Motorcycle Skid to Impact Combined Speed

JJBarnes · Newbie

Motorcycle Skid to Impact Combined Speed

skids4u · Member

You should be all set with a combined speed formula.

omegacrash · Veteran Member

You may want to consider a different approach. You have the motorctycle's speed at impact using a vault formula. Take that speed and use it in an initial velocity formula. I believe the formula is Vi = Square root of Ve squared minus 2ad. Ve would be the motorcycles speed at impact, a is gravity which is 32.2 multiplied by your drag facor, d would be the length of the skid marks left by the motorcycle. Remember to use FPS not MPH in this formula. Double check this formula because I'm going from memory on this and make sure it is correct. See if this answer is close to the combined speed formulas answer. There is always a way to check your answers by using a different speed method or by using a delta V formula. Your answers may not be exactly the same but it should be in the ballpark.
Also, when you do a vault formula for a motorcycle using the vault of the rider, the angle of the back of the gas tank is your take off angle. Just an FYI.

-- Edited by omegacrash at 14:27, 2008-08-04

lewpytel · Newbie

I think what you are asking is what technique should you use to determine the PRE-SKID speed of the motorcycle.

If that is your question, then use the combined speed formula, that will give you the pre-skid speed of the motorcycle. You have already done the hard part by obtaining the speed at impact and the speed from the pre-crash skids. Now, just do one more calculation to obtain the pre-skid speed.

Example: 30 mph at impact, 40 mph from skids yields 50 mph pre-skid speed.

ashman165 · Veteran Member

*blink blink*

chuckh027 · Newbie

If you have the vault speed which is an exact speed, and you calculate the speed from skids prior to impact using the vault speed calculated as the end speed in the formula for calculating the speed from skids you have the initial speed at the start of the skids. Why would you need to use a combined speed formula at all if you already have the initial speed prior to skidding?

ashman165 · Veteran Member

Well, vault speed isn't an exact speed. It too, I hate to tell you, is an approximation.

I think what's going on is that there has been some amount of kinetic accounted for pre-vault. And then there's some energy accounted for from the so-called vault. So the guy wants to know how to put it all together to get the speed at the beginning of skids.

Lewpytel used the 3, 4, 5 triangle to make his point.

I typed blink blink because he, like so many others (including some schools), give examples and conclusions without bothering with any of that pesky stuff in the middle. By pesky stuff in the middle, I of course mean all of the work.

So, essentially, his answer is 30 and 40 somehow get mixed up to be 50.

Without any of the work, there's no showing how this happens. Unless someone happens to recognize this application of the Pythagorean Theorem, then it's completely useless.

From that, I would like to make a simple point: if someone is asking how to use a formula, or what formula to use, it seems rather obvious they're asking for help. So, help them. If what you're going to type doesn't actually help them at all, spare us the ink.

The tricky part is that if they're asking how or which formula to use, there's a strong presumption of their general lack of understanding of the relevant things. This forum, at its very best, serves as a way for those of us with more experience/education/guessing skills/whatever to help people who aren't as familiar with this stuff.

JJ, here's the definitive answer:

Skids4u was 100% correct. The rest of the posts here are defective for some reason or another. You don't need to be in feet per second, or even miles per hour. Hell, you needn't even work within "speed" concepts. You're accounting for energy. Just keep in mind that any energy calculation(s) you make is/are speed equivalent kinetic energy.

The vault angle isn't necessarily the angle of the gas tank. Though that's not a bad idea, it isn't necessarily correct. It completely depends on the geometric orientation of the crash.

omegacrash · Veteran Member

Jonathan,
Just FYI, there was nothing wrong with my method. The formula for initial velocity itself may be wrong because I was typing it from memory but my method was an accurate way to determine a speed. The speed is not going to be dead on, but it's close. As for using a motorcycles gas tank for the riders angle of take-off, this is a good method up to a certain point. Granted, if the gas tank exceeds a certain angle, it may be off. I have found it to be accurate and it was shown to be accurate in testing that was done in the field. But once again, it is not an exact science and being off a couple of degrees is not going to make that big of a difference. There is no "one way" to determine a speed in a crash. In fact, it is always a good idea to use whatever means you have to see if the answers come out the same or close to each other.
I enjoy reading your views and you seem like a very intellegent person. You do come up with some good arguments.
Craig

BlueB · Veteran Member

Omega,
Spot on with two options firstly the answers are not an exact figure and should be recognised as that. After all we are second guessing the data we are using.
Secondly the use of several routes to assess the speeds is vital. If I don;t try one potential formula, you can be sure that the 'opposition' will. If we use several formual then we can show a range of speeds which may differ, but hopefully will deal with the query.
The days when the court asks for an exact speed or time should be well into the distant past. Lets be honest, if our scenraio fails because of a couple of mph difference then we didn't have much of an argument anyway.

ashman165 · Veteran Member

Thanks, Craig. I'm really not; I just manage to convincingly fake it for moments at a time with the right celestial alignment and favorable winds.

But, as I said, every post here was faulty for some reason or another. Some of the error was casual, but faulty nevertheless. Specifically, no formula is relegated to having to be restricted to mph, kph, fps or any other speed measure. Simple dimensional analysis will show this to be true. I believe IPTM even has the alternative formulae in their ready reference guide. Another post was infirm for suggesting that we get exact speeds from any formula we use. That simply isn't the case. I digress.

Craig, I don't dispute that the slope of the gas tank area might well be the take off angle, or reasonably close thereto. That, however, isn't a general, presumptive rule. The dynamics of each collision are different. We all know this, presumably.

I did make a specific reference to the geometric orientation of the bike, which is key. First off, in this case, we don't know from the information given if the bike was even upright at collision. For that matter, we don't know if it was traveling with the front forward. This renders any advice about how to determine the take-off angle as purely speculative, which isn't the province of science.

But to address the specific point with the geometry involved, I'd have to use linear algebra, differential equations and multivaried calculus to *prove* my point. The short version is that it's entirely possible, indeed quite likely under the right circumstances, that the rear of the bike can be rotated pitch-wise with speed greater than the former driver has and slap him. This would dramatically alter the dynamics of the collision. I have within the last year or so dealt with 2 such cases.

When this occurs, the analyses become rather complex (not in the mathematical, planar sense - well, not necessarily). The resultant miscalculation can be somewhat striking. Particulary given the two cardinal rules of taking measurements in airborne situations.

This goes back the conversation we had about training/education in the CoR post David started.

Blue, I am confused by your post here. As a general proposition, there is some arbitrary, finite point at which a "scenario" loses validity altogether. This happens on either side of the true speed - be it low or high in estimation. A few miles per hour one way or the other can render an entire argument faulty. Granted, it's not likely we are to ever come up with the exact speeds on purpose. But that doesn't mean we can't get close, rather close at that.

This doesn't, however, make a proper argument which works within acceptable bounds of error faulty. But going outside of that arbitrary, finite number most assuredly does.

Second guessing one's work (also known as considering all possible alternatives) is the sine qua non of scientific work. In all science are error and assumption. This is unavoidable.

omegacrash · Veteran Member

Jonathan,
You're right, I am assuming the motorcycle struck the vehicle in an upright position with the front wheel. It could be a whole different scenario. It very well could have happened like the example you gave.
It's good to be second guessed. Every one's input is important and can contain valuable information. "two heads are better than one".
And you're also right on the fact that formulas can be redesigned for FPS or MPH. The particular formula I gave was in FPS. There is an alternative for MPH. It's just that my preference is FPS. Can't tell you why, it's just one of those things
Take Care
Craig

-- Edited by omegacrash at 11:07, 2008-08-29