Fish tailing/Tank slapping?

[louemc]

Way back when Ben Spies was racing in Superbike (I think the track was Road America, but I'm not sure) The camera was right on his wheels when he was braking into a turn, his rear wheel was just enough off the track (for quite a distance) to see that it was in the air. As Ben turned into the Apex, he also used body English to shift the airborne rear wheel to the side, to the exact correct angle, for when he set it down under power, to drive into the corner (ala Flat tracker style).

Having the tires on the track is over rated,

 

[ohio]

If the rear wheel is unbraked, it will also be going faster than the front, so why won't it try to swap ends in that situation as well?

No it won't. It is still directional, and so will track with the bike towards the headstock. As soon as it is locked, it is not directional and will go wherever the inertia of the bike pushes it.

 

[ohio]

I have a hard time picturing a big cruiser pulling a stoppie.

On a cruiser, because of the long wheelbase and low center of mass, you reach the limits of traction of the front tire before you achieve sufficient deceleration for the center of mass to pass the front contact patch.

On a sportbike, because the short wheelbase and high center of mass, you will endo before you surpass the limits of traction of the front tire.

Not sure if that's helpful because I feel like I'm telling you something you already know. In short,
- if you can't stoppie your bike, you will benefit from using rear brake all the way to ITS limit of traction.
- if you can stoppie your bike, you will only benefit from using a rear brake if you are in a low traction condition (e.g. gravel road, wet surface), or for specific technical maneuvers (e.g. backing it in).

 

[MCSFTGUY]

Here's what I'm really questioning here: telling new riders with no training that they shouldn't use the rear brake in panic stops. I'm also wondering how much of this "evidence" is based on actual studies, and how much of it is IES (Internet Expert Syndrome). We have one poster with what: a year of riding experience? On one bike? Holding herself out as a braking expert. That's bad, because for every one of us loudmouth over-posters, we have 100 or more riders--a lot of whom have little knowledge or experience--reading and listening to the loudest, most insistent of us. Even if we have no idea of what we are talking about.

Here's what I believe, because the MSF has tested and reviewed it for 30 years: use both brakes when you stop, every time. Apply gradual pressure to both, just short of lock-up. You practice so you get familiar with where that lock-up point is. If the back brake locks, so what: hold it on until you're stopped. If you practice maximum braking, you will shorten your stopping distance. And yes, I believe using the back brake--even if it only adds 5% of the braking power--may save your bacon in an emergency, so use it.

Nicely said Gabe! Everybody has a lot of opinions on braking and if it works for them cool... but the physics behind braking... AKA negative acceleration is pretty straight forward and it says both brakes are best. Sorry....

The rear tire will swing back and forth behind the bike when it is locked up. Long story short, it is a friction thing... there are two different kinds of friction in play, one for your tire rolling nicely along the ground, and one for your tire locked up and sliding along the ground. Because there are two friction flavors in play at the wheels, and they don't have an equal value things get a little funny.

Go to YouTube and try a search for "trailer swing" On a big rig with a tractor and trailer it is possible to lock up just the trailer tires and you will see the trailer swing back and forth behind the tractor.... same things with the bike just a bigger vehicle. The big rid is known as a dual track articulated vehicle. A motorcycle is known as a single track articulated vehicle. The dual verse single track only comes into play when vehicles are stopped. The rear won't pass the front and the OP did exactly the right thing..... hold the handle bars straight and stay on both brakes.

This is the short story there is a lot more to it. OH, and MSF doesn't do testing, they rely on the studies and testing of others. WRT motorcycles, there is not as much out there as we would all like to see. But some people are work on this.....

 

[Gnarly Cranium]

We have one poster with what: a year of riding experience? On one bike? Holding herself out as a braking expert. That's bad, because for every one of us loudmouth over-posters, we have 100 or more riders--a lot of whom have little knowledge or experience--reading and listening to the loudest, most insistent of us. Even if we have no idea of what we are talking about.

You still have not explained to me how it is that you think fishtailing occurs, if my analysis is so completely incompetent.

I may not be an expert, but I'm not an idiot. I retain quite a bit from the various physics courses I've taken.

 

[Gabe]

You still have not explained to me how it is that you think fishtailing occurs, if my analysis is so completely incompetent.

I may not be an expert, but I'm not an idiot. I retain quite a bit from the various physics courses I've taken.

Didn't say you were an idiot and meant no disrespect. Just questioning your bona-fides as an expert in motorcycle dynamics.

I wouldn't venture to say how fishtailing occurs because I'm not an expert in motorcycling dynamics, haven't studied the issue, and don't like to talk about things I don't know about. Just because I don't know the answer doesn't mean you do.

 

[Gnarly Cranium]

Didn't say you were an idiot and meant no disrespect. Just questioning your bona-fides as an expert in motorcycle dynamics.

I wouldn't venture to say how fishtailing occurs because I'm not an expert in motorcycling dynamics, haven't studied the issue, and don't like to talk about things I don't know about. Just because I don't know the answer doesn't mean you do.

Ahh, okay then.

My riding experience is very limited, absolutely, and if somebody can explain the flaws in my reasoning, great. I'm trying to learn. It helps to discuss what I can along the way.

 

[MCSFTGUY]

I may not be an expert, but I'm not an idiot. I retain quite a bit from the various physics courses I've taken.

HA HA HA... I knew it! Let me guess... engineering... mechanical, civil, EE???? Or is it computers.... either or have to go over to the physics building every once and a while.

Think back to the friction lectures... friction is one of the five basic mechanical forces and static friction is about 10% less than dynamic friction. Locked rear is a static friction, front is dynamic friction so you have a case of less force on the rear tire at the contact patch than you do on the front tire at the contact patch. Since it is a single track vehicle with uneven force applications away from the center of mass, instability. But, not enough to upset the motorcycle and a lesser force will not pass a stronger force... simplest way I know how to say this, so the rear just oscillates back and forth behind the front. Weird feeling and you have to fight the car habit to release and steer into it.

Actually saw a crash involving an old goldwing, rider trying to out run the cops... way too fast heading toward a turn he could not make... almost 200 feet of lock rear wheel skid... bike stayed up and ran off the road into a parked car. Most beautiful serpentine from the back wheel moving back and forth across the straight front wheel track. Rider was hard on the front brakes also so I had some impending marking on the road that I could see.

 

[Gnarly Cranium]

HA HA HA... I knew it! Let me guess... engineering... mechanical, civil, EE???? Or is it computers.... either or have to go over to the physics building every once and a while.

3D animation, actually. Most of my classmates have no maths whatsoever (it's not required for us, for some idiotic reason) but I took a year of precalc-level physics for kicks. My dad is the structural engineer in the family, I'm just an art student.

Running from the cops on a goldwing? That sounds amazing. How hard did he hit the car after all that skidding?

 

[Lovedoc]

Didn't say you were an idiot and meant no disrespect. Just questioning your bona-fides as an expert in motorcycle dynamics.

I wouldn't venture to say how fishtailing occurs because I'm not an expert in motorcycling dynamics, haven't studied the issue, and don't like to talk about things I don't know about. Just because I don't know the answer doesn't mean you do.

Gabe, WTF??? Ad hominem attacks are the lowest form of debating. She has done her homework and understands what's going on. Further, MANY very experienced and even fast riders who threshold brake all the time agree with her in this thread.

HA HA HA... I knew it! Let me guess... engineering... mechanical, civil, EE???? Or is it computers.... either or have to go over to the physics building every once and a while.

Think back to the friction lectures... friction is one of the five basic mechanical forces and static friction is about 10% less than dynamic friction. Locked rear is a static friction, front is dynamic friction so you have a case of less force on the rear tire at the contact patch than you do on the front tire at the contact patch. Since it is a single track vehicle with uneven force applications away from the center of mass, instability. But, not enough to upset the motorcycle and a lesser force will not pass a stronger force... simplest way I know how to say this, so the rear just oscillates back and forth behind the front. Weird feeling and you have to fight the car habit to release and steer into it.

This is just fucking dumbass pseudoscientific gobbledygook and delivered in a condescending fashion to boot. You might be snowing the history majors but you've taken a little fact and come to absolutely the wrong conclusion. Of course you won't low side if you make steering corrections to prevent an ass-around. Inexperienced(ie MOST) riders may fail to do that. Very few will be able to ignore that skid and increase front brake pressure and actually decelerate rapidly. Further, it surely won't work in a curve or if you need to maintain directional control at some point in this stop. Locked rear puts you on a ballistic course.

saw a crash involving an old goldwing, rider trying to out run the cops... way too fast heading toward a turn he could not make... almost 200 feet of lock rear wheel skid... bike stayed up and ran off the road into a parked car. Most beautiful serpentine from the back wheel moving back and forth across the straight front wheel track. Rider was hard on the front brakes also so I had some impending marking on the road that I could see.

Alas, a rider with some skills but no real command of the front brake... or the rear

Have you ever actually skidded a bike?

 

[Lovedoc]

Ohh and why you can't use the front properly with a locked rear.

Everyone agree that the front provides 90% at least of the stopping power?

