Off-road Go-Karting.

Yamaha 125cc dirtbike
Yamaha 125cc circa 1970s

I built my first Go-kart when I was about seventeen. My dad did all the welding but I did all the planning and found all the parts. My friend Jase gave me an old 125cc dirt bike, the frame was bent but the engine was still good. It took long months to save for parts and longer for my dad to put in the time to weld for hours on end.

Eventually we got the thing together, I started it up and lost control almost immediately and ran over my little brother's Ninja-Turtles pedal kart, mangling it. To Do: Buy little brother new pedal kart.

The next test was out in the bush on a disused dirt road with me and 3 or 4 friends. I'd had my doubts about weather a 125cc engine was powerful enough to push the thing but they were soon put to rest as it took off down the track kicking rooster tails of dirt into the air. Steering on the rough surface was a bit of a struggle with the rough approximation of Pitman-arm steering, and the lack of suspension meant coordinating gear shifts required careful planning and dexterity.

That machine didn't last many outings, eventually something in the engine broke and it was put to rest in quiet corner of our yard. The other thing that sucked a bit was that in order to transport the thing I had to scum lifts off my mate Scott who drove a ute and when we'd get there we only had one go-kart so we'd all take turns standing around waiting for our next 15 minute shot of fun.

Sidewinder Buggy
Sidewinder Buggy

A few years later my Brother in law, Rob got the plans to build a Sidewinder and set about building one in his back yard. I got the bug again and scrounged together all the parts from my first kart as well as a 250cc road bike engine. My friend Jase also got in on the action with a 650cc shaft drive engine from a Yamaha XJ650. This time we had three karts in progress at the same time and over a six month period got them all to working condition. Mine was a lot more sophisticated than my first attempt with a full roll cage, disk brakes, pinion steering and full instrument panel. Rob and I borrowed a car trailer and took our karts out for a run one weekend, his ran like a beast but unfortunately mine had some issues, it would intermittently stall under power. I replaced plugs and coils and checked and rechecked the electronics but I never got to the bottom of the problem. Coming from a crashed road bike I assumed something in the electrics was at fault but couldn't afford to get it properly serviced by a bike mechanic and besides the old problem of transporting the bulky kart was making it difficult to take the thing out. Jase and Rob both sold their karts and mine sat in pretty much the same spot Mk1 had when it was put out to pasture.

The short summary of all this would be, yes I like off road go karting and I like building them too but the difficulty of storage and transportation and restrictions on where you can use them kinda put a dampener on the whole thing. Hence years later, when the bug began to nibble away again I set myself a couple of ground rules: One, the kart has to be small enough to transport easily and Two, I don't want to spend months and hundreds of dollars scrounging parts before I can actually ride the thing. My solution to this was the yum-cha built Drift-2 also known by about twenty other names. These things retail for less than AUD$1000 (much less if you do your research) and are more or less ready to go. Also, they're small enough to fit in a 6x4 trailer or a 1-tonne ute tray or with a squeeze even in a station wagon.

Drift-2 Go-Kart
Drift-2 Go-Kart

I took mine out for a first run at an off road riding park about 30km away from my home. The first thing that struck me was the gearing is much too high for the soft sand and rough terrain, the cheap 'n cheerful centrifugal clutched single-speed transmission is clearly a weakness in the design. The 200cc-ish engine seems to have a bit of power but it feels like trying to drive a car off-road while stuck in 4th gear; too much speed and not enough torque. At full clip the roughness of the terrain will definitely give your kidneys a massage and also eject you from the driver's seat on sharp cornering if your not paying attention. Getting it back home, there's a stack of things that have come loose or are wearing in a bad way and need some attention. This is a pretty common complaint of most Chinese-built off road machines, while the products are progressing well in sophistication and performance, there do seem to be some issues with build quality and overall durability. I'm not complaining too much, I bought the kart more as a platform to build from and tinker with rather than a maintenance free option, ...and it was cheap.

So with the rambling history lesson out the way, now you can see why I started looking for affordable ways to improve a major flaw in the Drift-2 design.

see: diy icon DIY simple 2-speed transmission for the full story.

Update: Suzuki GN250 Conversion.

6.5hp clone engine
Bye 6.5hp, hello ~20hp!

A while after the 2-speed trans' project, a friend put his GN250 up for sale, cheap! I couldn't resist and managed to jam the bike into the back of my station wagon before he changed his mind. Being the middle of summer and having plenty of other things to do, it sat under covers for about 6 months. Meanwhile, I drew up rough plans for how to fit the thing onto my tiny kart and nutted out most of the essentials; like where to put the jack-shaft?

