praedet Posted March 18, 2006 Author Share Posted March 18, 2006 Well, I was actually using the mu of the pad a few of them ship with, but I agree with you. I was just trying to easily illustrate how having stock rear pads and a BBK w/ upgraded pads can get vey poor... I will work on this in a little while. Is the code format better or worse? Ted Link to comment Share on other sites More sharing options...
Mach V Dan Posted March 18, 2006 Share Posted March 18, 2006 This is a great thread! praedet, kudos to you for assembling all the data. I wanted to chime in with a couple of observations. 1. Calculations only get you so far. There are a lot of real-world variables that have an effect on braking, and many of them are going to play just as big a part as pad mu or the difference between 70/30 and 72/28 bias. In my experience, the best way to figure out what works is to come up with your best guess using the calcuations, then TEST that guess in the real world. If you tromp on the brakes and the rears go up in a cloud of locked-tire smoke...back to the drawing board. 2. Ideal brake bias is dependent on weight transfer under braking. If you lower your car, it's going to dive a lot less under braking, and if you leave the bias alone, you'll tend to lock up the front brakes! Imagine that you had a super-hard suspension with NO dive under braking. You'd want to have a 50-50 front/rear bias. If you put on firmer, lower springs, you'll want to try to move the bias towards the rear to keep the braking force consistent with the weight over each axle under hard braking conditions. 3. In my opinion, the ONLY reason (okay, the only non-appearance reason) to do a big brake kit on the Legacy GT is for TRACK USE. The car has plenty of brake torque, enough to lock up the wheels at any speed, at least one time. And the size of the rotors is plenty big to dissipate all the heat I've been able to make even on a twisty mountain road. But at the road course, the heat loads are much, much higher, and that's where a larger kit is going to be able to better dissipate that energy. 4. Somebody mentioned above that grippier pads would generate more heat. That's not the case! Stopping the car from a certain speed creates the SAME AMOUNT of heat no matter what pad you use. The pressure involved may be different -- the grippier pad will require less pressure from your foot -- but the heat generated is exactly the same. My follow-up to these calculations would be to gather some empirical data. For those of you with big brake kits, how about some experiments with an observer (or maybe even a video camera) watching to see how the car behaves with the ABS off under hard braking? Do the fronts lock up first? The rears? Are they even? Does it change when the brakes warm up? --Dan Mach V FastWRX.com Link to comment Share on other sites More sharing options...
jsalicru Posted March 18, 2006 Share Posted March 18, 2006 Well, I was actually using the mu of the pad a few of them ship with, but I agree with you. I was just trying to easily illustrate how having stock rear pads and a BBK w/ upgraded pads can get vey poor... I will work on this in a little while. Is the code format better or worse? Ted It is indeed better code format. "some say, his arms are made of coiled adamantium fibers. And that he tops his cereal with nuts and bolts. All we know is, he's called the Jose." Link to comment Share on other sites More sharing options...
praedet Posted March 18, 2006 Author Share Posted March 18, 2006 Dan, Thanks for the post! I agree with every point you made. I am just trying to compile a list that will add a piece of information to the puzzle... It is very true that suspension can have a major effect on braking, as can tire pressure, tire grip, tire temperature, and sidewall strength. The empirical data would be great! I am working on going back to one mu now... Ted Link to comment Share on other sites More sharing options...
praedet Posted March 18, 2006 Author Share Posted March 18, 2006 updated... Shows data for each kit including the mu of the pad shipped with the kit. Bias is based on a mu of 0.35 for all pads shown. Ted Link to comment Share on other sites More sharing options...
Joel Gat Posted March 18, 2006 Share Posted March 18, 2006 Hello, This is a great thread! praedet, kudos to you for assembling all the data. I wholeheartedly agree! Great thread... I do have some comments on Dan's post. I'm hung over right now, so I'm going to pose questions rather than pretend to have the facts in my aching head 2. Ideal brake bias is dependent on weight transfer under braking. If you lower your car, it's going to dive a lot less under braking, and if you leave the bias alone, you'll tend to lock up the front brakes! Imagine that you had a super-hard suspension with NO dive under braking. You'd want to have a 50-50 front/rear bias. I'm not sure the effect is as big as you're saying. Without trying to hurt my brain with physics, the weight still transfers forward, and the lack of dive affects that transfer subtly, but doesn't prevent it from happening. I think? I certainly know that you would never want 50/50 bias in racing! Any imperfection in the ground, the chassis setup, the driver's inputs, etc., would send the car spinning. You always want the fronts to do the majority of the work, even if you could prevent that from happening. Understeer is stable, oversteer is not... Locking up the fronts means you continue in the direction you were traveling before (because a locked tire has less traction than a tire with slip). Locking up the rear means the rear stops decellerating as quickly as the front and the rear comes around. That's even the case with ABS - if the rears are being released more than the fronts, then the rear wants to travel faster than the front and the only way to do that is to come around. And forget trail braking if the rears are doing half of the work... 3. In my opinion, the ONLY reason (okay, the only non-appearance reason) to do a big brake kit on the Legacy GT is for TRACK USE. That's pretty much true of any Subaru. Though... if you have good twisties to drive on, I can certainly fade stock pads without trying hard. Pads and lines would improve feel and provide all the braking you need for anything but the track (even in the twisties, aside from a really long downhill, you usually have time for the brakes to cool, and you shouldn't be braking so hard that you generate too much heat to dissappate. But honestly, I would guess that something liky 75% of BBK sales are more about looks than about performance 4. Somebody mentioned above that grippier pads would generate more heat. That's not the case! Stopping the car from a certain speed creates the SAME AMOUNT of heat no matter what pad you use. I'm not sure... If the stock pads grip well enough to get you to the threashold of the tires, then certainly this is the case. Can the stock pads hold the car at the threshold of the tires (esp if you have good tires on), even with a haul down from 100 mph? I'm not sure - I've never tried. I would imagine, though, that at some speed, you'll exceed the heat range of the stock pads in a single stop... meaning you'd generate more heat if you had better pads. But under 80 mph or whatever, I'm sure that Dan is correct. Joel Link to comment Share on other sites More sharing options...
