4-arm Crank

Get your Head Around Gearing Options

There are very few real standards in cycling, and none that seem to persist for very long even once they are established. For example the manifold choices of bottom brackets; English and Italian but there are also the new ‘standards’, PF30, BB30, BB86, BB90, BB386EVO and others). With this proliferation of options it requires that whenever we talk about something on a bike, we should be clear as to what it is that we actually mean to avoid misunderstanding.

The focus of this article is the compact crank. Something akin to a compact crank has existed for a very long time, but it is comparatively recently that it has been given this name. Heeding my own advice from paragraph one, I should explain exactly what I mean when I use the term compact crank. Up until 2013 the definition was quite easy as it referred to any crank with a 110mm 5-arm spider. Why compact? Because the 110mm spider is usefully smaller than the regular-sized 130mm spider. But 130 is not the only full-size option and there were other standards before that.

Crank History

Going back to the 1960s we would find most bikes had a 144mm spider. The normal rings mounted on these were 42T for the small ring (the smallest one that will fit) and 52T for the large ring. The 10T jump between them was about as large as the era’s front derailleurs could manage. By the 1980s this had shrunk to the 130 mm size most of us would be familiar with. Standard gearing had spread to a 14T jump, with 39T for the small ring and 53T for the large ring. Shifting was still quite dodgy until after Shimano introduced the STI shifting system, incorporating shift mechanisms in the brake levers. With the downtube shifters that preceded STI, shifting could be (had to be!) finessed. Not only that, but because you had to remove one hand from the bars to shift you couldn’t do it standing, climbing steep hills or in most of the situations that really test shifting. Shimano’s response was to add ramps and pins to the backside of the big ring. Soon everyone followed suit because it worked so much better.

When Campagnolo went smaller they chose 135 mm – the only real differences being that the rings are not interchangeable and the smallest ring that will fit on a Campagnolo crank is a 39T, where the smaller 130 mm cranks can go one tooth less at 38T. For a good 20 years not much changed in the pro ranks. For the recreational market there were a lot of things going on, however. Mountain bike development continued unabated. On the dirt side of things the “standards” changed frequently as the sport matured. A lot of ideas crossed over to road cycling (and the other way).

The 110mm bolt circle that mountain bikes used for a decade had originally come from touring bikes. MTB racing led to some very good cranks being produced. In the 1990s some of these 110mm cranks found their way onto road bikes. First at the low end where beginners could benefit from the overall lower gearing offered. Later, onto the bikes of riders in the Grand Tours once they discovered that a higher cadence up steep hills keeps legs fresh for longer. Eventually the also appeared on high-end road bikes direct from the manufacturers. These 110mm cranks have a standard ring size of 34T for the small ring and 50T for the large ring (note the ever-increasing gap between rings, this one a 16T jump).

There are two ways to accomplish a reduction in low gear. The use of a smaller chainring is the first option while a larger sprocket on the rear wheel is the other option. Swapping sprockets is easy but there is a limit to how much lower the gear can be and it depends on what model of rear derailleur you have on your bike. The actual change you get depends on where you started – someone with a 21T low can get a much bigger reduction (33%) in low gear than someone who starts off with an 26T low (8%) if we assume that a 28T low is the lowest you can possibly go without changing rear derailleurs. Swapping from a 39T to a 34T small ring is not as simple (since it requires swapping the entire crank) – it reduces the low gear by 13%. For most people, the best way to approach this lower gearing issue is to change everything (more on this below).

Describing Gear Size

Gearing, like everything else in cycling, has at least two alternatives for discussion. The notion of “gear inches” is a hold-over from the heady days of the 1880s when the Penny Farthing was the bike and cycling was the mode of transport, one revolution of the cranks gave one revolution of the wheel and thus your gearing was dictated by how big your wheel was. A 50-inch gear is the gearing you would have if the big wheel on your Penny Farthing was 50” in diameter. It is generally somewhat independent of actual wheel and tyre dimensions too – a 50×15 is a ninety inch gear for all 700C wheels. Surely this one is a century past its use-by date?

The other common gear measure is “gear development” which is a measure of how far the bike travels in one revolution of the cranks. It is on the basis of development that junior gearing is restricted. To comply with a 6.5m limit, the bicycle may not travel more than 6.5 metres for one turn of the cranks. Regular junior racers and their parents will know that sometimes merely replacing a worn tyre results in exceeding the limit that was complied with using the old tyre. Gear development is pretty good but how many people can really visualise what a 10 metre gear feels like? Without a calculator can you tell me what size chain ring and which sprocket you would use to get close to it? I didn’t think so…

While I don’t think it will revolutionise our sport, or even our discussion of gearing, I am introducing a new notion to further our exploration of compact cranks and other gearing options. This is speed-at-cadence. While competitive cyclists tend to have a higher default cadence than utilitarian cyclists, most people understand that 60 rpm (one revolution per second) is pretty slow and 120 rpm (two revolutions per second) is pretty fast. The human engine has a very narrow operating window, which is one of the reasons we have so many gears on our bike compared to our car.

For the gear comparison I chose 90 rpm. This is roughly the foot velocity to which novice cyclists aspire. It is also the cadence that many coaching articles use as a target. It is relatively easy to calculate without a cadence meter, being 3 pedal strokes every 2 seconds.

