Rear brakes and chassis geometry - GSXR.com
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post #1 of 27 (permalink) Old 06-10-2014, 01:44 PM Thread Starter
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Rear brakes and chassis geometry

Finally done. Remember, this is *my* understanding of things. It is by no means comprehensive or to be taken as gospel.

The explanation

Man this is going to be involved; I am going to have to make diagrams and all that. Plus I am going to have to go very very deep into chassis geometry theory for people to understand why using the rear brake on the racetrack can actually be a good thing in very specific circumstances. Having a workable understanding of basic physics would also be a big help here.

I am going to preface this once again with a warning that this is a very advanced technique and not something that anyone should bother using unless they are at the absolute top of their game. Used improperly this will cause you to crash. Disregard my warning at your own risk. If you do decide to try this anyway, remember that I need a set of wheels for an '09 600 when you are parting out your bike :-p

Whew, I have to think of where I want to begin. I suppose we should start with some terminology. It is important to understanding this topic that the reader comprehends rake, trail, swingarm angle, ride height, wheelbase, and the gyroscopic effect as well as the effects each of these things has individually and in combination on how a motorcycle handles.

Understand that this will be a very condensed overview and will be by no means comprehensive. People write entire books devoted to this subject. Tuners devote their entire lives to learning the subtle nuances of chassis geometry. It truly is a mix of science and black magic. Ever listen to a MotoGP post-race interview? Often the rider will say something like "We were really able to turn some good laps once we got the suspension sorted out." This is everything in racing. Raising or lowering the front forks 5 millimeters is the difference between a bike that works, and one that doesn't. That is roughly half the thickness of your average smartphone.

It is also important to note that everything is a tradeoff. Changing any one of the characteristics I mentioned above will have a positive and a negative effect. The magic of suspension tuning is to find the exact right combination where you get the most positive effects while minimizing the negative ones. Hopefully this will give you some idea of the scope of just how massive and challenging this is. There are so many variables and I will not even be going into things like compression and rebound damping, spring rates, preload, etc.



RAKE
Rake is the angle of the steering head relative to a perpendicular line drawn from the ground. This is a fixed number since your steering head is a welded component of your frame. Most sportbikes sit around 23 degrees of rake. By comparison, most cruisers have around 26 degrees of rake.

Less rake makes the bike less stable, but turn in quicker. More rake makes the bike more stable, but turn in slower.

A little makes a lot of difference here. Remember this is in a 360 degree arc. Those 3 degrees is the difference between turning like a Harley and turning like a GSXR.



TRAIL
Use the above diagram as a reference. Draw an imaginary line straight down from your front axle to the ground, mark that spot. Next draw an imaginary line straight down your headstock. Continue that same line all the way down to the ground and mark that spot. Trail is defined as the measurements between those two points.

The effects of trail are perhaps the most important part of setting up a working chassis geometry. Like everything else, trail is a tradeoff. Less trail makes the bike less stable in a straight line but makes it turn in easier/quicker. More trail has the opposite effect, making the bike very stable but hard to turn.

Think of a shopping cart wheel. They are offset. Why is this? Because when you push the cart, the trail of that wheel forces it to go in the direction you want to go. If there were zero trail, the wheel could very easily turn sideways while you want to go forward!

The general rule of thumb here is to have the least amount of trail (so you can turn quick) while having enough trail to keep the bike stable in a straight line.

Have you ever experienced headshake? Where the front end bars move quickly from side to side? This is a result of a low trail number. This isnt always a bad thing though.

Things like steering dampers help us to have even more aggressive trail numbers, to the point where we would get headshake in a straight line but the damper keeps that in check. We are "cheating" the physics of chassis geometry in a manner of speaking.

Rake and Trail work together to determine a large part of how your bike turns. The goal is to find a balance that works between the two but generally speaking rake is set from the factory as they built the frame. Trail is what we have to work with.

