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Car Forum / Saab Cars / July 2005Brake Rotors
I have been told that replacing the original rotors with slotted or perforated rotors will result in cooler rotors, less warping, if any, and reduced wear of the pads. Is any of this true? Can someone give me a solid answer (pun not intended) ? Is it worthwhile changing the rotors? And, oh yes, I was told that just changing the front rotors is about 65 % of the effect. May I have your best opinions please? Thank you, Malcolm > I have been told that replacing the original rotors with slotted or > perforated rotors will result in cooler rotors, less warping, if any, [quoted text clipped - 3 lines] > > Can someone give me a solid answer (pun not intended) ? Yes it's true, however the stock rotors are more than adaquate for average driving. If you go out to the track, tow a trailer, or frequently go over mountain passes then you'll benefit from slotted or drilled rotors. > Is it worthwhile changing the rotors? See above, also if your rotors are warped or worn past their limit and you have to change them anyway, you may find the cost difference is not huge so it may be worth it if only for the cool factor. > And, oh yes, I was told that just changing the front rotors is about 65 > % of the effect. Slotted or drilled rear rotors are not common, if you make the switch, the fronts are the only ones worth worrying about, I'd say that 65% is a bit low, more like 80+% of the work is done by the front brakes. > May I have your best opinions please? > > Thank you, > > Malcolm > I have been told that replacing the original rotors with slotted or > perforated rotors will result in cooler rotors, less warping, if any, [quoted text clipped - 8 lines] > And, oh yes, I was told that just changing the front rotors is about 65 > % of the effect. Your car's OE brakes are very good. Avoid drilled rotors, they often develop cracks around the stress risers. Slotted make a difference, but unless you are racing or running your brakes very hot continously there isn't much point to it. > Your car's OE brakes are very good. Yes. > Avoid drilled rotors, they often develop cracks around the stress risers. Never heard such a thing, ever. In decades. Got something I could read about this phenomenon? It's odd that so many makers would use them if they were fundamentally flawed. (the preceding is my gentle way of saying "sounds like bullshit to me"). > Slotted make a difference, but unless you are racing or running your brakes > very hot continously there isn't much point to it. Drilled rotors have a noticably better stopping response to them. >>Avoid drilled rotors, they often develop cracks around the stress risers. > > Never heard such a thing, ever. In decades. Got something I could read > about this phenomenon? It's odd that so many makers would use them if > they were fundamentally flawed. (the preceding is my gentle way of > saying "sounds like bullshit to me"). I've seen it, but only on "home drilled" ones. In fact, a local garage has just such a cracked disk on their "interesting failures" shelf. Possibly the drilling was done in a careless manner which left uneven holes, I'm not sure.  SignatureGrunff >>>Avoid drilled rotors, they often develop cracks around the stress risers. >> [quoted text clipped - 5 lines] > I've seen it, but only on "home drilled" ones. In fact, a local garage > has just such a cracked disk on their "interesting failures" shelf. Well... there's home-drilled, and there's home-drilled. If I home-drilled rotors, it'd be on one of my milling machines with an indexing turntable, so it'd probably be different than some guy with his cordless drill. > Possibly the drilling was done in a careless manner which left uneven > holes, I'm not sure. Burrs would probably be a problem, and if they're not clean holes, there's all sorts of places for stress to build up. (thinks...) Uneven distribution of the holes could cause all sorts of interesting dynamic effects, and heat buildup during machining would need to be minimized to do this properly. > Never heard such a thing, ever. In decades. Got something I could read > about this phenomenon? It's odd that so many makers would use them if > they were fundamentally flawed. (the preceding is my gentle way of > saying "sounds like bullshit to me"). I take it then that you don't get around much. Have you seen what happens to drilled rotors in club circuit racing? >> Slotted make a difference, but unless you are racing or running your >> brakes very hot continously there isn't much point to it. > > Drilled rotors have a noticably better stopping response to them. Only if you have pads which produce lots of gas. Modern high quality carbon metallics don't gas a lot allowing you to take advantage of the larger friction surface of an undrilled rotor. When you say stopping response WTF are you talking about? Do you mean the amount of pedal pressure or the measured emergency stop braking distance? On ABS equipped cars I doubt you would see a difference. >> Never heard such a thing, ever. In decades. Got something I could read >> about this phenomenon? It's odd that so many makers would use them if >> they were fundamentally flawed. (the preceding is my gentle way of >> saying "sounds like bullshit to me"). > I take it then that you don't get around much. Have you seen what happens > to drilled rotors in club circuit racing? Why the attitude? Also, I didn't see the OP asking about this for a race application, so how is