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Discussion Starter · #1 ·
Soundman brought up an interesting point in his review of Avon Azaro ST tires: how engine braking might be a cause for increased rear tire wear.



Can someone describe (in layman's terms please for the physics-challenged ;D) what happens during engine braking? Any potential effect on tire wear would be icing on the cake....
 

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Engine braking: valves are closed while piston is coming up (compressing) so it's getting resistance to coming up. There is no explosion at the top of it's stroke to accelerate it back down (translating to revving of the engine and, if in gear, acceleration of the bike). There is a small effect of resistance even when the intake valve is open but the throttle plate is closed as the piston is going down ("sucking" in air).
The rear tire would wear down faster, but no faster than using the rear brake in a similar fashion.

EDIT: forgot to add the obvious resistance as the piston travels up to push the spent mixture out the exhaust valve and through the exhaust system.
 

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Discussion Starter · #3 ·
ShaggyZ-- Good stuff! :D So blipping of the throttle in downshifting (matching RPMs) results in less engine braking by lessening resistance in the pistons on the upstroke and imparting a force on the downstroke, correct? I imagine the same would be true of a gradual roll-off vs. chopping the throttle when engine braking without downshifting?

In your comparison to using the rear brake, that's what I'm driving at: the end result of the engine brake at the rear tire, or the application of a braking force on the rear wheel which produces negative acceleration.



I guess to be more specific, my questions are thus:

Is engine braking inherently tougher on tires than rear brake use?

Is it is easier to modulate braking pressure of either type (engine vs. rear) to smooth out the application of negative acceleration?




I am imagining in my feeble mind that sudden braking inputs which cause the rear to hop, skip, slide, chatter (whatever you description you prefer) is what might cause premature tire wear, regardless of braking method.
 

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Engine braking is not as easy to modulate as the rear brake.

Blipping the throttle smooths out the reaction at the back tire. Done right blipping and downshifting should be smooth and seamless. There should be no jerkiness, just extra engine braking because of the added RPM.

Engine braking changes as the engine decelerates. The sources of the braking are "ineffective" compression strokes and higher vacuum as the piston descends on the intake stroke and a smaller combustion (the engine should fire) stroke with the resulting smaller intake charge. When there is no ignition of the smaller charge backfiring often occurs. It takes no extra energy to force the exhaust gas out. In fact exhaust forces are smaller because of the smaller charge. As the engine slows the amount of braking decreases (fewer intake cycles), so you can add more rear brake.

If you are experiencing wheel hop or sliding your braking forces (engine, brake or both) are too high and too jerky. In the end there is no difference at the rear wheel between engine and brake braking as long as it's all done correctly and smoothly.
 

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+1 YESSSSS!

I am still having a b!tch of a time learning how to blip that throttle!

I can double-clutch a diesel rig like it was second nature .. but I'll be damned if I am still trying to get that technique down!
 

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Good Question!

Wear takes place (assuming that you are travelling in a straight line) when the rubber of the tyre is being scuffed against the road surface.

Scuffing takes place when the tyre is trying to go faaster than the load (bike and rider), or slower than the load.

Example - Faster than the load = laying rubber when accelerating
Example - Slower than the load = Skid marks

In both cases the amount of wear is directly related to the rate of change of speed and to the load

(In the physics sense assuming direction is constant rate of change of speed = acceleration [even if you are slowing down this is still acceleration to a physicist])

Considering the rate of change under braking, the wear surface (the tyre) is not effected by the source of the force acting on it, just by the magnitude.

In practice heavy braking is usually a combination of brake and engine, whilst gentle braking is often one or the other. The key to maximising tyre wear is gentle braking, which in turn comes from good anticipation.

I hope this helps!
 

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Discussion Starter · #7 ·
andyauger-- As always, your engineer's perspective makes sense even if I don't fully understand it (I re-read your post at least half dozen times the first time through).


chrisd-- Even though I giggled soda out my nose at 'slower than the load = skid marks' (I have a warped/adolescent sense of humor ;D), your wording makes sense, as does the distinction between the different types of wear that can occur when the rotational speed of the rear tire is faster or slower than the bike.


