Bicyclist Braking Capabilities and Limits Compared to those for Motorcyclists
Introduction
Studies related to motorcycle braking have demonstrated that most motorcycle operators are not capable of utilizing the full braking capabilities of their motorcycle (as defined by the tire friction and braking system components). Largely, this is because of the precision control needed to maximize the deceleration without causing the motorcycle to capsize. Maximal deceleration is achieved when both brakes are applied to the maximum extent possible without locking either wheel. Thus, failure to achieve maximal deceleration on a motorcycle could result from any of the following:
1) Applying the rear brake with enough force to lock the rear wheel. This results in reduced contribution from the rear tire to the deceleration, below what its contribution would have been if the rear wheel was just short of locking. Locking the rear wheel also results in reduced control of the rear of the motorcycle and could result in the motorcycle capsizing. Depending on the rider’s body motion and braking behavior, either a low-side fall, or a high-side fall could occur.
2) Applying the front brake with enough force to lock the front wheel. A motorcycle with a locked front wheel is not controllable for long, and a fall will occur. This fall could take the form of tucking the front end where the motorcycle quickly capsizes onto its side without first experiencing any significant yaw. If this does not occur, the fall could take the form of a pitch-over, where the deceleration is sufficient to throw the rider over the handlebars. Frank, Smith, Hansen, and Werner [1] reported that decelerations exceeding 1.0 g produced pitch-overs for a 1985 Kawasaki Ninja. Fatzinger, Landerville, Bonsall, and Simacek [2] reported a study of front wheel over-braking with sport motorcycles. One test that resulted in a pitch-up had a deceleration of approximately 0.8 g. Another test that resulted in pitch-over had a deceleration of 0.98 g.
3) The first two possibilities involve a motorcycle rider braking with too much force. The third possibility is that the rider will hold back and brake conservatively because of the risk of capsizing. This rider will not capsize but will fail to maximize the deceleration.
Thus, the need to maintain stability of the motorcycle limits the upper end of the decelerations that motorcyclists can be expected to achieve. In relationship to the stability of the vehicle, bicyclists can come up against even more restrictive limits on their braking capabilities since the higher center of gravity of the bicycle-rider combination (over and above that of the typical motorcycle-rider combination) makes bicycles more prone to pitch-over falls. Motorcyclists riding motorcycles with conventional braking systems will generally be capable of achieving a deceleration of 0.62 g ± 0.15 g on a dry road without causing the motorcycle to capsize [3, 4]. Many bicycle-rider combinations will generate a pitch-over in this range of deceleration [5]. So, the limits of bicycle braking are typically not set by the frictional characteristics of the tires.
Analysis
Cossalter et al. [6] developed a simple rigid body model for analyzing motorcycle braking dynamics, which can also be applied for analyzing bicycle braking dynamics. This model is shown graphically in Figure 1.
Figure 1 - Simple Motorcycle or Bicycle Braking Model
In this figure, the following variables are introduced:
Notice that the coordinate system for this model is oriented such that a positive acceleration in the x-direction slows the motorcycle down. The equations of motion for this model indicate that the rear wheel load will go to zero and the rear wheel will lift off the ground when:
Thus, the deceleration required to cause rear wheel lift-off and a subsequent pitch over varies based on the longitudinal and vertical positions of the center of gravity. DiTallo [7] presented and dimensions for 25 motorcycles and his data are included in the first two columns of Table 1. When a rider is added, the center of gravity will generally move rearward and upward. The third and fourth columns give approximate and dimensions for each motorcycle with the addition of a 172 lb rider. The value in the fifth column is then a calculation of the theoretical value for the longitudinal deceleration in g’s that would cause the rear wheel of these motorcycles to lift off. Suspension effects are neglected in these calculations. Since heavy braking will cause the front tire and suspension to compress and the rider’s weight to shift forward, the actual decelerations needed to cause a pitch-up will generally be lower than the theoretical values reported in this table.
Table 1 - Motorcycle Characteristics [7]
Broker [5] presented similar calculations to illustrate typical decelerations that would cause rear wheel lift-off for a bicycle-rider combination. He used a bicycle with a wheelbase of 42 inches, a combined bicycle-rider weight of 185 lb, and a combined bicycle-rider CG located 40 inches above the ground and 25 inches behind the front axle. These parameters resulted in a deceleration to produce rear wheel lift-off of 0.63 g. Wilson [12] presented similar illustrative calculations (with different values) of the deceleration needed to cause a pitch-up. The result in his example was 0.56 g. Broker next varied the location of the combined bicycle-rider CG to illustrate its influence on the longitudinal deceleration to produce rear wheel lift-off. These results are reproduced in Table 2.
