Car Accident Physics


Help! I am on my way to court and need an expert opinion! I was driving to work recently (old Mazda Protege, country road, clear day, no hurry)traveling east, and I came to a left hand bend in a country road, no warning signs, speed limit 45 or so. I curved left as I have a thousand times before, but it felt as though a hand had come along the left hand side of the car and was pushing it firmly to the right, away from the curve and off the road to the right. When I realized the car was not responding to my turning and that I was being pushed toward the narrow right shoulder, I decided to go with it and angled right, onto the shoulder. After about 50 feet I eased to the left to get back onto the road and again, the car angled hard left, across both lanes and into the ditch on the left side of the road. I was okay but the car hit a tree and was ‘totaled’. When the car was winched out of the ditch, all tires were clean, but the left front tire was flat, and had picked up a good deal of grass. I am of a mind that the flat is the clue/answer to my problem; I suspect I lost the left front tire just before or just into the left turn, and this is what pushed me to the right and off the road. While I would normally expect the car to pull in the direction of the front tire blow out (i.e., left blow out, to the left), it seems this would apply only in a straightaway. If the car is in a left hand curve and I lost the left front tire, wouldn’t the momentum of the curve pull the car in the direction it was ‘pushed’?

Puzzled, in North Carolina


But why are you going to court? Are you being charged with reckless driving? If so, your defense (IMHO, I’m not a lawyer) ought to have nothing to do with speculation about flat tires or physics but, rather, that the state must provide clear evidence of its charges.

Challenge the testifying officer whether he actually witnessed the accident (he didn’t), how he can state the accident was the fault of the driver rather than due to a mechanical defect. Keep hammering away that he wrote the ticket without a shred of supporting evidence.

You can follow through on your tire theory if you must but I suspect it will have no bearing on the course of the trial. It’s pure guesswork.


Whether or not the OP made a written statement to police (or verbal for that matter) will have some bearing here as well.

The flat tire (or picking up grass) is not proof of mechanical fault - many tires can blow after a vehicle leaves the road and makes impact with an object - lots of different forces at work there.

Was the vehicle inspected for any mechanical faults in the course of the investigation? If so, what were the results? I suspect that if there was anything found, you would be charged with a vehicle equipment offense instead of (or on top of) the reckless driving (if that’s what you are charged with - let us know).

Need more information, but if you’re being completely honest, it does sound like there may have been a mechanical fault with your vehicle.


Very, very very few crashes are witnessed by the investigating officer. Charges are brought if there’s a violation of traffic law. The evidence to support these charges comes from accident investigation. There is roadway evidence such as skid marks, yaw marks and gouges, vehicle damage evidence, occupant injury analysis and wittiness statements. Most traffic investigators have no problem at all proving the charges by building the case step by step with the evidence.

In your case, if the tire were flat prior to you losing control there would be clear roadway evidence of a deflated tire. A wobbly skid mark from the position (left front) would be clearly left on the roadway.

The officer will also use the departure angle that the car left the roadway to determine if it was a drift off (gentle angle) or a swerve off (sharp angle).

If there’s grass wedged in between the tire bead and the rim more than likely the tire became deflated after you lost control and were sliding sideways off the road.

Get a copy of the crash report and see what the officer says in the narrative. Did you tell the officer you thought a suddenly deflated tire caused the crash?


Many modern cars have an ECU that remembers the last 10 or 15 seconds of throttle opening, rpm, speed, brake use, etc. before impact. This can work for you or against you in court.


I’ll see what being a university physics student can do here.

Having a tire go flat while rolling will increase rolling resistance on that side, and will pull you in that direction, so losing a driver’s side tire wouldn’t result in your being “pushed” towards the passenger side.

However, changing the car’s direction of travel requires a certain amount of force, and since a road curve comes up quicker at higher speeds, the amount of force needed to change directions increases with vehicle velocity since the turn must be sharper, and thus the acceleration in the new direction is greater. To put it simply, my theory is that you were simply going too fast for the curve and couldn’t hold it since the force needed to change directions was more than you could apply. You might also have hit something just before the curve that made you slip, a wet spot or patch of oil or something.

When you went onto the shoulder I bet that’s when the tire blew, and that’s also when it picks up the clumps of grass. Then you got back onto the road, the tire is now flat and drags hard on the left side (which you aren’t prepared for and still might be traveling too fast to account for), pulling you that way and off the road into the tree.

So that’s my thinking of what happened here. Took the curve too fast, had to drive off the road, popped the tire, then dragged across into the tree.


The momentum of the car should continue pushing the car towards the outside of the curve. Good case in point here since you mention this.
About a year ago my son was with me and we were on out way into town. After I rounded a 35 MPH curve on a rural highway and got about a 1/4 mile down the road some fool in a green Taurus crossed the centerline on me and forced me to veer to the shoulder.

