Why is a road bike 15% FASTER THAN A MTB BIKE?

Question

Why is a road bike 15% FASTER THAN A MTB BIKE?

Answer 1

1. lighter- MTB has a bulkier and heavier frame than a road bike.

2. drop bars- you sit in the attack position which puts you lower and more aerodynamic

3. tires- road bikes have thin tires which make them easier to roll and they have less drag

4. gearing- road bikes are geared for faster riding over generally long distances

Answer 2

The reason is that road bikes are designed to go farther distances and to go faster.

This is accomplished by having a much lighter frame that any of sort of bike, allowing in to travel faster, and by having very thin tires, which reduce the amount of contact between the road and the tire. This cuts down on the total amount of friction, allowing the bike to go further in one pedaling motion

This next part is not factored into that percentage, but the combination of these two things allows the rider to use less energy while biking, which therefore lets the rider bike longer, and has enough energy to put more power into every movement.

Lastly, sometimes road bikes have more gears, which when used correctly, allows for smarter riding, which increases the speed of the bike.

Please note, road bikes are ONLY ideal of smooth, paved roads, so please consider the sort of riding you will be doing if you plan on purchasing a bike. Keep in mind that a hybrid bike is pretty much right in the middle between a mountain bike and a road bike, so in can go fast, but can take a bit more abuse- many think they are ideal for city riding.

Answer 3

Because a road bike is lighter, has a more aerodynamic frame geometry and has less rolling resistance due to smooth tyres.

Answer 4

This question has been asked many times and the answers follow a pattern.

Lets put a few ideas to rest.

1) A road bike is not faster than a mtn bike due to weight.

   The wattage it takes to maintain a given speed is no different between two riders if the total combined weight is the same. (all other factors being the same.)(my XC race bike is only 5lbs heavier than my road bike, and difference in total combined weight it is barely worth talking about.)

2) A road bike is faster for two reasons,

A) A road bike with its drop handle bars puts you in a more

     aerodynamic position, a mtn bike keeps you up and in the  

     air flow therefore increasing the wattage required for a

     given speed.

B) Rolling resistance, the narrow, smooth, high pressure

      tires of a road bike require less wattage than the    

      knobbies of a mtn bike, you can how ever replace the

      knobbies with road type tires but there is still some lose.

3) The gearing issue is only a concern for the hard core

      racer types, the every day casual rider will never achieve

      the speeds capable of the gearing a road bike has. The

      typical mtn bike with gearing (44/11) and a rider pedaling

      at 90rpm will be going 30mph.    

In short attacks on my mtn bike with road tires and 44/11 gearing I've seen 42mph on the flats, Yes I'm a XC racer and realize this is not an easy feat to do but it can be done. But back to the issue of 15% faster, its probably about right at as 15% only amounts to a few more mph. (20x1.15=23)

EDIT:

Michael C, Your points #1 & 4 are correct but #2 & 3 are not.

#2: Mtn bike frames are very rigid, my road bikes flex more than my any of my Mtn bikes. As far as power transfer goes most high end bikes have some sort of motion control and susp. movement is a non issue.

#3: Yes you use more energy to get a heavy bike up to speed, but the wattage required to maintain that speed is controlled by friction and aerodynamics!

You mention one of the posters stated "only thing that matters in terms of speed is the total combined weight of the rider and the bike" please read the post more carefully Its WATTAGE not speed!

"The wattage it takes to maintain a given speed is no different between two riders if the total combined weight is the same. (all other factors being the same.)("

As a side note you mention "This is why the "fastest" time trial bikes in the world aren't necessary the lightest, they're the most aero". Here is a fact, my road bike is 2lbs lighter than my TT bike, on the same 10 mile TT course I'm consistently faster on my heavier TT bike.(both bikes have the same gearing) It's the aero advantage not weight.

So by your own admission you negate the second part of your point #3.

Just to let you in on a secret, even though I go by Mtbikr and I compete for my sponsor in mtn biking. I race year round in road, crit and cyclocross, but I prefer off-road XC racing.

