The Lost Wheel Thread
Discussion
Over in the Lounge there has been a thread about workplace accidents, and one of the posts started a discussion about lost wheels from large goods vehicles, buses and coaches.
In my time have investigated a few of these, but the thing which started the controversy was this, especially bit in bold.
After investigating some incidents and seeing CCTV footage of these things whanging into oncoming traffic I got utterly paranoid about motorway night driving for a time. Thanks to better procedures and improved design it's less of a risk, but even so, one chimpanzee with an air impact nut driver can undo all the good work.If you follow the thread you will see various discussions.
Now as I've stated on that thread I cannot get my head around the explanations why the wheel should accelerate like that. Some of the circumstances I can understand, eg wheel rolls downhill, gains speed due to gravity, another wheel comes detached when spinning at high speed during drifting event.
Aiui the numbers behind the statement come from post accident analysis where the amount if energy absorbed in the collision has been estimated from damage to vehicles and objects after making allowances for materials, design, modifications and corrosion. From that the speed of objects and impact force has been estimated. I do not know to what extent there has been selective presentation of numbers to generate greater reader impact, sorry about the pun.
There it sits, a big fat don't know about the physics of it here.
What I do know, again said in that thread, it's irrelevant if a wheel accelerates or not, if it detached at 90kph, and even if it slows to 70kph, with your approach speed of 110 kph, you have a combined closing speed which means a very bad day for someone.
Over to you. No conveyor belts, by order.
In my time have investigated a few of these, but the thing which started the controversy was this, especially bit in bold.
FiF said:
RTB said:
Another truck related story: My dad used to drive oil tankers (4 and 6 wheelers) delivering heating oil. He'd come back from a delivery in south Manchester (to a factory I think) and was coming down the A6 through Hazel Grove and Highlane, with lots of traffic as it was school chucking out time. The wagon steering didn't feel right so he went steady. When he got back to the yard he drove the wagon into workshop and had one of the fitters jack it up to check it. As soon as the fitter touched the front wheel it fell off...
Not sure of the current stats but every year 7 people used to die from wheels coming off large goods vehicles. Here's a sobering quote from the Inst Road Transport Engineers on wheel security "When a wheel becomes detached from a heavy vehicle it may simply come to rest without causing any further damage or harm. However, in the wrong circumstances, when wheels become detached from a moving vehicle, they can accelerate up to around 150 km per hour, going out of control like a bouncing bomb, reaching a height of 50 metres before colliding with other vehicles or road users at an equivalent force of 10 tonnes."After investigating some incidents and seeing CCTV footage of these things whanging into oncoming traffic I got utterly paranoid about motorway night driving for a time. Thanks to better procedures and improved design it's less of a risk, but even so, one chimpanzee with an air impact nut driver can undo all the good work.
Now as I've stated on that thread I cannot get my head around the explanations why the wheel should accelerate like that. Some of the circumstances I can understand, eg wheel rolls downhill, gains speed due to gravity, another wheel comes detached when spinning at high speed during drifting event.
Aiui the numbers behind the statement come from post accident analysis where the amount if energy absorbed in the collision has been estimated from damage to vehicles and objects after making allowances for materials, design, modifications and corrosion. From that the speed of objects and impact force has been estimated. I do not know to what extent there has been selective presentation of numbers to generate greater reader impact, sorry about the pun.
There it sits, a big fat don't know about the physics of it here.
What I do know, again said in that thread, it's irrelevant if a wheel accelerates or not, if it detached at 90kph, and even if it slows to 70kph, with your approach speed of 110 kph, you have a combined closing speed which means a very bad day for someone.
Over to you. No conveyor belts, by order.
Sorry, forgot this, bit of background reading in addition to the Lounge thread
https://www.soe.org.uk/downloads/1452081366-Wheel%...
https://www.soe.org.uk/downloads/1452081366-Wheel%...
As covered on the original Lounge thread I was always a subscriber to the wheel slows down, vehicle slows down more rapidly, but then there's the trailer argument, and was shown a CCTV footage where the wheel really did appear to accelerate.
But as above, a big fat dunno from me, apart from particular circumstances already mentioned.
But as above, a big fat dunno from me, apart from particular circumstances already mentioned.
FiF said:
As covered on the original Lounge thread I was always a subscriber to the wheel slows down, vehicle slows down more rapidly, but then there's the trailer argument, and was shown a CCTV footage where the wheel really did appear to accelerate.
But as above, a big fat dunno from me, apart from particular circumstances already mentioned.
The wheel almost certainly benefited from being in the slipstream of the towing vehicle, which would have also presumably slowed on purpose once the issue was known.But as above, a big fat dunno from me, apart from particular circumstances already mentioned.
