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 'Grabbing a handful' - what does that mean?
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jilp
Male Junior Member
84 Posts


Mexico, Mexico
Mexico

BMW

R1200GS

Posted - 12/02/2010 :  4:54 PM
Hi James,
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This is a much more likely representation of what happens when you 'squeeze too hard' trying for a world record quick stop.
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So there are two basic variables in this process, time to get to max deceleration, and force applied (distance traveled of the brake lever?). Does this means that If I can achieve 0.3 sec of time to max deceleration then I could apply less force (squeeze less hard) and get the same stopping distance than extending the time to reach max deceleration but squeezing more hard? Is it safer to practice with less squeeze force when trying to squeeze faster to not lock the front tire? Sorry for my confusion.

Thanks a lot
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 12/05/2010 :  8:20 PM Follow poster on Twitter  Join poster on Facebook as Friend  
quote:

posted by jilp

... Does this means that If I can achieve 0.3 sec of time to max deceleration then I could apply less force (squeeze less hard) and get the same stopping distance than extending the time to reach max deceleration but squeezing more hard? Is it safer to practice with less squeeze force when trying to squeeze faster to not lock the front tire? Sorry for my confusion.


Sorry I didn't respond sooner - I simply missed your post.

Yes, you can use less maximum squeeze pressure if you squeeze faster and still stop just as quickly. And, yes, it is safer to practice with less squeeze force, but not much. Assuming you already know how much squeeze force you can use and not lock up the front tire, if you don't use more than that when you squeeze quicker, you do not increase the odds of tire lock but, instead, shorten your stopping distance.

When it becomes less safe is when you are very close to 0.3 seconds of squeeze time as then you are close to applying braking pressure before your weight transfer activity has actually completed.

Some words about that ...

Weight transfer takes ZERO time to occur. That is, the INSTANT that you are decelerating at a particular rate, weight transfer has occurred based on that rate. But it still takes real time for your shocks to compress and for that weight to be fully felt by the tire's contact patch. In addition, because your shocks are compressing, your bike's wheelbase is shortening. That, by itself, increases your front tire's traction.

Let's say that you and your bike together weigh 800 lbs. and that at a dead stop that weight is distributed 440 lbs. on the rear tire and 360 lbs. on the front tire (a weight bias of 55% to the rear.)

If your rake angle was 30 degrees, then every inch of compression of the front shock would shorten your wheelbase by almost one-half an inch. (Less, because after the compression your rake angle would be less than 30 degrees.)

If your bike's wheelbase was 66 inches long, then before braking your CG would be 29.7 inches from your rear tire contact patch and 36.3 inches from your front tire contact patch (55% weight bias to the rear). But after your front shock compresses, let's say 4 inches, the wheelbase would only be about 64 inches long. Then your weight bias would be closer to 54% toward the rear, and that means that even without weight transfer there would be 368 lbs. instead of 360 lbs. on the front tire.

That may not seem like a lot, but it illustrates the point that SOME of the added weight to the front tire during braking is the result of shortening the wheelbase instead of weight transfer, and shortening the wheelbase takes REAL time (is not instantaneous.)

A couple more facts and then we'll put it all together for you. We will assume that your CG is 33 inches high (giving you a weight transfer ratio of 0.5) and that your tire rubber on a good clean asphalt surface can handle almost 1.2g's of braking force before it loses traction.

If you could instantly brake (use no time to reach your maximum squeeze pressure), then as soon as you developed 432 lbs. of braking force (1.2 times 360 lbs.) on the front tire, it would skid. But within 0.2 seconds, or so, you would have completed weight transfer as a result of your deceleration rate. If you reached a deceleration rate of 0.8g's, that means that 0.8 times your weight transfer ratio of 0.5 times the total weight of you and your bike = 320 lbs. of weight is transferred from your rear tire onto your front tire. Your front tire, then, can handle a braking force of 1.2 times 680 lbs. before skidding or 816 lbs. But as we earlier saw, we added another 8 pounds to the front tire load because we shortened the wheelbase. So we could actually handle 1.2 times 688 lbs. or more than 825 lbs. of braking force.

If you know what it feels like to get a deceleration rate of 0.8g's, then you know what it feels like to get a braking force of more than 800 lbs. from your front tire. But until weight transfer has had time to compress your shocks, you only have about 360 lbs. on the front tire so you can only apply about 432 lbs. of braking force with it. Obviously, the tire will skid if you apply enough braking pressure to try to create more than 800 lbs. of braking force in less than about 0.3 seconds.Just as obviously, you can apply that much braking pressure after the weight transfer has compressed the shocks.

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jilp
Male Junior Member
84 Posts


Mexico, Mexico
Mexico

BMW

R1200GS

Posted - 12/06/2010 :  9:59 AM
Wooow, very clear explanation James, as we say in Mexico, you did it with apples for me to understand!

Thank you very much!
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 12/06/2010 :  7:35 PM Follow poster on Twitter  Join poster on Facebook as Friend  
Oops - I missed your post as well.

quote:
Posted by scottrnelson

... the amount of weight on the front wheel will be less initially, then increase as the full weight of bike and rider compress the springs to the maximum amount, then it will decrease slightly as the front end rebounds to its final position.


Hmmm. I guess we see the dynamics quite differently.

quote:
...the amount of weight on the front wheel will be less initially,


I agree.
quote:
then increase as the full weight of bike and rider compress the springs to the maximum amount,


Here we start to disagree. The full amount of weight transferred will be reached BEFORE the springs compress to their maximum.

quote:
then it will decrease slightly as the front end rebounds to its final position.


Here we really disagree. When the shocks begin a rebound from maximum compression, there will then be MORE downward force (apparent weight) on the tire and resulting traction as you will then have weight PLUS the decompressing springs loading the tires. It will return to the full weight transfer amount when that rebound ends and minor oscillations continue until the damper in the shocks stabilizes.

In other words, you could use maximum braking pressure before you get max shock compression, and maintain it while the oscillations dampen out.

This concept of overshooting at the top and bottom of shock compression is actually not particularly significant if your shock oil flow restrictor valves (dampers) are working properly.
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scottrnelson
Advanced Member
6886 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 12/06/2010 :  10:30 PM
quote:
Originally posted by James R. Davis

Here we really disagree.
Thanks for straightening me out on this. I didn't actually put a lot of thought into it (and put a disclaimer on it, knowing that it might be wrong). I would like to see instrumented measurement of the weight transfer sometime to truly understand what happens as the bike pitches forward.
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 12/06/2010 :  10:50 PM   Join poster on Facebook as Friend  
These people had pretty sophisticated equipment to measure all sorts of data in brake testing of motorcycles. They did 820 stops from 100kph in all.

One of the sensors measured half travel of the front suspension. Look at the chart for the 30 shortest stops. They measured about .34 of a second to half travel.

In my thinking, conveniently close to the .3 of a second mentioned on this thread to 'grab a handful".

http://www.fmq.qc.ca/pdf/amorce-freinage_eng.pdf
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gymnast
Moderator
4263 Posts
[Mentor]


Meridian, Idaho
USA

Harley-Davidson

Sportster Sport

Posted - 12/06/2010 :  11:21 PM
Roger, excellent link, thanks for sharing.
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Peer Review: Blocked

Posted - 12/06/2010 :  11:28 PM   Join poster on Facebook as Friend  
Anytime gymnast, although that source has been referenced on this site before a few times, I think.

It's got a lot of interesting bits, to me, in it. Big heavy cruiser vs sportbike braking for one. Declutching to shorten a stop for another.
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 12/07/2010 :  11:49 AM Follow poster on Twitter  Join poster on Facebook as Friend  
I want to try to make it clear why you can squeeze the front brake lever as quickly as 0.3 seconds without significantly increasing your risk of skidding the front tire given that it can take as long as 0.7 seconds to fully compress your shocks during aggressive braking.



This diagram above shows (the red line) how much downforce (essentially weight) is on the front tire over time given that the total weight is 800 lbs., the weight bias is 55% toward the rear, and a 0.5 weight transfer ratio (height of CG divided by length of wheelbase), and that maximum braking pressure (squeeze) occurs in 0.7 seconds and causes a maximum deceleration rate of 0.89g's.

We see that before any deceleration actually occurs, there is a total of 360 lbs. of downforce on the front tire. We also see that after 0.7 seconds, there is a downforce of 716 lbs. on that tire.

Now we look at the blue line. It represents the maximum braking force that the front tire can produce over time before it skids. This is calculated as 1.2 times the downforce.

Though braking pressure (the straight black line) is shown as linearly increasing, that is a simplification of reality. It suggests that you squeeze harder in a linear fashion - which is approximately correct.

The red and blue lines are curved, however, to demonstrate that weight transfer reaches the tire contact patch in a non-linear fashion.

Notice that at no time does the braking force generated by the front tire even approach the blue line - skidding is not likely.

Now let's look at what happens when we reach maximum deceleration rate with a faster squeeze time of 0.5 seconds.



Note that for a brief time near when the maximum deceleration rate is achieved the braking force exceeds the weight on the front tire, but that it does not exceed the maximum force required to cause a skid.

Finally, let's look at what happens when the squeeze time is as little as 0.3 seconds.



This is a concept discussion - the numbers are estimated and approximate. No attempt was made to estimate the effect of shortening the wheelbase.

Is this too subtle a concept to be easily understood? Is there a way to better explain or demonstrate it?
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jilp
Male Junior Member
84 Posts


Mexico, Mexico
Mexico

BMW

R1200GS

Posted - 12/08/2010 :  10:08 AM
James, as allways, excelent lecture!

Could this thread be posted as sticky? there are so much useful information in it for all of we, bikers, who want to understand the real braking issue. It should be preserved for easy access.

Regards
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 12/08/2010 :  10:29 AM Follow poster on Twitter  Join poster on Facebook as Friend  
This material is all extracted from volume two of the book I'm writing, so it will not get 'lost'.

I may add it to the safety tips articles as well.

This is not the full story, by any means, but it demonstrates the concepts that I've tried to convey here. A couple of important additional points should be made, however.

This last presentation presumes that all braking is done with the front brake. Obviously, a rider is well advised to use both brakes whenever doing a quick stop, because you need some anti-yaw force from the rear tire to help keep the rear-end of the bike tracking with the front, and because to the extent that you get any braking force from the rear tire, you decrease the braking force needed from the front tire.

In terms of the diagrams above, the red and blue lines are not effected in any way when you use both brakes instead of just the front one, the gray area (total braking force) remains the same, but the black line which shows braking force, would be broken into two different lines (black for front and white for rear brake) and the black line would be farther away from the red and blue lines - meaning, skidding would be much less likely.

Shown below is an example where the front brake provides 90% of the braking force.



Finally, as I mentioned earlier, the effect of shortening the wheelbase caused by compressing the front shocks is to make both the red and blue lines higher than shown. That, of course, also reduces the likelihood of skidding.

Just to make sure we are all on the same page ... it's when the black line crosses the blue line that the front tire skids.
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 12/22/2013 :  11:29 AM Follow poster on Twitter  Join poster on Facebook as Friend  
It's been a couple of years since my last post here on this topic and I thought it was time to revisit it - and add some value.

Volume two of my book was completed and published two years ago and did include virtually all of the material found in this thread. Additionally, though this thread uses squeeze times ranging from one full second to as low as 0.3 seconds, and emphasizes how significantly lower the maximum braking pressure required to achieve a quick stop as compared to when taking more time to reach maximum, in the book I took great care to emphasize that 0.4 seconds of squeeze time is as fast as I would recommend trying to achieve, for safety reasons.

The concept of 'threshold braking' was not previously discussed in this thread, but was talked about extensively in others that were launched as a result of this one.

The traditional definition of threshold braking has been "decelerating at a rate as close as possible to that which would cause a skid, without actually skidding". That definition, in my opinion, is dumb and dangerous. A much healthier (and safer) definition is "decelerating at ANY rate which exceeds the rate you would achieve if skidding". To understand the distinction consider the following graph showing the meaning of 'Coefficient of Friction':



Our roadways have a pretty large variance in terms of Coefficient of Friction depending on the materials used in their construction, their age, wear, and 'surface litter' (from surface moisture to sand and other particulates on that surface). The graph shows one such surface as if it is representative of most (though there is evidence that the average 'sliding or dynamic' CoF around the country is closer to 0.76 than 0.825) - it clearly shows that the 'sliding or dynamic' CoF is about 25% lower than is the 'rolling or static' CoF.

'Coefficient' is just a $20 word that essentially means 'ratio' or 'as compared to'. In the case of Coefficient of Friction, it means 'The amount of horizontal pressure (from turning, braking, or accelerating) compared to the normal vertical pressure (weight)'.

That is, from the chart you see that if the horizontal pressure from braking, for example, is more than 1.1 times the weight on a tire, that tire will begin to skid. It also shows that when that tire skids it will develop a horizontal pressure of about 0.825 times its weight.

Since g's are also a coefficient - meaning an acceleration or deceleration rate that is compared to free fall near the earth's surface, the graph also says that if you brake at a rate in excess of 1.1 g's (on that surface), you will begin a skid and that the deceleration rate during that skid will be about 0.825 g's.

If you use the traditional definition of threshold braking, you would attempt to decelerate AT AS CLOSE TO 1.1 g's as possible, without actually hitting, or exceeding 1.1 g's. If you use my definition, you would attempt to decelerate at ANY RATE between 0.825 and 1.1 g's - anywhere in the gray area of the chart.

If you slow down at any rate in excess of 0.825 g's (on this surface), you are stopping in less time and distance that you would if you were merely skidding! Using the traditional definition, you have a ZERO MARGIN OF ERROR - a tiny bit too much brake pressure and you skid!!

In a car, so long as you don't hit anything, so what if you skid to a stop? But a motorcycle loses all directional control and stability if the front wheel skids - IT CRASHES.

Ideally, you would like to stop as quickly as possible and in as short a distance as possible, without crashing. A motorcycle equipped with ABS tends to stop (during a quick stop effort) at a rate just a bit below 1.0g's. You can do the same or even better with a motorcycle that is not equipped with ABS, but you NEVER want to skid that front tire.

Here is a chart from Mechanical Forensic Engineering Services, LLC, published in 2007, that shows the distribution of skidding or dynamic Coefficient of Frictions measured on a wide variety of roadways in the U.S.



If you practice quick stops at 20 MPH and learn what squeeze time and pressure is required to average 0.8g's during that effort, then if you use exactly the same squeeze time and braking pressure in order to quick stop from any starting speed above 20 MPH, you automatically will achieve threshold braking during that emergency - decelerate at a rate within the grayed area of the first chart.
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wmcooper
Male Junior Member
33 Posts


perry, ga
USA

Honda

shadow aero

Posted - 07/06/2016 :  2:59 PM
Can't believe what I just read on another forum. Is it ok to post a link here to the comments there? Guy totally got the wrong idea about braking!
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/06/2016 :  3:02 PM Follow poster on Twitter  Join poster on Facebook as Friend  
Of course.
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wmcooper
Male Junior Member
33 Posts


perry, ga
USA

Honda

shadow aero

Posted - 07/06/2016 :  3:13 PM
ok, I wanted to check before I posted a link to another forum. The guy that started the thread is obviously confused about how much brake to use in an emergency stop. He seems to think he should stomp on the rear brake and not use the front.

http://www.hondashadow.net/forum/53...#post4642785
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/06/2016 :  3:26 PM Follow poster on Twitter  Join poster on Facebook as Friend  
I have been an expert witness in more than one case where that was exactly the belief presented by the motorcyclist.

My response, which I go on to demonstrate, is that 'you should never, ever, not once in your life, aggressively use the rear brake'.

That is Gospel advice, friends.
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wmcooper
Male Junior Member
33 Posts


perry, ga
USA

Honda

shadow aero

Posted - 07/06/2016 :  3:37 PM
I only recently took the MSF course, well it was about a year ago. They taught to use both brakes at the same time. If you lock the rear then keep it lock, if you let off on it you'll high side but if you lock the front you should ease off it enough to release the lock but then continue to squeeze.
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Alan_Hepburn
Male Standard Member
191 Posts


San Jose, Ca
USA

Honda

1994 GL1500SE

Posted - 07/06/2016 :  4:45 PM
What about the case of linked brakes? On my Goldwing the rear brake pedal operates the rear brake, and the left front brake. The front brake lever operates the right front brake. I've been applying moderate pressure on both controls for my stops - should I be modifying my braking?
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/06/2016 :  5:25 PM Follow poster on Twitter  Join poster on Facebook as Friend  
No change.

You NEVER aggressively use the rear brake. You virtually always use both brakes. You modulate/moderate (reduce) the pressure on the rear brake over time.

You assume that almost all of your stopping power comes from the front brake,
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James R. Davis
Male Administrator
17283 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/07/2016 :  8:47 AM Follow poster on Twitter  Join poster on Facebook as Friend  
quote:
Originally posted by wmcooper

I only recently took the MSF course, well it was about a year ago. They taught to use both brakes at the same time. If you lock the rear then keep it lock, if you let off on it you'll high side but if you lock the front you should ease off it enough to release the lock but then continue to squeeze.



As many people here know, I find plenty of fault with the MSF and its teachings, but the fact is that they have to teach FUNDAMENTALS to NEWBIES, so I believe that particular bit of advice from them is almost adequate and correct.

What it lacks is that no matter which brake you lock, you should release the FRONT brake pressure. If you leave that brake engaged, then you are setting yourself up for a crash as you will have more braking pressure coming from the front brake than the rear and if your bike begins to yaw, it will continue to do so aggressively.

As you gain experience and control, then if you happen to lock your rear brake AND NOTICE IT WHILE YOU ARE STILL TRAVELING IN ALMOST A STRAIGHT LINE (meaning that the rear tire is almost aligned with the front tire), of course you can release the rear brake. But be sure you have a good grip on your handlebar as you will experience a kick (or a pretty impressive jolt) as the rear tire regains traction and immediately gets pulled into alignment.

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