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I am currently riding a Foldie with 20in wheels and also a 700C road bike, and I see that (Subjectively) I can ride ~5kmph faster for the same effort on the road bike. I am curious to know how much of this is due to wheel diameter if at all.

I understand the foldie weighs a lot more, but that should not be a significant factor on the flat. The foldie should also be less aero (Since i am upright, also the fat tyres) but even that should not be significant at the speeds I am riding (15-25). There might be some effect based on component quality as well.

I have heard that smaller wheels hit bumps harder enough to cause more rolling resistance. I also wonder if the foldie wheel having to spin faster than the road bike for the same speed has anything to do with it.

TLDR: I am curious to know what makes my road bike consistently faster than my foldie, and how different factors weigh in, especially the wheel diameter.

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    Combination all all of the above. Above about 20km/h aerodynamics is the biggest power hog - tire rolling resistance would be the next, weight (on the flat), wheel size and wheel quality are each small but all add up. – mattnz Jul 6 '18 at 10:45
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    With almost identical wheels on a hybrid and a tourer, and similar bike weight, the tourer is faster at a comfortable cruising effort. Aerodynamics become significant at lower speeds than you think – Chris H Jul 6 '18 at 11:34
  • So the all else being the same does diameter have a significant effect? I saw one more site saying the rolling resistance effect of smaller wheels doesnt matter as much until much smaller.. – Karthik T Jul 6 '18 at 11:59
  • The first thing to understand is that a smaller wheel must turn faster to achieve the same speed. Either you have to crank faster or run in a higher gear. – Daniel R Hicks Jul 6 '18 at 12:02
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    FWIW this is a subject that the book Bicycling Science by Wilson covers in-depth. Summary is that yes, if all else is equal, which is a big if with some non-obvious elements, yes larger wheels are faster/more efficient. – Nathan Knutson Jul 6 '18 at 17:54
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There are at least four factors involved:

1. Tire pressure

The thinner the tire, the higher pressure it requires, and the higher pressures it can handle. More pressure means less deformation, and less deformation means less energy loss. Road bikes typically have very thin tires that allow pressures around 10bar, if I'm not much mistaken (you can look that up on the side of any tire), touring style tires are around 5bar, mountain bike tires even lower.

2. Tire geometry

The diameter of your wheel also plays a role: With a smaller wheel diameter, you get a shorter contact area with the road. That's simple geometry. Because the total area of the contact only depends on tire pressure, a shorter contact area means that it's wider. And a wider contact area means deeper compression of the tire, which again means more work that's expended for massaging the rubber.

3. Posture (aerodynamic)

At 25km/h, the strongest force holding you back is the air drag. And the more upright you sit on your bike, the more air you are accelerating as you pass by. Road bikes typically have a low bend sitting position because they are designed to be ridden at much higher speeds where air resistance is an even greater concern.

4. Posture (ergonomic)

To put your full weight onto your pedals, your center of mass has to be over the front part of the pedaling circle. With an upright sitting position that's generally not the case. The result is that some of your pedaling forces are translated into forces that try to rotate your body and to shift it backwards, which you have to compensate with your arms and butt.

The forward-bent position of a road bike does a much better job at putting your center of mass where you need it, and thus reduces the parasitic forces, allowing you to just use your strength to turn the cranks. This translates into more power output with the same subjective effort.

  • I am going to accept this answer due to its detailed nature, but gonna add a TLDR to say that all things being equal, larger wheels are a bit more efficient than smaller wheels, but not significant enough to be noticeable to laymen? Would that be a reasonable summary? Since I have a lot of ppl asking me does bigger wheels mean faster, and they expect a lot more significant difference than this, and looks like the rest of my bike might have as much or more of a part to play here. – Karthik T Jul 9 '18 at 3:52
  • In my case I think both tyres would have been around 80-100 psi though. – Karthik T Jul 9 '18 at 8:07
  • @KarthikT Forget my last comment, it was bullshit. I just redid the calculations, and came to the conclusion that the connection between tire diameter and pressure is approximately linear. So, a 20'' tire at 7bar will feel like a 28'' tire at 5bar. Or 100psi * 20'' / 28'' = 71psi. I'd say that's still noticeable under controlled conditions, but not in day-to-day riding. – cmaster Jul 9 '18 at 8:13
  • Thanks for updating @cmaster, and for what its worth by "obvious to layman" i mean "This wheel is 1.5 times as big, it can go 1.5 times as fast on the bike". that kind of noticable – Karthik T Jul 9 '18 at 8:50
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What I found with 20" wheel bikes compared with 26" wheel bikes is that the gearing is often not scaled to the size of the wheels. With that I mean that the gears are the same on a 20" wheeled bike as on a 26" wheeled bike, leaving you to spin your legs round like crazy when going down hill or with a following wind and not increasing your speed over that of the help you get.

That is much more important than the size and surface of the tires.

And it is something to check a bike with small wheels out on before you buy.

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    I guess i didnt mention in the question but gearing was something I wasnt considering because I wasnt maxed out on either bike at my speed so it wasnt an issue. I believe the chain ring for my foldie is much larger than the little one on my road bike which roughly balances it out. – Karthik T Jul 9 '18 at 3:48

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