Chassis Geometry 101, and why using the rear brake on the racetrack can be ok. - Page 2 - ZX6R Forum
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post #16 of 39 Old 10-18-2014, 10:51 AM
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I hope to not get too off topic with this question, but to those that say its better to leave the rear brake alone, what then is the purpose of it being there in the first place? Are we talking about just avoid using it during race/track days and canyons, or just in general?

I'm genuinely interested because I've never had a problem with it.
FWIW, I use the back when I have run out of front, or know I need that little bit extra stopping power that may be available. I ride street only, rarely at 80+ % in cornering situations as that would put me so far above the posted limits that I'd end up tee boning someone pulling out of a blind driveway in my path.

If I've done it right, I've already used engine braking to some degree before I've started applying the front brake. Once I've done that enough to have the initial 'dive' from the front -- if the back is still on the ground, it's available to add a bit more braking force.

Most self taught riders tend to use the back more than they should. Your legs aren't really doing all that much, and since your legs are more powerful than your arms, it's easy to apply the rear.... since there is such a strong probability that the brakes will be applied with much more force than the front, the rear rotor is smaller, and the pad area in the caliper is less so you cannot lock it up as readily. Even with all of that, it's still relatively easy to skid the back tire.
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post #17 of 39 Old 10-19-2014, 11:01 AM Thread Starter
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Apparently there is some misunderstanding here.

This is an advanced technique, used for track riding only.

On the street, it doesnt matter which brake you use as you should be stopping using less than 10% of the actual bikes ability to slow down.

For street, I use the rear brake when I am holding myself on a hill and I want my hands free, or if I encounter dirt/gravel and need to slow down.

That is it.

As I said, in a panic situation if you are trying to stop as quickly as possible then your rear tire should be in the air, meaning your rear brake is utterly useless. If your tire is not in the air, you are not utilizing the full potential of your front brake which is the bulk of your stopping power.

ON THE TRACK this technique can help take an advanced rider to the next level. Nothing more.
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post #18 of 39 Old 10-20-2014, 02:29 PM Thread Starter
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I have gotten some PMs with a few questions that I thought might be better answered here.

First, and quickly, why my screen name is PainfullySlo :-p. Speed is relative. Sure, I am faster than most but I am also slower than the fastest people. I dont want to be, so as far as I am concerned until I start to beat them regularly I am still just painfully slow. Actually, one of my earlier helmets had Kanji on it that literally translated into "slow" =)

Now on to the important questions. For starters it would be to discuss what happens when a rear tire slides. Does this mean you are crashing? But we see the MotoGP/Racer guys slide the rear (and even front!) all the time but they dont crash. Why?

There was a time, earlier in my racing when I used to describe my bike as handling "like it was on rails" as a way to say just how firm and planted the bike felt, it felt good and was very confidence inspiring. The faster guys smiled and nodded much like an adult amusing a child who just stated something ridiculous and I went on my merry way none the wiser, no doubt a few jokes were had at my expense but I was happy and my racing was progressing. The truth is that I wouldn't have been ready if they told me anything other than what I knew at the time.

The truth is that when you start to get really fast, your bike is rarely stable; it is more like riding as surf board than a motorcycle. Your bike slides around, wiggles, wobbles, and at times is downright violently trying to spit you off. That is the price of riding close to the edge.

The difference between when my bike was handling on rails and the way it is now is back then, I was adhering to the laws of physics. Now I am learning to bend them.

It is a dangerous road, and can have catastrophic results if not done properly but when used correctly, a rider can indeed cheat the laws of physics to go faster and do so quite safely...once you get past the "OH MY GOD I'M GONNA DIE" every 3-4 seconds.

Now, this is nothing new to the "fast guys". At my local track, we call them collectively "the aliens". You know the guys. Ridiculously fast, and they make it look so easy, like they were just taking a quick ride to the corner store and back. The kind that pass you on the inside of a turn when you swear you only left 6" of pavement and felt like you were going to break the sound barrier, yet they stroll on past you like you were going backwards in time.

