[akumachu]

Originally Posted by aerobod

The risk is from a stone squeezed under the edge of the tread next to the inner sidewall, effectively “pinging” the stone towards the centre underside of the car, as opposed to being thrown off the centre of the tread.

I see, so you are thinking that the tire edge pinch the rock so it goes laterally toward the cooler.
But the car is moving forward though right? Also the edge of the tire still rotate, not just the tread, so there is going to be some vector of force applying to that pinch velocity so the rock can't just go laterally completely.

Not saying that isn't possible but just seems almost physics defying to me. That's why I am trying to see what other folks opinions are.

[aerobod]

Originally Posted by akumachu

I see, so you are thinking that the tire edge pinch the rock so it goes laterally toward the cooler.
But the car is moving forward though right? Also the edge of the tire still rotate, not just the tread, so there is going to be some vector of force applying to that pinch velocity so the rock can't just go laterally completely.

Not saying that isn't possible but just seems almost physics defying to me. That's why I am trying to see what other folks opinions are.

The stone can move in a cone from the edge of the tyre, the bottom of the oil cooler will be within that cone. As the tread edge presses down on the stone as the tyre rotates, it will tend to project it inwards and forwards rather than backwards, but the shape of the stone and how much the edge of the tread is overlapping the stone will determine the direction it moves in.

The analogy is pressing your finger down on the side of a small ball with the finger tip raised slightly with the ball resting on a slightly rough surface - which direction can you make the ball fly in?

Still a low probability that a stone will be fired into the cooler and damage it, but the risk is there.

[akumachu]

Originally Posted by aerobod

The stone can move in a cone from the edge of the tyre, the bottom of the oil cooler will be within that cone. As the tread edge presses down on the stone as the tyre rotates, it will tend to project it inwards and forwards rather than backwards, but the shape of the stone and how much the edge of the tread is overlapping the stone will determine the direction it moves in.

The analogy is pressing your finger down on the side of a small ball with the finger tip raised slightly with the ball resting on a slightly rough surface - which direction can you make the ball fly in?

Still a low probability that a stone will be fired into the cooler and damage it, but the risk is there.

Would the force from the edge press is strong enough to pierce through the metal fin and then damage the metal tubing under?

That's very unlucky.

I think the skid plate is certainly good for the one scenario I worry most, which is curbing the underside or running over debris.

[aerobod]

Originally Posted by akumachu

Would the force from the edge press is strong enough to pierce through the metal fin and then damage the metal tubing under?

The tyre can impart quite a velocity to the stone due to the spring effect of the tyre.

Some basics for an approximation of that velocity:

Weight on tyre - 4500N
Contact patch area - 0.015m^2
Tyre spring rate - 400N/mm
Stone diameter - 15mm (0.015m)
Stone volume - 0.00000177m^3
Stone mass - 0.00477kg

If the stone is trapped mainly under the edge of the tread, due to the rigidity imparted by the tread steel belts the 15mm height of the stone will likely distort the contact patch over a 30mm radius semi circle an average of 7.5mm deep. This equates to 0.094 of the contact patch area and a stored spring force of 282N (spring rate * deflection * proportion of contact patch).

This stored force can impart an initial acceleration of 282 / 0.00477 = 59,119m/s^2. (a = F / m). This force acts over a distance (s) of about 10mm (0.01m) until the stone is ejected, which leads to a velocity of 34.4m/s, 123km/h, 77mph (v^2 = 2as).

I think a stone with some sharp projections travelling at 123km/h can easily puncture the fairly thin alloy tubes in the cooler.

[Carefree]

Everybody turn in your “homework”

For you purists that’s:
(home)x(Fs)

[Bret]

Sometimes projectiles defy logic.

[akumachu]

Originally Posted by aerobod

The tyre can impart quite a velocity to the stone due to the spring effect of the tyre.

Some basics for an approximation of that velocity:

Weight on tyre - 4500N
Contact patch area - 0.015m^2
Tyre spring rate - 400N/mm
Stone diameter - 15mm (0.015m)
Stone volume - 0.00000177m^3
Stone mass - 0.00477kg

If the stone is trapped mainly under the edge of the tread, due to the rigidity imparted by the tread steel belts the 15mm height of the stone will likely distort the contact patch over a 30mm radius semi circle an average of 7.5mm deep. This equates to 0.094 of the contact patch area and a stored spring force of 282N (spring rate * deflection * proportion of contact patch).

