Crank length More spin or more power

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Crank length More spin or more power

Postby brokenbus » Tue May 03, 2011 11:35 pm

I am upgrading my crankset mainly so I can get an external bottom bracket which I am lead to beleive is more weatherproof than the internal type and the current one is stuffed. Anyway its currently a 170 and I can get 175's for cheaper than 170's. Am I going to appreciate being able to spin quicker with the 170's or the extra leverage with the 175's when using it as a mountain bike - going through mudholes and up steep hills etc. I am 182cm tall if thats relevent and its for a Giant Boulder. I would love to upgrade the whole bike but the GLW has other ideas.
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by BNA » Wed May 04, 2011 12:14 am

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Re: Crank length More spin or more power

Postby HappyHumber » Wed May 04, 2011 12:14 am

I think it's quite negligible and hard to measure. I think a lot of it personal perception. But on the idea of extra leverage when you're using it as a MTB - you will also loose some clearance consequently on rougher terrain. But if you're not wanting to stretch the $ a bit further to get the 170, I don't think you're going to notice much really going to 175


I'm sure others can contribute more thought fodder for you, but my experience has been starting out on geared roadies on 175s, I then went to 165s as is popular convention for a lot of Fixed Geared bikes and althought that's a bit like comparing Apples and Oranges. My preference now is a happy medium of about 170, which I could re use on a SS or FG bike without too much worry (BB drop & frame geometry notwithstanding)
I'm 187cm tall, FWIW...
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Re: Crank length More spin or more power

Postby scotto » Wed May 04, 2011 8:34 am

someone worked it out once. difference between 170 and 175 is about 3%
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Re: Crank length More spin or more power

Postby Bentnose » Wed May 04, 2011 9:57 am

I be thinking that if you are 182cm tall you'd want use a 175mm crank anyway, unless you have realy short legs. I'm 183cm, have average length legs and always used 175mm cranks as I've been led to believe this is the right length for my leg length
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Re: Crank length More spin or more power

Postby trailgumby » Wed May 04, 2011 10:09 am

My youngster has just gone from 170mm cranks and 32T cassette to 175mm and 34T cassette when he inherited my old bike when I bought an new one this month.

He has noticed an improvement in climbing ease, and I've noticed he doesn't flake out as readily on longer rides.

He is 15 and about 180cm.

For me, 170mm cranks feel "funny". Banging your pedal can be avoided by ratcheting over an obstacle instead of attempting a full pedal stroke. Remember to lower your saddle by 5mm to compensate.
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Re: Crank length More spin or more power

Postby HappyHumber » Wed May 04, 2011 12:05 pm

Probably immaterial to the OP's question.. but I think the issue and 'popular opinion' is often at the whim of industry conventions or marketting; From the dozens of earlier 80s and further back bike I've collected and pulled apart - most cranks are generally no more than 170, Similarly, you're battling to find drop bars wider than about 40cm.

Now these days 175 seems to be the norm, or default, in a lot of cases and drop bars are unikely a to be narrower than 40cm.

I dunno, just something that's been at the back of my mind with all the cobwebs & dust bunnies.
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Re: Crank length More spin or more power

Postby sogood » Wed May 04, 2011 1:12 pm

1) Length of crank does not affect power. Power comes from the rider, not the bike.
2) Shorter crank will lead to more spin at equal power.

The difference in leverage (ie. Gearing) is 2.94% increase by going to 175mm from 170mm crank.
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Re: Crank length More spin or more power

Postby damhooligan » Wed May 04, 2011 1:17 pm

sogood wrote:1) Length of crank does not affect power. Power comes from the rider, not the bike.
2) Shorter crank will lead to more spin at equal power.

The difference in leverage (ie. Gearing) is 2.94% increase by going to 175mm from 170mm crank.


1) the power does come from the rider, but it can make a difference to the rider to transfer that power onto the bike.
Or can it not ??
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Re: Crank length More spin or more power

Postby HappyHumber » Wed May 04, 2011 1:20 pm

sogood wrote:2) Shorter crank will lead to more spin at equal power.


Unsuccessfully fighting the urge to bring up fixed gear riding again; as well as cornering & clearance issues the shorter crank on FG bikes allows the legs to spin out with less strain. eg. It's less stretch & strain barrelling down a hill at a given cadence on a shorter rather than a longer crank.

So, in supporting sogoods wonderfully succinct assertion... you'd theoretically be able to improve your cadence or spin more easily with a shorter crank.
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Re: Crank length More spin or more power

Postby twizzle » Wed May 04, 2011 1:25 pm

There are online calculators for working out appropriate crank length for given femur/shin bone length.

Going from shorter -> longer cranks is also a great way to introduce knee problems as it takes a while for the body to adapt to the changed knee motion.
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Re: Crank length More spin or more power

Postby mitzikatzi » Wed May 04, 2011 2:06 pm

Shimano LX FC-M582 Crankset at that price 170mm or 175mm are a done deal. About $105 posted $350 at my local Anaconda shop. They have a couple of other Shimano Crankests on sale in 170mm but are octolink.

