Test of brakes vs. engine

In article
>,
XR650L_Dave > wrote:

> On Oct 15, 6:36*pm, Alan Baker > wrote:
> > In article
> > >,
> >
> >
> >
> > *XR650L_Dave > wrote:
> > > On Oct 14, 1:44*pm, Alan Baker > wrote:
> > > > In article
> > > > >,
> >
> > > > *XR650L_Dave > wrote:
> > > > > 2003 subaru outback automatic, car weighs about 3600lbs. Brakes are
> > > > > new with about 1000 miles on them, and are bedded-in about as well as
> > > > > subaru brakes ever get (new rotors, ceramic pads).
> >
> > > > > Testing went pretty much as I envisaged.
> > > > > I will say up front I was not willing to cook the heck outta my
> > > > > brakes, but I gave them a pretty good workout.
> >
> > > > > Also let me say that almost anyone that this happens to that has a
> > > > > plan can probably emerge unscathed (but with a damaged car), but this
> > > > > test is only about brakes vs. engine, which wins.
> >
> > > > > What I expected was that at speed, 55, 65, 70 or so, the brakes would
> > > > > be able to shed a lot of speed off the car very quickly, but that the
> > > > > ability of the brakes to do so would actually fall as the cars speed
> > > > > decreased, due to 3 reasons:
> >
> > > > > 1) the brakes heat up and fade (very quickly with full throttle
> > > > > applied)... expected by all, I imagine
> > > > > 2) the car, being an automatic, downshifted as speed fell, resulting
> > > > > in more torque being applied to the wheels
> > > > > 3) as the car goes slower and slower, the rate at which the brakes can
> > > > > remove energy from the system goes down dramatically, while the rate
> > > > > at which the engine can add energy to the system remains constant (not
> > > > > envisioned by most here saying the brakes will always win)
> >
> > > > > I felt these 3 effects would combine such that the car would slow down
> > > > > considerably, reach 1st or maybe 2nd gear, and that the car would stay
> > > > > at a fixed speed until the brakes faded sufficiently that the car
> > > > > would resume accelerating.
> >
> > > > > To test this, I really needed to test only one thing- in 1st gear,
> > > > > throttle full, can the car stop?
> > > > > Well, on my subaru, no powerhouse, the answer is decidedly 'no'. I did
> > > > > this several times to make sure I would test the brakes through the
> > > > > temperature with the best friction, and for cold, warm, or hot the
> > > > > best the brakes could do was a draw, which of course if I did it long
> > > > > enough the brakes would fade and the car would accelerate.
> >
> > > > > I did some speed runs from 55, 65, and about 70 and had about the same
> > > > > results. I did *not* cook my brakes to the point where they actually
> > > > > faded and lost effectiveness, since in 1st gear the car could not
> > > > > stop.
> >
> > > > > So, reacting to a stuck throttle only by braking, here's what would
> > > > > happen with my car:
> > > > > 1) throttle sticks full-on
> > > > > 2) speed goes up, to, say 70 or 80 (doesn't really matter, except for
> > > > > figuring total heat load)
> > > > > 3) driver applies full brakes, nothing more
> > > > > 4) car quickly slows to, say 30 or even 25mph, downshifting as it
> > > > > slows
> > > > > 5) driver figures he has it under control
> > > > > 6) brakes heat sufficiently they lose effectiveness
> > > > > 7) car accelerates
> > > > > 8) driver regrets not having made out their will
> >
> > > > > If the driver begins by applying the brakes less than fully, the total
> > > > > heat load will be all the greater and the brakes may even fade before
> > > > > the car slows into the lower gears.
> >
> > > > > If at *any* time the driver pumps the brakes, there is no way the
> > > > > unboosted brakes will be able to slow the vehicle. I tested that and
> > > > > in 2 pumps I was standing on the pedal, accelerating, without faded
> > > > > brakes.
> >
> > > > > Now, a big-engine rear-wheel-drive will probably lock the fronts and
> > > > > spin the rears, but for FWD or AWD that's not going to happen
> > > > > (obviously).
> >
> > > > > Positing a situation where you could not shut the car off or place it
> > > > > in a lower gear/neutral, the best thing to do would be to apply the
> > > > > brakes fully, get to the lower speed, and drive into the ditch or
> > > > > against something so that the car would get stuck off-road or behind
> > > > > something so it could not move.
> >
> > > > > A small 1200cc econobox, yeah, you'll probably overpower the engine
> > > > > with the brakes in 1st gear, but my subaru is no powerhouse, and the
> > > > > engine wins.
> >
> > > > > A more powerful vehicle, even with good brakes, brakes will lose.
> >
> > > > I'm sorry, but I don't believe you.
> >
> > > > --
> > > > Alan Baker
> > > > Vancouver, British Columbia
> > > > <http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>
> >
> > > I don't ****ing care, unless you would like to call me a liar to my
> > > face.
> > > My good name, even on some anonymous bulletin board, is more important
> > > to me than being right.
> >
> > I'd tell you I don't believe you to your face, but then, you keep your
> > identity anonymous, don't you...
> >
> >
> >
> > > If you believe I am lying, that puts you in the list of folks I will
> > > no longer discuss the issue with.
> >
> > > That's also the list of people I wouldn't **** on if they were on
> > > fire.
> >
> > Wow....
> >
> > I'm heartbroken.
> >
> > --
> > Alan Baker
> > Vancouver, British Columbia
> > <http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>
>
> Let me know when you're in NY state.
>
>
> dave

Happily.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>