Fish tailing/Tank slapping?

[Lovedoc]

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.....

In a more respectful tone, how do you know really knows what stops you fastest? Seat of the pants G-meter, or maybe measured distance? The first might not be accurate, and the second might vary by quite a few feet each trial. With perceived G force, how do you know you got to max braking early and kept it there? Maybe using the rear delays getting the most from your brakes?

On a sportbike, you know you are at max decel and overshot slightly when the rear rises. Moving back on the seat is the only way to get more. The rear may rise very quickly too.

Your HD is going to benefit much more from the rear brake than a sportbike, I'm sure you use it well. Doubtful with the long wheelbase and mass of your bike you could even stoppie before the front exceeds frictional constraints.

I vote for linked ABS, and a BARF trial that includes onboard GPS accelerometers, using only a couple types of bike, a long wheelbase model and a sportbike.

 

[Cheyenne]

With perceived G force, how do you know you got to max braking early and kept it there?

It's all you will ever have to practice with in the real world, so you better get it fine tuned, or you're gonna wish you had more than likely.

 

[MCSFTGUY]

In a more respectful tone, how do you know really knows what stops you fastest? Seat of the pants G-meter, or maybe measured distance? The first might not be accurate, and the second might vary by quite a few feet each trial. With perceived G force, how do you know you got to max braking early and kept it there? Maybe using the rear delays getting the most from your brakes?

On a sportbike, you know you are at max decel and overshot slightly when the rear rises. Moving back on the seat is the only way to get more. The rear may rise very quickly too.

Your HD is going to benefit much more from the rear brake than a sportbike, I'm sure you use it well. Doubtful with the long wheelbase and mass of your bike you could even stoppie before the front exceeds frictional constraints.

I vote for linked ABS, and a BARF trial that includes onboard GPS accelerometers, using only a couple types of bike, a long wheelbase model and a sportbike.

Great question(s)

I will get back to this later tonight or tomorrow with an answer... I might have been BARFing at work a little bit today... and maybe it is time to go home now so I don't have time to write the answer....

And... this will give me a chance to think through the answer on the commute home.....on two wheels of course.

 

[MCSFTGUY]

In a more respectful tone, how do you know really knows what stops you fastest? Seat of the pants G-meter, or maybe measured distance? The first might not be accurate, and the second might vary by quite a few feet each trial. With perceived G force, how do you know you got to max braking early and kept it there? Maybe using the rear delays getting the most from your brakes?

On a sportbike, you know you are at max decel and overshot slightly when the rear rises. Moving back on the seat is the only way to get more. The rear may rise very quickly too.

Your HD is going to benefit much more from the rear brake than a sportbike, I'm sure you use it well. Doubtful with the long wheelbase and mass of your bike you could even stoppie before the front exceeds frictional constraints.

I vote for linked ABS, and a BARF trial that includes onboard GPS accelerometers, using only a couple types of bike, a long wheelbase model and a sportbike.

First off, I have to say don't do this without the proper training, expertise equipment and test location.

If I wanted to get the most accurate information for what the maximum possible stopping rate for a motorcycle was I would use an accelerometer that gave me a visible graph of acceleration rates over time. I would put this machine on my motorcycle, get up to about 40 mph and then slam on both brakes as hard and fast as I could so that I would lock up both brakes and skid. I would then make sure to brake or slow in the proper fashion so as not to crash. Doing this will give me a specific readout on the graph which would look like this, it would start out at zero and the instant I started to brake the graph would start to climb and it would continue to climb to a maximum value. After reaching this maximum value the graph would start to fall. Let's say the graph peaked at 0.9g and then fell to 0.85g. Let's say there are out riggers on this motorcycle so I can hold this skid until stops. The graph would stay at 0.85g, a straight line across the graph until the motorcycle came to a stop, at which point it would fall back to zero. The 0.85 g on the graph is where the tires on the motorcycle started to skid, and since the motorcycle skids to a stop the graph stays as a straight line. The areas between where the graph peaks at 0.9g and then drops to 0.85g is the transition period between moving from the static friction, rolling tire friction, to the kinetic friction, skidding tire friction. (Side note... this testing is what shows both brakes is the most effective way to stop in the shortest distance.)

This is the whole reason behind ABS, the graph shows us that static friction is greater than kinetic friction so in order to stop the fastest/and or with the most control, ABS is designed to keep the tire from moving into the kinetic friction, skidding tire friction mode.

Side note, yes some riders can stop faster and better without ABS on some bikes than with ABS so you are thinking what I just say is a bunch of crap and it really is not. Here is why, in order to maintain the static friction ABS has to do lock release cycles. It senses the wheel lock up so it releases it. How much release and how long the lock-release cycle is effects the time/amount of maximum braking. Whereas a person, who is good, can hit and hold that point of maximum braking... the peak of the graph. There is actually a name for this type of braking, "Impending Braking." Since the person does not have a lock-release cycle, which effectively lowers the braking efficiency, they can stop better. Enough said here because this is a topic all of its own.

Another way to check braking, simple distance. I set a braking point and the goal is to brake when I hit this point... with enough practice this is something that can be accomplished. I can come in at any speed.... 15 to 150 mph and I moderately brake. I mark this spot. I come through again at the same speed as before, same speed for every test, but this time I brake a little hard so I stop a little shorter than my mark. I mark this newspot. I repeat this process always trying to stop a little shorter than the time before. I do this until I reach a point where I am impending braking, or I can't stop without locking up the brakes. What this has done is establish the distance of the most effective braking and from there you can calculate the acceleration rate.

I believe a method like this is used by the test riders in Motorcycle Consumer News to test the stopping of the bikes they have, they do 60 to 0 testing.

Regarding this distance testing, can the be errors, yes. But, believe it or not, in the whole picture these errors are small... there are reasons for this.

This is the really short readers digest version of a much longer story. I hope that it has provided answers to your question(s).

I am a both brakes every time braking person. This is what I have practiced and this is what I have seen to be the most effective way to stop a motorcycle. But, the concerns voiced by others regarding control and skidding the front or rear tire are valid. This is why practice is a necessary thing for braking.... or ABS.

Paul, now a quick question for you, at one point you mentioned something about front forks flexing, or something like this, associated with a skidding rear tire, can you point me in a direction where I can read/learn more about this? This is something I would really like to try and wrap my head around because I have some questions about it.

Nice dialog, made me stop and think a few things through.

 

[NorCal Factory]

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.

Put the keyboards down, get out into the sunshine and see what works best for you and your bike. Practice until it becomes a reflex and you will be safer for it.

On a sportbike, most people in an emergency situation will not be able to modulate the rear brake enough to add value while doing maximum front braking.

If you want to get fancy on the track (especially in high speed downhill braking like T9 at Infinion or T2 at Laguna), you can start to use the rear brake for a second before applying the front brake. It will compress the shock, and change the angle of the rake which will allow harder front braking before unweighting the rear and loosing rear steering. It is a way to reduce rear wheel hopping and chatter.

On the street I practice that while squeezing the tank with my knees at the same time to keep the weight off the wrists. That also helps keep the rear from lifting.

Even though I practice this on the street a lot to develop a reflex, it is still a lot to do at once under max breaking conditions. That is why most people say - just use the front brake.

Practice a couple of hard 30-0 stops every time you ride.

 

[MCSFTGUY]

Put the keyboards down, get out into the sunshine and see what works best for you and your bike. Practice until it becomes a reflex and you will be safer for it.

On a sportbike, most people in an emergency situation will not be able to modulate the rear brake enough to add value while doing maximum front braking.

If you want to get fancy on the track (especially in high speed downhill braking like T9 at Infinion or T2 at Laguna), you can start to use the rear brake for a second before applying the front brake. It will compress the shock, and change the angle of the rake which will allow harder front braking before unweighting the rear and loosing rear steering. It is a way to reduce rear wheel hopping and chatter.

