# Next 2 stroke build idea.

Even costs that much on alibaba?
Depends on the seller and shipping. The cheapest I have found is \$310. Most are around 380-420.

Based on the pictures from this listing the ktm 50 has a 16 tooth on the driving gear and a 56 on the driven gear. (To double check me, count it up.)

(56÷16)crank to countershaft reduction, times the sprockets (44÷9) gives a reduction of 17.11

Assuming this ratio, with a redline of 10k rpm, and a 700x42 tire we get a 48mph top speed. Way too fast! With the stock 7hp, a 7 mile wind up and a UCI banned super tuck you might be able to get that.

Swap out to a 56 tooth sprocket? We get a final reduction of 21.78 and that brings the top speed down to 38mph, still a little quick for a bike, but more manageable. Still bogged down until you're at 30mph though.

Let's say a s3x is used as a jackshaft: 3.5 internal reduction on the engine, 22÷9 from the engine to the gear box and a 56÷22 from the gearbox to the wheel. The s3x has three ratios: direct, .75 and a .625

We get an 21.78 in direct drive and a 29.04 in the middle ratio and a reduction of 34.84 in the lowest ratio. At 10k rpm that would be the 38mph, 28 mph and a 23 mph top speed.

However: torque to the rear wheel is a more useful measurement. Assuming 8hp at 10k rpm, we can work backwards to get 4.2 ft-lbs or torque, multiply this torque by the given ratios, then divide by the radius of the wheel and we get(roughly) 78 ft lbs in high gear, 104 in medium, and 125 in low.

Bump those numbers down by 15 percent for drive train losses (An IGH loses about 10 percent of power to friction and flinging grease/oil around) and remember that's only at 10 thousand rpm, but it does show why gear boxes are cool, but also why this engine doesn't really need one.

Links to eBay may include affiliate code. If you click on an eBay link and make a purchase, this forum may earn a small commission.
Based on the pictures from this listing the ktm 50 has a 16 tooth on the driving gear and a 56 on the driven gear. (To double check me, count it up.)

View attachment 211122

(56÷16)crank to countershaft reduction, times the sprockets (44÷9) gives a reduction of 17.11

Assuming this ratio, with a redline of 10k rpm, and a 700x42 tire we get a 48mph top speed. Way too fast! With the stock 7hp, a 7 mile wind up and a UCI banned super tuck you might be able to get that.
View attachment 211123

Swap out to a 56 tooth sprocket? We get a final reduction of 21.78 and that brings the top speed down to 38mph, still a little quick for a bike, but more manageable. Still bogged down until you're at 30mph though.

Let's say a s3x is used as a jackshaft: 3.5 internal reduction on the engine, 22÷9 from the engine to the gear box and a 56÷22 from the gearbox to the wheel. The s3x has three ratios: direct, .75 and a .625

We get an 21.78 in direct drive and a 29.04 in the middle ratio and a reduction of 34.84 in the lowest ratio. At 10k rpm that would be the 38mph, 28 mph and a 23 mph top speed.

However: torque to the rear wheel is a more useful measurement. Assuming 8hp at 10k rpm, we can work backwards to get 4.2 ft-lbs or torque, multiply this torque by the given ratios, then divide by the radius of the wheel and we get(roughly) 78 ft lbs in high gear, 104 in medium, and 125 in low.

Bump those numbers down by 15 percent for drive train losses (An IGH loses about 10 percent of power to friction and flinging grease/oil around) and remember that's only at 10 thousand rpm, but it does show why gear boxes are cool, but also why this engine doesn't really need one.
For starters, 38mph isn't really "way too fast." Hell, my phantom can do every bit of 50mph. I regularly cruise along at 35mph on it without breaking a sweat. A Phantom will also happily rev out to 10,000rpm, with the power peak being 9500. The 50SX engines can go well past 10k. I've seen them at 12-14k, but most of the clone engines peak at 9500-10000, just like the Phantom. The difference is, they carry more power past peak, usually giving a useable power curve up to 12,000. They are also rated at 6.5kw, which is 8.7hp. My phantom makes less peak power, but way more torque, and at much MUCH lower RPM

I have already done all the math on this too.

56 to 16 is a 3.5:1 gear reduction, compared to the Phantom's 82 to 20 - aka 4.1:1, so we are only talking a roughly 17% difference in reduction.

The 50sx engine can have a 9, 10, 11, or 12 tooth drive sprocket. I will stick with 10 simply because it makes the math easy, but 9 is always an option.

If I installed a 50sx clone on my bike and ran it to the same redline as my Phantom, it would have a top speed of 58mph. A 46 tooth sprocket would restore the overall drive ratio between the two engines.

The problem with the 50SX is that, unlike the phantom, it doesn't have much useable torque below about 7k rpm, and my phantom cruises along at 35mph around 6500rpm

This is all based on my current bike with it's 700C wheels as well. Drop it down to a 26 inch wheel and I gain even more total reduction from the final drive ratio.

