Compact cranks

The current fad in road bicycles is to move to 110mm BCD “compact doubles”. Compared to a 130 or 135mm BCD this lets you run slightly smaller chainrings. A typical 130mm BCD double has 53/39 chainrings, where a typical compact double has 50/34 chainrings. I’ve talked about this many times on the BOB and RBW lists, but a conversation yesterday with Jan Heine reminded me of it and I thought it would make a good blog entry.

I’ve been experimenting with compact doubles on and off for 5 or 6 years. I started with a 110mm double (this was a fairly common crank in the 80s). My first bike setup like this had 50/34 chainrings with a 12-25 7sp cassette. Just looking at those numbers this sounds like a pretty reasonable setup: a high gear of 112″ and a low gear of 36″. Indeed the range worked well for me.

The problem with this setup is that my normal gears lived in the crossover range of the drivetrain. This meant that I was always shifting between the chainrings. Even worse when shifting chainrings I also had to shift many cogs on the rear due to the large difference in chainring sizes. This becomes more obvious when you chart out the full gearing:

50 34
12 112.5 76.5
13 103.8 70.6
15 90 61.2
17 79.4 54.0
19 71.1 48.3
22 61.4 41.7
25 54 36.7

From riding singlespeed I know that my normal cruising gears are in the 60-70 gear range. 70″ is the right gearing to go about 18mph at about 85rpm, a comfortable cadence. When you look at this gearing chart you can see that this gear is near the end of the cassette. So if I’m out the outer chainring and come to even a moderate hill I’m quickly dropping down into the 34t chainring. Likewise if I’m on the small ring and crest the hill I instantly need to shift into the outer chainring. Worse was that I ran this combination on a short chainstay bike (a Miyata 912 with 39.5cm chainstays) so using the 34t ring with the 2 smallest cogs or the 50t ring with the two largest cogs was noisy and inefficient. After about two weeks with this setup I changed the bike back to a triple.

This gearing is suited for someone, but it is someone who normally rides an 80″ to 90″ gear, not a cyclist who normally rides a 60-70″ gear. Even a 53/39 double would be better here because at least I could do most of the riding on the 39, just changing chainrings for long flat sections or downhills.

A Ritchey 94/58 crank setup with 46/31 chainrings

A couple of years ago I started experimenting with compact doubles again. This time I chose cranks with an even smaller BCD: 94mm and 86mm. 94mm BCD was commonly used on mountain bikes in a 94/58mm BCD triple. The smallest ring for 94mm BCD is 29t, but those are hard to find. 30t and 31t are a little easier, and 32t is very common. You can get large rings up to about 48t pretty easily. 86mm BCD was used on touring triples in the 70s and 80s where all three rings were mounted to the same BCD with extra long bolts. These cranks were made by SR and Stronglight. A typical setup would be 48/38/28 or 50/45/28. 28t is the smallest ring made.

Another change was putting on wider range cassettes. All of my bikes now have 8sp or 9sp cassettes. That one extra gear makes a noticable increase in cassette range. Here is what a 94mm double with 46/31 chainrings and a 12-28 8sp cassette looks like:

46 31
12 103.5 69.8
13 95.5 64.4
14 88.7 59.8
16 77.6 52.3
18 69.0 46.5
21 59.1 39.9
24 51.8 34.9
28 44

29.9

Look at what has changed. My cruising gear is now in the center of the cassette with the 46t chainring. The gear range on the 46t chainring extends down to 44t, low enough to let me climb most hills. The smaller large chainring also let me get a smaller small chainring with good shifting and that now goes low enough for almost any hill when unloaded. I did give up a little on the high end, but I almost never use gears over 100″ so this is not a problem.

This setup works great. I can do 95% of my ride in the 46t chainring, only shifting the rear cassette. I shift the front only for long hills. On my normal 10 mile commute this means that I’m only in the small ring once,where I’d drop into it 8 or 9 times with the 50/34 setup.

The hard part about getting real compact doubles is finding the cranks. If you want to experiment with this your best bet is finding a 86mm triple crank. The Trek touring and triple bikes sold in the early to mid 80s often came with the 50/45/28 cranks. These are reasonably easy to find on the used market and are nice production bikes which ride well. You may be able to find the entire bike used at a garage sale or swap meet for the price of a new crank. To make a compact double just remove the 50t large ring and replace the chainring bolts with double bolts instead of triple. You’ll also be able to put in a shorter bottom bracket (I think 107mm or 110mm is what I used) and you’ll end up with a lower Q-factor. Try this out with the 13-28 6sp freewheel that the bike probably came with and I think you’ll be a convert.

If you are lucky enough to have a Ritchey Logic Compact Triple then these cranks also make great compact doubles. They are extremely well made,the silver ones have a nice finish,and the cranks have no built in spacers for the granny ring, so they easily convert to running as a double. The hard part is setting up the bottom bracket. As a triple these cranks want a 107 or 110mm bottom bracket. When you remove the inner chainring you need to get even shorter. The shortest commonly made bottom bracket is 103mm and these are not easily found anymore (Shimano sold one for the 8sp Dura/Ace cranks). This is still not quite short enough.

