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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 09-14-21, 06:49 AM
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63rickert
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The bike does not climb. The rider climbs. The rider climbs with a bike. The bike is not an independent actor with some ability to climb. The bike is an object.

Gravity is a constant. Pedaling forces are not even remotely constant. The instant pedaling force is reduced forget about momentum and forward inertia. Gravity takes over instantly. The bike must be accelerated again after every slightest lapse in pedaling perfection. Climbing is an exercise in continuous acceleration. Which is hard.

The most accurate and helpful part of this discussion is the observation that red bikes are faster.
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Old 09-14-21, 07:26 AM
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A cyclist is not an electric motor with a constant supply of current. A cyclist is a reciprocating engine with no significant flywheel. Now redo your force and mass calculations to include that.
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Old 09-14-21, 06:17 PM
  #53  
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Originally Posted by 63rickert View Post
A cyclist is not an electric motor with a constant supply of current. A cyclist is a reciprocating engine with no significant flywheel. Now redo your force and mass calculations to include that.
Just because a cyclist's power output is a bit lumpy doesn't change the physics one little bit.
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Old 09-14-21, 06:28 PM
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Originally Posted by 63rickert View Post
The most accurate and helpful part of this discussion is the observation that red bikes are faster.
Not if the red parts are rotating. The Tour de France has never been won on tires with red sidewalls.
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Old 09-14-21, 06:37 PM
  #55  
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Originally Posted by 63rickert View Post
The bike does not climb. The rider climbs. The rider climbs with a bike. The bike is not an independent actor with some ability to climb. The bike is an object.

Gravity is a constant. Pedaling forces are not even remotely constant. The instant pedaling force is reduced forget about momentum and forward inertia. Gravity takes over instantly. The bike must be accelerated again after every slightest lapse in pedaling perfection. Climbing is an exercise in continuous acceleration. Which is hard.

The most accurate and helpful part of this discussion is the observation that red bikes are faster.
Most riders can hold a reasonably steady power output. They might well have the odd surge or dip in speed, but I wouldn't say climbing is an exercise in "continuous acceleration". Not unless you want to make it harder than it needs to be.
Any half reasonable dynamic model (e.g. Best Bike Splits) can tell you how long it will take to climb up any known hill/mountain given your average power and other basic parameters I mentioned earlier. It's not that hard.

If one bike climbs better than another it's either because it weighs less, has less rolling resistance, less aero drag (not so critical, but still counts for something) or somehow allows the rider to output more power e.g. better position.
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Old 09-14-21, 07:21 PM
  #56  
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Originally Posted by PeteHski View Post
Most riders can hold a reasonably steady power output.
Power output isn’t anywhere near reasonably steady for anyone throughout the pedal stroke. It’s unsteadiness is so predictable that it’s how power meters count RPM.
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Old 09-14-21, 08:50 PM
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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.
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Old 09-14-21, 09:16 PM
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Smoothed over more than two pedal strokes, the gravity work term is giMs, g = gravitational constant, i = incline in percent grade, M = combined mass of rider and bike, s = rider speed.

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Old 09-14-21, 11:34 PM
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Originally Posted by chaadster View Post
Power output isn’t anywhere near reasonably steady for anyone throughout the pedal stroke. It’s unsteadiness is so predictable that it’s how power meters count RPM.
Interesting. A bike rider in contact with reality.

A concept. Top dead center. Bottom dead center. Now how does a reciprocating engine with no flywheel and all manner of linkages attempting to defeat the purpose cope with TDC and BDC?

