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Fitting a 'sub-compact' road chainset to BB90 bottom bracket?

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Fitting a 'sub-compact' road chainset to BB90 bottom bracket?

Old 12-07-19, 09:11 PM
  #26  
anotherbrian
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It looks like same setup as Absolute Black. Outside chainring is on outside but chain line is moved inboard so that there is no interference between the chain and spider/bolts when in the small ring.


Originally Posted by unterhausen View Post
I take it the Shimano spline is a straight spline instead of a tapered spline, so the bb can be shimmed. Doesn't look bad to me.

Something is still hurting my head about these. Could you post a picture from the non-drive side?
They used the same bolt hole circle, I guess the problem with putting a smaller chain ring on these cranks is just the height of the spider between the rings.
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Old 12-07-19, 09:30 PM
  #27  
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A bit tricky to get the nondrive side...

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Old 12-08-19, 02:41 AM
  #28  
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Ha, guess that photo from the Croder site played tricks on my eyes!
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Old 12-08-19, 08:09 AM
  #29  
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Very interesting method of getting a smaller chainring. I think there are 33 tooth chainrings with no change in chain line, but I never thought that was worth pursuing.
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Old 12-08-19, 07:33 PM
  #30  
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Originally Posted by unterhausen View Post
Very interesting method of getting a smaller chainring. I think there are 33 tooth chainrings with no change in chain line, but I never thought that was worth pursuing.
TA makes them in 7075 aluminum, which partly explains the cost. I've been considering it should I wear out my 34T...
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Old 12-08-19, 08:20 PM
  #31  
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Well, I got my Sunday ride done. Moderately fast group ride, a bit of rolling with a short climb in the middle along a segment I like to call Cow Dung Land (for obvious reasons). Stayed on the big ring for most of the ride except for the Cow Dung Land climb, no problems keeping up with the group and I think I have a better chain line with the 48 than with the 53. Spinning uphill with the 32 felt pretty good, even managed to get a new PR on the segment (though admittedly may be placebo effect... we'll see how it feels couple of months down the road...).

Two downsides though... first one is definitely no cross-chaining. Either big-big or small-small will result in chain rub on the FD. Not really a biggie, since I tend to avoid that kind of extreme combination anyway.

Second downside... got a flat on the rear tyre during the ride. I typically shift down to small-small to make it easier to dismount, and later remount the tyre but I think it is easier for the chain to drop on the 32? I lay the bike down on its (non-drive) side while changing the tube, didn't really pay attention to the chain until after remounting the tyre and noticed it dropped off the front ring. But maybe this is just a one-off? We'll see couple of months down the road (if I get a flat again... damn wire or staple or whatever that crap was!!)

Last edited by atwl77; 12-08-19 at 08:24 PM.
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Old 12-09-19, 08:39 AM
  #32  
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did the shop re-size the chain?
Hope you don't have any more flats. I think it's pretty common for me to lose my chain if I have to change a flat. It's just part of the wonderful experience
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Old 12-09-19, 08:46 AM
  #33  
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Yes, they did shorten the chain, though I forgot to ask by how much.
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Old 01-24-20, 09:31 AM
  #34  
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To return to this subject: for my specific situation, I think a GRX FC-RX810-2 (48-31, 11speed) is the best option. However, I also have a 10-speed bike. Would I get away with using the 11-speed crankset on this also?
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Old 01-24-20, 11:02 AM
  #35  
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my understanding is that it will work. Possibly a little better with the matching 11 speed chain

I just found out that Stages power meter only works with the 810 and the 810 doesn't come with a 46 tooth chain ring. I'm somewhat less enthused about the whole thing right now.
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Old 01-04-21, 06:07 PM
  #36  
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Option for running 32t w/o changing the chain line

One year later, I know. Just thought it would be good to add another solution to this thread. You can change just the chainrings to 48-32 and keep the same chain line using the Gearoop KOM Challenger chainrings. Or do what I did and just change out the small chainring to 32t (which you can buy separately), so now I'm running 50-32 on my Shimano 105 cranks. Yes, the shape of the small chainring is no longer perfectly round, but I can't say that it's noticeable.
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Old 01-05-21, 09:48 AM
  #37  
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I recently built up a new bike with Easton cranks, using 46/30 rings. So far, no complaints. They've got a modular system that lets you swap spiders (or integrated rings). On the original subject, I don't think this could be used with BB90, because Easton cranks have a 30-mm diameter, and the OD for BB90 bearings is 37 mm. You'd need to have little tiny ball bearings in the cartridges to fit the remaining space.

As Unterhausen said upthread, 46x11 is a big gear (though admittedly, a 46t big ring looks kind of puny). When I got started in cycling, 52x13 was the biggest gear you commonly found. 46x11 is bigger than that.
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Old 01-05-21, 06:03 PM
  #38  
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I have tried the Absolute Black 46 & 30 chainrings on an Ultegra 6800 compact crankset. I have also installed a 48 & 32 GRX crankset on my Canyon Endurace.

I was dropping the chain and have shifting issues with the Absolute Black chainrings. The bike has STAM eTap and the shifting of the oval chainring is not ideal.

The 48 & 31 GRX crankset is ideal on a disc brake bike. But the chainline on a rim brake bike would be offset and problematic.


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Old 01-07-21, 10:30 PM
  #39  
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Originally Posted by adamrice View Post
I recently built up a new bike with Easton cranks, using 46/30 rings. So far, no complaints. They've got a modular system that lets you swap spiders (or integrated rings).
The RaceFace 'cinch' 104/64bcd spider works with the Easton 'cinch' crank. TA Specialites Chinook Outer and Inner rings allow to you mix combos from 46t to 22t.
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