With the rear locked, you HAVE to make little steering inputs to keep the bike straight up. You CANNOT do that with your body weight on your hands, yeah, we should be squeezing the tank but even that doesn't keep the weight off completely and is difficult to do all the time. You'll either ease off the front so you can sort of manage the skid, or let off the rear so you can operate the front. The first gives up the power of the front, the second might work out but's it's more complicated than just using the front all the way. Both are actually contrary to MSF dogma.

Actually I think MSF dogma boils down to "pretend you're an expert when you brake and never lock the rear. With luck, you might make it to that level"

 

[clutchslip]

.....Since it is a single track vehicle with uneven force applications away from the center of mass, instability. But, not enough to upset the motorcycle and a lesser force will not pass a stronger force... simplest way I know how to say this, so the rear just oscillates back and forth behind the front.

Just sharing! This reminded of what my best motorcycle mentor taught me.

I was worried about my suspension and other riding skills and explained the rear moving around to my mentor. He simply stated, "if the wheel is off the ground, it won't be moving around as much. The tire on the ground provides traction to the end that is still going faster, so it moves around more." He is right. Try it.

I use my rear brake at the beginning of "high-speed" braking, to slow the rear down. It seems to keeps the 'swapping' thing from getting uncontrollable.

I can hardly wait for the BARF testing. I hope it makes it to DOT highway safety. I bet ers can do a bang-up job.

 

[aphyr]

I'm sort of a physicist. (Hey, I'm published in Phys Rev Letters... ;-))

If you ignore secondary problems (i.e. can you maintain control), I think the answer is that your maximum deceleration occurs just prior to a stoppie. Before that the rear brake is still mildly helpful. Assuming a stable suspension state, the total linear force (or braking power) on the bike is proportional to the angular force trying to lift the rear of the bike off the ground. When the suspension is shifting forwards and the back wheel is lifting off, you're necessarily decelerating faster than when the bike is stable and level.

However, as soon as the rear wheel is off the ground, the effective COG of the bike relative to the contact patch shifts, and you *can't* decelerate as quickly. The best braking position is with the bike's mass as low as possible; i.e. just before lifting the rear wheel.

This is all independent of how you brake--100% front, 100% rear, anything in between.

If you haven't lifted the rear wheel off the ground, you haven't taken advantage of the maximum linear deceleration possible. On a lot of sportbikes, the front tire contact patch and brake system are more than adequate to execute a stoppie. You can therefore assume that they can deliver the maximum braking force achievable. The only time where the rear brake could conceivably contribute additional force is before the suspension reaches a steady state and the rear wheel lifts. You might be able to reduce the stopping distance by a little bit by braking quickly to that threshold point.

Again, this is an ideal scenario: our ability to modulate brake power, balance the bike, for the brakes to reach optimal force, etc., all factor in. I suspect that's why you'll see varying experimental results.

 

[MCSFTGUY]

This is just You might be snowing the history majors but you've taken a little fact and come to absolutely the wrong conclusion. Of course you won't low side if you make steering corrections to prevent an ass-around. Inexperienced(ie MOST) riders may fail to do that. Very few will be able to ignore that skid and increase front brake pressure and actually decelerate rapidly. Further, it surely won't work in a curve or if you need to maintain directional control at some point in this stop. Locked rear puts you on a ballistic course.

Have you ever actually skidded a bike?

WOW... little harsh.

If anything I said in my "fucking dumbass pseudoscientific gobbledygook and delivered in a condescending fashion to boot." was wrong, please feel free to correct me. I am perfectly willing to admit when I have made a mistake about something.

In the mean time here is some info about friction.

http://en.wikipedia.org/wiki/Friction

Wikipedia is not the best source of information for things, but this is not bad and this is relatively straight forward piece on friction, and they have break out stuff on Static and Kinetic Friction.

Now here is a really big one, if you read any of this Wikipedia stuff, show me where weight comes into play. So all of the stuff about weight transfer and increased traction is not really right........

And yes... I have done braking and tire skidding a few times.

 

[Lovedoc]

HA HA HA... I knew it! Let me guess... engineering... mechanical, civil, EE???? Or is it computers.... either or have to go over to the physics building every once and a while.

Think back to the friction lectures... friction is one of the five basic mechanical forces and static friction is about 10% less than dynamic friction. Locked rear is a static friction, front is dynamic friction so you have a case of less force on the rear tire at the contact patch than you do on the front tire at the contact patch. Since it is a single track vehicle with uneven force applications away from the center of mass, instability. But, not enough to upset the motorcycle and a lesser force will not pass a stronger force... simplest way I know how to say this, so the rear just oscillates back and forth behind the front. Weird feeling and you have to fight the car habit to release and steer into it.

