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Tw2.ch3 :: Made My Head Hurt


gogogusgus
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Hey All

Here's the Executive Summary question:

 

From TW2's Chapter 3, I've apparently not understood that a weight shift to the rear can be a phenom distinct from the back of the bike going down.

 

Can you recommend a thread I should consult to get a better understanding if this distinction?

 

Thanks

Justin

 

 

Less-short version :

So here at the dog park, listening to TW2, someone impersonating Mr Code said an upside-down-ness:

"most riders blv that the back of the bike goes down when they accelerate. It doesn't."(p14)

I heard and now see on p13 " ... throttle on ... transfers weight onto the rear ..."

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ktk_ace

 

Thank you. I'm not sure I understand where SEAT enters the equation. Perhaps I'm asking for a mountain answer to a molehill question. I think my head-swivel might stem from a subliminal assumption that

 

  • weight xfr to rear ==> bike's hindquarters squatting

but I think I might be unclear about TW2, where it says that:

  1. on acceleration both front and back of the bike rise (point your bike against a wall and throttle on)
  2. on acceleration, weight xfr to the rear makes the rear suspension less compliant (section about using this to get spin on driveout)

Perhaps I stated my original question poorly:

  • Maybe "From TW2's Chapter 3, I've apparently not understood that a weight shift to the rear can be a phenom distinct from the back of the bike going down."
  • Should have been "a weight shift to the rear does not nec cause the back of the bike to go down"

?

 

Since I hope I'm not being a pinhead on this, I'll look forward to pointers to prior threads.

 

Thanks

 

Justin

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This is my first reply to a serious question and I want to disclaim it right up front. See where it says "Squid" by my name? 'Nuf said... But here goes:

 

Your point 1; Both front and back would TEND to rise. Sure, but from different forces. The front as expected from the reduction of weight due to a rearward shift. The rear because of the application of throttle. Take a look at your bike. Notice the angle formed by the swing arm and chain. The cog is lower than the fulcrum of the rear suspension. On throttle application you "pull" the rear wheel down. This will cause the rear to rise in the absence of any other force. But there are other forces that come into play and so the rear will TEND to rise.

 

For point 2: Lets make two assumptions. One, your acceleration is due to throttle application. Two, you're cornering. Your throttle application has not only shifted weight to the rear but has stiffened the suspension (less compliant). Ideally, this amount of throttle has placed your suspension in it's sweet spot, mid-throw. Remember that TW2 is making an admonition about too much throttle application. "Whacking" on the throttle will cause a too stiff rear and loss of available traction.

 

It goes further on about spinning the rear out of the turn but I leave that till after I lose the squid label.

 

For what its worth. What do others think?

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Hey Justin,

 

I think I see where you're getting confused. A simple test to help get this all clear in mind: Take your bike and either place the front wheel against a wall, or hold the front brake on. Then lightly apply throttle, gently engage the clutch... and notice what happens to the rear of the bike (that is, the pillion seat, tail area.) wink.gif

 

 

Just to be clear there - when you apply throttle the tail of the bike will not squat down - it will actually raise. Try it out and see for yourself!

 

 

As far as the weight transfer to the rear of the bike under acceleration, I think you would already understand that correctly. A prime example is a power wheelie - strong acceleration adds so much weight to the rear that there is zero weight on the front wheel (has to be if it's floating in the air, right? biggrin.gif )

 

 

So there you have it, hope that helped.

 

Bender, you lost me a 'fulcrum'. laugh.gif

Nah, that all seems right to me, just getting a bit late and I didn't want to read too much. tongue.gif

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Action and reaction. If the rear wheel tries to push down, it will not get anywhere because the road refuse to move. It's still trying, though, and pushing like mad. This creates a force that runs through the swingarm to the chassis, where it creates a lifting force. So rear wheel tries to go down and instead the rear of the bike (seat etc) goes up.

 

Note that if the rear axle sits above the output shaft, the rear will be pulled down. Also note that if the force trying to push the wheel down, which happens with normal geometries, is less than the weight transfer, the rear of the bike can still sink. Torque applied is greater in 1st than in 6th gear also, and more power and lower gearing further enhance the lifting moment. Also, the steeper the swingarm slopes, the bigger the raising effect. This is also the major reason why racing bikes have adjustable ride heights and swingarm pivot positions; it allows them to tune the amount of raising force created under acceleration.

 

For most mortals, however, this is of little importance. What you need to know is that your bike will prefer to corner with a 40/60 weight bias ;)

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