Will The Second Engine Start Automatically?

Will The Second Engine Fire Up Automatically?

  • Yes

    Votes: 0 0.0%
  • No Way

    Votes: 0 0.0%
  • I don't Think So

    Votes: 0 0.0%

  • Total voters
    2

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Hello. I'm throwing this out into the universe, hoping someone has had a similar problem and solved it. I'm installing two Tanaka 47R engines on my 7-speed bike. Each has its own identical chain drive, sprockets and 5:1 reduction gearboxes. Gearboxes bolt onto each engine and have 11-tooth sprockets. They each chain to separate 72-tooth driven sprockets at the bike's bottom bracket. All three sprockets at the bottom bracket spin on a freewheel, and a single 24-tooth chainring sprocket drives the 14t-28t rear cassette.

The bike hasn't run yet. Being that each gearbox has a clutch, I figured that would keep both engines to run separately, with their individual throttles.

Twin engines CAN run independently of each other. I know this, because I built a cruiser with an engine on each tire(friction drive.)Their throttles need not be synchronized.
It's like two football players pushing a small car. One guy's using all his might, and the other player's faking it and just has his hands on the car.

It was a given that both engines' drivetrain would be spinning, even if I chose to fire up just one engine. I figured since they both have clutches which engage at about 1500rpm, the dead engine wouldn't be spinning its own clutch.

Knowing that both engines' drive trains would be turning, an engine speed of 3000rpm will have the rpms reduced by the time it gets to bottom bracket. The engine spins at 3000rpm. Its gearbox reduces it to 600 rpm. The 72t/11t further drops the revs to 92rpm.

Then something odd happens. The 92rpm from the bottom bracket now spins the non-running engine's gears. From 92rpm, it rises to 600rpm at the gearbox's shaft.....then spins that clutch and compounds(not reduces) the revolutions to 3000rpm at the dead engine's clutch!

Talk about the tail wagging the dog! LOL!

So the dead engine's clutch spins to 3000rpm, engages the engine and fires it up, if the ignition switch is on!

My question is.....do you think this will happen? I can't install 5000rpm clutch springs on the dormant engine. Then I wouldn't be able to run both engines at low speed.

Eventually, I'll find out, when the bike's done and running. I'd like to know now, if possible.

Thoughts?
 
Last edited:

kaneto

Member
Jun 6, 2016
32
1
8
Bulgaria
...

So the dead engine's clutch spins to 3000rpm, engages the engine and fires it up, if the ignition switch is on!
...
Did that actually happened or you are just wondering if it could happen?
It seems really unlikely to me. Even if the freewheel somehow get seized and starts to spin the clutch it should only spin the clutch bell and not the shoes, so the clutch shouldn't be able to engage on it's own.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,525
136
63
Thanks for responding.
Yeah, a few guys from another website agreed with you.
No, it didn't happen. I was wondering if it would.
 

MEASURE TWICE

Well-Known Member
Jul 13, 2010
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The only thing I thought of but it is not something you questioned on specifically. Say both engines were running and then you shut one off. If you were moving, then the stopped ignition on one engine may just stay with shoes locked to the bell on that engine and be an engine brake. This would be the case till you stop moving or go slowly. I use my off road bike with centrifugal clutch purposely to engine brake, but with the ignition on. At idle it releases the shoes. If I want to coast without engine braking just slowing enough releases the shoes from the drum.

Double engine braking could really help slow you down if it can be controlled.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,525
136
63
How are you, MEASURE TWICE! It's been years!
I've never experienced engine braking ,but I only played with engines 47cc's or less.
I imagine that clutch springs normally contract at idle or with engine off, thereby disengaging the engine from the drivetrain.
 

MEASURE TWICE

Well-Known Member
Jul 13, 2010
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How are you, MEASURE TWICE! It's been years!
I've never experienced engine braking ,but I only played with engines 47cc's or less.
I imagine that clutch springs normally contract at idle or with engine off, thereby disengaging the engine from the drivetrain.
Getting by now!
There are times that hills so steep I can't keep the engine braking going like a hand lever clutch at idle. I usually just shut the engine off and hold on the brakes, trying not to slip on gravel over hard pack dirt roads. It also gives me indication of how at least part of my drive train is since engine noise is gone.