Crank Arm Size

runbikesink · Feb 18, 2011

I am getting ready to retire my little Giant to the trainer and buy a new road bike. I never once thought about Crank arm size until now. I have a Cervelo P3 48 with 650 wheels and 54/39 chain rings with a 172.5 crank. I also have a too big, size small, Giant TCR road bike with 50/34 compact and the same 172.5 crank. I'm faster on the tri bike even on the hills and I've always been frustrated with the road bike. I'm not at all sure why I'm slower on it, but I'm pretty sure for one thing it's too big, I also seem to prefer to mash rather than spin and have always wondered about the compact crank.
 
Anyhow, the bike that i was all ready to buy is a Woman's specific bike. ( I'm 5' 3") and I noticed the crank arm size is 165, a big difference than what I'm used to. It seems that given my size the 165 is probably better for me, but I'm afraid I'm so used to the big crank arm I'm going to hate it.
 
Has anybody had any experience with this? Changing crank size or knowing what significance difference it makes?
 
 
 
 
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TriAndStopMe · Feb 18, 2011

No clues on the crank... just wanted to say, "Hi!" Good to see you here

cak73 · Feb 18, 2011

I've just switched from a 172.5 to a 165 in order to improve my position on my bike; both were 50/34 compacts cranks. I don't have a power meter and I haven't been out on the road yet, though...New England. 
 
The height/position of your saddle relative to the crank will probably change a bit. With a good bike fit, I would guess that the only difference you'd notice is the power you generate. 
 
BUT, I am far, far, far from being the best person to answer your question.

runbikesink · Feb 18, 2011

Thanks Cheyrl, Since it's a new bike, I will have to be fitted anyhow and I suppose the whole thing will feel weird to start.
Thanks Triandstopme, it's been awhile. I had to send in for my log on password

Runbikedrown
Kim

mrscoby78 · Feb 18, 2011

*giggle* 

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by runbikesink <a href="/forum/thread/72667/crank-arm-size#post_1984477"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style="border-right:0px solid;border-top:0px solid;border-left:0px solid;border-bottom:0px solid;"></a> 
Runbikedrown
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I don't know squat about bikes. But hi! <img alt="blob6.gif" src="http://files.kickrunners.com/smilies/blob6.gif" style="width:31px;height:31px;">
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runbikesink · Feb 18, 2011

LOL...Hi back!

jroden · Feb 18, 2011

I think a lot of riders decide they are a masher without really trying the alternatives. In any event, the 165 should not feel hugely different, but swapping out to the one on your misfit bike is easy, then sell it as a frame and fork and sell a part at a time on ebay

runbikesink · Feb 18, 2011

Jroden,
 
You mean put the crank arm from the giant on my new bike...Isn't the crank arm attached on ones side? I'm not at all sure.I'm guessing. It's not here. The only thing I know for sure, is I am faster on the Cervelo. I did a half Ironman with 7000 feet of climbing on the Cervelo even though the race director strongly recommended no tri bikes. I never once wished for my road bike with the compact.

jroden · Feb 19, 2011

Well, you are comparing a lot of variables, gearing, crank armlenfth, geometry and aerodynamics, plus maybe you just had a good day. In any event if the crNks are the same design, they are easy to swap but if your bike has not been delivered just have the shop swap to whatever size you desire, thats part of the deal. If you can climb fine without a compact, then use a conventional crankset

runbikesink · Feb 19, 2011

Haha...If you are talking about the half IM, hardly a good day. It was tough and left my legs completely shot for the run, but I had a lot of company. Only about 30 females did it as it has a rep for not being very pleasant. My Tri bike got me 5th place on bike time for the gals, but i'm pretty sure I would have dropped several positions on the road bike. Thanks for your help. I was a little disappointed to find out that my choices were so limited. I wanted to get the 52/36 compact instead of the usual 50/34, but I guess it won't work. Cranks are the new Ultegra and they don't make rings to fit the bolt pattern. The bike is specialized, and they make the right size, but it doesn't work on that particular bottom bracket so I am told.