Both of you appear to say that it doesn't matter whether it's the rear brake or engine compression which provides the braking force, it is the magnitude or smoothness of the braking input which has the greatest impact on rear tire life.



I appreciate your responses, guys, and I'm in full inquisitive kid/nerd mode... ;D



Let me throw out another branching question or two:


How is engine braking pressure applied to the rear wheel? (I know it's through the chain, but how?)


What happens to the chain, and what affect does chain tightness have on engine braking?


What happens, if anything, with the swingarm/shock/wheel/rider distribution of weight under either rear brake or engine compression braking? How does this differ from an application of braking from the front of the machine (front brake)?
 

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No one hit it on the head yet.  I'll spell it out.

First, I'll start with saying that during engine braking, there are forces being applied against the direction of rotation in the motor (due to the valves being closed per already been explained...). This negative torque is applied through the drivechain via the sprokets and chain.  The forces on these components are in the opposite direction of their rotation, which upon acceleration or constant speed riding they experience forces in their direction of rotation.

In short, this negative torque that is applied through the drivechain is applied to the wheel and tire against the road surface.  The tire is now facing kinematic and static friction forces in the opposite direction of normal tire rotation.  While this certainly won't help tire mileage, does it significantly hurt it?  You'd have to ask the RD&T engineers that.
 

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Discussion Starter · #9 ·
supaphatty J said:
No one hit it on the head yet. I'll spell it out.

First, I'll start with saying that during engine braking, there are forces being applied against the direction of rotation in the motor (due to the valves being closed per already been explained...). This negative torque is applied through the drivechain via the sprokets and chain. The forces on these components are in the opposite direction of their rotation, which upon acceleration or constant speed riding they experience forces in their direction of rotation.

In short, this negative torque that is applied through the drivechain is applied to the wheel and tire against the road surface. The tire is now facing kinematic and static friction forces in the opposite direction of normal tire rotation. While this certainly won't help tire mileage, does it significantly hurt it? You'd have to ask the RD&T engineers that.


Are you saying that engine braking produces a force that pushes the chain backwards (a clockwise force against the counter clockwise force applied to the sprocket) ala a hydrostat? ???

I understand the concept of applied force in a direction of rotation under acceleration (even cruising at sustained speed is very slight acceleration at the chain/sprocket due to the resistance of air, mass, and friction). But removal or reduction of that applied force is not the same as applying a force in the opposite direction of rotation, even though that is the end result when you bring the tire contacting the road surface into play.



I'm not having a go at you man, just trying to understand the concept... :)
 

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If you are really good at front brake modulation...you can keep the rear tire off the ground during heavy braking on corner entry and avoid rear tire wear altogether! Who uses the rear brake other than to stop a wheelie over? ;D Blip and rip! 8)
 

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Yes, the engine "pushes the chain backwards." That's the physics part of it. Because the engine is placing resistance on the drive chain, which slows you down, there is a negative torque value, otherwise you wouldn't slow down. Forces are reactive.
 

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Discussion Starter · #12 ·
supaphatty J said:
Yes, the engine "pushes the chain backwards." That's the physics part of it. Because the engine is placing resistance on the drive chain, which slows you down, there is a negative torque value, otherwise you wouldn't slow down. Forces are reactive.

That just doesn't make sense to me. A resistance to (continuing) motion when you shut down the throttle, yes, but an opposite force? ???
 

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Six Chin Skinny said:
That just doesn't make sense to me.  A resistance to (continuing) motion when you shut down the throttle, yes, but an opposite force?   ???

Forward force is this big <----------------------------->

Opposite (rearward) force is this big <-------------->

^Thats a visual for resistance. Opposite force eats away at forward force, give it enough time, it'll cancel out.
 

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I can't downshift and rev match pretty well to the point of getting the bike to a slow crawl with out the use of brakes. Now I'm really trying to down shift into 2nd or 1st and get the back end to squirm out to the side a bit before powering into turns. Less of a blip at higher RPMs should be the cure.

;D
 

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Discussion Starter · #15 ·
I don't understand how the cessation of a force is the same as an opposite force... ???


Yes, the end result is the same: lowered energy output until resumption of a static position, in this case, standing still.



So you guys are saying that the chain is being pulled one direction (in rotational axis) while under application of force (acceleration), and pushed in the other direction when that force is reduced (throttle shut down)?
 

