h5k

fast928 got me thinking. I have experienced what
I claim to be the wet braking problem:

half second of no braking power during certain
wet conditions. I have seen claims on this forum
anywhere from .25 seconds to 3 seconds.
Well, today was perfect conditions: wet snow
with no accumulation on I-25 and US-36.
My run is about 30 miles and I drove around
longer to do some experimenting.

It's me! Fast928 posed the question: is it happening
to everyone else on the road too? well, my last
two cars were 2 different subarus (both with
4 whell disc brakes). I always complained
they were mushy in any condition. Moving
up to the a4, I was astounded at the grip of
the brakes. I learned to hit them much more
gently (first 5000 miles I would jerk the car
because I was on the brakes too hard).
Well, now that conditions outside change,
I am expecting grabby brakes...but like with
all cars, one has to clear the water off.

So I performed about 25 experiments today.
(I tried to drive about a minute without braking,
then hit them at different attack speeds and
force).

First of all, as near as I can tell, one half second
is probably too long of a complaint for me.
However, in that first moment of braking, the car
DOES start slowing down (and I don't think it
was a result of simply taking my foot off the
accelerator). AND it _was_ completely reminescint
of my subaru brakes (under any condition).
But if I was ON my brakes, the difference was
actually imperceptible (sp?). 100% of the times
where I said I am going to make the car
slow down "this" much, it did, in the amount
of time I expected.

Now, I am not speaking for everyone. I have
seen vehement claims of 3 seconds with lots
of force on the pedal. Personally, if that
happens to me, the car will be sold within
one half hour or less! That is an engineering
disastor. My not adapting to condtions...
well, that is user error. Earlier I reclassified
the problem in my mind: even if i have to
press harder to get it to work, it is an
engineering problem to have such distinclty
differing characteristics out of the brakes
(which are a well understood problem).

My question to wet braking problem experiencer's
is this: is the a4 the highest performing car
you are _used_ to (to qualify that, I would
expect logging at least 10,000 miles on a car
to be _used_ to it). Maybe it is the mental adjustment
up to the better brakes, then a shift in habits
which does not accomodate wet conditions.

Personally, I am no longer concerned about the
problem.

h5k

Mr. Mom

that the majority of the cars that are experiencing this are equipped with either the sport wheels (pre-2000) or aftermarket wheels. His explanation was that these wheels allow more air (and moisture)to get into amd around the brake calipers. This excess moisture is directly related to the lack of braking for a couple of seconds. Hopefully, with the 31 complaints (please people, if you have experienced this contact the NHTA (again I may have left out a letter S perhaps)they have received so far at their website an investigation may begin. If you are having trouble finding the site, please search the forums as its in there and make your position known.


Speak up, be heard, and prevent any unnecessary accidents!!!

TIA,


Peter

CraigB

...the wheel size/type does not help/hinder moisture from reaching the backside of the rotor (where the shield should be). That guy is not much of an engineer, IMO.

CraigB

Stace C

increased airflow (due to the larger openings) pull more air through the wheel, and bring more moisture over the rotor. I'd assume pressure under the car is higher than pressure outside of the wheels, so, air would flow in that direction naturally, and might be increased with different wheels. I could be way wrong, as I am not an engineer.

AT

airflow around a spinning wheel is a major contributor to drag on a car. also, spokes can be designed so that when they spin air is drawn across a pressure differential to maximize brake cooling. of course that also affects airflow and pressure distribution underneath the chassis, where flow patterns hugely affect a vehicle's high speed behavior and stability.

a properly designed "performance" wheel (one that isn't just for looks) should take into account many of these factors while optimizing drag and brake cooling.

i am babbling now but stace you are absolutely right. the design of wheel spokes, even though the back of the wheel could be identical, vastly affects how air is drawn through and around them, and can affect how much moisture gets deposited on the rotors.

the wet braking phenomenon seems random and intermittent, but i've always suspected wheel design to be a big part of what's happening, because that is the one variable that people never mention. i'm curious to know what kind of wheels people have on when they experience wet braking.

ErikR

Sure, the modeling is hugely complex. Moving bed wind tunnels are rather expensive too. Fortunately empiricism is cheap. I had a 98.5 test bed, using both a closed-face 15" and the relatively open 16" sport wheel. In both cases I had the identical wet braking symptoms. It happens both summer and winter, mag chloride or not, in wet conditions. It did not happen in dry snow and it did not happen in light misty rain.

bbmass

and noticed problems with wet braking and with my new 17", which is a much more open wheel, it doesn't seem to be any worse

Mr. Mom

that the sport wheels with their more open design, compared to the non-sport weheels, allow more air and consequently, water in and around the brakes. He also went on to say that during testing of the Z34, which alo had an open wheel design, allowed water into both sides of the brake caliper as opposed to just one.


Don't take the information I'm reiterating as verbatum. And yes, I do believe he knew what he was talking about.


HTH,


Peter

CraigB

...with an inner shield, I think the problem would be drastically reduced, if not eliminated. My M3 has inner shields and I've never had a wet braking problem, even with a very open 17" 5-spoke.

I just wish Audi would acknowledge that there is a real problem. Thanks for the info! 8^)

CraigB

Mr. Mom

compared to earlier models. He also mentioned that the design for Audi's requires quite a bit of labor to remove them compared to other types of vehicles too.



HTH,


Peter