On the street I practice that while squeezing the tank with my knees at the same time to keep the weight off the wrists. That also helps keep the rear from lifting.

Even though I practice this on the street a lot to develop a reflex, it is still a lot to do at once under max breaking conditions. That is why most people say - just use the front brake.

Practice a couple of hard 30-0 stops every time you ride.

I do... not 30 mph for me, 25 mph because I have measurement markers on my pracitice street. Practicing does work... I had to go hard on the brakes two days ago for a lane changer and stop to turn driver, I heard the back tire chirpping from locking and releasing... I got impending brake.

2 points to make.... street riding "habits" are different from track habits...not a lot of fatal crashes on the track.

You don't need to go fast to establish the proper braking habits.... brake application is the same no matter what speed you are ride at.

 

[Lovedoc]

Paul, now a quick question for you, at one point you mentioned something about front forks flexing, or something like this, associated with a skidding rear tire, can you point me in a direction where I can read/learn more about this? This is something I would really like to try and wrap my head around because I have some questions about it.

Nice dialog, made me stop and think a few things through.

I reread my post and realize saying the forks twist was confusing. Actually, the front will try to track straight, as the rear yaws. This will create a steering force and the bike will turn, not self-correct. Frontbrake pressure will complicate things greatly. You have to correct this, difficult while you have weight on your hands. It is difficult to get a rear to skid in a straight line. The swingarm will flex however with rear brake pressure adding to your difficulty with skidding in a straight line. You will probably wind up backing off front brake or rear brake or both, ruining your stop.

The point I think sportbike riders are making isn't that the rear doesn't work. It's just on our unstable, short chassis bikes, it adds relatively little, yet vastly complicates the stop and requires great finesse to do properly. Certainly a novice or even intermediate rider is unlikely to succeed.

Another issue on forums is that most posters think they are good brakers. A generally accepted max decel with good tires and surface is around 1.1 or 1.2g, a sportbike will stoppie before that. A study I referenced in one these threads stated that experienced riders with ABS and an afternoon of training could barely exceed .8g, not bad but I wouldn't be smug about it. Hard to reason with someone who thinks they don't need to be taught anything.

Eventually the GPS systems will proliferate and riders will be able to see how well or poorly they do. I suspect many would be unhappy. Even trackdays usually have one or two vendors.

 

[MCSFTGUY]

Another issue on forums is that most posters think they are good brakers. A generally accepted max decel with good tires and surface is around 1.1 or 1.2g, a sportbike will stoppie before that. A study I referenced in one these threads stated that experienced riders with ABS and an afternoon of training could barely exceed .8g, not bad but I wouldn't be smug about it. Hard to reason with someone who thinks they don't need to be taught anything.

Eventually the GPS systems will proliferate and riders will be able to see how well or poorly they do. I suspect many would be unhappy. Even trackdays usually have one or two vendors.

There is more than one study out there that says riders don't brake well. I know some guys that did there own study on this and are finishing their paper for an egghead transportation safety journal right now

You bring up a very interesting point about sports bike verse other styles of bike.... due to their design they are more susceptible to some of the little nuances associated with the dynamics of the machine. Something else just occurred to me, braking mechanics for a sports bike rider have to be different from other riders. By definition of friction and braking, a sports bike has the potential to do a stoppie before max braking is reached.... seems to me that there are a lot of YouTube videos that show this.

I can tell you that if the rear tire is locked, it will never track straight.... it is being pulled foward by the still rolling front wheel... I have to think about this some more because I am having a little trouble wrapping my head around the back and forth oscillation of the rear tire effecting the front wheel. This goes against everything I have seen and done... but like I just said, the sports bikes are more susceptible to some of the little things.... I kind of have to get past my experience with things.....

 

[Gnarly Cranium]

Er don't mind me... random sloppy scribbles.

