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Crank-arm length and gearing

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Crank-arm length and gearing

Old 07-06-22, 09:03 PM
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smasha
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Crank-arm length and gearing

I'm trying to get my head around this, and thinking out loud, so I can check that I don't have it backwards.

Having a new bike built up, and changing the crankset and related parts (FD, shifter) before it leaves the shop.

The new crankset is not 100% ideal, based on what I think I want. The overall range of gears, from bottom to top, is a little bit lower than what I think I want. ie, the chain-rings are a little bit smaller than what I think I want.

Is it reasonable to think that 170mm crank-arms, instead of 175mm crank-arms, will compensate for that, a little bit?

eg, if the gearing was higher than I wanted, longer crank-arms could help me "grind" or "push" through it. Right?

So with shorter crank-arms, I'm thinking I should be able to spin faster, and better utilise the gearing that's lower than I want.

Of course, this assumes that there's any noticeable difference between 170 and 175 cranks-arms, which people can debate.

Does this make sense? Or do I have it backwards?

If the gearing is a little lower than I want, then shorter crank-arms (within reason) make sense?
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Old 07-06-22, 11:51 PM
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Shorter crank does makes it easier to spin higher cadence faster.

But getting accustomed to higher cadence can only be fully satisfied with training. It's not really that bad. Higher cadence is better for leg endurance.

As a bonus, you might find shorter cranks are more comfortable when you get used to it (less bend at the knees and better leverage for the legs).
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Old 07-07-22, 08:55 AM
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Mostly depends on your experience.

Gearing is most easily changed with the cassette. IME crank length is more dependent on leg length than anything. What's the groupset, ring sizes?
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Old 07-07-22, 09:05 AM
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Don't equate crank arm length to leverage. Sure it does add something but that alone isn't anything to judge why to get a longer or shorter crank IMO.

As mentioned a shorter crank will let most people spin a faster cadence with more comfort. If you only care to spin a slow cadence, then you'll probably prefer a longer crank.

I've got a 34.5" inseam and I use 165mm cranks. Tried longer for quite a few months but never liked them. Even test riding two bikes a size apart, one with 172 mm cranks and one with 175 mm cranks, I felt worn out after ten miles on the longer crank. The second 10 miles with the shorter cranks felt much better. When I ordered the bike and they built it out, I had them put 165 mm cranks on it and I've not regretted it yet. I do feel I have a large range of cadence. Though 120 rpm seems to be where I top out. Maybe I should work on getting to 130 rpm or better during accelerations.

If you have suitable cranks in long and short, you should just try them both for several months each or a 1000 miles to get some data to compare and just see how you really perceive them over time and not just a one or two ride experience.
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Old 07-08-22, 02:21 AM
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For me, it looks like 80RPM is on the low end of a comfortable cadence, 120 is on the high end of a comfortable cadence, and 100 is about the sweet spot.

The bike is being built up with an 11-42 10-speed cassette and 40-30-22 crankset. I may swap out the cassette for an 11-36 after I ride it for a bit. Ideally I'd want a slightly larger crankset, maybe 44-34-24. With the crankset slightly smaller than I want, I'm thinking if it's easier to spin faster, I can make better use of the gears being a little bit lower than I'd otherwise want.

On paper, top-gear (40/11) gives me about 50kph (31mph) at 99rpm cadence, which should be decent for a commuter/touring/grocery bike. And on the low-end, I should be able to maintain a very comfortable cadence going up-hill with loaded panniers.

I'm much more comfortable spinning than grinding, and even on crazy steep climbs I tend to stay on the saddle.

Not sure if I'm overthinking it too much… But I've got a good feeling about how it's shaping up.
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Old 07-11-22, 01:01 AM
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Originally Posted by koala logs View Post
Shorter crank does makes it easier to spin higher cadence faster.
In a lower gear! No one ever includes the last part. They make it seem like shorter cranks make you stronger. They do not. Your power does not increase, so if you reduce your leverage by using shorter cranks and thus increase your cadence to compensate, you will find the same gear harder to turn. You 'might' be able to use a lower gear AND a faster cadence to achieve the same road speed as before. Maybe, because cadence is an intrinsic thing. Just because you CAN pedal a faster cadence with shorter cranks does not mean you will want to. Competitive riders want to. They must. Commuters might. Recreational riders probably will not.
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Old 07-11-22, 01:14 AM
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Originally Posted by smasha View Post
For me, it looks like 80RPM is on the low end of a comfortable cadence, 120 is on the high end of a comfortable cadence, and 100 is about the sweet spot.

