Two Speed Automatic Transmission

msrfan · Apr 14, 2013

I got an idea from watching multi-speed RC cars shifting at certain RPMs into a higher gear. At a race last year I mentioned to Briggsbiker, Anthony, that it would be cool to build one for my race bike. He said he had one on an old Rupp mini-bike and that it worked okay. I researched it online and discovered Maxtorque was producing the proper components and they were being offer by someone for restorations. Anyway, I set out to duplicate their system. I started by buying the only part that was different than any other minibike, the double sprocket clutch sold by BMI. Somewhere along the way I chose to enclose it all into a removable case as it was way too bulky to have the clutch on the motor and the jackshaft on the frame. Also I wanted to include a pedal start mechanism integrated into the same case.
I started with the case, forming pieces of 1/8'' flat strap into rounded end caps and flat sides to weld onto them. Added rings to hold bearings and mounting tabs.

Got all that welded together.

Then I had to make a similar part for it all to slip into.

msrfan · Apr 14, 2013

Next step was to assemble all the parts that go inside the case. I had to calculate the shaft spacing so all the chains with different size sprockets would have the same slack. The low and high speed spacing came out to exactly three inches with the room I had in the case, and the starting chain was a little baggy, but was okay because it was just used for pedal starts.

After that, I fit it to the frame by cutting out the tube under the seat and machining transition parts to mate to the trans holder and added a connecting strap to tie it to the engine to minimize the tendency to twist under power and retain adjustability and later mount a chain guard to.

Got all that welded in.

msrfan · Apr 14, 2013

Decided to add frame strengthening gussets for some added support.

Built up a set of wheels with a larger front brake from a 125 dirt bike and chain drive rear to replace the previous belt drive.

Got it back up on two wheels.

msrfan · Apr 14, 2013

I had to turn my carburetor sideways because the trans took up a lot of room.

Reconfigured the cable holders for the throttle and compression release. Made a new intake tube.

Fit the chains (all size 41).

Added metal blocks to my forks to fit my new brake drum and also to hold a stiffener strap.

Put a piece of rubber on top in case of bottoming.

msrfan · Apr 14, 2013

Installed a catch can as per new racing rules.

Had one day to test it and it took my whole block to shift, so I changed a sprocket and got it shifting sooner. Needed one more sprocket change, but was out of time. Got to go race tomorrow.
Had a great time racing. The trans worked perfect but wound out a little far in first so when it finally did shift, I was at the tower on the long straight and never topped out before I had to lay on the brakes for the first turn. It also shifted right at the elevation change just in time to brake again and in the back sweepers when I was able to take them at speed. The new brake drum is very strong and allowed me to go deeper into a turn. The gearing is a tad high and I could not launch off the start or out of turns as well as I wanted.
I spent months and a few hundred bucks developing this transmission and had my friend, Dan, machine a lot of parts. Two of the gears inside have one-way bearings riding on a specially made shaft in order for it to work. Overall I'm very happy with the results. With a little (or a lot) more tuning, it will perform up to my expectations. Now, if I can learn to corner a lot faster, I will realize it's potential.

SuperDave · Apr 14, 2013

I'm curious as to what gauge metal you made your gearbox out of. Having cut the seat tube, my concern would be to maintaining the overall structural integrity of the bike as a whole. Hard to tell from the pics. (most didn't show up here)

How does it shift? Spring loaded centrifugal? And is it a slider/cog(like an automotive stick shift) or magazine/derailleur(bicycle)???

msrfan · Apr 14, 2013

I also had to add a pedal chain tensioner, as both chains didn't adjust evenly.

msrfan · Apr 14, 2013

Hi SuperDave, I used 1/8'' steel flat strap to form the case with. The adapters I made to make the transition from the case holder to the frame tube took care of keeping the integrity and the connecting strap from the engine to the trans keeps it from wanting to twist. It shifts automatically. As the RPMs from the first clutch shaft increases to a certain speed, the second clutch grabs and the one-way bearing in the low speed sprocket lets the high speed sprocket overide it and go into high gear. No cogs, no slider, just a smooth shift. The motor winds out and the looses RPMs during the shift back down into the power range and the bike speed increases considerably. Careful gearing and clutch spring/shoe selection should make it perform to whatever track speeds will allow.

