The Lost Wheel Thread

[4x4Tyke]

SpeckledJim said:

Aren't we discussing why something happens, which doesn't actually happen at all?

A rolling wheel slows down, unless something is actively speeding it up.

That is the real question, can we foresee anything that might speed it up, I think there is a edge case where that could happen.

Let's take the thought experiment a step further, but let's remove the drop factor. The wheel is in contact with the ground.

Scenario 1: Driven Wheel
Scenario 2: Undriven Wheel

The studs break progressively until only one is left for X time. What happens?

We've changed the centre of spin, created what amounts to a level, an offset gear.

I think the driven wheel, could have more angular momentum imparted, but I'm uncertain.

Just like we'd see with a sling.

Edited by 4x4Tyke on Friday 5th October 16:59

[Kawasicki]

TartanPaint said:

I feel like we're, pardon the pun, going round in circles.

I'm trying to imagine an experiment.


Imagine 3 identical wheels, suspended from an imaginary, static wheel-releasing dropper mechanism of some sort, hanging from the ceiling. Some sort of Barnes-Wallace Lancaster bomb releaser.

Spin wheel 1 forwards.
Don't spin wheel 2 at all.
Spin wheel 3 backwards.

Drop them all. What do they do?

Wheel 1 will roll/bounce forwards and come to rest away from the dropper.
Wheel 2 bounces a bit, wobbles and falls over somewhere directly underneath the dropper.
Wheel 3 rolls/bounces backwards and comes to rest away from the dropper, in the other direction from wheel 1.

I'm stating things here as I imagine them to happen, but feel free to argue if I'm wrong.

Wheels 1 and 3 have more energy because they have a rotational energy component, which wheel 2 does not.

Now do the same thing again, but with the imaginary wheel-dropping Lancaster bomber device flying along above a road at 70mph.

Drop them all. What do they do?

Wheel 2 first. It has some kinetic energy, but no rotational inertia, because it's not spinning. It bounces along at 70mph and slows down as air resistance and friction get the better of it and it'll fall behind the dropper.

Wheel 3 doesn't keep pace with the dropper at all. Although it has more energy than wheel 2, it's angular momentum is directional (momentum is a vector). It'll go shooting off backwards relative to the dropper. Every time it touches the road, its "backspin" will slow it down sharply. That's sort of intuitive, right?

Wheel 1 has a load of additional rotational kinetic energy, the same as wheel 3. But its angular momentum is acting in the opposite direction this time. In other words, in the direction of travel of the dropper.

I think that because wheel 1 and wheel 3 have the same amount of energy (total, kinetic plus rotational), wheel 1 will shoot forwards just as hard as wheel 3 shoots backwards.

Edited by TartanPaint on Friday 5th October 16:18

whether the wheel gets a forward boost of acceleration depends on the direction of the force at the contact patch upon first contact, if the wheel circumference is moving at a speed less than the ground contact speed it will get a braking force...and if it is moving at a speed higher than the ground it will be accelerated forward...so even the forward spinning wheel could get a jolt backwards if the circumferential speed is lower than the speed of the ground upon contact.

the length of time this braking or accelerating operates for is related to the stored rotational energy in the wheel

the wheel dropped from an airplane is way more complicated than a car losing a wheel...but it is a nice experiment all the same

i did all these experiments thousands of times...with lego technic as a kid...has anyone ever tried to make lego technic fly? I failed.

[Kawasicki]

SpeckledJim said:

Aren't we discussing why something happens, which doesn't actually happen at all?

A rolling wheel slows down, unless something is actively speeding it up.

yes.

but join in anyway, you might learn something!

[FiF]

Kawasicki said:

TartanPaint said:

I feel like we're, pardon the pun, going round in circles.

I'm trying to imagine an experiment.


Imagine 3 identical wheels, suspended from an imaginary, static wheel-releasing dropper mechanism of some sort, hanging from the ceiling. Some sort of Barnes-Wallace Lancaster bomb releaser.

Spin wheel 1 forwards.
Don't spin wheel 2 at all.
Spin wheel 3 backwards.

Drop them all. What do they do?

Wheel 1 will roll/bounce forwards and come to rest away from the dropper.
Wheel 2 bounces a bit, wobbles and falls over somewhere directly underneath the dropper.
Wheel 3 rolls/bounces backwards and comes to rest away from the dropper, in the other direction from wheel 1.

I'm stating things here as I imagine them to happen, but feel free to argue if I'm wrong.

Wheels 1 and 3 have more energy because they have a rotational energy component, which wheel 2 does not.

Now do the same thing again, but with the imaginary wheel-dropping Lancaster bomber device flying along above a road at 70mph.

Drop them all. What do they do?

Wheel 2 first. It has some kinetic energy, but no rotational inertia, because it's not spinning. It bounces along at 70mph and slows down as air resistance and friction get the better of it and it'll fall behind the dropper.