This guy was running about 80 MPH+ in my best estimate and while stopped my son and I looked back thinking this guy was going to kill himself on that 35 MPH curve. He hit the turn (partially obscured by high weeds) at high speed and we saw a cloud of dust and weeds rocket upwards. Uh-oh.
We whipped around, went back, and discovered this car had shot to the outside of the curve taking out a few reflector posts, an electric line pole, and demolished the car on the right front. Neither the moron behind the wheel or his wife was injured for some reason.

One could make the ananlogy of excessive speed causing the same problem as a blown tire since the end results are the same.

Another possibility here. When is the last time you checked the tire pressure? It’s quite possible that on a sharp turn the bead of a seriously underinflated tire could pop loose momentarily and cause an instantaneous flattening of the tire.


One problem with Budd and OK’s explanations: The tire that was found to be flat was the inside tire, meaning that it was relatively unloaded, compared to the right front tire. Therefore, it would be unlikely to have suffered catastrophic deflation due to loading, and its main effect on the car would be to exacerbate the understeer already inherent in this FWD car.

I think the real problem was excess speed and/or gravel or something else that decreased traction on the roadway, not a flat tire. This was followed by overcompensating for the slide by the driver.

I’ll also ask the OP: Why are you going to court over this?


First, I think coming here for an “expert” opinion is a mistake. Many of us are amateurs, but even the experts here sometimes venture out of their area of expertise and give advice in other areas.

As a professional driver, I will tell you what I think. The driver is not just responsible for operating a vehicle safely. He is also responsible for the condition of the vehicle. That is why professional drivers conduct a pre-trip inspection. If you recall driver’s education class, they instructed you to conduct pre-trip inspections and properly maintain your vehicle. This being the case, it doesn’t matter to me if it was a flat tire, a broken suspension component, a wheel falling off, or a broken steering component because all of these things are preventable, with the exception of design flaws. So perhaps the best plan might be for you to look for a design flaw that caused your collision. (Did you notice I didn’t call it an accident?)


Guilty, untill proven inocent !


Sounds like speed induced understeer.


“Guilty, untill proven inocent !”

No, no; that’s France.


And exactly what have you been charged with?


And, Mexico.


The OP may not have been charged with a crime. It might just be a civil case, you know, the kinds of cases that Judge Judy hears on TV for our entertainment.
In a civil case, the standard of proof is “preponderance of evidence” not “reasonable doubt”, meaning that 10 out of 12 jurors have to agree to the verdict instead of all 12 of them.


In a civil case the jurors argue about it until everyone gets tired and wants to go home. That settles a lot of arguments.


I’m a criminal defense attorney and I’ll speculate that the OP is charged with a misdemeanor such as “careless & imprudent” or some equvalent traffic offense. The State can make their case with the circumstantial evidence that the officer observed at the scene. The OP will have the choice of pleading guilty or pleading not guilty and proceed to trial. If the OP pleads guilty or is found guilty (beyond a reasonable doubt) there will likely be a fine, probation, and points if the State has a point system. If the OP wants to hire a lawyer to handle the plea arrangement, generally a better outcome can be negotiated. If the OP wants to try the case, then a lawyer should be hired or the likelihood of acquittal becomes negligible.

Bottom line, this is a minor issue that will in all probability result in a plea of guilty, a fine and points.

Just speculation, mind you.


this is a minor issue that will in all probability result in a plea of guilty, a fine and points

I’m not an expert nor did I stay at a Holiday Inn last night. However, I have been to traffic court a few times. This may not apply everywhere but more than one Judge, in their instructions to the collection of traffic offenders, made reference to a distinction in the pleas of guilty vs no contest. If you plead guilty (and will be found subsequently guilty) the conviction will be admissible in any future court proceedings. They indicated that this was not necessarily the case when pleading no contest and being found guilty. After the first time I heard this, the very first person to be called pled guilty and I always wondered why they made that choice right after hearing this potential downside to doing so.


When someone pleads “no contest” it usually means that they believe themselves to be innocent but also believe that the state has enough evidence to secure a conviction. It is still a plea of guilty, the defendants are found guilty and the resulting conviction is no different that a straight plea of guilty. The theory behind a no contest plea is to allow a defendant to do what he or she believes is in their best interests. If someone would rather plead to a probation offer than go to trial and risk incarceration, the courts have said that they can do that. In my state and in most states that I am aware of, there is no significant difference and no contest pleas are certainly admissible in court for impeachment and for enhancement purposes. In otherwords, whether you plead guilty straight up or plead no contest you are going to have a conviction and a criminal record.


You realize, Countrydriver, that you will now be called on to personally respond to any legal questions posted here. Maybe you could get a cut from Dewey, Cheetham and Howe for your time and expertise!

It’s been a couple of days since this thread was started. I wonder if the OP has become the guest at his/her local hoosegow. What’s the deal, rogersville??