Now I don't know if i want the compact or the standard. I live where there is a lot of hills. At any rate, at least the crank arms can be switched out, so I'm not stuck with the shorter ones if I don't like them.
 
Thanks for the info

jroden · Feb 19, 2011

If you want the bigger chainring then and option is a standard crank but get a 38 small ring and either stick with the stock 53 or swap to whatever you want. Chainrings are cheap on ebay and as long as the bolt circle is right they should work fine. To get your gearing to work, you need to get a larger rear.cluster, one tooth should do it. 
Take a minute to go to a website like sheldon browns gear inch calcularor and figure out how many inches you need to get up hills using your current setup, then enter the new chainring size and determine if you need to add one or two teeth in the back to compensate. 
You must be pretty freaking strong to be horsing up big hills on a conventioal crank, good for you!

TriAndStopMe · Feb 20, 2011

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<div class="quote-block">Runbikedrown
 
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Hah!

runbikesink · Feb 21, 2011

I don't think I am extraordinarily strong, I'm just a lousy spinner, and for whatever reason, do not like the drop from the big ring to the little one on the compact. I don't know why. I'll stay in the big ring for a long time before switching, and have had more than one person bring that to my attention. I don't think that is a good thing, but it is what it is. And yes, I do switch a couple gears before I do, otherwise I lose my chain. I use a big cassette in the back to compensate. I'm just going to buy a standard crankset and have them put it on and keep the compact in the event I have any problems. However, those short crank arms (165) are on the compact. I rode a bike with them yesterday and I don't like it. I really did feel as if I was turning in little circles. Will the crank arm on the standard crankset fit on the compact one? I'm getting the bike shipped form out of State as the local bike shop I do business with does not carry Specialized or anything that I could find that I liked in my price range. I'm also getting a pretty good deal through a friend.

niemsco · Feb 22, 2011

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by runbikesink <a href="/forum/thread/72667/crank-arm-size#post_1984977"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style="border:0px solid;"></a> I don't think I am extraordinarily strong, I'm just a lousy spinner, and for whatever reason, do not like the drop from the big ring to the little one on the compact. I don't know why. I'll stay in the big ring for a long time before switching, and have had more than one person bring that to my attention. I don't think that is a good thing, but it is what it is. And yes, I do switch a couple gears before I do, otherwise I lose my chain. I use a big cassette in the back to compensate. I'm just going to buy a standard crankset and have them put it on and keep the compact in the event I have any problems. However, those short crank arms (165) are on the compact. I rode a bike with them yesterday and I don't like it. I really did feel as if I was turning in little circles. Will the crank arm on the standard crankset fit on the compact one? I'm getting the bike shipped form out of State as the local bike shop I do business with does not carry Specialized or anything that I could find that I liked in my price range. I'm also getting a pretty good deal through a friend.
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You felt like you were spinning little circles because you where. With a shorter crank arm, you are going to have to spin slightly more rpms to produce the same power output as the 172.5 (just seems logical to me). 165 seems excessive, I'm 5'5 barely and I have a compact at 170. I use to ride 175 no problem. Depends on type of rider you are, and you don't seem to be a spinner. My optimal power is in the mid to low 80 rpms, so you sound like me a bit.

