Tuesday, November 27, 2018

Double chainrings finally practical for fast bents? 12s, XD drive, and super-compact 2x.

Of all the bikes out there, fast recumbents need the widest gear range of all. We go faster on flats and downhills than any other platform, but we can not stand on climbs, and we and our bikes are generally heavier. More than anyone out there we need wide range triple chainring drivetrains. Right?Right!

But there are problems with this. No one ever made a triple (3x) drivetrain with the range we need. the rear derailleurs can not handle all the extra chain in the small small combinations. I have a 54/39/27 crank and a 11-32 11s cassette. In the 27t chainring, almost half of the cassette leaves the chain slack! I can still use the gears, so it's not a big problem, but that's not ideal.

Lately things have been getting worse. Triples are going away in favor of all manner of doubles (2x), and, shudder, 1x! My set up leaves me with a gear range of 130.2-22.4 gear inches. I have calculated that to duplicate my current range of gears with 1x requires a 50t chainring, and a 10-60t cassette! To get the nice steps between gears it would need to be a 17s cassette too! That is well over a decade out, if ever. To make matters worse, you cannot entertain the thought of electronic shifting with a triple.

I have considered a compact double a few times in the past, but with 11s or even 12s I would want a 52/34, and would need an 11-40t cassette. The gaps between the small cogs would be too big so I would need a separate 11-32t or smaller cassette for flat rides and races. The great thing about my current 3x setup is that I can do everything with one cassette, so I don't want to give that up. Plus a 52/34 is not something any front derailleur will officially shift, let alone an electric one.

So what has changed? 4 things.
  1. SRAM is going to 12s for road groups. 
  2. The SRAM XD drive freehub bodies are gaining popularity. They allow the use of a 10t small cog on the cassette. SRAM is expected expand use of this from MTB to road with their new 12s road groups. It will be called XDR. The R is for road. 
  3. Super-compact 2x cranksets are now a thing, with 46/30t rings. There are even 46/30t rings being made to fit on some current compact 2x cranksets! Thank you gravel market!
  4. I have decided I can give up the 54/11 gear (130.2 gear inches). I am running fatter tires every year, which helps mitigate this a bit. I am getting older too. 
How does this all work together to give you the range and the reasonable gear spacing you need? 
  1. 12s just gives you one more gear range while maintaining reasonable gaps between gears. For me a 11-28t cassette has reasonable gaps in 10s, and an 11-32t is good in 11s. So it makes sense that an 11-36 is good in 12s. 
  2. XD drive means I can actually go to a 10-36t cassette. 
  3. Super compact rings mean that while we still see a 16t max gap between the 2 rings, as we get smaller in ring size, 16t gives a greater percentage gear difference. The difference between a 52 and 36 is 44%, but a 46/30t is over 53%. So in smaller chainrings a 16t gap, which works with most 2x front derailleurs, gives us more range. 
Behold the comparison (thanks to Sheldon Brown). New double on the left, old triple on the right. This is for a 700x28 tire. I give up less than one gear on top, dropping from 130 to 122 gear inches. My low gear drops a hair from 22.4 to 22.1 gear inches. As far as reasonable gaps between gears, I go from a range of 7.1-15.8% to 7.7-16.7%. Tolerable. In the critical area for riding hard on flat ground (roughly 80-110 gear inches for me) I still have 4 gears. Instead of using the 54t with the 13, 14, 15 and 17, I will be in the 46t ring and the 11, 12, 13, and 14t cogs. I spend a lot of time in the 54/14 combination in training rides/races, now it will be the 46/12. 

This chart shows you can get almost the same gears with a 26" or 650c wheel with a 50/34 chainring combination.

This chart shows you can get almost the same gears with a 24" (520mm) wheel with a 55/38 chainring combination. Now the gap between the rings is starting to exceed 16t, which may not work as well with a double front derailleur. You may want a 56t as well. 

  1. Lighter for sure. 
  2. Simple front shifts.
  3. Simplified shifting in general.
  4. Electronic capable. 
  5. Less boom flex in the small ring, because it's bigger than the inner ring on the triple was. 
  6. No chain slack?
  7. Triples are getting harder to find. 
  1. Slightly faster cog/ring wear. 
  2. Slightly more drivetrain rumble from the small cogs. 
  3. More boom flex in the big ring, since its now smaller than what the triple had.  
  4. As with most steps to more cogs on the cassette, chains get narrower, and the tolerances in the parts doing the shifting need to be tightened up, which costs money. 
  5. Not much available in oval chainrings yet. Absolute Black makes some, but they are not adjustable to a recumbent position like Rotor's Q-rings. 
  1. Yes, the 10-36t cassette I show is fiction. I made it up. I bet a 10-32 will exist shortly. Not sure about a 36. Lets hope the gravel market continues to drive innovation. 
  2. Even if it does exist, it may only be intended for a 1x system, and a rear derailleur capable of shifting a 10-36t may not exist in a road group. Hopefully SRAM keeps with their history of making road shifters work with and 10s MTB rear derailleurs, and that 10s rear derailleurs will work on 12s+, unlike Shimano. Or use a JTek Shiftmate 9
The future: 
  1. When/if monotube recumbent frames get stiffer the boom flex issue in the big ring will diminish. 
  2. If at the same time they are also available to fit riders better, the ability to climb at lower speeds will improve, so there may be a desire for one or 2 more low gears say a 40 or 42t cog. By the time that happens I may accept a compact plus 42/26 or 40/24 2x combination for the chainrings instead. 
Crank/chainring/spider options:
  1. Super-compact cranksets:  
  2. More super-compacts and smaller!
  3. Engin cycles makes a super compact spider for SRAM cranks, but its not on their site: Picture:
  4. You can even just remove the outer ring from your triple and install a 46t ring in the middle spot. Leave the outer position empty or get a chain guard. Since no one makes a 12s compatible triple, ring spacing may be an issue. Dana at did set up a bike like this years ago.
Why chainring size affects boom flex.

Let's cross our fingers!!

PS. Just came across the 3T 11s 9-32 Bailout cassette!! So if we take the above principle one step further.....

...and this cassette might exist!! See that as the rings get smaller, a 16t tooth gap is now 61.5%. That's how we get enough range out of a double.

No comments:

Post a Comment