Paul Swenson
08-04-2006, 11:16 AM
I just ran across this on Cyclingnews. SRAM has new recommendations on determining chainlength on FS bikes. Apparently there was a number of JRA rear derailleur explosions.
SRAM urges rethinking of proper chain sizing
By James Huang
Sizing a chain used to be a rather straightforward affair, with any one of the several accepted techniques yielding chain lengths that are typically within a link of each other. In general, if the chain could accommodate the large chainring-large cog combination without sagging in the other extreme, you were good to go.
The prevalence of full-suspension mountain bikes, however, has complicated the matter as the effective chainstay length can change dramatically as the suspension moves through its travel. Usually, this length increases during compression, and the change can be particularly pronounced on certain suspension designs and/or longer travel rigs. Therefore, if chains are sized using the old standard methods and the suspension is compressed when in the big-big combo, the results can be rather "unpleasant", and often costly.
In response to what has clearly been a marked increase in "JRA rear derailleur explosion" (industry jargon for 'Just Riding Along' ) complaints, SRAM has released the following "Chain Sizing Made Easy" guidelines, which are applicable regardless of chain make or model:
1. Wrap a full-length chain around the largest chain ring and largest cog combination, bypassing the derailleurs.
2. If applicable, chain sizing should be performed when the rear suspension produces the maximum effective chainstay length. This usually, but not always, occurs at full compression. Deflating the rear shock, removing the coil spring, or unbolting one end of the shock may make it easier to determine this point.
3. Add one inch of chain to the minimum length required to accommodate this gear combination.
4. Remove excess links as necessary.
5. Route the sized chain through the drive system and connect as directed by chain manufacturer.
Keep in mind that this method may result in insufficient chain tension in some cogs while in the inner chainring, but you probably already know better than to run in those cross-gear combinations anyway. Besides, the elimination of a few mostly-unusable gears is a much more appealing alternative to having your entire rear end shatter in spectacular fashion.
For more information see www.sram.com (http://www.sram.com)
http://www.cyclingnews.com/tech.php?id=tech/2006/news/08-04
SRAM urges rethinking of proper chain sizing
By James Huang
Sizing a chain used to be a rather straightforward affair, with any one of the several accepted techniques yielding chain lengths that are typically within a link of each other. In general, if the chain could accommodate the large chainring-large cog combination without sagging in the other extreme, you were good to go.
The prevalence of full-suspension mountain bikes, however, has complicated the matter as the effective chainstay length can change dramatically as the suspension moves through its travel. Usually, this length increases during compression, and the change can be particularly pronounced on certain suspension designs and/or longer travel rigs. Therefore, if chains are sized using the old standard methods and the suspension is compressed when in the big-big combo, the results can be rather "unpleasant", and often costly.
In response to what has clearly been a marked increase in "JRA rear derailleur explosion" (industry jargon for 'Just Riding Along' ) complaints, SRAM has released the following "Chain Sizing Made Easy" guidelines, which are applicable regardless of chain make or model:
1. Wrap a full-length chain around the largest chain ring and largest cog combination, bypassing the derailleurs.
2. If applicable, chain sizing should be performed when the rear suspension produces the maximum effective chainstay length. This usually, but not always, occurs at full compression. Deflating the rear shock, removing the coil spring, or unbolting one end of the shock may make it easier to determine this point.
3. Add one inch of chain to the minimum length required to accommodate this gear combination.
4. Remove excess links as necessary.
5. Route the sized chain through the drive system and connect as directed by chain manufacturer.
Keep in mind that this method may result in insufficient chain tension in some cogs while in the inner chainring, but you probably already know better than to run in those cross-gear combinations anyway. Besides, the elimination of a few mostly-unusable gears is a much more appealing alternative to having your entire rear end shatter in spectacular fashion.
For more information see www.sram.com (http://www.sram.com)
http://www.cyclingnews.com/tech.php?id=tech/2006/news/08-04