A Reanalysis of WREX 2016 Test #8 and an Exploration of the Limits of Wheelbase Reduction Methods
Introduction
In my book, Motorcycle Accident Reconstruction (2nd Edition), I made the following statement about WREX 2016 Test #8: “This collision is conducive to analysis using the Dodge crush and motorcycle wheelbase shortening.” I should have gone into much more detail related to this statement, because Peck et al. [1] had noted about this test: “The motorcycle suffered broken components in the front end, with a documented wheelbase reduction of 3.3 inches. However, the wheel could be moved with respect to the frame, due to the damage, making the documented wheelbase reduction irrelevant.” As you can see in the photograph of the motorcycle below, the left fork tube of the motorcycle fractured. So, which is it: Is this test conducive to analysis with wheelbase reduction methods or not?
On the one hand, we could look at the damage to this motorcycle post-test and say: “Can’t do wheelbase reduction methods; that there motorcycle broke.” And, of course, that’s not inaccurate. But, of course, motorcycles often do break when they crash into other vehicles. So, perhaps we can do better than that. That was the spirit in which I simply plugged the 3.3 inches of measured wheelbase reduction into the wheelbase reduction equations reported by Peck et al. [1] and reported the result. The equations in Reference 1 represent the most up-to-date set of equations for applying wheelbase reduction methods. And, as would be expected, the calculations underestimated the motorcycle impact speed. As I noted in my book, there is some ambiguity about whether this collision should be classified as a fender or a wheel impact. Either way, though, the wheelbase reduction equations contained in Peck et al. underestimated the speed when the measured 3.3 inches of wheelbase reduction is entered into the equations. The equation for fender impacts yields a speed that is 7.7 mph lower than the actual speed (38.6 mph versus 46.3 mph) and the equation for wheel impacts yields a speed that is 2.1 mph lower than the actual speed (44.1 mph versus 46.3 mph).
But to get to a more rigorous methodology, let’s ask this question: Under what conditions and at what time in the test did the fork tube fracture? So let’s spend some time with this test and see where it leads us. This test involved a 2012 Harley-Davidson FXDF Dyna Fat Boy motorcycle impacting the driver’s side rear of a stationary 2005 Dodge Durango at a 90-degree angle and at a speed of 46.3 mph. The motorcycle impacted the Dodge approximately 7 feet behind its CG and the collision resulted in approximately 44.5 degrees (0.78 radians) of counterclockwise rotation of the Dodge. The post-test condition of the Harley-Davidson is shown in the photograph below. The Dodge exhibited a maximum crush after the test of approximately 9.1 inches, measured on the rear fender.
Two Characteristic Impacts: One Good for Wheelbase Reduction Methods, the Other Not
When it comes to deciding whether or not to apply wheelbase reduction methods, we can first consider the characteristics of the impact. In the majority of the tests on which wheelbase reduction equations are based, the struck vehicle is stationary and the front wheel and fork of the motorcycle deform directly rearward into the engine, radiator, or frame. That’s not to say that the deformation stops there, but it certainly starts there. This is the type of deformation pattern that is conducive to analysis of wheelbase reduction. In the real-world, of course, the struck vehicle is often not stationary. If the struck vehicle has sufficient velocity perpendicular to the travel direction of the motorcycle, then when the front tire of the motorcycle contacts the side of the vehicle, the tire can stick to the side of the car and get pulled along with the car as it continues along its path. This results in twisting of the fork rather than directly rearward deformation. When the fork get twisted, at a minimum, the wheelbase reduction will be asymmetric left-to-right, and often, one or both of the fork tubes will fracture and the wheelbase reduction will not be able to be reliably measured. These types of collisions are not conducive to wheelbase reduction.
It is important to recognize that the deformation pattern in WREX 2016 Test #8 initially falls in the category of collisions that are conducive to analysis with wheelbase reduction methods. The struck vehicle is initially stationary, and thus has no velocity perpendicular to the travel direction of the motorcycle, and the front wheel and fork get deformed directly rearward. It is not until later in the test that the handlebars, fork, and wheel get torqued to the left and the left fork tube fractures. To see this, consider the video frames below, which show the relative motion of the two vehicles during the test. In the first two rows of images, the front wheel and fork of the H-D deform directly rearward and the wheel impacts the frame/engine of the motorcycle. Then, in the third row of images, the Dodge rotation from the collision picks up and the wheel of the motorcycle begins torquing counterclockwise. An additional video frame that shows this torquing from a different vantage point is also included below.
Experimenting with a Methodology
So, what would happen if we analyzed this test using wheelbase reduction methods assuming that the wheel was pushed as far back as it could go before it began torquing (which actually did occur in this test)? Based on the geometry of this motorcycle, if the tire gets deformed back into the frame, the wheelbase reduction would be approximately 6.7 inches (see graphic below). If the tire was compressed during the impact to the point that the wheel contacted the frame, the wheelbase reduction would be approximately 10.9 inches (see graphic below). A review of the video seems to show the wheelbase reduction prior to torquing being between these two values.
