I have been getting a lot of questions on gearing lately. With all the options these days (10 sizes of cassettes, 2 options in cranksets, not to mention the confusion in mountain bikes between 26 and 29 inch wheels) it can be very confusing! So how do you go about weeding through all the options to find out what is the best option for your riding? Hopefully we can shed some light on the all the options. Today we will talk specifically about road and triathlon bikes. Mountain bikes will be in the next post.
Cranksets: Compact VS Standard
A compact crankset runs a slightly smaller chainring bolt diameter (110mm) to allow it to run slightly smaller chainrings. Typically a compact crankset will run a 34 tooth inner chain ring and a 50 tooth outer ring. That varies from a standard crankset that will typically run a 39 tooth inner chain ring and 53 tooth outer ring. Essentially, the smaller 34 tooth ring is 13% smaller than the 39tooth of a standard crankset; and the 50 tooth big ring is 6% smaller than the the 53 tooth ring of the standard crankset. The compact crankset basically gives you slightly smaller gearing allowing for better climbing.
Great, so what effect will this have on my speed? Assuming a relatively standard 12/25 rear cassette while pedaling at 90rpm, the lowest gear of a compact crankset will give you a speed of 9.6MPH versus 11MPH when using a standard crankset. The top end speed of a compact crankset (50×12) while pedaling 90RPM will be 29.3 versus 31.1 when using a standard crankset. What this basically tells us is that if you are riding a lot of longer climbs, think Death Ride, Everest Challenge, or Mount Washington hill Climb, than a compact crankset can be extremely valuable and allow you to spin more up the climb. Not getting all Armstrong on ya, but spinning on a climb is always more efficient than mashing gears. Alternatively, if you are doing a lot of crits that extra ~2MPH in a sprint can be the difference from standing on the top of a podium to being mid-pack.
But what if you want to do both and don’t have a full truckload of team bikes to choose from? We can change the cassette. With 10 speed drivetrains we have a lot of options in rear cassettes. In fact, there are 10 separate cassette ratios listed on the SRAM website to choose from. When using a compact crankset, if we switched to a 11/25 cassette instead of the standard 12/25 cassette we would actually end up with a bigger gear than the previous example. Our top end speed with a 50 x 11 gear at 120RPM (what you should be reaching in a sprint) would be 42.6MPH. A 53 x 12 gear would be at 41.4MPH (2.9% lower). So a compact crankset with a 11/25 cassette will give you a bigger top end gear and a lower climbing gear versus a standard cransket and 12/25 cassette. In my opinion, this is the most versatile setup for the budget strapped rider. If we went the other way and went to a 12/27 cassette we still wouldn’t be able to get the same low end climbing gear we would have with the compact crank. A 39 x 27 at 90RPM would be 10.2MPH while a 34 x 25 at 90RPM would be 9.6MPH (5.8% lower).
Is there a downside?
There are a few negatives with the setup mentioned above. Your spacing between the gears on the rear cassette is slightly larger which can be an issue when looking for that ideal cadence. The other issue is that if you are blessed with bionic, Cavendish style quads of steel you may need to go with a standard crankset and a 11tooth small cog. That combo at 120RPMs would give you a top speed of 45.2MPH. That is Pro-Tour stylie… That rider probably wouldn’t need the lower gears a compact crankset either unless they are climbing the Angliru.
If you want to play around with the numerous combinations of gearing available, check out Sheldon Brown’s great gear calculator.