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Why does one bike climb better than another?

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Why does one bike climb better than another?

Old 09-16-21, 09:57 AM
  #76  
PeteHski
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Originally Posted by 63rickert
Does 32 feet per second per second sound Newtonian? Does any here have the faintest idea how to apply that? It is third grade arithmetic if you can structure the problem reasonably. Just a little story problem for you - if climbing at ten feet per second on a 15% grade how long can you coast (momentum) before the bicycles comes to a full stop? Most will know from experience it is less than a second, please calculate a correct answer or sit down. Does the answer give you a little clue as to why “slightly lumpy” power is completely different than continuous power?

Climbing at 60 rpm is one crank revolution per second. There’s a big fat power pulse when the right crank is at 3 o’clock, half a second later the left crank is at 3 o’clock and another power pulse. It is an eternity between those power pulses when the grade is steep. Good pedalers will get power from the 2 o’clock position to the 4 o’clock position. Great pedalers get something from 1 o’clock to 5 o’clock. Best pedalers get something (something, not a lot) on the upstroke, most do not even get out of their own way on the upstroke. No one gets full power through TDC/BDC. Does not occur. Fantasists on this thread can assert that cyclists put out power just the same as an electric motor does, it does not occur in the real world.
So let's humour this then shall we? Okay so g = 32 ft/s^2, 15% gradient = 8.53 degrees. So component of g acting down the slope = g x sin 8.53 = 4.7 ft/s/s. So if you coast from an initial speed of 10 ft/sec, it's actually going to take about 2 seconds to stop. I've ignored rolling resistance and wind resistance here.

So what have we learnt here then? Well if you stop pedalling on a 15% gradient you decelerate from 7 mph to zero in about 2 seconds. But if you continue to pedal at the same rate then you just carry on moving at 7 mph. Your power may be a bit lumpy, but given sensible gearing you should be able to apply force on each pedal in roughly 0.5 sec intervals (60 rpm cadence). So at worst case (assuming those pedal forces were instantaneous force blips every 0.5 sec), you would only decelerate/accelerate by 1.6 mph between each pedal stroke. But in reality your pedalling is not actually as lumpy as this extreme. The pedal force is spread out more evenly over those intervals and over the course of a few seconds your power becomes quite consistent. Your speed might vary by +/- 0.5 mph, which is irrelevant when discussing things like rotational inertia of bicycle wheels.
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Old 09-16-21, 09:59 AM
  #77  
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Originally Posted by chaadster
I think you’ve lost track of the discussion; we’re more than 70 posts in, but if you go back and read the OP, there is no mention of speed, only feelings, like “the Buzz feels like a pig,” it “feels like I’m pulling trailer,” and “I keep dropping gears and it feels like my quads are burning.” It’s pretty plain that the OP’s Doppler climbs better than their Buzz because it feels better, and the OP, in asking if it’s just geometry, was trying to figure out why it is the Doppler feels better when climbing.
It's all just semantics in basically saying that it feels "slower" than the other bike. I was just trying to address the red herring of rotational weight in this context. I agree the discussion has gone off tangent as they always inevitably do!

The OP even mentioned it as a significant factor:-

Originally Posted by BadgerOne
Rotating weight is of course a big one
Well it isn't a "big one" actually, especially not in the context of low speed climbing

Last edited by PeteHski; 09-16-21 at 10:09 AM.
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Old 09-16-21, 09:59 AM
  #78  
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Originally Posted by tomato coupe
You just flunked your own quiz; it takes just over 2 seconds to coast to a stop, not less than a second.
Agreed, he was talking bs.
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Old 09-16-21, 11:49 AM
  #79  
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Aw man....I thought it had already been decided that the problem was because my Buzz is matte gray! Some of the anodized parts are also turning a deep bronze color, so that definitely isn't helping with the uphill prowess either. The Breezer's metallic blue with silver components definitely lessens the effort required to push the thing uphill.

I haven't tried painting the bike red yet, but I did switch the tires from the worn-out Kenda K1024s to Michelin Protek in the same 40c size, another heavy and stiff wire bead tire with puncture protection. Maybe I'm nuts but the effort seemed notably reduced with the new tires. I suppose it could be true that the Kendas were really just awful tires. If anyone wants to find out for themselves, CRC has them in 35c for ten bucks a pop, which is probably about what they are worth anyway.