If the wheel did somehow increase it's momentum then the single detectable way to create energy would be to have wheels falling off, which would be the most bizarre power station in the world!
My take on it:
Wheel is squashed when on the car, has a particular velocity, and a particular angular momentum.
When it falls off the wheel reverts to its unladen regular diameter, and loses a bit of angular momentum to an increase in velocity.
(Wheel turns slightly slower but has an increased circumference, so speeds up)
Wheel is squashed when on the car, has a particular velocity, and a particular angular momentum.
When it falls off the wheel reverts to its unladen regular diameter, and loses a bit of angular momentum to an increase in velocity.
(Wheel turns slightly slower but has an increased circumference, so speeds up)
I think people here need to consider the angular momentum of the wheels when it detaches from the car.
The wheel already has a velocity in the same direction of the car, we'll (wheel hehe) call that v, say 30 mph.
It also has a rotational (angular) velocity, how fast it is spinning, lets call the 'av'.
When the wheel leaves the car its v and av need to be taken into consideration, it seems likely to me that Av will always be bigger than v.
If the wheel simply flies through the air then both v and av will degrade due to losses we can attribute to air resistance, and the wheel will finally come to rest due to gravity and frictional forces.
However if the wheel hits the road and has an angular velocity greater than its initial velocity 'v' then some of that angular velocity will be imparted to its other velocity and cause it to accelerate.
Let's consider the wheel detaching whilst the car is airborn and the driver had his foot on the brake. In this situation the wheel would have no angular velocity (it wasn't rotating), but the moment the wheel hits the road some of its forward velocity 'v' will be translated into angular velocity, spinning the wheel.
The product of v and av will be different according to circumstances. There is no logical reason to assume that at the moment the wheel detaches that its 'av' component cannot far exceed its 'v' component, and therefore the wheel could in fact accelerate massively and go bouncing down the road.
The wheel already has a velocity in the same direction of the car, we'll (wheel hehe) call that v, say 30 mph.
It also has a rotational (angular) velocity, how fast it is spinning, lets call the 'av'.
When the wheel leaves the car its v and av need to be taken into consideration, it seems likely to me that Av will always be bigger than v.
If the wheel simply flies through the air then both v and av will degrade due to losses we can attribute to air resistance, and the wheel will finally come to rest due to gravity and frictional forces.
However if the wheel hits the road and has an angular velocity greater than its initial velocity 'v' then some of that angular velocity will be imparted to its other velocity and cause it to accelerate.
Let's consider the wheel detaching whilst the car is airborn and the driver had his foot on the brake. In this situation the wheel would have no angular velocity (it wasn't rotating), but the moment the wheel hits the road some of its forward velocity 'v' will be translated into angular velocity, spinning the wheel.
The product of v and av will be different according to circumstances. There is no logical reason to assume that at the moment the wheel detaches that its 'av' component cannot far exceed its 'v' component, and therefore the wheel could in fact accelerate massively and go bouncing down the road.
AshVX220 said:
Could it also be that the wheel is no longer part of a much greater mass, therefore the energy available for just moving the wheel is less?
Close.Once free of the greater mass the energy at that instantaneous monent is the same but acting on wheel (lesser mass) alone.
Think sabot tank rounds...
Kccv23highliftcam said:
AshVX220 said:
Could it also be that the wheel is no longer part of a much greater mass, therefore the energy available for just moving the wheel is less?
Close.Once free of the greater mass the energy at that instantaneous monent is the same but acting on wheel (lesser mass) alone.
Think sabot tank rounds...
I'm not sure that the example of sabot rounds is particularly helpful. Surely the whole principle of those is that the projectile is a much smaller more dense material, the total weight of the projectile being less than a full bore round, but still subject to the same forces from the propelling gas, therefore it accelerates to a higher speed.
The light sabot part is discarded after leaving the barrel in order to reduce aerodynamic drag and thus result in the higher projectile speed being maintained. It's not as if the projectile loses the sabot and then goes even faster from that point.
If I've misunderstood how sabot rounds work then please correct me.
As an off topic aside we have fired sabot rounds from a sniper rifle into armour protected ballistic body gel as part of developments into vital organ plates for vests, bugger me they made a mess even through steel armour.
The light sabot part is discarded after leaving the barrel in order to reduce aerodynamic drag and thus result in the higher projectile speed being maintained. It's not as if the projectile loses the sabot and then goes even faster from that point.
If I've misunderstood how sabot rounds work then please correct me.
As an off topic aside we have fired sabot rounds from a sniper rifle into armour protected ballistic body gel as part of developments into vital organ plates for vests, bugger me they made a mess even through steel armour.
I've been thinking about this and it seems there is a 'bounds' issue.
