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Decreased performance after crankset change

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Decreased performance after crankset change

Old 06-20-19, 06:54 AM
  #26  
m_michael50
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Originally Posted by Iride01
I confused by your description of your gears front and back. Did you change the size of your chainwheels and cassette? If so, what were they before and what are they now?
I changed the crank and went from 50/34 (175mm) to 52/36 (172.5).
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Old 06-20-19, 02:32 PM
  #27  
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Changing two things at once like this can really make it hard to see what's happening. However, I think your main thing is you just don't have the same gear ratio changes available to you in the places you need them. When cruising now, you might not be able to get that gear ratio that allows you to put out your optimum wattage at a torque you can maintain with your muscles.

Do you know what gear ratios you where using the most with your previous set up? Can you get the same ratios with your current setup? Would changing to a different cassette get you those ratios back?
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Old 06-20-19, 08:36 PM
  #28  
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I don't have much to offer, but I will say this. I bought my first road bike. It had a 52/36 and a 12/23 cassette. It was a 7 speed. Kinda rough on hills. I had to walk some, still do for that matter. I'm still working on my motor. So the frame didnt fit me and I sold the bike. Its replacement was a much newer bike with a compact 50/34 and a 12/25 cassette. This one is a 10 speed. My point is I have and continue to struggle with the gearing. I don't feel strong at all and my times show it. I am having to try and learn to peddle faster in a higher gear. Right now it still feels un natural but I am finding that its bringing my times much closer to where I was. Try to be patient and learn to ride what you have and see if you can make it work.
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Old 06-20-19, 11:02 PM
  #29  
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Originally Posted by m_michael50
I changed the crank and went from 50/34 (175mm) to 52/36 (172.5).
This. He says he mostly climbs hills. A compact crankset is ideal for hilly train...with a 52/36 he loses some range and makes it harder therefore "going slower." I love a compact crankset personally and wouldn't want anything else especially if it is hilly around your area. Personally wouldn't go with anything else unless I lived in the country and had all flat terrain.
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Old 06-21-19, 02:23 AM
  #30  
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Switch back to the compact chainring setup. The crank arm length doesn't matter that much. Mine range from 170 to 175. I'm 5'11", 33" inseam. I hardly notice the difference after a few minutes on the bike. I kinda sorta prefer the 172.5 but if someone switched without telling me I'm not sure I could tell the difference from the 170.

The 175, yeah, I can kinda tell because it's on my hybrid with platform pedals, and the hybrid frame is slightly too big for me. It's really for someone 6' or taller. I have to be careful with the saddle height or I'll get hip and lower back pain.

But my cadence tends to be like clockwork regardless, usually right at 90 rpm after I warm up. Doesn't seem to matter what the crank length, chainrings or cogs are. I'll shift until I get the cadence that feels natural to me, even with the hybrid and platform pedals.

I'm even thinking of trying a 165mm crankset, just out of curiosity.

And if you're not already doing so, try varying your full body exercise routine. I've been back in physical therapy for six weeks (shoulder and neck problems from previous injuries). It's helped other issues with cycling. My posture is better, I'm better balanced on the bike and not favoring my right side so heavily. I'm just more comfortable overall on the bike now. It's mostly just resistance bands and light weights, nothing too strenuous, a couple of sessions a week. And lots of stretching and range of motion at home to improve flexibility.
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Old 06-21-19, 03:08 AM
  #31  
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Originally Posted by Maelochs
Your cadence is not dependent on crank length.
Originally Posted by wphamilton
I'll also point out that changing the crank length has no impact on cadence other than psychologically. That's not to mean just you; the tendency for most people is to increase their cadence at first when they shorten the crank. Because the foot speed is less for a given rpm. So the "cadence fixing" guys didn't really help.
Cadence can be affected by crank length. Shorter cranks mean less leg movement for each revolution, and also increase the amount of force required to turn a given cadence and a given power output, so with shorter cranks it can be natural to use a lower gear and a higher cadence to keep the foot speed and pedaling force similar.

That said, switching from a 175mm crank to a 172.5mm crank is only a 1.4% difference. Hardly anything.

If you really want to boost that cadence, throw on a 130mm crank or something. Except, it would still be pointless. The whole purpose of pedaling a high cadence with low gears is to reduce the force that your legs need to push with. But if you're only reducing your gear choice to compensate for the force increase from the shorter cranks, you're back to where you started.
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Old 06-21-19, 05:29 AM
  #32  
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Originally Posted by HTupolev
Cadence can be affected by crank length. Shorter cranks mean less leg movement for each revolution, and also increase the amount of force required to turn a given cadence and a given power output, so with shorter cranks it can be natural to use a lower gear and a higher cadence to keep the foot speed and pedaling force similar.

That said, switching from a 175mm crank to a 172.5mm crank is only a 1.4% difference. Hardly anything.

If you really want to boost that cadence, throw on a 130mm crank or something. Except, it would still be pointless. The whole purpose of pedaling a high cadence with low gears is to reduce the force that your legs need to push with. But if you're only reducing your gear choice to compensate for the force increase from the shorter cranks, you're back to where you started.
The force is a red herring. Think power - the same power required of you, in the same gear, at the same cadence, with either crank. Wanting to have a higher cadence is purely psychological, and that only because your foot-speed is faster on the longer crank at the same cadence, and you want to speed it up to match on the shorter. As soon as you get used to it, the psychological impetus is gone.
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Old 06-21-19, 10:57 AM
  #33  
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Originally Posted by wphamilton
The force is a red herring.
Controlling the balance between force and the amount of leg movement is why gearing exists at all. Force is never a "red herring" in a discussion of how people self-select gears on their bicycle.

