Mountain bike vs Road bike speed

[Surfdog93]

I don't currently own a road bike and was curious how much faster a road bike might be on a flat double track hard-pack surface.
On my 29er(s), I can average 19+ mph over the 11 mile trail

[Poncharelli]

The major difference could be in aerodynamics, depending on body position difference. There would some difference in tire drag from knobby to smooth tire as well.

My road bike is about 15 pounds, and the acceleration difference is huge. But at constant speed on flat road, weight doesn't matter much, it's all aerodynamics and tire drag. EDIT: I almost forgot, more weight will add more tire drag (Fd=uN), but probably not super significant.

IME, the difference is about 1-2 mph on similar road. My riding position is a bit upright on MTB and wider handlebars don't help (more projected frontal area).

Cyclocross bike would be ideal on double track hard pack though.

[pucked up]

Wouldn't the gears on a road bike have a big difference? MTB gears are smaller than road bikes. You'll have to pedal faster on a MT bike to keep up with a road bike.

[Poncharelli]

Wouldn't the gears on a road bike have a big difference? MTB gears are smaller than road bikes. You'll have to pedal faster on a MT bike to keep up with a road bike.

I believe at 19mph, you wouldn't be spun out on a 29er with 2X10. Definitely not with a road or cyclocross bike. So at that speed neighborhood, gears shouldn't be a factor.

[SSdirt29]

tires and wheels, rotating weight is a big factor. Also tire pressure.

[Surfdog93]

Ponch,

I can certainly see the aeorodynamics coming into play and for that reason I do focus on staying low(er), and as you said bike weight is probably not a big issue (once it's rolling, it's rolling), but what about the 2.2 XC tires with 25 psi compared to race or cyclocross tires with much higher pressures ?

I am running 1 x 10 on my FS and HT and definitely do not spin out. At 19mph, I usually have 1 or 2 gears to go.

[litany]

Ponch,

I can certainly see the aeorodynamics coming into play and for that reason I do focus on staying low(er), and as you said bike weight is probably not a big issue (once it's rolling, it's rolling), but what about the 2.2 XC tires with 25 psi compared to race or cyclocross tires with much higher pressures ?

I am running 1 x 10 on my FS and HT and definitely do not spin out. At 19mph, I usually have 1 or 2 gears to go.

Depends on which "XC" tires you are using. If you think a nevegal is XC then a lot, a race king maybe very little. It depends greatly on the course as the bumpier (talking about bumps small enough that just the tire alone can absorb it or absorb a lot of it) it is the more it will favor the wider lower pressure tire. Where I ride its a lot of hardpack but there are little bitty bumps everywhere in which case a cyclocross tire will probably have more resistance than my 2.2 tires at 20 psi. The key is basically you don't want the bump to actually move the bike, it's more efficient if just the tire moves.

[Le Duke]

A 42x11 at 90rpm, on 29x2.0" tires, will have you moving at 29.6mph.

The problem is your legs, not the gearing.

[Poncharelli]

Ponch,

Ibut what about the 2.2 XC tires with 25 psi compared to race or cyclocross tires with much higher pressures ?

here's some good information on rolling resitance of tire types:
Rolling Resistance of Bicycle Tires - Bike Tires
Rolling Resistance | Schwalbe North America

Probably the only way to calculate speed difference for your specific tires, is to have the data for your tires, and know how to do the calculations. Not an easy thing to do.

Tire Rolling Resistance has a calculator, but only for specific tire types.

BTW, my favorite MTB tire is probably one of the slowest rolling out there: Continental Mountain Kings. But they are worn down to semi-slicks now, so they roll faster every day.

[wrcRS]

I usually average 3-5mph faster on my road bike. Not even a fair comparison if you ask me.