wayold

I'm sure this question has been answered endlessly here, but bear with me. I'm looking for my next bike - nominally a gravel bike and I'm trying to decide on flat vs. drop bar. Now I already have a flat bar gravel bike (an old rigid MTB with 38mm slicks on 26" wheels) and it does OK on mild dirt roads, but it's kind of a dog on pavement compared to my road bike (10 year old Trek Domane on 28mm GP5000s). Just riding around on level pavement I'll typically average around 16 mph on the Domane, but maybe 12-13 on the flat bar bike for around the same level of effort. My question is why?

Is it the less efficient body position obtained with flat bars or some magic of more efficient geometry on the road bike? I'm not in a very efficient position on the road bike either with the handlebars raised up to the nearly height of the saddle, but maybe?
Is it aerodynamics? Maybe, but I ride pretty slow and again, I'm not all that clean on the road bike.
Tire differences? I'm sure the GP5000s are better, but that much better?
The road bike is much lighter (18 lbs vs 28 for the gravel) but I wouldn't think this matters too much on flat terrain. Both have new and well maintained drivetrains.

So why is my flat bar bike so much slower than my road bike?

blacknbluebikes 

or, I can turn it around to: how do I get my 1992 trek 820 hardtail CrMo mountain bike to "get the same speed as my CF 11sp roadbike for the same wattage / effort?" Well, first thing, I'd have to ditch the 2 1/4 semi-knobbies and get some thinner, slicker rubber on the road (someone will debate this) then I'd probably need to help the bike shed about 12 lbs (someone will debate this), then I'd probably put a longer stem on it to get my upper body a bit lower (no one will debate this). Generally I'm about 12-13mph on the old trek and 15-16 on the big boy bike, if I'm using the same legs.

PeteHski

It's both the aero and the tyres combined.

Iride01 

A lot of small things probably add up to make the difference. You mentioned some of them. Certainly weight has a bearing on how fast you can accelerate or climb. So it you do either of those a lot during your ride, then your energy reserves can get low. Geometry between the two bikes might have you in a less powerful position over the cranks on the flat bar bike.

We can suppose all sorts of things.

mstateglfr 

Riding Position.
Rolling resistance.
Perceived effort being different from actual effort.
Weight.
Mentality.


Any of those. All of those.

Troul 

b/c carbon = fast

freeranger

Greatly oversimplified--your road bike is designed to be faster. Geometry, tires, riding position, gearing, etc.etc.etc. on a road bike is designed to make it faster. Don't try taking it out on the trails and expecting it to perform like a mtn.bike.

Jughed

Ride by some reflective glass on both bikes, or have someone take pictures of you and notice the difference in body position. The road bike, even with the bars high, probably has a longer reach and narrower bars. Couple that with tires and small aero gains from the bike itself - you will be faster.

And unless you have a power meter - level of effort is hard to compare from bike to bike.

msu2001la

I bet if you ran the same tires on both bikes, the differences would be a lot narrower. GP5000's are some of the fastest tires on the market.

wheelreason

As far as the bikes go, the tires/wheels and position probably have the most effect, but if you are averaging 16 MPH on a road bike, the biggest cause is lack of power. Assuming you don't have any physical issues, are not very "mature", have a decent base and been riding more than a few months, you should be at or very close to 20 MPH. A rigid MTB with "road wheels" will not be very much slower than a decent road bike on a flat course.

msu2001la

There are far too many potential factors to make a blanket statement like this, but the most notable one is that the OP didn't say that 16MPH avg was the fastest average speed they could ride, just that this represented a "typical" speed when they were "just riding around".

mstateglfr 

Bro.

Shadco 

Paint the MTB red. You can thank me later.

.

RChung

If you really want to know exactly why, there are tests you can do to determine this -- we actually understand quite a bit about how differences in drag eat up the power you produce, and it's possible to measure those differences. There aren't many of them to measure but in order to get good measurements you have to be pretty careful about your experimental technique, so most people don't bother. It's kind of a pain in the butt to do, so only people who really want to know are willing to do it.

That's about the "demand" side of why a particular bike would be slower or faster -- how much of the power you put out gets used up by different things. There's also a "supply" side that affects how much power you can put out. Once again, it's possible to measure that, but it's also a pain in the butt.

