Steer for the Rear - Ch13 of TOTWII

[Lnewqban]

All excellent answers, gentlemen!

Based on those, and talking in forces and traction terms only, why are the rear tires sliding in this video, while the front tires are not?

 

[Hotfoot]

On ‎12‎/‎20‎/‎2017 at 4:53 PM, Lnewqban said:

If you can do it without inducing a slide of the rear tire, it is only because your tires have not been loaded enough (with cornering speed) in the previous stages of cornering: your entry speed was lower than it could have been.  In other words, you will be trying to compensate a slow entry with a fast exit.

Well said.

[Jaybird180]

I don't know why the rear tires are sliding in that video but the common thing I see just before the slides is a movement in the riders head&body back to the centerline of the bike. I'm wondering what's going on with that R1 as everything shook including the fairing stay.

[JohnCBoukis]

The question is difficult because the problems are not obvious to me, some of the riders appear steady on the controls, and also some of these appear to be going uphill (although that could be the camera perspective) which would add helpful load to the rear wheel.

This has me thinking about the bicycle coasting question. Are several of these riders coasting from too little throttle? You asked for force nomenclature, the rear would be unloaded, generating less friction than the centripetal force of the turn. The front wheel would have excess weight and thus adequate traction.

[Jaybird180]

You bring up an interesting point. A rider COULD cause loss of rear traction by inadequate roll-on. Seems counterintuitive to the pedestrian.

[Spaghetti]

You slide the rear going into the turn (or going into the lean). Visually it almost looks like a slow motion sequence of the rider applying leaning forces on the bike.

Once the bike is committed to the turn a rear slide can only cause stability problems. On the exit it can cause a high side.

[Hotfoot]

On ‎12‎/‎22‎/‎2017 at 10:31 AM, Jaybird180 said:

I don't know why the rear tires are sliding in that video but the common thing I see just before the slides is a movement in the riders head&body back to the centerline of the bike. I'm wondering what's going on with that R1 as everything shook including the fairing stay.

Here's some hints on these rear tires slides: LISTEN to the engine, how smooth is the throttle application when leaned over?

Watch the rider on the R1s throttle hand - what does he do when the rear tire starts to slide?

What control could a rider be using that could cause the rear tire to slide on the entry?

What could a rider do on the corner exit that would cause the rear tire to be under much greater load than the front?

I'd also ask - especially in the case of the black guy on the black bike near the beginning of the video - what is the condition of the tires, and are they adequately warmed up?

[Lnewqban]

On 12/22/2017 at 5:33 PM, JohnCBoukis said:

The question is difficult because the problems are not obvious to me, some of the riders appear steady on the controls, and also some of these appear to be going uphill (although that could be the camera perspective) which would add helpful load to the rear wheel.

This has me thinking about the bicycle coasting question. Are several of these riders coasting from too little throttle? You asked for force nomenclature, the rear would be unloaded, generating less friction than the centripetal force of the turn. The front wheel would have excess weight and thus adequate traction.

Regarding the bicycle coasting question: The forces acting over the contact patches in the case of coasting are only the portion of the total weight that each tire carries (vertical force vector pointing down) plus the lateral force trying to deviate the bike from a linear trajectory (horizontal force vector pointing towards the center of the circular trajectory described by the bike). Fortunately, the magnitude of that lateral force for a particular tire depends on the portion of the total weight that that tire carries. The other two factors on which that horizontal force depends are the radius of that circle (the smaller the radius, the stronger the force) and the square of the speed (the faster the bike, the stronger the lateral force, squarely.

The possible reasons for the slides of those rear tires in the video are many.  As Hotfoot pointed above, the rear tire could be worn, improperly inflated, too hot or too cold or under excessive braking or accelerating forces.  Even when both tires were made of the same material (possible same brand and pattern) and being rolling over similar surface and trajectory, the available traction of the front tires in each case were higher than the one for the rear tires.