Table 2 – Bicycle Decelerations (g) that Produce Rear-Wheel Lift-Off [5]
These calculations illustrate that there is a spectrum in two-wheeled vehicles in their susceptibility to pitch-up or pitch-over falls. The threshold to generate a pitch-up of a cruiser or touring motorcycle is well beyond the deceleration that most riders would be able to generate, and so these motorcycles are generally not susceptible to pitch-over. Sport motorcycles are more susceptible, having pitch-over thresholds in a range that can be generated when a rider of one of these motorcycles quickly grabs the front brake lever. Bicycles are more susceptible still, given their increased CG height over most motorcycles (when combined with a rider). This higher CG position is largely due to the weight of the rider being significantly greater than the bicycle. For this same reason, the pitch-over thresholds for bicycles will be much more dependent on the body position the rider selects at any given moment. For example, the further the combined CG is from the front axles, the higher the deceleration required to generate a pitch-up. A rider can move the CG further rearward by positioning their body further back relative to the bicycle.
While the stability limits of a bicycle/rider combination are more restrictive than those for a motorcycle/rider combination, bicycle and motorcycle braking share some common trends related to braking. Bicycles are like motorcycles in that the front and rear brakes are actuated independently. As with motorcycles, rear only brake applications on a bicycle produce lower decelerations than front only or combined front-rear brake applications.
Experimental Studies related to Bicycle Braking
Beck [9] reported testing with 7 mountain bikes and 1 road bike being braked on dirt trails and pavement using three techniques: 1) front brake application with rapid and repeated locking and unlocking of the front wheel; 2) rear brake application with lock-up; and 3) combined front (with repeated locking/unlocking) and rear braking. The road bicycle was only tested on the pavement surfaces. One of the mountain bikes was equipped with disk brakes. Table 3 lists the means and standard deviations of the decelerations reported by Beck. The reported decelerations were corrected for the slope of the test surface by the author. Beck’s testing utilized expert and intermediate riders, and he found that the expert riders were often able to produce higher decelerations than the intermediate riders. There also appears to be bicycle-to-bicycle and surface-to-surface differences in the data.
Table 3 – Mean and Standard Deviation Decelerations Reported by Beck [9]
Famiglietti et al. [10] reported bicycle braking tests conducted with eight bicycles: two full-suspension mountain bikes, two hybrid bikes, one beach cruiser, one BMX bike, one road bike, and one single speed bicycle. This study utilized a single test rider and a single test location consisting of smooth, dry asphalt in Torrance, California. The authors documented skid to stop decelerations under maximal application of the rear brake, the front brake, and a combination of the front and rear brakes. Starting speeds varied between 11 and 21 mph. Across all bicycle types, the decelerations for combined front and rear brake applications ranged from 0.40 to 0.71 g. Across all bicycle types, application of the rear brake only produced decelerations of 0.25 to 0.37 g.
Joganich [11] described the use of a video-based system for measuring the deceleration of a bicycle under brake application by a single 6-foot tall, 175 lb test rider. The helmeted rider was an experienced cyclist, with over 10 years of road and mountain bike racing. The testing was conducted in an asphalt parking lot without any traffic present. The bicycle was a 2012 Fuji Altamira 2.0 road bicycle. Rear braking only and front and rear combined braking were evaluated from speeds between 10 and 20 mph. Joganich stated that “this testing scenario reflected a highly alert cyclist braking under an emergency scenario.” However, no actual emergency was presented to the rider. Initially, “a camera flash provided a visual stimulus for brake initiation.” This method was later abandoned when the test rider had difficulty seeing the flash. For the tests that incorporated front brake application, the cyclist was instructed to use the front brakes “to the fullest extent as he felt comfortable without risking being pitched over the handlebars…” For the rear brake only tests (6 tests), Joganich reported decelerations of 0.24 g ± 0.02 g. For front and rear combined braking (4 tests), the decelerations were 0.41 g ± 0.07 g. Joganich also reported mechanical delay times from the time of brake initiation until wheel lock-up. For the rear brake only tests, he reported mechanical delays of 124 ms ± 22 ms. For the front and rear combined braking, he reported mechanical delays of 129 ms ± 33 ms. Some tests exhibited significantly longer mechanical delays in the 400 to 600 ms range. Joganich reported that “all testing resulted in distinct skid marks, which allowed for accurate skid length measurements. Additionally, the only skid marks were from the rear tire.” The front wheel did not lock up in any of the tests, and the author concluded that “the absence of front wheel lockup logically suggests that the test subject was cautious to some extent to prevent a loss of control, even though he was a proficient bike handler and was encouraged to maximally brake.”
