Rennlist - Porsche Discussion Forums - Advice needed: weird vibration + long brake pedal travel only when hot on track...

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Quote:

Originally Posted by Motul (Post 15302198)

Hello,
In reference only to your brake fade/soft pedal issue. As others have stated once the brake fluid reaches its boiling point, the liquid turns into a gas/vapor, gas/vapor is compressible, a liquid is not. That is why you're potentially getting a soft pedal. This problem will persist since the gas/vapor will remain in the lines until you bleed the braking system. You can further avoid this by stepping up to our RBF 660 which has a higher boiling point than the SRF.

Below is an excerpt from conversations we've had here on Rennlist in the past about Motul RBF products vs the SRF that you may find useful in track applications:

If you flush your brake system often, RBF 600 or RBF 660 will out perform the SRF:

RBF 600: Dry Boiling Point: 594 F
RBF 660: Dry Boiling Point: 617 F
Castrol SRF: Dry Boiling Point: 590 F

If you're looking for an extended drain product with a longer service interval versus higher short term performance, the SRF is a better choice as it has better moisture stability when water is present in the brake system. Brake fluid is very hygroscopic which is prone to absorbing water. Water lowers the performance level of brake fluid when present in the system.

Our position is in most racing applications, water is a non factor since brake fluid is drained regularly so we put more stock in dry brake fluid performance vs wet. And as you can see, the RBF 660 has a 27 F advantage vs the SRF.

All wet boiling point tests measure brake fluid at 3% water concentration- this would be considered a long drain service specification and again, not really a factor in short drain race applications.

If you have any questions, feel free to drop a line here.-Nick

Thanks Nick! Most folks on here that track their cars do so in an occasional weekend "DE" fashion, say once a month or two. And each of those days is maybe (5) 15-20 minutes sessions on track that is definitely not at race level stresses. What would you consider a recommended bleed / flush interval for usage like that? Thanks.

Quote:

Originally Posted by Mech33 (Post 15302393)

While there are many factors to consider (humidity levels, car storage conditions, amount of street driving etc) it's hard to say. In dry-cool storage conditions with limited street use, 1x-2x a year is a good window if you track regularly.-Nick

Mech33...what did you end up finding was the issue for the vibration while braking? thanks

Quote:

Originally Posted by Mech33 (Post 15294524)

4) As I keep pushing it and drive them hotter still, then the vibration gets noticeable all the time, even when not touching the brakes! And here's the kicker: once I hit this step (4) where the vibration becomes noticeable all the time, the brake pedal travel starts to ~double but only if I don't touch the pedal for a while (e.g., after a long straightaway). (!!) The feeling is extended travel, but then the pedal becomes firm at the end of that travel once I take up the pedal distance in that single stroke.

This sounds like a perfect match to a severe knock-back description. Is there vibration like from an unbalanced wheel in general? Also, how certain are you about the connection to temperature vs. other factors (e.g., running over rumble strips more aggressively)? If it's definitely heat-related, I'd follow the heat - inspect rotors, all mounting hardware and surfaces where rotor hats contact the hub and the wheel, maybe try different rotors (maybe can borrow someone's spare or used up set to avoid getting a new one just for that). The second place would be fine inspection of all contact surfaces between hub, rotor, and wheel and the centerlocks - any deformation or even dirt there may cause play in a wheel that would vibrate pads out of place. The worst pad knock-back I had was from a badly unbalanced wheel, fwiw. Bearings could be suspect too, but you car is too new for it to be an issue, unless they are defective.

If heat is not a 100% factor, it adds the possibility of any suspension play contributing. But that would have an effect right away, not after multiple laps.

Quote:

Originally Posted by mdrums (Post 15507710)

Mech33...what did you end up finding was the issue for the vibration while braking? thanks

I never found the exact culprit, unfortunately. I took the braking system back to 100% stock rotors and pads, and the issue has been gone ever since. I suspect it was an unlucky rotor issue that developed an axial run-out (either via physical warpage or actual play) only when very hot (hence the pad knock-back), but that's just my hairbrained theory.

Quote:

Originally Posted by Mech33 (Post 15508869)

Happy to hear the problem is solved. I did not notice it was an old post when I originally responded. Rotors develop run out when hot if the mounting hardware is not allowing for sufficient play to accommodate heat expansion. This can happen either due to a bad design or some of the mounting points being over tightened or stuck losing the ability to move with heat expansion.

Quote:

Originally Posted by Mech33 (Post 15508869)

Thanks...well glad the car is fine. I agree...probably what ever aftermarket rotors you had was the issue. I had a issue on my GiroDisc when I used Ferodo da25000 pads...developed a vibration. It was the Ferodo pads leaving deposits on the rotors. After bedding in Pagid Yellows and driving around on the street and going back to stock pads everything smoothed out.