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      03-19-2011, 07:09 PM   #1
paradocs98
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Followup--poor StopTech pedal feel

Refer to my post below for details--I had a StopTech BBK installed on my E90 M3 last week and was concerned about brake pedal sponginess and some dead space at the top of the pedal travel after the installation. The job was done by a well-regarded BMW independent that is also a Dinan dealer/installer. Based on my suspicion, advice from you guys, and a phone call to StopTech in Calif, I figured it was probably due to residual air in the system.

I took the car back to my shop a couple of days ago. They agreed that the pedal felt off, but were doubtful that it could be air because they use the same setup as BMW dealers--a German power flushing unit in conjunction with the GT1 diagnostic computer that runs a brake fluid flush algorithm to ensure no air is trapped in the ABS system.

They went ahead and flushed the fluid again, both manually and using the power flush/computer. They said they didn't see any obvious air coming out, but I'm sure there must have been some because now the dead space is improved and there is no sponginess, just power and solid bite. Interestingly, there is still a little bit of play or dead space at the top of the pedal travel. The shop owner thinks it's just the way StopTechs feel, because he explained to me that the OEM BMW setup and some aftermarket kits like Brembo use a spring tensioner that keeps the brake pads snugged up against the rotors, resulting in an immediate bite, while StopTech does not use a spring, instead allowing the pads to "float" a bit, being held in place by the locating pins coming from the rotors, and resulting in the slightest bit of pedal travel before biting. Anyone have thoughts on this? I haven't had a chance to call StopTech back to ask why they chose this design--perhaps to allow for some thermal expansion of components during hard/track use? Maybe the Street Performance pads also require a bit of heat? The brakes definitely seem to bite quicker and more aggressively the harder you use them. I'm looking forward to seeing how they do at Watkins Glen in May--track days, the whole reason I put these on in the first place.
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      03-19-2011, 07:57 PM   #2
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I'm not sure what your tech is describing. The only spring to which he refers is an anti-rattle device that has no effect on pad travel relative to the rotor. When you release brake pedal pressure, the pads are no longer pressed against the rotor with hydraulic force, but, they don't go anywhere; i.e. they are still positioned against the rotor. There is no "float", unless you have a pad knock-back issue, which is primarily a problem restricted to track cars using sticky tires and with suspensions that allow higher cornering forces than a stock M3. Regarding a "little bit of play" at the top of top of pedal travel, I've never seen a brake system that did not have such. Your analysis of pedal feel should be based on how things work from the point at which you first feel resistance, which is never going to be the first moment of contact.
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      03-19-2011, 08:41 PM   #3
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Thanks for your input, elh.
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      03-19-2011, 09:43 PM   #4
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1. I've had the StopTech BBK for over a year now and to me the pedal response should be very similar if not the same as OE.

2. As elh0102 says, the pads remain very close to the rotor at all times unless you have a bearing that is starting to go. I track my car twice a month and never really experienced this issue either.

3. The BMW OE set up is a single pot slider system which has no springs whatsoever to 'snug' the pads against the rotor as you can see from the diagram.



4. The StopTech Street Sport Pads are designed to be used primarily for the street (but is very good on track also). Therefore it has very good cold bite as opposed to full race pads.

5. Make sure you bed in your rotors/pads properly. That may give you some more initial bite. http://www.m3post.com/forums/showthread.php?t=396763
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      03-19-2011, 11:39 PM   #5
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Quote:
Originally Posted by paradocs98 View Post
...now the dead space is improved and there is no sponginess, just power and solid bite. Interestingly, there is still a little bit of play or dead space at the top of the pedal travel...
There's play at the top of the pedal throw because the master cylinder needs it. There's a fluid duct from the brake fluid reservoir that enters the master cylinder from the side or the top. When you push down on the pedal, you start the piston moving into the master cylinder. The first bit of piston travel closes the duct off as the piston slides past. Until this open duct to the reservoir is cut off, the brake pedal has no back pressure. Once the duct is fully closed off by the piston, the piston starts to build pressure against the the pads in the calipers, but there's still a bit of play because the pads back away from the rotors by the amount of any float or runout in the rotor and wheel bearing assembly. The two factors together are good for an inch or a bit more of brake pedal travel. Once you're actually pushing the pads against the rotors, things firm right up. Note that none of this has anything to do with the design or construction of the calipers.

Quote:
Originally Posted by paradocs98 View Post
...The shop owner thinks it's just the way StopTechs feel, because he explained to me that the OEM BMW setup and some aftermarket kits like Brembo use a spring tensioner that keeps the brake pads snugged up against the rotors, resulting in an immediate bite, while StopTech does not use a spring, instead allowing the pads to "float" a bit, being held in place by the locating pins coming from the rotors, and resulting in the slightest bit of pedal travel before biting...
For pure race cars burdened with excess bearing or knuckle flex under braking and cornering, there are "anti-knockback" springs that can be installed in the caliper cylinders to gently push the caliper pistons and pads against the rotor when the brakes are not applied. This is a race-car only application, and I don't know of any street vehicle that's so weak that it needs this kind of intervention. No BMW does.

What I can guarantee is that the pads are not "held in place by the locating pins coming from the rotors". There are bolts and anti-rattle springs that keep the pads from banging around in the caliper pad pockets (or falling out onto the road) when the brakes aren't applied, but that has nothing to do with pedal travel. On this point, I have no idea what he's talking about.
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      03-20-2011, 11:08 AM   #6
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OP needs to find a new shop.
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Let me get this straight... You are swapping out parts designed by some of the top engineers in the world because some guys sponsored by a company told you it's "better??" But when you ask the same guy about tracking, "oh no, I have a kid now" or "I just detailed my car." or "i just got new tires."
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