Mid-drive/jackshift practical? Or weird gears in general

[Risto Varanka]

Maybe somebody should do a my-gear-system-has-more-inches-than-yours FAQ site. So far I see 3
options for managing with odd wheel sizes and performance requirements:

1) internal gear hub -adds friction, if it doesn't, it's expensive -adds some weight too -works
well, practical in some situations

2) oversize chainrings at front -shifting can be sub-standard to your taste -not as flexible or big
gear range

3) mid-drive/jackshaft -adds some friction

Wondering about the third option... Can you just replace your chain idler wheel with a pair of
sprockets, so you get changed gear ratio with little effort and little cost? You could even change
the sprockets: for record racing, mountain touring... If you don't have a chain idler, you could
drill a hole in the frame and just install the new system... I wonder if these mods would be hard to
do in some workshop by yourself?

--
Risto Varanka | http://www.helsinki.fi/~rvaranka/ varis at no spam please iki fi

 

[Tom Sherman]

[email protected] wrote:
>
> Maybe somebody should do a my-gear-system-has-more-inches-than-yours FAQ site. So far I see 3
> options for managing with odd wheel sizes and performance requirements:
>
> 1) internal gear hub -adds friction, if it doesn't, it's expensive -adds some weight too -works
> well, practical in some situations
>
> 2) oversize chainrings at front -shifting can be sub-standard to your taste -not as flexible or
> big gear range
>
> 3) mid-drive/jackshaft -adds some friction
>
> Wondering about the third option... Can you just replace your chain idler wheel with a pair of
> sprockets, so you get changed gear ratio with little effort and little cost? You could even change
> the sprockets: for record racing, mountain touring... If you don't have a chain idler, you could
> drill a hole in the frame and just install the new system... I wonder if these mods would be hard
> to do in some workshop by yourself?

I happen to prefer No. 3, which is the standard Earth Cycles setup. You will need a tensioner for
the front chain - this could be made from a rear derailleur or possibly the Easy Racers chain
tensioner could be used.

Most idler wheels do not have much loading on them - I doubt that either the mounting or the
bearings on most would be up to use as a jackshaft. Industrial quality bearing would be prudent, as
would a strong and stiff mounting of the jackshaft axle to the frame.

The step-up arrangement is advantageous on a rear suspended bike or trike, as the jackshaft may be
placed at the suspension pivot, so pogo from pedaling forces should not occur.

Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

 

[Steven C. Hodge]

Tom Sherman <[email protected]> wrote in message >
[email protected] wrote:
> >
> > 3) mid-drive/jackshaft -adds some friction
> Most idler wheels do not have much loading on them - I doubt that either the mounting or the
> bearings on most would be up to use as a jackshaft. Industrial quality bearing would be
> prudent, as would a strong and stiff mounting of the jackshaft axle to the frame. Tom Sherman -
> Various HPV's
The Rotator bikes use a middrive that is a 6spd cassette with a front and rear bearing that are a
slip fit on a 10mm shaft. Shifting is done with a rear derailleur. This means the bike has to have
two rear or Right sided shifters, one for the rear wheel and one for the mid drive. They have been
using BMX axles but these are about 0.01" shy in diameter. When my Ti Pursuit was
rebuilt/reincarnated on a 2.125" monotube from the original 1.875" the middrive axle was a bit
loose. Made a new one out of silver steel and it broke at 850miles. Replaced it with a 4140 steel
axle and it has bent through a few degrees, a problem fixed by rotating the axle 180deg in the
socket. This suggests that there is "considerable" torque on the midaxle and that a larger axle size
should be considered than 10mm. 4140 axle now has 2800mi with no other problems as yet. The axle
screws into a tube that is welded to the monotube at right angle to the monotube and is internally
threaded. Steve

 

[Rorschandt]

[email protected] (Steven C. Hodges) wrote in
news:[email protected]:

> Tom Sherman <[email protected]> wrote in message >
> [email protected] wrote:
>> >
>> > 3) mid-drive/jackshaft -adds some friction
>> Most idler wheels do not have much loading on them - I doubt that either the mounting or the
>> bearings on most would be up to use as a jackshaft. Industrial quality bearing would be
>> prudent, as would a strong and stiff mounting of the jackshaft axle to the frame. Tom Sherman -
>> Various HPV's

An idler that deflects the "top run" of chain around frame members and the like recieves a lot of
stress. I know this from my experimental aluminum trike. The force is approximately equal to that
exerted to the pedals. (such as on a TeraTrike)

> The Rotator bikes use a middrive that is a 6spd cassette with a front and rear bearing that are a
> slip fit on a 10mm shaft. Shifting is done with a rear derailleur. This means the bike has to have
> two rear or Right sided shifters, one for the rear wheel and one for the mid drive.

I'm currently using a set of Shimano bar-end shifters with the only difficulty being that the left
shifter does not index. I'm using two 9spd cassettes, both work well across the range. Does the
Rotator design support the shaft on only one side? That will make a big difference. I'm using a 12mm
6061-T6 shaft, and am considering replacing it with a 4130 shaft, although there have been no
problems as of yet. My mid-drive is supported on both sides. Many hubs have smaller axles made of
6061-T6, but of course it is supported on both ends.

rorschandt