Russell · Apr 14, 2013

None of the gears look like freewheel but it seems that at least 2 of them must be to operate properly? Could you show a close up of these gears.

culvercityclassic · Apr 14, 2013

That bike build is really nice, I did see the bike is action and performed very well. Great engineering that's for sure. You and your friends builds are always worth checking out, the details are awesome. Thanks for sharing this design.

Trey · Apr 14, 2013

msrfan- Very impressive work! Thanks for the post.

msrfan · Apr 14, 2013

Russell said:
None of the gears look like freewheel but it seems that at least 2 of them must be to operate properly? Could you show a close up of these gears.

Hi Russell, the one-way bearings are of needle type and don't take up much room. One is in the low gear sprocket and the other is in the pedal start sprocket, both riding next to each other on a 20mm drill rod shaft. All inside the trans are for #35 chain. If I remember, I'll post more photos next time I have it apart. It won't be too long because I want to experiment with the clutch engagement RPMs.

msrfan · Apr 14, 2013

culvercityclassic said:
That bike build is really nice, I did see the bike is action and performed very well. Great engineering that's for sure. You and your friends builds are always worth checking out, the details are awesome. Thanks for sharing this design.

Thanks Jeff, although I don't know if it will ever catch your monster bike. Probably should just get a Comet at 1/3rd the price but wouldn't be near as much fun as making my own transmission. Cool to hang out with you and the rest of the racers yesterday.

msrfan · Apr 14, 2013

Trey said:
msrfan- Very impressive work! Thanks for the post.

Thanks Trey. It was really fun using old mini-bike technology and adapting it to my race bike.

maniac57 · Apr 14, 2013

Very cool trans! I've been dreaming of a centrifugal two speed that fits in a ht clutch case...
(Hint, hint)

Russell · Apr 14, 2013

msrfan said:
Hi Russell, the one-way bearings are of needle type and don't take up much room. One is in the low gear sprocket and the other is in the pedal start sprocket, both riding next to each other on a 20mm drill rod shaft. All inside the trans are for #35 chain. If I remember, I'll post more photos next time I have it apart. It won't be too long because I want to experiment with the clutch engagement RPMs.

Mrsfan,
Thanks for the info. Really clean & compact fab job! Are the spag bearings just press fit to the gear hubs or attached another way? Two speeds IMHO is all you need for general cruising.

msrfan · Apr 14, 2013

Thanks Russell. Yes, the sprockets are bored to press fit 26mm. The sprag bearings are 26mm OD X 20mm ID X 26mm long and have the one-way needles in the center with a row of regular needles on both sides. Very compact and available very inexpensive on ebay. The shaft they ride on has a 2 1/2'' section of 20mm and each end is turned down to fit 3/4'' ID support bearings and one side is keyed to secure the inside clutch and the outside sprocket.

msrfan · Apr 14, 2013

maniac57 said:
Very cool trans! I've been dreaming of a centrifugal two speed that fits in a ht clutch case...
(Hint, hint)

Sorry maniac, i'm afraid you're on your own unless someone utilizes a planetary system like the old Bendix kickback hub or the Sachs Torpedo automatic hub. Thanks for the comments.

harry76 · Apr 15, 2013

How have I missed this one? love it!!!!!!!!
Your work, as always is innovative and flawless. Everything you build looks so factory, which is meant as a compliment. Great work!!!!!

msrfan · Apr 15, 2013

Thanks harry, that's great praise coming from a builder like you. It turned out a little bulky but once in the frame it's like it belongs there.

Two Speed Automatic Transmission

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$50 Cruiser

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Old, Fat, and still faster than you

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