The bike is being built up with an 11-42 10-speed cassette and 40-30-22 crankset. I may swap out the cassette for an 11-36 after I ride it for a bit. Ideally I'd want a slightly larger crankset, maybe 44-34-24. With the crankset slightly smaller than I want, I'm thinking if it's easier to spin faster, I can make better use of the gears being a little bit lower than I'd otherwise want.

On paper, top-gear (40/11) gives me about 50kph (31mph) at 99rpm cadence, which should be decent for a commuter/touring/grocery bike. And on the low-end, I should be able to maintain a very comfortable cadence going up-hill with loaded panniers.

I'm much more comfortable spinning than grinding, and even on crazy steep climbs I tend to stay on the saddle.

Not sure if I'm overthinking it too much… But I've got a good feeling about how it's shaping up.
I am not sure why the shop is building something you aren't sure you want. That "On paper, top gear ... .... not going to happen." I'm not quite as strong as the millenials that cruise past me on my commute but I am not weak. Nevertheless I would only pedal top gear downhill, or with the momentum from a recent downhill bleeding off. It is not a cruising gear, for mortals like you and me. That said, your stated gearing is on the low side. I doubt you will be happy with it. the 44/34/24 is more reasonable, especially with the dinnerplate rear cogs! But, even so, you will probably cruise in the middle ring. Big ring will be for limited (in length) spurts of speed to slide through a timed out green light, or yellow one!
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Old 07-11-22, 01:38 AM
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Originally Posted by Leisesturm View Post
In a lower gear! No one ever includes the last part. They make it seem like shorter cranks make you stronger. They do not. Your power does not increase, so if you reduce your leverage by using shorter cranks and thus increase your cadence to compensate, you will find the same gear harder to turn. You 'might' be able to use a lower gear AND a faster cadence to achieve the same road speed as before. Maybe, because cadence is an intrinsic thing. Just because you CAN pedal a faster cadence with shorter cranks does not mean you will want to. Competitive riders want to. They must. Commuters might. Recreational riders probably will not.
It just feels easier on the legs and many of my still unbreakable climbing PRs were done on a bike with short cranks (150 mm)

My current bike of 3 years has 170mm. 170mm was determined to be the correct size for me based on inseam of 83 cm. However, I still perform better and apparently with higher FTP on shorter 150 mm crank spinning higher cadence at lower gear.

I'm simply short on luck finding affordable 150mm 1x crankset the reason why I'm still using 170mm. And even after 3years, I would have adapted to the 170mm crank by now but I till make more power on the 150 crank.
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Old 07-11-22, 11:52 AM
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Originally Posted by Leisesturm View Post
I am not sure why the shop is building something you aren't sure you want. That "On paper, top gear ... .... not going to happen." I'm not quite as strong as the millenials that cruise past me on my commute but I am not weak. Nevertheless I would only pedal top gear downhill, or with the momentum from a recent downhill bleeding off. It is not a cruising gear, for mortals like you and me. That said, your stated gearing is on the low side. I doubt you will be happy with it. the 44/34/24 is more reasonable, especially with the dinnerplate rear cogs! But, even so, you will probably cruise in the middle ring. Big ring will be for limited (in length) spurts of speed to slide through a timed out green light, or yellow one!
There's a gap between what I think I want, and what's available. There's also the fact that I haven't ridden this new bike, so nuances of the geometry and sizing and fitting may affect what I think I want.

