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Q&A: What gearing should I use on a singlespeed mountain bike?
April 11th, 2008 by Tim Grahl 
Ed. - This question came in from a reader who needs some advice. If you’ve got an answer and/or opinion, please leave it in the comments. Also, if you have a question you’d like answered, email diggers@commutebybike.com.
From Anthony:
I’m going to give commuting a go and need some advice.
I’m a singlespeed mountain biker and do not have any geared bikes. I test drove my route today and it’s 12 miles with essentially no hills. The only hills are highway overpasses, so I’m sire I can push a big gear over with no problems since they are short, plus I will have a rolling start. The bike I’m going to use for commuting is a GT 9′r (Ed. a 29 inch singlespeed mountain bike) and is currently set up as 32×18 with Schwalbe Big Apple slicks. I don’t even want to try commuting with 32×18 because I will never get there… It took me 30 minutes to drive the route today and with 32×18 it would probably take over 2 hours.
I need a taller chainring, so this is where I need some advice. I can either get a new set of road crankset or just a bigger chainring for my current mtb cranks.
Jensonusa has a clearance on Shimano 105 cranks, which are 53×39 for $45. I don’t need anything highend, so this seems like a killer deal.
If I get a mtb chainring I will go with the big ring, which should be a 44t.
So, my question is if I go with road cranks is it unrealistic to try commuting on 53×18? I have some 20T cogs, so I could do a 53×20. I know once I’m rolling I can push this gear, but I’m concerned about starting. Is it too tall?
Or, should I just go with 44×18?
I know this is subjective, but I’m looking for some general advice so I can get started.
April 11th, 2008 at 12:46 pm
I would say a 53×20 or 44×18 — 53×18 is absolutely huge, and 44×20 is pretty small for a basically flat route. I commute over varied terrain on a 47×16 fixed gear, which is pretty much exactly the same as the 53×20 (about 77 gear inches). I would guess that your tires and set up would make you want something a little smaller than my gearing, and that the relative flatness of your route would sort of even that out.
If you want to play with more gear combinations and get a sense of their relative size, check out Sheldon Brown’s gear calculator: http://sheldonbrown.com/gears/
April 11th, 2008 at 2:14 pm
39 x 12
April 11th, 2008 at 2:53 pm
I’ve got a fixed gear set up as 39/16, and I find that works quite well here in DC where we have some hills. If nothing else will teach you to spin, that will, and once you do it’s all good. For the record I’d cruise on that at about 18 mph, so not slacker speed, I think
I’d recommend using the 44.
April 11th, 2008 at 3:17 pm
That’s a good point to mention — terminal speed for me is about 36mph downhill, and I can cruise steadily at about 20mph, probably a reflection of my having bigger gearing than AJ. It’s also worth mentioning that fixed gears have a certain mechanical advantage that freewheels do not — if you have a flip-flop on your bike, it might be worth getting a fixed cog to see how you like that, too.
April 11th, 2008 at 8:59 pm
I used to use 38/16 with a 26 incher. It was the perfect gearing for my route which was not too hilly. Now, I have a much shorter route, but have a few gears (4 to be exact), because it is too steep. I was stretching chains too quickly.
With 29’s, you might like 38/18. That way you only have to change the chainwheel. If you can get away with it, single speed is the ideal setup for a commuter bike. Much less upkeep. Also get some form of flat protection. I recomment puncture resistant tubes from Performance Bikes.
April 11th, 2008 at 9:49 pm
I can’t advise on the gearing choice, but maybe on some math. These choices are all ratios. If you divide the chain ring by the rear sprocket you’ll get the number of times the crank turning once would turn the rear wheel. So a 40 in the front and 40 in the rear is a 1:1 ratio, which would be the same as a 32 chain ring to 32 rear sprocket.
Anybody have an idea what the fastest leg motion could be on a bike before your joints give out. And I’d have to find figures on the typical torque a cyclist can impart.
44:22 is the same as a 32:16 . So the sizing doesn’t seem to be as important as the ratio between the two sprockets combined with you tire circumference. The 53/20 and 47/16 mentioned above are actually different ratios and would feel much different.
