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:

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 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:

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