Mach V Dan Posted March 19, 2006 Share Posted March 19, 2006 I'm not sure the effect is as big as you're saying. Without trying to hurt my brain with physics, the weight still transfers forward, and the lack of dive affects that transfer subtly, but doesn't prevent it from happening. I think? I certainly know that you would never want 50/50 bias in racing! Oops, my theoretical example isn't quite right. To have a situation where you had no weight transfer to the front, you'd need a car that was all the way flat on the ground -- like a sheet of plywood. But if the center of gravity is above the ground, you'll always have weight transfer, even with a completley rigid suspension. In fact, given a 3200-pound car with a 110-inch wheelbase, a center of gravity 25 inches off the ground, and a braking force of 1.0 G, I calculate a front/rear weight transfer that would effectively make the front wheels weigh 2327 pounds, and the rears weigh 873 pounds -- a 73%/27% split that is very close to all the bias on the braking systems we're talking about. Assuming the suspension allows the car to actually move, that weight bias becomes more front-weighted, although I admit I can't figure out how to calculate it right now. --Dan Mach V FastWRX.com Link to comment Share on other sites More sharing options...
Joel Gat Posted March 19, 2006 Share Posted March 19, 2006 Hello, Oops, my theoretical example isn't quite right. To have a situation where you had no weight transfer to the front, you'd need a car that was all the way flat on the ground -- like a sheet of plywood. But if the center of gravity is above the ground, you'll always have weight transfer, even with a completley rigid suspension. Now I agree with you completely In fact, given a 3200-pound car with a 110-inch wheelbase, a center of gravity 25 inches off the ground, and a braking force of 1.0 G, I calculate a front/rear weight transfer that would effectively make the front wheels weigh 2327 pounds, and the rears weigh 873 pounds -- a 73%/27% split that is very close to all the bias on the braking systems we're talking about. That sounds more like what I would expect. Assuming the suspension allows the car to actually move, that weight bias becomes more front-weighted, although I admit I can't figure out how to calculate it right now. Hahah... can't help you there... we just had a post-St. Patty's Day drinking celebration (any excuse, eh?). Damn Naja's in Redondo Beach, CA - 77 different beers on tap, 107 different bottled beers. But yeah, suspension compression slightly affects weight transfer, but the effect is pretty small compared to the shift you get from the CG being above the ground. So it might be a few more percent. Joel Joel Link to comment Share on other sites More sharing options...
praedet Posted March 19, 2006 Author Share Posted March 19, 2006 ^^^Just a little, you are correct. It is very dependent on the springs in front also. Next though, you need to track down info on the Legacy's Brake systems proportioning. I know in the past, it was different between the wagon and sedan... Ted Link to comment Share on other sites More sharing options...
Joel Gat Posted March 20, 2006 Share Posted March 20, 2006 Hello, Okay, last week I dropped off my car at StopTech to let them have it for a week (hey, they're only a short 700 mile round-trip detour from home and SWA is dirt cheap). They did their thing and came up with a kit for the OXT. First, yes, the 328mm kits fit under the stock OXT wheels (pictured). Yes, there's radial clearance room for the 332mm kit, but not axial - you would need a 2-3mm hubcentric spacer. Second, I don't care for drilled rotors and I think Red doesn't fit the totally-stock outwards appearance of my OXT, so I had them take off the test-mule kit (yes, I believe _that_ strongly that a BBK is not necessary until you get the suspension and tires and head out to the track). Forget looking at gift horses in the mouth... I'll take slotted rotors and black calipers when I get around to buying a set. http://www.stoptech.com/images/Tim/obxt1.JPG http://www.stoptech.com/images/Tim/obxt2.JPGhttp://www.stoptech.com/images/Tim/obxt3.JPG I asked for a bunch of data from them, it'll be forthcoming. I'm on the road still (posting via EVDO - gotta love this wireless broadband while driving in the middle of nowhere!), so I can get more info as needed and post it later. Ignoring friction, the StopTech BBK makes 89.4% of stock front brake torque. Once you account for the recommended pads, however, it's roughly 96% of stock front brake torque. What additional data do you need for the charts or for general knowledge? Joel Link to comment Share on other sites More sharing options...