I removed the fixed right shoulder from a Dura/Ace 103mm bottom bracket so that I can slide the bottom bracket about 3mm to the left. This gives me a much better chainline when running as a double. These bottom brackets use a removable right “cup” so I can move my modified cup to another bottom bracket unit when this one fails (I have a few spares). If you like this setup then the best alternative is to buy a Phil Wood BB. They’ll make whatever length you can dream of and list a 102mm that should work well in this application since Phil Wood BBs already have an adjustable chainline.

The TA Carmina is also available in a 94mm BCD double. This is a nice crank, but looks very modern and costs over $300.

Leave the 110mm doubles to the racers who actually cruise all day in 85″ gears. Most recreational cyclists will find a 48/38/26 triple to be better than a 52/42/30 triple and likewise they’ll probably find something like a 46/30 double to be better than a 50/34 double. Try it and I think you’ll like it. If enough people try it and like it maybe we can get a special run of 94mm BCD double cranks from Kogswell or Rivendell.

alex

10 Comments

  1. Dave Cramer says:

    I”d be delighted to find a nice 94bcd double, silver and low-profile.

    Right now I”m running 46/33 chainrings with a 12-32 9sp cassette, which is working well for me. This is with one of the new Sugino XD2 doubles available from Yellow Jersey (they no longer have to grind off the inner chainring mounts).

    Dave Cramer
    Brattleboro, Vermont

  2. Nate says:

    Great story Alex. How can i get a double with the 46/31 gearing? Can I buy a mountain triple and simply remove the small ring?

    Thanks

  3. AlexWetmore says:

    Sadly it is difficult to get a 46/31 double today. You have a couple of new options:

    White Industries cranks use custom outer rings and let you use any BCD inner ring.

    TA Cyclotouriste cranks are still being made in very limited quantities. http://www.velo-orange.com has them. These can be setup with a small inner ring.

    TA still makes a 94mm BCD double spider for their cranks. http://www.peterwhitecycles.com sells these.

    You can buy used 94mm BCD mountain bike cranks such as the Ritchey Logics that I used. It can take some care to make sure that you”ll get good chainline with a double.

    You can find some used 86mm BCD cranks. They were sort of common on touring and sport touring bikes in the early 80s. They often came setup with 50/45/28 rings, if you remove the 50 and change it to a double you get a nice 45/28. Spare rings are a little hard to find, if you go this route I”d stock up on rings too.

  4. Geof says:

    It looks like Stronglight still makes a MTB double with 94 mm BCD. The Oxale Two model is what one should look for.

    http://www.stronglight.com/stronglight/page.php?nom=produit&keyProd=oxaletwo

    It comes with 44/29 chainrings. They make a 46t chainring for the outer ring as well.

  5. AlexWetmore says:

    Great find on the Stronglight Oxale Two. It looks like it is widely available in the UK for reasonable prices (around $140). Maybe someone will import it to the US as well.

  6. Geof says:

    This website will sell them in the US for $135.

    http://www.xxcycle.com/oxale-two,,en.php

    I believe that it is a French company so I have no idea how much shipping would be.

  7. Harry Lackland says:

    Having ridden a compact crankset (FSA Gossamer 50/34) for the last three years, I gnash my teeth that I didn”t have one 28 years ago, when I began cycling. It’’s well nigh perfect for me, except for one thing: once a year, I need a smaller inside chainring. It’’s for the Mt. Equinox climb, which is bloody steep: average grade 12 percent, maximum 28 percent. Using the biggest cog that Campag makes–a 28–in combination with the 34 chainring, I have a 33-inch gear. It’’s worth noting that that’’s the gear the 2006 Equinox winner, Joe Moody, used. But it’’s too much gear for me. I need something about halfway between a 33 and the 1-1 that a lot of Equinox riders use. To the best of my knowledge, there is no compact triple crankset on the market, and if there were I couldn”t bring myself to hang one on my beautiful handbuilt steel frameset. (I already feel like an aesthetic sell-out for sticking a blackTaiwanese crankset on my steed.) The day I turn to a triple crankset is the day I start wearing knee-high white tube socks. If only I could bolt a 30 or a 32 onto the inside bolts for that one day each August!

    I”m technically maladroit. Ideas, anyone?

  8. SuperDave says:

    You can use a 33T on your inner 110mm BCD compact cranks and Campy now makes a 29T largest cog, that should solve your issues for that “one day…”

    Enjoy,
    -SD

  9. Eric says:

    It’’s been a few years… but I”m not going through the exact same thought process and also arrived at the 94 bcd crankset with 46/30 rings as the best solution for a touring bike with Sram shifters. Check out Topline or Cook cranks – also the Suntour Microdrive. These are all 94, pretty easy to find on Ebay in 175mm or longer, a bit tougher in other lengths. The Toplines/Cooks really are beautiful.

  10. A few years have passed and there are definitely more options. I”ve been looking at the Middleburn cranks for my rSogn. You can do a 27-40, 29-42 or 32-44 compact double, with the inner ring acting as a spider. With a 12-36 or 11-36 cassette, it closely approximates the wide range of a mountain triple. Current price is $238 with chainrings.

    http://www.middleburn.co.uk/chainrings_duo.php

    Available in the US here:
    http://www.mtbtandems.com/Middleburn.html