SWINGARM ANGLE


Refer to the picture above. Swingarm Angle is the angle between a horizontal plane through the front swingarm pivot point and where the rear axle is. This setting has a direct correlation to how well the bike "hooks up" or gets traction.

We need to tune the swingarm angle to counter "squat", or the weight transfer to the rear of the bike as we get on the gas. Some weight transfer is good and it helps to improve our traction but too much can cause the bike to run very wide under power or worse, loft the front wheel while at lean. This makes for some great photography but sucks when it comes to actually turning the bike.

The main force we are considering here is thrust, the force of the wheel pushing the bike forward (and up). The up part is what counters the squat so we call this "anti-squat". Because the swingarm is below plane, the thrust actually pushes the bike up. Picture a ladder propped up against a house. If you push on the bottom of the ladder (thrust it forward) the top of the ladder goes higher on the house, right? Now, if that ladder were flat on the ground and up against the house, no amount of thrust is going to get that to move up. This is why it is important that we maintain a swingarm angle below the plane of the pivot.

The big trick and tradeoff is to find the right amount of angle that counters just the right amount of squat so that you maintain a stable and easy geometry when accelerating out of a turn.

Sportrider.com did a fantastic writeup of swingarm angle. Read it here: http://www.sportrider.com/more-fun-geometry/?image=0

RIDE HEIGHT

We call it ride height, but really the purpose that it serves (other than making sure you arent dragging parts on the ground ) is to counter squat. The goal would be to remove as much squat as possible from the ride while still maintaining handling characteristics. Remember where I said everything is a tradeoff? (at least 10 times?). Guess what? This is one too :-p

Raising the ride height counters squat BUT it also increases your swingarm angle and raises your center of gravity. A well-sorted motorcycle will have enough ride height to obtain the optimal rear swingarm angle while providing adequate ground clearance while having the lowest possible center of gravity. Not so easy, eh? There is a reason why racers pay BIG dollars to their tuners. The few people in the world who really understand this (I am not one of them) earn more money than the president of any country. The reason Valentino Rossi could go as fast as he did (up until this year) was Jeremy Burgess...his tuner.

Last edited by PainfullySlow; 07-03-2014 at 09:00 AM.
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post #2 of 27 (permalink) Old 06-10-2014, 01:46 PM Thread Starter
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**saving this spot as I am sure I will go way past the character maximum in the first post**

WHEELBASE
This is the measurement between the front and rear axle. It is measured when the bike is at rest and without any weight on it. What a lot of people dont understand is that this is NOT a static measurement, it changes depending on what you are doing on the bike. Understanding this is a key component in learning how to maximize a motorcycles handling potential.

A shorter wheelbase has the effect of making the bike easier to transition (quickly going from leaned over in one direction to the other) at the cost of some mid-corner stability. Longer wheelbase the opposite. Harder to transition/turn but more stable. Also your wheelbase has an effect on your ability to keep the front wheel down under acceleration. Drag bikes stretch their swingarms to make it harder for the bike to wheelie while our comparatively short wheelbase GSXRs love to loft that front wheel in the air.

Since the swingarm is on a pivot (where the swingarm attaches to the frame) and that swingarm is on an angle, it can alter the wheelbase of the bike as the swingarm travels up and down in its range of motion. I made some pictures to help illustrate this.

This picture is with zero degrees of swingarm angle. That is; the axle for the rear wheel and the pivot where the swingarm attaches to the frame on on the same plane. The green line marks the center of the swingarm pivot, the yellow line marks the center of the rear axle.


This next diagram I overlaid the same swingarm but placed it at a 25 degree angle. This is exaggerated but it serves our purpose. Notice that the swingarm pivot (green line) is in the same place for both the zero degree example and the 25 degree example. The red line is the new center of the rear wheel axle. As you can see, the red line is closer to the green line. This illustrates how the swingarm angle can change the wheelbase. Follow?