This stored force can impart an initial acceleration of 282 / 0.00477 = 59,119m/s^2. (a = F / m). This force acts over a distance (s) of about 10mm (0.01m) until the stone is ejected, which leads to a velocity of 34.4m/s, 123km/h, 77mph (v^2 = 2as).

I think a stone with some sharp projections travelling at 123km/h can easily puncture the fairly thin alloy tubes in the cooler.

That's some serious stats line. Where do you even get the spring rate of the tire? Must be some manufacture stats.

I would say a 15mm stone is a quite a big one too, but what I am surprised by is the 30mm radius estimate. Did you mean 30mm circumference with a radius of 7.5mm for the geometry?

Another part is that surely there is speed you are calculating but the direction, aka velocity is what really important here, since we are trying to estimate the velocity hitting the oil cooler, and depending on your ride height, the angle to hit it will be different, so is the amount of force.

Assuming all those estimated number being correct as well as formula used, a horizontal velocity isn't going to apply any force to the cooler that sits higher.

Maybe it will involve some form of funny rock shape hit the ground and then ricochet upward. The ricochet will take always some kinetic energy, which that physics is beyond my knowledge.

I think we need a ballistic expert here lol

And now I am going to go out and check my cooler since I had heard some funny metal sounds with my windows down before.

[akumachu]

Originally Posted by Bret

Sometimes projectiles defy logic.

Glad that the guy isn't hurt or killed:
"Guy shoots a 50 Caliber rifle at a steel plate. The bullet ricochets back at him, hits the ground, and then strikes the shooter in the temple."

I guess ricochet does take away a good amount of kinetic energy

[M_Power Rob]

Originally Posted by akumachu

That's some serious stats line. Where do you even get the spring rate of the tire? Must be some manufacture stats.

I would say a 15mm stone is a quite a big one too, but what I am surprised by is the 30mm radius estimate. Did you mean 30mm circumference with a radius of 7.5mm for the geometry?

Another part is that surely there is speed you are calculating but the direction, aka velocity is what really important here, since we are trying to estimate the velocity hitting the oil cooler, and depending on your ride height, the angle to hit it will be different, so is the amount of force.

Assuming all those estimated number being correct as well as formula used, a horizontal velocity isn't going to apply any force to the cooler that sits higher.

Maybe it will involve some form of funny rock shape hit the ground and then ricochet upward. The ricochet will take always some kinetic energy, which that physics is beyond my knowledge.

I think we need a ballistic expert here lol

And now I am going to go out and check my cooler since I had heard some funny metal sounds with my windows down before.

Since you seem to be very much against what is believed to have happened, I am waiting to hear your take how it happened.

[aerobod]

Originally Posted by akumachu

That's some serious stats line. Where do you even get the spring rate of the tire? Must be some manufacture stats.

I would say a 15mm stone is a quite a big one too, but what I am surprised by is the 30mm radius estimate. Did you mean 30mm circumference with a radius of 7.5mm for the geometry?

Another part is that surely there is speed you are calculating but the direction, aka velocity is what really important here, since we are trying to estimate the velocity hitting the oil cooler, and depending on your ride height, the angle to hit it will be different, so is the amount of force.

Assuming all those estimated number being correct as well as formula used, a horizontal velocity isn't going to apply any force to the cooler that sits higher.

Maybe it will involve some form of funny rock shape hit the ground and then ricochet upward. The ricochet will take always some kinetic energy, which that physics is beyond my knowledge.

I think we need a ballistic expert here lol

And now I am going to go out and check my cooler since I had heard some funny metal sounds with my windows down before.

30mm radius is realistic, if you try to lay a stone under the tread of a tyre that is half that height, as the belts prevent the tyre from fully conforming to the shape of the stone.

