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Discussion Starter · #1 ·
How about this one? Just been sent it from another (m/cycle) forum where the article mentioned was included in the latest MAG magazine.
Apparently the defendant was a past lecturer to the military & governments on electromagnetic equipment.

"The writer tells that he recently persuaded a court that he had not been riding at 100mph, as alleged by a police officer using a hand held radar gun.
He argued with considerable experience that the officer had recorded the speed of the upper part of his rear tyre. This apparently moves at twice the speed of the bike across the ground.

After much deliberation the court agreed...

Good one... "
 

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It is of course bull..., the very top section of the tyre is but the part that will reflecting the radar isn't.:D
 

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Well done... a speeder and liar gets let off.

I'm a biker and a keen driver but it pees me off when people think its good that they have flouted the law.
 

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Many years ago my mate was pulled over for speeding.
He was subsequently up in court for speeding at 100mph :eek:

He took his punishment like a man, paid the fine, no argument.

He then told me that he was actually doing 140mph when pulled over !


Sounds like those speed guns are pretty inaccurate to me.



I guess I'll never find out for sure, as I'm a dawdler :D



TURK
 

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noo noo, probably, in theory
when it's touching the ground the point on the tyre isn't moving forward at all (or it would skid), so at the top possibly needs to be 2x to average out as any point obviously moves forward = to what the bike does

major spurious argument in practice though - but nothing new there for our la la land court system ..................................
 

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Discussion Starter · #8 ·
Many years ago my mate was pulled over for speeding.
He was subsequently up in court for speeding at 100mph :eek:

He took his punishment like a man, paid the fine, no argument.

He then told me that he was actually doing 140mph when pulled over !


Sounds like those speed guns are pretty inaccurate to me.



I guess I'll never find out for sure, as I'm a dawdler :D



TURK
As is usual, his speedo was over reading, big time. I had a friend who moaned at being booked for doing 80mph (clocked) in a 70 zone. We had similar bikes so I asked him how come he didn't notice his speedo reading 90+ mph.
 

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Well done... a speeder and liar gets let off.

I'm a biker and a keen driver but it pees me off when people think its good that they have flouted the law.
LOL! Remember that comment when you next break the speed limit (if you ever do of course) - just hand yourself in and confess otherwise you will not be able to live with yourself LOL:
 

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My bike's speedo reads 10% over they designed the bike then changed the sprocket size....
So working on that theory, when abroad I obey (ish) the speed limits :D 50 in a 50 90 in a 90 and 130 in a 130 mph limit
 

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noo noo, probably, in theory
when it's touching the ground the point on the tyre isn't moving forward at all (or it would skid), so at the top possibly needs to be 2x to average out as any point obviously moves forward = to what the bike does

major spurious argument in practice though - but nothing new there for our la la land court system ..................................
If you want to get into the physics then remember that speed is relative, the whole tyre is travelling at 50mph and is also rotating. The bottom isn't stationary and the top isn't traveling at 100mph - if it were the tyre would be in 2 pieces. From the perspective of the radar gun the situation would appear as you have described.

Anyway, the radar gun would have bounced off the rear face of the tyre and not the top, which is travelling at 50 mph wrt to the gun (if the rider is to be believed). The top of the tyre may well give the impression of travelling at 2x speed but the radar has only a very small aspect on this portion of the tyre, and the returning signal would be dispersed as it's is not at a flat angle to the gun. The radar signal returning from the flat of the rear wheel and the rest of the bike would swamp out any spurious readings.
If the device used was a laser then there is slightly more scope for this but still highly unlikely.

Unless I see document proof of this story I'm still calling BS on it. It's just a day dream....:D
 

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noo noo, probably, in theory
when it's touching the ground the point on the tyre isn't moving forward at all (or it would skid), so at the top possibly needs to be 2x to average out as any point obviously moves forward = to what the bike does

major spurious argument in practice though - but nothing new there for our la la land court system ..................................
That sounds way off bud. All points of the wheel need to moving at the same rate or the wheel will deflect and distort.

I'm sure in my mind that this is about the angle you view it from but how do the speed guns work?
 

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LOL! Remember that comment when you next break the speed limit (if you ever do of course) - just hand yourself in and confess otherwise you will not be able to live with yourself LOL:
Actually mate, i don't break the speed limit... It's there for a reason, the van doesn't like 70mph, and small 250-400cc motorcycles provide much bigger thrills than their larger counterparts at much more common speeds.

I tend to think along the lines of 'what if someone DID step out?'
 

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Hang on hang on I think I've sussed his argument and it's a bit lame in my book. Cumbriaman maybe heading in the right direction.

Looking side on as this makes it easier to explain.

The WHOLE wheel is moving forward at say 50mph

It is of course rotating. Now as it rotates the bottom of the TYRE move as backwards while the top moves forwards.

Therefore it could be argued that one a single point on the tyre when at the top is moving forward at a speed of 50mph PLUS the speed of rotation. Similarly the bottom can be considered to be moving at 50mph MINUS the speed of rotation.

Now looking directly at the wheel as if it were coming towards you the whole wheel is travelling at but a point on the tyre is also travelling towards you at the speed of rotation. So I'm guessing the guys argument revolves around the speed of rotation of the tyre moving towards you.

Rubbish if they swallowed it.
 

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So I'm guessing the guys argument revolves around the speed of rotation of the tyre moving towards you.
Yes, but the only portion of his tyre that is travelling at 50mph+50mph is the very top section, this isn't visible when viewed from behind, as would be the case if being radar speed checked.
The section of tyre the radar gun would "see" is traveling forward at 50 mph and upwards at 50 mph. The upward motion would not be measured by the radar as it is measuring the time delay for the signal to bounce off the tyre and return, it cannot register rotational speed, and I don't think any court would swallow that so I'm calling BS on the whole story.

I think it's just another BS urban legend...I:
 

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Many years ago my mate was pulled over for speeding.
He was subsequently up in court for speeding at 100mph :eek:

He took his punishment like a man, paid the fine, no argument.

He then told me that he was actually doing 140mph when pulled over !


Sounds like those speed guns are pretty inaccurate to me.



I guess I'll never find out for sure, as I'm a dawdler :D



TURK
most, not all police will drop the speed. anything over 100mph is an instant ban ,with the threat of jail time too
 

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Discussion Starter · #19 ·
Originally Posted by DubDick View Post
I think it's just another BS urban legend...


I'm with you but worth 5 minutes to think of the physics of it.

OK, when I can I'll get the article from the MAG (Motorcycle Action Group)magazine where it was a reported feature (i.e. not in the letters section. ;) )
 

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DubDick - tyre speed: it's the forward speed that he claims is being measured. the road isn't moving so neither is the point of the tyre touching it. think of that fairground ride with seats rotating on arms that are fixed to a central one that also rotates, you are repeatedly still & then shoot across.
imagine a tyre being spun around a stick attached to the outer surface, where attached it's not moving, the rest is but it wouldn't break up.

agree re the gun, maybe in the case of a motorbike the court decided that it's not certain exactly what it is measuring in practice so can't be proven
 
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