The key is learning to control your panic response. Read that sentence over and over until you get it committed to memory. Control your panic response. Your body's natural survival instinct that will cause you to do things that your motorcycle will not like, often with bone-shattering results. When you feel your bike start to slide, your body will tell you to tense up. Grab those bars and wrestle control back! That would be very, very wrong. A rider needs to remain loose and in control at all times. Practice this, learn to force yourself to not tense up when something goes a little off, and you will find that you will have a lot more fun AND the added benefit of not crashing :-p

I am going to take a step back and lay down another statement: No rider in the world, no matter how talented or skilled, can make a motorcycle perform better. Put your ego on the shelf here because no matter how amazing you (or I) think we are, we will not/cannot make that bike do anything that it is physically incapable of doing.

The simple fact is that a motorcycle in motion is the equivalent of engineering art. It knows exactly what to do to not crash. It will merrily handle just about anything that you can throw at it and come out just fine on the other side if the meat in the seat doesn't do anything too stupid to upset what it is doing.

The best riders in the world interfere the least with a motorcycles natural tendencies. Again, read that sentence over and over until you believe it, because it is truth. Rossi, Marquez, Pedrosa...they are all amazingly talented guys and it is largely because they know when to just let go and let the bike sort itself out.

That is the magic. That is the big secret of sliding the rear and not crashing. Do. Nothing.

I want to make a few comments that might seem out of place in this discussion.

Over-controlling your motorcycle is dangerous.

Over-reacting is similarly dangerous.


Your motorcycle, more times than not, attempts to correct instability by itself requiring NO INPUT WHATSOEVER from the rider.

Centrifugal force does NOT push you away from the center of a curve.

Virtually all riders know that if a bike begins to slide in a turn you should turn your front wheel in the direction of the slide. What too few riders seem to know is that *YOU* don't have to do anything and the bike will, of its own accord, turn the front wheel in the direction of a slide. Your only real job is to not inhibit that self-correcting effort by the bike.

Should you try to 'steer into the slide' and either over- or under-shoot the amount of turn required to offset the slide you place the bike into an even less stable configuration. In other words, over-controlling is dangerous.

Similarly, over-reacting to a bit of instability almost invariably makes things worse. When you ride over rain grooves and your front-end becomes squirrelly, if you put a death-grip on your handlebars you merely cause the instability of the front-end to be broadcast through your arms into the rest of the motorcycle. If your rear-end squirts briefly to the side (slides) while in a curve, corrective action on your part can turn it into a disaster just as easily as it might 'cure' the problem.

While the rear wheel continues to spin there is essentially no danger that your bike is going to fall down - gyroscopic forces are tremendously strong. Further, unless your slide is the result of hitting an oil slick or ice, you have not LOST traction, just diminished it. You are still able to accelerate (or VERY MODESTLY decelerate) while in a slide.

And though it certainly feels like centrifugal force is attempting to push you away from the center of a curve, in fact what it does is attempt to make you go in a straight line tangential to that curve.

Thus, as your slide progresses there is less and less centrifugal force at play. That means that more and more traction is becoming available to the tire. In other words, if you do NOTHING (other than allow your front-end to steer itself in the direction of the slide), the odds are overwhelming that the slide will end of its own accord.

There are three things that you could do:

Slow down - WRONG, WRONG, WRONG - this causes weight transfer to the front and will reduce what traction you have left and the bike will almost certainly end up on its side.

Nothing but allow the front-end to steer itself into the slide - works most of the time and requires no skill whatever.

Modestly accelerate - increases rear-wheel traction and shortens the slide - but requires a gentle touch (skill).

Honest! The best course of action for almost anybody is to let it slide.

Look at any motorcycle race film and you will observe that 100% of the turns are negotiated with the rear wheel sliding! Nothing magic about that, now that you know what's going on. Right?

When someone now tells me how their bike "handles on rails" I tell them that they just aren't going fast enough. Yes, it makes me sound like an egotistical prick, but I then talk to them to explain these things I am about to write down.
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post #19 of 39 Old 10-20-2014, 02:52 PM Thread Starter
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Now, guys and gals that have ridden in the dirt are laughing that I had to write all that down up above. Dirt riders steer with the back tire all the time, most of them just don't think about it because it is 'just what you do'. Traction is rarely solid on the dirt so that style of riding simply becomes sliding the rear everywhere.

The trouble comes when the rear is planted one moment, and has diminished traction the next. The trouble isn't the bike, it is the rider and coping with the sensation that we all immediately recognize as "Oh shit, I'm gonna crash" when the truth is that there is actually quite a good sized window of grey area where you are sliding but not crashing. I call it my 'slush fund of traction'. You finance people can chuckle, it's ok.

These days, I am actively seeking to break the rear tire loose coming out of a turn. By doing so, I know that I have achieved maximum potential speed. I am accelerating just as fast as I can possibly go.

But wait, why then do people crash? We have all seen (and some of us done) crashes where you lowside because the rear dropped out on you. You had traction one moment, and none the next.

Well, there are a few contributing factors that can cause this and again, we will all need to put our ego's aside because WE are the reason for crashing (barring something reducing traction suddenly like ice/oil/water on the surface).

First, I will give you a little chuckle, taken from my Middleweight SuperSport race from June of this year.

MWSS - YouTube

Why did I crash? A 'helper' in my garage neglected to turn on my rear tire warmer and it was stone cold. Race tires suck sweaty monkey nuts when cold. They are about the equivalent of riding in the rain on slicks. Needless to say, I reaffirmed my earlier decree that no one touches my bike but me so this issue will not come up again.

So, other than the times when traction is simply not there due to tire temps, water, oil, etc...the rider is the cause of nearly every other crash.

Bar input, chopping the throttle, too heavy on the throttle, leaning past the coefficient of friction point...basically doing anything other than just letting the bike do what it needs to do can and will cause a crash.

The main culprit is bar input. I see 'professional' racers make this mistake all the time, and they complain and swear about how the moon was full, or the blinker fluid was low on their bike...and I am sad for them because they will never grow as a rider until they can find the root cause and fix it. I guess it is too difficult to look in a mirror and place blame for some.

Anyway, we heave on the bars pretty hard when tossing the bike into a turn. I would be quite embarrassed if there were a microphone in my helmet when I am racing because I literally grunt and scream when I throw my weight on a bar to initiate a turn. The key is that just as quickly as you throw all that force into the bar to get your bike to lean, you have to release all that pressure and have no weight on the bars at all once you have attained your lean angle.

If you are going into a turn at close to the maximum lean angle you do not have the luxury of introducing any opposing force. The tire needs all of its grip just to keep the bike on its course. You should be completely loose on the bars at this point.

I had a guy I was teaching tell me that it wasn't possible so this was what I showed him on the next laps out.



If it werent for controlling the throttle, there would be no need to have your hands on the bars at all. Again, the bike knows what it needs to do. LET IT HAPPEN!

More to come...
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post #20 of 39 Old 11-15-2014, 09:02 AM
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PainfullySolo, that post above was very informative.
I read before that steering input/force is very crucial. And to think of the bars as handle bars, not so much a steering wheel.

Once in the turn dont put force in the bars, support your self with your outside leg and weight on the tank. Even more try and lean the bike with weight not the handlebars.

Ive only been riding since this past march (2014), however its something Im very interested in, and love learning about. I take riding very seriously. And im looking to hit a track near me next season.
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post #21 of 39 Old 11-15-2014, 10:56 AM
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Originally Posted by PainfullySlo View Post

When we accelerate hard and the back end of a motorcycle rises, we experience the following conditions: Weight transfers rearward. Rake decreases. Trail increases. Swingarm angle increases. Ride Height increases. Wheelbase decreases.
I'm having trouble following this part. When you accelerate a motorcycle, doesn't the back end squat? If so, wouldn't the rake increase? Wouldn't the wheelbase also increase? Ride height decrease?
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post #22 of 39 Old 11-15-2014, 11:30 AM
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I'm having trouble following this part. When you accelerate a motorcycle, doesn't the back end squat? If so, wouldn't the rake increase? Wouldn't the wheelbase also increase? Ride height decrease?


Go to 7:25
http://m.youtube.com/watch?v=ndU65pUKn9k

The TOTW books should be standard reading.
The video is a little hokey but is technically good.
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post #23 of 39 Old 11-15-2014, 03:00 PM
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Slo, there was a "more to come..." tacked on after your last post; is that still on its way?
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post #24 of 39 Old 11-17-2014, 08:19 AM Thread Starter
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Slo, there was a "more to come..." tacked on after your last post; is that still on its way?
Ack, I am so sorry. I completely forgot about this thread. Too much going on :-(

I do have some other questions that were asked of me and I will put them down here as I am able.

If you or anyone has motorcycle related questions to ask, please feel free to do so. You can either post in this thread or send me a PM. I am far from a guru, but I will do the best I can to answer them for you.

Quote:
Originally Posted by green93tsi View Post
PainfullySolo, that post above was very informative.
I read before that steering input/force is very crucial. And to think of the bars as handle bars, not so much a steering wheel.

Once in the turn dont put force in the bars, support your self with your outside leg and weight on the tank. Even more try and lean the bike with weight not the handlebars.

Ive only been riding since this past march (2014), however its something Im very interested in, and love learning about. I take riding very seriously. And im looking to hit a track near me next season.
Yes, you should absolutely think of the handlebars as...handlebars =) I think I get what you are saying though, and that unlike a steering wheel, you do not actually steer with the bars...other than initiating a turn.

Your understanding is correct. You exert force on the bars only to get the bike to turn. The faster you go, the more force you can and will need to use to get the bike to 'tip in' to a turn (because you have all that gyroscopic effect to counter).

Once you achieve your lean, you need to be light on the bars. Your tire will be using all of its available friction to keep you on the pavement and your bike going through the turn. If you ask it to do more, like change direction by putting weight on the bars, well...something has to give and usually the rider wouldn't like the results.

FYI, nearly every single front end crash (washing out or losing the front tire in a turn) is caused by bar input. I would say in the 90%+ range. Practice being off the bars in turns and you will have a much happier time on the track.

Now, for your first time on the track you will have LOTS of things to worry about, and this shouldnt be a major concern at that point. You will feel like you are leaned WAY over, but the reality will be that you will have plenty of tire left to cope with a little bar input.

As you pick up your pace, it will become critically important that you learn to have little to no bar input at all but in the beginning you arent going to fall over unless you do something drastically wrong.

Relax and have fun at your track day, that will do you a lot better than worrying about excessive bar input at this stage of the game =).

Quote:
Originally Posted by exalted512 View Post
I'm having trouble following this part. When you accelerate a motorcycle, doesn't the back end squat? If so, wouldn't the rake increase? Wouldn't the wheelbase also increase? Ride height decrease?
No sir. Your thinking is sound if you were just worrying about the weight transfer, but we have chassis geometry to consider and it gets a little deep into physics and can be hard to wrap your head around.

I made a quick picture which I hope will help.



So, in example A picture your motorcycle at rest. The 'swingarm pivot' is where your swingarm attaches to the frame of your motorcycle and the 'tire contact patch' is just that, where your rear tire sits on the ground. The green line represents the top of the motorcycle. It is important to note that the red line is the same length in both pictures.

When you accelerate a motorcycle, it does so through 'Thrust', that is, the force of your rear tire pushing, or shoving the motorcycle forward.

Because your swingarm is on an angle, as the tire pushes forward (and brings the rest of the motorcycle with it) it pushes on the pivot which will rise.

The yellow arrow indicates the thrust of the rear tire pushing forward. As this happens, that red line increases the angle that it is on and the tip of it goes upward. The blue line is the new 'top' of the motorcycle so you can clearly see that it has risen above the green line.

Does this help? I am doing the best I can to explain a pretty complex topic but I am no teacher. I can something else if further explanation is needed.

If you wanted to see this for yourself, the best way is to gradually work yourself up to a burnout. Now, I will toss in the disclaimer of 'don't try this at home' and all of the other stuff that you should know before attempting to do this. If you wreck your bike, it is on you.

