Sliding and Tumbling Decelerations of a Motorcycle
Day and Smith [1] reported a study of motorcycle sliding deceleration in 1984, analyzing the behavior of two downed motorcycles on various surfaces – a 1967 Honda CB305 and a 1973 Yamaha 550 Special. They towed the motorcycles using a rope with an in-line force gauge and documented the forces required to pull the motorcycles at 1 and 40 kph (25 mph). For pavement, these authors reported a sliding friction factor range of 0.45 to 0.58 during the 25 mph tests. For gravel, the friction factor was 0.68 to 0.79 and for grassy earth, 0.79. Day and Smith noted that “during the testing, it was observed that projecting elements, such as the foot pegs and handlebars would tend to plow into the soil at low speeds, creating momentary high drag factors.” In this testing, the motorcycle started in a capsized position on the ground, and so deceleration from a fall was not a part of the friction factors reported by Day and Smith.
Lambourn [2] explored how sliding decelerations experienced by motorcycles differed between tests where the vehicle was dragged at low speeds and tests where the motorcycle was dropped at a higher speed and allowed to slide to rest. For the higher speed tests, motorcycles were dropped from a low platform (already on their side) or allowed to fall to their side from an upright position. In his literature review, Lambourn noted that some studies had reported a speed-dependence on the sliding deceleration, with the rate decreasing with increasing speed. He also examined this issue of speed dependence of the deceleration in his testing.
Lambourn reported that the decelerations (what they termed as a friction factor) measured “in the low-speed drag tests gave a value close to the high-speed sliding value. The friction was affected by the road surface texture, the presence of prominent side projections, and the wearing away of these projections during the slide. Some speed dependence was noted in the upright-launch tests which appears to be due to the ‘digging-in’ of the machine as it falls to the road, rather than an effect of the sliding friction itself.” He also concluded that “the reason for there being a clear speed dependence in the results of Becke and of Ashton, but not in the tests reported here, is almost certainly due to the fact that in both their experimental methods the motorcycles were dropped a distance onto the road surface. This would subject the machines to a large decelerating impulse as they struck the ground, which would considerably increase the average deceleration in low speed tests but be relatively unimportant in high speed runs.” Lambourn concluded that the sliding deceleration of a motorcycle was dependent on the roughness characteristics of the road surface.
Other authors have suggested that differences in test methodology – if the motorcycle drops to the ground in the test and from what height – account for some of the variability in the deceleration for sliding motorcycles and the apparent speed dependence in the decelerations. For instance, Baxter [3] stated: “One word of caution; read the test methodology as to how the friction values were obtained. In some drop tests, the motorcycle was pre-positioned laterally a few inches/centimeters above the road surface. [Friction values obtained] using this method are generally lower than a motorcycle falling from vertical (normal position) onto its side on the road.” Hague [4], Wood [5], and Walsh [6] also discussed the influence of the fall (capsize) on the deceleration. Hague stated: “The methodology of a motorcycle slide test can significantly affect the measured deceleration rate, apparently due to the speed lost in the initial ground impact. Those tests in which motorcycles were dropped from an increased height resulted in increased deceleration rates. During a road traffic accident, the motorcycle will also lose speed upon ground impact and testing should therefore try to mimic this process. Bearing this in mind, the most appropriate tests for the majority of collisions would be those in which an upright motorcycle was allowed to capsize from a normal height.”
This is different than the approach proposed by Wood and Walsh. They proposed splitting the speed calculations into separate phases for the capsize and the slide. Thus, within their method, the ideal test procedure for the sliding phase would not include a fall of the motorcycle. Walsh presented a method for incorporating the speed loss from a fall into the calculation of the deceleration from a test or the calculation of the motorcycle’s initial speed in a reconstruction. He also partitioned the test data into the following categories: motorcycles with crash bars, unfaired motorcycles, and fully or partially faired motorcycles. He found that the sliding deceleration for fully-faired motorcycles was 0.408g ± 0.036g. For unfaired motorcycles the sliding deceleration was 0.46g ± 0.092g. For motorcycles with crash bars the deceleration was 0.286g ± 0.037g. It is perhaps worth stating at this juncture that the critical issue for selecting a range of sliding decelerations for a reconstruction is to identify which components of the motorcycle were engaged with the roadway and the degree to which they were engaged (i.e., were they sliding or gouging). In theory, a motorcycle could have crash bars, but protruding components from the motorcycle could be gouging the roadway. In such a case, the sliding deceleration rate for a crash bar equipped motorcycle would probably not be applicable.
