Sliding Friction of Sport Motorcycles


While a great deal of research has been devoted to quantifying the coefficient of friction of downed motorcycles, in an effort to facilitate speed determination, the vast majority of that research is not related to fully-faired motorcycles. Here, we’ll dig into the studies that specifically address fully-faired, aka sport, motorcycles.  

Terry Day and Jay Smith pioneered research on the topic of motorcycle sliding friction in 1984, analyzing the behavior of two downed motorcycles on various surfaces, a 1967 Honda CB305 and a 1973 Yamaha 550 Special [1]. The authors 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 km/hr (25 mph). For pavement, the authors found a sliding friction range of 0.45 to 0.58 g’s during the 25 mph tests. For illustration purposes, using the heart of the range above (0.52 g’s), a motorcycle that goes to the ground at 45 mph will slide for approximately 130 feet before coming to rest. Of course, the bikes used here were standard motorcycles, where protruding metallic components will be the primarily points of contact.


So, let’s check out some faired results. In 1995, Barry Raftery slid an unknown motorcycle wearing Suzuki Katana fairings from an initial speed of 85 km/hr (53 mph) and noted an average deceleration of 0.26 g’s [2]. Another similar test, seemingly from a similar speed, resulted in the same 0.26 g’s. As a control test, Raftery took the same motorcycle, removed the fairings, and performed another test. The resulting coefficient of friction of was 0.33 g’s. Here, the coefficient of friction was calculated using the initial drop speed, and the documented sliding distance (mu = Vo^2/2d).  

  Kawasaki Ninja ZX-7, as used in Medwell's study. 

Kawasaki Ninja ZX-7, as used in Medwell's study. 

In 1997, Christopher Medwell et al. performed several tests using a fully-faired 1992 Kawasaki ZX-7, calculating the coefficient of friction in the same manner as above [3]. In two of the tests, the Kawasaki initially slid along the pavement, and then moseyed into a nearby area of grass, making them difficult to analyze. However, two of the tests were confined to the asphalt, with a release speed of 80 km/hr (50 mph). The motorcycle slid for 69.5 and 86.3 meters (228 and 283 feet) before coming to rest. The calculated coefficients of friction were 0.36 and 0.29 g’s, respectively. It’s worth noting, the 0.36 value was obtained during the test involving the right side of the motorcycle, which is the exhaust side. 

Bruce McNally and Wade Bartlett slid a fully-faired Suzuki Katana at IPTM’s Special Problems in 2003, and analyzed the results via frame-by-frame video and field data (known initial speed and measured slide distance) [4]. Video analysis yielded a coefficient of friction of 0.42 g’s while the sliding distance and known initial speed yielded a result of 0.39 g’s.   

Bartlett briefly presented nine additional tests performed using fully-faired motorcycles during IPTM classes over the years [5]. 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’s with a standard deviation of 0.08 g’s. 

In 2013, I worked with several members of CA2RS (California Association of Accident Reconstruction Specialists) to perform 14 sliding tests using modern GPS data acquisition technology, which captured the motorcycle speed at 10 Hz (ten times per second) [6]. From these speed samples, the coefficient of friction for the downed motorcycle could be calculated throughout the slide. Two of the tests involved fully-faired motorcycles, a 1989 Suzuki GSX-R750 and a 1991 Suzuki GSX600F, yielding coefficients of friction of 0.42 and 0.47 g’s, respectively. A sample test is shown here. 

  Woodcraft plastic frame sliders.

Woodcraft plastic frame sliders.

As far as I know, the values presented above are the only published sliding friction values for sport motorcycles to date. Missing from the literature, is documentation of the coefficient of friction of motorcycles equipped with frame sliders, a common sportbike modification. Frame sliders, usually comprised of a plastic composite, are mounted to the sides of motorcycles to mitigate damages during a fall. 

Over the course of two years, I collected data from track crashes at track days and racing events at New Hampshire Motor Speedway and New Jersey Motorsports Park. All analyzed crashes involved motorcycles equipped with a QSTARZ GPS data acquisition system (5 or 10 Hz) [6]. In total, data from 15 crashes were collected and analyzed. All 15 crashes involved faired motorcycles equipped with plastic composite frame sliders. The average coefficient of friction was 0.45 g’s with a standard deviation of 0.09 g’s. These numbers are more consistent with data from non-faired motorcycles, indicating that frame sliders will actually increase the coefficient of drag for sport motorcycles. A sample video from a track crash is shown here. Unfortunately, this one was me.

The data here should put you on the right track when analyzing sportbike crashes. If you’re aware of any other data that I’ve missed. Please shoot me an email and let me know.

Thanks for reading,

Lou


[1] Day, T., Smith, J., “Friction Factors for Motorcycles Sliding on Various Surfaces,” SAE Technical Paper 840250, 1984.

[2] Raftery, B., “Determination of the Drag Factor of a Fairing Equipped Motorcycle,” SAE Technical Paper 950197, 1995.

[3] Medwell, C., Shanahan, M., “Motorcycle Slide to Stop Tests,” SAE Technical Paper 970963, 1997.

[4] McNally, B., Bartlett, W., “Motorcycle Sliding Coefficient of Friction Tests,” Presentation at IPTM Special Problems in Accident Reconstruction, 2003.

[5] Bartlett, W., Baxter, A., Robar, N., “Motorcycle Slide-to-Stop Tests: I.P.T.M. Data through 2006,” Accident Investigation Quarterly, Vol. 46, pp. 18-20, 2007.

[6] 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. 62-78, 2014.