You had emailed me directly but I thought I would answer on the forum so that others can benefit or call b*ll $hi*t on me...
Originally Posted by ben_liu
Hello, and sorry for another gear ratio question
I am currently peddling a 32/16 ratio on a 29er but I need more speed as I find myself peddling like a madmen to reach 15 KM/h. I live in a relatively flat area with one 20 degree hill that is about 700 metres and I will have to go up/down for many years are I do plan on commuting to my school. My ride is about 7 KM each way. Would a 38t chain ring be suitable or even a 42? My LBS doesn't have 38 but it was recommended. Any tips?
I don't know how much advice I can offer you. As far as "specs" for you to use... it all depends. Fitness, terrain, weather, etc.
I don't know if it's the same everywhere but in the US people tend to use 'gear inches'
(link to Sheldon Browns site with lots of useful information) to compare various gearings/wheel combinations. This will give you an idea of how much *about* force/exertion you'll be putting down. The use of 'gear inches' is nice because it lets you examine the effects of gear ratio and tire size with one number. It neglects some things like tire rolling resistance and mechanical advantage from crank length. It comes from *old* technology and was used to compare *very old* style high-wheeled bikes where you had your crank on the the front wheel (no gears or chain). The gear inch was the equivalent diameter of the wheel you would be riding on a high-wheeled bike.
My commute is 8 miles (~13km) each way with about ~200m of ascent or descent depending on direction. I ride my fixed gear most of the time at 42x17 with a 700x32 tire (66.7 gear inches). I do drop down to 42x19 with a 700x35 tire (59.7 gear inches) in the winter as I find myself pedalling on ice and through snow. This is what works for me. With your mountain bike with 29" wheels (still a 700c wheel but traditionally with wider rims and tire) you'll probably be running wider tires with less air pressure giving you more rolling resistance (something the gear inch calculation will neglect). So if you wanted to ride with the same exertion as I, you might want to gear down (use an easier ratio). I guess, this is all playing into what I said earlier... "it all depends".
Gear inches is a preference. Some people like very high gear inches. I like them low as I've decided I need to improve my ability to pedal at high cadences.
For your situation, things to consider... If you are riding fixed (no freewheel, your tire turning means your crank is always turning) you need to find a gear that will let you climb a hill *but* also let you go down it w/o spinning out (tire and cranks turning too fast for legs). This is something you'll have to figure out with trial and error...
If 32x16 is too easy, I would start with the 38x16, and if that's too easy for your taste bump up to the 42x16. Keep in mind your chain will need to be longer if you are putting more total teeth (sum or chain ring and cog tooth count) into the mix. As changing chains can get expensive you might want to look at using a SRAM or some other chain with removable master links. If you can get a second master link (SRAM calls them "power links") it would be easy to put back in some links if needed (assuming you have them laying about). You might also want to price options on how much it will cost to switch chain rings versus cogs and what you have laying around...
If your going to put a 700x32 tire on the 29" mountain bike rim, make sure the rim isn't too wide for the tire