What makes road bike SO MUCH lighter than a mtb?

difference in weight for frame isnt that much comparing a hardtail with a road bike. wheels, 29inches on road bike isnt that light too. the crank is also not so light with the huge chain ring. handle bar is heavier than mtb. what makes a road bike a few kg lighter?

Here is the fundamental difference. A mountain bike is optimized for ruggedness. A road bike is optimized for weight and speed.

There is nothing to say that a mountain bike could not be much lighter. In fact, expensive, XC racing bikes are usually quite lightweight. Racers buy them for the best speed in full knowledge they are not buying a real robust bike. Should you take one of these bikes off some decent drops you would be likely to tear it up.

Designed for tame, paved roads, a road bike doesn't need to be at all robust. They can be optimized for the lightest possible weight and highest speed.

You might want to take a look at the following web site to see how light some people can build a mountain (or road) bike:

http://www.light-bikes.com/

Hope this helps.

Fork. In my opinion the fork on a ht mtb is where the weight is at. Also, when you look at individual components there is a few gram difference, but a few grams more in individual components adds up. Also Mtb typical have heaver rims due to the difference in width and the durability. 26" mtb rim is much more durable than a 700c found on a cross bike or road. Additionally the tires on a mtb are bigger and use more rubber and some have Kevlar bead, so there is a bit more weight there. When you add the small things up you get a good weight difference. Unless you are talking about a very low end road bike and a very high end mtb, the road bike will always be lighter.

So no one answers your question seriously. So I will answer you seriously. Roadbikes are more lighter because they are built for high speed and it's light weight frame is designed for smooth roads with better stability when running in high speed

while for mtb bikes the frame is slightly more heavy because of the reinforcement to critical parts it is designed for heavy punishment because they are used in rough roads and strong frame body is required.

The wheels and tires are indeed heavier than road bikes. The other big factor is the sus fork vs road fork - a several lbs right there. These 2 items alone can add 3-4 (or more) lbs on a light XC MTB. Then most all parts on a MTB is a little heavier it adds up. Of course if the MTB is dual sus that's more weight again.

Chain ring size means little as chain rings are light. But MTBs have 3 and the granny is steel usually. Rear cassette is huge and heavier as well. As you menioned the only lighter part is the bars.

A good light racing MTB can come in around 21 lbs. compared to around 17 on a road bike. (both can be lighter) As you can see the wheels/tires and fork make up the lions share of the weight difference.

mtn bikes have to be more durable to handle hills and jumps and rougher terrain. road bikes do not have to be made as durable because they travel on smoother roads. therefore road bikes can be made with lighter materials. i have a road bike and a mtn bike both made of aluminum and by cannondale, but the road bike has thinner aluminum walls and uses less aluminum so road bikes can be much lighter for this reason. also the rims are smaller and the forks are smaller both equalling less weight. hope it helps

mtb are made to be beat and jumped so they need biger tires and stronger frame and most old mtb are made of carbon steel so are the wheels but today there are not that much differents in the bikes for ex my mountain bike weigh about 27 puounds and my road bike weighs 21 pounds mostly because road bikes are made of some carbon fiber and if you would put carbon fiber on a mtb you would not have a mtb for long it would break when you jumped and skinny tires would not hold up to the jumps they would pop!!!!

You could be comparing a top end Carbon Fibre Colnago to a badly welded Steel mountain bike from Wallmart for all we know. It's all relative - the more expensive MTBs have better components and are lighter - exactly the same for road bikes.

You can get low, mid and high-range road *and* mountain bikes.

apples and oranges mate. what two bikes are you comparing, from the plethora of choices?

Road components use thinner, lighter tubes due to less of a need to withstand punishment. They lack suspension which adds more weight. The wheels use less wide, thinner rims, which results in less material uses. This of course means thinner tires and tubes, which also save weight. They also use thinner spokes, also resulting in weight savings. The componentry (Cranks, derailleurs, shifters, cogs, chainrings) are all built to a less rugged standard in an effort to save weight. So what makes a road bike so much lighter? EVERYTHING. Every component is designed to shave the maximum amount of weight without sacrificing safety. While steps to reduce weight are made in mountain bikes too, you certainly don't want your fork to snap off while going down hill over a log.
But if you are going for the greatest contributors towards the weight difference, I would say it is the frame, wheels, and tires.