170 v 175mm cranks

[Mr Panda]

it's interesting how 5mm (or 10mm on a full stroke circumference's diameter) can make a noticeable difference.

I'm around 6'2 and I have a 34" inside leg. so I'm hoping it'll still be suitable for me too.. ?

I'm pretty much the same size Steve, and I'll be buggered if i can tell the difference between 170 and 175. Go for it.

Aaron - the difference is 5

However - me niether, most of mine are 175 but have a coupple of 172.5s

yes it affects the gearing but not significant unless you're a top pro racer, which I'm not i dont think :xmas-big-grin:

 

[BobCatMax]

On a mountain bike, crank length doesn't make a big difference as you're not consistently pedalling with your body and legs in a fixed position, you're always moving around, backwards and forwards, weight off the saddle, then sat down, freewheeling, backpedalling etc. over terrain. On a road bike or bikes that you only use on the road over longer distances, it matters more. In this case, your legs are spinning over and over thousands of times in the same position, so if you are over stretching then you'll start feeling it eventually.
Really doesn't matter for an MTB.

you make an interesting point, as I run 175 on the slick tyred bike and 170 on the kona

 

[Anthony]

I run 175 on the slick tyred bike and 170 on the kona

That may be the wrong way round. I'm not sure 175 was much used prior to the mtb era, and 170 was the standard for road bikes. 175 was felt to be an advantage for mtb where greater leverage was needed for extreme climbs that you don't tend to find on roads.

 

[Carlos]

Not sure about that - I know a couple of old school racers (very top notch) who I'm sure have always used even 180 cranks on their racing bikes.
It's always down to leg length. I'm 5'6" with pretty damn short legs and a 175 on the road feels so wrong - I get knee pain after not long. 170 on the road every time for me (probably should really be 165!) but MTB I've always used 175 without any issues. One of these racers who I'm sure about, he's 6'3" or a little more and won't use anything below 175, really hates less than 180 though.

 

[the_duke]

Check out Sheldon "crank length" Brown's views Steve,

Different cyclists have different leg lengths. It seems obvious that crank length should be proportional, so long legged cyclists should have long cranks, short-legged cyclists should have short cranks....and yet, 99.9% of adult bicycles have crank lengths between 165 and 175 mm. Have the bicycle manufacturers joined in a great conspiracy to force everybody to ride the same length cranks, regardless of their needs?

This is a common misunderstanding. The "leverage" of a bicycle drive train, also known as "gain ratio" depends on the crank length, wheel diameter and the sizes of both sprockets.

Yes, if you go to longer cranks without changing any of the other variables, you will have more "leverage", which is another way of saying you'll have a lower effective gear...but on a multi-speed bike, you can change gears at will!

Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to.

I learned this the hard way when I bought a used mountain bike that came with 180 mm cranks. I found that it made my knees hurt every time I rode it.

On the other hand, there doesn't seem to be any deleterious effect from shorter cranks.

I've been experimenting with this a bit myself lately. For my fixed gear, I commonly ride 165 mm cranks with a 42/15 ratio on 700c or 27 inch wheels, when I'm riding fixed. This gives a gain ratio of 5.8.

My latest experiment is taking place on plastic Trek frame I picked up in a barter deal. I had a pair of TA 150 cranks that used to be on my kids' Cinelli BMX bike, so I've put these on the Trek. I'm running a 45/17, which gives a gain ratio of 5.9, just a bit higher.

When I first get on the bike after riding with longer cranks, it feels a bit funny at first, but within a very short distance it's just fine. I go just as fast, climb just as well. For a given speed, my pedal rpm is higher (though my pedal speed is the same) but the short cranks make it easy to spin much faster than I normally would.

After riding this bike for a few miles, when I get back on "normal" cranks, they feel a bit weird and long at first, then I get used to them after riding a couple of minutes.

I think people really obsess too much about crank length. After all, we all use the same staircases, whether we have long or short legs. Short legged people acclimate their knees to a greater angle of flex to climb stairways, and can also handle proportionally longer cranks than taller people normally use.

 

[Carlos]

Haha! What a ridiculous analogy! How many people climb stairs 1000s at a time, multiple times a week?!
This may be a point where I have to disagree with the Sheldon! True it doesn't make much difference with RPM or leverage but it does make a big difference with leg length. My legs will never acclimatise to 180mm cranks, my knees would disintegrate! Too much bend in the joints puts loads more pressure on them.

 

[the_duke]

You may be right but to be fair he does say,

"Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to."

 

[racer x]

For me 170mm cranks my knees are way better now if not fixed from any pain, im 5.10

The 170mm cranks spin up just a fast as any 175mm cranks ive had

 

[Carlos]

You may be right but to be fair he does say,

"Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to."

Yep, this is true. It's just that he seems to change his mind later! This is more important than he makes out.

 

[FluffyChicken]

...
I'm around 6'2 and I have a 34" inside leg. so I'm hoping it'll still be suitable for me too.. ?

Without reading the rest of the replies, the calcs for bike will almost certainly put you on the 175/180 border.
Mainly as I have the same leg but I'm much shorter and that's what they always tell me (these are MTB specific ones)
But I still use 175 as they are easier to get.

You might find the 170 do not stretch you enough during riding and cause possibly knee pain to tight muscles (would need to check the symptom though)