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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
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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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