Screwloose,
Thank you for the information.
Skid-Marks of around 25m in length were recorded by the traffic patrol officer. The police driver's statement did not mention anything about the ABS system - although I am becoming more and more convinced that the locking of the wheels had something to do with the ABS system or even the brake discs. He DID however say that he had heard from people before that the 'braking system' of the van was questionable. In a statement from his inspector, the following was deduced:
'the vehicle was intended as a goods carrying vehicle and hence the braking is for a load carrying vehicle. Consequently, if there is no load on the vehicle (at the time there WAS NOT) when the brakes are applied, the back end is very light and the brakes tend to lock up' - This adheres with what you have said.
The case was reported to the procurator fiscal, who in turn have charged the police driver with 'driving without due care and attention' because of one of the passengers in the receiving end of the car crash suffering an injury. Hence the PC's solicitor hired an engineering consultancy to act as an expert witness for the case to try and shift the blame from the PC onto the vehicle. This was an actual case two years ago but is being used to teach 5th year Mechanical engineering students consultancy. Although I am fundamentally a mechanical engineering student, I specialise in aero classes, and hence my knowledge of automobiles is limited! Every ounce of help is useful - and I am currently on a very steep learning curve to sharpen up my auto knowledge.
Any further comments?
Ps. Can you see anything out of the ordinary looking at the inspection pictures?
Many thanks.
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>Ps. Can you see anything out of the ordinary looking at the inspection pictures?
One of the rear tyres looks to still have mounting paste in he rim/bead area. It is possible that this tyre is very new, and may have still been coated in release agent, which wouldn't help adhesion.
If the ABS were working correctly, would the skid marks be continuous, or broken as the brakes are momentarily released?
I would argue that if the tyre were new, a professional driver would be aware of this, and make allowance.
I would further argue that a profesional driver will familiarise himself with the response of a vehicle in a safe environment prior to using the vehicle live. i.e. for a police driver, these excuses aren't really acceptable.
Number_Cruncher
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If the ABS were working correctly, would the skid marks be continuous, or broken as the brakes are momentarily released?
If the ABS was working properly there would be no skid marks because the wheel would not be locked.
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If the ABS was working properly there would be no skid marks because the wheel would not be locked.
Is this really true about leaving no marks?
I understood that ABS continuously cycles around the point where the tyre produces the most braking force. On a dry road, this would be about 20% slip, and, although I've never checked this, I would expect to see some marks left by such slippage.
Number_Cruncher
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>> >> If the ABS was working properly there would be no skid >> marks because the wheel would not be locked. >> Is this really true about leaving no marks? I understood that ABS continuously cycles around the point where the tyre produces the most braking force. On a dry road, this would be about 20% slip, and, although I've never checked this, I would expect to see some marks left by such slippage. Number_Cruncher
My post a couple below this explains the principle (taking wheel speed signal and using the second derivative to assess rate of deceleration of wheel). It is up to the ABS designers to set a threshold for pressure 'hold' and 'release' points - but you are correct it that on a dry road there will be some slip between the tyre and the road. This is a bit different to locking though and although you might see some rubber dust on the road you wouldn't see black marking.
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My post a couple below this explains the principle (taking wheel speed signal and using the second derivative to assess rate of deceleration of wheel). It is up to the ABS designers to set a threshold for pressure 'hold' and 'release' points - but you are correct it that on a dry road there will be some slip between the tyre and the road. This is a bit different to locking though and although you might see some rubber dust on the road you wouldn't see black marking.
I am (reasonably*) au fait about the way that ABS uses the angular acceleration to form a basis for its decisions, but am not sure what type of marks or evidence would be left at the scene under different types of skid or stop.
There must be wheel slip for a tyre to generate *any* force, but of course that is not locking, or 100% slip. If I were looking into this event, I would try to find out at what % slip do tyres begin to lay down measurable or observable amounts of rubber?
OK, if there are 25m long solid thick black skid marks, then, it is quite likely that the ABS didn't stop the skid, and may not have been working properly, or indeed designed correctly.
Number_Cruncher
* I say reasonably because my only involvement, beyond standard workshop repairs, has been only to the extent of helping a colleague develop a computer simulation of an ABS system for a midlands car manufacturer. I don't have test rig, or test track experience.
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Thanks for the help everyone,
To Number_Cruncher,
'On a dry road, this would be about 20% slip, and, although I've never checked this, I would expect to see some marks left by such slippage.'
True - although 25m of proper skid-marks were noted, which really do suggest a slippage of more than 20%?