The tyre spring rate is based on general knowledge from automotive sources, but can be estimated for a given tyre by comparing roiling radius under load vs actual tyre radius, then dividing that value into tyre load. For the 275/35-19 MP4S the number of revs per mile is quoted as 783, compared with the calculated radius giving 758.6. This rolling radius is therefore 0.969 of the value for the unloaded radius, or 10.5mm less. With a load of 4500N, that equates to a spring rate of 429N/mm

The velocity vector for the stone will be inside a truncated cone that is bound by the ground and underside of the car and has the apex at the starting point of the stone. As previously mentioned, the vector direction will be determined by the road surface roughness, the way the tyre sidewall presses down on the stone to load it, the stone surface shape itself and how the tyre sidewall releases the stone to apply the stored energy to it.

You can test this by pressing down on the side of a ball with your finger and seeing how it imparts a significant vertical motion when on a rough surface with high grip compared with skidding more horizontally on a smooth surface with low grip.

The acceleration imparted on the stone varies little with the vector direction, as it is determined by the energy released from the tyre and the distance the force is applied to the stone over. If the stone stays under the tread and doesn’t move, it doesn’t accelerate, if it moves sideways from under the tyre during the tyre returning to it’s undeflected state, then the tyre energy that was stored due to the deflection will be transferred to the stone.

[akumachu]

Originally Posted by M_Power Rob

Since you seem to be very much against what is believed to have happened, I am waiting to hear your take how it happened.

My take has been that it is more likely that some debris that is tall and hit the cooler. Or curbing the underside on a pavement.

I worry everyday since I have to do a U turn in my parking lot and the deck stair step is literally around that height.

I just want to better protect the cooler if I can convince myself the factors. Completely covering it isn't an option for me since I do autocross and track with the car so cooling is important.

[M_Power Rob]

Originally Posted by akumachu

My take has been that it is more likely that some debris that is tall and hit the cooler. Or curbing the underside on a pavement.

I worry everyday since I have to do a U turn in my parking lot and the deck stair step is literally around that height.

I just want to better protect the cooler if I can convince myself the factors. Completely covering it isn't an option for me since I do autocross and track with the car so cooling is important.

That is my RADIATOR that was damaged, not the oil cooler (dud you see the picture and read the warning I received?).

The underside of the car is blemish free. It did not bottom out, wasn't curbed or any other BS you implied.

[akumachu]

Originally Posted by aerobod

30mm radius is realistic, if you try to lay a stone under the tread of a tyre that is half that height, as the belts prevent the tyre from fully conforming to the shape of the stone.

The tyre spring rate is based on general knowledge from automotive sources, but can be estimated for a given tyre by comparing roiling radius under load vs actual tyre radius, then dividing that value into tyre load. For the 275/35-19 MP4S the number of revs per mile is quoted as 783, compared with the calculated radius giving 758.6. This rolling radius is therefore 0.969 of the value for the unloaded radius, or 10.5mm less. With a load of 4500N, that equates to a spring rate of 429N/mm

The velocity vector for the stone will be inside a truncated cone that is bound by the ground and underside of the car and has the apex at the starting point of the stone. As previously mentioned, the vector direction will be determined by the road surface roughness, the way the tyre sidewall presses down on the stone to load it, the stone surface shape itself and how the tyre sidewall releases the stone to apply the stored energy to it.

You can test this by pressing down on the side of a ball with your finger and seeing how it imparts a significant vertical motion when on a rough surface with high grip compared with skidding more horizontally on a smooth surface with low grip.

The acceleration imparted on the stone varies little with the vector direction, as it is determined by the energy released from the tyre and the distance the force is applied to the stone over. If the stone stays under the tread and doesn’t move, it doesn’t accelerate, if it moves sideways from under the tyre during the tyre returning to it’s undeflected state, then the tyre energy that was stored due to the deflection will be transferred to the stone.

So would speed of the car play into this at all? I guess I am trying to find out whether if this is would be more of a problem for a track vs the street.

I took a rock during the slow speed parade lap and my windshield got a cracked badly. That was a quoted $1600 repair. But meanwhile several other earlier highway rocks only left little divots.
So I know that's just rng, shape of rocks, but that because I literally saw it with my eyes.
The underside, it's always hard to tell.

[akumachu]

Originally Posted by M_Power Rob

That is my RADIATOR that was damaged, not the oil cooler (dud you see the picture and read the warning I received?).

