Thursday, April 26, 2007

Sanity in gearing

I'm back in the saddle again, happily. I've managed to churn out 160km since whining about being slack on Saturday. The extra miles have me feeling good on the bike and have really boosted my confidence for this weekend's trial. I've finally fallen in with a bunch of local riders of about my speed, ability and shockingly casual attitude, which is ironic because I'm leaving town in four weeks. Hopefully there will be a similar mob in Hobart.

Spending more time in the saddle watching the scenery go by has me pondering a few things. Why is the cable routing on Shimano SPD drop bar shifters so inelegant? Couldn't they have run the cables along the handlebars like Campagnolo do instead of leaving them hanging out in the wind? Would I be happier with down tube friction shifters just to make thing prettier? The big issues clearly occupy my mind.

Anyway, I'm working on a cunning plan to upgrade my crankset so I can use the old gear to build up a road bike for Mrs Surly Dave. (She gets a road bike, I get a new crankset, we're all winners). Being someone to completely over-analyse everything, I've hit the spreadsheets. I ride at an average speed between 19km/h and 23km/h. Now and again with a tailwind I'll nudge towards 30km/h. Up hills I'm slower - 10 to 15km/h depending on the hill. So I need gears which help me ride in that range.

OK, so here's where things get technical. I generally pedal with a cadence between 70 and 90 revolutions per minute. Let's call it 80 for the sake of the argument. My gearing ranges from 109 inches down to 29, or to put it another way, I travel between of 9.2m to 2.3m per pedal revolution. With a 52/39/30 triple crankset and a 12-27 cluster, my bike is set up to propel me at speeds of between 13km/h and 50km/h. The trouble is, I seldom go this fast. I very, very rarely use my top chainring. (The yellow highlights on the gear chart below show the gears would use at normal speeds, some are duplicates. Click to make big).

It works out that about eight of the gears on my 30-speed are never used except for when I'm in a hurry on long downhill. This is, of course, not counting the duplicates and the gears which can't be used because of chainline issues. Being charitable, that takes out another 12 combinations. So on a 30 speed bike, I've got about ten useful gears.

So it turns out that getting whole bunch more gears doesn't deliver anything more for the average cyclist except added weight and complexity. Wow, big surprise!

But assuming for a moment I'm a typical, or even sane, cyclist who seldom exceeds 30km/what would be the ideal setup for me? (I don't want to get rid of the lower gears, they come in handy for getting up hills.) It turns out to be 44/32 front chainrings with a 12-27 ten speed rear cluster. That gives me 92 down to 29 gear inches and an effective speed range of between 40km/h and 12/km/h at 80 rpm. I'd guess that such a setup would cover just about every casual and recreational cyclist in the world except those lucky few who have a downhill commute with a tailwind home.

But do you reckon you can buy such a setup? No way. Not even in the compact double cranksets so popular of late. Why not? Presumably because when you ride the Tour De France, you need a 53 tooth front chainring to give you a huge 120-something inch top gear to charge down mountain passes and in the sprints. It's madness, madness I say.

2,296km so far this year.

1 comment:

fishbones said...

I've ended up with a similar setup - corncob(ish) from 13 to 18, 21, 27.
But I've got 52/42 at the front - a) I'm a biopace nut and b) I do have quite a few downhills on my commute. I have got some overlap, but I can't find a 39t bio in 130bcd. And having some overlap avoids double shifting right in the middle of the range.

Having said that, if I went for a narrower chain abd added 11,12 & 34 I could dump the front derailleur all together. Which would be a worthwhile improvement.

But what would I put on the left side of the downtube?