James R. DavisPeer Review:
Posted - 01/03/2007 : 3:31 PM
If you can do a stoppie ...
you CANNOT stop as quickly as others
By: James R. Davis
Motorcycle Consumer News published the results of tests on various motorcycles and showed the following summary as to best stopping distances they measured:
Those stopping distances reflect virtually perfect rider skill in brake usage. They do NOT reflect best brakes. You can assume that any current production motorcycle has brakes that are more than adequate to allow you to stop your machine in the shortest possible stopping distance and time as they are all capable of locking your wheels regardless of speed or weight and best stopping requires using less braking energy than is required to lock a wheel.
From time to time on my board I am confronted with otherwise very knowledgeable and highly skilled/experienced riders who make declarative statements that are simply not true such as:
- Sportbikes can stop more quickly than any cruisers
- If you put dual discs on a bike you can stop more quickly
- Weight determines how quickly you can stop
- If you lean forward while braking you increase your ability to do a stoppie
- Any bike can toss you over the handlebars if you brake too hard
- Brakes are what limit how fast you can stop
- Sportbikes can stop more quickly than cruisers because they have better rubber
- A bike with ABS will always stop more quickly than one that does not
- Sportbikes don't have ABS because their riders are more skillful users of their brakes
I know some of you agree with at least some of these assertions but they are all, each and every one of them, false.
For those of you who have read my earlier tips you already know why most of those assertions are incorrect but for those who have not I will try to be concise and provide some facts for you to consider in evaluating these claims. My objective is to get you ALL to use both brakes when you stop, NONE of you to use just the rear brake, and all of you to better understand YOUR motorcycle's braking dynamics.
As an aid I will use a model I built a couple of years ago and make that model available to each of you here.
Some bikes CAN do a stoppie while others CANNOT. (Clearly I mean under normal circumstances. If you are 6'6" tall and weigh 350 lbs., then you getting on almost any bike can make that bike capable of doing a stoppie.) If you ride a Harley-Davidson, your scoot is not normally capable of doing a stoppie but if you ride a 'crotch rocket', it is.
Why is that important? Because if your bike is capable of doing a stoppie it is NOT capable of stopping as quickly as a bike that cannot do so. There is no good reason for you to be shy in the use of your front brake! On the other hand, if you ride a sportbike you know that it is quite capable of doing a stoppie and you MUST be more skillful in the use of your brakes in order to NOT do an unintentional stoppie.
But you have all seen stoppies movies, I'm sure, and with rare exception they do not result in the rider getting spit over the handlebars so surely that means that ending up on the ground eating asphalt is really not that big a risk, is it?
Ask anybody who is learning how to do a stoppie if that is true. (I think doing a stoppie intentionally is crazy, but that is just my opinion.) What I will say, because it is important to know, is that doing a stoppie means that you have transferred enough weight from your rear wheel to your front wheel that NONE remains on the rear wheel and that is why the rear wheel comes off the ground (a stoppie.)
What causes that weight transfer? There is a ratio that you should know very well. It is the height of the bike's Center of Gravity (including you) as compared to the length of your wheelbase. You will find that the ratio is close to 1:2, or 50%. THAT number determines how efficiently weight is transferred for any given rate of deceleration. So, for example, if your CG is 33 inches above the ground and your wheelbase is 66 inches long, you have a ratio of 1:2 which means that at a deceleration rate of 1g you will transfer 50% of the total weight of the bike from the rear wheel to the front.
So, if your bike, including you, weighs 1000 pounds, and you manage to attain a deceleration rate of 1g (32.17 feet per second per second), then 500 pounds will be shifted from the rear wheel to the front. If the total static weight on the rear wheel is less than or equal to 500 pounds, that rear wheel comes off the ground.
[I know, you can easily measure the length of your wheelbase but have no way of determining how high your CG is. Use as a rule of thumb that the height of your CG is 2 inches higher than the top of your seat if you are sitting on your bike.)
