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I've been puzzling over this for too long without coming up with a good explanation, so I'm hoping somebody here can cure my ignorance and free my mind for other puzzles.

Nearly always when I'm leaning a bit harder than a surface with non-uniform roughness or wetness will allow, it is the rear tire patch which starts to move out, not the front. (This will happen on an isolated wet spot, crosswalk marking, or small sandy spot. When the area of crappy traction is large, I tend to be upright enough to not slide as I like to have some reserve.) What puzzles me, filled with notions like coefficient of friction, is that for a given lean angle, the ratio of side force to normal force ought to be very close to the same for front and rear contact patches. So why doesn't the front begin sliding before the rear sometimes? Is it just because the rear tire is a little bigger? Do the side forces somehow distribute differently between the wheels than the downward forces? Does the front contact patch deform less?

Thanks for any insight.
 

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It's because the rear tire is under power. The additional force will break it loose more easily.
 

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Discussion Starter #3
gas, perhaps

It's because the rear tire is under power. The additional force will break it loose more easily.
That may well explain some of the rear tire slipping, but I wonder if there is something more happening. I really try to maintain even speed around corners where good traction looks to be a bit iffy, such as where those plastic crosswalk markings are, or when it is soon enough after a rain that wet patches remain. When the roadway is dry and uniform, I tend to accelerate through turns, but that is not when I notice these rear tire slips. Maybe the gas needed to not decelerate is adding to the rear contact patch sliding force, but that seems to be a significantly smaller force than the centrifugal force.

Does anybody else see rear tire slip favored when maintaining speed through turns?
 

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Maybe the gas needed to not decelerate is adding to the rear contact patch sliding force, but that seems to be a significantly smaller force than the centrifugal force.

Does anybody else see rear tire slip favored when maintaining speed through turns?
But both tires receive the same force at steady speed due to cornering lean. The rear get the additional force from the engine so the force on the rear tire is greater. Coefficient of friction is independent of area so contact patch size is not involved. A larger contact patch has less weight per square inch so that negates the larger area. I think just about everybody here who has had a tire slide on a wet day and is aware of which slid first will report the rear breaking free first on a steady speed turn. Paint stripes and manhole covers are slippery when wet. Slow down when turning over them.
 

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+1 with the above. I would also add when decelerating around corner, especially in lower gears, the motor will act as an engine break slowing down the rear wheel faster than the front. I would avoid accelerating or decelerating around corners on questionable surfaces. Keep an even throttle, maintain a decreased lean angle. When roads are wet you really want to stay out of the center of the lane (the oil slick) especially when approaching intersections. The oil slick becomes invisible when wet.
 

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Weight distribution, tire pressure and geometry, and different tire compounds front vs. rear are certainly factors. Also, the front forks act as a lateral shock absorber when the bike is leaned over, while the rear is relatively stiffer laterally. There may be something you can do with rear tire pressure and rebound damping to better keep the rear contact patch actually in contact with the road surface. While the coefficient of friction doesn't change, the amount of energy that a tire can impart to the road without shearing the rubber increases as the contact patch increases, which is why dragsters have large rear tires.
 

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But both tires receive the same force at steady speed due to cornering lean. The rear get the additional force from the engine so the force on the rear tire is greater...
Just to expand on that:

To keep a steady speed you need to apply power. I would understand "Neutral on the throttle" to mean that I am neither accelerating nor decelerating. But this means that power is being applied to counteract drag and other forms of friction. To maintain speed when cornering you need to apply a little more power than when going in a straight line as the act of changing direction uses power (you are accelerating to the right or left when cornering even though your speed might remain constant.) The higher the "G-forces" are when cornering, the more you need to lean, and to remain at steady speed you need to add more power.

..Tom
 

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tire circumference

To add what to Tom says, which is always right IMHO, when you lean the bike over in a corner the circumference of the tire in contact with the road is smaller. So without any deliberate change in throttle, and therefore no change in engine RPM, the bike must slow down. Therefore some deceleration forces are affecting the bike and may cause the rear tire to slip first before the front.
I believe that "throttle maintenance" is the correct term; a rider needs to increase the throttle in a corner to compensate for the change in tire circumference to maintain the same speed.
People that know me will know my conservative riding style; and riding at the raged edge of friction isn't my idea of safe plying. But I do understand some stuff.
Drifting through corners and sliding along while feeling one's way on a knee puck must be a real blast on a racetrack. I'm now probably more than 40 years to old to learn how to do it.
 

