The Wheels of Chance

A non-cyclist friend of mine described this blog as “geekified to the nth degree”, and now I’ve made it even geekier by referencing “The Hitch Hiker’s Guide to the Galaxy” in a post title. But I have no shame as the number 42 actually is important to me and my approach to cycling. All will be revealed.

Most bikes come with one of the standard crank and chainring combinations. If it’s a road bike it might have a compact double 50/39,50/34, 53/39 or a triple 50/39/30. On specialized touring bikes you’ll find combos like 48/36/26, 46/36/24 or if they use a MTB crank something like 44/32/22. The cassette on a road bike might be a 10 speed 12-25 while for touring it might be 11-32. There are almost infinite combinations and complications which get in the way of the basic truth that a range of gears from 100″ to 20″ will satisfy most cyclists and will definitely be appropriate for a tourer. Those “gear inches” numbers I just mentioned are calculated by taking the ratio of the number of teeth on the front chain ring to the number of teeth on the rear sprocket and multiplying by the wheel diameter. So 100″ is a hard gear to push and at 80 rpm you’ll be moving at 24mph while in a 20″ gear you’ll be spinning away and at 80rpm the bike will be moving at 5mph.

I rode 48/36/24 chain rings with a 11/32 cassette for a long time giving me a range of 118″ to 20″

That would seem to be just fine, “but no – wait a minute” there are other things to consider like the gear that you ride in most of the time and how often you want to be changing gears. Also there’s the issue of cross chaining. This is where there’s a big angle in the chain and occurs when you use a big chain ring and big rear sprocket or small chain ring and little sprocket combinations. So the 48t ring on my original crank was best used with the outer 4 rear sprockets, after that the idea is to change to the middle 36t chain ring and use the middle 4 rear sprockets. This keeps the chain fairly straight, but as I like to ride around 70″ most of the time I found myself at the extreme range of the outer and middle chain rings. The 48t ring was just a bit too big and the 36t ring was just a bit too small. I also just hated changing using the front derailleur.

So it occurred to me that a 42t chain ring would be perfect for 95% of my riding and a 24t or 26t ring would be useful as a granny to haul up steep climbs. A 42/26 double would reduce front ring changes and combined with an 11/34 cassette would give me a great touring range of 103″ to 21″. Here is the resulting gearing.

103.1	63.8
87.2	54.0
75.6	46.8
66.7	41.3
56.7	35.1
49.3	30.5
43.6	27.0
37.8	23.4
33.4	20.6

The 42/26 is like a compact double, but with smaller rings to give the lower gears favoured by tourists. The smallest inner ring that fits on a conventional 110mm bolt circle diameter compact crank has 33t. So to implement this super-compact double a 110/74 double crank would be perfect, but one didn’t exist (Sugino have since come out with one http://velo-orange.blogspot.com/2009/03/taipei-cycle-show-update-3.html). I could do it on an old TA cyclotourist crank, but then the obvious hit me. Simply remove the outer ring of a 110/74 triple and put the 42t ring on the middle location and the 26t ring on the inner position. I chose 26t over 24t to stay withing the range of a compact double front derailleur, ie 50-34= 42-26= 16. I’d have to move the bottom bracket cartridge out a few mm so that the middle ring lined up with the 17t sprocket on my 11/34 rear cassette, but with a Phil Wood square taper BB that’s easy. I took the rings off my Sugino 110/74 triple and replaced them with two shinny 42t and 26t TA rings. I had to use single ring crank bolts and the front derailleur had to be mounted with a larger than usual gap to the outer ring to clear the chainstay, but it works really well and is now my default gearing for all my riding. My chain is straight in 66.7″ gear which is the gear I use to go along at 16mph all day. I move back and forth on the rear sprocket according to the terrain and climb most stuff without going down to the 26t ring.

Of course you have to be prepared for some strange looks from some other cyclists when they see that your outer chain ring has 42t, but you’ll know that you actually thought about your gearing rather than slavishly following the latest fad.

So 42 is the answer and the question is

“What’s the largest chain ring on your bike?”

Filed under: Wrenching