Also, consider that if your wheelbase did NOT change as your swingarm moved, why would we need slack in our chains? =)

So, consider that there is an "optimal" wheelbase that each rider wants that gives them the right combination of handling characteristics that they are looking for. Time to throw another variable into the mix. Gear ratio.

A 16 tooth front/44 tooth rear gives you a ratio of 2.75. A 15 tooth front/41 tooth rear gives you a ratio of 2.73. Close enough that no one would notice a difference however, a 41 tooth rear is significantly smaller than a 44 tooth rear, allowing a tuner to push the rear axle out (longer wheelbase). The 44 tooth, since it is larger uses more chain length to go around it and thus requiring that the wheel be moved forward (shorter wheelbase) so not only does a racer need to determine the best gear ratio for getting around a particular track, they then need to determine which gear combination gets them to the desired wheelbase. Fun, right? :-p

GYROSCOPIC EFFECT

This video should help to explain the gyroscopic effect.
https://www.youtube.com/watch?v=UiTUiop9etk

When your wheels are spinning they exert a tremendous amount of force. This force is why motorcycles just dont fall over when in motion. It also means that the faster you go, the more stable the bike becomes (wheels are rotating faster, more gyroscopic effect) with the tradeoff being that the faster you go, the harder it is to turn the bike.
Check out this video. The bike does not fall over because of the gyroscopic effect.
https://www.youtube.com/watch?v=kWfCw9hIKE8

There are other factors that contribute to how strong the gyroscopic effect is, the most notable of which is the weight of the wheel or "rotational mass". The heavier the wheel, the more gyroscopic effect, the more stable (and harder to turn) the bike becomes. Compare a relatively heavy cast aluminum wheel which is what our GSXRs come with to say, a carbon fiber wheel. The lighter wheel has a lot less mass which makes the bike less stable but easier to turn, right? Isnt less stable bad? Sure it is, which is why we would add some trail to make the bike more stable...or change ride height to give us some more rake. Are you beginning to see how everything is interconnected?

So why use lightweight wheels at all? Well, the goal is to go faster, and to get to top speed sooner and that comes down to the Moment of Inertia (MOI). This is a measurement of how much force it takes to get a wheel spinning. The less MOI, the easier it is to make the wheel spin...This is also known as faster acceleration =)

So, we obviously want to accelerate as fast as possible, so we want the wheels with the lowest MOI. We then need to adjust ride height, trail, and swingarm angles to compensate. Whee!

The actual point of this discussion...sort of

So, as was mentioned above, when you accelerate on a motorcycle, the back end of the bike rises.

Lets digest that for a moment. A lot of you right now are going "What the eff? When I am accelerating the weight transfers back and it should squat". Physics says yes. Chassis geometry says no.

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. The motorcycle becomes more stable and harder to turn. The goal is to set ALL of the above settings such that you can get the maximum stability and traction at this time. The problem in doing so is that it causes the bike to "stand up", "run wide", or "unable to finish a turn". The more gas you give, the more the bike wants to move towards the outside of the turn. Finding the delicate balance of all of this is the goal of on-the-gas suspension tuning.

When we brake/decelerate we experience the following conditions: Weight transfers forward. Rake decreases. Trail decreases. Swingarm angle increases. Wheelbase decreases. Rear ride height increases. The motorcycle becomes less stable and easier to turn. The goal is to set all of the chassis settings such that you can turn as easily as possible while having enough stability that you don't crash. This is the goal of on-the-brake suspension tuning.

So, we can't change suspension settings on the fly...so as if it weren't enough to tune on-the-gas and on-the-brake suspension, it is the tuners job to find the settings that allow you to do both of those things well. No easy task I can assure you.

Get to the point already...

And finally, the home stretch.

So we now know that the goal of every suspension is to allow the maximum ability to turn which comes at the tradeoff of becoming unstable. Can we all agree that the point which needs the most stability is at the apex/slowest point of every turn?