Donohoe reported sliding decelerations for a 1982 Kawasaki KZ1000 Police Special. Testing with this motorcycle, which was conducted at the Los Angeles Police Department’s Specialized Collision Investigation Detail, utilized a flatbed truck with a lift gate on the rear [7]. The lift gate was positioned parallel to the roadway and approximately 6 in above the road surface. The motorcycle, which was facing along the direction of travel, was dropped an upright position with its front tire on the lift gate and its rear tire on the roadway. The roadway was a residential, asphalt roadway adjacent to Dodger Stadium. The initial speed of the motorcycle was measured with radar. Donohoe reported five tests with sliding decelerations between 0.38 and 0.50.
In 1995, Raftery slid an unknown motorcycle wearing Suzuki Katana fairings from an initial speed of 85 kph (53 mph) and reported an average deceleration of 0.26 g [8]. Another test, seemingly from a similar speed, resulted in the same 0.26 g. As a control test, Raftery took the same motorcycle, removed the fairings, and performed another test. The resulting deceleration was 0.33 g. The deceleration was calculated using the initial drop speed and the documented sliding distance. Raftery’s methodology involved suspending the motorcycle from a boom at the rear of a tow truck, driving the tow truck up to the test speed, and releasing the motorcycle from the boom. It appears from Raftery’s description of his tests that he suspended the motorcycle from the boom with the wheels rolling on the ground and when the motorcycle was released it fell to the road surface.
Carter [9] tested eight different motorcycles on three different surfaces (asphalt, dirt, and gravel) to determine their sliding decelerations from target speeds of 48 and 97 kph (30 and 60 mph). All of the tests run on off-road surfaces utilized a target speed of 48 kph (30 mph). The motorcycles that Carter tested included the following motorcycle types: standard, cruiser, sport, and touring. In all, Carter reported 50 tests. He reported that “some speed effects were observed, i.e., for higher speeds, the slide coefficient was lower (likely due to heat softening of structure contact points with the pavement).” Also, “for full fairing equipped motorcycles the slide coefficient was consistently lower than for non-fairing equipped motorcycles” and “deep gouges left by the motorcycle in the surfaces corresponded with higher slide coefficients.” Carter attempted to improve on prior studies by developing a test rig that allowed for consistent positioning and release of the motorcycles. The motorcycles were positioned front wheel forward and on their left or right sides. The motorcycles were released from a position with the lowest point on the side of the motorcycle approximately 5 centimeters above the ground. As Carter noted, “this release height was chosen to minimize the impact forces upon release, therefore restricting (to the extent possible) the tests only to energy dissipated during sliding.”
Medwell [10] performed four motorcycle sliding tests using a fully faired 1992 Kawasaki ZX-7 Ninja. He stated that “the tests were designed to approximate, as closely as possible, the motion of a motorcycle falling over from an upright position. The motorcycle was positioned upright on a fabricated platform mounted on the right side of a pickup truck…The height of the platform was adjusted so that its underside was as close as possible to the roadway surface. This test setup resulted in the motorcycle tire contact surface being approximately 90 mm above the roadway. The motorcycle was held upright by an assistant riding in the bed of the pickup truck. The truck was accelerated to the test speed, then the motorcycle was released and allowed to fall over sideways onto the road surface.” In two of the tests, the Kawasaki initially slid along the pavement but then traveled into a nearby area of grass, making them difficult to analyze. However, two of the tests were confined to the asphalt. Both had a release speed of approximately 80 kph (50 mph). The motorcycles slid for 69.5 and 86.3 m (228 and 283 ft) before coming to rest. The calculated decelerations were 0.36 g and 0.29 g. The 0.36 value was obtained during the test involving the right side of the motorcycle, which is the exhaust side. Like those reported by Raftery, Medwell’s decelerations appear to be low relative to other studies reviewed here.
