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      11-03-2010, 11:36 PM   #1
sipman
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Long pedal travel

Hi all,

I did my first HPDE at the beginning of the summer. I used Porterfields pads all around.
I was at Thunderbolt at NJMP and entering turn 1, my pedal would be well below the level of the accelerator. This made it terribly difficult to heel toe. So after some thought, I figured I must have boiled my fluid.
I'm getting Castrol SRF put in on friday, before I go to Sat's event at NJMP.

But yesterday, I swapped out from the stock pads to the porterfields. I noticed tonight, that I'm again well below the level of the accelerator, and in fact, it feels like the pedal is hitting a some sort of a "stop". I press harder, and the pedal is stuck (won't go further), but it was relatively easy to get to this "stop".
I never had this issue with stock pads. The pedal is really firm, way up high on the travel and Heel toe is really easy. In some 15-0 stops, I'm getting ABS to kick on, but I'm still really low on the travel.

Could this be a pad problem? Do Porterfields compress a lot? Could I have messed something up in the master cylinder when I used a C clamp to compress the piston in the caliper?

Thanks for the advice. As an aside, I left DSC on during the event as it was my first time, And then I left it on because I couldn't heel toe with such a low pedal. I'm hoping not to let the computer drive for me, so I'm willing to take it off on the track, but I don't understand if that would explain why tonight, the pedal goes so low after some street driving
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      11-04-2010, 10:00 AM   #2
R32
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which porterfields?
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      11-04-2010, 02:02 PM   #3
Trev
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After you swapped the pads did you re-bleed the system?

One other thing you said "Could I have messed something up in the master cylinder when I used a C clamp to compress the piston in the caliper?"

If you didn't open the caliper bleeder then you'll be pushing old fluid back up the system, this isn't recommended but normally won't hurt anything.

Personnnally i'd recommend when changing pads todo the following.
1. Extract old pads.
2. open bleed screw 1/2 turn
3. compress caliper piston back into bore
4. close bleed screw
4. insert new pads
5. push brake pedal a few times till pistons fully contact new pads.
6. bleed calipers using whatever method your happiest with.
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      11-04-2010, 06:29 PM   #4
MFGJR
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Quote:
Originally Posted by sipman View Post
I noticed tonight, that I'm again well below the level of the accelerator, and in fact, it feels like the pedal is hitting a some sort of a "stop". I press harder, and the pedal is stuck (won't go further), but it was relatively easy to get to this "stop".
This is exactly how my pedal feels all the time with stock pads (and plenty of pad material left). It feels this way stilling still, cold, so it's not boiled fluid. The sytem's been bled eight ways from Sunday, by me and the dealer, and the dealer's checked the master cylinder. I've been in another Z4MC that's the same way, too.

Anyone have a solution other than a BBK!

By the way, I'm running Ultimate Pedals with the brake pedal pad shimmed up with six washers to get a reasonable pedal position for heel-toe on the track.
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      11-04-2010, 08:37 PM   #5
sipman
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I'm running the R-4 pads.

MFGJR, I don't have this feeling when I have stock pads on, only when I have the Track Pads on. I was thinking about doing the Shimming Trick, same as you. But I'm wondering if there's another reason why this happens only when the track pads are on.

I've got the car being tech-ed tomorrow, and I've described the problem to my techs. What I think it might be, correction, what I hope it is, is that the pad is hanging up. If you take a cross section of the disc, it would be shaped like a "I".
What I mean is the stock pads are wearing away the metal where they contact it, but I think the Porterfields are slightly larger that the stock pads. Therefore, they might be hanging up on the high spots at the inner and outer parts of the disk. If this is the case, I'm going to shave the pads so that the large arch and the small arch fall inside the two lips that are created by the old pads.

If this isn't the solution, then I don't know what it can be. The Porterfield Tech guy recommended that I have a dedicated set of track rotors, that will wear with the pads. Any one out there doing this?

I know the proper solution is to re-bleed the brakes. But I'm moving to Castrol SRF fluid and I'm hoping that I can avoid having to bleed before every event.
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      11-04-2010, 09:24 PM   #6
MFGJR
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Sipman,

But if the track pads are hanging up on the rotors' ridges, wouldn't they sit farther off the rotor face and result in a higher pedal vs. the stock pads? And, wouldn't the pads wear away very quickly at that spot and confrom to the rotor, if that was their only contact point? The brakes are equally effective with the Porterfileds on there, right? Seems like they wouldn't be if they weren't contacting the rotors fully.

This is maybe a really dumb question, but are the track pads the same thickness (including the backing plate) as the stock pads? There's obviously not a 1:1 relationship between pedal and piston travel, and brand new pads give a noticeably higher pedal most times.

The dealer's service manager mentioned something to me about a "three stage" master cylinder, about which I'm clueless. Could a little less pad thickness have you into a different "stage" in the master cylinder, giving a different feel and travel? Can anyone explain this "3 stage" thing?

I'm also trying to figure out if track pads without a dedicated set of rotors is workable. At about $1,000 for a set of rotors, that's kind of expensive for a guy who does a couple track days per year!

