why do dirt bikes need such frequent rebuilds?

IMO, it's because you have a light weight, high output machine that is thrashed offroad, up hills, through creeks, over jumps.... The engines are revved higher more often and are constantly subjected to a dirty, muddy environment.

Your average streetbike doesn't see anything remotely like that. And the ones that do certainly show more wear.

Not all dirt bikes need to be rebuilt often . If you had an XR, KDX,TTR,DR, etc.. type of bike that was well maintained and ridden comfortably on easy trail rides, they can last an extremely long time. Now if you have a race bike or are racing a bike where most of the priority is getting as much of the power to the ground with as little wieght as possible well things are just going to wear out and start getting sloppy sooner.

It really doesnt matter if it is a bike, a quad , car, truck, or even a lawn mower if you are constantly stretching the throttle to its limits looking for more , going over the roughest terrain you can find or have the nerve to try and then add mud water and or sand it isnt going to stay new long.

Motocross bikes are built to win races and are extremely durable if you consider that top fuel dragsters get rebuilt every 5 seconds of intended use.

The more performance you squeeze out of any engine the more maintenance it will require. Most race engines on the pro level of any motorsport are almost always gone through after every race. Top fuelers only run for couple of minutes total between rebuilds but that the price you pay for all out performance.

Steve

Originally posted by rooster1713
A streetbike never needs rebuilds, except after a really long time. What is different about a dirt bike?

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Typically a street bike doesn't get used very hard. Once in a while it will have a top speed blast but rarely.  If you race a road racer you will probably do one or two topends a year just on a club level. 

My friend just toasted his YZF topend.  Barely had 10,000 miles on it.  Course you gotta change the oil and adjust valves especially when you ride it hard.

I sorta recently sold a 1985 KX1100.  It only had 22k miles on it.  I doubt many of them were hard miles.  Just Sunday cuising.