Bartlett [11] reported motorcycle drop tests from Motorcycle Crash Reconstruction classes conducted at the Institute for Police Technology and Management (IPTM) from 1987 to 2006. These tests were conducted on asphalt or concrete, but the surfaces varied from class to class. The drop techniques also varied from class-to-class. Bartlett observed: “The results are a chaotic mix of sliding and tumbling, not unlike real motorcycle crashes.” Bartlett’s dataset initially consisted of 237 drop tests using 107 different motorcycles. Twenty tests were discarded because the reported drop speed, slide distance, and deceleration were inconsistent with each other. Additional tests were excluded in which the motorcycle was dropped from a pickup bed or in which the motorcycle slid off the road surface onto the off-road terrain. The final dataset included 162 tests with 99 different motorcycles. Bartlett reported that the decelerations trended slightly higher with increasing speed and that the overall average deceleration for all the tests was 0.521 ± 0.140 g. Bartlett also combined his dataset with other available datasets. This resulted in 386 tests for which Bartlett reported decelerations of 0.480 ± 0.134 g.
In 2003, McNally and Bartlett slid a fully faired Suzuki Katana at IPTM’s Special Problems and analyzed the results via frame-by-frame video and field data (known initial speed and measured slide distance) [12]. Video analysis yielded a deceleration of 0.42 g while the sliding distance and known initial speed yielded a result of 0.39 g. Bartlett has also reported nine additional tests performed using fully faired motorcycles during IPTM classes over the years [11]. The individual results were not detailed in the paper, but combined with the data from Raftery, Medwell, and McNally, the total set of 14 tests had an average coefficient of friction of 0.37 g with a standard deviation of 0.08 g.
In 2004, Hague compiled data from prior studies where motorcycles capsized and then slid to rest [4]. Hauge concluded that “the analysis shows that a more accurate estimation of deceleration rate can be made if the motorcycles are split into two different categories, based on the presence of fairing, crash bars and/or panniers.” He noted that, while “one might expect partially faired motorcycles to have lower deceleration rates than unfaired machines…the two categories exhibit similar deceleration rates. Perhaps also initially surprising is that fully faired machines equipped with panniers gave similar results to the partially/unfaired motorcycles.” In relation to crash bar equipped motorcycles, the only available tests were those conducted by Lambourn [2], in which the decelerations varied between 0.25 and 0.35. Hague noted that, “As expected, fully faired and crash bar equipped motorcycles decelerated at relatively low rates. The crash-bar-equipped results are probably artificially low because they were all dropped from a very low height. If they had capsized from an upright position, they would have lost additional speed on striking the ground which would increase the average deceleration rate, more so at lower speeds. Although deceleration rates as low as 0.2 have been suggested for crash bar equipped machines there appears to be no published data to support such low values.” Hague reported an average deceleration for partially faired and unfaired motorcycles of 0.39 and for fully faired motorcycles of 0.27.
Peck, Focha, and Gloekler analyzed 15 actual crashes of sport motorcycles equipped with frame sliders and 14 controlled tests of motorcycles of various types not equipped with frame sliders. Frame sliders, usually comprised of a plastic composite, are mounted to the sides of motorcycles to mitigate damages during a fall. The crashes occurred during track days or races at New Hampshire Motor Speedway and New Jersey Motorsports Park. The motorcycles were equipped with GPS data acquisition systems measuring the motorcycle speed at either 5 or 10 Hz. The authors reported sliding decelerations of 0.45g (SD = 0.09) for the track crashes and 0.48 g for the controlled tests. According to the authors, their data showed that “frame sliders do not lower the drag factor of a sport bike, but actually increase it.” The authors cited prior data for fully-faired sports bikes without frame sliders with decelerations significantly lower than what they found for the motorcycles with frame sliders. Three of the track crashes involved sections where the motorcycle slid on hard packed dirt. These dirt slides had decelerations of 0.45g, 0.61g, and 1.11g. Another insight that this study developed was the fact that there is variability in the severity of a motorcycle’s first contact with the ground, and thus, variability in how the initial ground contact contributes to the overall deceleration. For example, a motorcycle would generally strike the ground with less severity during a low-side fall than during a high-side fall.