So here it is. A racer is coming into a corner hot, hard on the brakes. The bike is incredibly unstable as he begins to tip into the corner which he can do faster and easier because of the aggressive geometry settings he has. He trail brakes as long as he dares and then he gets on the gas before the apex because he wants to transfer that weight back to a more neutral stance and not weight the front as much. The problem is that he is still unstable from compressed geometry settings of the braking forces.

By gently dragging the rear brake...not enough to slow you down but enough to cause your geometry changes...he lessens his rear swingarm angle, lowers the center of gravity (ride height), increases his wheelbase, increases trail, and increases rake. ALL of these things make the bike much more stable.

All of that writing for the last 2 sentences =)

This technique allows an incredibly talented and very advanced rider to "cheat" chassis geometry, much in the same way that a steering damper allows us to "cheat" trail settings. He can run super-aggressive geometry that would be right at the point of crashing but right when he needs that maximum stability from the bike, he uses the above technique to give him the stability when he needs it, and the ultra-tight handling when he doesnt.
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Last edited by PainfullySlow; 07-03-2014 at 10:24 AM.
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post #3 of 27 (permalink) Old 06-10-2014, 02:08 PM
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Thanks for putting this together. Waiting for the next chapter.
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post #4 of 27 (permalink) Old 06-10-2014, 02:14 PM
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outstanding sir

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post #5 of 27 (permalink) Old 06-10-2014, 09:06 PM
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So far so good. Best visual (and maybe safe?) way to see anti-squat is on a dyno. With the front of the bike strapped down, watch the rear of the bike as it accelerates... it noticeably lifts up. Bear in mind that the lift is a result of engine torque. The rear end squatting is a result of weight transfer from forward acceleration. On the dyno there is no forward acceleration (or.... at least we hope not), so the lift is really noticeable.

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post #6 of 27 (permalink) Old 06-10-2014, 09:35 PM
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Sub'd as well. I know this is advanced riding techniques, but I think learning this stuff will help my basic understanding of how the bike is supposed to work.
You da absolute man PS. Looking forward to reading more.

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post #7 of 27 (permalink) Old 06-11-2014, 07:44 AM
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post #8 of 27 (permalink) Old 06-11-2014, 07:50 AM
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Much appreciated PS. These write ups people do really make this forum even that much better. Alot of this is beyond my riding ability but like Snow said its good knowledge to have.

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post #9 of 27 (permalink) Old 06-11-2014, 09:55 AM Thread Starter
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Subscribed.

So far so good. Best visual (and maybe safe?) way to see anti-squat is on a dyno. With the front of the bike strapped down, watch the rear of the bike as it accelerates... it noticeably lifts up. Bear in mind that the lift is a result of engine torque. The rear end squatting is a result of weight transfer from forward acceleration. On the dyno there is no forward acceleration (or.... at least we hope not), so the lift is really noticeable.
Great example! The "why" is critically important here. Also, anyone who has done a burnout will also have experienced this effect.

I am glad that people will get something out of this. To be honest, most of this is beyond my ability, or I am just now starting to delve into parts of this, but the knowledge of how this all works and comes together makes me a better rider. I hope that it will do the same for some of you.

I am happy to do this, just bear with me as there is still quite a ways to go.
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post #10 of 27 (permalink) Old 06-11-2014, 02:20 PM
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This is a very informative thread. Thanks for doing the write up PS.
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post #11 of 27 (permalink) Old 06-28-2014, 02:32 AM
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A good example of why the rear brake can be dangerous

https://www.youtube.com/watch?v=d-kKJFRQp4s

This rider's technique is poor but what really makes him crash is the fact that he locked up his brakes after he panicked about his entry speed (just a guess, but seems likely.)


