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Mid-drives for wide range gearing



 
 
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  #1  
Old October 15th 10, 05:34 PM posted to alt.rec.bicycles.recumbent
Opus[_2_]
external usenet poster
 
Posts: 414
Default Mid-drives for wide range gearing

I have been working on a mid-drive to use in a variety of applications
from drag racing to Battle Mountain to loaded touring. One thing I
have found is that using a mid-drive it is possible to get very wide
gearing with miniscule jumps between gears using standard bicycle
parts with a minimum of special components. The one I'm working on now
has the equivalent of a quintuple front with a 9 speed rear using 2
rear deraillers.

For touring the top gear doesn't need to be too high, a suitably low
gear for climbing steep hills is a must, but when touring with a
velomobile that gearing range must be extended in both directions
which is where the mid-drive shines.

The system I'm building now uses a lefty drive BMX crank that fits JSA
square taper cranks (Shimano), on a bomm that pivots on a bearing that
rides on the shaft for the mid-drive, with a rear derailler tab
mounted on the left side of the boom. The boom swings on the shaft to
allow adjustment for riders of different leg length without the need
for breaking the chain and adding or removing links, and the tab on
the boom means that the geometry of the front half of the drive
doesn't change when the pedals are moved back and forth to change for
differently sized riders. Chainwheels and drive side sprockets can be
changed to move the range up or down to suit the user without needing
to make major changes to the driveline, using the tooth range of the
deraillers to take up the adjustments

For the touring VM I'm using a 5 speed 11-34 cluster on the driven
side of the mid-drive with a single floating drive side sprocket. On
the rear wheel I'm using a 9-speed cassette that runs from 22-30
teeth, or basically the entire range between jumps on the front half
of the drive in single-tooth jumps. This allows the rider to get the
precise ratio they need to maintain a smooth cadence but still have
the range they need for climbing and descending mountains. The rider
can shift both deraillers at the same time without chain management
worries because the systems are independent. Using a 34 tooth
chainwheel and a 30 tooth drive sprocket, with a VM equipped with a
20" drive wheel will have a gear range of 20-85" with 44 single-tooth
jumps between gears. Obviously if you want more range then using a
cassette with larger steps between cogs will get you a wider range at
the loss of tiny steps between gears. Going 11-34 rear with the same
20" wheel and chainwheel and drive sprocket gives a range of 18-168",
replacing the drive sprocket with a 22 tooth gives a more sane top
gear of 124" but gives a stump-pulling 13" low gear for moving a
loaded VM up walls.

So, there you have it. Extremely close-ratio gears over a wide range,
or an insanely wide range with gearing jumps that are more normal to
standard bicycles, with the ability to move that range up or down by
changing a single sprocket in an easy to get to location without
having to change chain lengths. You could ride your VM to Battle
Mountain, change one sprocket and have the drivetrain to race at
Battle Mountain
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  #2  
Old October 16th 10, 11:04 PM posted to alt.rec.bicycles.recumbent
Tom Sherman 起豺_2_]
external usenet poster
 
Posts: 2,312
Default Mid-drives for wide range gearing

On 10/15/2010 11:34 AM, Opus the Poet wrote:
I have been working on a mid-drive to use in a variety of applications
from drag racing to Battle Mountain to loaded touring. One thing I
have found is that using a mid-drive it is possible to get very wide
gearing with miniscule jumps between gears using standard bicycle
parts with a minimum of special components. The one I'm working on now
has the equivalent of a quintuple front with a 9 speed rear using 2
rear deraillers.

For touring the top gear doesn't need to be too high, a suitably low
gear for climbing steep hills is a must, but when touring with a
velomobile that gearing range must be extended in both directions
which is where the mid-drive shines.

The system I'm building now uses a lefty drive BMX crank that fits JSA
square taper cranks (Shimano), on a bomm that pivots on a bearing that
rides on the shaft for the mid-drive, with a rear derailler tab
mounted on the left side of the boom. The boom swings on the shaft to
allow adjustment for riders of different leg length without the need
for breaking the chain and adding or removing links, and the tab on
the boom means that the geometry of the front half of the drive
doesn't change when the pedals are moved back and forth to change for
differently sized riders. Chainwheels and drive side sprockets can be
changed to move the range up or down to suit the user without needing
to make major changes to the driveline, using the tooth range of the
deraillers to take up the adjustments

Can you post pictures on one of the free Internet services?

For the touring VM I'm using a 5 speed 11-34 cluster on the driven
side of the mid-drive with a single floating drive side sprocket. On
the rear wheel I'm using a 9-speed cassette that runs from 22-30
teeth, or basically the entire range between jumps on the front half
of the drive in single-tooth jumps. This allows the rider to get the
precise ratio they need to maintain a smooth cadence but still have
the range they need for climbing and descending mountains. The rider
can shift both deraillers at the same time without chain management
worries because the systems are independent. Using a 34 tooth
chainwheel and a 30 tooth drive sprocket, with a VM equipped with a
20" drive wheel will have a gear range of 20-85" with 44 single-tooth
jumps between gears. Obviously if you want more range then using a
cassette with larger steps between cogs will get you a wider range at
the loss of tiny steps between gears. Going 11-34 rear with the same
20" wheel and chainwheel and drive sprocket gives a range of 18-168",
replacing the drive sprocket with a 22 tooth gives a more sane top
gear of 124" but gives a stump-pulling 13" low gear for moving a
loaded VM up walls.

Or you could use a Schlumpf bottom bracket for really extended gearing
range, without going to the complications of three shifters and
redundant gears that adding a conventional front derailer and triple
crank would result in.

So, there you have it. Extremely close-ratio gears over a wide range,
or an insanely wide range with gearing jumps that are more normal to
standard bicycles, with the ability to move that range up or down by
changing a single sprocket in an easy to get to location without
having to change chain lengths. You could ride your VM to Battle
Mountain, change one sprocket and have the drivetrain to race at
Battle Mountain


I thought recumbents were standard bicycles.

--
Tom Sherman - 42.435731,-83.985007
I am a vehicular cyclist.
 




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