Bicycle Mechanics - Optimum Deraileur Drivetrain
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For a deraileur equipped lightweight city bike with 650B X 42m Hetre tires, what would be an excellent shifting set up for a gear range of approximately 20-95 inches? I might be stuck using a V-O crankset, what would I be sacrificing compared to a Shimano crankset?
12-31-10, 11:32 AM
Which Shimano crankset? there have been many sorts.. :50:
if you can get a TA Cyclotourist Crankset, they allow 2 chainrings of wide difference
to be paired together.
On a bike tour down the Cal coast my companion from Britain had a 50tooth ring and a 28 tooth ring.
A hybrid drive train using schlumpf planetatry 2 speed crank is another approach
his 2 overdrive cranks are 1.6X and 2.5X the size of the small chainring
He also manufactures, in Switzerland, a reduction geared crank, I have one of those
That I fitted on my Brompton Folding bike, to accompany its 3 speed hub..
In my case the chain never leaves the chainrings on either end, as both ratio changes are internal..
My 54 t ring is in effect a 21.6t granny .. the 50t ring size divided by 2.5 comes out an even 20t.
on my 16" wheel bike.. low is 17" high is 77 " which is excellent for my purposes.
In derailleur bikes triples with a 58 94 pairing , or a 74 110 bolt circle pairing will work too .
in the 80's, attracted by shiny objects, i used a Campag triple as a double..
Middle 52, inner 36, outer a cyclocross chainguard.
it is 144, the inner an 86 mm , machined into the crank arms
Gipiemme triple arms were thickened to suit.
back then 13 t was as small as cogs on derailleur bikes were.
There's no one answer. You could go double or triple depending on your riding style and preferences. Glancing at this gear chart (http://home.i1.net/%7Edwolfe/gerz/gerz26.html) (which may be off a bit based on actual tire diameter) you can see that a 44/12 yields a decent high as would a 40 or 42/11. At the other end a 22/28, 24/30 or 26/32 would all give you the low you need.
I prefer not to go above 28t on freewheels (just a preference, not saying it creates problems otherwise) and like tight gearing for use on the road, so I'd probably do something like a 12/28 cassette with a triple 22/34/44 crankset. I haven't worked it out in detail so that's something you need to do for yourself. For a mtn bike I'd probably opt for a double with a wider cassette, and use outer chainring position for a bashguard.
You have lots of options, so use a gear chart to determine your endpoints then slot in the various choices for the middle based on available hardware.
So far, I have never had success with using a double ring with a large gap of 20+ teeth for wide range gearing. I think a triple is the way to go unless somebody has a combination they know can work well. Chain suck on the downshift was a big problem with the dual chainring. I am unfamiliar with the latest equipment (I know all about TA!), so I don't know what to expect. What I need are recommendations for equipment groups that people know from experience will work. Price is not a major concern. I have heard the 650B wheel size could present some front deraileur clearance problems with mountain bike groups, it is this kind of stuff I need to know. Thanks for the help!
12-31-10, 06:34 PM
Another calculation .. more wheel sizes http://sheldonbrown.com/gears/
On my touring bike which has 700c wheels 50/14 was a 95"
24/34 or 32 was the low. 6 speed freewheel so 17 20 24 28 in between,
a 40 or a 38.. campag race triple is a 24,40 50 dropping the 30..
74 130 BCD lets the middle one be a 38
optimum seems a Rohloff hub for Me, now
down shifts when youre nackered to a stand still
A Rohloff hub is supposed to work well for this application. No experience with one and lots of people don't like the friction and noise on a road bike. I will try one soon, but I might be too old a dog for this new trick.
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