LTI 20/20 thwarted again.
Discussion
Rubber-Ducky said:
Actually, the contact patch MUST be moving relative to the road in order to provide the required drive. That being the case, the part of the tyre in contact with the road will actually have a negative velocity (be travelling backwards). Additionally, the distance from the axle to the contact patch will be less than the distance from the axle to the top of the tyre thus the top of the tyre will be travelling at slightly more than double the bike's velocity.
Optimum tyre grip occurs at approximately 10% slippage (in the dry) - if tyres didn't slip on the road they simply wouldn't provide any grip. On the plus side, they wouldn't wear out, either!
Thats bOptimum tyre grip occurs at approximately 10% slippage (in the dry) - if tyres didn't slip on the road they simply wouldn't provide any grip. On the plus side, they wouldn't wear out, either!
ks, tyres don't grip by slipping 10%. They grip due to reaction between the two surfaces. If a tyre slips on the road its skidding, not gripping!I can give you the url for an enginneering forum if you would like a couple of tyre engineers to back that up.
98elise said:
Thats bks, tyres don't grip by slipping 10%. They grip due to reaction between the two surfaces. If a tyre slips on the road its skidding, not gripping!
Isnt it due to the tyre surface being rubbery so ahead of the contact patch its stretched, while behind its compressed (or is it the other way around)? The strecthing/compressing of the tyre gives a slippage factor under acceleration/braking even though the tyre surface isnt itself slipping?ks, tyres don't grip by slipping 10%. They grip due to reaction between the two surfaces. If a tyre slips on the road its skidding, not gripping!but the tyre distortions are not relevant to the point. The instantaneous centre of revolution should strictly be applied to a solid wheel.
However the point was that the bottom of the wheel is essentially stationary and the top of the wheel is moving at twice (bike) road speed (approximately).
Of course this is OT as it's not the reason that the LTI20-20 got the speed wrong (in the opinion of the judiciary).
Bert
However the point was that the bottom of the wheel is essentially stationary and the top of the wheel is moving at twice (bike) road speed (approximately).
Of course this is OT as it's not the reason that the LTI20-20 got the speed wrong (in the opinion of the judiciary).
Bert
BertBert said:
but the tyre distortions are not relevant to the point. The instantaneous centre of revolution should strictly be applied to a solid wheel.
However the point was that the bottom of the wheel is essentially stationary and the top of the wheel is moving at twice (bike) road speed (approximately).
Of course this is OT as it's not the reason that the LTI20-20 got the speed wrong (in the opinion of the judiciary).
Bert
However the point was that the bottom of the wheel is essentially stationary and the top of the wheel is moving at twice (bike) road speed (approximately).
Of course this is OT as it's not the reason that the LTI20-20 got the speed wrong (in the opinion of the judiciary).
Bert
BrianMillar said:
I resubmit my theory
Personally I agree, and another thing. There is video with supposedly an accurate clock / timestamp on each frame. All it would take is an accurate site survey taking the distance shown between camera and vehicle and check if that corresponds to determine if the vehicle in frame is the one being measured.BrianMillar said:
From the photo the distance from the camera to the bike and the car could be worked out.
Whichever vehicle is 178m away is the one that has been targeted by the laser.
So it should have been easy to prove or disprove if the bike was targeted.
Whichever vehicle is 178m away is the one that has been targeted by the laser.
So it should have been easy to prove or disprove if the bike was targeted.
Also measure the distance between two identifiable points along the vehicle path and determine the time interval from the video frames. After that it is simple mathematics.
Has anybody ever done that?
BrianMillar said:
I resubmit my theory
Usually we get a set of 3 timed photos to BrianMillar said:
From the photo the distance from the camera to the bike and the car could be worked out.
Whichever vehicle is 178m away is the one that has been targeted by the laser.
So it should have been easy to prove or disprove if the bike was targeted.
Whichever vehicle is 178m away is the one that has been targeted by the laser.
So it should have been easy to prove or disprove if the bike was targeted.
saaby93 said:
Usually we get a set of 3 timed photos to guess calculate the speed
Do you mean three stills taken from the video? Useless if you wish to contest the evidence. You need the FULL video, so that the alignment checks can be seen.Mark Lindsay said:
I believe there are 3 things that need to be in alignment - the red dot sighting, the laser and the video camera. (The red dot sighting has to run parallel with the laser and is fixed 5 cms above it. There is no adjustment. This is just a check that they are in alignment. This could get out of alignment by rough treatment, or a component failing. In this case this was done in a very slap-dash manner.)