The bounds being what exactly is happening at the moment the wheel detaches.
I think we can all agree that if the car was stationary, say on axle stands, and the engine was running driving the wheels, if the wheel were to let go then it would hit the ground spinning and accelerate, due to having a greater rotational velocity than it's forward velocity.
Likewise if a wheel detaches from a car doing 'x' mph when it (the wheel) is not spinning (braked), then it can only slow down.
Quite what happens to a wheel detaching from a car at 70 mph down a motorway is awkward, could the wheel end up going faster (down the road) than the car it just detached from was travelling - i.e. the wheel accelerates beyond 70mph?
The bounds being what exactly is happening at the moment the wheel detaches.
I think we can all agree that if the car was stationary, say on axle stands, and the engine was running driving the wheels, if the wheel were to let go then it would hit the ground spinning and accelerate, due to having a greater rotational velocity than it's forward velocity.
Likewise if a wheel detaches from a car doing 'x' mph when it (the wheel) is not spinning (braked), then it can only slow down.
Quite what happens to a wheel detaching from a car at 70 mph down a motorway is awkward, could the wheel end up going faster (down the road) than the car it just detached from was travelling - i.e. the wheel accelerates beyond 70mph?
JustALooseScrew said:
I think we can all agree that if the car was stationary, say on axle stands, and the engine was running driving the wheels, if the wheel were to let go then it would hit the ground spinning and accelerate, due to having a greater rotational velocity than it's forward velocity.
How I see it (and I am NOT good at this stuff..)
While attached to the trailer, the wheel is a flywheel. More energy is being put into the wheel than is being translated into useful work (rotation). The wheel doesn't spin faster than the forward motion of the trailer, because the excess energy input into the wheel is balanced by friction against the road slowing the wheel down (caused by the weight of the trailer and the drag from air resistance and friction). This balance of forces means the wheel, not surprisingly, turns at the speed the trailer is moving at relative to the road. That's all fairly intuitive except that the wheel is a flywheel and flywheels store excess energy.
Freed from the trailer's drag and weight, the wheel should behave like any other flywheel, and release its energy until it's all used up and the balance is restored in the system. I think it makes sense to me that this would be released initially as acceleration relative to the road, and probably some bounce like a tiddlywink with the force of the weight of the trailer removed. It will continue to accelerate until all excess rotational energy is all used up as forward motion and height gain. Then it's just like any other system... no energy input (ignoring hills) so friction and gravity win and it slows down and stops.
I think.
While attached to the trailer, the wheel is a flywheel. More energy is being put into the wheel than is being translated into useful work (rotation). The wheel doesn't spin faster than the forward motion of the trailer, because the excess energy input into the wheel is balanced by friction against the road slowing the wheel down (caused by the weight of the trailer and the drag from air resistance and friction). This balance of forces means the wheel, not surprisingly, turns at the speed the trailer is moving at relative to the road. That's all fairly intuitive except that the wheel is a flywheel and flywheels store excess energy.
Freed from the trailer's drag and weight, the wheel should behave like any other flywheel, and release its energy until it's all used up and the balance is restored in the system. I think it makes sense to me that this would be released initially as acceleration relative to the road, and probably some bounce like a tiddlywink with the force of the weight of the trailer removed. It will continue to accelerate until all excess rotational energy is all used up as forward motion and height gain. Then it's just like any other system... no energy input (ignoring hills) so friction and gravity win and it slows down and stops.
I think.
TartanPaint said:
How I see it (and I am NOT good at this stuff..)
While attached to the trailer, the wheel is a flywheel. More energy is being put into the wheel than is being translated into useful work (rotation). The wheel doesn't spin faster than the forward motion of the trailer, because the excess energy input into the wheel is balanced by friction against the road slowing the wheel down (caused by the weight of the trailer and the drag from air resistance and friction). This balance of forces means the wheel, not surprisingly, turns at the speed the trailer is moving at relative to the road. That's all fairly intuitive except that the wheel is a flywheel and flywheels store excess energy.
Freed from the trailer's drag and weight, the wheel should behave like any other flywheel, and release its energy until it's all used up and the balance is restored in the system. I think it makes sense to me that this would be released initially as acceleration relative to the road, and probably some bounce like a tiddlywink with the force of the weight of the trailer removed. It will continue to accelerate until all excess rotational energy is all used up as forward motion and height gain. Then it's just like any other system... no energy input (ignoring hills) so friction and gravity win and it slows down and stops.
I think.
