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I know nothing about engineering.

I feel like I can expend the same or even less effort riding a mountain amd a road bike (if going slowly) but feeling is not reliable, so I hope to get some insight on this matter.

Let's assume the mountain bike has a hard fork, hard tail, really thin tires. At same price as the road bike. The two bikes are going for the same distance on a smooth road, with no wind. I don't know if climbing and decending make much difference.

  1. What I guess is, if I am going slow, so the aerodynamics of a road bike will not make much difference, won't I using almost same effort to cover the same distance?

  2. Gear ratios affect fuel consumption, so can I manage my gear ratio and speed to save total work I expend? Will a mountain bike will require less work to cover the same distance?

  3. When riding in the city where you have to frequently stop and start, does a road bike need more effort than mountain bike since it has higher ratio gearing?

  4. Is there some way to test this without a lab environment? I can't find a way to measure how much work I'm doing, and total work I expended.

Please feel free to educate me, I just want to know more.

Where I got this weird idea was from my wife:

She got a hard tail hard fork DIY XC bike, with carbon frame and fork. it was decently light, at about 10KG, slick tire with high pressure and very narrow. It cost bit more than US$1600.

But, she only rides it if we are going out town. She commutes with a US$45 lady bike, she said it was not much difference riding this bike, if under 4km.

I tried her lady bike, it was pretty easy riding short distance in the city, but very hard at longer distance on account of riding posture, and very small bike -- riding slowly of course.

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    "hard tail hard fork" can be said a "rigid" bike. – Criggie Mar 23 '18 at 20:24
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    Assuming everything else is the same, the lighter bike would be less effort to ride. However nothing is the same. On a "lady bike" you're quite upright which recruits different muscles. For a shorter ride the differences would be unnoticeable, but over a longer ride you want to be using all your leg muscles. – Criggie Mar 23 '18 at 20:28
  • The test equipment you would want is a "power meter" on each bike - they are not cheap; would cost 10x what the ladybike cost. – Criggie Mar 23 '18 at 20:30
  • This question is very broad and doesn't have a definite answer, so its not a good match for this Q&A style. Try asking more specific questions, if we don't already have them on the site. – Criggie Mar 23 '18 at 20:31
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    A typical "road bike" is more efficient than a typical mountain bike on the "open road" because it's lighter, has thinner tires, has a range of gear ratios more suited to road conditions, and permits a better posture for distance riding. The wind resistance of the bike itself (vs that due to rider's posture) is only significant if you're in the TdF. You can't easily change bike weight and posture, but tires and gear ratios depend on how the bike is outfitted. – Daniel R Hicks Mar 23 '18 at 21:05
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Whelp, how to answer this? Depends what you mean by 'less effort' I guess.

I'll give the short answer up front. Road bikes are more efficient (therefore less effort to ride) than mountain bikes on road surfaces. We know this because road bikes have been optimized for speed and efficiency for a couple of hundred years. Geometry mess, riding position, wheel size, tire width and gear ratios all contribute to this.

To address points 2 and 3 about gearing. Everybody has pedaling cadence and gear ratio that is most efficient for them. A casual rider would use a lower gear ratio, a pro racer will use a much higher one. The availability of lower ratios does not make a bike more efficient (as long as they are low enough for the rider), Road bikes have enough gears to allow selection of an efficient ratio and their lowest ratios are sufficient to allow easy acceleration from a stop. Mountain bikes have lower ratios than road bikes to allow riding up steep inclines (and to pull their greater mass around).

Is there a way to test this? Yes. The advent of (relatively) affordable and accurate power meters make analysis of rider performance available to even casual riders.

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Simply put and ignoring a lot of variables, the answer to the question is that it doesn't matter which bike you ride.

First of all, remember the law of conservation of energy: that matter and energy can neither be created nor destroyed. As applied to bikes, that means that you cannot put in "less" or "more" effort to do something like climb a hill. All you can do is adjust the leverage on the gears. Remember Archimede's hypothesis that with a long enough lever you could lift the Earth? By shifting into a lower gear, you are effectively extending out a lever arm that makes lifting the weight (i.e. climbing a hill) easier. But, it also means you have to push that lever much further to lift the weight (i.e. turn the pedals many more times to go the same distance). With a higher gear, you expend more energy upfront, but you get a faster return.

Now, in terms of raw physical forces, the amount of energy it takes to move a given mass (you + the bike) a given distance doesn't change. When human bodies are taken into account, the picture is significantly complicated. That is the subject of research on athletic performance, about which there is plenty of literature. For cycling, usually it boils down to minimizing lactic acid buildup, which occurs during anaerobic (sprinting) exercise. That's grossly oversimplified, but the point is that it really depends on your body's fitness which mode of travel will be more efficient for you. However, as far as physics is concerned, it doesn't matter one bit.

Some other notes:

  • Aerodynamics is everything.

  • It takes about the same amount of effort to move a 15lb bike as a 30lb bike at a given speed. The difference is in acceleration.

  • Most people have a fair weather bike and a beater/commuter bike. The latter usually lives a harder life, while the former usually has more expensive components. Therefore it usually just makes fiscally more sense to reserve the nice bike for longer rides in good weather.
  • A more aggressive riding position on a road bike distributes your body's weight more evenly between saddle, handlebars, and pedals. A commuter bike with an upright posture focuses all the weight and disturbance of rough roads firmly on your lower back. An upright sitting position is hell on your body over time.
  • Power meters are expensive toys that hardly anybody actually needs. For those saying there are affordable ones out there, maybe there are, but not on a bike mechanic's wages!
  • 'As applied to bikes, that means that you cannot put in "less" or "more" effort to do something like climb a hill.' Could you clarify what you mean by that? As written, it's false. Going faster up a hill requires doing more work against air resistance; riding a heavier bike up a hill requires lifting more weight against gravity. – David Richerby Apr 23 '18 at 11:04
  • I'm talking about gears, no other variables. Changing gear ratios ONLY changes the leverage on the wheel. For a given speed, and a given distance, you will do the SAME amount of work regardless of what gear you are in. That's work as in, joules. Now if you're comparing two different bikes with different aerodynamic properties, different tires with different rolling resistances, and different bodies with different lactic acid thresholds etc., then you see different results. – Nathaniel Hoyt Apr 24 '18 at 15:30
  • OK. I tried to edit to make a clarification along those lines but found that I couldn't. The question seems to be more about comparing different bikes but your answer seems to be more about comparing different gears on the same bike. But the question does talk about different gear ratios so I'm not really sure what's going on. I agree that what you've written is correct in the circumstances you describe in your comment. – David Richerby Apr 24 '18 at 15:58

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