARD. At least as hard as my 65mph 50cc Peugeot, which has the clutch and variator absolutely dialed for maximum acceleration.

That Pro Sr. clutch setup, though, won't work at all for Colorado Springs. Plenty of 10% or greater grades in the area, plus 20% less oxygen than at sea level (means 20% less power), it simply won't climb them much faster than walking speed, slipping the clutch the whole time.

Two main options to deal with this: engage the clutch at a much lower speed, like 4000rpm. That's somewhere around 13-15mph with my current gearing (16:57 primary with 10:60 sprockets, geared for about 50mph at 15,000rpm). The sims show a dip in the torque at 8k, so at lower engine speed there is more torque, so easier to accelerate up to clutch bite. At that point, I may still struggle to blast up hills, but if I can climb into that dead spot at 8k, I'd at least be doing ~25mph which is the speed limit on most of those start-from-a-stop hills anyway. If I encounter a faster hill on a bigger road, no problem, I'm already on the pipe and can blast up it without issue. Not a terrible option, and can be improved immensely with something like ATAC or a trombone pipe (future projects once the bike is more complete). It's the "reliable" route, and conveniently the clutch springs from the KTM 50 SX Mini Adventure (significantly down-tuned version) swap right into the stock clutch, and should be set up for 4k-5k operation.

The other option is to raise the slip speed - I have a Tomar clutch on hand for this, much better than the stock three-shoe. It's set up for about 9500 out of the box, and has a good amount of easy adjustability (springs can be tensioned without removing the clutch, and options for spring size, too). By 9500, the engine should be making good torque again, making it no trouble to blast up hills (albeit loudly). Butt dyno on that first test ride tells me there's not a hill in the area that can stop me with this setup. But really tough on the clutch.

Here's a crappy flyby video, but you can at least hear it come onto the pipe:

Great work, man. Beautiful build!

I thought I was gonna hate this but it’s really well done! Keep workin on it and dial that thing in. Great work.

Ha thanks guys. I did this totally expecting some people not to like it, but hey, that wasn't on my list of goals ;)

So far, I feel like I've at least gotten significantly better with my angle grinder, finally figured out how to get the most out of the $20 crappy Harbor Freight vertical belt sander thing I found on clearance with missing pieces, and learned that aluminum can be a joy to work with... Oh yeah, starting to master straight-ish cuts with the hacksaw.

Chapter 11 - Gauges, Part 1

Since a big aim of this build is to get a whole lot of performance out of a 50cc single-speed, and I'm no master tuner, really it means the goal is for me to learn by doing. My day job involves instrumentation, testing, and data processing/analysis (on freight railcars), and it's something I've always been drawn to, so definitely applying it to this build. At this point in my 2-stroke tuning knowledge, I can usually get a carb in the ballpark to make decent power and not blow up. The only failure I've had was due to a damn clamp-mount VM18 constantly breaking its neck on my Peugeot (creating massive air leak, but enough flow through the carb to still keep it running! Snapped the crank at like 12k RPM due to seizing). That said, I have tons of room for improvement, and getting numbers helps me progress with my understanding of things.

So, this bike will have a Trail Tech Vapor (wheel speed, engine speed, coolant temp), EGT, and wideband O2. I know the wideband is not always going to give a real view of the situation, but it should be a sort-of-useful device while I'm on the pipe, at least. Eventually I hope to build a very simple brake dyno (perhaps literally with a brake assembly from a car or something), but in the meantime I can get some power numbers from the accelerometer and/or GPS in a GoPro. More on that later.

To start, I needed a place to mount the gauges. I'm using clip-ons, so the four threaded holes on the fork's top plate come in handy here. The Vapor was pretty easy:

Chapter 13 - Wiring, Part 1

Since the frame got stripped bare, and there was really no point in trying to hack up the stock General harness, and the KTM has only two wires (ground for the coil/TCI?/CDI? magic box, and power/trigger from the stator), I figured I would start from scratch and make exactly the harness I need. Goals here:

Practice with planning ahead, instead of slapping stuff together because it's a decent workout for the brain, and a good habit that mopeds have a way of making you want to avoid (for me, at least).

Ease of use because I don't plan to run the wideband or EGT all the time. Disconnecting and removing them should be quick and easy. And I expect the harness will see a few full removals and reinstalls for things like paint, more fabrication work, etc., so I want to be able to pull the whole thing out of the bike quickly without having to cut any wires.

No bullet or spade connectors, except where needed, like on the ignition box. I want weatherproof, easy to disconnect, durable. I chose Weatherpack here, because I had a kit on hand, and they're cheap and readily available. There's better stuff out there for sure, but for now this will work. Not a big deal to upgrade to better connectors later if needed.

Practice building a quality harness, one that either looks good or looks (mostly) invisible, or both. Added durability goes hand-in-hand with this. So I'll be using some sort of sleeving for it, which also forces me to plan ahead (see first goal).

First step, then, is to plan! Did the best I could here (using some electronics CAD here would have been a good idea, but whatever), including adding rough splice locations ahead of time. The electrics on this bike use two relays (one for gauges, one for lights), 5 switches (keyed ignition, kill button, lights, high beam, and gauges), and quite a few connectors, maybe 18 or so, to disconnect the various bits:

Used that to start laying out the wire on the bike. This part took a ton of time, since I wasn't entirely sure where everything was going to live. This is also a good point to mention that this is set up to not rely on any power generation - this bike won't be used for hours of riding on end, and with something like a 5000mAh battery, the headlight (30W, supposedly, 45W high beam, LED all around) should last a couple hours. Taillight is the Treats vertical fixation, so not much power draw there. Wideband uses something like 3A since it has to heat the sensor, but that won't be used too often once the bike is tuned.

Here's most of it layed out on the bench. A few of the splices got a little rearranged, but for the most part the initial sketch of the layout was pretty close to what I ended up with:

(edited)

Chapter 15 - Wiring, Part 3

In case you wonder, here's what everything goes to: