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Old 12-12-2012, 09:13 PM   #91
catweasel67
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The maximum braking

The maximum braking allows to achieve the maximum deceleration, that is:


The deceleration is a function of the wheelbase p, of the vertical h and horizontal position of the center of gravity and of the coefficients of friction, not of the vehicle mass.

The ratio between the front wheel braking force and the total available braking force depends only from the geometrical properties and the coefficients of friction:



Figure 6. Deceleration lines and braking ratio
wheelbase p=1.4 m; height of the center of gravity h=0.7 m; horizontal position of the c.o.g. b=0.7 m

The above figure shows that the deceleration increases with the coefficients of friction. Due to the load transfer the front wheel braking force is bigger than the rear wheel braking force.
The braking ratio between the front and the rear wheel are expressed by the red lines.
The horizontal axis represents a rear wheel only braking condition; the vertical axis a front wheel only braking condition.
If the coefficients of friction are low, the importance of the rear braking force is not negligible, as in high friction condition.
The maximum deceleration before the forward tilting is 1 g.
As an example consider braking a vehicle with a deceleration equal to 0.5g; it is possible to reach the desiderate decelerationusing different ratio braking. Braking using the front wheel only, requires a front coefficient of friction equal to 0.68 (point A).
If the used braking ratio is 80% on the front wheel, 20% on the rear wheel, the same 0.5g deceleration requires a front coefficient of friction equal to 0.55 and equal to 0.4 on the rear wheel (point B).
Which is then the optimum braking to achieve the 0.5g deceleration?
If the maximum coefficients of friction are the same for both the wheels, Figure 7 shows that the maximum deceleration is achieved when both the tires are used in the same way.
As an example consider a coefficient of friction equal to 0.8 for both the tires; the maximum deceleration (0.8g) is achieved with a braking ratio equal to 90:10.
Using the front brake only, the maximum deceleration is equal to 0.67g; using only the rear brake is equal to 0.29g.
In slipping condition the coefficient of friction is equal to 0.4, the maximum deceleration is equal to 0.4g and the optimum braking ratio is 30:70.
Se il fondo stradale è più scivoloso e i coefficienti di aderenza di entrambe le ruote risultano pari a 0.4 la frenata ottimale si ha con una diversa ripartizione (30/70) e fornisce una decelerazione pari a 0.4 g.

The 45° line represents the condition mf= mr and is the optimum braking; this line intersects different braking ratio lines as function of the desiderated deceleration.


Figure 7. Braking action of dry(0.8) and wet(0.4) surfaces

Figure 8 shows that the optimum braking line is tangent to the 50:50 braking ratio; it do not intersect the ration curves with the rear wheel braking force bigger than the front wheel braking force.
The above consideration is valid even if the static load is bigger on the rear wheel.
The optimum braking line is always tangent to the ratio curve having the same values of the static loads ratio.

As an example, if the static loads ratio is 45:55 (45% on the front wheel, 55% on the rear wheel), the optimum ratio curve is the 45:55 and is tangent in the origin to the optimum braking line.

Figura 8. Braking action of dry(0.8) and wet(0.4) surfaces
wheelbase p=1.4 m; height of the center of gravity h=0.7 m; horizontal position of the c.o.g. b=0.7 m


or. put another way.

use both brakes to maximise braking.
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Old 12-12-2012, 09:16 PM   #92
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Originally Posted by catweasel67 View Post
But you understand that a newbie won't break as aggressively as you so would benefit from learning to use both to maximise their braking until they get close to your declared skill level?
My thoughts on why rear brake usage during panic stops is bad.

1. You're added _twice_ the amount of task for stopping you now need to manage 2 separate brakes systems and tire level tractions.
2. The majority of the bike's stability is coming from the gyro of the running engine and the spinning wheels. If you lock the front and the engine is still spinning and the rear tire is still turning you have a very good chance of reducing pressure and riding out of it because the bike is still stable and tracking straight.

You lock the rear which is very easy to do while hard on the front and kill the engine you've lost much of your stability and your ability to re-accelerate and if you then lock the front on top of that the bike is completely unstable. To avoid killing the engine you now need to manage a 3rd control.

The nature of an emergency is that you aren't planning for it. It's hard to properly modulate 7 controls (front brake, rear brake, clutch, steering, body position, throttle) during a full on stop when doing planned full on stop, like on a race track.

It's going to be damn near impossible to pull it off when you didn't except it, because face it. If you where in a position to do it all right you'd not be in an emergency stop because you'd have seen it coming and already taken action to avoid the situation.
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Old 12-12-2012, 09:35 PM   #93
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Originally Posted by slartidbartfast View Post





He is not applying the rear brake with his right foot at the time that photo was taken. So what! Many GP bikes also have a left thumb control for the rear brake. There are very few top riders who don't use the rear brake - I've never seen an top rider on a bike without all that extra unsprung weight of a caliper and disk on the rear - and you KNOW they'd get rid of any extraneous parts if they weren't necessary.

^ this....
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Old 12-12-2012, 10:00 PM   #94
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Originally Posted by viverrid View Post
99.99%
Bullshit!

You do not need to assign a false fixed percentage to it...
Sound familiar?
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Old 12-12-2012, 10:21 PM   #95
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Originally Posted by ParaMud View Post
I am not talking about going around in a parking lot and using the back brake, I am talking about on the freeway and coming to a stop.

I have been telling my girlfriend not to use the rear brake on the street. Her front brake is more than enough to stop and she just needs to smoothly keep applying more pressure on it to stop, and even harder to stop faster.

The point of telling her not to use it during normal stops is because during an emergency brake, it is very hard to feather the rear brake and not lock it up and by not using it everyday, she won't slam down on it during an emergency brake out of habit.

So what is your opinion on this matter on teaching beginners about braking?

Personally, I very rarely use the rear brake.
Let me try this from a different angle for ya...
If you were 100% sure that you were doing the exact right thing by teaching your girlfriend to brake this way, you wouldn 't have posted up asking for everyone's opinions (unless you were just trying to stir the pot). You've heard many opinions, most of them telling you that you are wrong, but some that agree with you. Since you're not going to get any consensus here, and since you're not 1000% sure....my recommendation to you is to quit teaching your girlfriend and let a professional instructor do it.
Good luck.
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Old 12-12-2012, 10:25 PM   #96
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Quote:
Originally Posted by catweasel67 View Post
The maximum braking

The maximum braking allows to achieve the maximum deceleration, that is:


The deceleration is a function of the wheelbase p, of the vertical h and horizontal position of the center of gravity and of the coefficients of friction, not of the vehicle mass.

The ratio between the front wheel braking force and the total available braking force depends only from the geometrical properties and the coefficients of friction:



Figure 6. Deceleration lines and braking ratio
wheelbase p=1.4 m; height of the center of gravity h=0.7 m; horizontal position of the c.o.g. b=0.7 m
....


or. put another way.

use both brakes to maximise braking.

Thanks. This is really educational and useful. And I think it ends the argument.
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Old 12-12-2012, 10:32 PM   #97
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Originally Posted by Wakizashi View Post
Thanks. This is really educational and useful. And I think it ends the argument.
Yeah, except it's not the entire story and because of it's over simplification ends up being basically wrong.

http://dinamotoweb.dimeg.unipd.it/di...%20Beijing.pdf

"Figure 3 (a) shows that the optimal braking maneuver (dash line), which uses also the rear brake, is able to reach the
ellipse of adherence, except when stoppie condition occurs. There is a net gain in the maneuver performance which is
quantified by the highlighted yellow area in the Figure. However, the optimal braking strategy acts to the tire limits and
including the fact that the load torque of the engine in many cases is high enough to lock the rear wheel, only few racing
riders are able to put into practice this optimum braking strategy. The same cannot be said for acceleration maneuvers
since traction force is only available at the rear wheel."

Now make sure you properly read figure 3a because for straight up and down braking it shows stoppies occurring meaning there is no gain from rear brake usage.

crofrog screwed with this post 12-12-2012 at 11:17 PM
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Old 12-12-2012, 11:01 PM   #98
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3. Optimal front–rear braking
In racing motorcycles the role and the importance of rear
braking is very debated as, in practice, during a hard
braking on a straight line, most of the drivers make no use
at all of the rear brake. This brake is typically used for
better managing the attitude and the stability of the
motorbike during a bend. The reason is threefold:
  •  Due to the (almost) total load transfer to the front tire at
    mid-low speed in a hard-braking maneuver, the braking
    capability of the rear tire is comparatively small.
    
