Crank arm length: 172.5 vs 175

[AdrianDJ]

Hi Guys

 

According to ergofit i need a 172.5 crank length for my road bike, i currently have a 175.

 

Is this significant or should i not worry about it? it obviously affects the drop as my seat should then be set 2.5 cm lower?

 

My crank is the brand new shimano 105 (black) which has done 240ks (ok dont shhot me for using "brand new". Should i sell and get a 172.5 or keep?

[Bman]

gosh im far from the pro? but really, is it gonna make that much of a diff? and do you really want to break the bank when you have a brand new like 105 there mate?

[The Saint]

Dude I ride a 175 crank on the tandem and a 172.5 on the single - I didn't even know the tandem had a 175 on it until I re-did my seat height calculation - no difference as far as I am concerned.

[Harry Potter]

You will not feel the difference. I bought a bike and only after a year I found out that my crank arm lengths differ, the one was 175 and the other 172.5. I bought the bike like that did not even notice

[Cyclewizz]

I've ridden with a 170 a 172.5 and with a 175and I honestly can't tell you the difference.

Don't think that 5mm can make such a difference.

 

Rather spend the money on steak..........

[AdrianDJ]

true Bman, so i might be a bit pedantic.

 

Tarmac - wow, mm makes more sense! not much of a diference there then im 178 and my bike is large.

[AdrianDJ]

hahaha thanks guys. the 175 will stay

[The Saint]

Closest way to calculate your seat height is the following. Measure your inseam - push the tape into the bone between your hol and your nuts and down to the floor (no shoes on)

 

Take the measurement multiply by 109% and then minus your crank arm length (17.5 in your case)

 

That will give you a measurement as close a dammit from the top of your saddle to the middle of the BB. From there it is mm adjustments to get your seat height correct.

 

Hope this helps

[AdrianDJ]

thanks saint.

[Andyr249]

Thanks for that Saint!

[Tuxstir]

Um just a correction on your sizes their you would only have to drop your saddle 2.5mm lower not cm wont make much of a dif

[Tiny K]

Closest way to calculate your seat height is the following. Measure your inseam - push the tape into the bone between your hol and your nuts and down to the floor (no shoes on)

 

Take the measurement multiply by 109% and then minus your crank arm length (17.5 in your case)

 

That will give you a measurement as close a dammit from the top of your saddle to the middle of the BB. From there it is mm adjustments to get your seat height correct.

 

Hope this helps

 

or just multiply the inseam by 0.883 = center BB to top of seat height. Makes sense that you have another calc that takes crank arm length into account... kinda

[FowlStArt]

thats 0.25 of a centimeter, not 2.5cm

[splat]

Closest way to calculate your seat height is the following. Measure your inseam - push the tape into the bone between your hol and your nuts and down to the floor (no shoes on)

 

 

That is called your taint.

 

'Cos it ain't your hol and it ain't your nuts...

[AdrianDJ]

Spalt - hahahahaha

 

about the cm vs mm - yes i was having a bit of a blonde moment! but this was still very helpful

 

thanks again

[splat]

Dr Jeroen Swart wrote this article for Bicycling Magazine

 

What’s with different crank lengths, 175, 172.5?

 

 

Optimal crank length is a topic on which there is still no firm consensus. The principles of a lever arm apply to crank arm length. As you will know from using a crowbar or a wheelbarrow; The longer the lever, the greater the load that can be lifted. Similarly, a longer crank arm requires less force to generate a similar torque than a shorter crank arm. This naturally provides the rider with an increased ability to accelerate and also allows the use of a higher gear ratio than would otherwise be possible. All good right?

 

 

 

Not really. A longer crank arm requires each muscle to contract through a longer range of motion. As muscles produce peak forces at a specific length (normally in their mid range of contraction), the longer crank arm forces each muscle into a range on each end where it is weaker. This means that power is lost at the top and bottom of the pedal stroke even though it is gained in mid pedal stroke. Another down side is that the larger diameter of the pedal stroke requires a greater change in the inertia of each leg during each rotation which may offset any gain in torque by lowering efficiency. Thirdly, the greater range of motion needed in each joint when using a longer crank arm places the rider at a higher risk of overuse injury. The patello-femoral joint of the knee is particularly at risk when overly flexed at the top of the pedal stroke. Generally, longer legs negate some of these negative consequences which is why it is often recommended that taller riders ride longer crank arms.

 

 

Mountainbikes generally have a crank length which is 2.5mm longer than the equivalent road bike due to the slow speeds and lower cadence and the requirement for a sudden increase in torque to maintain forward momentum over rough terrain.

 

 

One particular school of thought that has changed 180 degrees is that of time trial bikes. In the past the “experts” recommended longer crank arms to allow the rider to grind along in a higher gear ratio. However, this forces the hip to flex more than it otherwise would. As the torso on a time trial bike is lowered to gain an aerodynamic advantage, the hip is already over flexed, robbing the rider of power as the crank reaches the top of the pedal stroke. A shorter crank arm reduces this effect and allows the rider to adopt an even more aerodynamic position while producing the same power output by using a higher cadence.

 

 

 

Confused yet?