Three Chain, Two Speed Transmission Illustrated

Inventorpardue

Inventorpardue

Member
#1
#1
In the early 70's I bought a used Sears minibike, with a sprung seat. It had a Tecumseh 4HP and the three chain, two clutch, two speed transmission system similar to other minis of the time. It was the only one we ever saw back then.

The chains were badly rusted and the overunning bearing was toast. A trip to bearings and drives for parts, and it was running well. It would outrun my cousin's Cyclops 5HP Briggs-powered go-kart, both holeshot and top end.

Illustrated how the two speed transmission functions, figured to share it here.

Jon
 
Neck

Neck

Growing up is optional
#2
#2
Most Two Speed Jackshaft setups had a special two row clutch rather than a separate sprocket on the crankshaft.
 
markus

markus

Well-Known Member
#5
#5
Jon, are you looking for one?? I spoke to you a few years ago I believe (I live down in englewood). There is a guy in Bradenton that has a few of the comet 2 speeds-what the sears bike used. He also has some of the Rupp/max torque style as well I think. He keeps up with what stuff is selling for so no screaming deals but hes got them. can forward you the info I have for him if you need it.
 
Inventorpardue

Inventorpardue

Member
#6
#6
Neck, thank you! I was going from memory... was 14 when replacing those rusted chains. Thanks for the clarification, will try and re-draw it with this new knowledge.

So the low speed gear on the overrunning bearing was the larger gear?

And both crankshaft clutch gears are connected to the clutch bell?

AndrewP that looks exactly like the one I had. Mine had purple paint, with a purple seat trimmed in white. That brings on the memories.

Markus, yes we spoke some time back about a frame you had. The Gulf Coast Minibike Club was attempting to form a group. I prefer a TAV nowadays and was reminiscing when I drew this. Will research Comet vs. Rupp/Max just for the understanding. I am not looking for one. Saw new Rupp 2-speed parts advertised though.

You all are a wonderful resource! Thanks for the feedback.

Jon
 
Inventorpardue

Inventorpardue

Member
#8
#8
:thumbsup:

Thank you very much AndrewP for posting those photos. My Sears had a rear wheel band brake. Thanks to your photos, I see the Sears Roper had the brake on the jackshaft.

My memory from 1975 is now refreshed! I see how to properly illustrate the 2-speed Sears system now. I may animate it just to experiment with software.
 
RobertC

RobertC

#9
#9
Inventorpardue said:
Neck, thank you! I was going from memory... was 14 when replacing those rusted chains. Thanks for the clarification, will try and re-draw it with this new knowledge.
......You all are a wonderful resource! Thanks for the feedback.
Click to expand...
Hello Jon,

The folks on here are great resources, and they'll 'keep you straight' too :laugh:

This two speed setup has been a 'mystery' to me, i understood the concept, and looked at a few from a distance, but could not really understand how it all worked together.

Your illustrations are great, and have helped me to grasp how this works. I look forward to seeing the revised version w/ tweaked descriptions.

Thank you for taking the time to illustrate and explain this as you have :thumbsup: .
 
Clayon

Clayon

#12
#12
Good info, that PDF really helped explain it well. :thumbsup: This is probably a dumb question, but what is the advantage of using this 2-speed setup over a centrifugal clutch linked straight to the sprocket on the back wheel? I know torque will be better, but is the top speed better? the same? or slower due to the increase in torque? Thanks in advance for the answer. :thumbsup:
 
A

Adrenaline

Member
#15
#15
Clayon said:
Good info, that PDF really helped explain it well. :thumbsup: This is probably a dumb question, but what is the advantage of using this 2-speed setup over a centrifugal clutch linked straight to the sprocket on the back wheel? I know torque will be better, but is the top speed better? the same? or slower due to the increase in torque? Thanks in advance for the answer. :thumbsup:
Click to expand...
The same reason a car uses multiple gears. A single speed drive always has compromise. Gear up and you lose take off and hill climbing. Gear down and you lose top speed. A 2 speed allows you to gear for take off and top speed both.

Now my question is couldn't it be built with the free wheel as an output off the jack-shaft rather then an input to the jack-shaft? this would require 2 wheel sprockets but could allow a normal primary clutch(or even a tav) to be used instead of a twin sprocket clutch.
 
Inventorpardue

Inventorpardue

Member
#16
#16
Clayon asked "what is the advantage of using this 2-speed setup over a centrifugal clutch linked straight to the sprocket on the back wheel? I know torque will be better, but is the top speed better? the same? or slower due to the increase in torque?"

That "first" gear ratio allowed the mini to pull a signifigant holeshot in a drag race. By the time it shifted to second gear, the engine was already at full revs, and the engine clutch was locked up. By then any other mini or gokart was a hundred feet or more behind it, so it surged on ahead.

Bottom line, it achieved top speed sooner than single speed bikes. This advantage prevented anyone else from catching up even we went on for a mile or more at similar top speeds. I am pretty sure a torque converter bike would have defeated it, but nobody had one.

Looking at AndrewP's mini (mine was bone stock) it does not appear the final gear ratio is any different than most single-speed minis. That first gear holeshot was the big difference, I suppose.

I doubt any huge margin of top speed difference is possible with a stock engine no matter what gear ratio you run. Certainly this setup got up to top speed quicker than a single speed clutch and chain drive, with a plain old lawnmower engine.

If you have an engine with enough to torque to pull a lower top gear ratio, this setup would be easier on the clutch while getting off the line, and get you off the line much quicker.
 
Inventorpardue

Inventorpardue

Member
#17
#17
Eliminator said:
Jon, There is an original sears with the two speed setup for sale right now on Boston Craigslist.. take a look !!
Click to expand...
Wow, that bike is in better shape than the one I bought as a 14 year old in the 1970's. Mine had an engine-mounted gas tank, chrome handlebars are identical, and a smaller perforated chrome chain guard.
 
Inventorpardue

Inventorpardue

Member
#18
#18
Adrenaline said:
Now my question is couldn't it be built with the free wheel as an output off the jack-shaft rather then an input to the jack-shaft? this would require 2 wheel sprockets but could allow a normal primary clutch(or even a tav) to be used instead of a twin sprocket clutch.
Click to expand...
I may illustrate that just as a proof of concept. Interesting.
 
A

Adrenaline

Member
#19
#19
Inventorpardue said:
I may illustrate that just as a proof of concept. Interesting.
Click to expand...
i'm thinking a tav plus 2 speed might work well to reduce the pulsing/slip from 1st to 2nd as the secondary clutch starts to engage(note i only know bout said pulsation/slip from hearsay) by the tavs function bridging the gap between gears. My demented mind says it could be perfect for like a briggs 305 in an old Vespa. Have 2nd kick in about 50-60 for highway cruising. Governer and lawnmower throttle for cruise control. Twist grip throtle set up as override for gov. for city riding. Damn now i need a scooter to try it on.
 
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Inventorpardue

Inventorpardue

Member
#20
#20
TAV or Clutch to 2-Speed

Concept illustration with a TAV and a standard mini clutch:

see drawings towards end for clutch version

One part not off-the-shelf is the low speed jackshaft sprocket that must attach to the large outside diameter of the overrunning bearing. It was easy to draw, and skip including that important detail.

Also drew it with a 2nd jackshaft, all-clutch version, etc.
 
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