Please explain this
- Thread starter oldman45
- Start date
Walter Rego
Member
I'm still trying to figure out how a nail or screw that is lying on the road flat can get picked up by a tire and go in point first. I get about one per year, just had one yesterday that luckily didn't puncture the belt and leak.
And why are there so many nails and screws lying on roads ?
And why are there so many nails and screws lying on roads ?
DeathGrip
US Veteran
I gotta laugh.When I was a kid some lady pulled into the service station with 14 roofing nails in her tires!She wanted me to plug them.She didn't want new tires and I told her I couldn't fix them.She pulled out of the station with three flats.
I don't recall seeing any rocks thrown forward by the tires of my motorcycles, but they sure can throw water or grit from the road surface. Some of it will blow back in my face, so I have to think it would also be possible with a rock.
In over 40 years of motorcycling, I have found that almost all tire punctures occur on the rear tire. The explanation usually offered was that the front tire would kick up the nail, screw, or whatever and set it up so that it could puncture the rear tire. I always found that plausible. For a while in the '80s or so, some motorcycle writers would recommend the use of long, ground-brushing mud flaps on the front fender to knock down and incapacitate those bouncing nails and screws. Either that or a rubber skirt under the engine. Some people would talk about doing it, but it never became what I would call a fad. I wonder how the idea might have caught on if we had the internet back then.
In over 40 years of motorcycling, I have found that almost all tire punctures occur on the rear tire. The explanation usually offered was that the front tire would kick up the nail, screw, or whatever and set it up so that it could puncture the rear tire. I always found that plausible. For a while in the '80s or so, some motorcycle writers would recommend the use of long, ground-brushing mud flaps on the front fender to knock down and incapacitate those bouncing nails and screws. Either that or a rubber skirt under the engine. Some people would talk about doing it, but it never became what I would call a fad. I wonder how the idea might have caught on if we had the internet back then.
5Wire
Member
Little tires, big tires makes no difference. The circumferential speed of the tread will be the same as the vehicle's speed. Centrifugal force is exerted on an object along a radial only while attached to the rotating object, the Mr Newton takes over and the freed object travels in a straight line. It cannot go faster than it already was unless "acted upon by an outside force," so sayeth Isaac.
I vote for space junk, sue NASA!
I vote for space junk, sue NASA!
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Thrown rocks from front wheels is much more likely with front wheel drive vehicles and becomes more likely when the tire is turned. Still, a rock cannot be thrown forward from a tire that is encased by fenders. Look at a front tire and measure where a rock could possibly let go and hit the ground. There is a very small window of opportunity there. By the time the rock bounced up, the vehicle would be well past the rock.
A motorcycle has more of an opening forward of the fender on the front tire and would more likely see rocks thrown forward.
I am still puzzled about what it was that hit my windshield twice in a short distance. Right now, I am leaning toward hail from a high level cloud, humming birds or something like very tough bugs.
G.T. Smith
US Veteran
I had a little car called a LLoyd. I was about 18 at the time. My girlfriend and I were going down the road at about 70 when I heard something like a glass gallon jug breaking. I pulled of the road and found that a bolt had came loose from the inner front U-joint, (front wheel drive axle), and had shot through the headlight on the drivers side. Must have been going faster that the car, right?
Had a drive shaft come out from under a 340 Dodge Demon and pass me on the right. It didn't go very far forward, but it did go forward.
The hood from my '55 Ford Victoria went straight up and back.
I think you may have driven through a little meteorite shower. I have heard the weather man call for freezing fog, I bet that would do it.
Peace,
gordon
Had a drive shaft come out from under a 340 Dodge Demon and pass me on the right. It didn't go very far forward, but it did go forward.
The hood from my '55 Ford Victoria went straight up and back.
I think you may have driven through a little meteorite shower. I have heard the weather man call for freezing fog, I bet that would do it.
Peace,
gordon
LVSteve
Member
If you saw red, then I would say most likely a meteorite.
The average person would be amazed and how many meteors enter the Earths athmosphere, daily.
snowman
Member
I just assumed he was telling us that he was upset at the damage being done to his truck. Maybe he'll stop back and tell us what he meant.
Andy
Exactly as you assumed is how it was meant.
Things have gotten so deep around here that I likely need hip waders with some of the suggestions.
Gordon, not sure what broke your headlight but I assure you it was not a bolt from your drive shaft.
Certainly some things can be "slingshotted" from a car but they do not go far. And anything thrown forward will not have the momentum to continue for long so the vehicle will pass over it.
As to the rock theory, yes it sounds good but not possible. I will do some photos when I get back from south Louisiana today to show that is not possible. For what it is worth, I drove on 22 miles of pea gravel road, bouncing on washout to get to my destination last night and will drive back today. My headlights were on but I did not see one rock go forward of my vehicle. On the way back, it will be daylight so I will look again. I heard a lot hitting under the truck but none went forward I saw.
geoff40
Member
Maybe the wheel/tire was not the source of the rock. Underside of the front bumper, pebbles lodged somewhere in the front/nose area of the truck being forced out of their hiding place by the speed of the air movement perhaps. If you're in a wrecker, there has to be plenty of places in the nose that they could have come out of.
5Wire
Member
Not to mention how did it get enough relative speed in the opposite direction to ping the windhield!Y'all are leaving out centrifugal force.
I think rotational speed of the tire also comes into play, compounded somehow with the truck's speed, but I'm not learned enough to know how.
