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I have a very comfortable ~500€ basic mountain bike with 2.2" tyres and 27.5" wheels on which I can maintain an average of 24-25 km/h on a windless day on flat asphalt when I'm feeling strong. I cannot keep pedalling in top gear continuously. Instead, I keep switching between the hardest and next to hardest gears.

How much easier would it be to go fast on an affordable road bike with narrow tyres? Sure it has "faster" gearing, but that alone won't help me since I don't spin out on my current bike either. I am simply not strong enough. Would I see a real speed improvement from a road bike, and if yes, approximately how much? Does that improvement come mostly from a more aerodynamic (but perhaps uncomfortable) position, less rolling resistance, better position for pedalling, or something else? I assume all of these matter, but there is likely a dominant factor.

I have never had the chance to ride a road bike, and I am simply curious. No, this question does not stem from any practical need such as commuting. However, it would be nice to know if a more efficient bike would allow me to reach more places within a reasonable amount of time during this pandemic when I am hesitant to take public transport. I suspect it wouldn't (without better fitness).

EDIT: Another way to phrase this question is, does it even make sense to think about a more efficient bike until I regularly spin out on my current one?

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    Welcome to the site - this is a well-written question. Do take a moment to read the tour to catch up on the details of how the site works.
    – Criggie ♦
    May 21, 2021 at 0:15
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    People - do please remember comments are for clarifying and improving the question. Answers go in answers, below. Please post your answers as answers. If you feel your answer is too short to be an answer, then expand it.
    – Criggie ♦
    May 22, 2021 at 10:47

8 Answers 8

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  1. Aerodynamics are very important in cycling, and the faster you go, the bigger a factor aerodynamic drag becomes, because it increases with the square cube of your speed, but other sources of drag (mechanical resistance, rolling resistance) increase linearly. So putting your body in a more aerodynamic position is important, and not something you can really replicate on a mountain bike. At the speed you're going, aerodynamics are not trivial, but not overwhelmingly important.
  2. Rolling resistance is the second-biggest source of resistance. You could put high-quality slick tires on your bike and get faster right now.
  3. If you are commuting in a city, your travel time has more to do with the number of stoplights you hit than your top speed. If you race from one stoplight to the next and then spend more time at the stop, you won't reach your destination much faster. Your "light-to-light" time would need to improve enough that you would beat the stoplight.
  4. "does it even make sense to think about a more efficient bike until I regularly spin out on my current one?" That's not a good way to look at it. Bikes will generally have top gears that you'll never use except on a downhill or with a tailwind. If you regularly spin out in your top gear on level ground in still air, your top gear is too low for your riding conditions.
  5. You would probably be faster on a road bike, or you'd ride the same speed for less effort. How much faster is hard to say. 25 km/h is a pretty good speed for urban riding.
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    +1, but I think you underestimate the importance of aerodynamics at 25km/h on a MTB.
    – mattnz
    May 21, 2021 at 2:04
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    I do not know if it is comfort (making the cyclist capable of pushing harder, or sustaining harder push for longer) or aerodynamics (providing less area to air drag, therefore more power transmitted to motion) but ... aerobars helped me a lot while commuting on a regular urban bike, I was 4/5 km/h faster (on average, long flat distance without disturbances and dangers in the Netherlands). extreme examples: bit.ly/3fznngK
    – EarlGrey
    May 21, 2021 at 7:29
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    @EarlGrey: I think it’s both. When you lean forward there is a tendency to pedal harder to support your upper body. You’d really need a power meter to find out how much.
    – Michael
    May 21, 2021 at 7:53
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    I would add suspension as an additional factor that you loose energy to on a mountain bike.
    – Karlokick
    May 21, 2021 at 9:26
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    A slight error in your first paragraph: Aerodynamic drag is a force, and it doesn't vary with the cube of velocity--it varies with the square of velocity. Power is a force times a velocity, so the power needed to overcome aero drag varies with the cube of velocity. Likewise, the power needed to overcome rolling and mechanical drag varies linearly with speed so those components of drag are constant (or nearly so) with velocity.
    – R. Chung
    May 21, 2021 at 13:37
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Try a quick poke of your values into http://bikecalculator.com/

Assuming you're doing 165W, weigh 80 kg, riding a 15 kg bike with knobbly MTB tyres, and using a "bar-tops" position returns 24.4 km/h

Changing the tyre choice dropdown from "MTB" to "clinchers"[*] gives you 27.87 km/h, a 10% improvement for no other change.

Moving to the hoods gets you 29.05 km/h and drops is 31.38 km/h

Noteworthy, dropping 5 kg off the bike or the rider only gets you an extra 0.16 km/h on the flat.

Do be aware that this calculator is idealised and the values are not exact. But the percentage changes should be a useful guideline.

It would be uncommon to spin out on your bike while doing normal riding; a gear like 48:11 is rarely used. They exist for the occasional times you do have a stomping-good tailwind or are on a downhill/undulating riding.