It’s also worth mentioning that the reported crush for the Dodge was measured on the fender, not the wheel, so for this experiment, I’ll classify this as a fender impact. The wheelbase reduction equation for fender impacts is as follows:
Using the wheelbase reduction needed to bring the tire into contact with the frame (6.7 inches) would yield a total crush for this test of approximately 15.8 inches. If we use that crush value within the wheelbase reduction equation for fender impacts, this would yield a speed of 42.9 mph. The actual speed was 46.3 mph, so the calculated speed is 3.4 mph lower than actual. Using the wheelbase reduction needed to bring the wheel into contact with the frame (10.9 inches) would yield a total crush for this test of approximately 20 inches. If we use that value within the wheelbase reduction equation for fender impacts, this would yield a speed of 48.2 mph, 1.9 mph higher than actual. This is interesting because the actual, pre-torquing wheelbase reduction in the test seems to be between 6.7 and 10.9 inches, so if we could obtain the actual wheelbase reduction, we would likely end up with a calculation that very accurately calculates the impact speed of the motorcycle.
Where does this exercise lead us? If we take the minimum wheelbase reduction that we are able to prove occurred (the 6.7 inches). Then, what we end up with is calculations that UNDERESTIMATE the known impact speed of the motorcycle. They don’t overestimate the speed. That’s logical, of course, because the fork tube broke after, or when, the wheelbase reduction had already reached its maximum. Bartlett [2] has observed that a limitation of a wheelbase reduction method “is its relative insensitivity at higher speeds: after the front wheel and fork have collapsed to the frame and engine block there is little additional motorcycle deformation possible.” Adamson et al. [3] had made a similar observation that “the maximum wheelbase reduction is limited by the deflection of the fork and the rim. Speeds higher than those tested would be expected to result in less wheelbase reduction than a linear extrapolation of the present results due to direct contact of the front wheel with the engine.” In other words, if the front wheel and fork get pushed back as far as they can go, then wheelbase reduction methods are likely to UNDERESTIMATE the motorcycle impact speed.
Summarizing the Limitations of Wheelbase Reduction Methods
On the one hand, we could say: Wheelbase reduction methods should not be applied if the motorcycle wheelbase reduction cannot be reliably measured. BUT, this statement is too simplistic, because it is ambiguous to time. Cannot be reliably measured when? Suppose we have a case where one of the fork tubes is broken, such that the wheel will essentially flop around and a post-crash measurement of the wheelbase reduction would be unreliable. On the other hand, suppose that there is physical evidence that the front wheel and tire of the motorcycle was initially pushed directly rearward into the frame, engine, or radiator of the motorcycle. Well, in a case like this, we could quantify the rearward movement of the front wheel and tire (and the resulting wheelbase reduction) needed to bring the front wheel or tire into contact with these components. Applying this measurement within the wheelbase reduction equations would be likely to underestimate the motorcycle speed (or the closing speed of the collision, if the struck car had velocity towards the motorcycle). So, a more nuanced statement would be: If the wheelbase reduction of the motorcycle cannot be reliably measured after the crash, then you will need to come up with a different way to estimate what the minimum wheelbase reduction would have been. If you can quantify the minimum wheelbase reduction that would have occurred during the crash, then that can be used to yield a low-end, conservative estimate of the motorcycle speed. In other cases, front end components of the motorcycle will be severely damaged and perhaps fractured, but the front wheel of the motorcycle post-crash is being held in place, and the motorcycle can be moved without significantly altering the wheelbase reduction that would be measured. In these cases, wheelbase reduction methods can likely be applied. That is, as long as the primary deformation mechanism that occurred to the motorcycle was rearward deformation of the front wheel and fork, as opposed to twisting of the fork.
References
Peck, L., Bartlett, W., Manning, J., Dickerson, C., Deyerl, E., “Eleven Instrumented Motorcycle Crash Tests and Development of Updated Motorcycle Impact-Speed Equations,” SAE Technical Paper 2018-01-0517, 2018, doi:10.4271/2018-01-0517.
Bartlett, W., “Motorcycle Crush Analysis,” Accident Reconstruction Journal, 19(2): 25–28, 2009, ISSN 1057-8153.
Adamson, K. S., “Seventeen Motorcycle Crash Tests into Vehicles and a Barrier,” SAE Technical Paper 2002-01-0551, 2002, doi:10.4271/2002-01-0551.
Acknowledgements
As usual, Connor Smith assisted me with the analysis reported in this post. If you want more information about this analysis, you can reach out to me or him (csmith@explico.com). If you want more information about this analysis AND you want all the good dirt on me, then just reach out to Connor.