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Date: 14-Jun-1996 20:23:24
From: Pete Read <>
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
>brake rotors.

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

                                       / Hat 
                         |----------|/        / Rotor 
                         |          |        /                                   
                         |          |       /         
            -------------|          |-------------   
            -------------  \      /  ------------- 
                            \    / 
                           Air Flow 
                           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 discussion.

  1. >... 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 flow.

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

                                       / Hat 
                         |----------|/        / Rotor 
                         |          |        /       / Cooling Air Inside Rotor 
                         |          |       /       / 
            -------------|          |------------- / 
            <-  <-  <-  <--:     :-->  ->   ->  -> 
            -------------  :     :   ------------- 
                           :     : 
                           Air Flow 
                   Direct route through rotor center. 

E28 M5 (and M6) Vented Rotor with Air Flow Director Cap

                                        / Air Flow Director 
                     |   |----------|   | 
                     |   |          |   |            / Cooling Air Inside Rotor 
                     | <---: (  ) :---> |           / 
            ---------- : | :      : | : ---------- / 
            <-  <-  <--: | :      : | :-->   ->  -> 
            -------------| :      : |------------- 
                           :      : 
                           Air Flow  

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.

Pete Read
'88 M5

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