Disclaimer: I am not expert; only an enthusiast of Physics, Math and motorcycling. Without being scientifically rigorous, this is what I have learned from books and from riding many miles in all weather conditions:

1) Understanding what is happening in the relatively small areas of both surfaces where rubber and asphalt meet (contact patches of both tires) is extremely important, as much for avoiding crashes as for high performance riding. The proficient rider develops a fine sensitivity about this; he/she can almost feel what is going on there, based on visual evaluation, comparative experience and feed-back from suspension and steering.

2) Traction or grip is the capability of both materials in contact (rubber and asphalt) to persist on staying together, not dramatically and uncontrollably sliding respect to each other, whether or not lateral forces (excerted in any direction that is parallel to the surface) are applied onto that contact patch. Physics tells us that such capability only depends on two things: nature of materials in contact (molecular structure and superficial roughness) and force that is exerted in a direction that is perpendicular to the contact surface (also known as normal force). After many laboratory tests, a "coefficient of friction" is obtained for each combination of materials, which is just the rate between the magnitude of the lateral force that is able to induce an slide and the normal force. That coeficient is a fixed number, which means that both forces are proportional and that in order to achieve more traction, you must increase the perpendicular or normal force. Example: double normal force in magnitude gives you double traction or friction, being the contact patch able to resist double lateral force in magnitude. 

3) Considering only the available traction of each contact patch of our machines: While rolling at high speed over a less than flat and perfect asphalt surface, our tires deviate some degree from that pure Physics concept of traction.

Static perpendicular or normal force: Static weight on each contact patch is the available perpendicular or normal force . That is only true for a horizontal surface; if there is camber of the track or road, that normal force becomes smaller than the weight. If the bike is going up or down a sloped road, there is more static weight on the lower tire and less on the one located higher. 

Dynamic perpendicular or normal force: Then, there is a dynamic load, which is fluctuating (greater and smaller than the magnitude of the weight in static conditions). When a tire rolls over a crest of the road's surface, its contact patch "feels" a higher weight or load (the suspension compresses and the available traction improves), the opposite happens when the tire rolls over a valley. When we accelerate or decelerate (brakes or engine brake), we dramatically change the distribution of the total weight of bike, fluids and rider between both contact patches and suspension (from 50/50 up to 0/100). Notice that it is much more difficult for the overloaded suspension to follow the crest and valleys of the track's surface, also that the overloaded tire suffers more dramatic changes in tire profile and contact patch area.

Coeficient of friction: Between a rolling tire and asphalt, this is not as constant as the lab experiments demonstrate. We are not dealing here with a loaded chunk of flat rubber that resists slidding. In the case of a rolling tire, any contact patch is rapidly disappearing to be followed by the next contact patch a few inches ahead (reason for the side-walking of the tires while cornering hard, when combined with lateral deformation of each contact patch, the next one lands off track). There is the molecular attraction of the two materials, but there is also the number of macro-imperfections of the asphalt and how deep the rubber "flows" into those; hence, bigger area and higher pressure of contact improves traction. Then, we have the dependence of the rubber on temperature: too cold and does not deforms well enough to dig into the surface, too hot and the integrity and resilience of the rubber degrades and slippery oils start to come out. There is also the time factor: being a flexible solid behaving as a very viscous fluid, rubber needs time to "flow" down and sideways and to get twisted: too quick the load application and the rubber reacts as a hard rock (imagine the water resistance on your body diving from 30 foot trampoline versus walking into a pool); hence the advice about smooth control inputs. Finally, foreigner material that eventually infiltrates between both surfaces, such as sand, water, Diesel, dramatically reduce the coefficient of friction (bassically sliding friction becomes rolling friction, as those molecules and small particles act like little balls).

