Fox Doodlebug 2-Speed Conversion

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ubernoob51

Active Member
#1
#1
I'm doing exactly what the title states, putting a 2-speed Max-Torque jackshaft intended for a Rupp onto a Fox Doodlebug. With a 2 1/2 horse Clinton Panther.

Jim at Max-Torque already converted a double sprocket clutch to a 5/8" shaft for me, should be arriving soon.

I've drilled and tapped the PTO shaft on the Clinton.

I had to move the brake caliper/sprocket of the Doodlebug to the other side of the frame, basically cut and welded the existing bracket.

I also had to clearance the frame near the jackshaft mount to clear the 1st gear sprocket by heating up the frame and hammering the tube down.

I notched the fender on both sides. The left side, to clear the 2nd gear clutch, and the right side, to clear the relocated chain to the wheel sprocket. I will probably weld a piece of steel back in, the fender as it is looks a little weird.

Waiting on a bunch of parts to come in before I put it all together!
 

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U

ubernoob51

Active Member
#2
#2
For the naysayers:

I'm going to be using an NOS '69 Rupp secondary clutch, which means if the motor doesn't rev high enough to shift, then I can easily put softer springs to lower the 2nd gear shift point.

The tires are pretty small, and first gear should be low enough to get me a decent boost in acceleration.

Second gear will pretty much be 1:1.

Attached is a picture of the drilled/tapped crank.
 

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Jim Donovan

Active Member
#5
#5
Alex -- you must have been a terror at Christmas not wanting to wait to open your presents. Heck it only left the shop on Thursday and if you think you can speed up the post office which almost had to shut down for lack of getting a paycheck, I wish you luck. The clutch looked very nice on the 5/8" crankshaft here in the shop. The only problem is the clutch is a two piece, shoe assembly separate from the drum and sprocket, so you have to do just what you did and that is drill and tap the crankshaft. All of us will be interested in how the engagement goes for both clutches. Since we make them both there is things that can be done to increase or lower engagement of either of the clutches but you have to tell us what is happening once you get out on the street and give it a ride.

A good project and you will have fun results. I work with all the old 2-cycle engines thanks to vintage karting that introduced me to the McCulloch, West Bends, Power Products, Clintons, and Homelite. There is nothing like the sound and smells when you are at the track looking at these 60 years old karts zipping around the track. It makes you feel like you are 16 again.
 
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ubernoob51

Active Member
#6
#6
Thanks for the reply Jim!

The jackshaft is supposed to come in today, so I can get started on that part of the project, and run the chain from it to the rear wheel.

Will keep you all posted.
 
sparkwizard

sparkwizard

#7
#7
Where would someone like me find that one-way low gear? The rest is fairly easy to find, but that sprag sprocket is a tough one.
I have a couple of old smoke motors on the shelf....
 
U

ubernoob51

Active Member
#8
#8
sparkwizard said:
Where would someone like me find that one-way low gear? The rest is fairly easy to find, but that sprag sprocket is a tough one.
I have a couple of old smoke motors on the shelf....
Click to expand...
Black Widow Motorsports sells the kit, they may be able to sell a sprocket separately. I know they sell the sprag bearing and inner race, not 100% sure about the sprocket though.
 
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Jim Donovan

Active Member
#13
#13
WOW, Alex -- your secondary clutch on the jackshaft is really a collectors item. That is the first time I have seen the two shoe version on a minibike. The ones we do today are all the six shoe style. The only other one I have seen is on my board here at work of the various clutches Max-Torque has made over the years. I will give you a lot of credit getting that bike all together and working so quickly.
 
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ubernoob51

Active Member
#14
#14
So today my oldest nephew and I took the Doodlebug and the Rupp TT-500 out for a quick test drive.

His consensus is that it's much faster than before. Cool. Although since I have an upgraded Teke H50 stroker and some gearing changes, I can still pass him like he's standing still.

I showed him how to oil the clutches, and we parked both bikes.

Considering the fact that even with a 350 pound rider (me) it manages to shift, I'll consider it good enough, and leave this bike for my nephews to ride. No need to restore it since they're kids.

Mission accomplished!
 

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Rockjaw

New Member
#15
#15
Hi, new to the group. Question, is the clutch on the jackshaft of these 2 speed setups a regular centrifugal clutch like you would install on the engine?
 
RobertC

RobertC

#17
#17
sparkwizard said:
I always assumed they were. I hope someone will let us know for sure.
Click to expand...
The way I understand it, the two speed setups are centrifugal clutches, however with different engagement speeds.

Once you reach the engagement speed of the 'second gear', there is something called a sprag sprocket, that allows the unused clutch to freewheel.

Please those with more knowledge correct me , or add to what I have said.

@Jim Donovan

Thank you.
 
sparkwizard

sparkwizard

#19
#19
I had a Boonie Bike in the seventies. It had a dual sprocket clutch on the engine , 12 teeth, 35 chain. One went to a sprocket on a jackshaft on a one way sprag, with 24 teeth. The other sprocket had no chain on it, so I bought a clutch and mounted it on the jackshaft, put a second chain on it and it worked. It was kind of spooky, doubling it's speed right at the point when the governor was limiting the revs.
Now, I want to find another one of those one-way sprockets.
The 24 tooth sprocket turned the jackshaft at 1/2 the speed of the crankshaft, so the clutch on the jackshaft was not engaging until the shaft spun twice as fast as the egagement speed of the one on the crankshaft. When second clutch engaged, the jackshaft spun faster than the low gear sprocket, so the one-way sprag let the jackshaft spin faster.

That clutch I installed, did not last long. It never fully engaged because that jackshaft never reached 2000 RPM.
 
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Rockjaw

New Member
#20
#20
Ginz61 said:
This has been posted before but I think it explains it pretty well.

https://www.parduebrothers.com/minibike/twospeedminibiketransmission.pdf
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Ginz61 said:
This has been posted before but I think it explains it pretty well.

https://www.parduebrothers.com/minibike/twospeedminibiketransmission.pdf
Click to expand...
Thank you Ginz61 for posting that! It all makes sense. I thought the clutch on the jackshaft was different and the drum fastened directly to the jackshaft. I had a Lil Indian with the 2speed and I'd like to build something now. Thanks again
 
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