4 Wheel Disk Brakes, better? Why - and AWD?

jerry_njOctober 8, 2009

I'm looking at new cars and was surprised to find many 2009 models (and 2010) now come standard with 4 wheel disk (disc) brakes, no more rear drum/shoe breaks. What makes this better?

While on the subject of newer stuff, I see more, and like the idea, AWD. However, this seems to give lower gas mileage, is that just because of the extra weight? Here I note that the Suzuki SX4 AWD offers a three operating modes: FWD only, claims better mileage (so much for the weight theory? Is it less friction?) and automatic AWD, plus, here's the real kicker, Locked or 4WD, for those times the extra traction is needed. We have a Subaru Forester and like the AWD for those times it is needed, but it is full time, always on AWD and the gas mileage is good, but not great... 2.5l 4 opposed cylinder engine in this case. As I recall, Subaru sells the full time AWD as giving more control even on dry pavement, i.e., in corners the power to whichever wheel matters.

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john_g

4 wheel disc brakes offer a number of advantages, the first of which is maintaining brake balance under different situations such as normal, and very heavy brake use. Disc brakes also lend themselves to being more effective with ABS systems, and lastly have the advantage of reducing unsprung weight.

As far as AWD vs 2WD, the main difference would of course be drag. Try pushing an AWD car around a parking lot, and then do the same with a similar sized 2WD. For the most part you will notice a slight difference in a straight line, but turn the wheels and you might notice you can no longer push the car because of the extra drag in the AWD/4WD.

As far as directing "more" power to the wheel that spins, lets just leave it at the idea that it's not as intelligent of a system directing power as the advertising might make you think. When one wheel loses traction, the other has no choice but to push the car harder because the torque being applied to both the wheels is the same. So it's not that one gets increased torque, its simply better envisioned as the other slipping wheel not helping.

BTW for testing purposes (and some other uses) there is typically a fuse holder under-hood on the Subaru to disengage the AWD, and make it just a FWD car. However extended use of this option has been shown to overheat the solenoid that controls that function and once damaged you now have no AWD. To shut off the AWD, you install a fuse into the holder. Removing the fuse allows the AWD to function again.

    Bookmark   October 10, 2009 at 3:57AM
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jerry_nj

Thanks, I like AWD better with your description. I figured it was just a normal differential with the ability to shift power to the wheel that doesn't slip, rather than to the one that does. In any case, I didn't see both wheels driving, or all 4 driving at the same time.

We have a Subaru Forester, and I've never thought about disabling the AWD and likely never will.

My questions were driven by my current interest in replacing my 1999 Mazda Protoge with a 2009 Suzuki SX4 Crossover which has 4W disc breaks and a switchable AWD: i.e., one can select 2WD to improve gas mileage. I wondered how that would help, it seems, as I think you describe, when in the 2WD mode that there is indeed less (viscous) load on the engine due to coupling the drive power to the rear wheels. The SX4 has a switch to also put the drive in automatic AWD and in 4WD, at speeds below something like 35 mph. Looks like a nice system and is included in the 7 year 100,000 mile factory limited warranty.

I also test drove a 2010 Mazda3i, wow, what a sophisticated car, I thought "a BMS with improved reliability". I know the Mazda3i and the SX4 are very dissimilar vehicles, but I did like my 1999 Mazda and figured I'd check the new model out. The price on the new Mazda is firm around here, there are some great deals on the 2009 SX4s and it offers a lot of stuff I like too, including the AWD. We do get some troublesome snows here in NJ.

    Bookmark   October 10, 2009 at 9:49AM
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jerry_nj

Looking back after posting, make the "BMS" "BMW", the German car.

    Bookmark   October 10, 2009 at 9:51AM
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