Also the smallest/hardest cog helps with fitting the cassette inside the frame - if your smallest was a 14T then it might hit the frame, so would need to be inboard anyway loosing you a gear, may as well put 11 or 12T cog there.

[*] Sorry that is confusing - the bikecalculator site has a list of tyre "styles" being MTB/Clinchers/Tubulars. I've presumed that to mean "knobbly wide MTB tyres" then "normal road tyres" and "fancy racing tyres" in progression. So yes the MTB option is likely to be a clincher bead, which is confusing.

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    Of all my bikes I probably use the tallest gear the most on my MTB, when riding roads to/from the trails. That's because it's the lowest at (I think) 46/11 vs 50/11 and 48/11 on my other bikes all with similar size wheels and all with triples. If spinning out was the issue, which it might be after switching to slicks, you could probably change the chainrings, though watch out for clearance issues. Spinning out is odd though - how fast you can spin depends on how much power you're putting in as well. There are nice graphs in Wilson: Bicycling Science
    – Chris H
    May 21, 2021 at 8:45
  • I don't follow regarding 'changing to clinchers'. MTB and road tyres of many sizes and shapes can be clinchers. Do you mean to suggest changing to a slick road tyre (28mm~32mm?) instead?
    – Lamar Latrell
    May 21, 2021 at 19:09
  • "Changing the tyres to clinchers" - since virtually everyone outside a pro-peleton is already riding clinchers, this statement needs some elaboration.
    – whatsisname
    May 21, 2021 at 21:57
  • @whatsisname Sorry that was confusing - yes the bikecalculator site has a list of tyre "styles" being MTB/Clinchers/Tubulars. I've presumed that to mean "knobbly wide MTB tyres" then "normal road tyres" and "fancy racing tyres" in progression. So yes the MTB option is likely to be a clincher bead, which is confusing.
    – Criggie ♦
    May 21, 2021 at 22:52
  • @LamarLatrell (comment reply above)
    – Criggie ♦
    May 21, 2021 at 22:53
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Not a scientific answer - but one I have a fair amount of personal experience with. I actually like riding my MTB on the road, because it makes steady training efforts easier (you can pedal more on downhills) and allows me to do training rides with less fit riders whilst still getting a benefit myself.
It's also often a more pleasant experience on the gravelly potholed goat tracks they call roads around here!

MTB is a 29er fitted with some part worn 2.2" x-kings. Road bike runs 23mm gp4000's

When riding at a steady endurance pace over rolling terrain, my rides on the MTB typically come out at 22-23km/h and road bike 27-29km/h. I believe if i picked up the effort to a higher level (lets say a 1 hour max effort) the gap would widen as I definitely really notice the MTB is much less efficient (both aero and draggy knobbies) once over 25km/h

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    +1 for how it feels, which is often important. I doubt you've ever tried it but I'm planning to have a go with some aero bars on the MTB for road drags
    – Chris H
    May 21, 2021 at 9:41
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    @ChrisH when i've done longer road/mixed surface rides on the MTB i've used 'puppy paws' a fair bit on long flat sections. It's noticeably faster (but quite uncomfortable without arm rests) and if i were going to try some sort of ultra distance event i'd definitely fit aero bars
    – Andy P
    May 21, 2021 at 12:11
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    I've never felt safe in that position. Perhaps my arms are too slippery. I've done a century or two on my MTB, when a gravel bike would have been OK, but I have plans to do a 200km taking in my 2 closest trail centres, which are in opposite directions. Only about 1/3 would need an MTB. Not ultra distance but far enough, and although I've got mini bar-ends and ergo grips I'd like the hand position and aerodynamics of the aero bars.
    – Chris H
    May 21, 2021 at 12:19
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    Good point, you don’t need a fast bike to train efficiently (especially if you have a power meter and know your actual intensity). However I think an advantage of a fast bike for training is that you can ride longer distances in the same time/effort, which makes getting out of the city easier/faster and overall gives you more change of scenery.
    – Michael
    May 21, 2021 at 19:15
  • @ChrisH have you tried grabbing by the fork crown, not using the handlebars at all? I like riding in that position when on safe, good-view asphalt straights.
    – leftaroundabout
    May 23, 2021 at 1:24
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I believe the greatest impact comes from two factors which have not been mentioned yet:

  1. Posture. The crouched position on a road bike puts your center of weight forward. This gives your weight a better lever on your pedals, allowing you to put much more force on your pedals without tearing at your bars. It also puts a lot of tension in your body, which makes it much more ready to output significant work. How much your speed benefits from the posture change depends on the strength of your cardiovascular system (because that's the only limiting factor for sustained speed on a road bike).

