When doing bikepacking races and ultra-distance cycling, it’s best to have gears that are comfortable to pedal in all situations and that won’t cause you to tire unnecessarily quickly due to using an excessively low cadence. It’s therefore advisable to to equip your bike with extra-low gear ratios so that even long, steep climbs can be summited with ease even when heavily fatigued.
To calculate the size of the easiest gear needed to allow a certain cadence to be maintained, you first need to decide what minimum speed you want to be able to maintain. To do so, find out what the average gradients are for the steepest long climbs on the route. For instance, some climbs on the 2016 Transcontinental Race (TCR) route had sustained sections of 10% gradient at elevations above 1500 meters (the Grosse Scheidegg in Switzerland and Passo Giau in Italy). It’s then necessary to either know what rate of climbing you can sustain (e.g., a VAM of 800 vertical meters per hour – this metric is available on many bike computers, is shown on activity tracking websites like Strava, or it can be calculated manually) or your sustainable power in watts (e.g., 200 watts, which can be obtained using a power meter or is estimated on activity tracking websites).
If you know your sustainable VAM then you can use the following equation to obtain your sustainable speed at a certain gradient:
Speed = VAM / (10 * gradient)
Where speed is in km/h, VAM is vertical speed in vertical meters per hour, and gradient is in %. Putting the numbers from the 2016 TCR route into this equation gives: 800 / (10 * 10) = 8 km/h.
Sustainable power can be turned into speed at a certain gradient using this calculator. The same values can be entered in the boxes on the left as were used in the Factors That Determine Cycling Speed section: weight = 85 kg (rider + bike + equipment), CdA = 0.43, Crr = 0.004, and drivetrain losses = 5%. If a power of 200 watts is entered in the lower box then the calculator estimates the sustainable speed to be 7.8 km/h.
The effect of elevation should not be forgotten, which is covered on the Environmental Factors page of the Factors That Determine Cycling Speed section. Research has shown that a cyclist’s available power is reduced by about 10% at 1500 meters elevation compared to sea level due to the reduced oxygen density, so a rider who can sustain 200 watts at sea level will only be able to sustain 180 watts at this elevation. The same correction can be made for vertical speed, meaning that a more realistic sustainable value at this elevations is about 720 m/h. These revised estimates would yield a sustainable speed of 7.0 or 7.2 km/h.
For the 2016 TCR route, having a gear that can be ridden comfortably at 7 km/h (4.3 mph) would therefore be advisable for this individual. The preferred minimum cadence then needs to be decided. Research shows that what cadence a cyclist prefers to use doesn’t have a massive or consistent effect on their power or fatigue as long as it is within a reasonably broad range, which is why I generally don’t use cadence sensors on my bikes anymore. However, most experts agree that using a cadence lower than about 60 rpm for an extended period puts extra strain on the muscles, thereby causing unnecessary fatigue, so I do focus on cadence when determining what gear ratios I want to have.
So, are the gear ratios that you currently have sufficient? To determine the cadence needed to pedal a certain gear at a certain speed, gear calculators such as this or this can be used. Using a regular road compact crank and the largest road cassette would give a front chainring of 34 teeth and a rear cog of 32 teeth; using these values with a wheel size of 700 x 25mm gives a calculated cadence at 7 kph of 52 rpm. This individual may therefore want to look into gearing options that are outside of regular road gear ratios, which are discussed below, but first I discuss what are the fastest/highest gears that are likely to be needed.
Whereas lower gears will certainly be needed during an ultracycling race, faster gears may get used very rarely because it’s better to conserve energy and not pedal when the speed goes above about 45 kph rather than wasting energy fighting the wind for a relatively small increase in speed (this is discussed on the Riding Technique & Efficiency page). Even if you choose to pedal at speeds much faster than this during training, after a couple of long days on the bike, it is likely that you’ll have switched into a pure energy conservation mode and you’ll rarely pedal on significant downhills.
At a cadence of 90 rpm (which is very comfortable and most people can handle significantly more), a 50-tooth chainring and a 12-tooth cog gives a speed of 47.4 kph (29.6 mph). Having a 50-tooth chainring (or larger) may therefore be unnecessary, and using a smaller large chainring has the advantage of allowing the cyclist to stay in the large ring for longer as the speed decreases on a gentle climb while maintaining a decent chainline, thereby reducing the frequency of shifts down to the smaller ring, which can disrupt the rhythm.