Over time, I may well swap out the cassette and/or chain-rings and/or crank-arms and/or ???, but as a starting point, this is how it's shaping up.

re your previous comment, yeah, "pedalling faster" has a few caveats… But ultimately, it's about matching the gearing to my sweet-spot for maintaining power at a comfortable cadence, and if the gearing is a little too low, then shorter cranks should help facilitate better utilisation of that gearing.
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Old 07-11-22, 12:01 PM
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OP's original question:
Originally Posted by smasha View Post
The overall range of gears, from bottom to top, is a little bit lower than what I think I want. ie, the chain-rings are a little bit smaller than what I think I want.
Is it reasonable to think that 170mm crank-arms, instead of 175mm crank-arms, will compensate for that, a little bit?
A: I visited "Sheldon brown's gear calculator" page, and based on the calculations there using "gain ratios," the difference between 170mm and 175mm arms equates to about a one tooth difference in the chainring.
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Old 07-11-22, 01:14 PM
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Originally Posted by timdow View Post
OP's original question:


A: I visited "Sheldon brown's gear calculator" page, and based on the calculations there using "gain ratios," the difference between 170mm and 175mm arms equates to about a one tooth difference in the chainring.
Exactly. That was a point I intended to address in another post. The percent difference in effort is roughly 3%. A person might notice a 3% harder to press computer keyboard but a 3% harder (or easier) to pedal bicycle just wouldn't be worth it to me for all the expense and hassle involved in changing out a crankset. The minimum I would consider is 10%. "Go big, or go home" is how it is sometimes put.
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Old 07-11-22, 03:50 PM
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Sort of hard to equate comfort to 10 percent gains of some performance number. If I'm more comfortable and willing to pay for the change it won't matter if I don't get more performance out of it. However even just a few mm shorter cranks do leave me with at least a sense of less work done for the faster times I have on the same route.

Even if that is just 1% difference, it's worth it to me for a lifetime of cycling. If I was only doing one ride, not so much.
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Old 07-11-22, 07:54 PM
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Originally Posted by Leisesturm View Post
Exactly. That was a point I intended to address in another post. The percent difference in effort is roughly 3%. A person might notice a 3% harder to press computer keyboard but a 3% harder (or easier) to pedal bicycle just wouldn't be worth it to me for all the expense and hassle involved in changing out a crankset. The minimum I would consider is 10%. "Go big, or go home" is how it is sometimes put.
In my case, based on parts availability, it's either:
a) get the gearing not low enough, with 175 cranks
b) get the gearing possibly a little too low, with 170 or 175 cranks

Until the bike is built, and I ride it up and down some hills, I can only make educated guesses about what's "best". After that, then I can consider different cassettes, chain-rings, etc.

Accounting for the pragmatic consideration of parts availability, and if not getting "the best" gearing, at least getting "the best" starting point to make adjustments from, I think option "b" is looking good.
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Old 07-11-22, 11:31 PM
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Originally Posted by smasha View Post
In my case, based on parts availability, it's either:
a) get the gearing not low enough, with 175 cranks
b) get the gearing possibly a little too low, with 170 or 175 cranks

Until the bike is built, and I ride it up and down some hills, I can only make educated guesses about what's "best". After that, then I can consider different cassettes, chain-rings, etc.

Accounting for the pragmatic consideration of parts availability, and if not getting "the best" gearing, at least getting "the best" starting point to make adjustments from, I think option "b" is looking good.
It doesn't have to be a trial and error process. Decent drivetrain components are not exactly disposable. The drivetrain you are settling for gives you a 98" top gear. It kind of works. A nice change from the usual ridiculous 113" top gears ... but ... you can do better. I can just about cruise in that on level ground! It would be spun out in any significant tailwind and any downhill worth calling it that. You are right about the 42T big cog. That's for a 1x drivetrain. With a triple you don't need such a monster bailout gear. The 36T gives you around 16" with the 22T granny. Almost too low to balance, but you would use it once you get used to it. The 18" with the 22Tx 32T just above it works better as an ultimate low. My cargo bike has that combo. I use it a lot.