  • The simultaneous optimal management of the front and
    rear brake is a very hard task even for a professional
    driver.
    
  • If the engine is engaged during braking, the load torque
    of the engine (especially on high-performance highcompression 4-stroke engines) in many cases is high
    enough to lock the rear wheel; this phenomenon can be
    alleviated using mechanical devices called ‘‘anti-hopping’’ clutches, which can be considered as a raw antilock braking system acting on the engine-induced
    braking torque.
From http://www.elsevierscitech.com/pdfs/CEP_Corno.pdf

Pro-racers have problems accurately modulating both brakes but n00bs in a 2 day program can learn it easily right?

So which is it MSF the program is so basic and the riders skills are so low that you don't have time to teach things like trail braking.

Or the program is so good that it can teach "advanced skills" like accurate rear brake usage?

Or your program is so cruiser oriented that it's under serving the sport bike community.


The study shows that both brakes provide .3 seconds better braking performance from 300km/h which is about 23ft. That's entirely because at high speeds aerodynamic forces. The cross over point occurs at 200km/h above 200km/h the aerodynamic forces keep the rear down and front tire traction is the limiting factor, below that speed rear wheel lift is the limiting issue.
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Old 12-12-2012, 11:23 PM   #99
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Originally Posted by cybrdyke View Post
Let me try this from a different angle for ya...
If you were 100% sure that you were doing the exact right thing by teaching your girlfriend to brake this way, you wouldn 't have posted up asking for everyone's opinions (unless you were just trying to stir the pot). You've heard many opinions, most of them telling you that you are wrong, but some that agree with you. Since you're not going to get any consensus here, and since you're not 1000% sure....my recommendation to you is to quit teaching your girlfriend and let a professional instructor do it.
Good luck.
So I should send my girlfriend to what? What type of training? MSF? As what the instructors say at the end of the class "Congrats you can now ride around a parking lot.". Send her to a track school? At the school she will learn to use the front brake....
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Old 12-12-2012, 11:47 PM   #100
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Originally Posted by ParaMud View Post
... Now lets talk about locking up the rear brake.

Will locking it up slow you down faster than not touching it?
Yes. Braking without locking it up provides the most stopping power, but locking it up does provide some braking power as well. The problem with locking it up is the loss of control and potential for falling down. Which is why (a) using it is good, and (b) PRACTICING using it so you know how to use it is important.

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Old 12-13-2012, 12:07 AM   #101
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Originally Posted by crofrog View Post
Not really, doing the same wrong thing for 300k miles is no different than doing the same wrong thing for 5 miles (not saying what your doing is wrong per se).
But repeatedly doing something risky successfully for 25 years in the real world, where doing it wrong could and likely will fuck you up sooner or later, means doing it mostly right. I try to not have to emergancy brake, but I can't predict absolutely everything all the time, especially in heavy traffic like SoCal perpetually has. So it happens now and then. I consider the emergency stop to be a *very* important skill, as any time you need to do it, the consequences of doing it wrong are likely to be heavy. So I have taken care to know what I'm doing there. And the method I outlined is the best one there is.

Quote:
Originally Posted by crofrog View Post
How do clutch plates wear when they are fully engaged?
In a literal sense, the clutch comment is from the fact that engine braking usually involves downshifting, which involves the clutch and more specifically involves modulating the clutch with some slip as you re-engage it after the downshift so as not to break the rear wheel loose. Which does cost some clutch wear. More generally, engine braking involves reversing the stresses on the entire drivetrain, and pushing backwards on everything, which really isn't a great idea. So for best care of the bike, it's better to not do a lot of engine braking, and if you want to apply some braking force at the rear, use the rear brake, as that's what it is designed for, and not the engine, which is not optimized for that purpose.