The daily temp' dropped from ~35° to 25°C and in a week-long flurry of activity I tore down the bike and my Kart and welded up an engine bracket. I scavenged parts from my 2-speed transmission to use in the jack-shaft (I can always rebuild it with new parts if I want to use it and the 6.5hp engine in something else). I bought another 15-tooth sprocket to match the one on the engine, (so I could use the original parts). Luckily, the GN250's 520-guage sprocket is almost exactly the same as metric #10 (10B-1) except about 2.5mm narrower, so I machined the extra off to make a matching pair. The forward running chain is metric #8 (08B-1) scavenged from my 2-speed box as I had about 3 feet of it left over from that project.

With the engine mount and jack-shaft sorted I moved on to modding the Kart. The first really obvious thing was that the original engine mounting plate and rear bumper-bar had to go. I chopped them off and welded on some rails to hold the new engine bracket. These have about 60mm of adjustment allowing for plenty of chain-link add/removal. In addition I built a spring loaded chain tensioner from part of the original one, this one has a beefier spring and applies a useful amount of pressure to the drive chain via a skateboard wheel.

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Video 1: Gear shift test

With the engine in place lining up the sprockets and fitting the chains was straight forward. I got the jack-shaft made much (100mm+) longer than required, figuring I could always shorten it later or use the extra shaft to fit something useful.

Kart with GN250 Kart with GN250
Photos 1 & 2: View of Kart and close-up drive side - click for details

Kart with GN250 Kart with front shocks mod
Photos 3 & 4: Close-up rear end and (later) front end mod - click for details

Build summary:

- Fabricating an exhaust from the bike's one was a bit of a chore using an arc welder, the end result is functional but ugly and might get swapped out for something pretty down the track?

- I fabricated a basic battery holder and bolted all the electrics onto a plate behind a guard made of sheet metal.

- I used the fuel tank from the 6.5hp engine and mounted it to the roll bar.

- The gear selector swapped sides because it's easier to use two rods instead of one and two cables. The clutch lever is on the stick because it makes sense, see: Video 1.

- I made a quick switch plate to fit the starter switch and essential lights. I put the GN250's handy little gear indicator in for kicks.

- The kill switch is now a toggle and the old momentary push button is now a horn. I kept the horn for emergencies so the driver can "beep for help" when (not if) the kart breaks down in the middle of the scrub.

- There's a piece of 3mm x 30mm strap acting as a diagonal brace between the frame and engine, this should help prevent the mounting rails from bending under the engine's mass, I'm keeping an eye on it to gauge stress.

- The rest is mostly cosmetic stuff such as: a guard behind the seat to reflect exhaust heat, an air filter made out of spray-can lids and a mower filter core. I re-welded the rear bumper bar back on but now beneath the engine rails.

The Kart has put on some weight with this conversion, it's going to be more difficult to transport but I think the extra power and having gears with a "real" clutch will be worth it. It goes out for a test run in a few weeks, I guess we'll see?

Kart shocks comparison
Bye rigid frame, hello shocks!

I already have plans for the next major mod to be on the front end, I'm keen to get some shocks in there somewhere, but I haven't figured out exactly how I'm going to go about it yet.

Update: Suspension Mod.

The front end mod happened (see: diy icon this guide for details). I added some shocks while keeping most of the original parts. The pivot point is close to the centre and the shocks give about 70mm of travel on each wheel. I tested it with 2 paving bricks under one of the front wheels and all 4 wheels were still in contact with the ground, this is about what I was hoping for.

I took it out for a test run recently and it's immediately clear there's a crapload more power and no more centrifugal clutch slip when things get too hard. In fact I lost control at one point and forgot to grab the clutch, the kart bumped into a tree and the back wheels just kept turning until it dug itself in. First gear is almost redundant it's very slow (probably < 10km/h) but it is useful for crawling up steep inclines. Second is better for general cruising and third is great for belting along and drifting corners. 4th and 5th are best used in flat open areas, I haven't GPSed the speeds yet but 3rd feels about the same as the Drift-2's stock ratio with a lot more responsiveness from the increase in power.

The suspension makes it ride like a whole new machine too, now when I hit the rough stuff the kart climbs over it without trying to eject me every time. My kidneys hurt a lot less after an hour on the track. I recommend some sort of suspension mod to anyone with a rigid-framed kart if you're going off-road.

I'm pretty happy with the results but no doubt I'll still find things to change, adding a reversing motor might be a fun project, drop by to see how it pans out.

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