No, wave your hand and climbing is as simple and straightforward as riding on the flats. Which is hardly straightforward either. Momentum! Inertia! And pay no attention to conflicting forces. GPA outrates gravity. Gravity is a great respecter of persons.
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Old 09-15-21, 04:25 AM
  #60  
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Originally Posted by chaadster View Post
Power output isn’t anywhere near reasonably steady for anyone throughout the pedal stroke. It’s unsteadiness is so predictable that it’s how power meters count RPM.
That's not why power meters need to "count RPM"
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Old 09-15-21, 04:50 AM
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Originally Posted by jayp410 View Post
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.
Thanks for this and I agree with all of the above (it appears we must have studied the same laws of physics!). My degree is in Mechanical Engineering, so basically applied maths and physics. I also worked for decades in motorsport, where the question of rotational inertia vs static mass crops up quite frequently.

Another simple way of thinking about bike wheels is as two fairly light flywheels. Like all flywheels they tend to smooth out any micro-variations in power input. If you ignored the detrimental effects of overall increased weight, heavier wheels would actually be a benefit in this sense. But some people make way too much of a big deal about minimising rotational inertia and its real world effects. Especially when it comes to steady state climbing performance, where only total weight really matters.
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Old 09-15-21, 04:54 AM
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Originally Posted by 63rickert View Post
Interesting. A bike rider in contact with reality.

A concept. Top dead center. Bottom dead center. Now how does a reciprocating engine with no flywheel and all manner of linkages attempting to defeat the purpose cope with TDC and BDC?

No, wave your hand and climbing is as simple and straightforward as riding on the flats. Which is hardly straightforward either. Momentum! Inertia! And pay no attention to conflicting forces. GPA outrates gravity. Gravity is a great respecter of persons.
No idea what your point actually is here? How does this relate to one bike climbing better than another with the same rider?
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Old 09-15-21, 06:32 AM
  #63  
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Also remember, as I’ve mentioned in several other, threads, that our bodies must also power the internal work of pedaling and moving our feet and legs.

Experimenters have shown this internal work to be essentially independent of the external work being done and to be a highly increasing function of pedal rate, both exactly as we would expect. Typical values range from 0.1 watts/kg at 50 rpm to just over 1 watt/kg at 110 rpm.

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Old 09-15-21, 06:36 AM
  #64  
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Originally Posted by PeteHski View Post
That's not why power meters need to "count RPM"
Correct, but I was making a simple point that output is not steady, and underscoring the fact that climbing performance (or let’s call it RPC(limbing)E) is, for most riders probably affect by feedback through the bike, i.e. as has been discussed, how the bike feels.

It is the same point jayp410 postulated when they wrote, “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.”
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Old 09-15-21, 06:59 AM
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I’d expect that instantaneous climbing speed would vary less when a rider is geared down and pedaling a high cadence with more angular momentum of legs in rapid motion and less time between power strokes. I’d expect more variation in instantaneous speed when pedaling slowly up a hill, like when I’m working hard climbing a hill on one of my SS bikes and can’t gear down.

I rode my “road” bike for the first time since I took a hard fall a month ago. Been riding the MTB for the last few weeks.

It felt so much quicker and faster. Part of that is surely to do with feeling more vibration from 32mm tires, Also a total weight difference of 4 pounds.

But mostly I think it was the rotational effect of lighter tires and rims combined with less energy being lost in the tires.

The combination being a bike that climbs faster and generally accelerates much faster whenever I’m pedaling harder.

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Old 09-15-21, 07:15 AM
  #66  
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To have a tidy theory pretend that riders have a constant power output.

Then pretend that gravity is not a constant. Gravity takes little vacations during low power/no power portions of the pedal stroke. Because ‘momentum’ cancels out universal constants. Because the theory is tidier if that pesky gravity is swept out of the way. Just calculate a simple lift and we are done.
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Old 09-15-21, 12:15 PM
  #67  
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Originally Posted by 63rickert View Post
To have a tidy theory pretend that riders have a constant power output.

Then pretend that gravity is not a constant. Gravity takes little vacations during low power/no power portions of the pedal stroke. Because ‘momentum’ cancels out universal constants. Because the theory is tidier if that pesky gravity is swept out of the way. Just calculate a simple lift and we are done.
Ah I see, you are going to counter argue Newtonian physics with ermmm..... "Bro-Science". Not going there thanks.