Answer 5

One of the previous posters has stated that the only thing that matters in terms of speed is the total combined weight of the rider and the bike.  This is not entirely true.  It depends on what we are defining as "faster."  If we're talking maximum speed, then yes, the weight doesn't matter.  A 50 pound cruiser going downhill will go slower only because of aerodynamics.  This is why the "fastest" time trial bikes in the world aren't necessary the lightest, they're the most aero.  Go look at the weight of high-end triathlon bikes, they're around 17-18 pounds while the lightest road bikes are generally in the 14-15 pound area.  

If we're also considering to some degree acceleration, then a lighter road bike with a heavier person will get up to speed faster than a heavier bike with a lighter person.  The reason this is comes from the fact that wheels, crank, gears, etc, are all lighter and thus accelerate faster.  Therefore, if the heavy and light bikes both reach the same maximum speed, the light bike will have reached that point sooner and will always have the lead.  If I put a 10 pound weight on my back and ride a 15 pound carbon bike and a 25 pound bike without the the weight on my back, I can react faster and thus get up to speed faster on the 15 pound bike.

As for the original question, why is the road bike faster?

#1.  Thin, smooth, high pressure tires = lower rolling resistance

#2.  Stiffer non-suspension frames have less flex and therefore transfer more energy to the wheels.

#3.  Lighter components all around mean that acceleration is faster and less energy is wasted maintaining greater momentum at the same speed (momentum = mass x velocity)

#4.  More agressive road geometries mean that the rider is stretched out farther horizontally, which results in greater aerodynamics.

I'm not hating on mountain bikers, I'm sure you can all ride plenty fast, but on an all road course, a proper road frame with road tires will complete the course in a shorter time than a heavier, similarly equipped mountain frame with the same rider.

Answer 6

like other said, it is not so much about weight but about less friction on the tires. think about the psi and the less contact resistance between the tires and the road.

Answer 7

mtbikr is right about the weight. The only time weight becomes more of a concern is on steep or long hill climbs. On the flats, it has an affect on acceleration, but if you're cruising along some extra weight is of little concern (within reason of course) There's a little more to it than that but in general relatively minor weight variations is minor compared to the advantage of wheels and tires found on road bikes. At higher speeds the aero affect of the road bike position becomes important.

Since I was the one that stated the 15%, I'm glad mtbikr (and implicitly others) agree. Like I mentioned before I tested it myself on a 24 lb MTB with aggressive knobbies pumped to 70 PSI over a lightly rolling 8 mile smooth course with one moderate hill vs a16 lb road bike with 23cm tires pumped to 110 PSI and low wind conditions. (I also rode this same course on my 21 lb road bike with my times about the same)

Now I would extrapolate from my experience, the road bike advantage would be less than 15% on a perfectly flat course with no wind. The more hills in the couse the road bikes advantage would climb a above 15%. This would occure due to 3 things:

1. Weight penalty on the uphill. This is where weight makes some difference.

2. Tire friction on the downhill. A fast decent greatly increases the friction of the fat knobbies.

3. Areodynamics. The fast speed on downhills penalizes the MTB because you catch more air vs the road bike.

And a final word. I would say at pro riding speeds the road bike advantage would increase for reasons 2 & 3 above. Likewise, I'd say at lower speeds like 14-16 mph the advantage would be less.

EDIT:

Micheal c, it's not accurate to say a lighter bike is faster accellerating when the total weight is =. 95% of faster accelleration would come from light rims and tires/tube - rotational weight. But accelleration does not affect terminal speed or average speed unless you through in many speed variations in a ride.

Answer 8

Weight factor.

Road bikes are more lighter and designed for less wind resistence.

But the MTB is more rugged for unpaved terrain.

So while you're repairing your deli-thin tire because of a flat, I'll see ya' with my knobby wheels spinning.

Answer 9

- Thinner wheels, which are run at a much higher pressure - less rolling resistance.

- Lighter frames and wheels, often have lightweight carbon parts (forks, seatpost) with no heavy suspension parts.

- Small, lightweight components - No big disc brakes.

- Drop handlebars provide are more aerodynamic riding position.

- Gearing is more suited to higher speeds - larger chainrings, smaller rear cluster (11-23T instead of a MTB's 11-30 for example).

Answer 10

A lighter frame, and most importantly, skinny tires that create less drag on the road.

Answer 11

Because it has a lighter frame and tyres that are about a third of the width. I'd be surprised if it was only 15%.