1973 Volkswagen 412 braking test

[Randall]

At this video -- https://vimeo.com/182331995 -- there is a panic braking stop by a VW 412 starting at 0:11. It starts skidding at 0:12 (you see tire smoke) and comes to a stop four seconds later. The last two seconds before it comes to a stop you can clearly see that the front and rear wheels have stopped rotating. I'm guessing it skidded at least 75 feet.

My question to you: does a vehicle that locks up the wheels during braking (thus skidding) take longer to stop than if the wheels had kept rotating until coming to a complete stop? My guess is 'Yes' but I could find no proof by searching the topic.

[Mike Fisher]

I would say skidding is the shortest stopping distance.
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[KTPhil]

Full skid on dry pavement gives the shortest stopping distance.

But it also gives the least control. ABS systems seek to sacrifice a little distance in order to maintain steering control throughout the stop. They let up on the brakes when they detect a wheel skid, the immediately re-apply them. This cycling of the brakes happens about 10 times a second, leading to an uncomfortable rumble or pulsation feeling during the stop.

Old-school driver training taught us to pump the brakes on wet pavement, which was the "manual ABS" process using a human computer, which is at best 10x slower cycle time than the modern computer-controlled ABS. Still better than skidding off the road!

[Chad1376]

A rotating wheel stops significantly faster than a skidding locked up wheel. This is why every modern car is required to have ABS. The coefficient of static friction is higher than sliding friction. Maximum braking traction occurs "just" before lockup, while the wheels are turning.

Performance oriented drivers (without ABS) learn to modulate the brakes. When the wheels lock up, let off the brakes just enough to regain traction, and you will feel the car decelerate harder than when the wheels were in full lock.

For non-ABS cars, brake modulation to maximize deceleration is a learned skill, and requires practice. Your average Joe Schmoe will naturally mash the brakes to the floor in a panic situation, because the learned muscle memory is not there.

All "pumping" the brakes does is give the driver intermittent control and intermittent non-skidding braking.
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[cdennisg]

[KTPhil]

Comparisons are mixed. Results were published in the April 1971 Motor Trend magazine compared early anti-skid braking systems.

Chrysler's Sure-Brake system, which worked on each front wheel plus the rear as a pair, showed the shortest distance with a full skid on dry pavement or loose gravel:

On the other hand, the Mercedes/Teldix Anti-Bloc-System, which worked on all four wheel independently, and at a higher pulse cycle rate, showed A-B-S to be the best under all conditions, while maintaining full steering control throughout the stop:

These two test sets were conducted under different conditions and on different vehicles, and can't be compared directly. But the results within each test can be compared again each other.

[Chad1376]

That's pretty old tech. I can only imagine how rudimentary 1971 ABS is, compared to modern systems. I also question how "skilled" the Skilled Driver is.

Here's a great (race oriented) video from someone who knows how to drive at the limits.

https://www.youtube.com/watch?v=2oU9OSINyTY

Go to 4:20 - Threshold breaking.
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[cdennisg]

[KTPhil]

I think the advances since 1971 (especially the Mercedes system) have been in affordability (in the control electronics) and the reliability of the wheel sensors, not the actual performance of the system itself.

Modern systems are known as "ABS" which is a German acronym used by Daimler-Benz. The Motor Trend article ended with the announcement by Mercedes that their patented system would be made available to all manufacturers, royalty-free, as an advance to overall automotive safety. I might even go out on a limb and wonder if all but the latest systems are evolutions of the Mercedes system.

More recently it has been integrated along with traction control, adaptive 4WD, and other systems and no doubt represents another leap in responsiveness and control.

Here is an article with photos from the same tests reported in the 1971 Motor Trend article (courtesy of google translate):
https://translate.google.com/translate?sl=de&t...mp;act=url

It talks about a 2nd generation system in 1978, which passed via Bosch to Porsche and BMW, long before American cars started adopting such systems. Of course, a $400 option to a $10K Mercedes is a lot easier to take than when added to a $2K Pinto!

[raygreenwood]

[KTPhil]

[Chad1376]

[KTPhil]

Wish I still had contacts in the LAPD... there is a great course in Granada Hills (only about half an hour from me) where they train officers in handling and pursuit. They can water it down in segments and simulate many road conditions.

That area was used on a Jay Leno's Garage episode with Tim Allen, if I remember correctly. Probably on many other shows, too.

That area used to be underwater, until the '71 San Fernando earthquake damaged the earthen dam, and the lower Van Norman reservoir was drained and never refilled. They said one more aftershock and the evacuated area of 40,000 homes, would have been flooded!

[KTPhil]

[cdennisg]

[KTPhil]

[cdennisg]

[KTPhil]

[cdennisg]

[raygreenwood]