Conducted a test this weekend on some very hard compacted snow with my 2003 Passat (normal tyres) and my brother's Audi A5 (winter tyres) to see whether there was actually a noticeable difference in stopping times.
Not that surprisingly, the A5 with winter tyres stopped in almost half the distance my Passat took. However, when watching I was puzzled by what I feel through the brake pedal and what I see at the wheel.
With ABS triggered, the brake pedal goes very firm and buzzes under your foot. The buzz is at least 30Hz. However, the wheel is actually locking, releasing, and locking again approximately 2x per second.
Can anyone explain the contradiction between what I feel and what I see?
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I don't see why it has to be a contradiction.
The return pump could easily be running at 1800 rpm, for example.
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The buzz is at least 30Hz. However the wheel is actually locking releasing and locking again approximately 2x per second.
How did you measure these frequencies?
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The ABS pulses at a few times a second. The higher frequency you are feeling is probably the vibration from the pump.
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L'Escargot: I can guess 30Hz from the "buzz" through my foot. I videoed the test, so I know that the wheel was locking and releasing just under 2x per second. As it stands, I would have expected to feel an equivalent 2Hz buzz to correspond with the video.
I discovered that using light and repetitive pumping I could easily beat the ABS stopping distance by approximately 30%.
My understanding is that a modern ABS is supposed to hold the wheels on the threshold of locking, not bluntly lock-release-lock-release which is exactly what mine appeared to be doing.
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ABS is relatively simple. It detects the reduction in rotational speed of a wheel by means of a sensor, and momentarily releases hydraulic pressure on the braking system on that wheel until the wheel rotates at the same speed as the others on the car. It then returns the hydraulic pressure to normal, and if the wheel starts to lock again, the cycle is repeated. It does this multiple times a second.
In order for the ABS to work, the wheel has to begin to lock so the reduction in rotational speed can be picked up by the sensors. On ice, there is so little grip that the transition from wheel rotating normally to wheel locking is near instantaneous. There's a big difference between gentle braking on ice and hard braking on a dry, grippy surface.
There is no grip detection or anything clever like that. It's a simple "release and reapply" system, designed to offer a faster version of old fashioned cadence braking.
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is the clicking noise the relay for the abs ? or do i have a problem !
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>>is the clicking noise the relay for the abs ? or do i have a problem !
Possibly the operation of the valves that control the intermittent application of the brakes.
If you had a problem the light would come on.
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I've experienced ABS on normal road conditions before and it did not vibrate the pedal near as much as the current car does on ice. Admittedly I've not experienced ABS on the current car (Mazda6) apart from on ice and at low speed.
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ABS gives a completely false sense of security on ice. Most people (including myself) mash the pedal to the floor at any sign of danger, the ABS kicks in, but with all four wheels appearing to be at same speed during a skid, it has almost no effect whatsoever, and you need to manually cadence brake to acheive any sensible safe stopping capacity. However you really need to have your wits about you to do this.
I really wish you could trun ABS off on snow and ice.
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I really wish you could trun ABS off on snow and ice.
So do I. I can see how it can be safer in normal (even wet) conditions allowing you to still steer. But at almost a walking pace, ABS has at times caused me grief.
I think I am right in saying a locked wheel gives most grip but no ability to steer around/away from problems. Hence ABS. In fact stopping distance goes up in an emergency with ABS but that is compensated for by the benefits. On ice.... it is not useful.
I also notice when my DSC kicks in I get a strange sound. I wonder if thats the sound caused by driving/sliding on snow? Not filmed what is actually happening to the car.
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On freshly fallen snow a locked wheel will stop you quicker than a car equipped with ABS - same applies to gravel. The "snow plough" effect of the tyres digging into the snow/gravel actually creates more stopping power than the tyre's grip in these circumstances. ABS increases stopping distance drastically on these surfaces, good systems actually defeat the ABS below 15mph for this reason.
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good systems actually defeat the ABS below 15mph for this reason.
Don't think - in fact sure - mine defeats ABS in these circumstances. I've been at 5mph and ABS cut in. But doing 5mph because I knew it would.
Now DSC seems to be useful I think.
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I don't like ABS at the best of times. If you trigger it in normal conditions you are probably driving badly anyway. In snow and ice it's useless and may even be a hindrance. In a real emergency a very short period of lock up kept under control will scrub far more speed off than the abs ever could and can if necessary be followed by a burst of cadence braking. In my opinion only as always of course.
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>>I think I am right in saying a locked wheel gives most grip
I don't think so, except in rare circumstances, like fresh snow.
IIRC maximum grip is achieved at up to 10% slippage.
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On dry roads, you get most grip at about 20% slip - this percentage reduces as the friction available on the road surface reduces, but, the general point is that in all but extreme circumstances, you get more braking force from a wheel which is still rolling than one which has fully locked.
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I'm certainly not qualified to offer up scientific or engineering proof of slip versus lock up.
But if you reverse the situation and induce wheelspin into acceleration the car goes nowhere fast and control is hazardous, if you keep it just under wheelspin point it accelerates far faster, that the eqivalent to locked wheel argument isn't it?
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>>that the eqivalent to locked wheel argument isn't it?
Yes, very similar arguments apply.
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NC if you are still reading would you mind answering something on Motorprops battery thread please.
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There is a very big difference in the cost of systems which release/reapply and those which maintain the wheel on limit of adhesion (probably in theorder of the cost of the car)
As an example diesel locomotives had a form of traction control which stops the wheels spinning on accelleration (5-10% coefficient of friction) so they don't burn the rail. The later large freight locomotives in the UK (class 59 on) have creep control which at slow speed literally maintains power so the wheels are just slipping (you wouldn't see the slip but you can hear it) so the train can pull more weight. It means one loco can do the work of two so the cost is justifiable.
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I really wish you could trun ABS off on snow and ice.
On my old car, I did just that, much to my satisfaction. Perhaps someone knows how to do it for yours?
Edited by FotheringtonThomas on 11/01/2010 at 11:17
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Ha - BMW bikes can do it, why not their cars ?
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The answer the original question ABS works by removing or reapplying brake pressure, sounds simple but it's not an analogue feedback system it's digital. The system has so many event cycles per second during each event (for the purpose of this explanation each event is 10 milliseconds) the brake pressure is decreased (anti lock) or increased (reapplying brake) by opening the pressurise/release circuits for fixed periods of time many times a second until the desired result is achieved, these events are the buzzing you can feel on the pedal. It does not simply release brake pressure until the wheels starts turning ? the pressure is released in hundreds of steps.
Why does the wheel locking not match those events? On a slippery surface the brake line pressure required to initially lock a wheel is far higher than that required to maintain a locked wheel, so in your example it took say 200 psi pressure to lock the wheel, the ABS detects this and starts letting pressure off in 10 millisecond (1 event) cycles until the wheel starts rotating again, it may not rotate until the brake line pressure drops to as low as say 20 psi at which point the ABS will now, in 10 millisecond chucks, start adding brake pressure until the wheel locks again. So in the slushy ultra slippy conditions, the series of release and pressurise events may take over a second before there's sufficient brake line pressure dumped to allow the wheel to turn again or added to relock the wheel.
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I thought ABS was about maintaining control, not necessarily reducing stopping distance?
FTF
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