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I don't know of such a comparison, but I can believe how tired you are.

I was given a mountain bike last year and it has big (2.5") knobby tires on it. The first time I rode it to work just to see what it was like I could not believe the rolling resistance those tires had on asphalt. If you are riding 30 miles on those things no wonder you are tired. Unless you do a lot of off road riding too you might invest in some "slicks" (smooth tread tires). You'll find the pedaling easier.

Mtn bike miles are 63% of road bike miles. No, kidding, I just made that up.

The other enormous variable here is simple tire pressure. If you're like me you may use between 35 and 45 lbs off road, then for trails or road pump up the tires to their maximum rating, you can significantly reduce the difficulty gap to a road bike. I think most decent mtn tires are rated to 60 or 65 psi.

there sure is a lot that goes into it. i don't know numbers with any certainty, but things to consider are the differences in rolling friction and aerodynamics due to rider position.

if you're on the road and going fast, you use considerably more energy than if you're going slow, since drag increases with the square of velocity, so that will also have an effect.

the knobs may not have as much of an effect as the inflation pressure.

ooh, and here's some actual numbers.

I don't think you will find a definitive statistic because of the variables (if you do find one, I would doubt it's accuracy).

Someone with some physics chops could probably do the math to show you the difference in torque needed to spin the different drive-trains/wheels/tires.

Rolling resistance is a huge factor though. I have done 100 miles on the road with out too many problems, but 60 miles on crushed lime stone seemed twice as long.

Rolling resistance is a huge factor though. I have done 100 miles on the road with out too many problems, but 60 miles on crushed lime stone seemed twice as long.

+1. Even the best limestone trails sap my will to live after about 40 miles.

it would depend on a million variables. your rate of travel, relative fitness, weight, weight of the bike, rolling resistance, the gears you are using, how hard you are pushing yourself, et cetera.

You could use a heart rate monitor and get a fairly good estimate.

I have done over 100-miles of mountain biking on rocky mountain trails in a day, and have done 100-mile road rides that left me feeling just as tired while burning more calories, and both of those rides made 112 miles on the limestone rail trail seem easy.

Yeah, there's been a lot of talk/analysis of this over the years. You might find a copy of "The Science of Cycling", or "Serious Cycling" by Burke. Also, if you google for bike power calculator you may find some interesting things.

Someone with some physics chops could probably do the math to show you the difference in torque needed to spin the different drive-trains/wheels/tires.

Nick, we just need power meters.

Rail trails are soul crushing. They're the next hardest thing to riding indoors.

Nick, we just need power meters.

That is the engineer in me, making everything overly complicated.

I asked my dad (ret. physicist) what he can come up with. It'll probably be more theoretical than practical, but he is a great teacher and tells good stories, I'll post what he says (if he doesn't).

ok, so i did some maths using (approximately) the data in my link.

using continental ultra gator skin tires (no numbers are given for diameter or inflation pressure), rolling resistance will adds about 7741 joules to a mile, at 18.6 mph.

using continental grand canyon tires (26" x 2.125") inflated to about 70 psi , rolling resistance adds about 7885 joules to a mile (no speed is given).

that's about a 2% difference. but i just looked and saw the mtb tire tests were done with a 40kg load, and the road tests were done with an 85kg load. whoops! the link suggests that the resistance may increase linearly with load, so let's assume it does. that gives 16755 joules per mile. that implies that mountain bikes would require 2.16 times as much energy as road bikes, based only on rolling resistance.

conclusion: the increase with load isn't fully linear, but i imagine that the difference is still pretty significant. there's also a lot of missing control data for these comparisons. i wouldn't be surprised if edmonds is close.

Most of my first commuting year was on an old mtb with knobbies. I then went to slicks, then got an old road bike.

Do yourself a favor and get an old road bike already. The difference is massive

You can play with the "Static Forces on Rider" calculators on Analytic Cycling. But the numbers are only as good as the input parameters, which you'll have to make WAGs on.

which you'll have to make WAGs on

Then you can move on to "wind on rider" which, as we all know, blows often from all directions at once (except from the back) while riding in the Pittsburgh area.

Somewhere within this maelstrom of math is the fact that on limestone trails the resistance of skinny high pressure tires goes up because they sink in, and the resistance of big knobby tires goes up because of all the factors mentioned above. So there must be an optimal tire size/pressure for limestone rail trails. Any of you Newtonians care to throw out an estimate for that?

wow thanks for the responses everyone, and that looks like some solid math HiddenVariable. I'm an idiot when it comes to calculations like this so I appreciate the help.

I've thought about getting slick tires but I'm moving out of the country in a couple months so I figured I'd just suck it up for a little while longer.

So there must be an optimal tire size/pressure for limestone rail trails

30-32mm, running 80-90 psi. Wasn't that easy?

xlnt!

Too much math for me.... All I know is that sometimes I really feel like my performance drops when on my MTB on roads/trails. I went from nubby to slick tires at high pressure and felt an improvement, but I was told that I can get even thinner ones.

@Matatarski: Where to are you going?

This is the quintessential bike forum discussion.

bikeygirl, a little slip like you can ride 19c tires at 110 psi. You'll feel like you're flying. At least, until you hit that first pothole.

19c might be a little narrow for crushed limestone, no?

Sad to say, I did the temporary trail on the north shore on 23c's... and I'm not a little slip by any stretch.

@bikeygirl, if you're asking where I ride to, it's just along the river...yesterday I went from North Oakland/South Side trail/North Shore trail to the end of the elevated trail construction and back by way of Jail Trail. If you're asking where I'm moving, Vietnam.

The more I think about this, this seems like an experiment we could probably do in the Bike PGH community. I could be wrong but we may just need a heart-rate monitor, someone on a mountain bike and someone of similar size on a road bike to figure out at what speed you get X heart rate on either type of bike on the same stretch of trail, and from there figure out how far you have to ride to burn the same amount of calories on each bike. Does that sound even close to being right?

@matarski, go with the above-mentioned power meters instead.

HR doesn't mean a whole lot even when it's the same person on the same bike, from what I understand...it's affected by too many outside factors. Power meters are pretty much objective measurements of power applied to wheel; that, plus well-calibrated speedometers of some form, will let you answer the question of relative power-to-speed betwixt the bikes. Do a dozen repeats on each over the same course, with a decent rest period between each one, and it might be vaguely statistically significant.

I agree with Dan. There is too many other variables with heart rate.

@Lyle: Flying = good!

@Matarski: Vietnam? That sounds pretty cool!

@Reddan

I think your close, but I would go with the following

Either set a distance or a time as a control(C) and the other as a variable(V) take reading from the power output(O) and ust the following Math

X = O*(C/V)

This should give a number X which is a relative power reading between the MTB and Road bike

you can also try this

1 watt = 44.25 ft-lbs.min

As a relative measure of power take the weight of the rider + the weight of the bike (for example lest call it 180 lbs total) have the rider bike for a set distance (2 miles 5280*2= 10560) so 180*10560 = 1900800 ft-lbs Divide the by the number of minutes it took the person to ride the distance and you will ft-lbs/min divide this and you will get an output in watts.

Do this for a number of different bikes and you will get numbers of relative outputs

Granted these numbers will only be good for judging bikes or riders between each other and not really of a use for anything else.

I think this will work

Dbacklover