Date: 14-Jun-1996 20:23:24
From: Pete Read <pete.read_at_boeing.com>
Subject: Re: Brake rotor airflow (M5 v. 750iL rotors)
Gary Ketner does some good thinking and says (politely):
>... I beg to differ with Pete Read about airflow through M5 v. 750iL
What we have here is a (my) failure to communicate. Your statements are
correct, but there's a disconnect between what I tried to communicate
and your points. Looking back, I'd say it's my fault for not being
clear in my sketches or write-up. I've probably confused others too, so
I'm glad you commented.
The subtlety you may have missed is the unusual outboard vent opening on
the stock M5 rotor. That's right, air enters the rotor vent from the
wheel side of the rotor, not the normal inboard side.
Normal Vented 750il Rotor
|----------|/ / Rotor
| | /
| | /
------------- \ / -------------
Entrance (from inboard side)
E28 M5 (and M6) Vented Rotor
Outboard Hat Cooling Holes
Air Flow / /
Entrance / /
\ |-------/--| /
\ | / |/
\ ( )
---------- | | ----------
Let me go through this. We're mostly in agreement. I'll try to add
more detail so you can see my reasoning and also help others follow
- >... the radial air flow through the internal venting slots in a
>disk brake rotor is driven by the internal vanes acting on the
>air inside the disk.
Agreed. The vented front rotors (versus solid rear rotors) are
essentially two solid rotors sandwiched together, separated by radial
cooling passages. The spinning vented rotor acts as an air pump,
drawing air through the center cooling passages. This cools both sides
of the rotor, from the inside, evenly. Air exiting from the rotor's
outer edge also cools the caliper assembly.
Note that rotors with curved cooling passages pump even more air, but
are directional, requiring different left and right rotors. E28 M5s
have straight cooling passages, so both front rotors are the same.
2. >Air is not forced into the hub and out through the disk; rather,
>air is expelled from the disk through its outer edge...
Agreed that the pumping action of the vented disk is the most
important factor, but there is significant airflow passing under
the car, and then out through the front wheel openings. Some of this
outboard moving air passes through the wheel, cooling the brakes. The
more airflow, the more brake dust on the outside of the front wheels.
This is one reason why front wheels accumulate more brake dust than rear
wheels. Front brakes work harder because of forward weight transfer
during braking, but the extra brake dust is also partly from greater air
Air flow through the wheels and out the wheel opening can be increased
several ways. One is by body shape. Looking down from above the car, a
more curved shape (narrow nose then increasing width back towards the
wheel opening) creates a low pressure area at the front wheel opening,
which draws out air. E28 M5s only have a little curve up front, but
it's better than some older slab-sided cars.
A second way is to create more (high pressure) air passages in the
vertical face of the front air dam. M5 foglights can be replaced with
air inlet ducts, but the valence behind the spoiler needs an air flow
opening (a 4" hole saw works if you're brave). I think M6s already have
an opening behind the fog lights (no cutting required).
Wheel mounted fans, as used on the E34 M5, draw more air through the
wheels. Steve Castle says BMW reported a 100 degree drop for the newer
wheels versus the old basketweave design.
I'm sure the Berlinetta NACA ducts that mount beneath the front spoiler
also help. I've heard the performance doesn't match the $400 price
though. NACA ducts are designed to created good flow with low drag.
But they need to be located in a high pressure laminar flow area to work
well (basically an upward-sloping smooth surface). The area beneath the
front spoiler isn't very high pressure or smoothly flowing air.
3. >...and replacement air is drawn in through the hub.
>The relevance of this is that the holes in the M5 rotor hat don't
>steal airflow from the disk. Perhaps the holes provide for easier
>access of air to the rotor, and may actually increase flow.
Agreed for most normal disks. But the M5 is slightly different. See my
diagram above. If you just tape shut the hat holes, no air can pass
from the inboard side of the hat to the rotor vent air flow entrance on
the outboard side.
Here's my guess about the M5 rotor design. The hat holes are intended
to help cool the wheel bearings during extended high-speed runs. The
natural outboard flow of cooling air (see #2) goes inside the hat, over
the wheel bearings (hub assembly), and out through the hat holes. The
outboard air entry location for the rotor vents actually helps pull even
more air out through the hat holes.
The design seems to be more oriented towards keeping the wheel bearings
cool (flow through hat) than keeping the rotor portion cool (flow
through center of rotor). The air flow directors ensure that the air
flowing through the hub holes continues on out through the center of the
rotor in this unusual design. Note that air flow directors won't aid
the cooling of a normally designed rotor (those with inboard rotor vent
air entrance points).
My reason for thinking 750il rotors cool better is the more direct
route for cooling air. Air flow through the M5 rotors (with air
directors) is much more circuitous.
Normal Vented 750il Rotor
|----------|/ / Rotor
| | / / Cooling Air Inside Rotor
| | / /
-------------| |------------- /
<- <- <- <--: :--> -> -> ->
------------- : : -------------
Direct route through rotor center.
E28 M5 (and M6) Vented Rotor with Air Flow Director Cap
/ Air Flow Director
| |----------| |
| | | | / Cooling Air Inside Rotor
| <---: ( ) :---> | /
---------- : | : : | : ---------- /
<- <- <--: | : : | :--> -> ->
-------------| : : |-------------
Circuitous route up through hat holes, then down through outboard vent
entrance, and finally through the rotor center.
4. >But they <hat holes> certainly do not let air that would otherwise
>pass through the rotor 'leak out', as in Pete's diagram.
Agreed that holes in a normal rotor won't let significant amounts of
air leak out. The exception to this is brake ducting with sealed-back
rotors (race cars). In that case, air is actually forced through the
rotors, in addition to the pumping action of the vented rotor. I've
tried to approximate this with my brake duct set-up. The brake ducting
almost contacts the caliper while directing air through the opening
behind the caliper.
Note that the "rotor" (flat part) and "hat" (hat shaped part) are all
one piece on the M5 and most production cars. High performance rotors
sometimes have separate aluminum hats.
The aluminum hats are either bolted to the rotor or connected via
dog-drive (dogs on hat fit into slots machined in the edge of the rotor
inner diameter). The idea is to reduce the stress related problems,
such as cracking and warping, caused by the different expansion of the
hot rotor section and the much cooler hat. Aluminum also reduces weight
and the joint between the hat and rotor slows heat transfer, keeping
wheel bearings cooler.
Frank Fahey makes high performance, direct bolt-on rotors with both
curved cooling passages (see #1) and bolted aluminum hats. After the
initial purchase, rotors can be replaced separately. Frank Fahey
Manufacturing 619.693-1872 Brake rotors and hats: $650 front set.
Replacement rotors: $250 front set. For comparison, 750il rotors are
about $100 for a front set.
Hope this helps. Let me know if it doesn't make sense.
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