A chainring of 46 teeth may therefore be more suitable, which gives 43.5 kph at 90 rpm with a 12-tooth cog or 47.5 kph with an 11-tooth cog, so is more than large enough for an energy-conserving ultracyclist. James Hayden, the winner of the 2017 TCR, independently came to the same conclusion as this and told me that he chose to put a highest gear of 46-12 on the bike that he ended up winning the race with. He also said his lowest gear of 34-32 wasn’t quite enough on certain climbs during the race, even for him.
Having determined the slowest and fastest gears that you need to be able to pedal at a comfortable cadence in all situations, you could then use a more complex gear calculator tool to evaluate which combinations of cassettes and chainrings offer the appropriate range. Here is an example output from this tool:
Ten or 11-speed 11-32 cassettes are now common and a lot of modern road rear derailleurs will work with a 32-tooth large cog, and if not then rear derailleurs specifically designed for such cassettes are offered by Shimano (the GS cage models), SRAM (WiFli models), and Campagnolo (in the Potenza group). When changing to a larger cassette, the chain may have to be lengthened by a couple of links (see Sheldon Brown’s instructions) and the rear derailleur height will probably need to be adjusted using the b-screw (again, see Sheldon Brown’s instructions).
The 11-32 cassettes offered by SRAM and Campagnolo have an unfortunate jump in cog size right in the middle of the cassette between the 19 and 22 tooth cogs, which causes a cadence change of 16%. For this reason, I prefer Shimano 11-32 cassettes, which have better spacing with 18, 20, and 22-tooth cogs.
The 11-tooth cog is normally superfluous for ultracyclists, even if their largest chainring has fewer than 50 teeth (see the High Gears section above). It’s difficult to find stock 12-32 cassettes, so one of my favorite setups consists of taking an 11-32, 11-speed Shimano cassette and replacing the first four cogs (11-12-13-14) with the first four cogs from a 12-25 cassette (12-13-14-15). I then have a custom 12-32 cassette that has a 15-tooth cog which I find to be far more useful than the discarded 11-tooth cog. Shifting across the unmatched 14-15-16 cogs is normally OK with a mechanical derailleur and remains excellent with an electronic derailleur.
Cassettes exist with cogs that are larger than 32 teeth that can work on road bikes. For instance, most 10-speed MTB cassettes are 11-34 or 11-36. Not many road rear derailleurs will work with that size of cog, but if you have Shimano 10-speed road shifters then a Shimano “9-speed” MTB rear derailleur will work perfectly and if you have a SRAM or Campagnolo 10-speed road shifter then a SRAM “10-speed” MTB rear derailleur should work.
For 11-speed cassettes, Shimano now sells an 11-34 cassette in the Ultegra group (since late 2017) and SRAM sells an 11-36 option, both of which can work with Shimano road 11-speed long-cage (GS) or SRAM WiFli rear derailleurs on some bcikes, but not all (the length and geometry of the derailleur hanger is the determining factor). The road rear derailleurs that SRAM sells specifically for their 11-36 cassette can only be used with one chainring. The newer Ultegra rear derailler (RD-R8000-GS) is officially compatible with the 11-34 cassette and two chainrings, and should also normally work with the 11-36 cassette.
Shimano sells 11-40 and 11-42 11-speed cassettes. I’ve made an 11-40 work with a road rear derailleur by using a Wolf Tooth RoadLink that effectively increases the length of the derailleur hanger. The derailleur is too low for a large portion of the cassette, which causes poor shifting with a mechanical rear derailleur, but the extra precision of an electronic rear derailleur makes the shifting work well.
I’ve modified a Shimano 11-40 cassette by replacing the first four cogs (11-13-15-17) with the first four from an Ultegra 14-28 “junior” 11-speed cassette (14-15-16-17) to get tighter spacing between gears instead of having gears that I’ll rarely use. When paired with a regular compact crank (34-50) this allows me to pedal at a decent cadence of 65 rpm at 7 km/h.
