Keeping Your Rear Wheel on the Ground
(is Tough to Do)
By: James R. Davis
Most motorcyclists prefer that their riding be uneventful. They might consider the ride itself an event, or that they are riding to or from an event, but along the way they want that ride to be CONTROLLED and without any other events that are potentially dangerous.
In order for a motorcycle ride to be ‘uneventful’ both tires must remain on the ground and not be skidding/sliding. That seems pretty obvious, I’m sure, because when one or both of their tires leaves the ground they are no longer in control of their machines and are then merely along for the ride until the tire hooks up again. When traction has diminished to the point that steering input becomes imprecise (or non-existent), or braking becomes ineffective (or non-existent), an event has occurred.
One of the least appreciated parts of your motorcycle, the one that has the most to do with insuring that your tires stay on the ground, is your shock absorber. That device is thought by most to ‘absorb’ irregularities of the roadway surface in an effort to make them essentially unnoticeable to the rider. Not true. (Your springs do that.) What your shock absorbers are designed to do is keep your tires in contact with the roadway, regardless of those irregularities. What good, after all, can your brakes or throttle or steering input do if the tires have no traction because they are off the ground?
Your shock absorbers do good work for you for the same reason that you are not crippled or killed every time you pull the trigger on a pistol or rifle – the bullet has far less mass than you and the gun. A compressed spring in your shock absorber can do only one thing – decompress – that’s the law. Like ignited gunpowder, that decompressing spring pushes EQUALLY on the wheel (bullet) and the bike's frame (gun and you).
Because your wheels are substantially less massive than the frame of the bike, they move farther and faster away from the spring than does the frame. In other words, after you hit a bump that compresses the springs in your shock absorber, the decompressing springs push your wheel away from the frame of the bike so that they regain contact with the ground much sooner than they would have had they been rigidly connected to the frame of the bike.
So all of that seems to indicate that shock absorbers are ideally designed to keep your tires on the ground, and for the most part you would be correct. But there is one condition in which their design works in exactly the wrong way and they cause your tires to be lifted off the ground instead of pushing them toward that surface – when you use your rear brake.
Huh?
Yep, it’s your rear shock absorber that starts most rear wheel skids, not excessive weight transfer.
I suspect that most of you think that if your rear tire comes off the ground during braking you are doing a stoppie. Rarely true. A stoppie is NOT an event defined by your rear tire coming off the ground. Rather, it is an event defined by your bike frame moving upward and lifting that wheel off the ground in the process.
Semantics? Not at all. A stoppie can only be caused by excessive use of the front brake which results in enough weight transfer from the rear of the bike to the front that the rear-end of the bike has a negative amount of weight left on it. Your rear brake, alone, cannot cause a stoppie. But your rear brake, alone, can EASILY cause your rear tire to lift off the ground entirely.
How is that possible? Well, let’s review the function of yet another motorcycle component – your swing arm. That connects the rear wheel with the frame of your motorcycle via a pivot point located near the middle of bike’s frame. When you accelerate that pivot arm pushes the frame of the bike forward AND UPWARD. (Yep, your bike’s rear-end gets HIGHER as a result of acceleration, not lower.) And when you use your rear brake the swing arm is pulled level and in the process pulls the frame of the bike DOWN as the wheelbase lengthens.
Said differently, when you accelerate your rear shock absorber extends and when you use your rear brake your rear shock absorber contracts.
Now recall, please, that your rear wheel is substantially less massive than is the rest of the bike. So when the rear shock absorber contracts during braking the rear wheel will move UP farther and faster than the rear-end of the bike can move DOWN. The result will be a dramatic loss of traction on the rear tire, and if the use of your rear brake is aggressive enough, the rear tire will be lifted off the ground.
So long before simple weight transfer caused by braking can unload all the weight from your bike’s rear end, the shock absorber will suck that wheel off the ground for you. And that is exactly what you do not want it to do. You want your tires to stay on the ground as we’ve already discussed.
Shortly after your rear tire lifts off the ground the lumbering mass of your bike’s rear-end falls enough to push that tire back into the ground. And what you have just observed is why your rear tire ‘chirps’ when you are too aggressive with the rear brake – it is losing traction and ‘bouncing’ on the roadway surface.
Bad, bad, shock absorber! Behave yourself!
Just one more reason that it is NEVER appropriate, ever, not once in your life, to aggressively use your rear brake.
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http://www.msgroup.org
(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)
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