Who am I kidding. Argue away, this is entertaining.
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Old 09-16-21, 04:02 PM
  #80  
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Originally Posted by jayp410
As someone with a Physics degree + some grad school, and one class short of a coincidental Math degree, 3.7 GPA... I agree with PeteHski. This is very simple stuff, yet there are misconceptions.

Kinetic energy in this system is linear motion (1/2*M*v*v) + rotational energy of the wheels (1/2*I*w*w). Where M is the mass of the entire system (rider, bike, wheels), and I is the interia of the wheels. The inertia of the wheels does depend on the wheel weight, and the distribution of mass within the wheel (inertia being proportional to square of the distance of the mass from the axis of rotation) but that is irrelevant because, assuming a relatively constant speed up the hill, the total kinetic energy does not change from the bottom to the top of the hill. Yes there are accelerations/decelerations on the way up, but the kinetic energy at the top is basically the same as at the bottom once a relatitvely steady speed is established. There is effectively zero net change in kinetic energy.

But there are micro accelerations, you might say. Micro accelerations happen due to the uneven forces, and yes, lighter wheels will accelerate faster, but that is irrelevant. The speed of the bike is changing some per pedal stroke, but what is really happening (from an energy standpoint) is that kinetic energy is being lost (slowing down in between pedal strokes) but that kinetic energy that is lost from that (both linear and angular) is being converted into potential energy as momentum carries you up the hill in between strokes. The kinetic energy is like a reservoir that is being drawn from and added to on each pedal stroke. A bike with very heavy wheels is going to have more angular momentum and correspondingly more rotational energy, and thus is going to slow down less between pedal strokes. So, heavier wheels are going to "help"/assist more in between strokes, even though they are harder to accelerate on the downstroke. The rider injects more energy into the system (or more accurately converts chemical potential energy into kinetic energy) by pedaling. Again, the rotational energy at the top of the hill is basically the same as at the bottom. This is different than a sprint from a standstill, where wheel inertia absolutely would matter because you would need to add net rotational energy during the sprint.

What does change as the hill is climbed is potential energy, which is M*g*h. Two bikes only differ in M (total mass) when it comes to potential energy. Wheel weight distribution does not matter in this.

The rider's power output determines how quickly the hill is climbed (or height is gained), as the power output determines the rate at which potential energy is stored.

All of the above ignores wind resistance and friction, of course. Some of the rider's power output is consumed by those things as well.

All that said, there could be secondary effects from wheel weight which contributes to how quickly a bike (or rider AND bike) can climb. A rider could be more efficient in delivering power if the wheels allow quicker accelerations, due to biomechanics or some mumbo jumbo that is nearly impossible to quantify except by testing, and it may vary by rider physiology and/or technique. With heavier wheels, the duration of the downstroke in which more force is applied could be lengthened, and the tire could deform more (over a longer time per pedal stroke) with heavier wheels, which could increase rolling resistance more while it's deformed. I wouldn't be surprised if those factors came into play, although I also wouldn't be surprised if there was little measurable effect from all of that, in which case the only benefit from lighter wheels would be less total mass. I still would want lighter wheels for that reason, just like I wouldn't want a bottle cage made out of lead.
An interesting experiment would be to take the favorite bike and do two climbs. One climb would have weights attached to the rims and the other would have those same weights, say, on the bottom bracket. Would one try feel faster than the other?
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Old 09-16-21, 04:55 PM
  #81  
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Originally Posted by PeteHski
The pedal force is spread out more evenly over those intervals and over the course of a few seconds your power becomes quite consistent. Your speed might vary by +/- 0.5 mph, which is irrelevant when discussing things like rotational inertia of bicycle wheels.
Yeah, the pedal force is spread out over a significant angular range to the point of having some power just about about all the time. Folks with fancy power meters can even show fancy angular plots showing how pedal force is substantial throughout each pedal stroke.

Otto
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Old 09-18-21, 02:54 PM
  #82  
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Originally Posted by PeteHski
So let's humour this then shall we? Okay so g = 32 ft/s^2, 15% gradient = 8.53 degrees. So component of g acting down the slope = g x sin 8.53 = 4.7 ft/s/s. So if you coast from an initial speed of 10 ft/sec, it's actually going to take about 2 seconds to stop. I've ignored rolling resistance and wind resistance here.

.
2 seconds is quite long enough that an assistant with a stopwatch will not be required. Go test your hypothesis. Find a 15% grade, get up to a speed of 7mph, coast, see how long you keep moving. It won’t be anything like 2 seconds. Anyone who wants to try can test this. Anyone who has ever climbed a 15% at any speed already knows how this experiment ends. But please do try this at home.

