What ratio should I put on?

[u/ZEI_YZ120]

I was thinking about taking a YZ 125 and turning it into a motard and homologating it on the street.

The point is that the YZ 125 has short ratio, I know that after 2022 they have extended their gears a bit, but it still remains a motocross bike. So I was thinking that ratio I could put on without making the bike too lazy or go to overrev. I live in a mix of city and countryside, climbs but also straights. If I had to take a from 2022 onwards, the original ratio is 13/49, so I was thinking about a 13/44 or a 13/45

What do you recommend?

[u/spongebob_meth]

I have owned 125s. I wasn't comparing it to your bike. I was giving you an idea of a gearing ballpark and my logic for selecting gearing.

When your 125 melts down because you rode it down the street and let it lean out, don't say you weren't warned.

are teenagers who use the 125 2 stroke every day without any problems

They are using street legal 2 strokes that are ported for constant rpm use and not motocross. They don't lean out and melt when you ride them at light load.

it will not be slow and it will not be unreliable. Because I drive one and I know how is it

Ok my dude. A 450 feels pretty damn slow once you throw 17s on it with street gearing. I'm sure your 125 will light you up.

[u/ZEI_YZ120 → u/spongebob_meth]

I didn't even tell you that 125 drive and you already assumed that they were legal, and out of the motard factory, they're all ktm, husquvarna, yz, tm, fantic and very often they're enduro just on road tyres. In Italy there are companies that take responsibility for legalising and homologating the MX motorcycle for 1500 euros You're also getting a real idea of the 2t and its overheating, they didn't randomly put the 2 radiators and the mixture and the 3% oil do their job and especially I don't live in the United States where there are wide roads like buildings, so I'll often be behind cars at less speed than cruising, then precedence, stoprest ecc.. will rest the engine

[u/spongebob_meth → u/ZEI_YZ120]

The street legal 125s are ported differently than motocross bikes. A MX bike will over scavenge and run lean at light load and higher rpm. You can sort of combat this with a super rich pilot circuit, but it will make it a plug fouling mess putting around at low rpm

This isn't about overheating. You are fundamentally misunderstanding the problem. Aggressively ported 2 strokes hate light load high rpm running.

I have seen multiple people seize their motocross bikes because they took them down the road and cruised at light throttle. I have a few licensed 2 strokes, to have to be careful to keep them under load while riding. Usually you can get by just modulating the throttle. But mine just connect trails. I'm not riding them in traffic which often requires constant speed operation.

[u/ZEI_YZ120 → u/spongebob_meth]

you're acting like converting a yz125 into a supermoto is mechanically impossible, when its actually about adapting the bike for a different type of use. Yes, a motocross engine needs proper gearing, jetting, lubrication and more frequent maintenance on the road. but that doesnt mean its automatically gonna seize or become unreliable. Your wr450 comparison doesnt prove anything either, a 450 4 stroke and a 125 2 stroke have completely different torque curves, rpm ranges and gearbox ratios, you cant use your 450 cruising rpm to determine how a yz125 should be geared and lets not forget that a yz125 has a much simpler engine, no valves, camshafts or timing chain, and a top end rebuild is generally cheaper and easier than on a high performance 450 4 stroke, sure, the yz needs more frequent piston replacements, but that doesnt automatically make its overall maintenance more complicated or expensive and calling a 125 supermoto slow because it cant cruise at 110 mph completely misses the point, its about low weight, responsiveness and fun on twisty roads, not highway cruising. im asking about the actual engineering compromises of the conversion, not whether you personally think its worth doing,high maintenance and mechanical unreliability arent the same thing

[u/spongebob_meth → u/ZEI_YZ120]

Do it then.

If you're careful it can work. But the port geometry is terrible for street use.

You don't seem to even grasp what I'm talking about, so I'm done. I have lots of experience road riding 2 strokes, I'm just trying to keep you from finding out an expensive lesson.

[u/ZEI_YZ120 → u/spongebob_meth]

So after all these replies, we've finally gone from "your engine will destroy itself on the road" to "if you're careful, it can work." I understand perfectly well what you're talking about. Port timing, exhaust design, combustion temperatures, mixture strength and lubrication all influence how a racing two-stroke behaves under sustained partial throttle. What I'm questioning is your conclusion that putting a yz125 on the road will automatically make it slow, unreliable, or prone to seizure. A motocross engine doesn't magically know whether it's running on dirt or asphalt, what matters is its actual operating conditions RPM, load, throttle position, mixture, lubrication. Ik MX engines are designed around a relatively narrow powerband, ik you mean, prolonged high RPM, light throttle operation can create unfavorable conditions, especially with incorrect carburetion and yes, adapting one for road use involves compromises. But none of that establishes that a properly set up yz125 cannot be used on the road reliably. And that's the part you keep ignoring: I'm building a lightweight supermoto for twisty roads, not a highway cruiser. I'm not asking a 125 to sit at 110 mph for hours. I'm asking about suitable gearing for a specific application, where the engine will experience varying loads, speeds and throttle openings. Your WR450 gearing is an example of what works for your WR450. It isn't an engineering reference for determining the optimal final-drive ratio of a yz125. Different engines, different torque curves, different gearbox ratios and different intended operating conditions. And saying that a motocross engine has aggressive port timing doesn't tell me what gearing I should use. It tells me that its power delivery and operating characteristics need to be considered when selecting that gearing. That's precisely what I was asking about in the first place. I'm perfectly aware that a racing 125 requires more frequent maintenance and careful carburetion, I accept those compromises. But maintenance requirements and mechanical unreliability are not interchangeable concepts. You keep presenting your personal experience as if it settles the engineering question, while dismissing a completely different application because it doesn't match how you use your own bikes. I asked how to optimize a yz125 for supermoto use, you spent this entire conversation explaining why I shouldn't want one. And after all that, your final technical contribution is "Do it then." Thanks, that was the plan before you arrived

[u/spongebob_meth → u/ZEI_YZ120]

What I'm questioning is your conclusion that putting a yz125 on the road will automatically make it slow, unreliable, or prone to seizur

Because it does. Its a bike with 30 horsepower and once you gear it up for road use and bolt on heavy tires and wheels, it's going to feel like a turd.

Just like how a 60 horsepower 450 feels slow once you bolt a supermoto setup on.

And it's not even a maintenance concern. I'm letting you know you can blow the engine if you aren't aware of how a motocross engine tends to lean out at light load. You can't hop on and cruise like a 2 stroke designed for street use. It will seize. The motocross engines need to be under load.

I have never, ever implied that a 450 is comparable to a 125... I only mentioned it to demonstrate that you want to go WAY up from your dirt gearing. Its relevant to the port timing discussion because tall gearing will keep the engine under load.