Braking Distance
 

I recently got a subscription to Road and Track and have been glancing at all their stats on different cars. It seems the average 60mph-0 braking distance of a regular ole sedan is around 120 feet.

In the back of the manual for my 1985 300D it says the car should stop from this speed at around 165 feet. Just an interesting tidbit, i looked up the stats on the Honda CRX from the same year, it has about half the weight as a w123 and the same brake distance. So i assume these four disc brake systems were quite good for the time, but what advancements in brakes have been made in 20 years to lose 40 feet from a 60-0 stop?

It is not only the brakes but the suspension. If you compare say my W126 to a W220 for example:
The W220 has larger front discs with bigger calipers, maybe 4-6 piston's? never counted.
They are also cross drilled.
The air suspension keeps the car extremly flat during hard stops.
The traction control uses every ounce of grip the tires can muster.

So it stops insanly fast, perfectly straight and will throw you into the dash while the car stays level! :D

Auto makers have not been sitting still for the last 25 years.

Not that W126's have bad brakes, I love the brakes on my car they are great! Tons of bite and the bleed off speed real fast.

I have a suspicion that the tires are the driving factor.

In straight line braking, the M/B can lock up it's brakes just like the brand new vehicles can.

But, the older M/B has 195/70/14 tires.

A new vehicle might have 225/55/16 tires. That's a lot more rubber on the road to assist with the stop prior to lockup.

Furthermore, if the vehicle has anti-lock brakes, the stop is going to be considerably shorter than an older vehicle without anti-lock.

Wouldn't ABS brakes have a lot to do with this? In ABS sytems the brakes never truly lockup and the intermittent braking action also helps maintain control, while reducing heat and fade. I wouldn't be surprised that they would shave 25% off of stopping distances. My 82 doesn't have them, but some cars had them as early as 84 I believe....not sure about the 85 300D?

There isn't any heat/fade if you just lock them up right away :D I've never managed to lock mine up, even with a severe press on the pedal, it certainly was stopping hard though, took much effort to not greet the steeringwheel :eek: Our van feels like it'd just have the rear end fly up and roll over forwards if you slam the pedal....kinda scary....my SD lifts up/goes down that way too but it doesn't give the "I'm going headfirst" feeling....

pawosd, I think something might be amiss with your car. On my 116 I could lock the tires at about 60 if I wanted to (experience). In that car, all 4 wheels would lock and it would slide but it would go slightly sideways so it went forward at an angle, if that makes any sense. It was like you see the stunt drivers in movies. According to the records I got from the PO, brand new calipers were put on in '98 at Midas! They cost $250 a piece, just for the caliper, not including the hose or labor.

My SDL, if I stomp the pedal down the wheels chirp but then ABS does its thing so it's a different story there. It does stop a lot better in the snow though! My SD had vacuum problems so you coudn't just keep pumping the brakes because after the 3rd time or so, the pedal started to get hard.

Thanks
David

Yeah I was just thinking of tires, Carlton.

My brakes are out of shape, they are hard to lock up, there seems to be too much of a dead zone in the brake pedal too, before the pads are bit on.

Around 6 months ago I did a brake flush and bleed and the pedal felt great, but it seems to have deteriorated now, I am thinking maybe I didn't tighten one of the bleeder valves enough or something. Oh well, I need new rubber brake hoses anyways...which are $50 a piece at the local dealership... haha. Maybe over the summer i'll do a brake rebuild.

This is the only W123 i have driven and have never really been impressed with the brakes. My mother's 300SDL W126 has excellent feeling brakes though, the effort is a lot lighter and just feel better all around... can anyone confirm the difference in feel between these two cars or is there just something wrong with mine?

You can get the rubber brake hoses for something around $15 I believe here off Fastlane (didn't look but I'm sure they'll be cheaper than $50). If you buy the aftermarket lines, one isn't going to blow apart on you BTW.

Thanks
David

My rotors/pads/all brake flex lines, and fluid, are all new/less than a year old....and it made pretty much no difference in how it stops, I've never really tried to actually "lock them up" but I'm quite sure it's capable based on the force I can get around half to 3/4 pressed pedal, its right on the edge of a lockup at that point....I just don't want to wreck my $93 apiece goodyear assurance tires :eek: I will say the '83 has a much more "intense" feeling of brakes than mine if you accidentally hit the pedal too quick/hard. My vacuum system seems to be fine and the pedal is not hard to push or anything, it just has a softer feel than the other one. I don't have ABS and have never had much problem in snow, the new tires helped a ton. Our van has no ABS either and it just loves to slide sideways and go nuts in the snow....not fun to drive in it in my opinion, the SD is way better. My only remaining thing I could change to affect the brakes.....would be the calipers, and they seem to work fine, dealer said they were ok too. Master cylinder holds pressure, I can pump it on our steep driveway with the car off and on 1-2 pumps I can get a very firm pedal that can hold the car forever without sinking down at all......The 83 does have newer calipers, which may explain its more powerful "feel" to the brakes......

It would be interesting to see a 126 or 123 in a braking test with modern 205 or even 215 tires (also, front suspension in new and perfect condition). With a skilled driver, I think the results would be pretty suprising to the pro-abs crowd.

Not possible.

Without ABS, you have the same hydraulic pressure at each wheel. When the first one locks up, a skilled driver needs to let off the brakes slightly to keep them all rolling.

With ABS, it prevents the first one from locking up and the other three can continue with increased stopping power because the hydraulic pressure remains at the limit of tire adhesion.

This is a simplified explanation, however, suffice it to say that a skilled driver is no match for a properly functioning ABS system.

abs...
 

all tests that i have read say that abs actually increases stopping distance a little on dry pavement. the safety increase comes from never having to lift the pedal because of one side locking and a skid starting. also in limited traction situations such as rain or snow it is much safer for same reason. also steering is not lost and pedal can be held down. i am not sure but the magazine guys i think may just lock them up and skid to a stop which obviously you wouldnt want to do if you could avoid it. (flat spotting the tires)

This would mean that the sliding friction of the tires on pavement is greater than the friction provided by the brake pads on the rotors, if the wheels are allowed to continue to rotate.

While not impossible, I have my doubts. :confused:

How would one go about measuring 60-0 stopping distance? My currant tires grip a heck of a lot more then mid 80's 205 rubber ever could. However if I stomp on the brakes I can still make the ABS kick on and the tires chirp a bit so the W126's brakes provide way more bite then the tires can handle.

ABS is the best thing that has come along since discs brakes. It makes a big difference.

measuring
 

i dont know how the magazines do it but i have heard of a method in which there is a device that shoots down a mark on the pavement when brakes are applied then you just measure from it to the stopped car. i suppose that the magazines have some electronic device to do it now. road and track always had a bike wheel looking device that they attached to the rear bumper for measuring such things but probably now they use something completely electronic.

if you could stop faster with abs then race cars would have it. they dont. when they have lost it they just lock them up and slide. this also keeps them sliding straight and not into the wall.