Motorcycle Speed from Skid Marks

Before you get your hopes up, nobody here will be able to help you if you are going to court.

The best you will get from anybody is a guess, unless you have much more information than you provide here.

Weight of motorcycle, road conditions (wet/dry/ice/snow/dirt/sand/mud), road surface roughness, contact area and materials (rubber or metal or skin) in contact with the road are just a FEW details any accident investigator will require.

Be ready to spend at least $1,000.00USD to get an investigator accepted by the courts.

Then, if you are going up against an insurance company make that $5,000.00.

Sorry, don't mean to rain of your bike.

GA, although I'm glad you said 'estimated' since the resistance to drag will depend on the road surface condition. If the bike was travelling fast enough, there should be a secondary skid mark, left by the rider in his wake (probably involuntarily).

GA SE But there is no such thing as 'deceleration':

https://www.lhup.edu/~dsimanek/scenario/physlang.htm

http://www.timesparadigm.com/beta.html

It is an acceleration in a different direction.

I don't agree. The rider will generally move in the same direction of the bike. Look for the brown skid marks and you can follow his progress. Brown lumps are a dead giveaway!!

What do you mean you don't agree? It is not a matter of opinion, it is a fact of physics:

IF the rider or the bike are moving, they have velocity, which is a vector with a magnitude and a direction. If nothing affects them, their velocity will not change. If there is a change in their velocity, in either magnitude or direction then an acceleration has affected them.

It could be friction, gravity, propellant etc.

If this Gentleman is preparing for a legal case, he may as well have an understanding of the correct terms that will be used in court.

Bearing in mind of course that there are multiple vectors involved, such as gravity; the effect of perhaps a kick -stand grabbing the earth; the bike going ass-over-tip, and swivelling on the handle-bars etc,,etc..etc.. All of these will act against the vector that you have described, especially gravity. So now you have something like 200 kg of machine + rider = approx 500 Kg in total travelling at say, 150km/hr. The coefficient of friction is not so obvious, since it will depend on which part/s of the bike are in contact with the surface, and where the centre-of-gravity is of the load (rider).

Since a competent rider will extricate him/herself from that muddle asap, assume this occurs immediately after impact..this resistance in itself will result in a measure of deceleration to to increased resistance/ loss of velocity anyway. At this point in turn, the riding surface contacts the leathers, coaxing the second skid mark into being...So now, the velocity of the bike has changed and probably the rider and the respective vectors as well. How do you reckon an acceleration has affected them if they are not free-falling ? ie the rider may be rolling, skidding, charging or other down the slope, an we know it is a gentle slope, not likely to increase the velocity of either the bike or rider. Neither are we yet aware of obstacles on the surface.

There is no way of calculating these changes, so they must be measured in situ, in order to arrive at an estimate. Like I said, follow the brown skid mark!

Naturally, I expect you will now disagree, physics or not!

Of course I disagree. You have mixed and misapplied too many terms and definitions to address.

Suffice it to say, although there are many and several variables that would affect the change in velocity of a rider and bike through the course of an accident. Accident reconstruction science can fairly accurately predict the initial velocities prior to ANY subsequent accelerations during the occurrence of an accident:

http://www.amazon.com/Accident-Reconstruction-Science-Alan-Watts/dp/1936360039

But alas and anon, there is little help we can offer the OP here. This is a 'hands on application' of engineering principles beyond the scope of speculation.

I had a look at that site before I ventured my first opinion, and am not overly impressed. Fact is that anything in friction will lose kinetic energy pronto especially if two surfaces are not geometrically smooth and parallel.

Nevertheless I got you to disagree (venture an opinion) on a matter of pure physics, which was your first reaction to my own stated disagreement. I accept your disagreement and consider the matter unresolved in this forum for the time being.

I guess the OP will need to approach others..perhaps the author of the book?

Sorry Hilton, but once again you misapply terms. This does not fair well in legal matters:

Kinetic energy is not lost, it is converted:

https://en.wikipedia.org/wiki/Conservation_of_energy

Peace.

Accepted....dammit!

By laid down do you mean it landed on its side and skidded down the road until it eventually stopped?

You would hope . . .

Too many variables, even if you gave more information.

I need to determine the speed of a motorcycle,Probably too fast for the road.