I paid $100 for a set of 44t and 34t chainrings once at my LBS
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Re: Crank length More spin or more power

Postby trailgumby » Wed May 04, 2011 2:53 pm

HappyHumber wrote:
sogood wrote:2) Shorter crank will lead to more spin at equal power.


Unsuccessfully fighting the urge to bring up fixed gear riding again; as well as cornering & clearance issues the shorter crank on FG bikes allows the legs to spin out with less strain. eg. It's less stretch & strain barrelling down a hill at a given cadence on a shorter rather than a longer crank.

So, in supporting sogoods wonderfully succinct assertion... you'd theoretically be able to improve your cadence or spin more easily with a shorter crank.


Mountain biking has the greatest variation in cadence of all the cycling disciplines - anywhere from 30 to 150rpm, all within a couple of hundred metres if the terrain demands it.

Given that grinding at low cadences is most risky for knee injury, that biases the crank length selection towards going longer in my opinion.

At 185cm, with my inseam length, the Zinn on line calculator came up with a suggested length of 192mm, so 175mm is distinctly on the short side.

My view is that 170mm is too short for the OP for off-road use, and 175mm would be more user friendly.

If 170mm came stock on the bike, it raises the question in my mind as to whether the frame might be a little small as well. However, if you are in between two frame sizes, it is better to err on the small side, as the bike is easier to manage on the tricky stuff if it is shorter.
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Re: Crank length More spin or more power

Postby sogood » Wed May 04, 2011 3:06 pm

damhooligan wrote:1) the power does come from the rider, but it can make a difference to the rider to transfer that power onto the bike.
Or can it not ??

Rider is the source of power, while crank/chainring/cog/wheels are but components that affects the gearing and nothing more. In other words, it's all about leverage ratio/mechanical advantage, nothing to do with the power. Power transfer is more related to bike fit, interface equipment, technique, friction etc.
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Re: Crank length More spin or more power

Postby toolonglegs » Wed May 04, 2011 3:24 pm

I run 180's on my mtb as well as sometimes on my road.
They help me in the really step stuff technical stuff...but I also noticed going from 175mm to 180mm the reduction in ground clearance.
So you become more aware of pedal strike.But I like to try and ride up everything... :D .
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Re: Crank length More spin or more power

Postby zero » Wed May 04, 2011 8:08 pm

brokenbus wrote:I am upgrading my crankset mainly so I can get an external bottom bracket which I am lead to beleive is more weatherproof than the internal type and the current one is stuffed.


more weather proof - no.

Both of the reasonable quality conventional taper bottom brackets were completely free and usable after 2 years of all weather commuting and offroad riding. a UN54 might well last you 10 years.

I've already had to remove and reassemble my external once and its less than a year old. Net stories abound of people opening em to find rusted bearings. The only thing I would say is ignoring the bearing vulnerability for a moment, is that I find removing the cranks to be far less of a chore, and I imagine its harder to damage the crank/spindle interface, and I prefer the idea that both bolts have to come loose for a crank to come loose.

Anyway its currently a 170 and I can get 175's for cheaper than 170's. Am I going to appreciate being able to spin quicker with the 170's or the extra leverage with the 175's when using it as a mountain bike - going through mudholes and up steep hills etc. I am 182cm tall if thats relevent and its for a Giant Boulder. I would love to upgrade the whole bike but the GLW has other ideas.
Cheers
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Your boulder may well have a 48/38/28 crankset (which is really a touring or hybrid commuter crankset), and if you are buying an external bracket MTB crankset it will be a 44/32/22. Reckon your main source of improvement will be the new crankset closing up the ratios slightly and having more useful "MTB" ratios, and less commuter ratios you probably can't max out anyway.

I find the 32 very amenable to general single track riding, and I think I'd find a 38 too high, and a 28 only low enough as a granny if I wasn't fatigued. (ie I'd get up the first silly hill in a race no drama, but the tenth one ?)

I find a combination of crank length and crankset width together to be more uncomfortable for my knees than crankset alone. My road bike has 177.5s but road doubles are narrower. A period spent not riding my 175s on my MTB whilst I waited for brake parts to arrive, seemed to need acclimatisation for my knees afterwards. They never feel like that jumping on the roadbike after not using it for a while.
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Re: Crank length More spin or more power

Postby HappyHumber » Wed May 04, 2011 9:44 pm

zero wrote:
brokenbus wrote:I am upgrading my crankset mainly so I can get an external bottom bracket which I am lead to beleive is more weatherproof than the internal type and the current one is stuffed.


more weather proof - no.

Both of the reasonable quality conventional taper bottom brackets were completely free and usable after 2 years of all weather commuting and offroad riding. a UN54 might well last you 10 years.