DiTallo [14] examined three different test methods for determining the drag factor for motorcycles sliding on their sides. This testing utilized 26 motorcycles sliding on an asphalt roadway in North Las Vegas, Nevada. The following three methods were utilized: (1) dragging an already capsized motorcycle across the pavement, (2) releasing a motorcycle from the rear hydraulic lift of a box truck and allowing it to fall to the pavement and slide to rest, and (3) towing a motorcycle behind a moving vehicle with its front tire held in a pneumatic clamp until its release. After release, the motorcycle would fall to the ground and slide to rest. DiTallo noted that many of the motorcycles had been previously utilized in impact testing at the 2016 ARC-CSI conference. These motorcycles included sport, touring, and motocross motorcycles along with mopeds. One of the sport motorcycles had frame sliders.
A total of 36 pull tests (Method 1) were conducted with 9 motorcycles. The motorcycles were pulled in varying orientations. This series of tests resulted in a range of drag factors between 0.36 and 0.64 and an average of 0.51. DiTallo also noted that the tests with tires leading exhibited an average drag factor of 0.54 and the tests with the tires trailing exhibited an average drag factor of 0.51. Drop tests (Method 2) were conducted with 12 motorcycles at speeds ranging from 30.7 to 41.1 mph. These tests produced drag factors between 0.28 and 0.70. Clamp tests (Method 3) were conducted with five motorcycles at speeds ranging between 29 and 43.9 mph. These tests resulted in average drag factors between 0.28 and 0.43. This group contained the sport motorcycle with frame sliders. In addition to parsing his data by test methodology, DiTallo also considered motorcycle type. He found that the sport motorcycles had drag factors between 0.28 and 0.61, the touring motorcycles had drag factors between 0.28 and 0.60, the mopeds had drag factors between 0.35 and 0.54, and the motocross motorcycle had a drag factor of 0.43.
References
Day, T. and Smith, J., “Friction Factors for Motorcycles Sliding on Various Surfaces,” SAE Technical Paper 840250, 1984, doi:10.4271/840250.
Lambourn, R., “The Calculation of Motorcycle Speeds from Sliding Distances,” SAE Technical Paper 910125, 1991, doi:10.4271/910125.
Baxter, A.T., Motorcycle Crash Investigation, Institute of Police Technology and Management, Jacksonville, FL, 2017, ISBN 978-1-934807-18-7.
Hague, D., “Calculation of Speed from Motorcycle Slide Marks,” Impact: The Journal of the Institute of Traffic Accident Investigators, Spring 2004.
Wood, D.P., Alliot, R., Glynn, C., Simms, C.K., Walsh, D.G., “Confidence Limits for Motorcycle Speed from Slide Distance,” Proc. IMechE Vol. 222, Part D: J. Automobile Engineering, 2008.
Walsh, D.G., Wood, D.P., Alliot, R., Glynn, C., Simms, C.K., “Motorcycle Capsize Mechanisms and Confidence Limits for Motorcycle Capsize Speeds from Slide/Bounce Distance,” 18th EVU Conference, Hinckley, UK, 2009.
Donohue, M.D., “Motorcycle Skidding and Sideways Sliding Tests,” Accident Reconstruction Journal, Vol. 3, No. 4, 1991, ISSN 1057-8153.
Raftery, B., “Determination of the Drag Factor of a Fairing Equipped Motorcycle,” SAE Technical Paper 950197, 1995, doi:10.4271/950197.
Carter, T., Enderle, B., Gambardella, C., and Trester, R., “Measurement of Motorcycle Slide Coefficients,” SAE Technical Paper 961017, 1996, doi:10.4271/961017.
Medwell, C., McCarthy, J., and Shanahan, M., “Motorcycle Slide to Stop Tests,” SAE Technical Paper 970963, 1997, doi:10.4271/970963.
Bartlett, W., et al, “Motorcycle Slide-to-Stop Tests: IPTM Data through 2006,” Accident Investigation Quarterly, Spring 2007, ISSN 1082-6521.
McNally, B., Bartlett, W., “Motorcycle Sliding Coefficient of Friction Tests,” Presentation at IPTM Special Problems in Accident Reconstruction, 2003.
Peck, L., Focha, W., Gloekler, T., “Motorcycle Sliding Friction for Accident Investigation,” Proceedings of the 10th International Motorcycle Conference, Institute for Motorcycle Safety, Essen, Germany, pp. 62-67, 2014.
DiTallo, M., et al., “3 Different Methodologies for Determining the Drag Factor for Motorcycles Sliding on Their Sides,” Collision: The International Compendium for Crash Research, Volume 12, Issue 1, September 2017, ISSN 1934-8681.