Rear tire slides first, why? - Stromtrooper Forum : Suzuki V-Strom Motorcycle Forums
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post #1 of 40 Old 03-19-2014, 01:50 AM Thread Starter
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Question Rear tire slides first, why?

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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post #2 of 40 Old 03-19-2014, 02:22 AM
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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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post #3 of 40 Old 03-19-2014, 02:57 AM Thread Starter
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Question gas, perhaps

Quote:
Originally Posted by greywolf View Post
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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post #4 of 40 Old 03-19-2014, 03:07 AM
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On dirt, it tends to be the other way around and staying off the front brake is a good idea.

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post #5 of 40 Old 03-19-2014, 09:48 AM
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Quote:
Originally Posted by Trepidator View Post
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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post #6 of 40 Old 03-19-2014, 10:24 AM
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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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post #7 of 40 Old 03-19-2014, 11:32 AM
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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.

Last edited by Kelly2012; 03-19-2014 at 11:34 AM.
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post #8 of 40 Old 03-19-2014, 11:38 AM
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Quote:
Originally Posted by greywolf View Post
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.

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post #9 of 40 Old 03-19-2014, 02:26 PM
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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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post #10 of 40 Old 03-19-2014, 02:58 PM
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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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