Benefits to a larger crank arm?

[Baracuda]

Interesting topic.

 

What about knee problems with changing cranks? I would assume not only do you have to lower or raise your saddle to accomodate a different length but also adjust it forward or backwards.

 

I used to train on one bike and race on another and I developed knee problems only to discover the saddle on my race bike was different to the training bike in terms of how far back I sat in relation to the 3 o clock/9 o clock losition. It was only 15 mm but my knees felt it. The cranks were the same length though.

 

Shorter cranks which results in having to put down more effort will also put more strain on knees.

 

Anyway just my thoughts on the matter.

 

I would sincerely like more advice on knee problems relating to crank length. 

 

I have two bikes with 175mm cranks and one with 170mm. I have continual frontal knee pain on the 170mm bike. The other two I am fine. I have been to physios etc and done all the exercises and it makes little difference. I can't really move my saddle further back as it is dropper post and the layback dropper posts cost a fortune. 

 

I would love to know if this is a common issue and if it is worth buying a further set of 175mm cranks. My bike has XX1 on it, so it is not cheap. 

 

Cheers

[Fat Boab]

I would sincerely like more advice on knee problems relating to crank length. 

 

I have two bikes with 175mm cranks and one with 170mm. I have continual frontal knee pain on the 170mm bike. The other two I am fine. I have been to physios etc and done all the exercises and it makes little difference. I can't really move my saddle further back as it is dropper post and the layback dropper posts cost a fortune. 

 

I would love to know if this is a common issue and if it is worth buying a further set of 175mm cranks. My bike has XX1 on it, so it is not cheap. 

 

Cheers

 

Same seat tube angle on both bikes?

 

Anything else different between the 2 bikes, their set-ups, or shoes you use between them?

Edited April 19, 2017 by Fat Boab

[Baracuda]

Same seat tube angle on both bikes?

 

Anything else different between the 2 bikes, their set-ups, or shoes you use between them?

 

Thanks for the reply, the 170mm is a Pyga 110 with a seat angle of 74 degrees and my other bike is a Giant XTC with a seat angle of 72 or 73. Shoes etc are the same. 

 

I suppose that all added to together - seat angle plus shorter crank length all adds to my saddle being further forward relative to pedals as they are being pushed down. I may get a Thompson lay-back saddle post to see if it makes any difference - that should shift my butt back about 20mm. I

[Trance Dance]

We're both riding 2015 Giant Reigns if I'm correct so same geo and BB height except yours is a bit more raked out.

 

I experienced significantly less pedal strike going from 175's to 170's and honestly couldn't feel the 3% difference. The jump from 175's to 165's would likely be noticeable but if you're not very tall I rate you'll get used to it after the first ride.

 

Interesting article on the subject of crank length and pedaling efficiency: http://www.cobbcycling.com/crank-length-coming-full-circle/

So the story about the 170mm and 175mm crank arms being the same length but with the holes set 5 mm back isn't true.

[Paddaman]

On crank length, I am of the opinion that longer cranks give more leverage, at the cost of increased knee and hip loads.  Knee problems often occur when great loads are placed on the knee when it is bent at over 90degrees.  The femur length tends to influence just how much the knee lifts and thus knee angle.  The general rule of thumb is that it is unwise to have cranks longer than half the length of your femur.  (ie https://www.stevehoggbikefitting.com/bikefit/2011/06/crank-length-which-one/ point 2)

 

So as shorter people have shorter femurs, they should consider shorter cranks.  As humans have generally grown bigger over the last century it is plausible that popular crank lengths will increase from 170 to 175.  (my bikes are 170mm except for my stock MTB which came with 175mm cranks on a small frame ??????)

[cdevil]

So the story about the 170mm and 175mm crank arms being the same length but with the holes set 5 mm back isn't true.

 

I highly doubt thats true, I'll have to compare my crank to a 175mm one to be sure. The older M770 cranks all have different weights for each length:

 

[Serious Panda]

I use both 170 and 175 on mtbs. I find that the 175 is better because on rough terrain the extra torque of the longer crank is nice to have. With the 170 cranks it is more important to be in the correct gear for a higher cadence, with 175 cranks it matters a bit less if the cadence slows down with a sudden increase in gradient. However, Annie Last use 170 cranks and she seems to do very well, so personal preference and riding style/cadence/power to weight ratio is probably a bigger factor. Shorter cranks puts less strain on the knees because of the knee joint angle but the cadence should also be higher than with 175.

[Trance Dance]

I highly doubt thats true, I'll have to compare my crank to a 175mm one to be sure. The older M770 cranks all have different weights for each length:

 

Shimano Cranks.png

Please check and confirm! I'd be very interested to find out. Looking at those weights, the difference between the 175 and the 180 is a lot more than the 170 and the 175.

[Paddaman]

Another interesting point to consider, especially with MTB and not so much with road, is saddle setup and riding style.

 

Tri athletes have been known to "push" bigger cranks in time trial position, which is well forward of the BB and often at lower cadence, so as to "save" the legs (hamstrings) for the run.

 

In XC longer cranks theoretically could also be used as there is a lot of standing and changing of position, and the race is relatively short

 

Longer marathons and ultra marathons, often on gravel, may require shorter cranks so as to "save" the knees, as the rider is seated in one position for longer periods of time (thus potential for overuse injuries).  The same may hold true for road cycling.

 

This is just a bit of musing, and may be totally incorrect.

[Serious Panda]

Longer marathons and ultra marathons, often on gravel, may require shorter cranks so as to "save" the knees, as the rider is seated in one position for longer periods of time (thus potential for overuse injuries).  The same may hold true for road cycling.

 

This is just a bit of musing, and may be totally incorrect.

 

I have always had problems with sore knees, until I started using oval rings. I have never had a sore knee since I switched to oval rings, for both 170 and 175. My personal preference is to use 170 for training and 175 for the event day/race snake tendencies.

[Fat Boab]

Thanks for the reply, the 170mm is a Pyga 110 with a seat angle of 74 degrees and my other bike is a Giant XTC with a seat angle of 72 or 73. Shoes etc are the same. 

 

I suppose that all added to together - seat angle plus shorter crank length all adds to my saddle being further forward relative to pedals as they are being pushed down. I may get a Thompson lay-back saddle post to see if it makes any difference - that should shift my butt back about 20mm. I

 

It does sound like your knee-forward-of-foot parameter will be different between the 2 bikes. This value is often related to knee pain. I'd suggest a bike fit on both bikes to translate the fit on the 'good' bike onto the 'bad' one, although, as you suggest, you may be limited with your saddle fore-and-aft.

 

I don't know if there's scope to swop the cranks over to try and negate the knee-F-O-F movement? That may cause problems for both bikes of course, so solicit advice from a competent bike fitter perhaps?

 

Good luck.

[udxcob]

Racing BMX in the 80's we always raced with 180mm cranks, i struggled with the shorter cranks on a MTB mentally

[Newbie321]

On 4/19/2017 at 4:33 PM, Baracuda said:

 

Thanks for the reply, the 170mm is a Pyga 110 with a seat angle of 74 degrees and my other bike is a Giant XTC with a seat angle of 72 or 73. Shoes etc are the same. 

 

I suppose that all added to together - seat angle plus shorter crank length all adds to my saddle being further forward relative to pedals as they are being pushed down. I may get a Thompson lay-back saddle post to see if it makes any difference - that should shift my butt back about 20mm. I

Did you ever find a solution? I'm in a similar position now and also struggling with the 170mm cranks and a more forward seat than what I am used to