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Six Chin Skinny said:
So you guys are saying that the chain is being pulled one direction (in rotational axis) while under application of force (acceleration), and pushed in the other direction when that force is reduced (throttle shut down)?
Yes. Theres a few other things in play, but basically yes.
 

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Discussion Starter · #17 ·
I'm sorry, but that makes no sense to me at all...


Yes the tire is a drag on the chain when you shut down the throttle, and that will impart tension on the chain in the opposite direction of normal rotation. But to say that the engine pushes the chain in the opposite direction to normal rotation when the acceleration force ends? I call b.s...
 

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ShaggyZ said:
Engine braking: valves are closed while piston is coming up (compressing) so it's getting resistance to coming up. There is no explosion at the top of it's stroke to accelerate it back down (translating to revving of the engine and, if in gear, acceleration of the bike). There is a small effect of resistance even when the intake valve is open but the throttle plate is closed as the piston is going down ("sucking" in air).
The rear tire would wear down faster, but no faster than using the rear brake in a similar fashion.

EDIT: forgot to add the obvious resistance as the piston travels up to push the spent mixture out the exhaust valve and through the exhaust system.

Shaggy, the valves are ALWAYS closed during the compression stroke, whether under engine braking or hard acceleration, there is more resistance in the compression stroke under hard acceleration than engine braking, theres a bigger charge in there to compress, valves opening and closing has NOTHING to do with engine braking other than controlling the pulses of air going thru the engine  , if it did, and engine wouldn't be able to accelerate

engine braking is as Andy sez, vacuum being pulled agains the throttle plate

the eggine is still rotating in the same direction, and is still getting enuf charge to keep the engine running at idle even with a fully closed throttle
 

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I was trying to keep it as simple as possible. I'm aware there's no air being pushed out of the intake valve. I was speaking of the "sucking" when the intake valve is open and the throttle plate is closed. There is resistance from the throttle plate.

You'll notice that I just repeated myself. :-\
 

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Six Chin,

It really is boiling things down to it basic components.

Bear with me while I go through a couple of definitions ...

Statics = $hit don't move, force is applied but it still is not moving.  What is the force doing?

Dynamics = $hit moves ... velocity is constant speed *wince* (you other engineers and physicists give me a break  :D ) and acceleration is change in speed/velocity and or direction.  Positive acceleration ... you are going faster, negative acceleration ... you are slowing down and to really blow your mind, if you are going at a constant rate around in a circle, you are accelerating because the definition of acceleration is a change in speed or DIRECTION!

Kinematics = $hit moves in very strange directions ... not really applicable here but if you crash, where all those parts are flying and where they land is a kinematics problem!   ;D

Then there is thermodynamics ... we ain't going there but that my friends gets very interesting!

So to break down 2+ semesters of engineering into a couple of paragraphs ...

Statics ... the bike is pushing down on the ground and the ground is pushing back up an equal amount.  

Dynamics ... the engine generates a rotational force (torque ... torque is real, horsepower is imaginary) that is translated to the wheel with chain and sprocket.  The wheel then translates this torque to the ground thereby moving the bike forward.  Torque can be positive or negative, define one direction as positive and the other as negative.

What makes the SV or other v-twins unique in their drivability is it is very possible to drive this bike with no brakes.  Give it throttle and the top of the chain is taut and it is driving the bike forward.
Release the throttle and the bottom of the chain is taut and it is the bike trying to drive the engine forward.  The reason for this is there is energy in a moving motorcycle and to slow it down, one can use the engine (remember the throttle is closed so the engine is delivering less energy than what the bike possesses at the time) or the brakes.  Brakes transform that energy into heat.  (thermodynamics anyone?  ;D )  These two forms of energy are called potential and kinetic energy.  

Friction from the brakes is very easily controlled.  

The slowing of the bike by using the engine on the other hand is not as easily controled because there are several variables in effect.  Drive chain slack or lash as it is called, transmission lash and finally the engine forces that andyauger eloquently described.

For those of you that don't care ... I apologize for the long winded thread.
Those that have an inkling ... there is some really neat and funny things that go on if you take the time to break it down into simple blocks.
 
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