any low RPM 4-stroke engines suitable for bike mounting?

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andy b.

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Jul 14, 2010
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I see some 4-stroke engines listed with 4000 max RPM, and some all the way up to 5000 and 6000 RPM. Are there any with top speeds in the 2800-3000 RPM range that still have some torque down to 800 RPM? If my memory serves me correctly on old engine theory, a rather long stroke would be needed for such an engine. I don't know if something like that currently exists in a small enough package to mount in a bike frame.

Thanks,

andy b.
 
scotto-

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
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I can't say for sure you'll find a 4-stroke that will turn those low of rpm's that would be managable unless it was direct drive. I know the centrifugal clutch on my little 4-stroke doesn't engage till almost 3,000 rpm. I'm sure someone here with 4-stroke knowledge will chime in...good luck. I think a Honda GX 100 may fit the bill though.
 
tyrslider

tyrslider

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GX 100 maxes out at around 4000 @ nearly 3hp. There are others too. Not all of these motors have low end grunt though, many just have a narrower powerband.

There are many different clutches and engagement springs avail for different configurations.

Cheers!
 
Bikeguy Joe

Bikeguy Joe

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Jan 8, 2008
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Now you have a job, andy b.

Search on find, then let us know. :) That rpm range is certainly do-able, but I am not sure what you'll find that will fit on a bike...
 
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andy b.

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Jul 14, 2010
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Bikeguy Joe said:
Now you have a job, andy b.

Search on find, then let us know. :) That rpm range is certainly do-able, but I am not sure what you'll find that will fit on a bike...
Click to expand...
LOL That's why I'm here, because I'm lazy and hoped all of you had already searched. :) Seriously though, I am searching and will let you guys know if I find something that has what I'm looking for.

scotto,

Actually, I would prefer direct-drive, that was one of the reasons I was looking for such low RPM. Not so much because of the extra cost of the reduction transmission, but I'm thinking of the smallest size solution I can come up with.

andy b.
 
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Gator Wrestler

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Jan 22, 2009
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I have seen an old B&S using a belt to drive a rim that was welded onto the rear wheel. The rim and wheel were both 26 inch and I think the engine was 158CC (5HP). The guy had a lever with an idler pully that put tension on the belt to engage the engine. It worked well and had good speed. The engine had the governor removed and a header exhaust. The cart guys have lots of experience with these engines, you may find the information you are looking for there. Here's a good place to start:
DIY Go Kart Forum | Choppers | Mini Bikes | Discussion and Plans

Good luck,
Tom
 
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andy b.

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Jul 14, 2010
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Tom,

Thanks for the link! What you describe is really what I'm looking to do. I have a few lines on some old Briggs engines, but I need to see if they will work. I'm not really looking for all that much speed. If the thing goes 30MPH that is faster than I'll probably ever go. That is one of the problems with a 5000 RPM engine, they all want to run at 40+ MPH. I have ridden bicycles faster than 40 MPH before, but that is not my goal with this project.

andy b.
 
Tad Bit Tipsy

Tad Bit Tipsy

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Andy, just go with a large sprocket on the rear, 56T or larger. Would give you the low end grunt and reduce your top end. A rear mounted Robin 33cc or Subaru should give you a smaller engine, but will still be plenty of hard on the pull with a large sprocket. But a 50cc will do just fine, too. I've seen a Robin chain drive hooked up to a 72T freewheel sprocket, the guy hit the mountains on it every weekend, towing camping gear in a yak trailer behind him. He'd carry extra gas in aluminum water bottles attached to the trailer mount points. I think his top end was around 15-18MPH maybe 20MPH on a downhill slope. He was able to go up some pretty steep hills by pedal assisting, straight over rocks and sticks, etc. L8R.
dnut
 
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andy b.

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Jul 14, 2010
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I'm looking for something to put in one of those Newsboys and use a belt drive to the rear wheel. I am slowly opening up to a chain drive, but I need a clutch if I do. If I use a belt drive I can just use an idler pulley to tension the belt and don't need a clutch. Those Robin engines do bare further investigating.

andy b.
 
Tad Bit Tipsy

Tad Bit Tipsy

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Yep, those newsboys are a small fit for 4-strokes. You could fit a Robin in there, your going to need some reduction, retrofit a gear box with a pulley. These have a 76mm clutch, which I think EZM's Q-matic would be compatible with, you'll have to ask them. But I've seen them running a pulley and belt with it, although it was a 50cc engine. I have a titan coming from Dax that is 50cc and it needs 9.5" of height and 8" of length, but the width is at almost 12" and I had to get wide crank and pedals, some frames don't need them, but they allow for easier pedaling anyway and well worth it if you plan on pedaling any distance without the engine once you convert. Good Luck!

P.S. cool thing about the Robins, is you can paint all over the cheap plastic parts and make it look retro, metal, etc. If you were going for that boardtracker look.
 
Tad Bit Tipsy

Tad Bit Tipsy

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G

Gator Wrestler

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Tad Bit Tipsy.
Both of the engines you link to are HP 2 stroke. The second one is used for it's high speed. I think what the OP is looking for is a slower RPM 4 stroke like the 3 HP B&S or the 2.5HP HF or RS. The 79cc HF would be a good candidate. These engines are usually governed at 3600RPM and when using a large pully to drive the rear wheel should cruise comfortably from 20 to 30 MPH.
 
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andy b.

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Jul 14, 2010
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Thanks for the continuing info guys! I think I may pick up one of the HF engines and play around with it while I figure out what I am going to do. There seems to be a lot of tips on this site for using them. I have some ideas, just in my head for now.

andy b.
 
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trophyyride

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Jul 28, 2010
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Minneapolis
andy b. said:
I see some 4-stroke remanufactured engines listed with 4000 max RPM, and some all the way up to 5000 and 6000 RPM. Are there any with top speeds in the 2800-3000 RPM range that still have some torque down to 800 RPM? If my memory serves me correctly on old engine theory, a rather long stroke would be needed for such an engine. I don't know if something like that currently exists in a small enough package to mount in a bike frame.

Thanks,

andy b.
Click to expand...
Hi andy. I think that the top speed is around 2900 RPM if I am not mistaken. .shft.
 
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andy b.

New Member
Jul 14, 2010
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PA, USA
I want to run a belt drive directly from a pulley on the engine crankshaft. Even with a 2" pulley on the engine and a 20" belt ring on the wheel, it would be about 42MPH at 5000 RPM. I don't really need to go that fast. I am slowly finding that there aren't a lot of slow, small 4-stroke engines, so I may have to run a gear reduction of some sort. I guess the good thing about that would be that my speed range would be wider if I used an engine that ran from 1400-6000 RPM and even a 2:1 reduction gearbox. It would also allow me to use a larger drive pulley as a 2" pulley is getting pretty small for a leather belt.

andy b.
 
MotorbikeMike

MotorbikeMike

Dealer
Dec 29, 2007
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Sacramento
Hmmmm, well, I have experimented much with belts, and chains, and can tell you that primary reduction, and a wider usable RPM band generally work out better than low RPM direct drive.

We can run from Zero to a little bit over 30 mph, easily, with no damage to the engine, good fuel economy, a legal sized engine, and a good time to be had by all.

What more could you want?

Mike
 
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