I could be totally full of crap here, and this may be completely nonsensical (sorry about the bad handwriting) but I got bored and drew a quick sketch of the main forces involved in fishtailing. These forces are always there when you decelerate, whether you're skidding the rear or not. If the friction between your rear tire and the ground is stronger than the sideways forces generated by a difference in angle between the trajectory of the front and the rear, the bike will track in line. If that horizontal force is stronger than the friction beneath the tire (or greater than the gravity holding it down!), the rear end of the bike will go out of line... and start fishtailing, or fly up in the air, whatever.

For anybody who hasn't done trigonometry yet: if you have a force pushing in a diagonal direction, it's the same thing as two separate forces pushing at right angles to each other. You just draw a triangle to find how big they are. For instance, if you're accelerating on your bike, the bike is pushing straight forward, while gravity pulls straight down, and the end result that you feel is a force pushing you diagonally down and backwards. This is what makes the world seem to tilt funny when you're in a plane on takeoff. Meanwhile, friction is always treated as a force directly opposed to the thing that's moving.


There's a zillion other fiddly factors involved, of course. The geometry of the bike, the height of its center of gravity above the ground, mushiness of the suspension weirdifying the shifting of the bike's weight from back to front as it brakes, hell maybe the contact patch of the front gets a little bigger as the weight on it increases, the rider's gut flopping around, gyroscopic stuff, blah blah. All that is crazy complicated, but secondary.

If you could hold the bike perfectly, absolutely straight, you could lock the rear all you want and fishtailing would not occur.

 

[Lovedoc]

Er don't mind me... random sloppy scribbles.

I could be totally full of crap here, and this may be completely nonsensical (sorry about the bad handwriting) but I got bored and drew a quick sketch of the main forces involved in fishtailing. These forces are always there when you decelerate, whether you're skidding the rear or not. If the friction between your rear tire and the ground is stronger than the sideways forces generated by a difference in angle between the trajectory of the front and the rear, the bike will track in line. If that horizontal force is stronger than the friction beneath the tire (or greater than the gravity holding it down!), the rear end of the bike will go out of line... and start fishtailing, or fly up in the air, whatever.

For anybody who hasn't done trigonometry yet: if you have a force pushing in a diagonal direction, it's the same thing as two separate forces pushing at right angles to each other. You just draw a triangle to find how big they are. For instance, if you're accelerating on your bike, the bike is pushing straight forward, while gravity pulls straight down, and the end result that you feel is a force pushing you diagonally down and backwards. This is what makes the world seem to tilt funny when you're in a plane on takeoff. Meanwhile, friction is always treated as a force directly opposed to the thing that's moving.


There's a zillion other fiddly factors involved, of course. The geometry of the bike, the height of its center of gravity above the ground, mushiness of the suspension weirdifying the shifting of the bike's weight from back to front as it brakes, hell maybe the contact patch of the front gets a little bigger as the weight on it increases, the rider's gut flopping around, gyroscopic stuff, blah blah. All that is crazy complicated, but secondary.

If you could hold the bike perfectly, absolutely straight, you could lock the rear all you want and fishtailing would not occur.

Thank you Ms Cranium. Bike #3 & #4 above shows the turning forces occurring when the rear leaves the line. A "fishtail" to the left generates the equivalent of a countersteer push to the right, you will have to correct that force or lowside. The front will turn right as the rear slides to the left. The opposing correction must occur while your weight is on your hands and fork compression shortens the geometry, so the steering forces are magnified and more difficult to correct. The correction may occur instinctively and lead to serpentine skid marks. You will likely have to back off one or more brakes, you will "feel" out of control, and your attention will be diverted from any maneuvers that might salvage the situation.

