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B tension.. chain length???(4 posts)

B tension.. chain length???Geof
Feb 18, 2002 5:14 PM
Can the B-tension be off by the chain length? Is there a direct relationship between the two? I've been playing this game for awhile and am baffled by the whole B-tension thing (I know what it does) it just seems that Shimano B-tension is basically worthless. I tend to run a slightly (one full link) short chain (seems to shift quicker) and I wonder if this has a major impact on B-tension. My Sram 9.0sl B-tension on my MTB visibly moves the der, thus allowing a very precise adjustment... not so with my Shimano. Also, is the B-tension adj any diff on a Dura Ace der???
"B" screw adjustmentKerry Irons
Feb 19, 2002 5:22 PM
It's not about chain tension, it's about the relationship of the top jockey pulley to the cogs. The chain tension is controlled by the derailleur, and varies quite a bit from the small/small to large/large combo. Any tension chaing from the "B" screw is so minor as to be meaningless.
That's what I thought but...Geof
Feb 19, 2002 7:51 PM
Can the length of the chain cause the der to be pulled up and forward thus loosing the potential for adjustment at the largest cog...

In other words, if your upper jockey pulley is touching the largest cog, and the B-tension is maxed out, can the chain have any thing to do with this? Or, could it be the rear cable housing being too long?

I was unable to adjust in the correct gap on a 12-27 cog on my Santa Cruz Roadster frame. I thought this to be rather odd. My 12-25 adjusts pefectly with a bit of room to spare on the b-tension. Is this common for 12-27's? BTW I'm running a 42-53 up front with the chain set up for that combo... I would think there would be enough "slack" to not have to worry about lengthening my chain...any thoughts?
Try thisKerry Irons
Feb 21, 2002 6:31 PM
Your setup needs to be understood by you. You can easily see the effect of chain lenght by shifting to the cog of interest, and then just pulling down on the lower length of chain. This will be the equivalent of shortening the chain. Whatever the effect is, lengthening the chain will have the opposite effect. It sounds like your chain is too short in going from the 25 to the 27 (maybe just barely long enough with the 25, but not enough for the 27). Why don't you just add a link and see what happens. Your comment of "unable to adjust in the correct gap on a 12-27 cog" is not the least bit clear. Same for your "12-25 adjusts pefectly with a bit of room to spare on the b-tension" reference. You leave us guessing about what exactly you're trying to say.