Sitting on your bike, put it in first gear. Hold the front brake lever in. You will keep the front brake on the entire time. Gradually let the clutch out until the bike starts to try to move forward. Give the bike more gas, slowly. Keep the front brake on. As you provide more and more gas, you will start to feel the back end of the motorcycle rise. This is the exact same process that happens when you get on the throttle on the track.

It will actually rise quite a bit because you are adding thrust without the normal counter of weight transfer.

Again, be extremely careful if you are going to do this but it is the best way to see for yourself just how thrust can make the back of a motorcycle rise.

**EDIT** Found a dyno video of mine that is REALLY crappy quality but it does clearly show the back end rising. Check this out:

http://www.youtube.com/watch?v=wjpVVCbykMc

or another of the GSXR. Better quality video, a little more difficult to see the back end rising, but it is there.

http://www.youtube.com/watch?v=3YjsZrZvwRo

Last edited by PainfullySlo; 11-17-2014 at 08:43 AM.
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post #25 of 39 Old 11-17-2014, 08:48 AM
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Originally Posted by PainfullySlo View Post
When you accelerate a motorcycle, it does so through 'Thrust', that is, the force of your rear tire pushing, or shoving the motorcycle forward.

Because your swingarm is on an angle, as the tire pushes forward (and brings the rest of the motorcycle with it) it pushes on the pivot which will rise.
This is what made sense after I started thinking about it. The video didnt really explain it and I'm not one to take answers of 'because'...lol. But when I started to really think about it, it made sense that the tire is essentially rolling up underneath the bike due to the acceleration.

Thanks
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post #26 of 39 Old 11-17-2014, 09:13 AM Thread Starter
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This is what made sense after I started thinking about it. The video didnt really explain it and I'm not one to take answers of 'because'...lol. But when I started to really think about it, it made sense that the tire is essentially rolling up underneath the bike due to the acceleration.

Thanks
-Cody
Yes, exactly, and that is a great way to explain it. I will try to remember that for future explanations =)
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post #27 of 39 Old 11-17-2014, 09:40 AM
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thanks man! great info!
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post #28 of 39 Old 11-17-2014, 10:29 AM
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Wow. Very impressive and detailed post. Thank you for bringing this to the forum. Way to good NOT to be stickied.

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post #29 of 39 Old 11-17-2014, 11:44 AM Thread Starter
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thanks man! great info!
Just glad that some folks may get something out of my ramblings =)

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Originally Posted by MistressOfMayhem View Post


Wow. Very impressive and detailed post. Thank you for bringing this to the forum. Way to good NOT to be stickied.
Thanks MoM. Just doing my part to pay it forward. With what I do, I need as much good karma in the bank as possible :-p
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post #30 of 39 Old 11-17-2014, 12:22 PM Thread Starter
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So a few years back I was watching world supersport and they were talking about Davies I think and said his bike is set up for braking. I understand set up for stability or set up for turning (at the basic level of understanding), but what do they mean set up for braking? The correct geometry for stopping quickest? Being able to turn while braking hard? I think he was lifting rear wheel consistently on braking.

Thanks
I just realized that we never addressed this question.

A lot of tracks are 'point and shoot' tracks which do not require a lot of trail braking or turning while braking. It is a lot easier to tune for if you brake > turn > accelerate but some tracks/corners require significant turning while on the brakes, something your bike doesnt want to do by default.

Decreasing radius and double apex turns iare the main types that come to mind (the corner gets tighter as you get deeper into the turn). For those of you that have ridden NJMP Thunderbolt, the double right apex turn 9 is a good example of this:



As I mentioned in my initial post, everything is a tradeoff. Generally speaking if you get a bike set up perfectly for a standard turn, then something has to give. That something usually means that turning while braking kind of sucks.

When you encounter a track where there is a significant amount of turns that require deep trail braking or direction changes while braking, you can tune your bike to do just that...at the expense of regular handling characteristics.

I have personally never had to set a bike up for this as the tracks that I ride are dominated by other kinds of corners so even though there will be some trail braking, it is far more important to me to get the best out of the bike on 90% of the course, rather than tune it for that 10%.

Hope this helps.
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