Now just because your bike with you on it weighs 1000 pounds does not mean that the weight is evenly distributed between the front and rear wheels. You can assume that slightly more than half of it is on the rear tire. And, of course, if you are carrying a passenger then far more than half is on the rear tire.
Anyway, this is sounding like you might have to do some work in order to understand the dynamics of your bike while braking. Not really. I will do all the math for you needed here and the model will do all of it for you should you wish to play with it.
Sportbikes are designed with a relatively high CG as compared to their wheelbase. Thus, that all important ratio I mentioned earlier is higher for sportbikes than for cruisers, for example. Where a 1g rate of deceleration might transfer 500 pounds of weight from the rear wheel of your 1000 pound cruiser, it would transfer a good deal more than 300 pounds on a sportbike weighing 600 pounds.
Okay, let's get into it. Here is an example run in which I have input values that are representative of a sportbike (a Honda VFR800).
Let me point out a few things to start with. In order to determine the fastest possible stopping distance and time you need only know the bike's speed, the amount of grade, if any, the bike is riding on, the static coefficient of friction available to the tires by the roadway surface, and the efficiency of braking. You do NOT need to know WEIGHT!
In that first run we started braking at a speed of 60 MPH, had a flat surface (no grade), the surface provided a static coefficient of friction of 1.12, and we used our brakes with 100% efficiency. As a result, this bike could stop in only 107.14 feet! But, in point of fact, it actually could NOT stop that quickly because before it reached a deceleration rate of 1.12g's it reached a deceleration rate of .95g's and did a stoppie. Thus, this bike COULD NOT stop as quickly and in as short a distance as was possible if it did not do a stoppie.
What we have seen is that if a bike can do a stoppie, it CANNOT attain a rate of deceleration sufficient to skid its front tire -assuming the roadway surface provides adequate traction.
How do we know that? How did we calculate at what rate of deceleration the stoppie would occur? Why we calculated weight transfer for the bike, of course, and at the rate of deceleration at which 100% or the weight on the rear tire was transferred to the front tire (and, thus, the rear wheel began to lift off the ground) turned out to be .95g's.
Our earlier discussion was about the effect of the ratio of how high the CG is relative to the wheelbase of a bike in determining when a stoppie will occur. So, in this next diagram I show you what happens if, all else being equal, the height of the CG on the bike (including rider, of course) was lowered by only .7 inches.
We see that the only result of that change was an INCREASE in the rate of deceleration this bike is capable of attaining before doing a stoppie. It rose from .95g's to .97g's. Thus, this bike could still not achieve the 1.12g rate of deceleration the environment would have allowed (because it does a stoppie before it gets there.)
The next diagram shows what would happen if instead of lowering the CG we raised it just .3 inches.
The resulting maximum possible rate of deceleration for this bike has dropped to .94g's. We have seen, in other words, that the deceleration rate at which a stoppie occurs is GREATLY affected by how high the CG is.
So what if the rider leans forward on his bike? Does that make the stoppie easier or harder to occur? By 'easier' I mean at a lower rate of deceleration.
Here we see a diagram of results given that the rider, in leaning forward, has reduced the height of the CG by .7 inches while at the same time shifting the weight distribution on the bike from being 54% on the rear wheel to being only 53.5% on the rear wheel. (A 1/2% change is a BIG change.)
We see that we have INCREASED the maximum rate of deceleration from .95g's to .96g's. In other words, leaning forward in order to try to make doing a stoppie 'easier' is counter-productive - it makes it harder to do a stoppie, not easier.
Now some of you still wonder why weight is not a factor in calculating stopping distance or time. For you I have included the final graphic. It shows the formula used to calculate the stopping distance. Feel free to challenge any aspect of the model I built for you, including the formulas. They are in the clear and can be viewed with Microsoft's Excel program.