drewster Posted March 20, 2006 Share Posted March 20, 2006 yep i think black and slotted rotors look much better. http://img.photobucket.com/albums/v291/urabus/subaru/stop_bbk.jpg i've been sort of waiting to see the numbers for the "S" brand here, but actually I don't really care, the BBK works for me, i'm happy, um end of story. oh yeah, it is good to know that you can run the stock wheels with the 328mm kit, now i have 3 sets of wheels that will work on the obxt, whoppee !!! Link to comment Share on other sites More sharing options...
praedet Posted March 20, 2006 Author Share Posted March 20, 2006 I need the piston sizes and effective rotor diameter. It would be nice to have the mu of the pad they ship with also... Ted Link to comment Share on other sites More sharing options...
Joel Gat Posted March 20, 2006 Share Posted March 20, 2006 Hello, Quick stop for more info: Relative front torques, StopTech BBQ for Outbacks: For the OB2.5i and 3.0R, 89.4%. For the 2.5XT, 90.1%. For the LGT, it's a totally different BBK, so Tim says the front brake torque is 99.0% of stock, ignoring friction. I will ping him for the piston sizes, effective rotor diameter, and mu. Joel Link to comment Share on other sites More sharing options...
praedet Posted March 20, 2006 Author Share Posted March 20, 2006 It'd be nice to find out about both set-ups since the outback version would fit the Legacy's stock rims also... Link to comment Share on other sites More sharing options...
Chappy Posted March 20, 2006 Share Posted March 20, 2006 Great info praedet. Ive read all six pages and have learned alot. I guess my question is what do I buy for a street application for my LGT? Stoptech, iON, or do I just upgrade my pads, lines and fluid? What pads do you recommend? I read about the Hawk HPS pads but they dont seem to be available yet for the LGT. I wanted to go with a Big Brake Kit but now it seems that I may be wasting some money in doing so. The stock caliper seems to have enough bite its just the pad fades a bit to much. Thanks for all the info and research. Link to comment Share on other sites More sharing options...
jsalicru Posted March 20, 2006 Share Posted March 20, 2006 I'd go with pads, lines and fluids only. Check around because they are indeed available. I installed a set of HPS pads on the rear of BOXRPWR's car. "some say, his arms are made of coiled adamantium fibers. And that he tops his cereal with nuts and bolts. All we know is, he's called the Jose." Link to comment Share on other sites More sharing options...
praedet Posted March 22, 2006 Author Share Posted March 22, 2006 Street pads I like my Endless SS-Ms, but they are expensive... I have run Bobcats on other cars and really liked them. I have never tried any Hawk pads... I definitely recommend trying pads, lines, and fluid first. You can always sell the stock set-up to a WRX owner if you do go to a BBK. Ted Link to comment Share on other sites More sharing options...
robinlsb Posted March 22, 2006 Share Posted March 22, 2006 My question is this, with ebd what difference does brake bias make when a computer is trying to substitute its own calculation regarding brake bias? "Belief does not make truth. Evidence makes truth. And belief does not make evidence." Link to comment Share on other sites More sharing options...
praedet Posted March 22, 2006 Author Share Posted March 22, 2006 Well, ebd only turns on once ABS comes on. When threshold braking, if your bias is perfect you will stop faster w/ no ABS activation. The same force applied to a poorly biased car would activate ABS and EBD. As PGT mentioned earlier, it is like snow tires with a stability control systems. The limits become higher... Ted Link to comment Share on other sites More sharing options...
Waxiboy Posted March 22, 2006 Share Posted March 22, 2006 Ted - Any comments on SSY and SSS? Link to comment Share on other sites More sharing options...
praedet Posted March 22, 2006 Author Share Posted March 22, 2006 Haven't tried them. From talking with Keefe/RaceComp, they don't have quite the temp range. SS-Y has the highest mu of the three (0.38-0.48) w/ the lowest temp capability (716 F) and SS-S has a mu between SS-Y and M (0.35-0.43), with a max temp closer to SS-M (932 F). SS-M will have the least dust by far, and has a max temp of 986 F and a mu from 0.3-0.4. Here is the post where I got that info... Ted Link to comment Share on other sites More sharing options...
praedet Posted March 22, 2006 Author Share Posted March 22, 2006 Just added pad info to the first page. Please PM me if you want to add a pad and have the info on it, or if you uhave info for the ones I already have listed... Ted Link to comment Share on other sites More sharing options...
robinlsb Posted March 22, 2006 Share Posted March 22, 2006 How can that be true regarding EBD,does anyone have a wiring diagram of EBD? Does it have a separate fuse? "Belief does not make truth. Evidence makes truth. And belief does not make evidence." Link to comment Share on other sites More sharing options...
praedet Posted March 22, 2006 Author Share Posted March 22, 2006 confused about the question... Link to comment Share on other sites More sharing options...
Waxiboy Posted March 22, 2006 Share Posted March 22, 2006 I think you got the SSS and SSY interchanged in the top post. Link to comment Share on other sites More sharing options...
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