Actually saw a crash involving an old goldwing, rider trying to out run the cops... way too fast heading toward a turn he could not make... almost 200 feet of lock rear wheel skid... bike stayed up and ran off the road into a parked car. Most beautiful serpentine from the back wheel moving back and forth across the straight front wheel track. Rider was hard on the front brakes also so I had some impending marking on the road that I could see.

WOW... little harsh.

If anything I said in my "fucking dumbass pseudoscientific gobbledygook and delivered in a condescending fashion to boot." was wrong, please feel free to correct me. I am perfectly willing to admit when I have made a mistake about something.

In the mean time here is some info about friction.

.

Overly harsh my apologies. Motorcycles articulate in the middle. They are not stable, especially sportbikes with short wheelbases and steep steering angles. When the rear yaws to the right, the forks also twist to the right. This is equivalent to countersteering the front to the left. This is not a self-correcting oscillation, it's a potential crash. As the rear yaws right, pushing the right handlebar, steering into the skid, can hopefully correct it. Even with the rear off the ground, you have to make ass corrections to keep the rear in line. And woe to you if the rear touches down with any rear brake pressure in place.

Thank you for your previous interesting and thought provoking posts, I've enjoyed your novel perspective. It's just that any incorrect riding advice delivered authoritatively on BARF can influence newb thought.

 

[MCSFTGUY]

Overly harsh my apologies. Motorcycles articulate in the middle. They are not stable, especially sportbikes with short wheelbases and steep steering angles. When the rear yaws to the right, the forks also twist to the right. This is equivalent to countersteering the front to the left. This is not a self-correcting oscillation, it's a potential crash. As the rear yaws right, pushing the right handlebar, steering into the skid, can hopefully correct it. Even with the rear off the ground, you have to make ass corrections to keep the rear in line. And woe to you if the rear touches down with any rear brake pressure in place.

Thank you for your previous interesting and thought provoking posts, I've enjoyed your novel perspective. It's just that any incorrect riding advice delivered authoritatively on BARF can influence newb thought.

Thanks Paul!

I am not trying to be arguementative but I have to ask, how do you know that I don't know what I am talking about? BARFers can come in many different flavors and backgrounds.

We may very well be talking about the same thing, just in different perspectives. Your comment about the forks twisting when the rear yaws is interesting... sounds like Tony Foale... I have to think about that one for a bit. I will get back to you.

 

[Lovedoc]

Thanks Paul!

I am not trying to be arguementative but I have to ask, how do you know that I don't know what I am talking about? BARFers can come in many different flavors and backgrounds.

We may very well be talking about the same thing, just in different perspectives. Your comment about the forks twisting when the rear yaws is interesting... sounds like Tony Foale... I have to think about that one for a bit. I will get back to you.

Again, apologize. I do believe you know what you're talking about with friction. Just need to account for the rotational movement at the steering tube and how that affects the yaw when you skid

 

[Z3n]

Look:

Here's how you solve this entire debate.

No need for a degree if physics.

Go out to a parking lot.

Set up a cone.

Accelerate to a fixed speed. As you pass the cone, brake with the front.

Repeat with only the rear.

Repeat with both.

What stops you the fastest? What are you most comfortable with? Do it a bunch of times, figure out what stops you more consistently.

Done.

You can talk theory, rolling resistance, locked traction, unlocked traction, how many cubits per parsec your bike puts down in negative acceleration, but in the end, the important thing is what technique stops you and how fast it does so.


I really need to go practice stopping on the sumo and see what I can find. I can roll stoppies on it no problem, but I'm curious as to if I'll stop faster with the front or rear brake.

 

[TTTom]

.... Motorcycles articulate in the middle. ...

I haven't had a motorcycle that articulated in the middle since I junked my Kawasaki 500 triple.

 

[MCSFTGUY]

Look:

Here's how you solve this entire debate.

No need for a degree if physics.

Go out to a parking lot.

Set up a cone.

Accelerate to a fixed speed. As you pass the cone, brake with the front.

Repeat with only the rear.

Repeat with both.

What stops you the fastest? What are you most comfortable with? Do it a bunch of times, figure out what stops you more consistently.

Done.

You can talk theory, rolling resistance, locked traction, unlocked traction, how many cubits per parsec your bike puts down in negative acceleration, but in the end, the important thing is what technique stops you and how fast it does so.


I really need to go practice stopping on the sumo and see what I can find. I can roll stoppies on it no problem, but I'm curious as to if I'll stop faster with the front or rear brake.

Already done, I know what stops me the fastest and that is what I have practiced!

And, how do you know that one of us doesn't have a degree in physics???

Sorry I could not resist this one.... just like the simple fish, I snap at the bait.....