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On Mon, 21 Jun 2004 07:08:11 GMT,
Orienteer wrote: Bit of a myth that ABS enables a vehicle to stop quicker, in fact it can have the opposite effect. It's purpose is to enable the vehicle to be steered while braking hard, which without ABS often results in a skid and loss of control. I don't have any figures for it but I suspect that when braking hard from high speeds (70mph+) ABS may well enable a car to stop quicker. Some (10+?) years ago there was an artical in SciAm about emergency stops in cars at motorway speeds and it was suggested that the best bet for the cars of the time might well be to deliberately skid. IIRC stopping distances from these sorts of speeds when skidding were about 20% further than the perfect stop. However, without ABS the braking is split in a fixed percentage between back and front wheels. The weight transfer to the front wheels can cause the rear wheels to lock putting the car into a spin. By deliberately locking all the wheels the car will stay pretty much in a straight line (motorways don't tend to have enough camber to be likely to put a skidding car into a spin.) ABS eliminates this problem and allows maximum braking on the front wheels. But ABS doesn't have to be a good thing. The one time I have skidded on the motorway I was very grateful for the noise. Picture the scene - me on empty motorway, slip lane joining. Slow lorry almost at end of slip lane that would be joining shortly after I had passed. Another car on sliplane that would be joining about the same time as the lorry. So I moved from lane 1 to lane 3 in order to give both vehicles joining room to join without having to adjust their speeds. But the car doesn't move into lane 2 to pass the lorry but continues into lane 3. Now I should have anticipated this but by the time I realised he wasn't going to stay in lane 2 I was about level with his rear door and going maybe 10mph faster. I didn't have time for the horn but my squealing tyres meant he only came about 2 feet into lane 3. (Skidding from 70mph to about 45mph leaves a big cloud of smoke!) Tim. -- God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t," and there was light. http://tjw.hn.org/ http://www.locofungus.btinternet.co.uk/ |
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