The underside of the car is blemish free. It did not bottom out, wasn't curbed or any other BS you implied.

I know the original poster was about the radiator but I thought we were talking about the skid plate, which is on the bottom.

Yeah, the front is a total different topic that doesn't require much discussion. I literally just wrote about that for my windshield.

[M_Power Rob]

Originally Posted by akumachu

I know the original poster was about the radiator but I thought we were talking about the skid plate, which is on the bottom.

Yeah, the front is a total different topic that doesn't require much discussion. I literally just wrote about that for my windshield.

No, you are totally missing what I posted. The damage to my radiator was the backside (that faces the engibe), not the front.

[aerobod]

Originally Posted by akumachu

So would speed of the car play into this at all? I guess I am trying to find out whether if this is would be more of a problem for a track vs the street.

I took a rock during the slow speed parade lap and my windshield got a cracked badly. That was a quoted $1600 repair. But meanwhile several other earlier highway rocks only left little divots.
So I know that's just rng, shape of rocks, but that because I literally saw it with my eyes.
The underside, it's always hard to tell.

Speed is likely not a factor for stones coming off the sides of the tyre in this way, as opposed to those thrown from the tread which are speed dependent. When we have gravel on the road here in the winter to aid traction, the stones that are projected sideways from a vehicle’s tyres when passing alongside or being passed, make just as much noise when hitting the paint, independent of the vehicle speed. Luckily they typically keep the gravel for snow traction to about 5mm diameter.

[akumachu]

Originally Posted by M_Power Rob

No, you are totally missing what I posted. The damage to my radiator was the backside (that faces the engibe), not the front.

I didn't realize you can get to the back side until I looked this morning.
No idea except the possibilty may be happening going on reverse.
Or maybe a little mouse with a screw driver playing xylophone with your radiator.

[akumachu]

Originally Posted by aerobod

Speed is likely not a factor for stones coming off the sides of the tyre in this way, as opposed to those thrown from the tread which are speed dependent. When we have gravel on the road here in the winter to aid traction, the stones that are projected sideways from a vehicle’s tyres when passing alongside or being passed, make just as much noise when hitting the paint, independent of the vehicle speed. Luckily they typically keep the gravel for snow traction to about 5mm diameter.

You made me worry so I looked this morning when I was getting gas.
I took a picture of my underside from the side.

The oil cooler sits even further up front than I remember.

I know there is always a possibility but looking at that, I feel much better since it just looked so unlikely a pinched rock could go in the direction back to the front by several inches and up to hit the oil cooler.

I say we should never drive reverse lol

[aerobod]

Originally Posted by akumachu

You made me worry so I looked this morning when I was getting gas.
I took a picture of my underside from the side.

The oil cooler sits even further up front than I remember.

I know there is always a possibility but looking at that, I feel much better since it just looked so unlikely a pinched rock could go in the direction back to the front by several inches and up to hit the oil cooler.

I say we should never drive reverse lol

The angle varies from 9 to 20 degrees from the tyre contact point to the oil cooler, which is consistent with the low-angle high-energy stones hitting the side of a vehicle and chipping the paint or leaving small dents from another vehicle that is passing several metres away, with winter gravel on the road.

The tyre also has a higher probability of projecting the stone forward when the car is moving forward as the the tread is clamping down on the stone. This is why the analogy of pressing on the side of a ball with a raised finger tip simulates the edge of the tyre rolling on to a stone, when you try this experiment the ball has a higher probability of going forward, inwards and slightly upwards.

You still have to be very unlucky to have the right stone hit the oil cooler with enough energy to put a hole in the cooler and large road debris is probably a higher risk. I believe there was a car in Germany that had a diagonal track marking the fins from the direction of the tyre contact point, on it's damaged G-series oil cooler.

[BimmerFix]

Someone explain it to me like I'm 5

Much ado about nothing

[shehatezme]

Update: took it to dealer. No leak in the oil cooler. The yellow is Possible paint transfer from something. Otherwise bone dry. Going to install my skid plate now.

[KC87ZB]

Originally Posted by shehatezme

Update: took it to dealer. No leak in the oil cooler. The yellow is Possible paint transfer from something. Otherwise bone dry. Going to install my skid plate now.

Excellent news