(Notice how in the beginning of the video when it shows him turning how he is not hanging off the bike at all. Seems like he didn't know how to corner, yet still wanted to take them fast.)
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post #12 of 27 (permalink) Old 06-28-2014, 09:13 AM
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Great thread you started ps. The knowledge is appreciated. The more you know how your bike works the better a rider it can help you become

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post #13 of 27 (permalink) Old 07-03-2014, 06:59 AM
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Great writeup!
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post #14 of 27 (permalink) Old 07-03-2014, 07:45 AM Thread Starter
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Thanks for the bump, Tiger. I had totally forgotten about this thread and I still have a lot of writing to do for it.

I have just been SO busy... I will get to it, I promise! =)


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post #15 of 27 (permalink) Old 07-03-2014, 09:17 AM
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Suspension helps so much with comfort level, which is king on the track. I really benefited by spending a day with a private instructor and he helped me get the bike to brake better by tweaking the rear. Stuff that wouldn't be intuitive.
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post #16 of 27 (permalink) Old 07-03-2014, 09:21 AM
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Yup....I don't think I'll ever try this. Awesome write up PS. I learned a bunch of stuff I never knew, and pretty sure I'll be able to translate a lot of this info to the track in tiny increment. Knowing HOW something works definitely allows you to use your tools more effectively, even if most of us will never be able to finesse the bike's setups that minutely.



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post #17 of 27 (permalink) Old 07-03-2014, 09:28 AM
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And yeah, I haven't touched my rear brake pedal in like 15 years. Hell I didn't even change sprockets in a full season of racing. My learning curve was still pretty steep when I had to stop.
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post #18 of 27 (permalink) Old 07-03-2014, 10:03 AM
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When you're hard on the gas and the rear end rises, rake decreases.... just sayin. But you are right about dragging the rear brake to induce a little squat to gain stability.

From low speeds the front will try and come up, and once contact is lost, it's a mute point anyway. But at speed, say 4th gear on up. The rear rises and the front is still at sag height decreasing rake.


Gyroscopic forces affect the motorcycle for sure, but it's not what keeps it upright. What keeps the bike upright is the same force that keeps a penny from falling over when it's rolling in a circle. Inertia. An object in motion tends to stay in motion along a straight line. Turning the bike requires lateral acceleration. Once in motion, it's actually easier to roll forward than fall over. The momentum follows the path of least resistance.

Pretty cool experiment if you want to look it up. Check out the zero gyro wheel. It has a low hanging weight off the axle to provide the "ground" bar. When the wheel spins forward, a proportional weight just inside the rim spins in the opposite direction. The result is zero gyroscopic effect. The damn thing rolls without falling over. Because it's CG wants to maintain its direction.
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post #19 of 27 (permalink) Old 07-03-2014, 10:23 AM Thread Starter
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When you're hard on the gas and the rear end rises, rake decreases.... just sayin. But you are right about dragging the rear brake to induce a little squat to gain stability.

From low speeds the front will try and come up, and once contact is lost, it's a mute point anyway. But at speed, say 4th gear on up. The rear rises and the front is still at sag height decreasing rake.


Gyroscopic forces affect the motorcycle for sure, but it's not what keeps it upright. What keeps the bike upright is the same force that keeps a penny from falling over when it's rolling in a circle. Inertia. An object in motion tends to stay in motion along a straight line. Turning the bike requires lateral acceleration. Once in motion, it's actually easier to roll forward than fall over. The momentum follows the path of least resistance.

Pretty cool experiment if you want to look it up. Check out the zero gyro wheel. It has a low hanging weight off the axle to provide the "ground" bar. When the wheel spins forward, a proportional weight just inside the rim spins in the opposite direction. The result is zero gyroscopic effect. The damn thing rolls without falling over. Because it's CG wants to maintain its direction.
Thanks for the input! You are absolutely right and I will correct the rake thing, that was a pure brainfart. Too many hours staring at the screen.

I simply overlooked intertia during the rest of my rolling description. As I said, far from comprehensive. This is as "layman" terms as a subject this complex can get.