I'm inclined to agree with zero. I'd be dubious about claims of better weather proofing for the external bearing units. I personally am a fan of the old concept of having a drainage hole in the bottom of the BB shell, and have retro-drilled most of my steel frames for this reason. Haven't done it to my aluminium MTB, though.

An engineer mate of mine who has more recently started getting into his riding was musing out loud to me about the possible benefits of the bearings being slightly larger in diameter and marginally further out of the BB shell in that they probably provide marginally better torsional stiffness (if my memory serves me correctly) than those smaller diameter mounted further inwards on the BB spindle.

Interestingly, he finds the popular bicycle terminology for parts quite amusing; Why exactly is the Bottom Bracket a "bracket"? When he tries to describe a part to me, he sounds like he's flipping through a CAD drawing library.
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Re: Crank length More spin or more power

Postby brokenbus » Wed May 04, 2011 11:53 pm

Thanks for all the responces. I am thinking I will probably go with the 175s. I seem to struggle up steep hills which maybe a combination of the higher gears 28 38 48 but it sounds like some extra leverage will help too. I hope I get the same benfits as TG's son. Does it work in your 40's!!!
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Re: Crank length More spin or more power

Postby Oxford » Thu May 05, 2011 8:21 am

brokenbus wrote:Thanks for all the responces. I am thinking I will probably go with the 175s. I seem to struggle up steep hills which maybe a combination of the higher gears 28 38 48 but it sounds like some extra leverage will help too. I hope I get the same benfits as TG's son. Does it work in your 40's!!!
Cheers
Nicko
Yes it does, I'm now just closer to 50 than 40 and I ride with 180mm cranks on the MTBs including my MTB fixie commuter. BEst thing I ever did going to 180mm MTB cranks, roadies are still 175 and 172.5.
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Re: Crank length More spin or more power

Postby Bentnose » Thu May 05, 2011 8:33 am

HappyHumber wrote:
Interestingly, he finds the popular bicycle terminology for parts quite amusing; Why exactly is the Bottom Bracket a "bracket"? When he tries to describe a part to me, he sounds like he's flipping through a CAD drawing library.


Isn't it called a bottom bracket because its a bracket to hang your cranks on? Why isn't a headset called a top bracket?
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Re: Crank length More spin or more power

Postby trailgumby » Sat May 07, 2011 7:34 pm

Only time I max out 44/11 on the mtb cranks is going down Roseville Bridge on the road at over 70km/hr. On the other end, I don't use 22/34T very often, but it is very comforting to know it's there when I need it when climbs get really nasty (like today at Awaba where you start to worry about tipping over backwards :oops: :lol: ).

Going from 28/28/48 to 22/32/44 makes a lot more difference than a change in crank length. Sorry, I kinda passed over the gearing when it was first mentioned, but I still reckon the 175mm cranks are worth doing.
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Re: Crank length More spin or more power

Postby ozynigma » Sun May 15, 2011 7:38 pm

brokenbus wrote:I am upgrading my crankset mainly so I can get an external bottom bracket which I am lead to beleive is more weatherproof than the internal type and the current one is stuffed. Anyway its currently a 170 and I can get 175's for cheaper than 170's. Am I going to appreciate being able to spin quicker with the 170's or the extra leverage with the 175's when using it as a mountain bike - going through mudholes and up steep hills etc. I am 182cm tall if thats relevent and its for a Giant Boulder. I would love to upgrade the whole bike but the GLW has other ideas.
Cheers
Nicko


I am just under your height and have always had 175's on all my geared mtb's. Save the money, order the 175's and you will probably find they feel great.

I have been trialling 180's riding single speed (slower cadence usually when under load) and I went to marginally smaller 172.5's on my new groupset for my roadbike (higher cadence usually when under load).
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Re: Crank length More spin or more power

Postby brokenbus » Sun May 15, 2011 9:57 pm

Set of 175s sitting on the bench at the moment. Just waiting on a BB tool to come so I can start the conversion. A bit of a rap for Jenson, ordered on Friday night and the courier showed up to deliver on Tuesday.
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Re: Crank length More spin or more power

Postby trailgumby » Tue May 17, 2011 1:55 pm

Hey, brokenbus, a small group of us are heading up to Kiwarrak for the Queen's Birthday long weekend. The recently marked out XC circuit would be well within your ability (it's more twisty than technical), a hardtail would not be at all out of place and there is not a lot of climbing involved unless you seek it out on some of the trails off the main circuit.

The only warning I'd make is that you won't view fire trails in quite the same light again. :D

Wanna tag along on one of the days? :)
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Re: Crank length More spin or more power

Postby brokenbus » Tue May 17, 2011 7:27 pm

Sounds good Trailgumby. I dont think I am going anywhere that weekend. Let us know some details when the time comes.
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