Tech tips, MTB 101 - Gear »
Analyzing the gearing on your mountain bike
The gearing combinations you use on a mountain bike can have a serious effect on the way that you ride. Most times, people take the gearing selection that come stock on their bikes for granted, but by taking a close look at how you ride and how you shift, you might find that a different gearing selection may work better for your pedaling style, terrain, and application.
Gearing basics
There are two wonderful resources I found that discuss gearing for cyclists. You should check these out first, and I’ll just summarize a few points here.
In a nutshell, mountain bikes typically have three chainrings up front and between seven and nine cogs in the rear. A gearing selection is made up of the combination of the front ring and a specific rear cog. So if you have three rings up front and nine in the back you (theoretically) have 27 different speeds.
There are, however, a few caveats. The combinations of the small ring / smallest cog and the big ring / biggest cog should be used very sparingly, if ever at all. These combinations create a severe chain line which causes excessive wear. If you chain is set too short, or if you have a short/mid cage rear derailleur there is also a chance that your chain will bind (or worse, explode) your rear derailleur.
In addition, several of the gear combinations actually overlap so you don’t really have 27 unique gears.
Why should you reconsider your gearing?
If you’ve been riding for any length of time you know that mountain biking is all about the MO, or momentum. In xc racing especially, we are always trying to maximize momentum to carry as much speed through any given section. Gears are changed in reaction to terrain changes in order to maintain an efficient cadence. The spacing on the cogs in the rear, and the rider’s use of these individual gears, play a significant role in maximizing your momentum on the trail.
When Shimano added 9 speeds to mountain bike cassettes it was viewed with some concern with regards to durability of the cassette. However, the xc crowd jumped on it because the additional gearing allowed for smaller steps between cogs. This meant that the relative change between cogs was smaller with 9 speed than with 8 speeds. This smaller ‘delta’ made it easier to maximize momentum.
The other day I was riding with a local expert. I saw a small cassette on his rear wheel and just chalked it up to him being an animal. But as we talked about it more I learned that he combined a compact crankset up front with a 12-27 rear cassette in order to get the smaller steps between gears offered by the 12-27 as compared to a typical 11-32 or 11-34. In addition this also allowed him to use a short cage rear derailleur which provided faster and more positve shifts. There can be a weight weenie benefit by switching to a smaller cassette as well.
The drawbacks of current mountain bike cassettes
One of the drawbacks of mountain bike cassettes is that they have to compromise the spacing between cogs in order to provide a good low-to-high range, especially with most cranksets currently coming with a 24 tooth (or 22 tooth) small ring. In order to provide low enough gearing to climb, off road manufacturers usually put at least a 32t cog and often specify a 34t cog in the rear. At the same time they need to provide a high enough gear for hauling ass and they’ll put an 11t or 12t as the smallest cog on the cassette. There is a fairly wide range between an 11t and a 34t. In order to span this range the delta in teeth between cogs becomes at least 4t.
Look at the delta between cogs on this cassette:
How to compare gearing
In order to compare one gearing combination to another you need some metric for comparison. One comparison is gear ratio with drive teeth divided by driven teeth or front ring teeth divided by the number of teeth in the rear cog. Personally, I prefer the gear inch metric as described on the Cycling Site gearing tutorial because whole numbers just seem easier to compare across gear combinations.
I followed the instructions on that tutorial and used the gear calculator at Sheldon Brown’s wonderful site to create several tables to allow comparison of some gearing options.
An Example
Here is my particular example. I currently run a Race Face Turbine LP crankset with compact gearing 22t-32t-42t up front. In the rear I’m running a SRAM 11-32 cassette.
I’m considering an Ultegra or Dura Ace 12-27 road cassette and either keeping the 22t up front or dropping to a 20t small ring.
Here are 3 charts showing these gearing combinations:
In comparing the first two charts, the biggest difference is in the lowest and highest gears. Of most concern to me is the lowest gear which has is 22/32 or 18 gear inches in my current setup and 22/27 or 21 gear inches. Those 3 gear inches could be the difference between cramping vs being able to spin on a climb in a race.
The difference between the highest gears doesn’t concern me as much, because it seems to be rare that I am in my absolute highest gear on the mountain bike. And if I had to I could go to a larger front ring such as a 44t without much trouble. In fact, I’d actually went down to a 42t with the hopes of being able to stay in the big ring longer, but I am finding myself spending too much time in the 42t and the next to the largest cog which does create an acute chainline and increases wear and resistance compared to a similar gear using the middle ring.
The last chart shows what would happen if I went to a 20t small ring. This would give me back a lower bailout gear for climbing. The one concern I have is the increased risk of dropping a chain when switching from the middle ring to the smaller 20t ring.
The real issue is the smaller steps that are provided using the 12-27 cassettes through a lot of the range of gearing. These smaller steps can make a bike difference when added up throughout an entire race course.
I actually have a 12-27 on my roadbike that I am going to swap to the mountain bike to see how it goes. Ironically, I had also been thinking of getting a compact crankset for my road bike recently to make climbing easier. However when I swap the 12-27t to the mountain bike, I’m going to put the 12-32 on my road bike killing two birds with one stone.
I’ll report back how it goes.
Conclusions
Gearing can have a huge impact on how your ride. You don’t always have to use the gearing that comes stock on your bicyle. By taking some time to really observe how you shift there might be a better gearing combination that works for you. There are several cassettes offered by the big S manufacturers and there are more customized cassettes that are available from places such as Cycle Dynamics to meet your needs.
Don’t be afraid to play around to see what works best for you. In fact, I saw a guy yesterday on a road bike with a customized corn cob cassette with on really large biggest cog. He found the corn cob gave me the best gearing for tearing up the flats and rollers, and the bailout cog let him spin up any climb. Did I mention that he smoked me on a 4.2mile road time trial by 40 seconds?
1. Posted by Mr. P | 7:02 am, 7 June 2006
Interesting write up. To take it one step further, riding a single speed shows one how much gearing is in our legs, position and cardio system. And how much is doable without over reliance on gears.
I would love to find a way to an efficient set up with only one shifter, front or back.