To everyone:
Further information (which may not be relevant): The van did hit a triangular island kerb (separating the incoming from oncoming lanes at the roundabout entry/exit road) shortly before colliding into the car - and hence the van was found with an extensively misaligned offside wheel (possibly due to the collision of the kerb - which was on the offside of the van) and also the longer of the two skid-marks was the offside wheel's mark. They also noticed a slight change in direction of the skid-marks after being straight for the most part - this is due to the fact that the PC lifted the brakes and re-applied them in order to try and steer the van (hence the slight change in direction of the skid-marks).
My question to everyone would be now; if the PC lifted his foot off the brakes after realising that the wheels locked and going into a straight skid, and then re-applied the brakes again - resulting in yet more skid-marks but obviously not in a straight direction due to his attempt to steer the van - then would that not suggest that since he technically applied the brakes twice, both of which resulted in skid-marks, then the wheels of the van must have locked both times and hence the ABS was not operational at both times of the incident?
aeromecheng
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aeromecheng
Ahh; now things are making more sense. The whole crux of this issue is those skid marks. Their very existance means that it can be positively extrapolated that the ABS wasn't working - at the time of the incident. [But nothing more than that.]
Obviously; an investigator would then ask for a report of what the ABS [and air-bag/SRS] ECUs recorded. [Some systems now contain crash recorders that burn-in the data if an airbag fires.] The presence of ABS fault-codes and the record of the vehicle's actual mileage travelled with the ABS warning lamp lit [or even removed!] could be very enlightening.
The pre-crash reports of the vehicle's wheels locking would indicate prior supervisory knowledge that the ABS on the vehicle was faulty. This [in itself a crime] could have been used to divert the legal case on to why this matter had not been rectified by the person responsible and might have been crucial to the driver's defence.
The fact that the back end was light and that the brakes were specified for a heavier load is irrelevant. The brakes of any vehicle with functional ABS do not - ever - lock. They may well activate the ABS at the lightest touch - particularly on slippery surfaces - but the leaving of skid marks means that ABS wasn't functioning: end of story.
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Another eyebrow raiser is that all the Vito Vans were retracted from police patrol duty shortly after being put into service! (from a statement)
Also, the vans were intended as police patrol, maybe not the ideal vehicle for car pursuits?
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I have worked on ABS design for a well-known company. The ABS should stop the rear wheels locking even if the rear is lightly loaded. Basically the ABS ECU monitors rate of wheel speed deceleration (second derivative of wheel speed signal) and if the rate of deceleration exceeds a pre-set threshold then braking pressure to the wheel in question is "held". If wheel continues to excessively decelerate then pressure is "reduced" (ABS pump operates) to ensure that the wheel does not lock. The system is designed to operate even where the braking effort required for locking is very low (braking on wet ice, for example).
The system may not function correctly for a variety of reasons, including:
1. Electronic fault
2. Hydraulic fault (sticking solenoids for example)
3. Mechanical fault (pads binding in their carriers meaning that they don't release when pressure is reduced).
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The ABS should stop the rear wheels locking even if the rear is lightly loaded.>>
Due to the fact that the system is designed to function in a high load situation where more weight is over the rear wheels than the front it may be that the ABS is not able to release enough pressure to stop the rears locking in a situation where the vehicle is very light and the front brakes are causing effective deacceleration and therefore weight transferance towards the front.
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Due to the fact that the system is designed to function in a high load situation where more weight is over the rear wheels than the front it may be that the ABS is not able to release enough pressure to stop the rears locking in a situation where the vehicle is very light and the front brakes are causing effective deacceleration and therefore weight transferance towards the front.
Load doesn't really matter. The ABS can completely release the braking pressure to the rear wheels. About 15 years ago I worked on an ABS system that was to be used on fire engines (for a well-known company near Leamington!). Because of the fact that can carry water the change is load is very great - the system was still able to cope.
Obviously if the vehicle is being braked hard then there are stability issues if it is not absolutely straight and you can experience yawing forces. That's why some modern ABS systems incorporate yaw sensors.
The most difficult braking condition is the so-called split-u (split-"mu") surface. MIRA, for example, have a split-mu test surface consisting of a strip of high-grip tarmac next to a strip of epoxy. The epoxy is water-drenched. Drive on that and hit the brakes without ABS and you'll do a few 360-deg turns! A good ABS will keep you more or less straight.
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Also, the vans were intended as police patrol, maybe not the ideal vehicle for car pursuits?
Was a van really being used in a pursuit (I noted the earlier comment about damage due to hitting a kerb)? If so, I find it incredible that this is 'allowed'. Surely the Police are failing in their Duty of Care responsibily by allowing a pursuit in an unsuitable vehicle.
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