In the not so distant past I exited a freeway into a rather large gas station during a rain storm at supposedly a too high rate of speed. (Exceeding reasonable and prudent speed for the current weather conditions.)

Speaking from experience; Despite my very long skid pattern which encompassed the entire length of the driveway from the sidewalk past all pump dispensing stations, there was not any visible, discernible marks anywhere on the wet concrete.

However; There were significant signs of abrasion on my left lower leg, the bike's left side, and my left palm.

It is very hard to accurately determine a motorcycle's speed during a crash when the surface is anything other than clean and dry.

Most police investigations are a culmination of their on-site evaluation and testing coupled with information extracted by their personal choice from an extensive accident DATA BASE.

If your liability risk is high or you are going after someone else in a legal situation, I suggest you follow Lyn's advice and hire an expert to do the investigation and testify.

It will not be cheap but it may be necessary and well worth the expense in order to limit your legal and monetary liabilities.

Good luck and be careful.

The motorcycle was moving up a gentle grade (1.4 degrees). At the top of the grade, the driver laid the motorcycle down on its side, moved to the upward facing side of the motorcycle, and rode it down the 1.4 degree grade on the other side. The surface of the road was dry and in very good to excellelnt condition. The length of the skid was 138 feet.

Well, I never thought of using a bike as a surf board before. To add to the list of imponderables, did the bike leave the ground at the top of the hill? FWIW the skid mark is not a long one, corresponding to braking a (fully upright) vehicle from less than 50 mph - UK Highway Code at https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/312249/the-highway-code-typical-stopping-distances.pdf
and assuming that the effective coefficient of friction of a bike on its side is lessened the initial speed does not seem to be high. Being on a downhill gradient would make the initial speed lower still. I suspect that the visible skid mark represents only a portion of the total slide.

The last time I laid my bike down , I was traveling at approx. 40 - 50 MPH. at which point the throttle stuck.

Possibly the cable stuck and I lost control to where I could not reach the kill switch!

I was headed for a large pine tree and an 8' tall cinder block retaining wall so I dumped it!

The road was clean and dry and sloped down approx. 5 degrees and curved to the right slightly.

My motorcycle, a 1986 Suzuki GSXR 1100 slid approx 275 feet from the point of where it tipped over and started its slide.

Maybe you can extrapolate some calculations from there but sliding on its side, the ABS plastic fairings provided a very slippery surface on the asphalt and the oil and fuel leaks helped lubricate the path also.

Here is a picture of me after the impact. The bike did a bit better as I only had to repair one plastic fairing and an engine side casing / cover.

Ouch!

To answer the question about the make and model of the motorcycle it is as follows:

It was a 2011 Kawasaki, Model ZX-10R.

I hope this makes a difference.

Thanks,

Alex

Weight 436# + rider and gas.

Capable of 200MPH.

Alex,

As other people have stated, there are so many variables that pinning down a specific speed is more than difficult.

I do believe that police crime labs and insurance companies either have accident reconstruction software or they contract that out if they deem necessary.

These are some variables you need to have available:

• Exact weight of the motorcycle including fuel and any accessories?
• Was the motorcycle modified in any way?
• Exact weight of the rider?
• Exact friction coefficient of the asphalt?
• Friction coefficient of the motorcycle traveling on its side?
• Was the rider accelerating or decelerating at the time of the accident?
• Did the rider brake at any time prior to the accident?
• What tires are on the motorcycle?
• How experienced is the rider?
• Who measured the travel distance?
• Was the rider injured?

As you can see, there is a lot more to this than just calculating the distance and the slope. If the motorcycle was bearing down on its metal frame or on the plastic fairings will make a huge difference in the travel distance. (possibly 40% different)
If this is just a casual question I would say they were doing approx. 20 - 30 MPH.If this is something you are going to want to bring into a court of law... Hire a professional accident reconstruction consultant. I do believe they start their billing at $5000.00 to $7500.00 up front and then bill hourly against that. Once you exhaust that amount, they will ask for another installment of $2500.00 each time the money is exhausted.
Here is an accident reconstruction company that specializes in motorcycle accidents!

I have always had difficulties with the police asserting that a vehicle was travelling at a specific speed determined by the skid marks. As other posters have identified there are way too many variables. Was the bike skidding on fairing or was a bolt/pedal sticking out and digging in? When did the weight of the rider stop influencing the calculation? In fact a good defence would be to get an assessment from 5 experts and show that there is no agreement. Unless they all used the same program! In that case you could argue that the program is not individualised to each case and is just a model to form a basis for further, deeper investigation.

