We often hear that 90% of the braking is on the front wheel of the bicycle, sometimes with the added comment that we therefore do not even need the rear wheel. Let’s look at that idea.
First maximum braking comes when both wheels are braking at 100%. 100% braking happens when the wheel is being braked as hard as it can be without losing traction. Now let’s break that 90% down into a more understandable statement. Braking force is conventionally stated in negative G. G equals approximately 32 feet per second squared. For simplicities sake lets say a given bicycle with an expert rider can brake at 1G. Now if the front wheel is doing 90% of that, I think that an expert can get 15:85 or even 20:80, but for the sake of this we will use that commonly stated 10:90 braking ratio (You did know that is what is what is meant by that 90% figure, right?).
OK, now we can look at the silly idea that you do not need the rear brake. Braking with just the front brake reduces your maximum braking power from 1.0G to 0.9G (an 11.1% loss) right up front. Now if I am correct about an expert being able to get a 20:80 braking ratio you do even worse you have reduced your total braking ability by 25% (Insidious things those percentages, aren’t they?).
An aside: I think that 10:90 figure is about right for a dropped bar rider who does nothing but squeeze the brake levers. My 15:85 would be about right for an upright rider with a setback seat. And the 20:80 would be about what an expert who slides as far back as he can to transfer as much weight as possible to the rear wheel.
So how do you get maximum braking in a pinch? First you side your body to the rear of the bicycle and transfer as much weight to the rear wheel as possible; next you brake both wheels as hard as you can without skidding. Those who say you should just lift your rear wheel do not understand simple physics for we have shown above that doing so loses 10-25% of your braking power and if the front loses traction you have lost 90% of the remaining braking power (it is generally thought that a skidding tire only has 10% of the traction a rolling one does). In reality you would be trying for about 95% braking on each wheel so you have a bit of a reserve if you hit a rough spot or something. With that technique you should have about 120% of the braking ability you would have with only the front wheel in the same conditions.
You will also notice that the higher a percentage of the braking you can move to the rear wheel the more total braking power you will have because you are not reducing the front braking power by doing so, only increasing the rear braking power by raising the rear wheel skid point.
Some other things to consider about braking: If you skid the rear wheel things may get a bit squirrelly, but if you skid the front you lose control of the bicycle and are likely to go down. I have never seen a bicycle take a header at speed from simply braking too hard, what usually happens is that the front wheel skids the rider loses control. The result looks like a header because it happens so fast, but it is not caused by rolling over the wheel as it would be if something got caught in the spokes as you would have to be going quite slow and have your weight well forward to have enough traction to do that instead of skidding the front wheel.
I have simplified things in this post. One could of course find the actual coefficients of friction for the tires involved and plug in real numbers to calculate things pretty accurately, but that would only prove accurate for that particular bike and rider, so the generalities I used above are of more interest to most of us.
If nothing else, I hope the above dispels the silly myth that maximum braking comes when the rear wheel lifts. Because at that point you are riding a unicycle with a loose seat and reduced braking power, not the safest thing to be doing in an emergency stop.