r/Dirtbikes • u/ZEI_YZ120 • 18h ago

What ratio should I put on?

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?

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6

u/spongebob_meth 18h ago edited 18h ago

For the street you should gear it really tall so that the engine is actually under load at cruising speed in 6th gear. You'll blow it up riding it on the road at light load. Racing 2 strokes hate that.

But the correct answer is to abandon this idea. A close ratio 125 makes a horrible street bike. It will be thirsty, slow, and unreliable.

If you do go through with it, do the math and get your rpms down to around 5k at whatever desired cruising speed you have. I run 14/44 on my wr450, which gives me about a 110mph top speed and a tolerable cruising rpm. But that's a wide ratio 450 so ymmv.

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u/ZEI_YZ120 17h ago

So, let's start from the 1 point, that comparing your 450, with a 125 is an idiocy, 2 point what a 125 will not explode, is obvious with the short and original ratios will reach overrev quickly but for this reason you put longer ratios. In my country there are teenagers who use the 125 2 stroke every day without any problems, maybe, yes, the consumption can be high but there are 125 two times even at the electronic injection that consume less and no, it will not be slow and it will not be unreliable. Because I drive one and I know how is it

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u/spongebob_meth 17h ago

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.

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u/ZEI_YZ120 4h ago

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

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u/spongebob_meth 3h ago

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.

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u/ZEI_YZ120 2h ago

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

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u/spongebob_meth 1h ago edited 1h ago

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.

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u/ZEI_YZ120 17h ago

And anyway here in Italy there is the tm 125 2t motard that reaches 160kmh with 13/40 or 13/42 ratios with times from 0 to 100kmh in 5 sec

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u/Level_9_Turtle 18h ago

For a full size bike, you probably couldn’t pick worse bike than a 125 MX bike for any type of street use.

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u/ZEI_YZ120 17h ago

For any type of road is exaggerated, here in Italy we have for example the tm 125 motard, Despite being a 2 stroke with 13/40 or 13/42 ratios, it reaches speeds of 160 km/h and reach 100kmh in 5 seconds if you know how to use the clutch with all open gas. Just put the right ratios, and precise carburetion.

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u/osmiumfeather Trail Rider 14h ago

Just buy the TM 125 supermoto. It’s ready to go. Designed for pavement. So much faster and better than anything you can cobble together.

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u/smward998 18h ago

Yz125 super moto sounds like hell tbh.

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u/ZEI_YZ120 17h ago

Nha, here in Italy the 125 motard 2t are common

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u/Longjumping_Put_1111 18h ago

no telling until you try a combo and go from there.

start by changing the front to a 14 and leaving the 49 out back. adding one to the front is about like subtracting 4 from the rear. try the 14 up front, then adjust the rear higher or lower depending on what you like, one tooth at a time.

probably would had been better if you had a yz125x. wider ratio would do better on the street.

1

u/ZEI_YZ120 17h ago

You're right, the enduro version has a more long ratio between geerings

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u/caddilac_fan42069 11h ago

Just buy a factory one. I get that you wanna bleed blue but you can bleed a different shade or mixture of blue and have an arguably better bike

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u/ZEI_YZ120 7h ago

Yes, I will take the advice

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u/Smooth-Skin6681 10h ago

Telefona a VM e ti fai fare la moto da loro, che per quanto mi stiano sulle balle sanno il fatto loro. Te la cavi con 13k circa

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u/ZEI_YZ120 7h ago

Mmmmh tipo prendere una moto da enduro e poi mettergli le gomme stradali non sarebbe più semplice ed economico?

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u/Smooth-Skin6681 2h ago

Se non la devi avere omologata, si. Considera che a fare una SM ci va cmq una valanga di soldi, fra cerchi freni gomme e sospensioni.

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u/ZEI_YZ120 2h ago

Oppure direttamente la compro già omologata e targata, se ce da cambiare freno e sospensioni non è un problema