that relevant? >>> Slotted make a difference, but unless you are racing or running your >>> brakes very hot continously there isn't much point to it. >> Drilled rotors have a noticably better stopping response to them. > Only if you have pads which produce lots of gas. Here I thought the gas in question was carried in on the surface of the rotor. (shrug?) > Modern high quality carbon > metallics don't gas a lot allowing you to take advantage of the larger > friction surface of an undrilled rotor. Which explains why nobody uses drilled rotors anymore, is that it? > When you say stopping response WTF are you talking about? Do you mean the > amount of pedal pressure or the measured emergency stop braking distance? > On ABS equipped cars I doubt you would see a difference. Then why do you claim that slotted makes a difference above? The geometry of the holes doesn't change what they do, if anything. I'm also finding your apparent hostility to be unwarranted and distracting from the main point. Thanks Grunff I will take your word for it you have been so well informed in the past. Best regards papa >When you say stopping response WTF are you talking about? Do you mean the >amount of pedal pressure or the measured emergency stop braking distance? >On ABS equipped cars I doubt you would see a difference. My understanding is?was that ABS did not increase stopping distance. Think about it! Government mandated increased stopping distance. I can blame the feds for a lot but not that one. Now... If slotted rotors reduced stopping distance without ABS why not with ABS. And on the question of pedal pressure or distance.. Have you actually seen published stopping distances related to pedal depression force and where does that one come from with power brakes? And is WTF a new engineering unit? Malcolm > >When you say stopping response WTF are you talking about? Do you mean the > >amount of pedal pressure or the measured emergency stop braking distance? [quoted text clipped - 18 lines] > > Malcolm It's not really a matter of stopping distance, but reduced fade. As a brake rotor gets hotter, the pedal effort required increases. A drilled or slotted rotor lets the hot gasses created escape, reducing or eliminating brake fade. As others have said though, the stock brakes are quite good so the benefits of special rotors for street use will be minimal. >>On ABS equipped cars I doubt you would see a difference. > > My understanding is?was that ABS did not increase stopping distance. > Think about it! > Government mandated increased stopping distance. I can blame the feds > for a lot but not that one. ABS will stop better than an average driver. A good driver can stop better than ABS in some conditions. If someone claims they can always do better than ABS, they're lying and/or delusional; it's impossible to threshhold brake on 4 wheels simultaneously, when you only have one pedal. > Now... If slotted rotors reduced stopping distance without ABS why not > with ABS. Well, ABS deals with wheel slip relative to the pavement, which slots won't change. So, you can get _to_ wheelslip faster with slots. I'd say it most likely helps, and would be easy enough to test. >>When you say stopping response WTF are you talking about? Do you mean the >>amount of pedal pressure or the measured emergency stop braking distance? >>On ABS equipped cars I doubt you would see a difference. > > My understanding is?was that ABS did not increase stopping distance. Essentially ABS *regulates* the stopping distance in panic conditions. A highly skilled driver can stop a car faster without ABS, but an unskilled driver is likely to just stomp the pedal, lock 'em up and stopping distance skyrockets. > Think about it! > [quoted text clipped - 3 lines] > Now... If slotted rotors reduced stopping distance without ABS why not > with ABS. I doubt there would be any difference either way. The primary limiting factor for stopping distance will not be related to the brake rotors. You can quite easily get enough braking force out of even stock rotors and pads to exceed the adhesion threshold of the tires. Considering this, I would say that it is the tires and their traction/friction that has a lot more to do with the braking distance than the rotors. -Fred W > And is WTF a new engineering unit? > > Malcolm It's the old US (non-metric) standard. To eliminate confusion: 1 WTF = 144 OS's ;-) >> And is WTF a new engineering unit? >> [quoted text clipped - 5 lines] > > ;-) Eliminate ????? >> 1 WTF = 144 OS's >> >> ;-) > >Eliminate ????? I thought the ratio was much lower than that, ex. 1 WTF = 12 OS's. In fact, some days it only takes a half dozen OS's to reach WTF status. > I have been told that replacing the original rotors with slotted or > perforated rotors will result in cooler rotors, less warping, if any, > and reduced wear of the pads. > Is any of this true? Counterintuitive, perhaps, but yes, crossdrilled rotors do last longer, as do the pads. It's all about heat buildup, and the perforations help with that. Additionally, the holes give the air layer that is present between the pads and the rotor, somewhere to go. It's not much, but it is real. > Is it worthwhile changing the rotors? If you're looking to replace rotors, then yes. If you've got perfectly good ones, there's less reason. > And, oh yes, I was told that just changing the front rotors is about 65 > % of the effect. Maybe more. The back brakes only do 10-20% of the braking. Dave Hinz |
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