Beats my stab at it While attached to the trailer, the wheel is a flywheel. More energy is being put into the wheel than is being translated into useful work (rotation). The wheel doesn't spin faster than the forward motion of the trailer, because the excess energy input into the wheel is balanced by friction against the road slowing the wheel down (caused by the weight of the trailer and the drag from air resistance and friction). This balance of forces means the wheel, not surprisingly, turns at the speed the trailer is moving at relative to the road. That's all fairly intuitive except that the wheel is a flywheel and flywheels store excess energy.
Freed from the trailer's drag and weight, the wheel should behave like any other flywheel, and release its energy until it's all used up and the balance is restored in the system. I think it makes sense to me that this would be released initially as acceleration relative to the road, and probably some bounce like a tiddlywink with the force of the weight of the trailer removed. It will continue to accelerate until all excess rotational energy is all used up as forward motion and height gain. Then it's just like any other system... no energy input (ignoring hills) so friction and gravity win and it slows down and stops.
I think.
Kccv23highliftcam said:
TartanPaint said:
How I see it (and I am NOT good at this stuff..)
While attached to the trailer, the wheel is a flywheel. More energy is being put into the wheel than is being translated into useful work (rotation). The wheel doesn't spin faster than the forward motion of the trailer, because the excess energy input into the wheel is balanced by friction against the road slowing the wheel down (caused by the weight of the trailer and the drag from air resistance and friction). This balance of forces means the wheel, not surprisingly, turns at the speed the trailer is moving at relative to the road. That's all fairly intuitive except that the wheel is a flywheel and flywheels store excess energy.
Freed from the trailer's drag and weight, the wheel should behave like any other flywheel, and release its energy until it's all used up and the balance is restored in the system. I think it makes sense to me that this would be released initially as acceleration relative to the road, and probably some bounce like a tiddlywink with the force of the weight of the trailer removed. It will continue to accelerate until all excess rotational energy is all used up as forward motion and height gain. Then it's just like any other system... no energy input (ignoring hills) so friction and gravity win and it slows down and stops.
I think.
Beats my stab at it While attached to the trailer, the wheel is a flywheel. More energy is being put into the wheel than is being translated into useful work (rotation). The wheel doesn't spin faster than the forward motion of the trailer, because the excess energy input into the wheel is balanced by friction against the road slowing the wheel down (caused by the weight of the trailer and the drag from air resistance and friction). This balance of forces means the wheel, not surprisingly, turns at the speed the trailer is moving at relative to the road. That's all fairly intuitive except that the wheel is a flywheel and flywheels store excess energy.
Freed from the trailer's drag and weight, the wheel should behave like any other flywheel, and release its energy until it's all used up and the balance is restored in the system. I think it makes sense to me that this would be released initially as acceleration relative to the road, and probably some bounce like a tiddlywink with the force of the weight of the trailer removed. It will continue to accelerate until all excess rotational energy is all used up as forward motion and height gain. Then it's just like any other system... no energy input (ignoring hills) so friction and gravity win and it slows down and stops.
I think.
It's freewheeling and will only use enough energy to overcome drag
TheDrBrian said:
Why is the wheel taking in extra energy?
It's freewheeling and will only use enough energy to overcome drag
The additional energy I was talking about was the bit which overcomes the drag. It's freewheeling and will only use enough energy to overcome drag
But you're right, whether it's a driven wheel or a free wheel does make a difference. Dammit. Edit: No it doesn't.
Edited by TartanPaint on Friday 28th September 11:27
Hi i think people are thinking this as a perfect theoretical situation. In my experience things don't tend to break cleanly and detach uniformly, so i would expect, in some situations where studs are shearing off allowing a rotating wheel to come loose, the wheel has detached from the centre hub and could be held by one or two studs then the centre of rotation has changed and this will impart a greater velocity into the wheel as the final studs shear and the wheel departs the axle area. Think what you see when a lorry tire sheds itself going down a road, the wheel jumps all over the place not maintaining perfect rotation.
idiotgap said:
JustALooseScrew said:
I think we can all agree that if the car was stationary, say on axle stands, and the engine was running driving the wheels, if the wheel were to let go then it would hit the ground spinning and accelerate, due to having a greater rotational velocity than it's forward velocity.
Perfect! (Wish I'd thought of that example) I assume we can all agree that a loose wheel can accelerate once detached.
JustALooseScrew said:
idiotgap said:
JustALooseScrew said:
I think we can all agree that if the car was stationary, say on axle stands, and the engine was running driving the wheels, if the wheel were to let go then it would hit the ground spinning and accelerate, due to having a greater rotational velocity than it's forward velocity.
Perfect! (Wish I'd thought of that example) I assume we can all agree that a loose wheel can accelerate once detached.
The lost wheel situation is a car/trailer in a steady state at speed X which loses a wheel which somehow manages to accelerate to speed X+Y and overtake the car which is still at X.
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