Twin 66cc 2 Stroke Build Thread

[Dom]

Hello, I am new here and from the UK but not new to motorised bicycles. I've built a couple of them using the standard kits before over the past couple of years.
However now I am building something I've wanted to do for a while, a twin engined motorised bicycle! I originally planned on fitting the engines side by side in the stock position but this was going to make the bike awkward to pedal and look more like a motorbike so I thought I'd see if I could mount them in front of each other and with some cutting and filling to the fins and crank case they fitted!

I've recently got a 135a Mig Welder and this bike has been my first chance to use it but I got a little practice first on an old frame and taught myself to weld and am pleased with the result.

The engines will share one chain (see the cutout in the bottom ones sproket/clutch cover) and I am using a 32t sprocket I bought off Ebay so am hoping for speeds of 50-60mph if I dare go that fast. haha

The controls will be connected to both so I'll just weld a longer handle onto to the clutch to make it usable for a normal person!

 

[frank66]

cool. let us know how it goes. take ur time. im afraid the heads will warp if alot of metals is removed. they are only just stiff enough from the factory.
good luck with the twin.

you could do a twin engine trailer too. im fond of the idea.

 

[YesImLDS]

Looks like you got something to work with! That's a huge frame too. How are you going to join them together? a 2 in to 1 out sort of sprocket system? Looking forward to see how it works out for ya.

 

[silverbear]

That' thinking out of the box, for sure. Welcome to the forum.
SB

 

[gokart25]

you should put 2 different sized sprockets on the rear wheel, with two throttles (left hand And right hand).
then once you reach the top speed of one motor, you can start using the other....

 

[Dom]

Thanks for the comments, I've only removed a small amount of the top of the fins which is also touching the metal frame so I don't think it will be an issue, I am considering possibly using two sprockets side by side with some sort of derailleur system but still having both engines run together as I've gone for two more for acceleration more than top speed.

Now to start stripping it down for a few costs of British Racing green!

Cheers

 

[frank66]

if you want to test it just a pull starter would do it. lookin out for you. maybe you will be ok after all. but for about 15 dollars the pull starts are handy to keep around. l8r

 

[cannonball2]

If I was gonna do that I would ditch both clutches and use straight keyed 15mm shafts linking the engines with chain and sprockets on the right side. Take the drive out of the lower engines sprocket to a jackshaft mounted behind the seat post with a good cent clutch then on to the wheel. Not a real difficult conversion. Would require a pull start.

 

[YesImLDS]

What if you got something like this

Then from there run both chains to 2 of the sprocket and then have a single drive chain for the rear wheel. If you were to do that you could make sure they are spinning the same rpms so you aren't over-revving one of them.

 

[cannonball2]

Problem is the engines run backwards due to the gearing, so my idea of running off the right side via chains wont do unless the timing on the ignition is flipped to reverse the running. Not a big deal. Running off the left sprocket to sprocket would be easy with the double sprocket. However the engines can be come untimed unless the clutches are positively locked. Timing the engines I would think would be for simultaneous firing.

 

[YesImLDS]

Ah I see what you are saying with the clutches. Maybe some sort of third clutch mechanism would work? Say on the sprocketing system itself so your clutches on the motors themselves would always be locked and you could set them 180 degrees of each other. I don't know how you would go about making a clutching system on the sprocket though. An idea though I guess.

 

[cannonball2]

I would do something like weld the clutch carriers together at the proper point. A good centrifugal clutch added after the engine on a jack shaft would do fine, something like a Tomar or similar, I expect even the old Maxtorque could handle two stock or nearly so engines.

 

[Davezilla]

The easiest way to synch the 2 engines would be to run them both off one way bearings into a common shaft to the wheels, this way the strongest engine will always be providing torque to the jackshaft and the other engine will begin to pull along with the stronger engine during acceleration and when it gets closer to top speed, this will also take the load off one engine if it is slightly stronger than the other so it's only job is to turn the jack shaft instead of helping the other engine and vice versa.
Another trick is to run a synchronizer tube between the 2 intake manifolds so both engines always get the same amount of air regardless of any differences in throttle position, of course, you block off the synch tube while tuning the engines to run as equally as possible, then each output shaft runs off a one way bearing into a common jackshaft, then the manifolds are tied together with the synch tube so both engines can perform as equally as possible at all times.
The one way bearings isolate each engine in case one shuts down or loses power, the other doesn't have to help the weaker engine and by adding a synch tube to connect both intake manifolds, each engine should perform very equally to the other so the same amount of power coming off each engine can feed into the jack shaft which feeds the power from both engines to the rear wheels.
This is the way most multi engine motorcycles are set up so both engines are feeding the same torque into a common shaft, but if one fails for any reason, the other engine only feeds that shaft and don't have to fight against the weaker or failed engine.

 

[MadMaxed]

im almost certain you would have to get the timing on both to match absolutely perfect also. 2 pistons not on the same crank is the same as running one engine. you wont get more speed out of that setup maybe torque. i dont see the point... i could be wrong tho

 

[YesImLDS]

im almost certain you would have to get the timing on both to match absolutely perfect also. 2 pistons not on the same crank is the same as running one engine. you wont get more speed out of that setup maybe torque. i dont see the point... i could be wrong tho

Got two engines and some time? Why not? You also will get more speed out of this because the extra power allows them to rev higher easier. This set-up if accomplished will probably just have an awesome power range and a decent top speed with some motors fairly over-revving themselves, but it won't be a strenuous over revving because of the extra help of the other engine will make it a bit easier.

As far as getting the timing to match he was talking about welding the clutch plates and all he would have to do is put one at TDC and the other at BDC. Because you wouldn't want them sparking at the same time just because of extra vibrations. Having them firing opposite of each other would make it more tolerable and just make sense. You don't see harleys firing at the same time!

 

[MadMaxed]

it is interesting for sure i have seen some v twinn engines for motorbikes. but non for commercial sale

 

[frank66]

i think a a twin engine trailer will be easier in the long run. then time them 180 degrees apart with a long single carb manifold feeding both engines.

the power stroke is about 120 degrees on two strokes wich is why triples are so fast. 3 times 120 degrees equalls 360.... perfect power spread.

 

[Dom]

Thanks for all the helpful advice, I've been a bit worried about the timing myself so now I think I'm going to go fit a timing chain between the two engines on the right side so that both clutches are still used with one big chain on the left as I'm not sure how well one clutch would take the extra power.

 

[MadMaxed]

Thanks for all the helpful advice, I've been a bit worried about the timing myself so now I think I'm going to go fit a timing chain between the two engines on the right side so that both clutches are still used with one big chain on the left as I'm not sure how well one clutch would take the extra power.

Franks idea of timing is what i believe to be the best way to time it.

 

[boxcar]

You will likely add only a tiny bit to the top end. Max RPM is max RPM.
But it will pull much better .
Witch will allow for a smaller tooth count sprocket . And ultimately a better top speed....
If you link the engines from the output sprockets , you will only need to control the down hill clutch.
The best information on this subject can be found on any karting sight.
We have been running twin 2 strokes in the Go kart world for years..... Most are vintage karts...
https://www.youtube.com/watch?v=macY5n_R3gE
I like the idea though.
I might try it on a 3 wheeler running centrifugal clutches...... ( no need for engine timing as the clutches will
compensate for the engine differences ) .