just another gosling
Touring gear-inch calculation
A question which often comes up on this forum is "What gearing should I use?" I have devised a calculation which might answer that question for prospective tourists. I use a hypothetical rider, bike, and gear in the example calculation below.
Let us suppose that the rider weighs 162 and rides a bike, which they intend to outfit for touring, which has an all-up weight of 30 lbs. Therefore, their total climbing weight is 192. Let us further suppose that the rider knows that they can climb anything they might encounter on their prospective tour with this bike in its sport weight of 30 lbs., with a 30T granny and 27T cassette cog. This calculates to 30 gear inches for a 700c bike. Then let us suppose that the tourist has weighed the gear they will add for touring, and it comes to an additional 40 lbs. So their touring climbing weight will be 192+40=232 lbs.
Let x = touring gear-inches.
x/192 = 30/232
x = 24.8 gear-inches or a 30T granny and a 32T cassette cog for this hypothetical rider and weight combination.
Put in your own weight, bike weight, gear weight, and current gearing to calculate what gearing you would use on tour.
If you don't know how to calculate gear inches, it's the tooth count on the front chainring divided by the tooth count on the rear cog, times the wheel diameter, usually 27" for 700c and 26" for MTB. The exact value for wheel diameter doesn't really matter in this calculation, since all you are after is the difference in gearing based on your experience with your current gearing.
Last edited by Carbonfiberboy; 06-08-11 at 11:14 PM.
Reason: Made it more hypothetical
This one works for me :
take out the word 'internal' and there's a second slightly different calculation program.
speed .. Me? Average, about 7mph , with a touring, campsite load, aboard ..
Sturdy to handle the load the bike is ~ 40 pounds, on its own.
622-40 tire , Low gear 0.7:1 , high 3.57:1
when I was lighter,30 years ago
about a 95" to 19" range
now, I use a 17.5" low rather than a 19"
[Rohloff's minimum external ratio 16:38, in a 26x1.75 tire bike ]
still sometimes it's better to walk up the hills ..
Last edited by fietsbob; 06-08-11 at 01:13 PM.
Carbonfiberboy, Before I built my first touring bike recently I searched for all of the things that make a touring bike a well rounded touring bike. Gearing was the most entertaining topic I searched and determined that anything in the 20-95 GI range ought to be about right. I'm willing to coast downhill and prepared to walk uphill as I don't see how every what if can be prepared for. Personally I feel the intermediate gears most often used are more important than the extremes.
Last edited by bradtx; 06-08-11 at 09:31 PM.
just another gosling
I have edited the OP to make it clearer that this is not about my choice of gearing, nor am I attempting to prescribe what gearing another tourist should use. I'm only presenting a method for any tourist to calculate a good climbing gearing, given that they know what they can do on their unladen bike.
Personally, I much prefer to ride my bike up every hill I encounter. I find it much easier than walking. Touring in intermediate gears I like to just find my rhythm and ride at whatever speed that is. It's always plenty fast enough. I do select a rear cassette and front rings that allows my front chain rings to split out the rear ratios, so that I have no or very few duplicate gear-inch ratios, but that's the subject of another post.
The calculations are bogus. If you lower the gearing, then you have to increase your RPM to maintain the same speed.
Also, in the real world a touring cyclist will encounter conditions that will increase the difficulty (fatigue, headwinds, etc) beyond what they might find while riding around unloaded. Can you really determine "what you can do on an unladen bike" with any degree of certainty? I could go find the nearest big hill and climb it, but what if my chosen tour includes crossing the Rocky Mountains and I have nothing similar nearby? The real answer is actually pretty easy - get the lowest gear you can can afford. If you can swap a road derailleur and cassette with 11-25 for a mountain derailleur and a cassette with 11-32, do it.
just another gosling
The idea is to climb at your ideal long ride effort. One cannot maintain the same speed climbing with a loaded bike that one uses for an unloaded bike. Yes, I can tell exactly what I can do on a long ride with an unladen bike, having been on many of them. Doesn't even have to be your touring bike, since you can include that weight change in the calculation.
Originally Posted by Thor29
It's quick to run a couple of limiting cases, say a light load of 20 lbs., and a heavy load of 80 lbs.:
x/192 = 30/212
x = 27 gear-inches or a 30-30 granny combo (closer gearing)
x/192 = 30/272
x = 21 gear inches or a 26-34 granny combo
A more interesting case would be a person who is not already a strong climber and might use a 30-32, or 25 gear inches, on their unloaded bike. Many riders have found 26-34 to be the lowest easy-shifting gear for a standard road setup. They could calculate their greatest easily hauled load by going:
21/192 = 25/x
x = 228
228 - 192 = 36 lbs. of gear. More than 36 lbs. and they should go to a mountain crankset.
Or take the case of my riding buddy who weighs 110 and prefers to climb at a high cadence. Her unloaded climbing gear is 26-28 or 25 gear-inches. To see what she could carry at the same cadence with a 26-34 (21 gear inches) she could go:
21/140 = 25/x
x = 166 lbs.
166-140 = 26 lbs. of gear, totally reasonable, especially for a person who shops in the child section.