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Old 09-16-21, 02:59 AM
  #68  
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Originally Posted by PeteHski View Post
Ah I see, you are going to counter argue Newtonian physics with ermmm..... "Bro-Science". Not going there thanks.
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 far this thread is wishful thinking and magical thinking and focus on third order minutiae. Yes, rim weight matters. Trivially. No, it is not important.

What matters is smooth pedaling. No one wants that answer because then they would have to learn how to pedal. They would have to do something different than what they do now. And expend energy thinking that through. Those who can’t figure out the effect of 32 feet per second per second are not going to figure out pedaling dynamics.

It will also matter if the bike has any possibility of storing and releasing energy to carry the rider through dead spots in the pedal stroke. Jan Heine has written rather a lot on that topic, so far he hasn’t even convinced himself the effect is large. On steep climbs even Heine would not try to find an effect past it might make the ride feel nice.

Otherwise the only question is what does the bike weigh? It might also matter how well the bike transmits power. Unless the bike is just badly maintained and out of adjustment they all transmit power well enough. If the frame is not busy getting hot, converting power inputs to heat, they are all about the same. Power losses in the bike are trivial and insignificant next to power generation by the rider.

At this point I will add that most bikes are badly maintained and out of adjustment. The new bike that feels so much better is not a better bike, it is just working better while new. For those who do not much take care of the bike getting new ones frequently means going faster. Now buy ten bikes in succession, each subjectively faster than the last, and there is no big change. It is about the rider, not the bike.

The bike does not climb. The bike is inanimate. The rider climbs.
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Old 09-16-21, 03:14 AM
  #69  
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Originally Posted by chaadster View Post
Correct, but I was making a simple point that output is not steady, and underscoring the fact that climbing performance (or let’s call it RPC(limbing)E) is, for most riders probably affect by feedback through the bike, i.e. as has been discussed, how the bike feels.

It is the same point jayp410 postulated when they wrote, “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.”
Well you have picked up on the only vague and speculative part of that entire excellent post! Mathematical models predicting climbing performance for a given rider power output, bike and hill profile are accurate enough without resorting to modelling micro-power/accel variations during pedal strokes or rotational moments of inertia of wheels. As far as I am aware none of them bother to do that simply because it doesn't affect the end result. It all averages out over any reasonable time frame e.g. a 3 second rolling average is enough to show a reasonably smooth power output when riding along attempting to hold a specific power level. For example if I target 250W for a long steady climb I can hold that power level quite easily within +/-10W with a 3-second rolling average filter. It only starts to look lumpy with a 1 sec averaging filter. Those micro-accelerations/decelerations during the pedal stroke have an immeasurable affect on my speed profile when climbing. It's not like a full bore sprint, where you really are accelerating your wheel set through a meaningful speed range that might actually make some difference to the total work required. Even then it's quite a subtle effect in the overall equation.
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Old 09-16-21, 05:14 AM
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Originally Posted by PeteHski View Post
Well you have picked up on the only vague and speculative part of that entire excellent post! Mathematical models predicting climbing performance for a given rider power output, bike and hill profile are accurate enough without resorting to modelling micro-power/accel variations during pedal strokes or rotational moments of inertia of wheels. As far as I am aware none of them bother to do that simply because it doesn't affect the end result. It all averages out over any reasonable time frame e.g. a 3 second rolling average is enough to show a reasonably smooth power output when riding along attempting to hold a specific power level. For example if I target 250W for a long steady climb I can hold that power level quite easily within +/-10W with a 3-second rolling average filter. It only starts to look lumpy with a 1 sec averaging filter. Those micro-accelerations/decelerations during the pedal stroke have an immeasurable affect on my speed profile when climbing. It's not like a full bore sprint, where you really are accelerating your wheel set through a meaningful speed range that might actually make some difference to the total work required. Even then it's quite a subtle effect in the overall equation.
Yes, I agree with that (and have no problem with the first law of thermodynamics), but in the context of the OP, climbing speed doesn’t seem to have been the basis for the assessment that one bike climbs “better,” so I’m more interested to keep the conversation focused on why bikes feel different, rather than the role of wheel weight location in climbing speed models, which are a settled matter insofar as I am concerned.
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Old 09-16-21, 05:54 AM
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Originally Posted by BadgerOne View Post
Why does one bike climb better than another?