Having checked this the operator needs to align the video cross-hairs with the red dot. This should be done at approximately the [same] distance the device is to be used, or further. There are adjustment screws to carry this out. This is to iron out the parallax error between the laser (LTi 20-20) and the video camera (Concept DVD) which are mounted next to each other. In this case there was no evidence that this was carried out at all, and, in my opinion, the operator didn't appear to realise the importance of it.
So the LTIs accuracy appears to have been thwarted by a clown operating it without an understanding of how it worked! Having checked this the operator needs to align the video cross-hairs with the red dot. This should be done at approximately the [same] distance the device is to be used, or further. There are adjustment screws to carry this out. This is to iron out the parallax error between the laser (LTi 20-20) and the video camera (Concept DVD) which are mounted next to each other. In this case there was no evidence that this was carried out at all, and, in my opinion, the operator didn't appear to realise the importance of it.
Clive Walsh, the operator would do worse than to read here! .
.F i F said:
BrianMillar said:
I resubmit my theory
Personally I agree, and another thing. There is video with supposedly an accurate clock / timestamp on each frame. All it would take is an accurate site survey taking the distance shown between camera and vehicle and check if that corresponds to determine if the vehicle in frame is the one being measured.BrianMillar said:
From the photo the distance from the camera to the bike and the car could be worked out.
Whichever vehicle is 178m away is the one that has been targeted by the laser.
So it should have been easy to prove or disprove if the bike was targeted.
Whichever vehicle is 178m away is the one that has been targeted by the laser.
So it should have been easy to prove or disprove if the bike was targeted.
Also measure the distance between two identifiable points along the vehicle path and determine the time interval from the video frames. After that it is simple mathematics.
Has anybody ever done that?
IF the alignment was out, I would think that viewing the video would sooner or later show discrepancies between the behaviour of the targeted vehicle, and the readings recorded above.
No visit or survey required.
Certainly the magistrates were happy having seen the video, and heard the testimony of the operator that there was more than reasonable doubt as to whether Mark Lindsay was speeding.
F i F said:
Also measure the distance between two identifiable points along the vehicle path and determine the time interval from the video frames. After that it is simple mathematics.
Has anybody ever done that?
That's what I meant - seems to workHas anybody ever done that?
Often there are 3 photos so you can do 3 checks
Mill Wheel said:
If Puff the Tragic was on the case, he probably plotted every blade of grass - even if it was two or three years after the event!
IF the alignment was out, I would think that viewing the video would sooner or later show discrepancies between the behaviour of the targeted vehicle, and the readings recorded above.
No visit or survey required.
Certainly the magistrates were happy having seen the video, and heard the testimony of the operator that there was more than reasonable doubt as to whether Mark Lindsay was speeding.
My point is a more general one, I think relying on three photos per saaby is not enough. It places a reliance too much on the measurement and there being an identifiable spot.IF the alignment was out, I would think that viewing the video would sooner or later show discrepancies between the behaviour of the targeted vehicle, and the readings recorded above.
No visit or survey required.
Certainly the magistrates were happy having seen the video, and heard the testimony of the operator that there was more than reasonable doubt as to whether Mark Lindsay was speeding.
If you know the location of the camera, even without distinguishing marks on the road it would be possible to traingulate position of vehicle at various points by vehicle position in relation to background, road furniture etc. Thus determine several positions and make accurate measurements on the ground.
It may prove the reading is accurate or not, but it's worth doing if faced with prosecution for a speed where you know that is wrong.
F i F said:
My point is a more general one, I think relying on three photos per saaby is not enough. It places a reliance too much on the measurement and there being an identifiable spot.
If you know the location of the camera, even without distinguishing marks on the road it would be possible to traingulate position of vehicle at various points by vehicle position in relation to background, road furniture etc. Thus determine several positions and make accurate measurements on the ground.
It may prove the reading is accurate or not, but it's worth doing if faced with prosecution for a speed where you know that is wrong.
I like your thinking If you know the location of the camera, even without distinguishing marks on the road it would be possible to traingulate position of vehicle at various points by vehicle position in relation to background, road furniture etc. Thus determine several positions and make accurate measurements on the ground.
