I have been reading a lot of posts lately (both here and elsewhere), about people locking up their rear wheel. Why does this happen, and how does one avoid it?

Secondly, in the MSF class they explained that when stopping, you should apply both brakes and downshift in a way that if you have to get moving again, you are in the appropriate gear. I understand that if you come to a stop, you need to be in first gear to get moving again. But for example, what if you were going at about 55 mph, and came into town and there was a red light. So you apply both breaks and start downshifting. But then the light turns green and cars start moving again. How do you know if you are in the proper gear for the speed? What happens if you downshifted too much and are in too low of gear for the speed at which you are moving? What if you didn't down shift enough and are in too high of gear for the speed at which you are moving?

Thanks,
Laura

I usually kick it all the way down to first or neutral, if the light turns green it takes less than two seconds to kick it back up to were you want to be.

There are a couple of common ways to lock up your rear wheel: brake hard enough with your rear brake, or snap the throttle closed when you're at high RPM.  If the conditions are right (ie, low traction situations), either one of these can lock the rear wheel.

For your second question, you will quickly get a feel for what gear you're in, by guaging your speed vs. how fast the engine is spinning.  But until then, when downshifting while still moving (like your traffic light example), the key is to release the clutch slowly, while applying a little throttle.  If the throttle isn't high enough, as more of the clutch grabs, the rear wheel will pull the engine faster.  If the throttle is too high, as more of the clutch grabs, the rear will pull the engine slower.

The key is to smoothly release the clutch; that will keep you out of trouble.  For example, if you've slowed to 40mph, but are in second gear, if you release the clutch quickly, your RPMs would jump , and you'd be jerked forward.  If you release the clutch slowly, you would be able to feel this happening, and you would be able to cluch back in, and shift up a gear or two.

Man, that's a lot of questions.  Let's see.  

First off, locking up the rear wheel can happen if you apply the back brake to hard, or if you downshift to a gear that is too low for the speed you are traveling.  Too much force slowing down the rear wheel and not enough grab on the road surface causes you to skid (aka lock up the rear).

Second, if your breaking and downshifting at the same time you don't need to be in any particular gear for a certain speed.  I can travel at 40 mph in 1st, 2nd, and 3rd gear.  You'll know if your in 1st b/c you won't be able to downshift anymore, but even if you're in 2nd or 3rd you should still be okay.  If you get back on the gas and you don't feel like you have enough power, quickly shift down a gear and you'll be okay.  If you downshifted too much you'll lock the back tire, and you may risk throwing yourself over the front of the bike (it all depends on your speed).  If you don't downshift enough it won't really hurt you, just do what I suggested earlier.

Another technique that I think a lot of people use is compression braking.  To do this you down shift one or two gears and let the clutch out easily.  As the clutch engages, the compression of the engine will slow the bike down.  I usually don't down shift two gears unless my rpms are less than 5000 during approach.  Probably even less than that, more like below 4000 (I don't know for sure, just try it for yourself.  Start downshifting at lower rpms, than the next time try at a little higher.  Keep experimenting until you find a comfortable zone to downshift 2 gears).  I also apply the breaks to get my rpms low enough to downshift, but I let the engine slow me down the most.  By doing this you know what gear you're in, and you're ready to take off if the light turns green.

Hope this isn't too long.  Though I think it is.

John L.

well one trick I do to make sure I'm in Second gear when I take a corner is COUNT!  hehe.  might sound stupid but just remember if your in 6th gear count how many times you shift down,  *click* one, *click* two, *click* three, so now your down to 3rd gear.  or just learn what rpms are what gear at what speed,  For example I am doing 45mph at 4000rpm in 6th so I know I'm in 6th.  Also think of maybe 1000rpm per gear (not exactly proportional but close)  so say your doing 45mph in sixth gear at 4000rpm, you want to accelerate but 6th isn't enough so you decide to get the engine up to 6000 before you rev it up, so you downshift 2 times to get the engine around 6000rpm.  comes with practice.

Thanks for the detailed explanations! Very helpful.

They are all right, but I don't know if they answered your question.

The reasons your rear wheel can lock up while braking is because as you brake, your weight shifts hard to the front. This puts much more weight on the front wheel. Obviously at the same time your weight is shifting off of the rear wheel. more weight = more traction. This is why your front hase 70% of the braking power and the rear, only 30%.

It's important to really sueeze that right lever when braking hard.

 -Anti

Edit: Wow Kevin, simple mistake, don't beat me up.

Anti's post helps to explain why the rear disc brake rotor is so much smaller than the front rotor, and why a lot of cars have disc brakes in the front and drum brakes in the rear.

Even though the rear rotor on the GS500 is small, it's still a powerful disc brake.  The rear wheel "lifts" under hard braking - not off the ground usually, but enough that the traction with the road is much reduced.  If you stomp on that rear brake, the friction and torque on the rear rotor will easily overpower the friction of the rear tire on the road.

Result?  The rear wheel "locks up" and you skid....

It's brake, not break! If you brake properly, you won't break anything.

I've watched some less experienced riders lock the rear wheel when braking, by not matching the engine rpms to the wheel speed when downshifting. It is a bit tricky to do, since you are already squeezing the brake lever with your right hand, and then you have to try to rev the engine with the throttle in the same hand.

Locked rear wheel high-side is a pretty common accident with cruiser types, who don't seem to like the front brake. I personally rarely use the rear, except in low traction situations, or sitting on a hill. On  a high Center of Gravity bike (non-cruiser), the rear will be a small percentage of the ttoal braking on a surface with good traction.

http://pdmec4.mecc.unipd.it/~cos/DINAMOTO/H1_High_side_braking/high_side_braking.html

http://pdmec4.mecc.unipd.it/~cos/DINAMOTO/frenata/Braking%20paper/braking_new.htm