jroden · Feb 22, 2011

<div class="quote-container" data-huddler-embed="/forum/thread/72667/crank-arm-size#post_1984977" data-huddler-embed-placeholder="false">Quote:
<div class="quote-block">Originally Posted by runbikesink <a href="/forum/thread/72667/crank-arm-size#post_1984977"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif"></a> I don't think I am extraordinarily strong, I'm just a lousy spinner, and for whatever reason, do not like the drop from the big ring to the little one on the compact. I don't know why. I'll stay in the big ring for a long time before switching, and have had more than one person bring that to my attention. I don't think that is a good thing, but it is what it is. And yes, I do switch a couple gears before I do, otherwise I lose my chain. I use a big cassette in the back to compensate. I'm just going to buy a standard crankset and have them put it on and keep the compact in the event I have any problems. However, those short crank arms (165) are on the compact. I rode a bike with them yesterday and I don't like it. I really did feel as if I was turning in little circles. Will the crank arm on the standard crankset fit on the compact one? I'm getting the bike shipped form out of State as the local bike shop I do business with does not carry Specialized or anything that I could find that I liked in my price range. I'm also getting a pretty good deal through a friend.
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Im not a great expert, but really do feel you would be well served by learning to use the gears on the bike correctly and make the chainring shift before you start to bog going up the hill. You want to be able to reduce the torque onyour pedals fora second u til the chain drops, or it is morelikely to pop off. Running some massive cassette in the back so you can mash along in the big ring all cross chained is a poor policy. 
The crank arms and the spider are all integrated into a crankset, they are normal, compact and triple. Each design has a different bolt circle and each brand requires a unique bottom bracket. 
Cranksets sell cheap on ebay, something like ashimano105 with bottom bracket is quite cheap and the spanner wrench to install it is very cheap. Just remember the one side screws in backwards, its marked.

runbikesink · Feb 22, 2011

I have an 11/28 on my tri bike with the standard gearing and I rarely ever use the 28, but it's nice to know it's there and I only use it in the small ring. I have no problem switching to the small ring on that bike...I don't cross chain.
 
 It is the compact where I stay in the large ring for a long time. I have an 11/26 on the compact and am probably occasionally guilty of using the 26 with the big ring because I hate shifting to the little one on that bike, which is exactly why I wanted to try something else.

AdCo · Feb 22, 2011

I just had a fit and was told I could probably go to a 165 from 170, but not sure If i can just swap out the cranks vs swapping out the whole triple..

runbikesink · Feb 22, 2011

If you have to switch out the entire crank set and it is possible, why don't you go to a compact? Less weight, less gears to deal with. Have you tried one before?

jroden · Feb 23, 2011

<div class="quote-container">Quote:
<div class="quote-block">Originally Posted by runbikesink <a href="/forum/thread/72667/crank-arm-size#post_1985054"><img alt="View Post" class="inlineimg" src="/img/forum/go_quote.gif" style="border:0px solid;"></a> I have an 11/28 on my tri bike with the standard gearing and I rarely ever use the 28, but it's nice to know it's there and I only use it in the small ring. I have no problem switching to the small ring on that bike...I don't cross chain.
 
 It is the compact where I stay in the large ring for a long time. I have an 11/26 on the compact and am probably occasionally guilty of using the 26 with the big ring because I hate shifting to the little one on that bike, which is exactly why I wanted to try something else.
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you make some trades with that cluster, note that you start taking jumps after 15 and taking large jumps after 21: 11-12-13-14-15-17-19-21-24-28. The advantage of the compact is you have such a small ring in the front you can run a narrower gearing in the back like a 11-23, so you have no jumps until you hit 17: 11-12-13-14-15-16-17-19-21-23.
 
your 39 x 28 gives you a 36.6 inch gear for climbing (smaller is easier) while a 34 x 25 gives you about the same at 35.7, a bit harder I guess. The attraction of the compact is you can run the smaller gearing in the back with fewer jumps of two or more teeth and still get over hills.
 
your 50 x 11 with give you 32 mph at 90 rpm, while switching to the 53 gives you 2 more mph, if you have enough muscle to push the bigger gear.
 
For me, I would not ride a 11-28 cluster on the road if I could possibly avoid it. the 23 or 25 clusters are pretty much the standard choice and the 23 is especially nice because it has a 16, which seems to be a "sweet spot" gear for many riders. If you have a lot of those "can't find the right gear" moments on the flats, your cluster may be contributing to this issue.

runbikesink · Feb 23, 2011

Thanks for the suggestions. I guess it's just an ongoing trial and error thing for me. We really DO have big hills around here and the thought of a 23 sounds hard even on a compact, but then again, I don't know jack about gearing. I'm going to start with the standard crank and the cassette that comes with it, and see how it works out. However, I have a feeling I will end up back on the compact and just play with the cassette sizes. It just sucks it has those little crank arms on it. The new bike actually comes straight out of the catalog with a 5/34 and an 11/28.

Crank Arm Size

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