It may prove the reading is accurate or not, but it's worth doing if faced with prosecution for a speed where you know that is wrong.
However all it shows is that at 179 metres there was something travelling at 63mph - which may have been one of the spokes.
With two photos ( one at 179 metres) you can work out the approx speed of the bike.
With the extra photo you can do some checking.
saaby93 said:
F i F said:
My point is a more general one, I think relying on three photos per saaby is not enough. It places a reliance too much on the measurement and there being an identifiable spot.
If you know the location of the camera, even without distinguishing marks on the road it would be possible to traingulate position of vehicle at various points by vehicle position in relation to background, road furniture etc. Thus determine several positions and make accurate measurements on the ground.
It may prove the reading is accurate or not, but it's worth doing if faced with prosecution for a speed where you know that is wrong.
I like your thinking If you know the location of the camera, even without distinguishing marks on the road it would be possible to traingulate position of vehicle at various points by vehicle position in relation to background, road furniture etc. Thus determine several positions and make accurate measurements on the ground.
It may prove the reading is accurate or not, but it's worth doing if faced with prosecution for a speed where you know that is wrong.
However all it shows is that at 179 metres there was something travelling at 63mph - which may have been one of the spokes.
With two photos ( one at 179 metres) you can work out the approx speed of the bike.
With the extra photo you can do some checking.
F i F said:
True, though what I am trying to do is draw virtual Gatso markings on the ground and take accurate measurementsbetween them, and at the same time verifying the 179 metres, or not.
As soon as you have your 190 metres the timed photos can be used to calculate the speed without any use of references on the groundsaaby93 said:
There could be more to it than that
If the bike is travelling at 50mph the tyre in contact with the road is at 0mph while the tyre top is going away from the camera at 100mph.
At some point in line with the cross hairs the spokes are travelling at 65mph away from the camera.
This effect doesn't apply to LTI2020 Lidar measurements. All subsequent arguments are voided too.If the bike is travelling at 50mph the tyre in contact with the road is at 0mph while the tyre top is going away from the camera at 100mph.
At some point in line with the cross hairs the spokes are travelling at 65mph away from the camera.
(however, such errors are possible with radar equipment)
The LTI2020 doesn't measure speed. The closest it gets is deducing the rate of change of distance to the surface the laser bounced back from over the individual samples (and even that is a grey area).
E.G: measuring the speed of a tyre spinning around at 100mph, on a static bike (say on a stand), will yield a displayed 0mph (assuming enough reflectivity, and no operator slip).
Directing the beam to the spokes of said wheel would likely result with jitter (random variance) of deduced distance, so the error trapping will usually void such readings (E03; no speed displayed). Even if not, the limited radius of the spokes (let's say 30cm) would yield a maximum error of 0.3*2 /0.39 * 2.237 = +/-3mph. However, as 14-7 rightly said, the spokes aren't visible in this case, so this error cannot apply in this case anyway.
I've experimented with various LTIs.
Edited by smeggy on Wednesday 4th May 12:59
F i F said:
Personally I agree, and another thing. There is video with supposedly an accurate clock / timestamp on each frame. All it would take is an accurate site survey taking the distance shown between camera and vehicle and check if that corresponds to determine if the vehicle in frame is the one being measured.
Also measure the distance between two identifiable points along the vehicle path and determine the time interval from the video frames. After that it is simple mathematics.
Has anybody ever done that?
Yes, but I wasn't universally liked for doing so: Biker Claims Only Doing 90 and not 150mphAlso measure the distance between two identifiable points along the vehicle path and determine the time interval from the video frames. After that it is simple mathematics.
Has anybody ever done that?
I don't have access to the necessary data to do it again for this particular case.
saaby93 said:
smeggy said:
the limited radius of the spokes (let's say 30cm) would yield a maximum error of 0.3*2 /0.39 * 2.237 = +/-3mph.
can you break down that calc?0.3 = radius (m) of the wheel (not the tyre. On reflection, this was a big overestimate; 0.2 would have been better)
x 2 = to get the diameter (width). From the LTI's POV, this is the greatest change of distance that it could detect for the spokes in this scenario
/ 0.39 = LTI measurement period, in seconds (although I did accidentally recalled the details for the Ultralyte because I was rushed. A TSM is 0.34)
x 2.237 = conversion from m/s to mph (3600/1609.3)
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