Now let's look at yet another result. We revert to the original input values with the exception that the static coefficient of friction of the roadway surface is reduced from 1.12 to .95. Now we can see just how fast this bike can ACTUALLY stop in terms of stopping distance. Since the front tire will lose traction, begin to skid, when the rate of deceleration reaches .95g's, which just happens to be the same point at which a stoppie will occur, we see that the stopping distance can be no shorter than 126.32 feet instead of the 107.14 feet that was possible given a better roadway surface.
You might wonder, does this mean the bike will both skid its front tire and do a stoppie at the same time? The answer is a decided NO! As soon as the front tire begins a skid it then has access only to the DYNAMIC coefficient of friction of that roadway which is at least 20% lower than its static coefficient of friction. Clearly a stoppie is no longer possible.
What you have seen is that because of the design geometry of this sportbike it is capable of stopping in a distance of no less than 126.32 feet because it will do a stoppie at .95g's, while simply changing the geometry of the bike (essentially lengthening its wheelbase and/or lowering its CG) it could stop in as little as 107.14 feet.
For those of you still paying attention ... braking efficiency has always been shown to be 100%. That means that the rider was able to get up to, but not exceed, the rate of deceleration at which skidding would occur. If he had used just his rear brake, for example, and got as close as possible to a skid of his rear tire, his braking efficiency would have been approximately 40%. This, in other words, is not an evaluation of how good the brakes are, but of how skillfully the rider uses those brakes. The results of the 10 best stopping distances presented earlier shows that the riders of those bikes obtained very nearly perfect usage of their brakes, or a braking efficiency of almost exactly 100%.
One of the members of my board made a comment about ABS in an earlier message and a question was asked about whether or not you could begin a skid with the front tire and then 'modulate' your brake to unlock the front tire and 'save it'. That is precisely what ABS does. In terms of braking efficiency for these analyses you can assume that ABS equipped bikes can routinely achieve an efficiency of 98+%. And that means that a skillful rider can outperform an ABS equipped bike, does it not?
Now, if you are still with me you should know enough facts to realize that each and every one of the assertions made at the beginning of this article is untrue. There should no longer be any reason for you to believe that any bike can toss you over its handlebars, just those that can do a stoppie.
And as to that ... when a stoppie begins the rear wheel lifts off the ground. THAT raises the CG while at the same time modestly shortening the wheelbase. You should now know that the result of doing those things is to INCREASE THE EFFICIENCY OF WEIGHT TRANSFER and that, in turn, means that it takes less and less braking force to maintain the stoppie. But more importantly, it means that if your bike ever begins a stoppie YOU MUST RELEASE BRAKING PRESSURE or the bike will toss you face first into the ground. It takes more braking energy to start a stoppie than it does to maintain it. Using the same braking energy after a stoppie begins merely makes the stoppie get more severe, then even more severe, then wipes you out.
How quickly you can stop *IS* your rate of deceleration. Clearly the higher you can get your rate of deceleration, the quicker your stop will be. If your bike can do a stoppie then it will reach a rate of deceleration that causes a stoppie before it reaches that rate which causes the front tire to skid. Thus, it CANNOT stop as quickly as a bike that cannot do a stoppie.
As to dual disc brakes ... your rate of deceleration is not limited by your brakes! (As we noted in the beginning, you can lock your wheels with the brakes you have so they are capable of stopping you as quickly as the environment will allow.) Your rate of deceleration is limited by how much of available traction you can use. If the amount of traction available supports a deceleration rate of, say, 1.1g's, then if your bike can do a stoppie you CANNOT stop that fast. So one or two discs is of no importance. (A second disc merely allows you to use your brakes longer before they overheat.)
In summary, maximum deceleration is limited by environment (available traction) for bikes that cannot do a stoppie and by the geometry of the bike, not its brakes, if it can do a stoppie. You want to stop more quickly? Lengthen the wheelbase or lower the CG if your bike can do a stoppie, develop better braking skills if it cannot.
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)