Rememeber making the rear turn more times with one chain ring revolution is harder on the legs.
If I get some time I’ll post a link to an excel sheet. There was a motorcycle magazine that created a spreadsheet of ratios and tire circumference so you could map the speeds in each gear and the maximum speed (given an rpm) all based on torque to the rear wheel.
It’ll take some converting over to bicycle format. And I’m not sure how I’d implement maximum input RPM
April 11th, 2008 at 10:09 pm
I stand corrected, Svenny is absolutely right — I was comparing the gear inches of a 53/20 on a 29″ wheel with my memory of the inches for 47/16 on a 700c wheel, not realizing that the two are evidently of such different diameters.
April 12th, 2008 at 1:26 am
39/17 works for me on a 29er.
April 12th, 2008 at 8:19 am
I’d stick with the lower ratio. It’s easier on your knees, and it won’t take you very long to go 12 miles if you are going 15 miles an hour, which you will be able to do on the 29er. Try this gear inch calculator (thanks to Sheldon Brown, RIP)….he linked to this site:
http://software.bareknucklebrigade.com/
April 12th, 2008 at 9:11 am
Try the 44/18 it should be close enough within a tooth or two on the rear.
April 12th, 2008 at 11:09 am
I used 39-18 on my 26er here in Reno (Elevation!), though all the fixie nerds push 48-16, if I were you ID go with the 44
April 16th, 2008 at 12:38 am
If Charlie The Bike Monger (www.CharlieTheBikeMonger.co.uk) doesn’t have an answer to this one, I’ll eat my hat.
April 16th, 2008 at 8:15 am
Thanks for all the replies.
I tired to mount a 44T to my current MTB cranks and the chain line was off by a mile. I thought about mounting the chainring inboard but my cranks don’t have the inside lip to hold the chainring in position, so I nixed that idea I went a head an ordered the 105, which should be in today. I cannot wait to get going..
April 16th, 2008 at 8:16 am
Sorry, I ment to mention that I will be giving the 39×18 a try. I will post my results when I have some..
Thanks again..
AC
April 17th, 2008 at 3:02 pm
Redcliffs:
I have always wondered: what is the mechanical advantage of fixed over freewheel (assuming single speed and same ratio)?
Thanks
April 17th, 2008 at 3:27 pm
Dweendaddy:
Because cranks turn with the wheel on a fixed gear, the bike’s momentum is always driving the cranks — thus, unlike on any freewheel set-up, momentum is helping you pedal. This can be a disadvantage on downhills, but on climbs, the fact that you’re moving actually helps you keep moving, if you see what I mean.
If you go to Sheldon’s gear calculator, he also has a choice that will give you rpm at any given mph. What this indicates is that in YxZ gear, A rpm = B mph. Ultimately, on a fixie, maintaining A rpm is easier once you are up to speed, because a certain amount of the force required to turn the cranks is being provided by the bike and your own mass.
My only thought in raising that earlier was that I don’t know whether a YxZ gear on a fixed gear would feel the same on a SS, or whether you’d want something smaller on the SS. Then again, AC’s ride is relatively flat, so it won’t be much of an issue.
AC:
Regarding chainline — if you were just changing the gearing on an existing SS set-up with a good chainline, what happened? Nothing that I know of about changing the size of your chainring should have changed your chainline. I suspect you may find that switching cranksets will create more problems than it solves — or, rather, if your chainline improves with the 105 set-up, it will just be luck that it lines up better with your rear hub.
April 17th, 2008 at 3:30 pm
AC: I meant to add this link to Sheldon Brown’s article on chainline, including directions for measuring and adjusting chainline and tables listing stock spacing for a variety of hubs, sprockets/freewheels and cranksets. http://sheldonbrown.com/chainline.html#chainline1 As will become instantly obvious to you, if it isn’t already, adjusting your front chainline is much easier than doing it in the back.
April 22nd, 2008 at 8:12 am
Thanks everyone!
I actually rode to work today for the first time ( yay! ) and decided to go with the 39×18, which worked out nicely. I changed my route slightly and I have several overpasses which required out-of-the-saddle climbing with the 39×18, so I would have been sucking big time with the 53×18.
Thanks!
AC