As regards to the MOT testing, its only any good at the time of the test not sometime after.
|
|
|
Don't forget ABS lengthens stopping distance (slightly) but because you can steer helps avoid accidents.
Maximum breaking force only available when wheels locked and your are skidding - max friction. But you cannot steer. And skidding when it's wet means lengthy stopping times and why you need to significantly increase distance between cars.
|
Sorry rtj70, but that's wrong, at least in part.
Maximum friction is the point right before a slide starts. The coefficient of static friction is always higher than the coefficient of sliding friction. Microscopic variations in the two surfaces lock into each other when there is no relative movement, providing more grip. That's why older trains used to have play in the couplings - so that each carriage was jerked forward, breaking the 'stiction', and that's why air suspension systems suffer at low speeds. Stiction is a difficult engineering problem to overcome.
Whether a car with ABS takes longer to stop than one without I don't know. Perhaps, but I would think only because the system is set overly cautiously, but I'd be surprised. If a car has 'dynamic brake distribution thingy', or whatever it's called, I'd expect it to stop more quickly, as I've not yet seen a car with four brake pedals. Or a driver with four feet. Although it would be hte logical progression from heel and toeing.
Gord.
|
Gordon,
Tyre to road friction is a bit more complex than that.
On a dry road, you get maximum longitudinal force when the tyre is slipping by about 20% - i.e. you have gone way beyond static friction before you get to the friction peak. On a wet road, this percentage is lower, but not zero.
Part of the reason behind this is that the rubber conforms to the road surface, and doesn't behave in a classical solid friction manner. For example, F = mu x N doesn't really apply, because for rubber, there *is* an area dependance.
The difficulty faced by ABS is that it can't tell the true speed of the vehicle (because all systems rely on measuring wheel speed, which, depending on slip conditions can be less than accurate!)
So, the ABS has to attack the problem pragmatically, by cycling the brake pressure up until it senses a rapid deceleration of the wheel, and then releasing until it senses the wheel is rotating again. By doing this cycle rapidly, the wheel passes through the region of maximum friction once per cycle. i.e., the ABS homes in on the maximum friction area of the slip curve, but can't actually position itself there.
If an ABS system is in cycling mode on all four wheels, then you are getting the equivalent of the dynamic brake distribution, via the iterative technique of the ABS controller (assuming the ABS controller does control all 4 wheels independantly, and isn't one of the older "select low" systems).
Among the problems with ABS is that people tend not to prod the brake pedal hard enough, and get worried when the ABS starts cycling, and let off.
Incidentally, with rail wheels, you are unlikely to see a friction peak,the characteristic is much more insidious, which is one of the (many!) reasons why wheel slip protection is difficult to implement on trains, and that leaves on the line are still problematic.
Number_Cruncher
|
|
All very informative, but I still think we are at the point of identifying if the car has got abs or not and if so was the light indicating it was ok or faulty.
|
yawn-----------------
www.highwaycode.gov.uk/09.htm
think this answers any question to be honest----------
ive heard it all before -----------------
|
Thanks for all the replies. I'm not claiming that it wasn't my girlfriend's fault - it was.
That said, I was under the impression that "for a majority of drivers, in most conditions, in typical states of alertness, ABS will reduce their chances of crashing, and/or the severity of impact." (en.wikipedia.org/wiki/Anti-lock_brakes ).
Therefore, if my car has got ABS, I think a bit of investigation can't hurt.
|
I don't think it has ABS mate.
|
|
|
|
Dont believe everything you read in wikipidia - half of it is tosh. Ive seen people alter it then quote it in arguments.
|
The work was on the rear of the car - and it's the front brakes that contribute the (vast) majority of the braking effort, and we can conclude that the front brakes worked just fine.
If the car had ABS then it would either have worked on the front wheels, or displayed a fault light on the dashboard - at least after the MoT work was done.
The idea that the bulb was OK during the MoT and blew immediately after the bushes were replaced is a very long shot.
I'd guess that the car doesn't have ABS.
I hope your girlfriend's OK after the crash.
|
I have rear ended someone in a 306 with ABS (entirely my fault I hasten to add!) When I hit the brake on a slightly damp road, but too close to the car in front, I was very aware of the pedal throbbing as the ABS did its best to help. The ability to steer clear would not have made any difference in this situation.
Given what has been said above, I suppose it is just possible that without ABS I might just have stopped fractionally sooner?
It sounds as if the car in question was not ABS equipped.
|
|
|
|
|
|
Interesting, I stand corrected. :-)
|
|
An ABS equipped car will outbrake a non-ABS car in the hands of most drivers-you would have to be very,very,very expert to beat the system.
|
probably very technically correct but I have to disagree.
In most lesser experienced drivers hands (or feet?) they will release the brake pedal when the abs is activated and even possibly panic causing the braking to take longer than under conventional system.
Most people with abs have never activated the system and do not know what to expect or experience and as such abs imo is over rated and possibly more likely to contribute to an accident. these same people treat it a s a magic stopping aid and drive faster and closer than they would if they didnt have it.
I do agree that experienced people with it are probably safer with it though. perhaps driver education is needed.
I think this is likely to be contested, and discussed, but its my opinion after talking to lots of people about abs.
|
|
|
|
|
|
|