Wheel 3 doesn't keep pace with the dropper at all. Although it has more energy than wheel 2, it's angular momentum is directional (momentum is a vector). It'll go shooting off backwards relative to the dropper. Every time it touches the road, its "backspin" will slow it down sharply. That's sort of intuitive, right?

Wheel 1 has a load of additional rotational kinetic energy, the same as wheel 3. But its angular momentum is acting in the opposite direction this time. In other words, in the direction of travel of the dropper.

I think that because wheel 1 and wheel 3 have the same amount of energy (total, kinetic plus rotational), wheel 1 will shoot forwards just as hard as wheel 3 shoots backwards.

Edited by TartanPaint on Friday 5th October 16:18

whether the wheel gets a forward boost of acceleration depends on the direction of the force at the contact patch upon first contact, if the wheel circumference is moving at a speed less than the ground contact speed it will get a braking force...and if it is moving at a speed higher than the ground it will be accelerated forward...so even the forward spinning wheel could get a jolt backwards if the circumferential speed is lower than the speed of the ground upon contact.

the length of time this braking or accelerating operates for is related to the stored rotational energy in the wheel

the wheel dropped from an airplane is way more complicated than a car losing a wheel...but it is a nice experiment all the same

i did all these experiments thousands of times...with lego technic as a kid...has anyone ever tried to make lego technic fly? I failed.

OP here, breaking my own rule, reckon you could simulate the wheel dropped from an aeroplane experiment very easily using a conveyor belt. Oh no I've said it now.

I agree with your analysis essentially, that if the speed of the contact patch at first bounce is higher than the speed of the whole wheel then it will accelerate, viz the example of a lost wheel during a drifting competition.

[Kawasicki]

FiF said:

OP here, breaking my own rule, reckon you could simulate the wheel dropped from an aeroplane experiment very easily using a conveyor belt. Oh no I've said it now.

I agree with your analysis essentially, that if the speed of the contact patch at first bounce is higher than the speed of the whole wheel then it will accelerate, viz the example of a lost wheel during a drifting competition.

i've just spent the last half hour throwing water balloons. Binary star style.

[cahami]

I’m grabbing at straws but. I can picture in my head a front wheel drive vehicle with the wheel pulling a couple of tons behind it when it is let loose from the vehicle the energy that was exerted into the tyre as a pulling force would be released and momentarily would accelerate the wheel until other resistant forces slowed it down. Bit like pinging an rubber band.

[98elise]

cahami said:

I’m grabbing at straws but. I can picture in my head a front wheel drive vehicle with the wheel pulling a couple of tons behind it when it is let loose from the vehicle the energy that was exerted into the tyre as a pulling force would be released and momentarily would accelerate the wheel until other resistant forces slowed it down. Bit like pinging an rubber band.

It would be a tiny amount as a tyre doesn't deform much on a normal passenger car.

[SpeckledJim]

cahami said:

I’m grabbing at straws but. I can picture in my head a front wheel drive vehicle with the wheel pulling a couple of tons behind it when it is let loose from the vehicle the energy that was exerted into the tyre as a pulling force would be released and momentarily would accelerate the wheel until other resistant forces slowed it down. Bit like pinging an rubber band.

But once the wheel is free the spring effect has nothing to act against. It will work as hard at slowing the hub as at accelerating the rim.

[Clive-sz8cz]

I have seen film of a bouncing bomb test in which the bomb bounced higher than the drop height and took the tail off the bomber killing all aboard so would some of the spin energy must have been used to propel the bomb vertically upwards? Once at height the bomb or wheel would gain energy again due to gravity but how that would translate in to an increase in speed I wouldn't have a clue as I did my physics O level 42 years ago! Just thought I'd mention it.......

[4x4Tyke]

Clive-sz8cz said:

I have seen film of a bouncing bomb test in which the bomb bounced higher than the drop height and took the tail off the bomber killing all aboard so would some of the spin energy must have been used to propel the bomb vertically upwards? Once at height the bomb or wheel would gain energy again due to gravity but how that would translate in to an increase in speed I wouldn't have a clue as I did my physics O level 42 years ago! Just thought I'd mention it.......

The bouncing bomb was given back spin, for two reasons, to make it lag behind the plane to stop that happening and also keep it next to the dam wall as it sunk.

IIRC that is an post war American plane, the germans also captured one of the bouncing bombs from a crashed Lancaster and tried to make their own version suffering the same fate as that Yank plane because they didn't understand the importance of the back spin.

Edited by 4x4Tyke on Wednesday 17th October 12:08

[SpeckledJim]

Clive-sz8cz said:

I have seen film of a bouncing bomb test in which the bomb bounced higher than the drop height and took the tail off the bomber killing all aboard so would some of the spin energy must have been used to propel the bomb vertically upwards? Once at height the bomb or wheel would gain energy again due to gravity but how that would translate in to an increase in speed I wouldn't have a clue as I did my physics O level 42 years ago! Just thought I'd mention it.......