4) Finally, considering only the horizontal forces that the contact patches "feel" as reaction forces back from the asphalt (those trying to slide or detach both surfaces in contact, inducing a skid or a slide) on each contact patch of our machines. There are three fundamental types of these horizontal forces: A) Longitudinal forces (horizontal force vectors pointing directly forward (accelerating) or rearward (decelerating). B) Lateral force (horizontal force vector pointing towards the geometric center of the circular trajectory). C) A combination of A and B above (horizontal resulting vector pointing at certain angle between the trajectory and radius lines), which happens when we simultaneously corner/accelerate or corner/brake. The magnitude of those forces are limited by the traction that is available at each moment, which can be represented as an imaginary circle around each contact patch. Each time that one of these forces grows beyond that limiting circle (available traction), the inexorable laws of Physics punish us with a skid or a slide (excessive entry speed, for example). Exactly the same result happens each time that the circle of available traction suddenly shrinks and its diameter becomes smaller than the the magnitude of any lateral force for that tire (cornering and crossing over a patch of sand, for example).

I find "A twist of the wrist 2" book to be very accurate about the Physics of motorcycling. It is not scientifically rigorous, but it seems to me that the principles that it explains were based on accurate and objective observations. I have extensively experimented with its tips, techniques and advise, and all work regarding performance and safety.

Once again, excuse me for the long post.

 

 

[Jaybird180]

On 12/23/2017 at 10:01 PM, Hotfoot said:

Here's some hints on these rear tires slides: LISTEN to the engine, how smooth is the throttle application when leaned over?

Watch the rider on the R1s throttle hand - what does he do when the rear tire starts to slide?

What control could a rider be using that could cause the rear tire to slide on the entry?

What could a rider do on the corner exit that would cause the rear tire to be under much greater load than the front?

I'd also ask - especially in the case of the black guy on the black bike near the beginning of the video - what is the condition of the tires, and are they adequately warmed up?

On the clips of smooth riders, the throttle application is also very smooth. The R1 rider as well as the black dude rolled off the gas when the rear began to slide. The guy on the Duc rolled out & straightened the bike just before his near miss of the road sign.

The guy on the naked bike (Aprilia maybe?) was on the rear brake on corner entry and it looked like the slide there was intentional.

Too much throttle could cause excessive rear tire load.

[Hotfoot]

The guy on the Ducati would really benefit from CSS Level 3, check out his transitions across the bike (and how the bike reacts) and his lock on, how secure does his lower body look? 

[DucPati]

On 12/26/2017 at 10:24 AM, Jaybird180 said:

The guy on the naked bike (Aprilia maybe?) was on the rear brake on corner entry and it looked like the slide there was intentional.

Yeah he was definitely backing it in on purpose. That could also have been from engine braking rather than rear brake.

[DucPati]

On 12/26/2017 at 11:33 AM, Hotfoot said:

The guy on the Ducati would really benefit from CSS Level 3, check out his transitions across the bike (and how the bike reacts) and his lock on, how secure does his lower body look? 

Has stomp grip on the tank, apparently doesn't know what it's there for - his outside foot wasn't even on the peg, and that was before he went off road. After watching a few more times, looks like the slip off the peg is what caused him to go off road. May have been preceded by target fixation on the sign, but hard to tell for sure from that video.

 

Anyone also notice that he put the front brake on, and kept it on until he was thrown in the air, and then back on again when he came down onto the bike again? Either he has a very light touch (it was only one finger on the brake by the looks of it), or ABS saved his butt.

 

Regarding Ducatis and locking on - because they are narrower around the tank it can be difficult to get a good lock on the way it's taught in lvl3 (in my experience on my 1098S anyway). Different rearsets would probably help. I have had my outside foot come off the peg in a left hander at ~200-220kmh while knee down, but I had a pretty solid grip with my leg/knee and it didn't upset me at all or seem to make any difference to the laptime.
 

[Jaybird180]

I saw the braking action and thought he got real lucky...again.

[Lnewqban]

Accelerating hard (enough to lift the front tire) at extreme lean is equivalent to hard braking at extreme lean: the tire will slide.

The very essence of the trail braking technique is the trade off between the longitudinal force of braking and the lateral force of cornering.

https://www.sportrider.com/sportbike-riding/riding-skills-series-traction-circle

https://www.motorcyclistonline.com/leaning-bike-code-break

Our natural gauges to feel and evaluate braking force are the degree of compression of front suspension and forward pressure of our weight on knees and hands.

Our gauges for evaluating cornering force are the lean angle and the pressure of our butt on the seat.