The Lost Wheel Thread

[SpeckledJim]

julian64 said:

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
.
.
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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

The energy in a squashed tyre goes into elevating the centre of gravity of the wheel and tyre as it unsquashes itself.

The tyre is, on average, further from the centre of the earth after being unsquashed, which accounts for the energy.

[otolith]

That claim appears to come originally from a 2006 Transport Research Laboratory report, by the way. Can't get my hands on the report to see where it got it from.

[julian64]

SpeckledJim said:

The energy in a squashed tyre goes into elevating the centre of gravity of the wheel and tyre as it unsquashes itself.

The tyre is, on average, further from the centre of the earth after being unsquashed, which accounts for the energy.

I can't argue with either of those as undoubtedly some will, as I explained the bounce probably comes from 1. . Probably the same could be said for any other explanation on this thread. What is important however is the significance of those wrt the significance of other places where energy can transfer.

In that I think my original explanation is by far the most likely

[SpeckledJim]

julian64 said:

SpeckledJim said:

The energy in a squashed tyre goes into elevating the centre of gravity of the wheel and tyre as it unsquashes itself.

The tyre is, on average, further from the centre of the earth after being unsquashed, which accounts for the energy.

I can't argue with either of those as undoubtedly some will, as I explained the bounce probably comes from 1. . Probably the same could be said for any other explanation on this thread. What is important however is the significance of those wrt the significance of other places where energy can transfer.

In that I think my original explanation is by far the most likely

I don't think we've established that this happens at all, much less how.

[FiF]

otolith said:

That claim appears to come originally from a 2006 Transport Research Laboratory report, by the way. Can't get my hands on the report to see where it got it from.

If you are referring to 2006 report PPR086 I have a pdf of the report and nowhere does that refer to any increase in wheel velocity.

Obliquely they reference as one of the data sources as the HVCIS fatal database which is an analysis of the STATS19 form completed by police collision investigators, there may be something in there.

Equally the bibliography references another TEL report from 2006 PPR085, but that's just a rdgulatory impact assessment so possibly not from there. I don't have a copy of that.

Other references include collision investigations from around the world, which would be a large task to survey, but seeing as they don't make the claim in the report.

Suspect we have to trawl back through old IRTE stuff, thrown all mine away in a clear out.

[otolith]

This is interesting - a wheel comes off a lorry. Bounces, tumbling, then it would appear that it hits the ground at the right angle for the spinning tyre to grip and propel it spearing off violently to the side.

https://www.newsflare.com/video/69495/crime-accide...

If you can imagine that happening in such a way that it is propelled even faster in the direction it was already going.

In this case, the tyre looks to be going a lot faster than the traffic on the other carriageway?

https://www.youtube.com/watch?v=WOtb3MvK-a0

I think these are basically freak "flying tyre" accidents.

[otolith]

FiF said:

otolith said:

That claim appears to come originally from a 2006 Transport Research Laboratory report, by the way. Can't get my hands on the report to see where it got it from.

If you are referring to 2006 report PPR086 I have a pdf of the report and nowhere does that refer to any increase in wheel velocity.

Obliquely they reference as one of the data sources as the HVCIS fatal database which is an analysis of the STATS19 form completed by police collision investigators, there may be something in there.

Equally the bibliography references another TEL report from 2006 PPR085, but that's just a rdgulatory impact assessment so possibly not from there. I don't have a copy of that.

Other references include collision investigations from around the world, which would be a large task to survey, but seeing as they don't make the claim in the report.

Suspect we have to trawl back through old IRTE stuff, thrown all mine away in a clear out.

Hmm, I'd love to know where the numbers come from, and how.

[Kawasicki]

otolith said:

This is interesting - a wheel comes off a lorry. Bounces, tumbling, then it would appear that it hits the ground at the right angle for the spinning tyre to grip and propel it spearing off violently to the side.

https://www.newsflare.com/video/69495/crime-accide...

If you can imagine that happening in such a way that it is propelled even faster in the direction it was already going.

In this case, the tyre looks to be going a lot faster than the traffic on the other carriageway?

https://www.youtube.com/watch?v=WOtb3MvK-a0

I think these are basically freak "flying tyre" accidents.

Nothing unusual there, nothing freaky either.

[Catatafish]

It's not that complicated. There are at least 2 forms of stored energy: rotational energy of the wheel, kinetic energy of its forward motion, and the compression from suspending the vehicle before detachment. Certain circumstances could see the wheel ending up moving faster than the vehicle.