It was impacted water that hit the plane, I think, not the bomb?

[otolith]

If a wheel detaches from a vehicle and simply keeps on rolling on a smooth road, it will gradually slow down and stop, as you would expect.

If "something" happens to the rapidly rotating wheel which causes it to stop rotating and convert the rotational kinetic energy into translational kinetic energy, it will accelerate. Back of a fag packet schoolboy mechanics suggests that if the wheel were a solid, uniform disk, and it went from having a translational and tangential velocity of v (i.e. rolling) to just a translational velocity (i.e. flying, no longer spinning) it would be going at about 1.2v. "Something" being perhaps hitting something that deforms the rubber and pings it into the air.

Realistically, wheels and tyres have higher moments of inertia than solid disks do, so more rotational kinetic energy, but then they don't perfectly transfer rotation to translation when they hit things.

[Liokault]

I've been looking at this on youtube, lots of video of wheels coming off...none of the wheel overtaking the original vehicle other than when that vehicle is decelerating.

[Mammasaid]

Liokault said:

I've been looking at this on youtube, lots of video of wheels coming off...none of the wheel overtaking the original vehicle other than when that vehicle is decelerating.

vis - https://www.youtube.com/watch?v=-UgDAZBNZDA

[SpeckledJim]

That car is braking for a corner?

[Mammasaid]

and still the wheel doesn't overtake until the car turns in.

[julian64]

I think the answer to this is easy.

Wheel on the lorry has exactly the same rotational speed as when its released from the lorry. However when released it has an elastic deformation of the circumference as no longer has the weight of the lorry on it.

1, The sudden elastic change in vertical height of the tyre cause the tyre to bounce
2. The sudden increase in circumference will cause the tyre at the same rotation as those attached to the lorry to be heading down the road faster than those attached to the lorry purely because of the bigger circumference, until of course other forces take over.

[FarmyardPants]

julian64 said:

I think the answer to this is easy.

Wheel on the lorry has exactly the same rotational speed as when its released from the lorry. However when released it has an elastic deformation of the circumference as no longer has the weight of the lorry on it.

1, The sudden elastic change in vertical height of the tyre cause the tyre to bounce
2. The sudden increase in circumference will cause the tyre at the same rotation as those attached to the lorry to be heading down the road faster than those attached to the lorry purely because of the bigger circumference, until of course other forces take over.

If the wheel/tyre combo increases in radius it's rotational velocity will decrease (just as a spinning ice skater increases his rotational velocity by pulling his arms in, and vice versa). For the rotational velocity to increase for a given moment of interia (let alone an increased one) would imply the wheel has gained energy from somewhere.

[FarmyardPants]

The diff can play a part in two ways (assuming it's not a LSD):

1. In the moments before the wheel detaches it might be easier to drive - eg if it's jiggling around on the ends of the studs and bouncing about/providing less grip. This would cause the diff to send more power to that wheel, just as it would had the wheel hit something slippery. That could give it a small boost just before it leaves the vehicle.

2. After the wheel has come off, the hub is free to spin and less engine power will be available to the opposite wheel, which will slow the vehicle even if the driver maintains a constant throttle, allowing the wheel to overtake.

[julian64]

FarmyardPants said:

julian64 said:

I think the answer to this is easy.

Wheel on the lorry has exactly the same rotational speed as when its released from the lorry. However when released it has an elastic deformation of the circumference as no longer has the weight of the lorry on it.

1, The sudden elastic change in vertical height of the tyre cause the tyre to bounce
2. The sudden increase in circumference will cause the tyre at the same rotation as those attached to the lorry to be heading down the road faster than those attached to the lorry purely because of the bigger circumference, until of course other forces take over.

If the wheel/tyre combo increases in radius it's rotational velocity will decrease (just as a spinning ice skater increases his rotational velocity by pulling his arms in, and vice versa). For the rotational velocity to increase for a given moment of interia (let alone an increased one) would imply the wheel has gained energy from somewhere.

I'm gonna go for the idea that it has gained energy from not being squashed. The air inside a squashed tyre is hot, The compression in the tyre wall is stored energy. Both of those will not be present in a tyre not squashed. That energy goes somewhere.

If the diameter of the wheel changes after it left the hub with all other factors equal I would agree with you, but I don't think it does
.
.
.

I do have to just say here I don't actually believe this happens in reality. I believe the wheel comes off the lorry and goes at exactly the same speed as the lorry. I don't believe the wheel overtakes because I don't think any of the forces above are significant. So mainly theorising here.

Edited by julian64 on Wednesday 17th October 15:16