In the original story problem 15% was thrown in as a confounder. You bit, as you simply don’t understand the problem. Force of gravity is a constant, it does not depend on the gradient. Gravity is not and never will be, on planet earth, 4.7 feet per second. It is invariably 9.81 meters or 32 feet

Last edited by 63rickert; 09-18-21 at 03:01 PM. Reason: Units of measure
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Old 09-18-21, 03:36 PM
  #83  
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Climbing is more about the rider than the bike.
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Old 09-18-21, 03:44 PM
  #84  
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Originally Posted by 63rickert
2 seconds is quite long enough that an assistant with a stopwatch will not be required. Go test your hypothesis. Find a 15% grade, get up to a speed of 7mph, coast, see how long you keep moving. It won’t be anything like 2 seconds. Anyone who wants to try can test this. Anyone who has ever climbed a 15% at any speed already knows how this experiment ends. But please do try this at home.

In the original story problem 15% was thrown in as a confounder. You bit, as you simply don’t understand the problem. Force of gravity is a constant, it does not depend on the gradient. Gravity is not and never will be, on planet earth, 4.7 feet per second. It is invariably 9.81 meters or 32 feet
It's pretty clear you don't understand your own problem. If the bike's acceleration did not depend on the gradient, you would coast to a stop at the same rate on a 45% slope as you would on a 1% slope.
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Old 09-19-21, 12:14 AM
  #85  
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Originally Posted by 63rickert
[Gravity] is invariably 9.81 meters or 32 feet
Gravitational constant varies a bit more than 0.5% over the surface of the Earth. It's lower at the Equator and higher at the poles.
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Old 09-19-21, 05:39 AM
  #86  
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Originally Posted by 63rickert
2 seconds is quite long enough that an assistant with a stopwatch will not be required. Go test your hypothesis. Find a 15% grade, get up to a speed of 7mph, coast, see how long you keep moving. It won’t be anything like 2 seconds. Anyone who wants to try can test this. Anyone who has ever climbed a 15% at any speed already knows how this experiment ends. But please do try this at home.

In the original story problem 15% was thrown in as a confounder. You bit, as you simply don’t understand the problem. Force of gravity is a constant, it does not depend on the gradient. Gravity is not and never will be, on planet earth, 4.7 feet per second. It is invariably 9.81 meters or 32 feet
Okay so clearly you don’t understand how gravity works on a slope. Here’s a clue. You have to multiply by the SIN of the slope angle to calculate the component of gravity acting parallel to the slope.

You are just making yourself look like a fool here as you don’t understand basic physics.
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Old 09-19-21, 05:46 AM
  #87  
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Originally Posted by tomato coupe
It's pretty clear you don't understand your own problem. If the bike's acceleration did not depend on the gradient, you would coast to a stop at the same rate on a 45% slope as you would on a 1% slope.
But even better than that, you would be able to climb 45% slopes as easily as 1% slopes. You know because gravity is a constant for all slopes right? 3rd grade physics innit.
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Old 09-19-21, 06:30 AM
  #88  
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Originally Posted by wolfchild
Climbing is more about the rider than the bike.
What an amazing insight.
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Old 09-20-21, 01:45 AM
  #89  
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better bike fit
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Old 09-20-21, 04:59 PM
  #90  
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Originally Posted by PeteHski
Okay so clearly you don’t understand how gravity works on a slope. Here’s a clue. You have to multiply by the SIN of the slope angle to calculate the component of gravity acting parallel to the slope.

You are just making yourself look like a fool here as you don’t understand basic physics.
."...the component of gravity acting parallel to the slope.”

Gravity operates in one direction. Down.

Divisible components of gravity?

What universe are you in?
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Old 09-20-21, 06:24 PM
  #91  
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Originally Posted by wolfchild
Climbing is more about the rider than the bike.
OK, to a point, but try riding a 40-pound beach cruiser up a steep grade and get back to us.
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Old 09-21-21, 12:54 AM
  #92  
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Originally Posted by 63rickert
."...the component of gravity acting parallel to the slope.”

Gravity operates in one direction. Down.

Divisible components of gravity?

What universe are you in?
Are you for real? I'm glad I'm not your physics teacher!

Here's your homework:-

https://www.studyphysics.ca/newnotes/...s/lesson25.htm
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