[dradernh]

Originally Posted by xingqianli

2) I changed the wheel size from 19'' to 17''. The free space around the rotors and calipers is smaller than before thus I may get less air flow and worse cooling. However, I'm not sure how big role this factor plays.

Now I'm using RBF 600 brake fluid. The dry boiling point is 312C. I did get mushy feeling on the brake pedal late in the day. But how can I tell if the fluid is boiled or not?

Given the physics of the problem, cooling airflow is a big factor. Typically, to ensure optimal braking performance on cars like ours you'd run a 3" brake duct hose directly into the centers of the rotors. Those provide a LOT of cooling air; so much so that on some tracks you'd block the ducts off if they were over-cooling the brakes.

From what I saw when the car was up on a lift, though, we're not going to be able to push a lot of air into the center of the rotors. IIRC, it's the steering knuckle that's blocking off most of the inside opening into the rotor - there was just a small eyebrow-shaped opening available. But as the race shop owner said when we were looking at it: "Something is better than nothing."

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My experience is that if you get a soft pedal and it stays soft, you've likely boiled the fluid, and the calipers need to be bled. However, if you're getting an intermittent soft pedal (e.g., it goes soft but doesn't stay soft), you're likely experiencing pad knockback.

That issue is explained better than I can about halfway down this lengthy (and excellent) page telling you everything you need to know about braking performance: https://www.essexparts.com/essex-des...87M2f80M3f82M4. I've used one of Essex Parts' AP Racing-based brake kits on a race car I ran, and they know their stuff, inside and out. If they made a kit for our car, I'd already have one installed on mine.

The solution for pad knockback is to tap the brake pedal two or three times with your left foot after you've gone through turns or over curbs that have knocked the pistons back. Those are the turns/curbs immediately preceding the braking zone in which you experience the soft pedal. That soft pedal then goes away because your use of the brakes has solved the pad knockback problem. This assumes it's not a brake fluid boiling problem that you're experiencing. (I'm not sure I'm articulating all this very well - let me know if you have any questions, and I'll do my best to answer them.)

I've experienced a soft pedal in my M240i once (and only once) during each event I've attended, except for the last event. Not once every lap, or every session, but just once per event. I've discussed that circumstance with two race shop owners with many decades of high-level racing experience between them, and both immediately said..."pad knockback". So I'm now practicing making light, left-foot tapping of the brake pedal second nature.