crank arm length

ednino

So my LBS think my knee & slight hip pain is down to the 175mm cranks on my bike (bought it second hand with them on). Im told a shorter crank & higher saddle will mean less angle at my knee

I wanted to buy Ultegra cranks anyway so that's fine with me

But do I go for 165mm or 170mm? Seems like such a grey area. Im a 32" leg if that helps :oops:

themekon

My inside leg is 30" and have 170 on my fixed bike and 172.5 on my best bike. I can't feel the difference. So you should be fine with 170s.

Wirral_paul

Difficult to say without knowing more details but what did the bike shop recommend? Are they experts on bike fit??

I'd have to say there's other areas that could be causing the issue rather than the crank length (and quite possibly more likely!!). Have you considered a proper bike fit from someone who really knows what they are doing - mine was done by Pedal Precision in Manchester. Richard is a Sports Rehabilitation Specialist first, a bike fitter 2nd - and like you i was getting knee and hip issues.

PS 31" leg - 172.5 mm cranks (ie only 2.5mm shorter)

ednino

Wirral_Paul wrote:

Are they experts on bike fit??

No lol

There is a Retul 3D bike fit place locally but not sure I can bring myself to spend that sort of money :shock:

Thanks for the suggestions guys

Wirral_paul

ednino wrote:

Wirral_Paul wrote:

Are they experts on bike fit??

No lol

There is a Retul 3D bike fit place locally but not sure I can bring myself to spend that sort of money :shock:

Thanks for the suggestions guys

It'd be the best money you ever spend if it cures your injury issues. You could always spend the same money on a crankset that will make virtually no difference mind you

DaveRobinette

i've always been led to believe that crank length is nearly irrelevant of leg length/height. but is more important for cadence as a longer crank length = longer pedal circumference (more vertical displacement) = a tendency for lower cadence, whereas a shorter crank length is better for a faster natural cadence as there is less vertical displacement.
this seems logical to me, but if someone more knowledgeable could correct me....
seems quite a jump from 175mm to 165mm though, i'd have thought 170mm would be enough.

bernithebiker

I vote for 170. That's what I use, and I'm 32' too (5'8 height). Anything that promotes higher cadence and less joint stress is a good thing in my book.....

crikey

Setting seat height is usually done from the centre of the Bottom Bracket to exclude the crank length from the equation. Then crank length can be selected according to preference; some like 170s, some 172.5s, some 175s.

Sprool

how do you know what length you prefer unless you have the chance to try all 3 on your bike? This oppotunity doesn't arise for most of us.

ednino

bernithebiker wrote:

I vote for 170. That's what I use, and I'm 32' too (5'8 height). Anything that promotes higher cadence and less joint stress is a good thing in my book.....

I think this is what i'll go for

Thanks guys, all very helpful.

I don't get how setting seat height is done from the centre of the bottom bracket :?
I've always done 109% of inside leg = from saddle to pedal

crikey

It's done that way to get a ball park figure that excludes the difference in crank length, cleat type, shoe type, pedalling style and so on.

You need to experiment anyway to get the correct seat height for you and your style of riding, rather than relying on a formula that dictates the exact height your seat should be.

(oh, like you appear to do... which probably emphasises that exact seat height is not as crucial a measurement as those magazines would have you believe.)

stueys

It makes some sense that smaller cranks would mean less movement in the leg, I know that TT guys tend to run smaller cranks as its easier to keep the cadence up when you're forward in the saddle and down flat.

I'd get a second opinion though. Difficult to see how 5mm is going to make a big difference. Plus the downside of smaller cranks is less leverage on climbs.....

crikey

Not to mention the effect of how big or small your feet are, where you put your cleats, what distance from pedal axle to foot, how thick your socks are, whether you pedal flat footed or toe down or heel down, how padded your shorts are, whether or not you are wearing tights as well, how soft your saddle is and whether or not you move forward or backwards on the saddle...

All these things will influence the exact distance between your pedals and your saddle...

styxd

ednino wrote:

bernithebiker wrote:

I vote for 170. That's what I use, and I'm 32' too (5'8 height). Anything that promotes higher cadence and less joint stress is a good thing in my book.....

I think this is what i'll go for

Thanks guys, all very helpful.

I don't get how setting seat height is done from the centre of the bottom bracket :?
I've always done 109% of inside leg = from saddle to pedal

Why dont you try adjusting your saddle height first, before you buy new cranks?

ednino

styxd wrote:

ednino wrote:

bernithebiker wrote:

I vote for 170. That's what I use, and I'm 32' too (5'8 height). Anything that promotes higher cadence and less joint stress is a good thing in my book.....

I think this is what i'll go for

Thanks guys, all very helpful.

I don't get how setting seat height is done from the centre of the bottom bracket :?
I've always done 109% of inside leg = from saddle to pedal

Why dont you try adjusting your saddle height first, before you buy new cranks?

Have tried this^

MichaelW

DaveRobinette wrote:

i've always been led to believe that crank length is nearly irrelevant of leg length/height. but is more important for cadence

Given a "normal" length crank, then changing crank length will affect cadence. Normal is relative to your leg length (or more precisely, femur length).
Do the stick-man drawing of a tiny rider on long cranks and a very tall rider on short cranks and examine the angle of the hip and knee joints. Correct crank size will keep these angles within bounds for the flexibility of the particular rider.
For medium riders, almost every crank is OK, 160-175. I am medium size (5'11, 132 inseam) and use both 170 and 175mm but prefer 170. For short and tall riders almost every crank is the wrong size, it is very rare to see a new bike correctly setup for the size of the rider.