Quote:
Originally Posted by crofrog View Post
I'd love to see some side by side test done with riders using both brakes versus just the front with repeated stops I think you'll find that adding the rear removes very little stopping distance while the chance of getting it wrong while stopping with the rear brake is allot more. You fuck up and lock both brakes you're going to have a much worse time than just locking the front.
That has been done MANY times, by various magazines and other groups. EVERY time, they find that using the both brakes makes you stop faster. On some bikes (cruisers, tourers, scooters) the rear brake adds a lot of braking power. On standards a middling amount. On sportbikes a small amount, but not zero. And on all but the best surfaces, the rear is more helpful. I think the last one I saw was from Cycle World, and a couple of the very best riders were able to equal their distances using the front only vs. both, but still only a couple of them, and only on sportbikes on good surfaces.

And no, if you lock the front and are less than a supreme expert, you are falling down regardless of what the rear is doing. So you won't have a worse time at all if both lock vs. just the front. Nope. Wrong. Fail.

Quote:
Originally Posted by crofrog View Post
My thoughts on why rear brake usage during panic stops is bad.

1. You're added _twice_ the amount of task for stopping you now need to manage 2 separate brakes systems and tire level tractions.
2. The majority of the bike's stability is coming from the gyro of the running engine and the spinning wheels. If you lock the front and the engine is still spinning and the rear tire is still turning you have a very good chance of reducing pressure and riding out of it because the bike is still stable and tracking straight.

You lock the rear which is very easy to do while hard on the front and kill the engine you've lost much of your stability and your ability to re-accelerate and if you then lock the front on top of that the bike is completely unstable. To avoid killing the engine you now need to manage a 3rd control.

The nature of an emergency is that you aren't planning for it. It's hard to properly modulate 7 controls (front brake, rear brake, clutch, steering, body position, throttle) during a full on stop when doing planned full on stop, like on a race track.

It's going to be damn near impossible to pull it off when you didn't except it, because face it. If you where in a position to do it all right you'd not be in an emergency stop because you'd have seen it coming and already taken action to avoid the situation.
Have you actually ridden a motorcycle before? Do you know what the clutch is? Even the newest rider knows to pull in the clutch while braking, and that's a universal reflex, requiring no extra attention, among people who have actually ridden more than once. Sheesh.


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Old 12-13-2012, 12:12 AM   #102
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Originally Posted by ParaMud View Post
So I should send my girlfriend to what? What type of training? MSF? As what the instructors say at the end of the class "Congrats you can now ride around a parking lot.". Send her to a track school? At the school she will learn to use the front brake....
There are intermediate courses, starting with the advanced MSF course, which does include instruction and practice in using the rear brake, including controlling the bike if you do lock it up. Also there are track schools that focus on street riders and skills, and street skill classes like Lee Parks' Total Control. I'd recommend any of those.

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Old 12-13-2012, 12:18 AM   #103
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Originally Posted by crofrog View Post
Last time I checked when I have 100% weight transfer during stopping my back brake is supplying 0% of the braking power.

Assuming your riding a bike that can lift the rear wheel (aka not a harely). If you don't have 100% weight transfer (or damn close to it) then you're doing a bad job of "emergency" braking.

I've seen a Heritage Softail with its ass 4 feet in the air. For that matter the with the ABS touring bikes ...good luck stopping on a CBR as fast as one of those things.

So far as it goes a buddy who had his rear brake fail in a race "For something that I never use, I sure as hell missed it when it wasn't there." and it wasn't like he went offroading, he finished 2nd.
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Old 12-13-2012, 01:21 AM   #104
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And no, if you lock the front and are less than a supreme expert, you are falling down regardless of what the rear is doing. So you won't have a worse time at all if both lock vs. just the front. Nope. Wrong. Fail.

Have you actually ridden a motorcycle before? Do you know what the clutch is? Even the newest rider knows to pull in the clutch while braking, and that's a universal reflex, requiring no extra attention, among people who have actually ridden more than once. Sheesh.


PhilB
If you momentarily lock your front wheel you release the front brake some. You don't need to fall over. Your statement makes it sound like you do not practice true threshold front braking.
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Old 12-13-2012, 01:25 AM   #105
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If you momentarily lock your front wheel you release the front brake some. You don't need to fall over. Your statement makes it sound like you do not practice true threshold front braking.
There are a lot of qualifiers there.
You can recover the front, but its a bit hairy, and the faster you are going the more time you have to correct it.

....assuming you are straight up and down. Because if you aren't that is about the fastest way I can think of to break a collar bone.

On the street anyway, in dirt, meh, you plow the front end all the damn time anyway.
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