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Technical and DIY
Started by Robert O'Donnell at 07-01-2006 10:08 PM. Topic has 5 replies.
  
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 07-01-2006, 10:08 PM
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Robert O'Donnell
Joined on 07-02-2006
Posts 6
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Slotted rotors changing braking characteristics
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I recently upgraded the brakes on my 2002 Sentra SE-R Spec V. For the last year I had been running with Hawk HP+ pads on the front, but otherwise OEM rotors, fluid, lines, and rear pads. Due to rotor wear and the always-present urge to upgrade/modify I recently changed to Powerslot rotors front and back, Goodrich lines, ATE Super Blue fluid, new Hawk HP+ in the front and Hawk HPS in the rear. The braking feel with this new setup is very different, and I am wondering if this may be a normal result of using slotted rotors.
Old set-up: in terms of "decelleration" vs. "pedal movement", there was a very distinct threshold. With light to moderate pedal pressure the decelleration was moderate (i.e. "unremarkable"), but as pedal pressure increased the pads would suddenl "hook up" or "bite" and haul the car down with alacrity. There was nothing squirrely about this threshold- it was completely repeatable and predictable. There were just two dstinct regions of braking performance with a relatively short transition between them.
New set-up: the threshold is gone, decelleration vs. pedal movement is very progressive and feels very linear. It feels like I have to push the pedal harder/further for maximum braking performance but I think this is just because I am still driving with the "threshold" habits and haven't fully adjusted to the new feel. There is no doubt, the current configuration stops gooooooood. But it does feel different.
The car only has ~36000 miles on it and the OEM lines were in good shape, so I don't think the lines or fluid have anything to do with the changed feel. I have come up with a hand-waiving rationalization based on my zeroth-order understanding of braking fundamentals but I'd probably embarass myself if I wrote it in this post, so I won't. :-) Anybody have any insight?
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24.6.29.87
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07-02-2006, 7:42 PM
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Fast Viper Dan
 
Joined on 04-14-2006
Santa Clara, Ca
Posts 14
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Re: Slotted rotors changing braking characteristics
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Slotted rotors allow gases to escape, that’s a good thing. Do the new rotors have cooling veins that are curved, as in directional? I’m not familiar with stock Sentra rotors but most OEM rotors have straight cooling veins. This makes them interchangeable from left side to right side. The cooling veins would be the biggest benefit to a rotor upgrade besides weight, that is if you went to an aluminum hat and floating rotor set up. The next best thing you could do would be to add some kind of cooling duct or even a curved scoop to throw air into the wheel from the inside. I have a set of Porsche air deflectors I use on my Viper. Something like that is a real cheap way to accomplish this.
As for peddle pressure, you should bleed your brakes after the first track session if possible. Then again at the end of the day. Once you are sure you have all of the air bubbles out you will be able to feel what the new set up will do for you. High temp fluid, steel lines and track pads will make the peddle feel firm and more predictable.
The diameter of the rotors is what will give you more control of threshold braking. Imagine a large bicycle wheel turning and tiring to stop it by pinching the spokes near the axle. Then try pinching out near the tire area. You will have a lot more leverage and control the farther out on the diameter.
Looks like your on your way to being the next ARC racer!![Big Smile [:D]](/com/emoticons/emotion-2.gif)
See you at the track,
Dan
1999 Dodge Viper GTS Silver/Blue
2000 Panoz GTS Silver/Blue
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75.7.23.37
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07-04-2006, 3:17 PM
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admin
Joined on 04-05-2006
Posts 4
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Re: Slotted rotors changing braking characteristics
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Robert, I swapped back and forth from blank rotors to slotted on my Camaro (same size, both good quality vaned rotors), so I've felt the difference. To me, I could feel what seemed to be more quick engagement of the pedal. Ie, when I hit the brakes, they seemed to grab sooner with the slotted rotors. I did not feel that the slots degrated the maximum "bite" at all, or that I had to push the pedal harder than with the blank rotors. So I saw them completely as an improvement in feel - whether or not they improved maximum braking or not is questionable to me.
Frankly from what you describe, assuming the brakes were comparably bled, the difference to me is the rear pads. That's the one variable you changed, and my guess is the difference in feel might be that you now have a bit harder braking on the rear, which feels less like standing the car on its nose as you stop. If you get rear wheel hop braking into T14 next week, then I'm right :)
Dave
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24.2.59.60
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07-05-2006, 9:43 AM
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Robert O'Donnell
Joined on 07-02-2006
Posts 6
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Re: Slotted rotors changing braking characteristics
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Hmm... good point regarding the rear pads; the car doesn't seem to stand on its nose as much during heavy braking and that may very well contribute to the overall difference in braking feel. With respect to pedal feel, as much as I hate to say it, I'm leaning towards Mike's suggestion that there may still be a bit of air in the lines. I bled the brakes after the hardware installation but even though no bubbles came out, the pedal travel was way too long so I bled them again and sure 'nough, lots of bubbles came out (but ONLY from the left-front line; dunno what that might mean). After that the pedal feel improved so much I thought I was done, but as I've driven the car more (i.e. with more heel and toe) I've come to the conclusion that for my "normal" runs the pedal position is slightly below where it was before. At least I think it may be. It's not an obvious thing... but in any case I will bleed the lines one more time.
For what it's worth, I was talking with the guys over at Livermore Performance and one of them had an experience with a Mustang with ABS- after installing new calipers and bleeding the lines the brakes felt fine until the first time ABS engaged and barfed some air into the brake lines. Maybe something similar happened with my car? The Sentra FSM does not mention anything special for bleeding ABS-equipped cars... once again, dunno, but I made sure that I engaged the ABS over the weekend, before this last bleed.
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