Why are mountain bikes harder to ride on roads?

Answer 1

"Harder" is a strange choice of words, most people find MTB easier to ride due to their more upright riding position. Not that riding a road bike is particularly difficult.

I guess what you really mean is why they are slower, or need more effort to reach the same speed as a road bike.

Many people blame the gear ratio but that's very rarely the reason.

First thing is usually tires. Wide, knobbly, thick-sidewalled and low-pressure MTB tires have a huge rolling resistance compared to a slick, high-pressure road tire.

Next thing is probably suspension elements.

Absolutely worst is a full-suspension MTB w/o lock-out. Every time you push on the pedals some of that energy goes into compressing the frame.

With a stiff road frame this isn't much of an issue, as the energy will eventually make its way into the drivetrain anyhow. But if you have suspension elements on the bike these will steal some of this pedalling energy from you. Bikes with lock-out (where you turn the suspension elements rigid)are better, but quite pricey.

Then there's rotating weight and stationary weight. Not much of a concern for top speed, but the lighter the wheels and the bike as a whole, the more nimble it will feel at take off. And road bikes, which are not expected to have to survive a 2-3 foot drop, can be quite a lot lighter.

Finally, as you begin to pile on speed (say, 12 MPH and above), aerodynamics begins to play a role. Chunky wheels, chunky frame and an upright riding position will begin to make itself know when compared to a sleek road bike and a hunched over position.

Another issue is original quality. MTBs tends to be available in what's called "department store bikes", or among some riders/mechanics BLOs(=Bike-Looking Objects) and can be quite horrible in terms of overall quality. Road bikes as a rule aren't available at quite that low level of quality, which screws up the comparison base. Pitting a $250 MTB against a $2000 road bike shouldn't have anyone surprised regarding the results.

With that said, don't forget the rider. Throw a pair of high-pressure slicks on a rigid MTB, set the bar up low and you should be good to keep up with roadies until they begin to stray over 20 MPH - unless they're simply much fitter than you.