Recently combing through the specs and noticed that the E320 CDI has the longest/worst of the Eseries in braking distance, 133 ft from 60-0. As for the bigger brothers, E550 or E55 or AMG, that braking distance is only 120 ft. That 13' could be quite useful in a panic stop.

So why is it that the E320 CDI is longer by 13'? Which factor is the primary reason why, tires (being smaller/narrower on the CDI) or brake system (smaller diameter rotors on the CDI)?

 

Pay more get more.

The cars you list can go a lot faster and indeed are sold as ultra high-performance cars. They can stop from speeds the diesel won't achieve. This does not mean the diesel is underbraked.

 

Ok, I get it pay more, get more. But what specifically acheived the shorter braking distance? It doesn't matter how much more faster the E500 E55 & AMG go, that's the engine. What in the brake system is the primary reason for the shorter distance? I'm thinking that it maybe more so due to the tires, ie. 245 & 275 staggered tires has a greater contact patch than the 225 CDI. And lesser so attributed to the 4 or 8 pot calipers.

 

Sure it does--lots more energy to bleed off at a higher speed. They don't put superior brakes on faster cars for nothing. Little bit bigger tires isn't enough.

The brakes are bigger, rotors and pads.

You'll pay a fortune for 13 feet if you convert.

 

No, no, engine size doesn't have anything to do with my question. 60-0 on nearly identical weight cars, its the same potential energy. Repeat, 60-0. Not from 120 nor 150 mph. Thus its the brake components and/or tires that make the difference on stopping distance. Now unless you have your right foot still depressing the gas pedal while you're braking.

 

This is not true. The E350 with sport package has the same brakes as an E550, the 350 being even cheaper than the 320. The only reason the CDI/Bluetec models take longer to stop is that they are not able to get the sport package in the US, although the components can be installed.

The 320 diesel models are limited to 155, or 130 in the US, just like the other non-amg E classes.

 

Contact patch, and rubber compounds. Basically friction.

Reverse things a bit. Assume for a moment that the rubber compound and tread patterns are the same between all the different size tires in question. Now, take the aforementioned E550, or 350 with Sport, and replace the wheel/tire combination on them with those from Sea's E320 CDI.

Stopping distance will increase on the 550/350-S with the 225's because you have reduced the contact patch of the tires. It will not matter at all how big the rotors are, nor how much force the calipers can exert on the rotors; if there isn't enough friction between the road and the tires the ABS is going to activate sooner or, if there's no ABS, the brakes will lock sooner with the smaller tires.

Now, to Sea's original question. The difference between the 60-0 distance of the cars is more than likely most attributable to the tires. Contact patch, tread pattern, rubber compound. I say "most" because there is the possibility that the brake system on the E320 may not be able to generate enough pressure to induce lock-up when the car is fitted with stickier tires. This is where the larger rotors and calipers (not to mention master cylinders, boosters, etc) come into play. In other words, if you can get the ABS to activate during your 60-0 test with the 225's, but cannot when using 245's, then the rotors/calipers/brake system are undersized for the job.

EDIT: There are, of course, other reasons for larger rotors and calipers. Heat disipation, mechanical leverage, applying even pressure across the pad surface, pedal feel. All this starts to become a factor as speed, vehicle weight, etc, increases. Heat induced brake fade is the biggest problem. It's one thing to haul a 2 ton vehicle down from 60 once, it's quite another to haul it down from 120, or to haul it down from 60 5 times in a row.

Dan

 

Ok, here are a few ideas to consider that may have been forgotten.

First off, the diesel version of the E is much more expensive to build and therefore costs more to buy. The difference I believe is somewhere in the vicinity of 4,000 euros compared to an E350. It was believed that America was not going to buy a diesel vehicle with that type of price difference. So the difference was adjusted, the approximate difference in price between an E350 and E320 in the US is about 1,000 USD. I believe they simply made up the difference elsewhere, such as less expensive brakes, I am sure there are many other things and Mercedes is making less of a profit on the diesels in the US.

If and when diesel takes off here we will get better options. Its probably going to be BMW that shows America wants sporty diesels too.

My $.02, hope it helps keep things in perspective.

Baron11