The wheel running along side the racecar dissipates the wheels rotational energy and its kinetic energy in a gradual way as it rolls along the track. If it struck a fixed, hard object, the tyre might partially or totally bite against the fixed object, which could liberate some of its rotational energy very quickly. This rapid conversion of its stored rotational energy could give it an impulse that might accerate it in addition to its rebound from its kinetic energy.

[SpeckledJim]

Catatafish said:

It's not that complicated. There are at least 2 forms of stored energy: rotational energy of the wheel, kinetic energy of its forward motion, and the compression from suspending the vehicle before detachment. Certain circumstances could see the wheel ending up moving faster than the vehicle.

The wheel running along side the racecar dissipates the wheels rotational energy and its kinetic energy in a gradual way as it rolls along the track. If it struck a fixed, hard object, the tyre might partially or totally bite against the fixed object, which could liberate some of its rotational energy very quickly. This rapid conversion of its stored rotational energy could give it an impulse that might accerate it in addition to its rebound from its kinetic energy.

Genuinely confused here.

If it's travelling at 70mph, and rotating at a speed commensurate to 70mph, what fixed object could it partially or totally bite against that would get it to 71mph?

[otolith]

A pothole, a bump in the road, a barrier.

[FarmyardPants]

Nanook said:

As the wheel detaches from the car though, the inertia of this particular rotating component will decrease, as its no longer attached to the rotating hub, shaft, brake disc etc.

Like the ice skater, the energy is constant, the inertia decreases, so the rotational velocity increases.

The first paragraph is wrong. The wheel has rotational inertia (Iw) when it is attached to the hub. The hub, brakes etc also have inertia (Ih) and the combined assembly has Iw+Ih, but that doesn’t mean that the wheel at the point of detachment suddenly has <Iw. They become two separate systems instead of one.

The driven wheels are constantly under rotational load in the sense that they are overcoming aero drag to keep the vehicle moving at a constant speed, and at the time immediately before the wheel comes off the diff will likely give the loose wheel an extra kick (wheel spin) because the loose wheel will be easier to rotate than the other wheel, as I said above.

[Mr2Mike]

FarmyardPants said:

The driven wheels are constantly under rotational load in the sense that they are overcoming aero drag to keep the vehicle moving at a constant speed, and at the time immediately before the wheel comes off the diff will likely give the loose wheel an extra kick (wheel spin) because the loose wheel will be easier to rotate than the other wheel, as I said above.

I don't think that scenario would occur under normal driving conditions, e.g. cruising down a motorway. However if you were wheel spinning furiously when it detached it could certainly gain a decent amount of speed.

[otolith]

Any contact which causes the wheel’s rotation to slow down is likely to accelerate it longitudinally.

[FarmyardPants]

Mr2Mike said:

FarmyardPants said:

The driven wheels are constantly under rotational load in the sense that they are overcoming aero drag to keep the vehicle moving at a constant speed, and at the time immediately before the wheel comes off the diff will likely give the loose wheel an extra kick (wheel spin) because the loose wheel will be easier to rotate than the other wheel, as I said above.

I don't think that scenario would occur under normal driving conditions, e.g. cruising down a motorway. However if you were wheel spinning furiously when it detached it could certainly gain a decent amount of speed.

When cruising down the motorway you are constantly powering the wheels to maintain your speed. They are pushing the car to move the air aside. It’s not just purely friction and pumping losses.

[Kawasicki]

otolith said:

Any contact which causes the wheel’s rotation to slow down is likely to accelerate it longitudinally.

I was driving on a track, with a trailer. One of the trailer wheels fell off, and overtook me as I braked for a corner...it rolled past and smacked into the Armco, which caused its rotation to slow. It didn’t accelerate, it stopped.

[otolith]

Was there anywhere for it to go?

[FiF]

I can understand the argument from otolith, it requires an impact which rapidly slows the wheel rotation but without applying a restriction to further lateral movement, as say an Armco barrier would restrict it. So perhaps a kerb which would also help to launch it into the air.

I'd like to see the numbers.

[otolith]

Yep, I think it needs a particular set of circumstances. I think you could probably set up a test rig to do it reproducibly. I think it can only happen in a flying/bouncing scenario, because it will no longer be rotating fast enough to roll at the speed it’s travelling, so it needs to be a bump large enough to launch it off the ground and bluff enough for the tyre to bite against it, but not so large that it stops it dead.

[otolith]

Kawasicki said:

Nothing unusual there, nothing freaky either.

Would you say that it’s usual for a bouncing object to bounce higher each time it hits the ground? The transfer of energy from rotation to translation is very obvious there.