This probelm drives me crazy! The brakes are effective as hell, but the pedal feels like crap and is too low for comfort. Arrrgh!
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      11-05-2010, 07:08 AM   #7
sipman
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I would think that your first question "wouldn't the pedal sit higher" would be a no. I think that the pedal returns to it's same height regardless of the pedal. But if the pads sit farther away from the rotor surface, that would result in a longer pedal stroke, hence the pedal ending lower than the accelerator in it's stroke.

I too agree that any variations in the pad surface should wear quickly, and the pad would then conform to the rotor's face. But when the pads were brand new, I had the same problem for 8 - 25minute track sessions and the problem never resolved itself.

What I've heard is that the pad itself will deflect. What I'm guessing is that the pad material is so strong that the two ends that are catching on the high spots are hanging there, but the piston is causing the center of the pad to deflect until it hits the rotors surface. This would be like standing in the middle of a 2x4 that is propped by it's ends on two bricks. If this is the case, then the pad would return to it's un-deflected state and keep the large gap between the rotor's face and the pad.

As far as equally effective. I don't know about that. I mean, from low speeds, I can engage ABS, but during those 8 track sessions, I never once got ABS to kick in. And seeing it was my first track session ever, braking out of the straightaway was scary as hell. I was scrubbing speed from 130 to about 60 in the brake zone, but my pedal was burried and I felt like I couldn't get more braking with more pressure.

I've never heard about the master cyinder having different stages. I wouldn't even know how that could be. Isn't it just another piston and cylinder design? To get staging, would that not assume a change in either the ration of the piston's travel in comparison to the pedal movement, or a change in the ration of fluid moved by the piston?
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      11-05-2010, 09:39 AM   #8
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Is this happening in a static condition or only at the track? If it's not happening in your driveway, it could be a pad knockback issue. Can you pump the pedal back to a normal engagement point?

Could just be a bad master cylinder. Also check the sliders and make sure everything is greased properly.

Anyway, just throwing some ideas out there.
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      11-05-2010, 09:56 AM   #9
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Quote:
Originally Posted by DrSeuss View Post
Is this happening in a static condition or only at the track? If it's not happening in your driveway, it could be a pad knockback issue. Can you pump the pedal back to a normal engagement point?

Could just be a bad master cylinder. Also check the sliders and make sure everything is greased properly.

Anyway, just throwing some ideas out there.
I have been having the same issue recently at the track and it is exactly as you describe, basically I have to "double brake" before I get any real stopping power. I pump the brakes once and then on the second time my brake pedal is nice and firm, but the first time it goes to the floor. I thought it could be a bad master cylinder but wasn't sure.
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      11-05-2010, 11:16 AM   #10
DrSeuss
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If you have to pump the pedal back to firmness, that's probably pad knockback. What's happening is your rotor is shifting undercornering so it's basically pushing the piston back into the caliper. So when hit the brake the first time, all you're doing is moving the piston back into contact with the rotor.

The stock caliper is a slider so supposedly they are more resistant then a fixed caliper. Anyway here's a good article on Stoptech's website.
http://www.stoptech.com/tech_info/wp_knockback.shtml

This happened to me when I was autox'ing and I ended up going through a cone wall. When I mentioned it to my co-driver (owner of the car), he was "oh yeah, I forgot to tell you about that."

A bad master cylinder will cause a sinking pedal because it won't hold pressure. The easiest test is to just stand on the brakes in your driveway. The pedal shouldn't move at all. If it is, you either have a leak in your brake lines or your master cylinder is bad. When my MC started to go bad so it would brake pretty decently but then it would gradually sink to the floor.

Both these incidents were for other cars BTW. I haven't had any brake related problems with my Z4.
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      11-05-2010, 01:59 PM   #11
Shereef_Osman
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Quote:
Originally Posted by DrSeuss View Post
If you have to pump the pedal back to firmness, that's probably pad knockback. What's happening is your rotor is shifting undercornering so it's basically pushing the piston back into the caliper. So when hit the brake the first time, all you're doing is moving the piston back into contact with the rotor.

The stock caliper is a slider so supposedly they are more resistant then a fixed caliper. Anyway here's a good article on Stoptech's website.
http://www.stoptech.com/tech_info/wp_knockback.shtml

This happened to me when I was autox'ing and I ended up going through a cone wall. When I mentioned it to my co-driver (owner of the car), he was "oh yeah, I forgot to tell you about that."

A bad master cylinder will cause a sinking pedal because it won't hold pressure. The easiest test is to just stand on the brakes in your driveway. The pedal shouldn't move at all. If it is, you either have a leak in your brake lines or your master cylinder is bad. When my MC started to go bad so it would brake pretty decently but then it would gradually sink to the floor.

Both these incidents were for other cars BTW. I haven't had any brake related problems with my Z4.
Thanks for the link, that was actually very informative, it is the exact issue I am having. I'm guessing probably partly cause I have the StopTech brakes on my car (Fixed vs Floating Calipers). But I suspect it shouldn't be as bad as I have, I must have something loose in my front end. I also noticed that my front rotors weren't wearing evenly (lots of hot spots). I wonder if that could be either because of the knockback or in fact be causing the knockback.
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