To get an empirical measurement you could take the same bike to the same spot, lay it down and pull it along the ground with a strap and force scale. Ignore the start load and look at the steady state measurement on the scales. Add the weight of the rider and get back to this forum for the maths and further advice.

Jim

You illustrate my point at #16 beautifully!

In my youth, properly clothed FOR BIKE RIDING, I managed to come off several bikes with nothing worse than some spectacular bruises and a lot of minor scratches on the bike and helmet. Though none of them were anything near the speed and weight of some modern bikes, which probably played a role.

My best ones could top 100MPH, but little more than that, when it was tsill legal in the UK to drive that fast!!!

My Ego got bruised too...

But there was not the traffic then.

Today you might end up under a "city slicker" and his SUV!!! One who never rode a bike of ANY sort, or his wife!!! (You can take that ANY way you like!!)

I lost the fun part while still in the Navy MANY years ago.

Here in Germany, the less than 10% of Bike Drivers on the roads, form aprox 50% of the road deaths......ALL ON THEIR OWN MOSTLY!!! THEY DO NOT NEED MUCH HELP......

Up to 200MPH+ even on our roads takes a heavy tribute in blood.....

I would expect similar statistics from anywhere else.......even when the allowed maximum speeds are far lower....

If that is your choice of vehicle, so be it! (you were not clear as to who, you or the opposition, was riding at the time!!)

Although such situations have been clearly well addressed as possible by several (to my mind at least) people here, with obvious outstanding good knowledge and understanding of the fundamentals.....e.g. you won't get much better info for US$00.00 I feel......you may be on a losing theme with your question.......

If it was you who survived, take it as a warning....especially if you have dependents!!

Best of luck. Do have a great day.......

Given that correctly applied brakes could have dropped the speed substantially before the wheels locked to create skid marks or tip the bike, there is no way to determine the original speed. If the bike was fitted with ABS this would also make a difference. Even if there was an accurate way of estimating the speed at the point where the skid started the prosecution could argue that the initial speed was much higher. The more experienced the rider, the more plausible this argument becomes. A good prosecutor would first establish the rider as an expert before springing this trap.

Alex, if you are trying to rebut an insurance adjuster or police statement I suggest a really good lawyer experienced in this sort of litigation. They will have a list of "expert witnesses" that will testify in court to the actual speed of the vehicle in question.

Just be aware, very few have ever successfully disputed a large insurance companies claim or a veteran police investigators estimation.

Good Luck. -- JHF

Another thing to take into consideration if you're going to court is that court counselors often use professional witnesses to argue their cases. These professional witness use arguments based on widely accepted theories that are easier understood by legal professionals and, while they are based on some properties of physics, don't always adhere to proper physics. A good lawyer will be a lot better investment than a physics professor in a courtroom.

Yeah, sounds like we should not go to global warming court.

The physics answer to your question is:

1/2 mv^2 = Fd

F = mg x coefficient of friction

Where 1/2 mv^2 is the kinetic energy before the bike went down and F is the force of friction, which is equal to mg multiplied by the coefficient of friction of the bike against the concrete. (g is acceleration of gravity -- 9.8 meters/sec^2), make sure the units cancel out. v would be in meters/sec, mass would be in kg. (although mass doesn't matter) Stopping distance (d) would be in meters. If the pavement was dry, the coefficient of friction would probably be in the range of 0.4 - 0.7. If it was wet, the coefficient of friction could have been 0.1 - 0.3. If it was icy, it could be much less than 0.1.

Since mass shows up on both sides of the equation, it doesn't play a role in the calculation. In other words, a 500 kg bike would skid just as far as a 100 kg bike, if everything else is equal.

Notice that velocity is squared... If you double the velocity, kinetic energy is quadrupled. (Stopping distance is also quadrupled!)

This equation is for flat ground.

That doesn't make sense, because from the beginning to the end of the skid mark, the speed of the motorcycle is continuously changing either in the downward direction as it comes to a stand or in the upward direction as it red-shifts out of there. And it's probably not exactly linear, either.

There is a thread nearby entitled, "How to Analyse Jerks"; perhaps the rider of the said motorcycle would stand up to some analysis there?