And I realize in some circumstances it is o.k. to slow down using the engine, by shifting down. But in most cases this is the more dangerous way to slow down.
The reason is that, by downshifting to slow down, you are depending on the traction of the ground/pavement to force the rear wheel to keep turning, albeit at a slowing rate. If the surface traction is not sufficient to keep the wheel turning against the force of the engine (that is trying to slow the wheel down) the engine power will exceed the road resistance, and the rear wheel will nearly completely stop.
In my case, I shifted from 3rd to 2nd gear, just a few feet before the steel storm drain grate. That grate had about zero traction for my knobby tires. When I went over the storm drain, the rear tire slowed to probably around 5 mph, but my motorcycle was still travelling at about 25 mph. When I reached the other side of the storm drain my rear tire met the far greater friction of the road -- still going nearly 25 mph, but with my rear tire rotating at about 5 mph. This was nearly a dead stop for my motorcycle, and it fell over about 4 feet on the other side of the storm drain, and I went over the front tire.
For more information on my incident, see this:
http://www.motorcycleforum.com/showthread.php?t=101529
The reason is that, by downshifting to slow down, you are depending on the traction of the ground/pavement to force the rear wheel to keep turning, albeit at a slowing rate. If the surface traction is not sufficient to keep the wheel turning against the force of the engine (that is trying to slow the wheel down) the engine power will exceed the road resistance, and the rear wheel will nearly completely stop.
In my case, I shifted from 3rd to 2nd gear, just a few feet before the steel storm drain grate. That grate had about zero traction for my knobby tires. When I went over the storm drain, the rear tire slowed to probably around 5 mph, but my motorcycle was still travelling at about 25 mph. When I reached the other side of the storm drain my rear tire met the far greater friction of the road -- still going nearly 25 mph, but with my rear tire rotating at about 5 mph. This was nearly a dead stop for my motorcycle, and it fell over about 4 feet on the other side of the storm drain, and I went over the front tire.
For more information on my incident, see this:
http://www.motorcycleforum.com/showthread.php?t=101529