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Most bikes carry more weight of front. You get most of your braking force due to weight transfer to front.
 

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Most bikes carry more weight of front. You get most of your braking force due to weight transfer to front.
That is true while braking due to the shifting weight, but not necessarily true when standing still. And when accelerating the weight shifts rearward.

As an aside, you really do want the rear tire to break loose first when cornering as it is a much more controllable slide condition. If the front breaks free on a corner first you'll be down before you know what happened.

When choosing unmatched tires you should always try to put your stickier tire on the front, and also do not want to inflate the front tire higher than the rear.
 

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obviosly your bike is toast..... I'll give you 100 bucks for it....
:devil_0:
 

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So why doesn't the front begin sliding before the rear sometimes? Is it just because the rear tire is a little bigger? Do the side forces somehow distribute differently between the wheels than the downward forces? Does the front contact patch deform less?

Thanks for any insight.
Wow! Interesting question almost best answered by blaming it on magic. Although I can't disagree with what you personally experienced, I have to disagree with some of the finer points of your analysis. A well designed bike, riding on properly matched, ideally inflated tires will lose traction in the rear first partly by design. It is relatively easy to save a rear wheel slide on the street but very, very difficult to rescue the front end if it runs out of friction. This did not escape manufacturers. Truth is, on some bikes, the front end will feel uncertain, especially when riding on unmatched tires or if the bike is improperly loaded, so that the CofG is out of range. Suspension setup and condition also has a big effect of traction balance, as does tire condition, age and temperature.
Did you know street tires were designed to go around curves while under power? This has the effect of transferring weight to the rear contact patch, improving rear tire grip. Application of power causes the rear tire to slip longitudinally so that it actually rotates faster than the front(!) Due to the effect of engine brake while decellerating of slip while under power, coupled with the fact that the rear tire, even at low speeds is never really pointed directly in the direction of motion in a turn, causing it to twist its contact patch along, the rear contact patch is almost always unstable.
The front tire, by comparison is not exposed to any thrust effects other than trail-braking (for those who do this) and is always pointed directly at the target, obviously. It therefore has a marginal advantage, which can be lost by:
sudden braking, contamination, bumps, elves, etc...

Hope this helps!
 

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Wow! Interesting question almost best answered by blaming it on magic. Although I can't disagree with what you personally experienced, I have to disagree with some of the finer points of your analysis. A well designed bike, riding on properly matched, ideally inflated tires will lose traction in the rear first partly by design. It is relatively easy to save a rear wheel slide on the street but very, very difficult to rescue the front end if it runs out of friction. This did not escape manufacturers. Truth is, on some bikes, the front end will feel uncertain, especially when riding on unmatched tires or if the bike is improperly loaded, so that the CofG is out of range. Suspension setup and condition also has a big effect of traction balance, as does tire condition, age and temperature.
Did you know street tires were designed to go around curves while under power? This has the effect of transferring weight to the rear contact patch, improving rear tire grip. Application of power causes the rear tire to slip longitudinally so that it actually rotates faster than the front(!) Due to the effect of engine brake while decellerating of slip while under power, coupled with the fact that the rear tire, even at low speeds is never really pointed directly in the direction of motion in a turn, causing it to twist its contact patch along, the rear contact patch is almost always unstable.
The front tire, by comparison is not exposed to any thrust effects other than trail-braking (for those who do this) and is always pointed directly at the target, obviously. It therefore has a marginal advantage, which can be lost by:
sudden braking, contamination, bumps, elves, etc...

Hope this helps!
Well, yes, that explains why I wear out front tires faster than rears :) . (Not!).

Mostly, it comes down to your riding style, you are riding in a way that's decreasing the chances of a front slide.

(I'm probably not)

Pete
 

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I don't know if this will help or not but here goes:

I have low-sided twice, on the same bike, and embarassingly worse yet, in the very same turn, and it was a 50-50 split between the rear tire sliding out vs. the front tire sliding out.

'Twas on my trusty old '73 Honda CL450 scrambler back in the early 80's. And it was the entrance to the parking lot at my work, which had a dip in it as it transitioned off the street, and the location was the High Desert of So Cal (Hesperia to be exact). On days and weeks when the wind was free, it was not uncommon for the dip to fill with desert sand and become treacherous.

