Nobody has successfully brought a modern rotary powered motocross bike to market.
So here I am, doing God's work.

When looking for my first car my parents stopped me from buying a V8 or turbo because they were "too powerful." I did my research and found that old Mazda rotaries were getting great quarter mile times — even better than the V8s I was looking at. I kept this to myself though and told my parents I had decided on an old Mazda RX4 with only a 1.3 litre engine. Naturally they were stoked.

What they didn't know was that the lightweight build and the extra firing events per revolution made that car punch well above its weight.
I have been hooked on rotaries ever since. The high power to weight, the lumpy idle and unique sound, the flames and pops on deceleration. These things are fun. Combining them with a dirt bike chassis would be bringing two worlds of fun together — and I've wanted to do it for a long time.

For years though, I put it off. Building a rotary engine from scratch at motocross spec is a serious engineering undertaking. Most available rotary knowledge centres on the Mazda 13B — not much exists for something the size of a motocross engine. Building a chassis from scratch to anywhere near current MX standards would take a world class engineering team and a budget that doesn't fit around a 9-5 and a family.

The idea sat on the shelf until something clicked at work.After years in an engineering firm I noticed that next to nothing is ever truly built from scratch. Every new project starts with components borrowed from previous ones — a mash of existing solutions until eventually a working concept is found. That realisation changed how I looked at this build entirely. Take the best available rotary engine on the market. Adapt it to one of the best chassis on the market. Prove the concept. Make the tweaks from there.That's what EccentricMX is.

The Build
I'm adapting an Aixro XR50 294cc Wankel to a 2025 Yamaha YZ450F using a custom primary drive housing — layout inspired by the Crighton CR700W. Custom spur gear primary train from the eccentric shaft through an idler to the YZF clutch basket, all contained in a billet housing designed from 3D scan data of both engines.The YZF gearbox, clutch, flywheel and stator are retained where possible.

The Aixro has a strong reputation in karting and UAV applications — compact, proven, reliable and awesome power to weight.

The YZ450F has arguably the best suspension setup in production motocross, a gearbox built for solid power, and an engine orientation that suits the rotary intake and exhaust layout better than the others. On paper it's the ultimate pairing.

The Aixro is a development engine. Long term the goal is manufacturing my own rotors and housings — both to hit the power I'm after and to make a future conversion kit commercially viable. These Aixros aren't cheap and buying new individual parts would blow the budget.

Who I Am
Mechanical designer by trade. Decades of building two strokes, 4WDs, rotaries and cars in the shed. Built a 13B Bridgeport turbo. Designed and had manufactured custom dirt bike components through a 5-axis CNC shop in China. Comfortable with the CAD to machined part process.

The goal here isn't a garage ornament. It's a rotary powered motocross bike that can be ridden, raced and abused like any modern dirt bike.

The Challenges
There are real engineering problems to solve. The ones I'm most focused on right now:Mating the rotary to the YZF gearbox cleanly without butchering the chassis.

Cooling — the Aixro is designed for a kart with natural load variation and cooling opportunity. A MX track at wide open throttle is a different environment entirely. And dust management — rotaries are sensitive to particulate ingestion in a way a four stroke simply isn't.
The filtration solution needs to be genuinely better than stock.There are more challenges beyond those but they're each worth a post of their own as the build progresses.

Where I'm At
Two XR50s acquired — one new, one used. A blown 2025 YZF donor on the bench, which was a basket case when I got it. Spent time with factory manuals getting it back to a known state.
3D scanning of the YZF cases is done. Converting point cloud scans to 3D CAD models is next, then arranging everything before the housing design begins.
Primary ratio calculations are complete — targeting 2.6 to 2.7 to keep the rotary's power curve in the right working range of the YZF gearbox.
I'll post CAD progress, scan data, machined parts when they arrive, and eventually a running bike. Based in the Perth hills in Western Australia so test tracks won't be an issue once complete.

Happy to answer questions as it progresses.One specific area I'm researching right now — running coolant through the eccentric shaft and sealing solutions for a rotating shaft coolant feed. Looking at CNC spindle coolant sealing as a possible approach. If anyone here has relevant experience I'd genuinely like to hear it.

Brad
EccentricMX


The Donor YZ450F as I bought it.


Blown up yzf engine components. The piston let go but the gearbox was luckily unscathed


Aixro XR50


Disassembling the Aixro for 3D scans




294cc rotor



Rough sketch that I have since dimensioned up. I'll use a mix of 3D scans and measurements


YZF engine case scan



Aixro rotor sitting loose in the yzf case for scale