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Discussion Starter · #1 · (Edited)
hi guys, i know this is a really simple qn. cos my nephew who is in school has been instructed to write an essay on this topic and to compare how they are built different and what makes the mountain bike much slower.

Thus he asked me, and i replied him these;

1. the tires are bigger?

2. the geometry is more slack, thus not so aerodynamic

3. they are heavier

4. the chain ring has fewer teeth

er..

is there anything else i missed out?

thks guys!
 

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Underskilled
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Don't forget the softer tyre compounds.

The air drag only comes into effect at higher speed.

Geometry generally isn't slacker, slack geometry makes a bike more stable at high speeds.
weight only affects acceleration, not top speed.

I Ride a Niner WFO on Schwalbe big apples, I can take most road riders without trying.

Don't forget suspension taking power out of the hard stokes.

oh and tyre pressure.

Riding stouts I literally knock 10mph off my top speed from the apple
 

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Underskilled
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rotating weight of wheels for acceleration too.
 

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Physics.....

Less mass, larger gear ratios, smaller contact patch (less rolling resistance), and a more aero position all lend to road bikes being "faster" than mtbs.

And CaveGiant, the point isn't about road "riders" being faster than mountain "riders," its about road BIKES needing less effort to go fast than mountain BIKES. If you were on a 16lb road bike, you'd be able to dust those "road riders" that you pass on your Niner even more easily...
 

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Any decent roadie on the flat could easily spin a 53-14/15 gear at 90+ cadence (pretty cruisy). This is about 43km/h. This is on the equivalent of a 26 inch MTB is about 105 cadence (a lot) in your 42/11 gear. In other words, in order to keep up with a roadie (who is just riding tempo) you would have to spin the f*#k out of your highest gear. The roadie can hold a conversation at this point, the MTB rider is into oxygen debt (ie cannot sustain this pace).

The reasons why you can't get to this speed are a combination of (a) not high enough gearing, (b) high rolling resistance on wide and knobby tyres and (c) wind resistance. Weight is a minor factor that only really comes into play on climbs. For example, on a 1 hour hillclimb assuming uniform power output (260w or so) at 8% gradient there is a loss of about 1.2 seconds per oz (28g).

For example, if a road and moutain bike arre climbing an 8% paved road, and start together, climb for one hour, and the road bike weighs 7kg and the mtb (hardtail to remove suspension bob losses) weighs 10kg, plus 2kg of camelback and water, plus say 1kg of heavier equiptment that MTB riders carry the road rider will end ~257 seconds ahead of the MTB rider(assuming 250w at 17km/h). This is a rough and ready set of calculations, but it gives a general outline of the weight factor.

I call BS on the guy who 'can easily take most roadies'. The fact is that on the road, road bikes are 15-20 km/h (+) faster for similar effort. Sure, that doesn't mean you can't burn your legs up and overtake an old guy or two, but a typical road ride lasts about 2 1/2 hours. Road riding is mostly about sitting back and riding tempo. I used to think I 'could take roadies' until I actually started counting. You tend not to notice the ones that zip by you, and make it some kind of personal victory when you do overtake somone who in truth is probably warming up or spinning down. You also don't even see the vast majority who start ahead of you and stay ahead. That is to say, it doesn't count if they are not racing. If they were, then they would call an early end to their warm up, put it into the big dog and ride away into the sunset, leaving the MTB rider to wallow in a heap of lactic acid.

weight only comes into play in acceleration and up hills, not maximum speed. Maximum speed on the flat is determined by wind resistance, power, gearing and rolling resistance.

This is a table of road tyres in rolling resistance tests. As you can see there is even quite a difference between road tyres. Tubes even make a small difference!

MTB tyres are even more diverse, but I cannot see any one MTB tyre being more efficient that any one road tyre. Wide and grippy tyres = slow. Small wheels also have some impact as bearing resistance comes into play (small wheels have to turn more times per metre travelled and bearing resistance increases exponentially with the number of revolutions per minute).

aerodynamics is a very big factor. to demonstrate this, get a road bike and a mountain bike and put the two on a trainer and do standing start to, say, 40km/h accelerations. The results will be about the same.

However, for the sake of a child's science assignment I would keep it simple. Stress the following: Power output - wind resistance - weight - rolling resistance. Then a breif explanation of gearing ratios. Just for reference, a typical geared MTB setup is 22-32-42 chainrings with something like a 12-28 cassette. A typical road setup is 39-53 with an 11-25 cassette. A conclusion to the report/presentation could be along the lines of design specificity. While on flat, straight road conditions a road bike will perform better, off road, with poor traction, over uneven terrain, a moutain bike will be faster than a road bike.

The question of what is faster in a straight line is not relevant. Neither bike is designed for that purpose (that is a time trial bike). Rather, it is a matter of designers assessing the needs for a product, applying physics and making two different products to meet different conditions.
 

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Underskilled
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Calling BS on me?

Just because you cannot do it doesn't mean others can.
Don't forget I am on road tyres.
 

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First and foremost -- It ain't the bike, it's the motor ;-)

Mechanically, the extra weight of the MTB, lower gearing, and the increased rolling resistance of the wheels (wider, knobby tires as well as lower tire pressure) will make the bike slower than a road bike for the same amount of pedaling effort.
 

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PscyclePath said:
First and foremost -- It ain't the bike, it's the motor ;-)
Very true. However, I have seen a LOT of people on this site claim that they can smoke roadies on their MTBs even with knobbies on. I'm sure some can, and I KNOW some are full of BS.

And as C Dunlop stated, you often don't really know if that person you "smoked" was warming up, cooling down, or had already ridden for 50 miles. Or maybe they are doing intervals...

It's like being able to bench press "more" than that big guy in the gym who might have already done 5 sets before you came in.
 

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CaveGiant said:
Calling BS on me?

Just because you cannot do it doesn't mean others can.
Don't forget I am on road tyres.
If this was pointed at me, the answer is no. I was merely stating that you, on a true road bike, with road gearing (50/34 or 53/39 chainring) are going to be able to go faster than you already can on your 29er mtb with 44T chainring. Its simple physics. Also, your "road" tires are what, 1.5" width? Throw a true road tire (23cm wide) on a true road rim, and you'll be able to go even faster. You'll see an instant increase in speed relative to power (or a instant decrease in power needed to go the same speed) by swapping to a road wheelset.
 

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Can we define the context of "faster" for the purposes of the essay? Put a mountain bike and a road bike on a rocky downhill, and the road bike isn't so fast . Put a mountain bike and a road bike on a flat paved road, and my money is on the road bike, given equal rider strength/fitness.
 

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you're comparing two different beasts, the road bike is faster on THE ROAD, the mountain bike is faster on THE MOUNTAIN. you're comparing apples to oranges. Why not ask why the road bike is slower than a mountain bike (when taken on a trail)

they both have their purposes and are aces at that.

pretty simple concept
 

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CaveGiant said:
I Ride a Niner WFO on Schwalbe big apples, I can take most road riders without trying.
Noobs inter-braggin' on the beginner forum.

Every time I check this forum, I get to chuckle. Thanks

Now where's a phone and BS's number. I have to make a call....
 

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CaveGiant said:
Calling BS on me?

Just because you cannot do it doesn't mean others can.
Don't forget I am on road tyres.
mmmm, my P.O.S., (and heavy Apollo MTB with slicks blows away my Scale with knobblies on the road. My undersized, unloved, elderly Peugot road bike leaves them both for dead. Same rider, different bikes. The tires are definitely the main difference - the Apollo has low level, well worn components and is heavier than the brand new, well fitting Scale but still rolls much faster.
 
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