So there I was arriving to work a little late one day and I turned right into the parking lot from the street a little hotter than usual and WHAM down I went, my right side slamming down onto the pavement and of course, since who wears gear when you are young and invincible, sanding a large patch of skin off my right forearm. I distinctly remember it was the front wheel that slid out, I was on the brake. Also broke the front brake lever, which I couldn't afford to replace at the time (this figures in another story of stoopidity which I shall regale everyone with at a later time).

Not less than a couple of weeks later, having had a long hard day at work (Metal Finisher aka "Abrasive Engineer") I was leaving the parking lot and decided to get a little too frisky with the throttle as I was making a left out onto the street... as I went through the dip with sand, WHAM down I went, my left side slamming down onto the pavement and of course, sanding a large patch of skin off my left forearm. I distinctly remember it was the rear wheel that slid out, I was on the gas. Also did NOT break the clutch lever, which enabled me to keep riding with a broken front brake lever which enables the events in a story of stoopidity a couple weeks later regarding no front brake and an unseen freight train at an open railroad crossing in the middle of the desert, which I shall regale everyone with at a later time. Perhaps next winter.

So the moral of the story the way I see it, without a lot of physics class hullabaloo, is that the traction budget of the tire needs to be taken into account, and whichever tire has greater accelleration or braking force applied to it while having equal side load force as the other, is the one most likely to overrun it's traction budget and thus try to sand a patch of useful skin off of your forearm, or other protruding bodily... component.
 

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If there is foreign material on the road surface that decreases the traction suddenly, then all bets are off. We were talking more about what happens on otherwise unadulterated roads that one encounters 99.9% of the time.

Like your wind blown desert sand, ice on New England roads (about now) or coal dust and diesel spills in the mountains of West Virginny, you will not have a chance because the road's available traction has been reduced too much. The front will wash out first (and you'll probably brush your teeth on the pavement) because it's the first wheel to get there and the traction is so limited.
 

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Discussion Starter #17
multi-quote response

Re "But both tires receive the same force at steady speed due to cornering lean. The rear get the additional force from the engine so the force on the rear tire is greater." (greywolf): I have to disagree. When at steady speed, the same fore/aft distribution occurs for lateral and vertical forces arising from turning and gravity.

Re "Coefficient of friction is independent of area so contact patch size is not involved. A larger contact patch has less weight per square inch so that negates the larger area." (greywolf): True, but the "coefficient of friction" idea is a simplification which ignores certain non-linear effects, such as heating during slip (which is more concentrated in a smaller contact patch, leading to higher temperature rise), and deformation of the patch under slip conditions. (Kelly2012 touches on the former in #7.) Given that it's a close contest between front and rear sliding base on pure friction theory, it seems reality's deviation from that ad-hoc theory is relevant.

Re "Paint stripes and manhole covers are slippery when wet. Slow down when turning over them." (greywolf) and "When roads are wet you really want to stay out of the center of the lane (the oil slick) especially when approaching intersections. The oil slick becomes invisible when wet." (bleedgrn): Good advice, thanks. Under such conditions, I hardly ever slide because I (try to) leave enough traction margin to account for the fact that I cannot see the difference between clean water and slightly oily water in time to adjust. The sliding I ask about is over small enough low-traction areas that the tire is over them for less than a tenth of a second. The recovery is assured over the following track with better traction. (I'm not sliding on manhole covers; I strictly avoid those while leaned.)

Re "To maintain speed when cornering you need to apply a little more power than when going in a straight line as the act of changing direction uses power (you are accelerating to the right or left when cornering even though your speed might remain constant.)" (V-Tom): Yes, but the extra power is not to supply centripetal force, which is essentially perpendicular to velocity and hence requires no work. Tire losses are increased, especially when limited slipping occurs, and the rear wheel must rotate slightly faster (as others note), so a bit more gas is needed. But applied extra force at the rear patch is only countering loss within that patch or the extra force developed at the front patch. As far as I can see, the only fore/aft in-line forces either balance out or (continue to) make up for air drag, which is a small effect at the 20-25 MPH speed I'm doing when these minor rear slides occur.

Re "Therefore some deceleration forces are affecting the bike and may cause the rear tire to slip first before the front." (568v8): That may happen sometimes, but I am getting the rear slide in the steady-speed portion of turns, (typically when crossing the marker lines for a crosswalk across the street into which I'm turning.)

Re "... conservative riding style; and riding at the raged edge of friction isn't my idea of safe plying." (568v8): I ride conservatively as well, and part of my practice is to understand where the limits actually are so that I can retain suitable traction margin without riding like little old ladies drive and know what can be done in emergencies. I intentionally approach slip over crosswalk markings because I want to know how slippery they are, but always while crossing them rather than riding along their length to assure recovery.