Going to your alternate 44/34/24 is better. Now your top gear is 108" and with 36T, your bottom gear is 18". Schweet. I'd ride that ... but you can (still) do better: <drum roll> 42/32/22 x 11 - 36. 104" on top and 16.5" on the bottom. What's not to like? And there isn't any magic about 10 teeth between chainrings. You could do 44/32/22 for a bigger top end and still keep the ultra low 16" for those all day long climbs with a load. You haven't even begun to tax the capabilities of modern derailleurs with the drivetrains we have composed thus far. I guarantee if you build the 42 (or 44)/32/22 x 11-36 that you won't be looking to swap anything. 170 vs 175 is a wash. I have bikes with both and I couldn't tell you which has what.

I can't believe with some looking around that you wouldn't be able to find the parts to build this drivetrain. Right now. Hell, go to a bike co-op and get some used rings to tide you over till you can do it right.
Edit: I don't think there has been any mention of wheel/tire size in this discussion. It almost goes without saying that I have been assuming 700C. But I think now is a good time to get it on the record.

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Old 07-12-22, 03:14 AM
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Yeah, 700c wheels, commuter/touring tires. Same as my old commuter.

The old bike has a 44-36-26 crankset (175mm), and 11-34 cassette. Based on my calculator, that gives a range of 22-116 gear inches.

I rarely need more than 116 gear inches, but going uphill with loaded panniers, I'd rather spin with lower gears than grind without lower gears. I'd like to have more gears on the low end.

So really, 44/11 (4.0) is a useful high gear (for me, on this type of bike), and something (???) lower than 26/34 (0.76) is what I want for a low gear.

The factory drivetrain that comes with the new bike is 44-32 crankset with an 11-42 cassette, which gives a range of 22-116, pretty much identical range to the old bike, but 2x10 instead of 3x9, so fewer (usable) gears.

So, thinking out loud, and crunching the numbers… And assuming that the frame geometry and fitting won't throw things too far out of whack, and making assumptions based on the old commuter, and with an 11-42 cassette… I probably want a 44-34-24 3x crankset, or maybe a 44-24 2x crankset, if that's feasible.

Or maybe a little more high gear and/or accounting for switching from 175 to 170 crank-arms, a 46-36-24 3x, or a 46-24 2x, again, if that's feasible.

For the weight difference, between a 2x and a 3x, I'd rather have the extra weight, and chains that last longer; a 3x allows for straighter chainline than a 2x.

Alright: If you can point me towards one of those cranksets, 175 or 170, without going too crazy on the price, let me know! I'm looking for Deore level, give or take, from any reputable manufacturer.

Or, since I found a good deal on 40-30-22 cranksets (Shimano FC-MT500 3X10, 170mm, 40-30-22T, 96/64 BCD), should I start with one of those and swap out chainrings? Of course, even that raises the question: Where can I get the chainrings I'd want?

Last edited by smasha; 07-12-22 at 03:17 AM.
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Old 07-12-22, 11:04 PM
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42(44)/32/22 x 11-36. Make it happen. Peace. Out.
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Old 07-13-22, 03:35 AM
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Originally Posted by Leisesturm View Post
42(44)/32/22 x 11-36. Make it happen. Peace. Out.
That could be what I'm looking for - https://www.gear-calculator.com/?GR=...32,36&UF2=2309

Old bike on top, 44-32-22 x 11-36 on the bottom.

I'l see how it goes using mostly readily available parts, and then figure out how/if it needs to be tuned from there.
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Old 07-15-22, 04:22 AM
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You're making things too difficult. Being able to pedal to 50kmh at a cadence of 99rpm, is PLENTY fast enough, for that style of bike.
Don't over think it or double guess yourself.

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Old 08-15-22, 11:42 AM
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Crank arm length is to get your knees into safe joint angles at max flexion, nothing more. Just wrote a paper on this on lermagazine.com (got the cover!)
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Old 08-30-22, 10:13 AM
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Experience of riders switching from 170mm to 150mm cranks is no change in ratios is required. Gears used are same as before. A 5mm change is unlikely to do much at all.

BFC above has it correct. I would have said 'useful' or 'comfortable' rather than 'safe', but that's me. If you can pedal the bike you will be fine. Pedaling mechanics, which is you, is way more important than fine details of bike mechanics.
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