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Monday, November 7, 2011

Gear Ratio's

So you want to build or gear your fixed gear with that money gear, the one gear that does it all, for every rider, in every condition. Does it even exist? Sure it does. 70 gear inches. Go ahead and test me on it... given enough miles I believe you’ll concur. After 4 years of hard core Fixed Gear and Single Speed riding, that would be my gear if I could only choose one. If I had to ride coast to coast, that would be it. It’s the gear I’ve seen more veteran fixie riders choose, especially ‘century’ riders. So if you’re looking for a starting place use a gear calculator like you’ll find at http://www.sheldonbrown.com/gears/ and set yourself up with 70 gear inches. You can get there 20 different ways, so before you buy a new chainring or rear cog/freewheel, check and see if you can make it work with something you already have... chances are you’ll get close enough. How close is close enough? 68-75 to start.
Just one of the cool things to come from Surly, the Dingle Cog.
http://surlybikes.com/parts/dingle_cog

If you're stronger than all your buddies combined, go a little on the big side, or if your just starting out, a little less. After you get all jiggy with it you can optimize your skid patches too, the more the better. Example, the most common fixie ring is probably the 48t, combined with a 18t cog/freewheel you get your 70.1 gear inches, right on the money but only 3 skid patches, ouch. I chose a 49t ring, combined with the 17t, 18t or 19t, for 75.8, 71.6, 67.8 gear inches, and 17, 18 and 19 skid patches. So some combos are better than others. But I’m not going be skidding you say, not yet… Just the power decelerating hard is enough to spread that energy to more than 3 places on the rear tire. In a perfect world you’ll need/want a couple gear ratios, given your frame can take up the chain slack and your brakes can reach. I use a 2 tooth swing from my low to high gear on my White Industries ENO hub.



2 teeth difference in a rear cog is equal to 8 gear inches with my front ring and enough to cruise all day at 17-21 mph and climb at mountain at 7-9 mph. I have a White Industries freewheel that has both a 17t and 19t side by side, you must use a 3/32” chain with these, not the 1/8” track chain. I use a 17t cog for fixed riding, I rarely climb in it, just rolling hills and flat land rides up to 100 miles. The 19t freewheel for climbing and when I’m tired, and the 17t freewheel for fast group rides and centuries.  

White Industries DOS Freewheel. Hands down, the nicest, and last, freewheel you'll buy! Rebuild able too!! Only available in 16/18t or 17/19t so plan accordingly...
http://www.whiteind.com/singlespeedgearing/freewheels.html

A good combination for those with flip-flop hubs and old road bike conversions is a fixed gear on one side and a freewheel 1-2t larger on the other. Most frames can handle a 1t tooth jump as well as the brakes without a need for re-adjustment. Then you can climb hills, descend and have a bailout gear if you're far from home and spent with a freewheel. If you have only a fixed gear hub you can still use a freewheel in place of a cog, just not the other way around.
Here is a fun one, the Sturmey Archer SX3, a 3 speed internal fixed gear hub introduced in 2010. With a 52t ring and a 16t cog I have a 53.6", 64.3" and 85.7" gears! Spaced a little far for my liking, and this is considered as a close ratio. The original, extremely rare, Sturmey Archer ASC fixed gear 3-speed hub was a true close ratio racing hub built in the early 50's.

With the right gear ratio you can go on a group rides and not get dropped on the hills or the flats, and that’s when you know you’ve found your right gear ratio! If you can keep up on the climbs but get dropped on the flats you need a taller/harder gear, if you can’t climb but do fine on the flats you’ll need a shorter/easier gear. My fixed gear is the ultimate riding workout, but I’m limited to a top speed of just over 30 mph, and only for a short period, and that’s where I find my freewheel comes in handy, just tuck into someone’s draft on a downhill and there is no top end limit…

If you look closely at my rear wheel you'll see my new custom 'knock-offs'. Still a work in progress, it allows me to make a quick gear change or flip the wheel without the use of tools.


4 comments:

nordic_68 said...

For geared dirt randos (forgive the topic change), have you embraced the new/old 46-30t double crank? I'm currently running a shimano compact at 46-33 which is as low as it can go. But I can see the benefit to the knees of having that 30t small ring...

Eric Althen said...

I used a road triple for a long time with a 30t small ring on my one do everything bike, and if I had only one bike today it would still be on there for sure. My cross bike uses a 46/36t that I love, but if I was doing a lot of climbing on it on really steep terrain I could see the appeal of a 33t ring. Thanks for the comments!

Steel Riding said...

I am glad I took a look at your blog after yesterdays ride. I watched you and the other single speed rider closely, wondering if I could keep up. I like my fixie because of it's "no forgiveness" ride. In fact, I am thinking of calling my bike "The Unforgiven"! I run that 44T White crank with a 16t rear cof on 700x23 tyres......72" according to Sheldon.

Eric Althen said...

You road that fixie like a Boss Eric! Your new bike Is beautiful, you did a nice job designing and building it! 72" is right on the $!