# Dirt Bikes Less Long-Term Reliability Than Street Bikes?

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I've heard that dirt bikes have less long-term reliability than street bikes. Assuming this is true, is it primarily due to higher stress levels in dirt bikes as opposed to inherent design? Or maybe a combination of both? Let's try this experiment: Compete a 250 cc 4-stroke trail (not motocross) dirt bike against a 250 cc 4-stroke street bike. Same rider. Let's meet half-way stress level wise. We'll only be doing gravel and hard-packed dirt roads with varying terrain (straight-aways, hills and ruts). Assume both bikes go through their fork compression full ranges but neither bike bottoms out on the forks. Assume both bikes vary speed from low to high but neither bike redlines out on RPM. Over long term riding under these conditions, which bike would be expected to have better long-term reliability?

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Two things determine longevity, assuming an equivalent level of materials integrity: Total horsepower hours/liter, and total number of engine revolutions. On horsepower hours/liter, it only counts if the engine actually produces a certain amount of power during operation, not that it has potential to create more.

If two engines produce 20 horsepower for an hour, and one is a 250 and the other a 500, the 500 will last longer. Street bikes last longer because they run much higher gearing at lower loads per liter.

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Two things determine longevity, assuming an equivalent level of materials integrity: Total horsepower hours/liter, and total number of engine revolutions. On horsepower hours/liter, it only counts if the engine actually produces a certain amount of power during operation, not that it has potential to create more.

If two engines produce 20 horsepower for an hour, and one is a 250 and the other a 500, the 500 will last longer. Street bikes last longer because they run much higher gearing at lower loads per liter.

Correct me if I'm wrong, but I believe you're saying that assuming equal engine sizes and same rated horsepower, a dirt bike engine will run at a higher rpm than a street bike under all speeds. That is, if both the dirt bike and street bike are going 10 mph, the dirt bike engine will be running at a higher RPM than the street bike. If both the dirt bike and street bike are going 50 mph, the dirt bike will be running at a higher RPM than the street bike. Is this the case? From an engineering standpoint, all mechanical parts exhibit what is called "wear-out". I don't remember the exact statistics, but if life cycle tests are performed on a large sample of mechanical shafts for example, lets say that 30% of the test samples fail before 50,000 revolutions. This would be defined as "wear-out" and cannot be designed out within cost, weight and size constraints. This shaft will be given a design life specification of 50,000 revolutions. Typically, all weapon systems are given a design life specification such as 10 years for example. Design life is where so many parts are beginning to wear out it's not economically feasible to keep replacing the parts but better to scrap the system and buy a new one from a cost standpoint. A design engineer will for example in the case of a shaft calculate how many revolutions the shaft will be expected to turn in turn in 10 years. Let's say it's 25,000 revolutions. He'll typically procure the shaft with a design life of 50,000 revolutions to have some margin. But even before the 50,000 revolutions more parts will begin to fail as the 50,000 revolution point is reached. If a dirt bike shaft experiences more revolutions than a street bike under the same operating conditions and time elapse, the dirt bike shaft will fail before the street bike shaft. Is this essentially what you are saying?

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If you examine only the question of gearing as equipped, your analysis of what I said is reasonably accurate, if oversimplified.

Assuming two identical engines with the same horsepower, one set up in a road bike, and the other as an off-road bike, run at the same speeds on the same road for the same amount of time, the fact that the off road engine is spinning faster means that the piston will have traveled farther within its bore over that time than the road engine's piston. Likewise, all bearings and any other directly connected drive components will see a higher number of total revolutions and, more distance traveled increases wear by that amount.

The part you ignored is that horsepower hours per liter. In this situation, you will often find that off road engines, particularly those in MX bikes, produce a much higher horsepower per liter than an equivalent road engine. If, over the course of the comparison, the power output of the two runs at an average of 35 per hour for 20 hours. Comparing a 450 producing that much to a 750cc road bike doing the same work, we see the 450 at 77.7 hp/liter and the 750 at 45.6 hp/liter. After 20 hours, the 450 will have posted 1554 hp hours per liter the 750 will have run up only 933.3. Most things being equal, the smaller engine will have pushed itself closer to the end of its live cycle than the larger because the engine carried a 66% larger load relative to its size, whether or not it was designed to survive it.

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If you examine only the question of gearing as equipped, your analysis of what I said is reasonably accurate, if oversimplified.

Assuming two identical engines with the same horsepower, one set up in a road bike, and the other as an off-road bike, run at the same speeds on the same road for the same amount of time, the fact that the off road engine is spinning faster means that the piston will have traveled farther within its bore over that time than the road engine's piston. Likewise, all bearings and any other directly connected drive components will see a higher number of total revolutions and, more distance traveled increases wear by that amount.

The part you ignored is that horsepower hours per liter. In this situation, you will often find that off road engines, particularly those in MX bikes, produce a much higher horsepower per liter than an equivalent road engine. If, over the course of the comparison, the power output of the two runs at an average of 35 per hour for 20 hours. Comparing a 450 producing that much to a 750cc road bike doing the same work, we see the 450 at 77.7 hp/liter and the 750 at 45.6 hp/liter. After 20 hours, the 450 will have posted 1554 hp hours per liter the 750 will have run up only 933.3. Most things being equal, the smaller engine will have pushed itself closer to the end of its live cycle than the larger because the engine carried a 66% larger load relative to its size, whether or not it was designed to survive it.

In regards to horsepower per liter, I believe you're saying that all riding conditions being equal the off-road engine is seeing an inherently higher stress level per revolution than the street engine per revolution. If both engines use similar parts in regards to part quality and design, the off-road engine would be expected to fail quicker because of the higher stress levels the parts are experiencing. So two things at play: (1) More revolutions of the parts in the off-road engine and (2) a higher stress level per revolution in the off-road engine. Am I getting this right?

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This is an interesting question

Engine reliability is only part of it because a good motor in a dog of a rolling chassis is no good. And vise a versa.

I think a roadie will do big K's and a dirt bike will take the hits

and they are two different worlds

Just my opinion

Cheers

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This is an interesting question

Engine reliability is only part of it because a good motor in a dog of a rolling chassis is no good. And vise a versa.

I think a roadie will do big K's and a dirt bike will take the hits

and they are two different worlds

Just my opinion

Cheers

Actually, in the original question I was including the reliability of the suspension as well. The question is really too nebulous to answer. But there was a pragmatic reason why I asked the question. I was really trying to determine whether I would be doing over-kill with a dirt bike for the relatively benign terrain I'll be on (dirt roads, gravel roads, trails, etc.). At age 64 I would not be doing the same things I might be doing with a dirt bike if I was younger. But the whole thing's relatively moot at this point. It looks like I'll probably be going with a dual sport bike.

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I didn't think it was just an engine type of question which is why I mentioned the hits

I know some older riders who love the BMW bikes that are ofroad freindly but are quite road

usefull also.

Out of my price range but they love them and are very user friendly

Cheers

Edited by Fishin Scott