Crank it out

Spend 5 minutes with any technically minded cyclist at the moment and the subject of crank length wont be far from the conversation.

Along with tyre width it is has been the biggest switch away from ‘old school’ over the past few years.

Pretty much all bike manufacturers use a variation of the following :

50, 52 or size XS and S come with 170mm

54, 56 or size M and M/L come with 172.5mm

58, 60 ,or  XL and above all come with 175mm

From a manufacturing point of view this is simple and cost effective, but from the physiological and personal point of view no two people are the same, so should they all be pigeon holed into a certain crank length? The simple answer is No.

Recently I started using a Purely Custom Fit Bike Pro 1, to give it its full title, for bike fitting and especially for testing different crank lengths. It can quickly go from 155mm up to 185mm in 2.5mm increments so is a very useful tool for finding what suits each individual best.

Personally I found that 170mm worked best for me so I have changed to 170mm on my road bikes. At 6’3” or 191cm this goes against what bike manufacturers would ever supply, but the differences that I have noticed are more stability on the saddle and what feels like a smoother pedal stroke. I also feel less tension in my hips (especially the right) and my average cadence has also gone up by around 5 rpm looking back on each spin since fitting them.

Filippo Ganna at 193cm also rides 170mm whilst Tom Pidcock (170 cm in height) now uses 160mm. Tadej Pogacar (176cm) uses 165mm He has been the catalyst for bringing this topic to the forefront of all technical cycling conversations and for pro teams to really look at it more seriously.

So, what’s the downside?

The first is cost. If you have 2 or 3 bikes you will have to change the cranks on all bikes. If you have crank based power meters this could cost you over 3000 euros. Without power you could buy a 165mm shimano 105 chainset for around €150. Spending this sort of money you need to get it right so having a bike fit where you can test different crank lengths beforehand can be a very good idea.

Yesterday we did exactly that for a client and it was very interesting to see that whilst initially it looked like he would be going from 172.5mm down to 165mm, that it turned out that 160mm was significantly more comfortable and better overall.

Another downside, if you wanted to call it that, is that in a race situation where someone attacks hard out of the saddle you can generate more initial power for a few seconds and open a gap on someone behind on shorter cranks. The initial power difference then fades and the person behind can sustain their power for longer so closes the gap and has slightly less fatigue when you are both side by side again.

Who should consider shorter cranks?

If you like to spin a gear at a high enough cadence shorter cranks are definitely better for you. If you have any hip, groin, knee or low back issues, shorter is once again better as they allow the hip angle to stay open more at the top of the pedal stroke and reduce the size of the pedalling circle. This also places less pressure on the knees.

If you are a Time Trialist, or long range attacker, then shorter cranks at a higher cadence will allow you to maintain a higher average power for longer.

If you are a criterium rider and like to pedal through the corners they help here too.

Triathletes with shorter cranks can get flatter on the bike, and also find it easier to run off the bike due to the more open hip angle.

Who should not change to shorter cranks?

If you like to grind a gear at a low cadence then there is no advantage to having shorter cranks and they will cause you to fatigue slightly sooner.

If you cannot find a gear low enough on steep climbs, especially out of the saddle,  then having less leverage with shorter cranks will be a disadvantage.

This is an example of what happened yesterday when we went from 172.5mm to 160mm. 

BEFORE with 172.5mm

AFTER With 160mm

As Bob the Builder might say in regards to shorter cranks and their benefits :

‘Can they fix it?’

‘Yes they can’

Barry