Discussion and Conclusions
The studies discussed in this section are summarized in Table 4. As with studies that examine the braking capabilities of motorcyclists, several points are important to consider. First, there is a distinction to be made between the capabilities of the bicycle (in terms of tire friction and braking system components) and the deceleration that a rider can or will achieve. On a dry paved surface, many bicycle riders will not fully utilize the braking capabilities of their bicycle. The upper limit of deceleration will often be limited by the deceleration that will produce a pitch-over. On the other hand, the limit may also come from the rider’s awareness of the pitch-over limit. In other words, the rider might hold back on front wheel braking, to avoid producing a pitch-over. The tendency for a rider to self-limit their deceleration to avoid producing a pitch-over could increase with experience. A bicycle rider could shift their body on the bicycle to increase the necessary deceleration to produce a pitch-over. With this technique, an experienced rider who understands how to shift their body during braking can increase the upper limit of braking. That said, when a bicyclist must respond by braking to a hazard that creates a sudden emergency, there may not be the time available for the rider to reposition their body before braking. In addition to these considerations, it is important to keep in mind that the studies summarized here were conducted with riders who were not confronted with actual emergencies. The testing was generally conducted on roads without hazards or parking lots. Thus, these studies capture what riders are capable of under ideal circumstances. When confronted with actual emergencies, some riders may not achieve the decelerations that riders in these studies did.
Table 4 - Summary of braking decelerations from various studies (dry asphalt)
When comparing motorcycle and bicycle braking, the following observations surface:
The limits of braking deceleration by bicyclists are generally lower than for motorcyclists, perhaps by approximately 0.2 g.
This limit derives largely from the bicycle/rider combination being more susceptible to pitch-over than the motorcycle rider combination.
This greater susceptibility to pitch-over comes from the complete reversal of weight ratios between the two types of vehicles. A motorcycle is typically heavier than the rider, usually by a significant margin. A rider is typically heavier than a bicycle, usually by a significant margin.
Given this, future bicycle braking studies could benefit from more time spent quantifying the CG position of the rider/bicycle combination, both statically and dynamically. This would give greater insight into the degree to which the braking limits in the studies are deriving from the actual pitch-over limits of the bicycle/rider combination versus the rider’s willingness (or lack thereof) to approach that limit.
With bicycles, even experienced riders are limited in their braking capabilities in a way that seems related to their awareness of the pitch-over limit.
References
Frank, T., Smith, J., Hansen, D., and Werner, S., “Motorcycle Rider Trajectory in Pitch-Over Brake Applications and Impacts,” SAE Int. J. Passeng. Cars - Mech. Syst. 1(1):31-42, 2009, https://doi.org/10.4271/2008-01-0164.
Fatzinger, E., Landerville, J., Bonsall, J., and Simacek, D., “An Analysis of Sport Bike Motorcycle Dynamics during Front Wheel Over-Braking,” SAE Technical Paper 2019-01-0426, 2019, doi:10.4271/2019-01-0426.
Ecker, H., Wasserman, J., Hauer, G., et al., “Braking Deceleration of Motorcycle Riders,” International Motorcycle Safety Conference, Orlando, FL, March 1-4, 2001.
Bartlett, W., Greear, C., “Braking Rates for Students in a Motorcycle Training Program,” Accident Reconstruction Journal, 20(6): 19-29, November/December 2010, ISSN: 1057-8153.
Broker, J., Hottman, M.M., Bicycle Accidents, Crashes, and Collisions: Biomechanical, Engineering, and Legal Aspects, Second Edition, Lawyers and Judges Publishing Company, 2017, ISBN 978-1-936360-58-1.
Cossalter, V., Lot, R., Maggio, F., “On the Braking Behavior of Motorcycles,” SAE Technical Paper Number 2004-32-0018, 2004, doi:10.4271/2004-32-0018.
DiTallo, M., Paul, E., Adamson, K., Green, T., et al., “Motorcycle Center of Gravity Data: Methodology and Reference,” Collision: The International Compendium for Crash Research, 12(1): 50-60, 2017, ISSN 1934-8681.
Wilson, D.G., Bicycling Science, Third Edition, The MIT Press, 2004, ISBN-13: 978-0-262-23237-1.
Beck, R.F., “Mountain Bicycle Acceleration and Braking Factors,” Accident Reconstruction Journal, Vol. 19, No. 4, July/August 2009, 49-56.
Famiglietti, N., Nguyen, B., Fatzinger, E., and Landerville, J., “Bicycle Braking Performance Testing and Analysis,” SAE Technical Paper 2020-01-0876, 2020, doi:10.4271/2020-01-0876.
Joganich, T., “A Video-Based System for Measuring the Braking Performance of a Bicycle,” SAE Technical Paper 2018-01-5032, 2018, doi:10.4271/2018-01-5032.
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