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nigel breeze

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Number of posts : 357
Registration date : 2007-12-23

PostSubject: gear lever linkages   Wed Aug 22, 2018 7:17 pm




Trevor, help!!
im having a bit of trouble with the gear linkage on my bike.Hopefully the thing can be helped by the above style system. i need less gear lever movement but more throw on the linkage to the gear box shaft. Is this going to help or have i not understood the picture correctly? cheers
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Trevor Amos



Number of posts : 884
Registration date : 2010-08-13

PostSubject: Re: gear lever linkages   Fri Aug 24, 2018 12:58 am

Nigel,
One serious problem with the selector mechanism in the 3 speed Bantam gearbox is the huge amount of lost motion within the mechanism during actual gear changing. The same applies to the 4 speed 175 unit but nowhere near to the same extent. Lost motion always leads to a long change lever movement, precisely the problem you are describing. I well remember changing gear on my work hack D14/4 by having to lift my foot clear of the foot rest to effect a positive change.
From a purely engineering standpoint what is required is to convert rotary motion, to linear motion then back to rotary motion and simultaneously provide enough mechanical advantage to overcome basic friction and spring resistance. Easy to recite but tricky to implement!
The device in your post when attached to a Bantam situation would work but looks complicated, and has lot of connecting points to create slop and lost motion but how large would the whole thing have to be? I tend to try and keep things simple, a conventional linkage such as on a RS Honda would do the do the job very well particularly with rose joints and robust adjustable linkage rods. You could measure up the system on Derek`s RS? A bespoke set up could be drawn out on paper and the various levers adjusted to provide the appropriate foot change motion that you want and corresponding movement in the gearbox. However, short levers (small mechanical advantage) require a larger input of force to achieve the same as a system with long levers, and it is a short lever motion you want for the foot change.
I don`t think I have been of much help but Bantams seem present a problem in almost every respect! †

Trevor.
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nigel breeze

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PostSubject: Re: gear lever linkages   Fri Aug 24, 2018 3:37 am

Thanks for reply Trevor,
yes ive had a linkage system off my other 4 speed engine on this other bike as it keeps the gear changes as standard .. 1 down the rest up but i feel its still not giving enough movement at the gear shaft linkage. ive tried the versed standard lever way which makes it 1 up the rest down and i donít feel comfortable with it. †Just roughly measured the free play between 1st and second and its a good 20mm of lever movement before you can feel it ready to engage and as you say having to lift you foot off the peg to get the next gear in. i suspect its the same between the other gears.Does the diagram give a ratio of 3:1 or 10:1. i can see one scale of 1 to 10 on †the top end and 0.1 to 1 on the lower end but visually it looks smaller. i can see your point above slop and loss †and in reality the moving parts may need some support. Just taken some pictures of both and will post when they get into my e-mail in box.




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