So when is the YZ350F/FX going to appear ?


47 replies to this topic
  • Not sponsored

Posted May 04, 2016 - 03:13 PM

#41

It's not the rotating mass, it's just the mass.  The engine itself weighs 75 pounds, and it's 2 feet off the ground.  If the gyro effect had anything to do with it, you should be able to run the RPM up to about 5000 and put your feet on the pegs at a standstill.  But you can't.


I'm no engineer, all I know is with substantial time on both the 350sxf feels substantially lighter than the 450sxf even though it was within 4 lbs, whatever the reason. The husaberg 450 I owned with the 70 degree motor felt lighter in motion than any 4 stroke I've ever owned (unless you had to pick it up) even though it was the heaviest by far so there must me some truth to it. Isn't that why Yamaha has the backwards slanted motor? To get the moving parts closer to where they effect the handling the least?

  • grayracer513

Posted May 05, 2016 - 07:10 AM

#42

No, it isn't.  The 3rd and 4th generation YZ450F and the current YZ250F are engines that were designed around the intake port.   Essentially, they started with the most optimal cylinder head port layout they could reasonably use, which involves an intake port that runs almost dead straight from air horn to the combustion chamber, with only a slight jog at the end to get around the valve head.  Likewise, the exhaust lacks it's usual one inch radius 70 degree turn upon leaving the exhaust valve seat. 

 

So, there you are, with this really odd shaped head that you know absolutely has potential to outperform the head on every other dirt bike made.  But, the throttle body is sticking up out of it at around 40 degrees instead of the normal 80-90.   How are you going to fit that in the bike?  You can't orient the cylinder at a conventional angle without intruding on rider space.  The space under the seat is limited, so that's out, but there's a lot of room between the crank and the steering head, so if the cylinder is placed so that the intake runs up into that space it solves the problem.  Angling the jug forward is out of the question, so it got angled back enough to fit the intake into the frame.  Fortunately, this also comports with the mass centralization concept they've been pursuing in previous  models.



  • Not sponsored

Posted May 05, 2016 - 08:16 AM

#43

No, it isn't.  The 3rd and 4th generation YZ450F and the current YZ250F are engines that were designed around the intake port.   Essentially, they started with the most optimal cylinder head port layout they could reasonably use, which involves an intake port that runs almost dead straight from air horn to the combustion chamber, with only a slight jog at the end to get around the valve head.  Likewise, the exhaust lacks it's usual one inch radius 70 degree turn upon leaving the exhaust valve seat. 
 
So, there you are, with this really odd shaped head that you know absolutely has potential to outperform the head on every other dirt bike made.  But, the throttle body is sticking up out of it at around 40 degrees instead of the normal 80-90.   How are you going to fit that in the bike?  You can't orient the cylinder at a conventional angle without intruding on rider space.  The space under the seat is limited, so that's out, but there's a lot of room between the crank and the steering head, so if the cylinder is placed so that the intake runs up into that space it solves the problem.  Angling the jug forward is out of the question, so it got angled back enough to fit the intake into the frame.  Fortunately, this also comports with the mass centralization concept they've been pursuing in previous  models.

I learned something new today... You an engineer or Yamaha mechanic? You always seem to have technical Yamaha info that the average guy wouldn't know, not being sarcastic just curious. Either that or you live in your mothers basement with your star wars figurines and spend your life on the Internet researching this stuff, regardless I usually come away enlightened. Keep up the good work!

  • grayracer513

Posted May 05, 2016 - 08:33 AM

#44

It's a combination of sorts.  40+ years of professional mechanical experience ranging from complete fabrication of motorcycles to automatic transmissions, including some time at a Yamaha shop.  Mom's been gone a long time, and never had a basement, and I never owned any Star Wars junk, but I am naturally inquisitive, and I do hunt down information about things that make me curious (which is nearly everything).

 

The polarity of a reciprocating assembly like the piston does have an effect on the nature of engine vibration, but as far as I know, reciprocating motion does not create angular momentum (the "gyro" effect) in any manner similar to a rotating assembly, mostly because rotation is constant and works in every direction on a single plane, while reciprocation is cyclic and focused along a single line.



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  • weantright

Posted May 05, 2016 - 02:30 PM

#45

What defies logic is that you believe the angular momentum of the crank can make the bike hard to lean when it isn't strong enough to keep it upright.

You are missing my point. On a 450 vs. 250 you have more mass on the piston and a longer stroke. This extra mass when spinning does make the bike stand upright, similar to the gyro effect the wheels give. No it doesn't make the bike stand on it's own. If you take a 250 and a 450 from the same brand that uses the same motor design and frame, the smaller bore will be more nimble then the larger. Unless the new Yamaha's are magical, it happens just like every other brand out there. Mass centralization is not relevant here since both 250 and 450 uses the same design. This is one reason many of the KTM riders are running 250f vs 450 in endurocross. Powerful light bikes that handle much better the their larger siblings.



  • rr558

Posted May 05, 2016 - 04:14 PM

#46

The SXF 350 is only like 2 lbs light than the 450SFX but it feels 10 or 15 lbs.  I would be all over a Yammie 350 if they did it right it took KTM about 5 or 6 years to get that bike where it is today but the suspension still sucks. 



  • grayracer513

Posted May 06, 2016 - 08:02 AM

#47

You are missing my point. On a 450 vs. 250 you have more mass on the piston and a longer stroke. This extra mass when spinning does make the bike stand upright, similar to the gyro effect the wheels give.

 

I'm not missing the point at all, I'm saying that the "point" is irrelevant because the angular momentum of the crank is insufficient to effectively resist leaning the chassis to the extent that it affects the ease of steering.  The mass of the piston or any of the reciprocating assembly is completely unrelated to the matter because it does not rotate, and therefore generates no angular momentum at all.  Only the larger, heavier crank will do that, and that small difference in extra weight is still inadequate to have the effect that you assign to it.

 

But you can believe anything you want to. 



  • rr558

Posted May 06, 2016 - 07:36 PM

#48

Um ok if you say so





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