Foot speed or cadence?
When you’re considering how the muscles in your legs fire, the amount of revolutions your cranks turn every minute is much less important than the speed your foot actually is moving at. If you ride shorter cranks, then your foot travels a smaller distance at the same RPM, therefore it has less speed. So to maintain the pedal speed on shorter cranks & allow your muscles to fire at the same rate, a higher cadence will be needed to achieve the same pedal speed.
Let me give you some examples & a more detailed explanation of what I mean by pedal speed, lets compare 165mm cranks & 175mm cranks:
- Distance travelled by pedal in one rev (∏xdia) – 165mm⇔1036.2mm – 175mm⇔1099mm
- Pedal speed at 90rpm – 165mm⇔1554.3mm/s – 175mm⇔1648.5mm/s
So when you ride smaller cranks & you retain the same cadence, your pedal speed is lower as your foot travels less distance in the same time as your foot moves through a smaller diameter.
We can illustrate that if you use a 53 chainring setup on 175mm cranks, then a 50 (compact) chainring setup on 165mm cranks is the equivalent to allow for the same foot speed. (Using Sheldon Brown website for 700x25c gear development)
- Gear Development – 50×12 ⇔ 8.8m – 53×12 ⇔ 9.3m
- 90rpm on 53×12 covers 837m in 1 minute
- 90rpm on 50×12 covers 792m in 1 minute
So in order to demonstrate that matching foot speed (and increasing rpm accordingly) for the shorter cranks will be equivalent to the longer cranks, we have to cover an additional 45m in that minute. So we divide the shortfall by the 50×12 gear development (45/8.8) & we get 5.1RPM. The 165mm cranks should have a very similar pedal speed at around 95rpm as the 175mm cranks at 90rpm.
- 165mm – (95.1RPM x distance pedal travels)/time = (95.1 x 1036.2mm)/60s = 1642.4mm/s
- 175mm – (90RPM x distance pedal travels)/time = (90 x 1099mm)/60s = 1648.5mm/s
- (Reference: 165mm cranks turning at 90rpm result in a pedal speed of 1554.3mm/s)
So what does this tell us?
- Shorter cranks require a higher cadence for the same speed of muscle contraction.
- Pro riders you see with notably higher cadence may simply be on shorter cranks than their rivals.
- You can ride the same cassette, but reduce weight in smaller chainrings & shorter cranks for same road speed.
This one is pretty simple & likely the least contested of the benefits of shorter cranks. Power output reduces as the thigh to torso angle gets ever more diminished, so as a rider tries to get into a more extreme position the amount of power they are able to produce gets reduced. This means there’s a sweet spot, where you’re fastest, losing as little of your power as possible, while being very aerodynamic. Shorter cranks help this in two ways.
- Your thigh to torso angle is increased as your thighs are not raised as high by the longer cranks, so allows you to get lower without compromising power output (or bashing your thighs off your ribcage).
- The UCI 5cm fore/aft rule is thwarted a little. Having shorter cranks means that the horizontal distance from your hip to your pedal axle is reduced with the cranks horizontal, so it would result in a similar position to sitting slightly further forward on your TT or pursuit bike.
This falsehood is incredibly widespread & almost accepted as a fact by many folks who would normally appear to have a clue what they’re talking about. In several studies there has been no measurable difference in power output between different length cranks. As a blatant example, track sprinters ride short cranks (rumours that some GB sprinters have experimented right down to 155mm) but they seem perfectly able to produce incredibly high wattages. If there were issues with leverage & being able to generate power, Chris Hoy would surely have been riding 180mm cranks, but he didn’t.
If this was really an issue, another part of the drivetrain would also experience the same issue with leverage, notably the chainrings & sprockets. So we’d all be climbing on large chainrings & very large cogs to get that extra ‘leverage’, but we don’t, because it doesn’t work.
If you imagine yourself doing squats in the gym, think about where you’re able to generate more strength, is it at the point where your knee is more bent, or less bent. This in itself should be able to dispel any perceived benefits a rider may get from longer cranks, with the knee bent at less of an angle you are able to generate more force when required & also avoid putting too much force through the knee at too great an angle. I’d suggest that some riders who suffer knee problems have been riding long cranks & that may be a component of their injury, especially as they get older.
Power To Maximum
This aspect of crank length is really interesting & potentially very important depending on your discipline. The Macdermid Study in 2010 showed that the time it took to reach maximum power on 170mm cranks compared to 175mm cranks took about 28% less time. The numbers seem small until you consider how it may affect you during an attack, following somebody elses attack or jumping in a finishing sprint. The study didn’t cover crank length shorter than 170mm, so it’s fair to deduce that there would be a similar improvement. It’s well worth a read, but here’s the numbers, which could create a gap between you & somebody of the exact same ability, simply due to riding different length cranks.
Time to maximum power:
- 170mm cranks – 2.57s
- 175mm cranks – 3.29s
At 50kmh, your bike travels about 10m in the time delay between the two, consider that for whether or not it’s significant.
The Gist Of It
There are numerous studies which show that there is no difference between power output on every length of crank, apart from maximum power (where for some reason 145mm seems to come out on top, with longer & shorter cranks recording lower values). As I’ve tried to explain, there are numerous other advantages & some world tour teams are now investigating & testing short cranks. I simply cannot find a single reason for riding longer cranks other than availability & that your bike is probably already fitted with longer cranks, so it would cost you to change them all. Maybe next time you buy a new chainset, give shorter cranks a go, you might be quite surprised.