|Mechanical Forensics Engineering Services, LLC|
Motorcycle Braking & Skidmarks
The deceleration capabilities of motorcycles is often a topic of debate among accident reconstructionists. As shown in the publications table below, there has been little research in the past decade on the topic of motorcycle braking, despite a dramatic improvement in the grip afforded by available street tires. In recent years, magazine-reported average decelerations from highway speeds are universally in excess of 0.9 g's, and even top 1.0 g's with some frequency. Furthermore, there are many people who aver that one can determine if the front brake was in use by examining a rear wheel skidmark for a "lazy-S" appearance. They claim that a straight rear skid is a clear indication of front brake use. This set of tests was conducted in part to test that theory.
|The Harley Davidson FXRT
All of these tests were conducted on an essentially flat, level, dry, paved surface. Speeds were recorded using Stalker radar equipment. (My thanks to Northeast Collision Analysis for the loan of the gun.) The first battery of tests, using a 1990 Harley Davidson FXRT equipped with Dunlop touring tires, resulted in the following graph:
Click for full-size photo
of FXRT skidmarks, 68 Kb
The Yamaha FZ600
Similar tests were conducted with a 1986 Yamaha FZ600. Sadly, two of the bike's front brake pads lost their friction material during testing, resulting in metal-on-metal contact which damaged the front brake rotors and resulted in poor overall stopping performance. The bike was equipped with Metzler street tires.
|The Suzuki Katana 750|
Another set of tests were conducted with a 1992 Suzuki Katana on a cool but dry day. The best stop was actually achieved with the front brake only. The bike was equipped with Dunlop-Sportmax street tires.
|The '08 Kawasaki EX650
In addition to the tests already noted in this article, and discussed below, a series of skid tests were conducted with a Kawasaki EX650 in Nevada, in March 2009, on dry lightly traveled asphalt pavement. My best stop was again achieved with the front brake only, at about 0.88g. The bike was equipped with OEM Bridgestone Battlax street tires. During a front-only test, the front wheel locked, leaving an 8-foot skid. The decel dropped from 0.88 to about 0.60g. The front tire had good speckling and a nice burned patch where it locked. The rear-only skids slowed the bike at about 0.39g. The chart below shows a rear-only skid test where I intentionally pushed the rear tire sideways near the end, getting the chassis rotated about 24 degrees before returning to the left (and overshooting a little). The front tire followed a straight path while I was doing that.
The shape of the rear-wheel skids from the FZ and Katana once they had reached low speeds could, indeed, be characterized as a "lazy-S", but at high speeds, a majority of the skidmarks were virtually straight. The HD rear skid was essentially straight all the way to stop.
Center of Gravity
The three bikes generated skids of decidedly varying darkness and width. The Katana's rear wheel skid marks were noticeably lighter when even modest front brake was used. The Katana's front tire left no discernable mark until it was locked, while the FZ's front tire left a clearly visible mark without achieving lockup. The Harley's rear wheel skid mark was slightly lighter when the front was used, and appeared very similar to the Harley front-only skid mark. The Harley's front tire left a slight but visible mark prior to lockup.
The rear only skids from the EX650 were fairly straight during the higher speed segments, but it started to get wavy near the end. The chart above is for a test in which I intentionally tried to kick the bike sideways back and forth near the end. The on-board compass showed that the chassis orientation changed by up to 24 degrees from the nominal travel path, making the mark very wavy, but the front tire continued to track in essentially a straight line.
In calculating dynamic behavior of motorcycles, one important parameter is the location of the center of gravity. One common "rule of thumb" is to assign the cg height half the wheelbase. In DOT HS-801-810, "Accident Avoidance capabilities of Motorcycles," Rice, Davis, and Kunkel listed the following measured CG heights:
Real Rider Performance
|MODEL||WheelBase||Seat Height||CG Height|
|Norton 850 Commando||56.8||32.8||19.8|
One of the USA's largest providers of reconstruction training, IPTM, conducts motorcycle recon courses quite often all over the country, and has done so for many years. During these classes, it is common to have one or two riders demonstrate stopping a motorcycle with front only, rear only, and both brakes. The data from many such tests was collected over the years, and was recently published ("Motorcycle Braking Tests: IPTM Data thru 2006", W.Bartlett, A.Baxter, N.Robar, ARJ 17(4), Jul/Aug 2007). The figure below shows a summary of hundreds of braking tests. See the ARJ article for a more comprehensive description of the testing. A few of the REAR-ONLY tests were much higher than would be anticipated, and deviate from a normal probability plot, suggesting that they do not belong to this same dataset. Having often been part of this type of testing, I suspect that the too-high values are the result of riders who inadvertently used some front brake during the stop. Deleting the 8 highest values (which appear to be high outliers) reduces the standard deviation to 0.05g, but does not significantly change the overall average of 0.37g. The slight curve offset visible in the figure is an artifact of the chart type.
A single long straight skidmark may well be caused by a rear wheel skid generated while the front brake was used (particularly if the motorcycle was still traveling at some speed at the end of the mark), but one can NOT be confident to the level of "more likely than not", let alone "beyond a reasonable doubt" that this is true. The "lazy-S" shape clearly indicates a rear brake skidmark, but without additional supporting information does not indicate "to a reasonable degree of certainty" that the front brake was not used. Similarly, a serpentine mark can be generated with front brake use, and is dependant on the operator's "body language". Without testing the actual tires in use on particular the bike, or in some cases examining the tires themselves prior to their being driven on much, it may not be possible to confidently identify which tire made a particular short mark, or the vehicle's braking condition. With representative exemplar marks made under known conditions (rear-only and front-with-rear) and using the actual tires in use on the accident unit, it would probably be possible to identify the braking condition at the time of the accident.
Some related publications
Stopping decelerations generated with the rear wheel only were about half of the maximum deceleration achieved. A motorcycle's rear brake contributes little (or even nothing) to the overall stopping power when efficient use is made of the front brake.
Locking the rear wheel requires little skill, resulting in a fairly tight range of possible values. Effective front brake use, however, is highly skill dependent, and though modern motorcycles can commonly reach 1g, most riders are not that skilled.
Motorcycle Accident Reconstruction, Obenski, Lawyers & Judges Publishing
Motorcycle Accident Reconstruction, Baxter, IPTM Publishing
Considerations for Improving Motorcycle Brake Systems, Zellner, SAE Paper 810408
Friction Applications in Accident Reconstruction, Warner, SAE Paper 830612
Aspects of Motorcycle Braking, Donne, SAE Paper 856127
Motorcycle Braking Methods, Tolhurst, SAE Paper 860020
Braking Performance of Motorcyclists with Integrated Brakes, Mortimer, SAE Paper 861384
Emergency Straight Line Braking of Skilled vs. Less-Skilled Moc'l Riders, Prem, SAE Paper 871228
Stopping Characteristics for Motorcycles in Accident Situations, Fries, et al, SAE Paper 890734
The Calculation of Motorcycle Speeds from Sliding Distances, Lambourn, SAE Paper 910125
Accident Recon Methodology Peculiar to Moctorcycles and Minibikes, Hurt, DOT-73051
Motorcycle speed estimates, Daily, ARJ Mar/Apr 1990
Drag Factors - Applications and Sensitivity, Adams & Knight, ARJ Jan/Feb 1990
© Copyright 2000 Wade Bartlett
Mechanical Forensics Engineering Services, LLC.
This page last modified on 21-NOV-2009