AIM Data Aq E training site

Jay Lutz

Jay Lutz

Well-known member
http://vimeo.com/aimsports/videos

This is why I'm so glad I bought my system from AIM....the support NEVER stops and keeps getting better and better and better. By accident I found this site which walks you through a few tricks inside the AIM RaceStudio 2 analysis software. 15 videos so far but more on the way.

Thanks AIM, thanks Roger Caddell.
 
Jay, I presume you either were in attendance for the webinar AIM "Practical Data Acquisitation" on the Spec Miata site or have since watched the video. During the video a point was made while talking about brake pressure by Roger that there was a blip of brake pressure (IIRC 68 psi) at a point in time when the brake pedal was never touched. Presumed to be curb/hole pad knock back.

Question for you being the brake engineer by profession. During pad knock back, what is a realistic distance for the pads to be moved away from the rotors?

Also I'v talked with folks that have caliper pistons on both sides directly on the pads & they say they don't have pad knock back. Is pad knock back a result of the one sided piston OEM calipers used that are required by rules?

Thanks
 
I'm no brake engineer, but my unofficial theory on knockback is that it's a function of wheel bearing "slop", not the caliper itself. As the car turns a corner and goes under lateral loads, the play in the wheel bearing causes lateral movement of the rotor inside the caliper and it's this movement that pushes the pads back. By how much, I have no idea and is surely different for each car and situation.
I would also think that a single piston sliding caliper should be less affected by knockback than a fixed-mount caliper, for that specific reason. Also, a larger rotor would have more knockback issues than an otherwise same brake setup with a smaller rotor.

My 240SX has never had knockback issues with stock or 3 different sets of Wilwood calipers on 2 rotor sizes (11 and 12").

My daily driver (Infinti G35) has a 4 piston StopTech brake kit w/13" rotors on it, and I've had annoying amounts of knockback since day 1 on that car. I can drive 50 miles of straight road and the brakes are nice and firm as soon as I hit the pedal, but as soon as I take a couple turns, I have to pump the brakes to get the pedal back up to the top. The harder I take a turn and more hard turns I make, the more knockback I have. I've never had the pedal go to the floor, but I've had it go annoyingly low. A couple quick pumps and the height is back to normal, but that's still disconcerting if I were rounding a corner and suddenly had to jam the brakes.
 
Matt has it largely figured out (also is a function of caliper bridge deflection, seal and groove design) and I find his advice to be sound. I would disagree with his statement about a single piston caliper exhibiting less pad knockback though...don't forget that the caliper bridge moves on the anchor plate on every pressure application and contact with the rotor.

Regarding the amount of the knockback I don't recall any numbers.....it's been a lot of years since I did any hands on lab work but its real and there are EZ non contact methods to measure the movement. You can roughly calculate it though if you know how much extra pedal or MC stroke it caused. Just calculate the volume of fluid change at the MC (MC crossectional area x stroke). That same volume of fluid was moved at the caliper except the diameter is much smaller on the caliper end. Calculate the caliper piston movement which will be much, much smaller.

As far as the introductory AIM seminar, I skipped it as I'm looking for a little higher level support....I'm already sold on DAQ as a tool. The referenced videos are a great help for users of all skill levels.
 
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