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Street bike vs dirt bike engine reliability..confused..

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Just out of curiosity I was wondering how inline 4 street bike motors can rev so high (for example a yamaha r6 redlines at 16,500 rpms) and hold up for many miles without wearing out the piston and rings? A modern single cylinder 4 stroke dirtbike motor can rev to at most about 14,000 and even in recreational use for trail riding the piston and rings have to be replaced quite often, and that's with keeping the air filter cleaned and the oil changed frequently! What is so different in design that allows a street bike motor to last at high rpms, running the same high compression pistons, etc. and not need to be rebuilt like a dirtbike? Is it because of having more cylinders, makes for less stress on just the one piston or a dirtbike or what is it?

Thanks.

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Just out of curiosity I was wondering how inline 4 street bike motors can rev so high (for example a yamaha r6 redlines at 16,500 rpms) and hold up for many miles without wearing out the piston and rings? A modern single cylinder 4 stroke dirtbike motor can rev to at most about 14,000 and even in recreational use for trail riding the piston and rings have to be replaced quite often, and that's with keeping the air filter cleaned and the oil changed frequently! What is so different in design that allows a street bike motor to last at high rpms, running the same high compression pistons, etc. and not need to be rebuilt like a dirtbike? Is it because of having more cylinders, makes for less stress on just the one piston or a dirtbike or what is it?

Thanks.

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Dirt bike people just have itchy fingers and can't leave their darn bikes alone...... which is why I keep mentioning that they always do rebuilds unnecessarily.  I'm sure dirt bike motors are damn reliable.

Edited by Honda_Power

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Street bikes spend 80% of their time at less than 50% of maximum revs and less than 1/4 throttle.

 

Dirt bikes spend 80% of their time at more than 80% of maximum rpms and more than 50% of theit time at full throttle.

 

Go ride your R6 around at full throttle and redline all day every day and see how long it lasts.

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The street variants tend to have heavier built engines.

The stroke and bore of the individual cylinders are also significantly smaller, which means less surface area on the rings as well as less torsional forces the connecting rods, piston pins, and crankshaft.

 

As far as the top end goes, I'm not sure. once again, maybe heavier valve guides...

 

I had a 4 cylinder bike, and I was almost never above 7000 rpm (rev limiter kicked in 14,000)

I expect my dirt bike to last a while before it needs to be rebuilt. I never wring it out or do anything wild.

 

Edit: Comparing to a kx450f.

Edited by CincoBoy
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It's the dirt.  It gets past your air filter and also into the oil breather.  The intake dirt is the reason valves wear out as the hardened coatings are worn off.

 

Also lower speeds are going to mean less airflow over everything and higher temperatures.

 


 

Go ride your R6 around at full throttle and redline all day every day and see how long it lasts.

 

About ten seconds before I eat a guard rail at mach 0.2!

Edited by OrangeYZ
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Dirt bike people just have itchy fingers and can't leave their darn bikes alone...... which is why I keep mentioning that they always do rebuilds unnecessarily.  I'm sure dirt bike motors are damn reliable.

 

 

They're extremely reliable. Modern dirt bike engines are paragons of high performance packed into a compact lightweight package.

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Street bikes spend 80% of their time at less than 50% of maximum revs and less than 1/4 throttle.

 

Dirt bikes spend 80% of their time at more than 80% of maximum rpms and more than 50% of theit time at full throttle.

 

Go ride your R6 around at full throttle and redline all day every day and see how long it lasts.

Really, do people think before asking? LOL
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Dirt bike people just have itchy fingers and can't leave their darn bikes alone...... which is why I keep mentioning that they always do rebuilds unnecessarily.  I'm sure dirt bike motors are damn reliable.

Also I might add....... it makes them feel as if their motors are these highly modified race spec motors, so they constantly rebuild them to reassure themselves of that, when in fact these motors are build for reliability too...... obviously, if companies release them to sell out to the public.  I guess not soo obvious to some.

Edited by Honda_Power

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Well in my experience with dirtbike motors, a crf250x for example, even when trail riding, with no sustained high rpm riding, and I always make sure my air filter is very clean and so do many other people, and I know no dirt contamination is getting past that filter. After about 80 hours of trail riding the rings are well worn past there service limits and the piston is worn as well. A street bike when riding the same way with no sustained high rpms can last upwards of 40,000 miles and even then the rings probably still aren't worn... Just confusing..

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Also I might add....... it makes them feel as if their motors are these highly modified race spec motors, so they constantly rebuild them to reassure themselves of that, when in fact these motors are build for reliability too...... obviously, if companies release them to sell out to the public.  I guess not soo obvious to some.

 

 

Here's a PIE chart to which you can refer to perhaps support the validity of your argument:

 

 

color91.jpg

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Also I might add....... it makes them feel as if their motors are these highly modified race spec motors, so they constantly rebuild them to reassure themselves of that, when in fact these motors are build for reliability too...... obviously, if companies release them to sell out to the public.  I guess not soo obvious to some.

 

 

facepalm4cr-1.jpg

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Biggest difference between the two is the bore and stroke. Small bore pistons that don't have to travel very far will be much more rev happy. That's how f1 v10s reach 20,000 rpm...but keep in mind it is only a 2.8 liter v10.

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The cam profiles on street bikes are less aggressive which makes them much easier on components and its also very difficult to work a street bike engine hard under normal riding conditions.  They usually only spend a few seconds at full power on the street.

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Street bikes spend 80% of their time at less than 50% of maximum revs and less than 1/4 throttle.

Dirt bikes spend 80% of their time at more than 80% of maximum rpms and more than 50% of theit time at full throttle.

Go ride your R6 around at full throttle and redline all day every day and see how long it lasts.

Chokey, I owned a 2003 r6 until 2012. In that time I logged over 50,000 very hard miles, including several track days per year. The red line was 15,500 rpm and saw it more often than idle. I sold it (personal reasons) still all stock and still running like a champ. It was just as healthy as when I got it.

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Alright, so if a street bike had a longer stroke like a dirtbike, running at the same high rpms that a street bike normally can run at, it would be less reliable?

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Here's a PIE chart to which you can refer to perhaps support the validity of your argument:

 

 

color91.jpg

My wife just made some great apple pie, any one want some of it?

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Chokey, I owned a 2003 r6 until 2012. In that time I logged over 50,000 very hard miles, including several track days per year. The red line was 15,500 rpm and saw it more often than idle. I sold it (personal reasons) still all stock and still running like a champ. It was just as healthy as when I got it.

Trust me, you won't get through to these people in here.  The people here rebuild their motors after every ride, hell I wouldn't be surprise if they brought a top end kit along for their ride, and halfway into the ride, they make a pit stop to rebuild the motor.

 

These are the same folks who changes oil and air filter after every ride.

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Alright, so if a street bike had a longer stroke like a dirtbike, running at the same high rpms that a street bike normally can run at, it would be less reliable?

Length of stroke is mostly what dictates the max rpm. Lengthen the stroke and you reduce the rpm capability.

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