Lastly, Horsepower doesn't determine gearing. Torque does, and a nearly 9hp 50cc 2 stroke like this isn't making a ton of low RPM torque. Hence the need for more gear reduction. It's ideal to keep the RPM's up to stay in the limited power curve. I would want to have my bike geared to maintain 30-35mph cruising speed by sitting at around 8000rpm, give or take a few hundred. Sitting below peak, but on the strong part of the powerband. I don't feel like going everywhere at 10k+

This particular engine on my current bike with a 56 tooth rear sprocket and a 10 tooth front at peak RPM would net me a top speed of just 43.3mph, 7mph slower than my Phantom can, and has done. It also gives me a 35mph cruising rpm just a hair over 8000rpm, just inside the power curve. If I find that to be too tall, I can then drop to the 9 tooth front and have 35mph at 9000rpm on the nose.

The whole point of the 3 speed hub is to use second gear as the "normal" gear, and then first for hill climbing ability, and third to drop the revs down.

The fun part is, I can run a 9t-16th drive on the hub, and then another 16 tooth to 56 tooth from the hub to the wheel and still have that same 35mph at 9000rpm cruising speed/rpm.

First gear would then net me a top speed of 29mph at 10000rpm. Second 38.9mph. and Third 48.7mph (still slower than my Phantom). Third gear would also give me a cruise speed of 35mph just below 7000rpm.

The best part is, if none of that works out, I can go up in driven sprocket size between the engine and hub. 18, 20, even 22 tooth if needed, which will reduce speed but grant more torque/rpm as I desire to suit said engine.

Lastly, remember that at the very beginning of this I stated that this build isn't meant to be anything serious, just a fun side project. I have a motorcycle to take my to silly speeds on 2 wheels. Sometimes it's just fun to build something simply because you can.

Links to eBay may include affiliate code. If you click on an eBay link and make a purchase, this forum may earn a small commission.
48mph top speed. Way too fast!

I do think that regularly riding in the high forties and low fifties mph is pushing bike frames and wheels close to the edge though. But I could be wrong.

I did find it interesting that these ktms have a similar gear reduction to the china dolls. Makes gearing them appropriately much easier than other non kit engines.

I am curious on how the pipe could be run. Going up and around means you have a hot exhaust pip between you and the bike, but going under the bike gets awkward with the pedals and relies on a good ground clearance.

I do think that regularly riding in the high forties and low fifties mph is pushing bike frames and wheels close to the edge though. But I could be wrong.

I did find it interesting that these ktms have a similar gear reduction to the china dolls. Makes gearing them appropriately much easier than other non kit engines.

I am curious on how the pipe could be run. Going up and around means you have a hot exhaust pip between you and the bike, but going under the bike gets awkward with the pedals and relies on a good ground clearance.
I don't disagree at all. I tend to stay below 40.

As far as the pipe goes. Not much different than the pipes that are used on the kit engines. Wouldn't be much different than an MZ65 pipe or a Jog pipe.

I don't disagree at all. I tend to stay below 40.

As far as the pipe goes. Not much different than the pipes that are used on the kit engines. Wouldn't be much different than an MZ65 pipe or a Jog pipe.
43mph for my setup is about the limit of comfortable riding, past that it gets too real, and feel like I should be wearing gear. ( I always wear a helmet though).

So, I am revisiting this thread because I managed to win a bid on one of those Sturmey archer S3X Fixed Gear hubs that @cloakedvillain posted up.

This brand new in the box unit with shifter and cable to be specific. https://www.ebay.com/itm/1263989774...d=&campid=5338458686&toolid=10001&mpt=5995925

I put in the minimum bid and walked away figuing I would never win it, but I did. Do I need it? Not really. Will I use it with this particular engine? Maybe, or maybe not. I got the whole thing for \$225, plus taxes and shipping, about \$250 when all said and done.

Is it a great deal? I dunno. It's \$150 cheaper than the next cheapest one, and half as cheap as the next one, so it can't be too bad. If all else fails I can sell it and probably at least get my money back if all else fails.

Links to eBay may include affiliate code. If you click on an eBay link and make a purchase, this forum may earn a small commission.
I am just trying to determine what the best bike for this build moving forward would be. I am considering doing a Hyper Cruiser based build, just like everybody else. Being a proven frame that has handled Phantom power after having another engine on it speaks volumes, and a cruiser frame with lots of room to work with also has its perks.

It's either that, or I may get a bit more spendy and go with a Sixthreezero "In the Barrel" fixie cruiser.

Either way, I need different wheels, and to add at least a front disc brake. I may even go with a rear drum hub wheel since I will be looking at such a large rear sprocket.

All Ideas at this point as I want to gather the other parts I need first, including the engine itself, before I commit to any bike in particular.

That is an interesting place to start. I like it.

A nice hub sitting on the floor waiting for a bike to be built around it. Cool. Or sick. Or dope. Or dank. Depending how old you are.

Cool. Or sick. Or dope. Or dank. Depending how old you are.
How many times must I say this...lol...Some of us are NOT old, we are merely Chronologically Gifted is all...lol...lol.

Replies
36
Views
1K
Replies
254
Views
8K
Replies
17
Views
828
Replies
9
Views
204
Replies
24
Views
2K