Having an accurate and precise power meter makes it easier and faster to do these measurements but it's possible to do them without a power meter. Possible, but even more of a pain in the butt.

wayold

As to how mature I am you can take a handy hint from my user name. Yes I know I'm old and weak, but I didn't come here to whine about my infirmities. The question isn't why I'm slow, but why I'm slowER on the flat bar bike. The reason I ask is that I have bunch of MTB parts and could build up a reasonably light flat bar road/gravel bike fairly easily. I don't want to go that way, though, if I'm throwing away a bunch of performance compared to a conventional drop bar gravel bike (assume identical weight, tires and relatively high handlebars for both bikes to make it as much of an apples-to-apples comparison as possible).

mschwett 

definitely up there in terms of gross generalizations. 20mph on flat ground with no wind and no stop signs, lights, traffic, turns, etc? sure. an average for an average, relatively fit rider on a road bike? no way.

looking at a very wide range of real world people's strava rides, i'd say the OP's 16mph is just about right for the real world with a bit of climbing here and there.

mschwett 

how do you feel about the assumptions used by bikecalculator in their web interface version of the models? 4-4.5mph difference on flat ground, clinchers and drops vs knobbies and bar tops within a power range of 120-200w. seems to match OP's experience pretty well.

tempocyclist

Aero drag and rolling resistance.

ClydeClydeson

Narrower tires, to a point*, will roll faster at higher speeds because they are lighter and have a smaller profile headed into the wind. ALso, road tires are likely to have a more supple lightweight casing than MTB tires, although you can get light and fast MTB tires and heavy pigs for road tires.
The biggest difference is likely to be rider position and aerodynamics - road bars are designed to keep your elbows in and your general riding posture lower, while flat bars generally feel best with your elbows out a bit and almost always result in a more upright posture.

If you put clip-on aero bars on a mountain bike and installed 35mm/1.5" wide lightweight tires and pumped them up nice and hard, your MTB will likely roll almost as fast as your road bike - the limiting characteristic will probably be the low gearing on the mountain bike, where top gear is probably close to the middle of the gearing range on a road bike.

* the argument about tire width and rolling resistance is as follows: traditionally, narrower tires have been seen as faster, but more recent tests have shown that, all other things being equal, wider tires have less 'rolling resistance'. The confusion comes from the fact that, above a certain speed, rolling resistance (the energy lost to flex in the tire tread and casing) is very small compared to other sources of drag, specifically aerodynamic drag. At high speeds, like over 30km/h, aerodynamic drag absolutely dwarfs all other sources of drag, and even the difference in aerodynamic drag between a 25mm tire and a 32mm tire is significant.
The 'rolling resistance' can actually go up with narrower tires because they have to deflect more to support the weight. This can be countered by putting more pressure in the tire, but on a less-than-perfectly-smooth surface (ie, the real world) you can then encounter 'suspension losses' - energy lost to surface irregularities pushing the bike and rider up and down instead of forward.

So to add this all up, most racers have realized that medium/narrow tires (between 25 and 28mm) are actually fastest, and most racing bikes are now equipped with rims that more closely match the width of these slightly wider tires as the shape of a tire mounted to a narrower rim has much more drag than the same width tire mounted to a rim the same width.
For we mortals cruising at 22km/h, many people find that a bit wider tire - like 32 - 40mm - is ideal because the lower speed makes the aerodynamic penalty less, and the advantages of wider tires - smoother ride, more traction, better on loose surfaces - outweigh the small aerodynamic penalty.

unterhausen

Have you heard of Unterhausen's law of cognitive bias?

msu2001la

Even if the riding position could be made more aero, riding a 26" MTB in aero bars seems really awkward compared to a road bike.

In terms of tires, the road bike comparison here is 700c x 28mm GP5000. Is there any 26" x 35mm tire that would even come close to that in terms of rolling resistance? I seriously doubt it.
Even if tires were equal (requiring a downgrade on the road bike I think), the 700c size seems like it would inherently roll faster than 26".

RChung

I used to use calculators like that before I figured out how to measure the actual drag parameters. (I wrote a calculator like that, to help explain drag to some of my friends). They can give you an idea of the magnitudes of the demand side, but they don't tell you anything about the supply side. In addition, there's a lot of variation in how riders fit on their MTBs and road bikes, and a lot of variation between two different tires of the same size. So these calculators are useful to understanding, but they're not good for proof, or ferreting out the actual sources of drag. Sometimes they can generate more argument than they can resolve.

Eric F 

My mediocre understanding of bike tire physics leads me to believe this is accurate.

genejockey 

Pretty sure we have a subforum for that.

wolfchild

The reason why your road bike is faster is because road bikes are designed for speed and efficiency, while MTBs and gravel bikes are designed more for off the pavement capability. Wheel size also makes some difference. A 700cc wheels will roll faster and better than a 26 inch wheelset.. While 26 inch wheels may accelerate faster, 700cc wheels are better at maintaining speed which means you cover the same distance a little bit faster,