  2. No suspension. Every suspension needs to include a dampening mechanism to ensure that the tire stays in contact with the road. You may not see this shock absorber in a mountain bike fork because it's built right into one of the stanchions, but it is there. And this shock absorber turns your precious kinetic energy into useless heat. A mountain bike needs to have suspension, because it is designed to handle big roots, stones, potholes and drops that would kill any road bike wheel. A road bike, however, is ridden on asphalt only, and thus can avoid the resistance from the dampening in a suspension by not having a suspension in the first place.

These two factors alone account for most of a road bikes perceived agility, and they are certainly amplified by the reduced air resistance due to the crouched posture. The down side is, that the crouched posture requires much more tension in your body, and the thin hard tires without suspension make riding over roots and potholes harsh and dangerous.

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    Most MTBs even at that price point allow the forks to be locked off, at which point the suspension just adds weight. And if you think road bike wheels are fragile, watch this (don't try that at home, but for normal people, see some of the stuff I do on my tourer but friends do on lighter road bikes)
    – Chris H
    May 21, 2021 at 9:37
  • Damping is not needed to ensure that the tyre stays in contact with the road. This only becomes an issue at near or below the resonant frequency, i.e. for big hits or at slow tempo. (And even then it's possible, unlike with motored vehicles, for the rider to provide the damping.) I normally run my suspensions at high pressure and all damping at the lowest possible setting, and then suspension losses are very small (negligible compared to aero, as long as I stay in the saddle).
    – leftaroundabout
    May 21, 2021 at 10:51
  • @leftaroundabout Being over-sprung negates the need for compression damping, but what about rebound? Surely you cannot react fast enough to manually dampen rebound in any scenario other than an isolated drop or jump or something.
    – MaplePanda
    May 21, 2021 at 18:21
  • @MaplePanda the point is, it is never necessary to dampen the high-frequency components at all, because even undamped suspension acts as a 2nd order low pass filter. You always need to take care only of the contributions that are slow enough so you can do it manually. Of course, that's still effort, and for fast downhill riding it definitely takes away difficulty if the rebound damping helps you. IMO it also takes away some of the fun – I prefer going a bit slower and bunny-hopping over the bigger hurdles, rather than just smashing the suspension into them.
    – leftaroundabout
    May 21, 2021 at 18:44
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Another issue which is implied by the other answers but not explicitly mentioned is the overall fatigue after riding a long distance by either a MTB or a road bike.

By personal experience: When riding my road bike with my MTB buddy of similar fitness level on his MTB, after ~70km at same speed, the difference in posture, aerodynamics and other factors mentioned above, makes it impossible for him to keep up.

PS. My first answer on this site, if not a good contribution please let me know to remove and put as a comment.

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    That's a great point, not yet mentioned. Welcome to the site!
    – Criggie ♦
    May 21, 2021 at 23:02
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This is not a complete answer but anecdotal "evidence" on changing just two variables in a practical use case scenario.

There was a couple of months where my commute to work ended with a 2.8 km shallow slope descent. Since I was going to the workplace I tried not to sweat so most of this descent I did just coasting. At first I did it on 26x2.2 knobby tires at about 40 PSI. The coast-down part took 15 minutes. That corresponds to an approximate average speed of 11.2 Km/hr

Then I changed to 26x1.5 slick tires at about 60 PSI, then the same stretch of the commute took 10 minutes, which is about 16.8 Km/hr.

I think I only changed the rolling resistance and a little bit of the bike's aerodynamics:

  • The slick tires where a somewhat harder rubber and where at higher pressure, but not as high as a road tire would be.

  • The tires being narrower but also lacking knobs should present less aerodynamic drag themselves, as the knobbier ones had fairly tall and squared knobs.

Other variables where kept the same. Same bike, carry items and clothing.

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I will add another anocdotal evidence because it does not match the experience of some other answers. Namely, the difference is much smaller for me.

I beleive looking at averages over lomg rides is mesningless because one is simply going to push harder with a road bike on the tarmac while an MTB bike on the road is more likely to be a trip for fun.

I did several flattish Strava segments on both — a cheap but new Sora-level road bike and an old 26er hardtail with very cheap Racing Ralphs (with bar ends and overall older traditional geometry)

Flattish, but a bit up/down forest road: 30 km/h MTB and 32 km/h road bike.

Open country road between two villages, slightly up/down but more down, finish sprint uphill: 36 or 37 km/h MTB and 39 km/h road bike 41 km/h with good tailwind.

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    Additional point - it can feel "safer", or at least more-controlled, to do a fast descent with the wider bars of a MTB than the narrower bars of a road bike.
    – Criggie ♦
    May 22, 2021 at 10:49
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I’ll add a non-technical answer for regular, layman folks, such as myself.

I didn’t think I would notice all of the science between a mountain bike and road bike. For a long time I thought it was unnecessary to have two separate types of bikes to store and maintain. But once I got on a road bike, it was night and day for me.

Everything was just so much easier. Going up hill was especially surprising to me because the ride was drastically better. With everything combined, commuting with a road bike just made the commute a much more enjoyable experience than with a mountain bike.

Hope that helps 🤞🏻

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