Motorcycle Friction

Dmoreno · Newbie

Motorcycle Friction

CrashXprt · Member

There have been motorcycle skid tests by various private firms, individuals, and police departments, but the documentation of the methods and reporting of results from these tests varies widely. Here's what I have handy:

A series of tests conducted by the LAPD was documented in an article by M. D. Donohoe in the July/August 1990 A.R. Journal. They did not do any tests that involved front braking only, but they did front and front/rear tests. There is not much about their testing protocol in the article, but there is enough to raise some questions about it. For example, for comparison purposes (apparently) they skid tested a 1986 Chevy Caprice police cruiser on a 4% downgrade and came up with drag factors ranging from .96 to .99. Not impossible, but those seem a little high for a heavy vehicle like a Caprice. I would contact LAPD to get the actual test data (contact info is in the article but is probably no longer current).

Since you referenced Northwestern, you are probably already aware that page 74-11 of the Blue Book offers some ways to estimate drag for motorcycles based upon drag tests from automobiles, but their methods always adopt the position that if there is any doubt, assume the lowest reasonable drag factor. A fair suggestion, but generally not as useful as full-blown test data. They refer to testing that they conducted, but only in a vague manner.

Here is a link that might be useful:
http://mfes.com/motorcyclebraking.html
The essay itself is a little sparse, but there is a fair list of papers/studies at the end of it that might offer what you need.

As for your drag factors, I would tend to believe what you are getting with your VC3000 (assuming you are using it correctly on a dry/flat surface and all that good stuff) is accurate even though they seem low. Traditionally, the problem with measuring motorcycle friction is how to go about measuring the distance during braking. Since most reconstructions involve observing physical evidence in the form of skid marks, tests should be conducted that measure skid marks so that a valid comparison can be made. However, that does not mean that the start of the skids occurred at the start of braking! In fact, most riders will avoid locking up their wheels until they have no other option because once they do, the stability advantage created by their rotating tires is lost.

Are your numbers average or peak drag factors? If they are averaged over the entire length of deceleration, then I would expect them to be lower than published values that usually consider visible skid distance. Did you measure any tire marks during your tests? I am interested to know how much variation there is between your Vericom data and the drag calculated from skid distance. If there was enough deceleration prior to the onset of skidding, it could explain why you are getting values that seem low in comparison to published data.

I hope this helps and good hunting...

omegacrash · Veteran Member

This is the information I have on drag factors for motorcycles. This information was received from IPTM during my motorcycle crash recon course.

REAR BRAKE ONLY - 81 TESTS - .38 AVERAGE

FRONT ONLY - 73 TESTS - .63 AVERAGE

FRONT AND REAR NORMAL APPLICATION - 24 TESTS - .80 AVERAGE

FRONT AND REAR MAX. APPLICATION - 57 TESTS - .83 AVERAGE

Albert T. Baxter and Neil Robar of IPTM are the motorcycle gurus and really know their stuff. They are usually more than happy to help with questions when they have time.

CrashXprt · Member

omegacrash wrote:

FRONT AND REAR NORMAL APPLICATION - 24 TESTS - .80 AVERAGE

FRONT AND REAR MAX. APPLICATION - 57 TESTS - .83 AVERAGE

Do you know how they went about determining "normal" application of brakes? .80 is a pretty high drag factor that is somewhat higher than a vehicle skidding on asphalt. While I expect your average motorcycle to be able to decelerated at a faster rate than your average car when they are braking at max, wouldn't "normal" braking be defined as whatever rate is comfortable for most riders? That is, say whatever the 85th percentile drag factor is for a sample of motorcyclists braking normally.

While I do not know your context for these factors, on surface, the .80 seems a little high for normal motorcycle braking. It stands to reason that normal braking of a bike must be at about the same rate as it is for normal braking of a car or car drivers would constantly be removing fast-braking motorcyclists from their front windshields.

That said, I think the .83 is a solid estimate for rear wheel locked and front wheel nearly locked. I am curious to know IPTM's support for a .80 normal motorcycle drag factor.

omegacrash · Veteran Member

The material I have from IPTM is very vague for this particular experiment. It does not specify what type of motorcycle or size was used. Going through my notes here, it appears engine size (in cc's) can make a difference in stopping time for a motorcycle. Also, whether the motorcycle is equipped with anti-lock or an integrated brake system. Also, I think the problem he was having using the vericom 3000 is that it is designed for automobiles, not motorcycles. I realize the physics are the same, but the make-up of the vehicles is much different. So, asI say, IPTM did not provide alot of info on the experiments. But I do know if they published it, it must be good information. You know as well as I do, if you use different vehicles for test skids to determine drag factor, you are going to get different results.
This is good stuff. I like debating things like this. Very educational. I respect your point of views.

ashman165 · Veteran Member

Dmoreno wrote:

I have been trying to find information on Motorcycle friction and can not find much new information. We recently went out with a Vericom 3000 and used a Police Kawi, we tried just some rear wheel locked skidding and came up with .123 and .161 friction. I know Northwestern has a table, but thats for sliding and dropping. I checked out SAE 2002-01-0551, 910125 and 840250; but none of those seem to deal with braking and staying up. The .123 and .161 seemed pretty low. Does anyone one of any good studies that I could take a look at!!!!

I don't know. It doesn't seem too low to me considering that the rear tire of a motorcycle doesn't offer up a significant contribution to the stopping ability of a bike.

And here's why:

In a car, the weight of the passengers isn't a substantial contibution to the load of the platform. On a motorcycle, the rider is a significant contribution to the total weight - perhaps even fully a third of it (or higher in some cases).

We all know, or should know, that when a vehicle's brakes are applied, the inertial forces at the top of the car get the message slightly after the parts of the car closer to the tires. This results in the change in the vehicle's orientation - the front dips down, the rear comes up.

In a car, though, there is still enough weight in the back to keep the tires grounded. Even if only the rear tires lock up, there is still usually a sufficient degree of mass in the back to keep the tires in contact with the road - forget for a moment that the locked tires will want to lead.

On a motorcycle, this isn't the case. The driver, who is alone a substantial portion of the vehicle's overall weight, is going to cant forward. This overload the front the tire and underloads the rear tire. Also consider that the bike itself is canted forward. So, there isn't a great deal of force left on that rear tire in comparison to the total amount present that that one tire needs to retard.

Whether we're talking about 10 - 20 percent, or 20-30 percent of gravity, I still wouldn't expect the rear tire to be a major factor in the motorcycle's ability to stop.