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post #20 of 27 (permalink) Old 07-09-2014, 06:24 PM
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great write-up Mr. Weyant. i already have a few books on order myself. i really enjoy implementing different techniques for shits and giggles and of course better understanding of feedback im receiving . im about to read (the Upper Half of the Motorcycle,Total control, and an unknown suspension tuning book). apparently i should figure that out first for a better understanding of everything in general. thanks again !
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post #21 of 27 (permalink) Old 07-11-2014, 06:02 AM Thread Starter
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great write-up Mr. Weyant. i already have a few books on order myself. i really enjoy implementing different techniques for shits and giggles and of course better understanding of feedback im receiving . im about to read (the Upper Half of the Motorcycle,Total control, and an unknown suspension tuning book). apparently i should figure that out first for a better understanding of everything in general. thanks again !
Glad that you got something out of my writings!

I would really recommend adding twist of the wrist 2 to the reading list. Total Control is good however, in my opinion, Twist2 is better.


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post #22 of 27 (permalink) Old 07-12-2014, 08:13 PM
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will do . i was scoping it out and ive seen the twist of the wrist video
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post #23 of 27 (permalink) Old 05-06-2016, 11:12 PM
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So much information, great post.
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post #24 of 27 (permalink) Old 08-03-2016, 07:35 AM
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I realize this is an older thread but I have to ask a question. As I trail brake into the corner ( I'll use the corner at the end of the front straight as my example) I use my clutch to drop from 5th down to 3rd I'm still hard on the breaks and in 4th as I turn in and dropping to 3rd just before the apex. using that slipper clutch the whole time to controll the rear.
so my question: Is this not working in the same manner as applying a little rear brake pressure? the reason it feels that way to me is that the rear is so light that slipping that clutch and dropping down gears makes the rear slide out. smooth, controlled and feels fine, but adding rear brake doesn't seem like the right thng to do here.
2nd thought. you did say in CERTAIN CIRCUMSTANCES. if I'm giving an example of the wrong time to use this technique then please forgive my redicculous question. but also let me know too!
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post #25 of 27 (permalink) Old 08-03-2016, 07:48 AM
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I've used engine braking when straight up before tip in but you're talking about using it from tip in to apex in place of rear brake? Seems it would be much harder to modulate.

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post #26 of 27 (permalink) Old 08-03-2016, 08:26 AM
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Originally Posted by thomas74p View Post
I realize this is an older thread but I have to ask a question. As I trail brake into the corner ( I'll use the corner at the end of the front straight as my example) I use my clutch to drop from 5th down to 3rd I'm still hard on the breaks and in 4th as I turn in and dropping to 3rd just before the apex. using that slipper clutch the whole time to controll the rear.
so my question: Is this not working in the same manner as applying a little rear brake pressure? the reason it feels that way to me is that the rear is so light that slipping that clutch and dropping down gears makes the rear slide out. smooth, controlled and feels fine, but adding rear brake doesn't seem like the right thng to do here.
2nd thought. you did say in CERTAIN CIRCUMSTANCES. if I'm giving an example of the wrong time to use this technique then please forgive my redicculous question. but also let me know too!
I sent a note to PS. I don't think he checks this site much any more.

There is a grey blur, and a green blur. I try to stay on the grey one. -Joey Dunlop
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post #27 of 27 (permalink) Old 08-03-2016, 08:33 AM
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yes, it think it would make it very hard to controll also. thats why I am also thinking this maybe a technique used in a different type of corner. like maybe the second half of a chicane when your not on the brakes at all but back on the gas varying the bikes speed to maintain the needed lean angle, say in a decreasing or increasing radius type exit, maybe here using both throttle and rear brake at the same time is what is meant. I can see how if your on the gas at lean dragging the rear may help extend that WB out just a bit more. just trying to wrap my head around the post to fully understand it.
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