Seems to me it might be a good idea to ask the OP what he's really asking here. Maybe I missed it, but I don't think anyone has asked the OP on clarification regarding what he means by "climb better".

The OP's original question is vague, and can be interpreted at least 5 different ways:

1. Why does one bike make me pedal harder than another on the same climb?
2. Why does one bike require more total effort (work) than another to make the same climb, regardless of time?
3. Why does one bike require more time at the same power input than another to make the same climb (alternately, why does one bike require more time to make the same climb at the same power level)?
4. Why can I make the same climb faster on one bike than another if I choose to vary my effort as I see fit?
5. Why does making the same climb on one bike leave me feeling more "beat up" or fatigued than making that same climb on another?

I'm sure there are other interpretations that don't come to mind that are equally valid. These are simply the ones that came to mind with a bit of thought.

From the info in the OPs original post, I'd guess #1 is what the OP is asking, or maybe #5. But that's only my guess; I don't really know. It could be one of the others, or the OP may not have thought the matter through that far. Dunno.

The questions don't necessarily all have the same answer. Extreme example: a low-slung 50lb 3-wheeled trike with 20" wheels and an exceptionally low "granny" gear would almost certainly excel in cases 1 and 5 above, if for no other reason than it could be pedaled at a much slower speed than most 2-wheeled bikes due to stability. But it would very likely be demonstrably worse than most other alternatives in cases 2 through 4.

Even for "similar" bikes, as others have noted specific details matter. Examples: different wheel sizes (as is the case for the OP's two bikes) result in different meters development for the same gearing. Tires can vary hugely in rolling resistance, both inherently based on brands/models/sizes as well as with tire pressures, tread patterns, and the surface on which used. Do the OP's two bikes have similar frame geometry and the same crank length? Do both have the same type and size handlebars? Maybe I missed them, but I don't remember seeing those kinds of specific details in the comments above. Those details are needed for reasonable analysis.

In theory, the first four of the questions I list can be analyzed and a decent answer given, provided you know enough specifics about the rider, the bike, and its components. The last may not be subject to valid analysis, since feeling more "beat up" or fatigued is itself a subjective judgement.

Just my $0.02 worth.

Last edited by Hondo6; 09-16-21 at 06:00 AM. Reason: Wording changes.
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Old 09-16-21, 07:40 AM
  #72  
PeteHski
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This is just getting silly now. The only way I can think of to interpret "climbing better" is climbing faster for a given amount of effort. It doesn't matter whether or not you actually climb faster as you might decide to use less effort. But "climbing better" essentially means climbing easier or faster. They amount to the same thing.
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Old 09-16-21, 07:50 AM
  #73  
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Originally Posted by 63rickert View Post

The bike does not climb. The bike is inanimate. The rider climbs.
That wasn't the question though. The rider isn't changing here, only the bike.
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Old 09-16-21, 09:10 AM
  #74  
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Originally Posted by 63rickert View Post
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.
You just flunked your own quiz; it takes just over 2 seconds to coast to a stop, not less than a second.
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Old 09-16-21, 09:36 AM
  #75  
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Originally Posted by PeteHski View Post
This is just getting silly now. The only way I can think of to interpret "climbing better" is climbing faster for a given amount of effort. It doesn't matter whether or not you actually climb faster as you might decide to use less effort. But "climbing better" essentially means climbing easier or faster. They amount to the same thing.
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.
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