Re "Most bikes carry more weight of front." (Scott E.): Not true for most bikes I've seen, and certainly not with an adult rider.

Re "you really do want the rear tire to break loose first when cornering as it is a much more controllable slide condition." (Fred W): Yes, and that's why I'm asking the question. I want that enough to desire some control over it.

Re "When choosing unmatched tires you should always try to put your stickier tire on the front, and also do not want to inflate the front tire higher than the rear." (Scott E.): That "stickier" tip is attractive. I wonder how to implement it, though. I also wonder how inflation interacts with slipping. Have you seen anything providing illumination on that?

Re "I have to disagree with some of the finer points of your analysis." (griffondude): Hmmm, that's a puzzle because I had not gotten to anything but the crudest initial look at the problem. Not because I cannot see the finer points but to avoid predisposing the discussion and to not pretend more knowledge than I have. (Since the problem still puzzles me, I figured I must be missing something.)

Re "A well designed bike, riding on properly matched, ideally inflated tires will lose traction in the rear first partly by design." (griffondude): I'd sure like to know my V-Strom was so well designed. Is there reason to believe so? I would also like to understand enough of what causes the "rear slide first" behavior to be able to assure it, or at least know when it is being compromised.

Re "Due to the effect of engine brake while decelerating of slip while under power, coupled with the fact that the rear tire, even at low speeds is never really pointed directly in the direction of motion in a turn, causing it to twist its contact patch along, the rear contact patch is almost always unstable." (griffondude):

I don't get your point "engine brake ... under power". I suppose that would put more normal force on the front wheel, giving it a lower ratio of sliding force to normal force, but I'm seeing the preferential rear slip during near-steady-speed turns.

The twisting of the rear contact patch is an interesting idea. That effect is amplified by its larger area on a normally built and loaded bike. I'm skeptical enough of the effect's magnitude to do a little experiment with the front wheel on a dolly so I can see how far the twist has to go to begin moving the outer edge of the rear patch. I suspect that will be more than the twist occurring in a normal radius street turn, but I'll report back when (and if!) I do that experiment.

Re "Mostly, it comes down to your riding style, you are riding in a way that's decreasing the chances of a front slide." (PeteW): I hope that's true, and if I knew what about my style makes it true, I'll be sure to keep doing that. Any idea as to what the trick is?

Re "So the moral of the story the way I see it, without a lot of physics class hullabaloo, is that the traction budget of the tire needs to be taken into account, and whichever tire has greater accelleration or braking force applied to it while having equal side load force as the other, is the one most likely to overrun it's traction budget and thus try to sand a patch of useful skin off of your forearm, or other protruding bodily... component." (Stoopy): I'm a big believer in learning from other people's mistakes. I'm looking for a way to be more likely to have the rear start sliding first (when any sliding is in the cards), and would love to learn how to arrange that, whether or not any physics or hullabaloo is apparent. Your "arm sanding" story helps motivate that desire, and, more specifically, shows that mixing hard turns with surfaces going up and down is fraught with hazard.
 

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A steady speed turn requires delivering engine power to the rear wheel. If both tires and the road under them are providing the same coefficient of friction and the front brake is not being applied, the rear will break free first.

If you want to make sure the rear slides first, you have to avoid putting the front tire on a slipperier surface than the rear and accelerate through the turn.
 

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A steady speed turn requires delivering engine power to the rear wheel. If both tires and the road under them are providing the same coefficient of friction and the front brake is not being applied, the rear will break free first.

If you want to make sure the rear slides first, you have to avoid putting the front tire on a slipperier surface than the rear and accelerate through the turn.
Applied physics that turned flat tracking into a sport.
 

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I stand corrected - on most bikes except pure racing bikes, when loaded the rear carries more static weight. You had me intrigued as motorcycle dynamics are not fully understood by even the most knowledgeable on the subject. So, I did a little old fashion research and came up with the following two C&P topics. I cited one but not the other, hopefully this is not a forum no-no.

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

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

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

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

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

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

There are three things that you could do:

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


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


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


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

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

Aren't you glad, now, that your motorcycle generally has over-steer rather than under-steer built in? There is essentially no 'fix' if your front-end slides out from under you while in a curve. But if the rear wheel continues to spin, it can slide a little and you can continue your ride, a little more 'puckered' but none the worse for it.
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