SRAM offers a 10-42 11-speed cassette, but with that gear range only one chainring is needed (a ‘single-ring’ or ‘1x’ setup). 11-36 or 11-40 cassettes are also options for single-ring setups. Some people like single-ring setups because they are slightly lighter and shifting is simplified. The major disadvantage is the large jumps between gears that cause large changes in cadence. This is not such a problem when riding off-road, but I find that it’s not ideal when riding on roads and trying to conserve energy.
If you’ve decided that you want lower gearing than that offered by a regular compact crank and road cassette, then instead of using one of the extra-large cassette options listed above, you could use a “super-compact” crankset.
Regular compact road cranksets have the chainrings attached using bolts spaced apart in a 110 mm diameter circle, which is called the Bolt Circle Diameter or BCD. Given this restriction, the smallest chainring that can be mounted has 33 teeth; Stronglight (Amazon) makes one for 5-bolt cranks and Spécialités TA makes one for the Shimano 4-bolt cranks. However, changing a 34-tooth chainring for a 33-tooth only gives a 3% improvement (or about 54 rpm instead of 52 rpm at 7 km/h with an 11-32 cassette).
Fortunately, there are a few super-compact road cranks available now or will be released in 2017 that have lower gearing than regular compact cranks:
- Praxis Works have been able to squeeze 32/48 chainrings onto a special 110 mm BCD spider.
- FSA are launching some super-compact cranksets called the “Adventure” models, with 32/48 and 30/46 chainring options at several price levels (SL-K, Gossamer, etc.).
- The Sugino OX Series are road cranks with chainring options as small as 30/44 (Gravel Cyclist posted an independent review).
- The IRD Lobo “adventure” crankset is very similar to the Sugino model and can be ordered with a large chainring between 40 and 46 teeth and a small ring with 28-32 teeth.
- IRD also do a cheaper Defiant model that comes with a single option of 46-30 chainrings and mounts to a square-taper bottom bracket.
- A huge variety of chainring sizes can be mounted to the White Industries VBC cranks (currently only available for square-taper bottom brackets, but a 30mm axle version will be available soon).
- The Spécialités TA Carmina and Vera cranks (only available for square-taper bottom brackets) allow various chainring spiders to be mounted, including a 94 mm BCD double that can take 29/46 rings.
- The Velo Orange Grand Cru classically-styled crankset takes a large variety of chainring sizes, including 46-30 (BCD = 50.4mm).
- Lightning make custom carbon cranks with an option to use a 94mm BCD spider, and so 29/46 rings.
- Most road triple cranksets can be used with only two chainrings mounted in the inner and middle positions, and nothing in the outer position. For example, a 28-tooth inner ring and a 44-tooth middle ring or 30 inner and 46 middle (such chainrings are available from Spécialités TA).
A 30-tooth chainring and a 32-tooth cassette gives a cadence of 59 rpm at 7 km/h, or with a 28-tooth chainring it’s 63 rpm.
Mountain bike double cranksets also offer low gearing options (e.g., 26/39 or 28/42). However, MTB cranks have longer axles than road cranks to give more clearance for the frame due to the wider MTB tires. Such cranks therefore cause the chainline to be wider and so most road front derailleurs can’t swing out far enough to shift between the chainrings. In addition, some people find that riding with their feet wider apart doesn’t feel as natural as when using a narrower road crank. Some people do use MTB cranks on road bikes without any problems, but I don’t recommend it because super-compact road cranks do now exist.
Due to the size of the large chainring on a super-compact crank (44-48 teeth), the teeth are not very high, and so it can be a problem to move the front derailleur low enough for crisp and reliable shifting. I recommend using some sort of chain guide to prevent derailments on all bikes, but this is even more important with this sort of setup. The most robust chain guide is the K Edge Cross Double XL (braze-on mount: Amazon or clamp-on mount: Amazon) and there is also the SRAM Chain Spotter (Amazon).
Hopefully this page has given you an idea of how to choose appropriate gearing and the options that are available. There is also a good article at road.cc about methods to get lower than regular gearing on a road or gravel bike. If you prefer to watch videos than read text then GCN have tried to cover this topic in the following two videos; unfortunately, I don’t think either video does a particularly good job at addressing the topic.
Last minor page modification:
Last significant page update: November, 2016
This is the final page in the Bike & Components section, which is in Ride Far, Part II: The Bike.