BigAl1

Might be helpful if I were to ever drive it in the snow! NOT going to happen! I chose to leave mine off.

123quattro

Why is that? There is a ton of air moving through your wheels at highway speed. If the roads are wet you are moving a lot of liquid through there too. That liquid has to come into contact with the rotor.

dr bob

The net effect may be more cooling due to evaporation, but moist air is less dense than dry air. Technical point that doesn't change your comment materially.

Speedtoys

What little spray that may be ducted in..not all will hit the rotor face, the ducting is towards the inner hub of the rotor, and the vanes pull air FROM the inner hub..the rear face of the rotor wont get wet. And what does..is hitting a fast spinning disc, that's very warm from the latent heat of the last braking plus wheel bearing heat directly radiated into the rotor...it would be evaporated almost instantly..not even a full rev would be required.

On a seriously rainy day, I have serious doubts that a rotor would at any time, be 'wet' given the, IMHO, spinning action, and evaporation from being on a warm metal plate with a fair amount of air being moved across the surface as well.


I remember that BMW commercial, which suggested visually that that light application of the brakes, would have water dripping from the bottom of the caliper. Uh huh.


All IMHO, and only off roading in the jeep, immersing the brakes, have I ever felt "wet brakes".

Speedtoys

Ah..duh..good point.


Flow & temp..you are correct.

123quattro

Next time it's raining hard go drive your car on the highway at 70 and don't touch the brakes for a while. Slow down and see what the brakes feel like. Now speed right back up and immediately slow back down again. I bet you it will feel different. I used to work in brake modulation at my last job. I'm not making this stuff up.

Speedtoys

Dont tell me.

Test 2 will be hotter.


But test 1 will be above ambient..there is hub heat in there..and spinning..with lots of airflow...test 1 still has dry rotors.

Ducman82

they are there to cost you more money.... :-P

brealytrent

I don't think they have much effect. My guess they are there to keep rims from getting black and eaten, since they don't really cover much anything else. My 924 had a pair on the front wheels. Dumped them, cleaned and painted the calipers red.

Cole

I've run several cars without them for various reasons and have never seen an ill effect.

dr bob

The shields are there to keep sand/rocks/pebbles/gravel from getting stuck between the pad face and the rotor face. A small stone there would be noisy, but more important would keep the pad from making direct contact with the rotor itself. It would drastically reduce braking on that caliper.

Water does a most excellent job of shedding heat when rotor temp exceeds a couple hundred degrees. The heat require to evaporate the water to steam is significant, and water generally doesn't leave a residue when it flashes. Meanwhile, most brakes work a lot better when they are warmed up some. A cold rotor with a film of water on it takes a few revolutions before the pad scrubs the surface dry. Steam from the water will clog the pad face area initially as the rotor starts to warm up. A few more revolutions and pads will be warm enough to do some real work. Those first few no-real-brakes-yet revolutions can be disconcerting in a panic situation.

That's my amateur analysis.

Speedtoys

DrBob:

I belive that as well, I just dont see a spinning rotor being WET on the surface...short of very low speed thru a VERY deep hole.

brealytrent

That makes sense, but how would anything besides possibly a grain of sand get in between the pad and the rotor?

Lizard928

The first time you hit the brakes after a long stint at 70 the pedal will fall further, and it will take a moment to shed all the water off. The pedal travels farther due to the water build up causing a slight amount of "knock back" on the pad.

I know what you are talking about and have experienced it many many times.

dr bob

The pistons in the caliper draw the pad back somewhere in the 1/16" range total when all the pieces are there and the calipers are clean. Further, anything in that size range can get wedged between the rotor and the pad, even if the 'big part' of the debris is bigger than the gap. The pad retracts from wherever it was pushed each time the pedal is applied, so you could end up with a bigger gap after a few hard pedal applications when the pad didn't move forward enough. Together, these mean that there would be room for something to get between the pad and the rotor face. Now push hard on the pedal. One side of the pad has the stone, so the pad is pushed toward the rotor with contact ultimately at three points or a point and an edge; less friction area no matter how the stone is in there. So now you have one pad doing work on that wheel, and the other pad is not making enough contact with the rotor to do much. And now, that piece of crud/sand/gravel is wedged in the gap, partially impressed in the pad surface so it can't just rattle on through and out. Eventually it will wear away, will wear away at the rotor, or get pushed into the pad material. One of these will allow more pad contact with the rotor, and brake force will be restored.