I just "modeled" this on my old Stumperjumper, seemed to confirm the phenomenon

 

[MCSFTGUY]

A "fishtail" to the left generates the equivalent of a countersteer push to the right, you will have to correct that force or lowside. The front will turn right as the rear slides to the left.
I just "modeled" this on my old Stumperjumper, seemed to confirm the phenomenon

Ahhhh the part I am having trouble wrapping my head around.... how does the lesser force on the rear translate through the pivot point to effect the front? The rear is literally being pulled forward by the front. And, the force here is friction, which by definition is in the opposite direction of motion, and motion for the motorcycle is forward. At the pivot point the motorcycle only feels a rearward pull in line with the motion.... I am having trouble putting an angle to the forces at the rear tire through the pivot point at the steering head, at an angle to cause the counter steering. In essences, the pivot point isolates the tire force on the rear tire from the front tire.

As I said before, look at trailer swing, it is the frictional equivilent of this only with 4 wheels, as long as the tractor is kept straight the trailer just oscilates back and forth behind the tractor as it is pulled forward... this is what I have seen happen with motorcycles no correction needed.

We must find video of this.....:|

 

[Lovedoc]

Ahhhh the part I am having trouble wrapping my head around.... how does the lesser force on the rear translate through the pivot point to effect the front? The rear is literally being pulled forward by the front. And, the force here is friction, which by definition is in the opposite direction of motion, and motion for the motorcycle is forward. At the pivot point the motorcycle only feels a rearward pull in line with the motion.... I am having trouble putting an angle to the forces at the rear tire through the pivot point at the steering head, at an angle to cause the counter steering. In essences, the pivot point isolates the tire force on the rear tire from the front tire.

As I said before, look at trailer swing, it is the frictional equivilent of this only with 4 wheels, as long as the tractor is kept straight the trailer just oscilates back and forth behind the tractor as it is pulled forward... this is what I have seen happen with motorcycles no correction needed.

We must find video of this.....:|

I think you and I are describing an elephant from different poles with our eyes closed. I have not mentioned friction at all. I'm simply stating that a yaw of the rear produces motion of the handlebars relative to the direction of travel. In this post, you imply this motion is a self-dampening wave and self-correcting.

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

I disagree. As the rear steps out, the vertical axis of the bike changes, both front and back, ie inducing roll. In turn, because of the castoring effect of the front wheel a turning vector is imparted. There is also the problem of tire aspect changing as well, and I'm getting a headache. I'm afraid I'm going to say something soon which involves math, bio major dontcha know. Perhaps you could consider the problem from this perspective and tell me where I've erred

 

[motobiatch]

[youtube]GvHtChodNk0[/youtube]

 

[MCSFTGUY]

I think you and I are describing an elephant from different poles with our eyes closed. I have not mentioned friction at all. I'm simply stating that a yaw of the rear produces motion of the handlebars relative to the direction of travel. In this post, you imply this motion is a self-dampening wave and self-correcting.

I disagree. As the rear steps out, the vertical axis of the bike changes, both front and back, ie inducing roll. In turn, because of the castoring effect of the front wheel a turning vector is imparted. There is also the problem of tire aspect changing as well, and I'm getting a headache. I'm afraid I'm going to say something soon which involves math, bio major dontcha know. Perhaps you could consider the problem from this perspective and tell me where I've erred

GEEZ I love this conversation, thanks Paul, your are really make me think things through. Your eval of the castoring of the front wheel and then turning is spot on. And, in a rigid body problem I would say your evalution is right on the money... except for the part about two vertical axis, no big deal. By definition, yaw is rotation around a vertical axis. In a rigid body that vertical axis would be through the center mass, rotation to the rear induces everything you described to the front. But, in the case of a motorcycle the pivot point at the steering head isolates the yaw... this pivot point becomes the vertical axis of the rotation and the rotation can occur without effecting the rest of the vehicle because it can pivot.

Static case, can you set your motorcycle upright, hold the handle bars in place, straight ahead and then simply move the rear tire around to one side without changing the orientation of the handle bars?

I know a guy, who knows a guy who has an uncle whose brother in law knows some people that might be able to get us a video from testing. Let me see what I can come up with.

I got a lot of respect for the bio-science people... I could never do all of that memory stuff, I was a a physical science guy, learn the rules and build from there.

GOOD STUFF.