Crank length ?

[ourclarioncall]

A new one I have to consider

Does it make that big a difference what length it is ?

[Danneaux]

Does it make that big a difference what length it is ?

For many people, no. For me, yes...crank length is critical to my knee comfort/health and is a big factor in achieving my preferred fast-light ("hummingbird") cadence. The magic number in my case is 170mm. I don't get along well with cranks longer or shorter than that.

Generally, longer cranks (172.5, 175mm, 180mm for example) better suit "pedal mashers" who prefer a slow, "power-stroke" cadence. Taller people with longer legs also tend to prefer longer cranks. Longer cranks can sometimes cause toes to interfere with a front mudguard, depending on top tube/front center length.

Shorter cranks (155, 160, 175mm) tend to better suit spinners and shorter people with shorter legs. Though they provide less torque leverage due to their shorter moment arm, some people simply like them better (Forum member Martin F likes shorter cranks) and they do provide better ground clearance to avoid a pedal strike, usually only a real factor if your frame has an unusually low bottom bracket and you also like to pedal around corners while leaning deep into them.

Here's another factor to consider with cranks: Q-factor, the distance between your feet ("stance width", formerly called "tread", when pedals are also considered). Some people don't notice any difference, for others it is a critical dimension for comfort and power application. I prefer a narrower Q-factor on my high-mileage randonneur bikes and can easily tolerate a wider one on my off-road and expedition bikes. By the way, if you tend to stand up on the pedals to climb or accelerate, it helps to match a wide Q-factor with relatively wider handlebars to counter the torque of pedaling. Grant Petersen of Rivendell at one time called this the "stilt-step factor".

In these days of "system" components, one often does not have the choice to fine-tune these dimension as in the past. You pay your money and get whatever results in the best shifting/chainline and then adapt to the crank length or Q-factor provided; much depends on whether you are riding an IGH or one of the wider cassettes. Tire width can affect these dimensions too, especially in the case of bikes with really fat tires and frames designed with wider stays to accommodate them. These are all factors I keep in mind when designing the frames I make myself.

If you can borrow or rent a variety of bicycles to test ride, it can really help to hone in on refining your preferences.

Best,

Dan.

[martinf]

Forum member Martin F likes shorter cranks

My knees like short cranks. I believe the short cranks and low gears put less stress on my knees, which gave me problems when I was pushing standard 170 mm cranks slowly in high gears in the late 1970's/early 1980's.

150 or 155 mm size help me maintain a relatively high pedalling cadence (about 90 rpm on average), in my case turning short cranks faster allows me to keep the same speeds as before I changed from the standard 170 mm crank length.

I found this out by accident when I borrowed short cranks fitted to my wife's bike, 150 mm was the "correct" length for her.
Theoretically I "should" have 172.5 or 175 mm cranks.

One advocate of short cranks is the designer Mike Burrows:
http://www.bhpc.org.uk/short-and-sweet-a-discussion-on-crank-length-by-mike-burrows.aspx

On the other hand, there are small people who get on well with relatively long cranks, so I think crank length is like saddles, a personal matter to be decided by trial and error.

Crank length influences gearing for me. With short cranks I do not need very high gears, a 78 inch gear equates to 34 km/h at 90 rpm. For short periods I can wind that up to 45 km/h at 120 rpm, for example when negotiating large roundabouts.

When I got my Raven Tour with a Rohloff it came with 38x16. Since then Rohloff has authorised lower combinations, but I haven't bothered to change yet.

38x16 has the 78 inch gear as 13th, so I have an 88 inch high gear that I hardly ever use. The lowest gear is 17 inch, which is 7.4 km/h at 90 rpm, I can comfortably drop my cadence to about 60 rpm on very steep inclines, which gives a "walking" speed of about 5 km/h.

[leftpoole]

Hello,
Much of these lengths and width of Q factor have been dismissed as theoretical these days.
For example Hollowtech cranks have 'outboard' bottom brackets which widen the so important Q factor.
Plus, so called Gravel bikes (!) which are a consumer con in my view (having built and used similar many years ago) now have a single chainring with up to 13 sprockets at the rear. All those years of ramming down our throats that chainline is just so so important seem to have gone!
So basically use what you chose and ignore other views including mine and others (in my expert opinion).
Happy New Year!
John

[Andre Jute]

You choose the crank length for your own comfort. Unless you have short legs, 170mm is a good choice to start with. I've never felt the need to change away from 170mm, but then I don't aspire to a butterfly cadence and, for an old rugby player, my knees are in amazingly good condition. With some setups the tread width, defined above by Dan, can influence the chainline, depending on where on the spider you fit the chainring; you can in addition get spacers for the chainring bolts. Rohloff requires you to have a chainline within 1mm of specification, a requirement that is probably more breached than observed. I went to considerable trouble, with help from other members, to observe it because I'm a masher who used to get ridiculously short lifespans on transmission components, so every marginal improvement is important to me, and a skew chainline is obviously a source of wear; on the other hand, George, who posts here as "mickeg", an engineer, isn't bothered about a 5mm chainline mismatch.

[JohnR]

If the cranks are too long for the frame size then there's the risk of toes colliding with front mudguard when turning a corner or heels clipping the chainstays. I've had that problem with a previous bike.

[PH]

I bought a second-hand bike a few years ago with 170 cranks when I've always ridden 175s, it never felt right till I changed them. That could have been imaginary, or the cause might have been the unfamiliar, maybe if I'd always ridden 170s the 175's would have felt uncomfortable, maybe if I hadn't known they were 170's I might not even have noticed. I think most people get on fine with the average for their size and only look into the options if they don't.  If you are sensitive to such things, don't forget to include the pedal stack height and of course your shoes. The crank length is to the centre of the spindle, you might easily have 10mm variation in where your feet actually are.