Technical > Transmission
chain slack
JimK:
I was zooming down a nice long hill with a few bumps along the way... when my chain fell off! I could put it back on quite easily without removing the rear wheel. Must be a bit loose!
I just checked - T1-T2 is about 75mm! So indeed, the instructions in the Thorn owner's manual look accurate! My idea right now is to put on a new chain... to rotate through a few chains & thereby minimize the wear on the sprocket and chainring. I saw somebody recommending five chains to match the rate of wear on the chains and sprockets. I have an extra new chain at hand, so that will be number two. After that.... we shall see!
I've only had my bike five months & I can't say I ride it very much - a bit surprised to have so much chain "stretch" already... but maybe it's too much slush and grit and not enough oil. I imagine I will be learning through experience!
Jim
JimK:
Truth is, I never measured the chain slack when I got the bike from the shop! Anyway, I just put on a fresh chain, and the slack was already T1-T2=50mm. So I took the plunge and tweaked the EBB - my first time fiddling with that.
Wow, just the slightest angular rotation and the chain tension changes hugely! It's very easy to pull the chain fully taut - a mistake, of course! So then I back it off a touch - oops, too much, I'm back to 50mm! Play a bit... I think it's about 25mm slack now - still pretty tight, but it turns freely, so that should be okay I think.
Here I am, plunging ahead and learning! I just hope I don't screw things up too badly!
Jim
JimK:
Since I have put on a nice new lubricated chain, I thought I would try lubricating my brain a bit too. How close will the little dents be in the EBB shell?
Let me try to estimate. I will zoom on on a section of chain from the measuring point to the chainring:
Now to attempt a bit of math:
The chain length stays constant as I reduce the slack by increasing the distance between the chainring and the sprocket. If I want to reduce the slack by say 24 mm, when the distance from the measuring point to the chainring is about 225 mm, then I need to move the chainring from the measuring point about 0.25 mm. The measuring point also moves from the sprocket that same 0.25 mm, so the total movement of the chainring from the sprocket is 0.5 mm.
The bottom bracket axle is about 5mm eccentric. The shell is about 20mm in radius. So at the position where the axle is at about 12 o'clock, a 0.5 mm movement of the axle should translate to an 2 mm movement of the shell.
So the dimples in the shell should be about 2 mm apart at their closest.
Anybody want to check my math?
Relayer:
--- Quote from: JimK on April 18, 2011, 08:22:08 pm ---I was zooming down a nice long hill with a few bumps along the way... when my chain fell off! I could put it back on quite easily without removing the rear wheel. Must be a bit loose!
I just checked - T1-T2 is about 75mm! So indeed, the instructions in the Thorn owner's manual look accurate! My idea right now is to put on a new chain... to rotate through a few chains & thereby minimize the wear on the sprocket and chainring. I saw somebody recommending five chains to match the rate of wear on the chains and sprockets. I have an extra new chain at hand, so that will be number two. After that.... we shall see!
I've only had my bike five months & I can't say I ride it very much - a bit surprised to have so much chain "stretch" already... but maybe it's too much slush and grit and not enough oil. I imagine I will be learning through experience!
Jim
--- End quote ---
Funnily enough my chain came off a few weeks ago, this surprised me a lot since my chain doesn't seem to be particularly slack, in fact it's positively taught compared to some pictured on this forum! However at the time I was also zooming down a hill and changing up into 14th gear and I think I hit the bump at exactly the same time - no recurrence so I won't fiddle with the EBB just yet, but in future I will be aware when hitting bumps at speed.
P.S. I am sure you math is good Jim :-\
julk:
JimK,
make sure the dimples do have a gap between them, otherwise they just run together in a groove and the eccentric can move easily.
I am sure the eccentric dimples on my stolen Thorn were further apart than 2mm, but I waited until the chain was just about dropping off before adjusting the eccentric.
I still have to adjust the new bike chain, not slack enough yet...
Impressive maths!
Great photos of your rides (previous posts)!
Keep up the posting.
Julian.
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