Wanting to have a higher cadence is purely psychological, and that only because your foot-speed is faster on the longer crank at the same cadence, and you want to speed it up to match on the shorter.
Increasing cadence (or increasing force) generally don't feel like an improvement in and of themselves; they're compromises that we make to reduce an excess of the other. When we shift to a lower gear, it's usually not because we want higher cadence, but because we want to reduce the force and feel like the increase in leg motion would be worth the tradeoff.
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Old 06-21-19, 11:29 AM
  #34  
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Originally Posted by HTupolev
Controlling the balance between force and the amount of leg movement is why gearing exists at all. Force is never a "red herring" in a discussion of how people self-select gears on their bicycle.
That's the general misconception that drives this I think. It's completely different from gearing, which allows you to exchange cadence for torque. With different cranks it is the same torque, but exchanges cadence for foot speed.

If that's not clear, use the sniff test on this. The actual "force" that you're concerned about is much lower than people would tend to think. When you're cranking along at around 180 watts, it's a few pounds. Maybe 5 pounds. The difference you're concerned about is a few ounces. It's too trivial an amount to make a difference in cadence.
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Old 06-21-19, 12:45 PM
  #35  
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Originally Posted by wphamilton
That's the general misconception that drives this I think. It's completely different from gearing, which allows you to exchange cadence for torque. With different cranks it is the same torque, but exchanges cadence for foot speed.
Torque and force are proportional, for a given crank length. Shorter cranks mean more force to produce a given torque.

How torque and force relate to each other with crank length is analogous to how cadence and foot speed relate to each other with crank length. They're two sides of the same coin.

The actual "force" that you're concerned about is much lower than people would tend to think. When you're cranking along at around 180 watts, it's a few pounds. Maybe 5 pounds. The difference you're concerned about is a few ounces. It's too trivial an amount to make a difference in cadence.
I completely agree that the 172.5 vs 175 different is too small to make a noticeable dent; my original response noted that the length is only a 1.4% difference, and I went on to describe how pursuing cadence changes by altering crank length is pretty silly in any case.

How did you come up with 5 pounds, though? A 175mm crank arm sweeps a path with a circumference of about 2*.175*3.14159 = 1.1m. A cadence of 90rpm would be 1.5 revolutions per second, which means that the pedals are moving at around 1.65m/s. Power is force times speed, so force is power divided by speed. So, 180/1.65 gives 109 newtons. 109N is about 24lbs, or 12lbs per foot. But that's average force: in most pedal strokes, the downstroke is where most of the power delivery happens, and the upstroke is usually ever-so-slightly-negative, so the force should peak at much higher than 12lbs.
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Old 06-21-19, 01:40 PM
  #36  
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Originally Posted by HTupolev
...

I completely agree that the 172.5 vs 175 different is too small to make a noticeable dent; my original response noted that the length is only a 1.4% difference, and I went on to describe how pursuing cadence changes by altering crank length is pretty silly in any case.

Then you agree with me, it makes only a psychological difference for cadence. It's a few ounces different even with your calculation.


That whole foot-speed/cadence explanation doesn't originate with me BTW. I came across it a few years ago, googling studies about the effects of crank lengths on pedaling mechanics, and it's a pretty common assumption among the researchers.


Parenthetically, as I recall crank length has little effect on bio-mechanical energy efficiency or performance, except for an amusing conclusion that the optimal length (by a quite small margin) for most people is quite a bit shorter than what any of us likely use. My practical conclusion is that you don't lose (or gain) anything by selecting whatever crank feels most natural with your own pedaling mechanics, but if you want to be OCD about it you won't go wildly wrong by choosing smaller. It's mostly a snark hunt.
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Old 06-21-19, 02:32 PM
  #37  
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Originally Posted by wphamilton
Then you agree with me, it makes only a psychological difference for cadence.
No, I would say that it makes a real difference to cadence for non-psychology reasons, but that this difference is extremely tiny for the crank length differences that we're discussing.

Parenthetically, as I recall crank length has little effect on bio-mechanical energy efficiency or performance

Right, for the reason I described: if you respond to shorter cranks by increasing self-selected cadence, the footspeed and force remain similar, so the stresses on the body remain fairly similar. In practice, this doesn't tend to be completely proportional to crank length...
https://www.ncbi.nlm.nih.gov/pubmed/11417428
...seemingly instead choosing to trade some force increase to keep from having to fire off our leg muscles in a super-fast cycle, but it's still significant.

Last edited by HTupolev; 06-21-19 at 02:42 PM.
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Old 06-21-19, 03:05 PM
  #38  
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I'm not going into dueling studies, but that is a very early one and lacks because of:

1. "Trained cyclists" only, (which always suggests training specificity to me) and
2. Looks at "maximum cycling power" which is not what we've (I've) been discussing.

Also I would dispute that the conclusions I mentioned earlier are due to differences in cadence, but not strongly enough to look them all up. If you're interested, be my guest and we'll look at them.
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Old 06-22-19, 04:26 PM
  #39  
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I think most riders could change from 175 to 172.5 w/o any setup changes, and not notice a difference.

Depending on which pedal you were coming from the newer Shimano pedals generally have a lower stack height. Which would offset the seat height difference due to the crank length change (raise seat for shorter crank, lower seat for smaller pedal stack = no change).

You didn't mention if you changed cleats when you changed pedals? Especially climbing heel drop can have a bigger impact on effective seat height than 2.5mm of crank length.
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