Let's face it though- if the rock was not moving faster than the truck, how did it get ahead of the truck?
BuckeyeChuck
Member
This is, in my opinion, the most salient point in the whole discussion. The earlier statement that any object thrown from a tire must be moving at the same speed as the vehicle is incorrect. The average velocity of any part of the tire is identical to the vehicle's velocity. The instantaneous velocity is decidedly not, ranging from zero velocity to a velocity exactly double that of the vehicle. I'll illustrate later.
Can a rock be hurled by a tire faster than the vehicle itself? The answer is an indisputable yes. It is only necessary to make an assumption that the rock would be held by the tire's tread. Once the assumption holds, then the rock will have the same velocity as the tangential velocity of the tire. Consider four points in the tire's rotation.
(1) At the point where the tread contacts the ground, the rock (and the tire) have zero velocity in any direction. Is this really true? Yes. The road has no forward velocity, so at the point tire contacts the road, it too must have zero velocity. Earlier, it was stated that the rock hurled by the tire must have the same velocity as the vehicle. This is demonstrably false because it ignores tangential velocity. Any rock stuck in the tire tread under the contact patch has zero velocity, regardless of how fast the vehicle is travelling.
(2) At the very top of the tire, the tangential velocity of the tire (and any rock in its tread) is precisely double that of the vehicle. If the car is moving 70 MPH, then the top of the tire is traveling forward at 140 MPH. Any rock stuck in the tread at that point will also have a forward velocity of 140 MPH.
This illustrates an important point: the tangential velocity of a tire on a vehicle moving at 70 MPH is always 70 MPH. However, because the tire is in rotary motion, this motion can be broken into its horizontal and vertical components. The horizontal component of that velocity *must* be added or subtracted to the vehicle's velocity to compute the *forward* velocity at a point on the tire.
On the bottom of the tire, 70 MPH vehicle velocity - 70 MPH tangential horizontal tire velocity opposite that of the vehicle = 0 MPH forward motion at the tire tread. Any rock dislodged at this point would have no velocity at all.
On the top of the tire, 70 MPH vehicle velocity + 70 MPH tangential horizontal tire velocity in the same direction as the vehicle = 140 MPH forward motion. Any rock dislodged at this exact point would move exactly double that of the vehicle.
(3) At the rear of the tire (the point closest to the rear of the vehicle), the velocity is exactly identical to that of the vehicle. This is because the tangential velocity of the tire is purely in the vertical plane, meaning horizontal velocity is exactly zero.
70 MPH vehicle motion + 0 MPH tangential horizontal tire velocity = 70 MPH total forward velocity. Any rock dislodged at this point would have forward motion identical to that of the vehicle. It would also have massive vertical velocity, causing it to rocket skyward. It would not, however, hit the car (assuming there were no fender to inhibit its motion). The vehicle's speed is maintained by the power of its engine. The rock, now devoid of any motive force and experiencing the wind resistance of 70 MPH forward velocity, would slow down very quickly and land well behind the vehicle.
(4) At the front of the tire, the effect is identical to that of the rear of the tire except that any rock dislodged at this point will slam into the ground with a vertical velocity of 70 MPH. It will still have a forward velocity of 70 MPH.
This is the nature of a tire on a vehicle. The tread (any rock stuck in it) goes from zero forward velocity at the contact patch to double the vehicle's velocity at the top of the tire, passing each time through 70 MPH at the front and rear of the tire (defined relative to the vehicle). 0-70-140-70-0. Over and over and over again. If the vehicle has a constant motion so that the tire rotates at a constant rate, then the average forward velocity is exactly that of the vehicle. But to say that any rock dislodged from a tires tread can only move as fast as the vehicle is incorrect because instantaneous velocity <> forward velocity.
There was earlier a discussion about throwing a baseball from the back of a truck. Is this possible? Of course. I can throw a baseball about 60 MPH. If I stand in the back of a truck going 70 MPH and uncork one of my eminently hittable fastballs, the ball will leave my hand with the following velocities:
60 MPH relative to my hand and the truck.
130 MPH relative to the ground.
Upon release from my hand, it will encouter more ferocious wind resistance than any baseball has ever experienced from the hand of Aroldis Chapman. Lacking zero force in the direction it was propelled, it will decelerate at a massive rate. If I throw it just right, the truck will overtake it and the ball will hit the truck. (Or the truck will hit the ball, I suppose.)
Now, back to the salient point quoted in this post: given the degree of enclosure of most vehicle tires, it seems to me to be very difficult for this to happen. There are a few vehicles where the tires stick out of the fender wells to a significant extent. I suppose it might be possible to have a freak occurrence that discharges a rock just the right point, flinging it forward such that it bounces up and is caught by the windshield. But to happen twice? Dunno about that one.
roscoguy
Member
I may have saved you the trouble. This a 1/2 ton 4WD pickup, so it may or may not have the same ground clearance and/or wheelwell clearance as yours.
The dowel on the left goes from the highest point possible on the tire that will allow it to miss every part of the truck. The right dowel, at the same rebound angle, clearly shows that there is plenty of room (~6-8", I forgot to measure
) for an object thrown from the tire to bounce up in front of this vehicle.I wouldn't expect a rock to rebound from a gravel road. It would almost certainly need a solid, paved surface to bounce off of.
Aticus
Member
Old man, you didn't, by chance, have a short discussion with Rod Serling before you started your journey that fateful night?
Pecos
Pecos
