Author Topic: converting rotary transmission to return-type  (Read 991 times)

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Offline JackA

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converting rotary transmission to return-type
« on: March 15, 2018, 07:04:44 PM »
I have also posted this question in the 50-60-90-100 section, but maybe this is a better place (in case not everyone scans the whole forum for new stuff, as I do).

I am looking to build a couple of 50cc engines for vintage roadracing.  For openers, I am leery of rotary transmissions for that use.  Looking at the parts diagrams (what a helpful site this is, just for those and the other manuals!), it looks like the Tora has only about 4 more parts than the Chibi to make the shifter a return type instead of rotary, and my next step will be to ask Richard if he has those parts.


The drums are also different, presumably for 3 vs. 4 speeds (?).  They both appear to differ from the Sport 50 drum, but query whether that could be used with the Tora stopper parts.

As an alternative, does anyone have any experience and/or photos of shop-made solutions for this?

thanks,
Jack
« Last Edit: March 15, 2018, 08:06:32 PM by JackA »

Offline BRT-GTR

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Re: converting rotary transmission to return-type
« Reply #1 on: March 16, 2018, 09:09:00 AM »
       Jack,
             We are not ignoring you, most members including me don't have personal experience of the smaller engines.
      Based on the onsite manuals, the Chibi and Tora specify a return type box, which model do you believe has the rotary type change. Get out clause  ;D :- Can't find a service manual specifically for the Taka.                            Brian
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Offline JackA

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Re: converting rotary transmission to return-type
« Reply #2 on: March 16, 2018, 06:44:14 PM »
Thanks, Brian.  Given the dearth of 50cc "vintage" racing ("post classic" to you, perhaps) in the US, I'm not surprised that there's much more lore about the 175s than the little ones on this board.  I thought the Chibi had a rotary box, as well as the Sport 50 and 60, while only the Tora (among the little engines) had a return type shift -- but  I have lots more reading in the manuals to do!  Lucky to have them here, too.

Jack

P.S.  And upon going back to the Chibi/Tora manual, I see that the Chibi does have a return box!  I may have been paying too little attention since it's only 3 speeds, and since I need to end up with 50cc I have been looking more for Sport 50 motors.  Thanks for the prompting!
« Last Edit: March 16, 2018, 06:57:07 PM by JackA »

Offline JackA

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Re: converting rotary transmission to return-type
« Reply #3 on: March 16, 2018, 07:11:12 PM »
By the way, for those who may be interested in this topic, since my further pleas for help (and maybe an eventual build thread) will continue to be small engine related and not really about the Chibi or Tora models otherwise, I think I will confine my further posting to the 50, 60, 90, 100 section.

Jack

Offline CL-100

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Re: converting rotary transmission to return-type
« Reply #4 on: March 17, 2018, 08:47:15 AM »
http://bridgestonemotorcycleparts.com/index.php?topic=4323.msg25248#msg25248

I'm not sure if this will help you but the link above is from a thread I started some time back.  I was assembling a 100cc engine from a box of parts and ran across a "shift stopper boss" that is found on the Taka.  It stops the rotary shift function of the drum, making it a conventional four down, four up transmission.

Offline JackA

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Re: converting rotary transmission to return-type
« Reply #5 on: March 17, 2018, 11:28:34 AM »
Thanks, CL.

Jack

Offline BRT-GTR

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Re: converting rotary transmission to return-type
« Reply #6 on: March 17, 2018, 12:17:05 PM »
            Jack,
        Just had a look at the dealer manual for the 50/60 sport models.  Although not clearly stated, it looks from the gear change drawings that they do have a rotary change mechanism.  Can anyone confirm this ?  I can see now why you posted your query,.                                                           Brian.
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Offline CL-100

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Re: converting rotary transmission to return-type
« Reply #7 on: March 17, 2018, 03:28:03 PM »
I have a BS50 Sport that's next in line for a full restoration after I finish my current 350 project.  My BS50 DOES have a rotary transmission.  I don't have it disassembled yet so I don't know if the stopper boss on the Taka would work on the 50 Sport. 

The stopper boss is just a mechanical stop piece that physically won't let the drum turn again in the rotary function which would allow it to go back to neutral after fourth gear. 

Jack, you might want to check with those on this site that have a SR100 factory racer as these bikes were also equipped with the stopper boss for the 90/100 rotary transmission.   They could give you an idea if the stopper boss would also work on your BS50 transmission but you might have to post a picture of the neutral switch end of that trans shaft.   If so, Richard might have them available for purchase.

Someone with a SR175 might also be able to let us know if their transmission includes the stopper boss.  I'm not familiar with their transmissions but Bridgestone may have also configured this factory racer with some type of stopper device.

Offline OldSwartout

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Re: converting rotary transmission to return-type
« Reply #8 on: March 17, 2018, 06:46:33 PM »
The SR175 just had the guide pin in the 5th gear shift fork permanently pinned inward to the shift drum groove, preventing the drum from rotating past fifth, as if the external lever on the DT175 was in the return shift position. Nothing more than a drilled hole in the fifth gear shift fork and a cotter pin.
Karl Swartout
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BS175 Roadracer
BS350 GTR

Offline JackA

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Re: converting rotary transmission to return-type
« Reply #9 on: March 20, 2018, 11:16:10 PM »
As far as I can tell from reading most of the manuals, etc., yes, all of the BS50s had rotary transmissions; and just to be clear, that's what I don't want for racing. 

Meanwhile, doing some more homework, I see that section 9 of the Optional Parts List for Bridgestone Motorcycles sets forth the names and part numbers for "Exclusive Parts for Return-Shift" for both the 90cc and 50cc engines.  Interestingly, almost all have the same part numbers, excepting only the "shift drum assembly" in each case and a "thrust receiver" for the 50.  Needless to say, I plan to ask Richard if he has any or all of those parts.  Otherwise, my best option seems to be using Chibi parts, as those are the most plentiful on eBay, and assuming that I can mix and match cogs to retain the BS50's 4 speeds.

Reading on in the Optional Parts List, section 12 lists "BS50, BS90 Speed Kit Parts."  I am not only going to ask Richard about those, I am going to cc Santa Claus....  At the least, I would like to acquire some aluminum cylinders.  Besides being over my race rules displacement limit, the Tora and Chibi cylinders are all apparently cast iron, but I seem to recall reading in the advertising literature that the BS50 had aluminum.  If that's the case, I would be happy to find any of those, even if the race kit cylinders have better porting.  It would be interesting to know more about the other race kit parts, as well, e.g. crank and clutch parts, in case stock parts can be modified to be similar.

Thanks again to everyone chiming in on this.  I realize that the 50s are not everyone's (tiny) cup of tea!  Once I get my hands on, and into, some engines I will try to post some pictures, and eventually maybe start a race bike build thread.  However, don't hold your breath on that:  I am not even dreaming about getting a BS50 racer going this year!  Our season begins in May, and I have plenty to do getting a couple of H*ndas back on the track.

Jack

 


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