Historical Options

144 BCD Cranks

In the table (below) I have included many of the chainring and sprocket sizes discussed above, and their speed at 90 rpm. Consider climbing  the steepest hills in your backyard with gearing from the 1960s era. Typically for this time a bike with 144 mm spider ran a 42/52 pair of rings and either a 13-17 straight block (if you were a hero) or a 13-23 (if you were a mortal). At 90 rpm even using his lowest sprocket our hero is travelling an amazing 28kph in the 42 chainring. That’s going to hurt! Our mortal is doing better at 21kmh, but this is still a velocity unachievable by most on steep climbs. The reality is that riders in the 60s ascended at very low cadence. At the other end of the spectrum top speed at 90 rpm is a low 45kph, so anyone wanting to really go fast had to do it with a high cadence.

144 BCD Cranks

130 BCD cranks campag

Our modern era bike with a 130 mm or 135 mm spider runs a 39/53 by default and the 130 has the option of dropping down to a 38T ring. I included both to give an idea of what difference that single tooth makes. With 10 or 11 speed gearing an 11-26 is typical for all-around use by amateur and professional alike. In low gear our speed would be 17kph at 90 rpm. This is getting closer to achievable but on a long or steep hill, this speed is in the realms of professional. Taking one tooth off the small ring results in a decrease of less than half a kilometre per hour. That might just make the difference between getting up that last hill on a long ride or not, but for most people the change is barely noticeable. In top gear our speed is 55kph. I can see many club cyclists achieving this sort of pace with a tailwind.

130 BCD Crank

110 BCD cranks sram

Finally we look at the compact crank option coupled with a wide-ratio cassette (34/50×11-32). The low gear produces 12kph at 90 rpm. That is a realistic speed and cadence for a long climb in the Alps or Rockies. At the end of a long ride, or facing a stiff headwind up a hill, this sort of gear would allow you to continue at a low speed when those without the low gear might have to walk. On the top-gear side we see 52kph at 90 rpm. Not quite as fast as with the 53T ring, but quite reasonable. As an historical comment, when compact cranks were first introduced the smallest sprockets were 12T and the 11T came out to meet the demands of the compact crank users. A 53×12 and a 50×11 are almost identical – early adopters didn’t give up any top-end speed but gained a lot of room at the low-gear end.

110 BCD Crank

4-arm cranks Shimano

The latest crop of 4-arm cranks recently introduced by Shimano, Campagnolo and a few smaller companies have taken a one-size fits all approach to cranks. Now, regardless of your chainring choices, there is a single crank to fit them on. Everything from a 34T low for climbing to a 56T monster for time trials sits on the same crank. As another historical comment, when compact cranks were first introduced the front derailleurs of the day didn’t shift between the smaller rings with alacrity. Many companies introduced compact-specific front derailleurs. This put you firmly in the ‘regular’ or ‘compact’ camp as there were too many items to swap to make it practical.

4-arm CrankGear table showing velocity (kph) for each gear combo at 90 rpm.

























































Equipment Issues

As gearing has evolved, more of the components have been designed to work with each other and good performance relies on these pieces being present in a set. For example, SRAM has a front derailleur that “yaws” as it swings from small to big chainring permitting the use of all 11 rear sprockets on either chainring. This only functions as designed when used with the accompanying shift lever and crank. The reality is that swapping one or two parts might function, but to achieve the optimum performance envisioned by the designers you will need to change every part in the system. Which parts that ‘system’ includes depend on the exact configuration on your bike to begin with. An example of this is when Shimano had a single front shifter for both double and triple chainring configurations (so to swap from one to the other did not require changing the shift lever).

Most of the people I help out with these equipment questions are headed on a big bike ride – usually overseas where there are some serious mountains. Their current gearing options are fine for their local terrain, but not quite low enough for their destination. To survive the cycling holiday it might be enough to make a tiny change. For a rider with a 39/23 low gear now, swapping to a 32T cassette drops the 90 rpm speed from 19 to 14kph, a large enough change for many cyclists. To achieve this requires swapping three components: chain, cassette and rear derailleur. The cassette obviously to get the new lower gearing, the chain because the current one will be too short on the new cassette and the rear derailleur because shifting on a wide-ratio cassette requires a long-cage derailleur.

Someone who rides long distances might find the change from 39/26 to 34/26 sufficient (in the table that change is just over 2kph at 90 rpm). In this case swapping the crank and repositioning the front derailleur would do the trick (unless they were already running a new 4-arm crank in which case they could just swap the rings and move the front derailleur). Hopefully you can see now that deciding what to do requires an assessment of both the bike and the rider, the current capabilities and the impact of the new terrain. The luxury option is a second bike equipped with the full low-gear option, reserved for those days or vacations where this gearing is necessary.

At the Giro d’Italia this year the route took them up some of the steepest mountain roads in Italy – such as the Zoncolan. From what I saw, different teams had different solutions for their riders but in general the GC guys had lower than normal gearing and the domestiques had much lower than normal gearing (so that when they joined the grupetto and rolled to the finish they could do so in relative comfort). It has become routine for pro-team mechanics to adjust the team bikes to suit the coming terrain.

The bottom line is that road cyclists have never been so spoilt for choice in gearing. If your current bike causes you to struggle in the hills, chances are you have an option to make life more pleasant. This used to be the domain of the triple crank. Triples are all but non-existent these days because a double chainring shifts so much better than a triple. From a practical side, the lowest gear achievable with a compact double is sufficient for anything but touring with a heavily laden bike (where the triple option is still to be found). 


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