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Old 01-04-2002, 12:01 PM   #1
Whit
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Default 4 wheel disc brakes better that disc/drum?

In stock configuration? ie, does the WRX wagon stop shorter that the TS or OBS.

I'm thinking LSD and rear disc brakes would be cool, but only if they significantly help.
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Old 01-04-2002, 12:53 PM   #2
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IIRC, the WRX has larger front brakes(11.4" vs. 10.7"), so I would imagine it would stop faster. LSD wouldn't affect braking, but would be desirable.

I would concern myself more with upgrading the fronts first, then do the rear at the same time if that is what you are considering.
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Old 01-04-2002, 01:06 PM   #3
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Ohhh, I didn't realize the fronts were different on the WRX.

What got me thinking was my brother just bought a Forester S with LSD and rear discs, and if I could find a wrecked one I might be able to swap the parts rather easily.

But I also seem to recall the brake master cylinder is different on cars with rear disc, any truth to that?
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Old 01-04-2002, 08:12 PM   #4
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The simple, logical answer to your initial question is, "Yes, otherwise everyone would still be using drums." There's a good reason manufacturers keep offering fewer and fewer models with rear drums.

Assuming the WRX had the same size front brakes as a 2.5TS, would the WRX stop shorter? Yes. Drums are not very good at modulation at the limit, and are way more prone to fading. So, the drums in the rear are going to reach their limits more quickly than discs, and would trigger the ABS on the rear brakes. Once you trigger the ABS you've lost some braking effectiveness, so you won't stop as short. This might not be a factor in a single hard stop from 60-0, but you do a few in a row and the car with rear discs will definitely stop shorter.

I'm not sure on the master cylinder question, but I would guess it is different on the cars with rear discs. Drums generally use more fluid, so the master cylinder on the cars with drums probably is designed to account for that. I'm basing that on my knowledge of Mustangs w/drums vs. Mustangs w/discs, though, so don't quote me on it.

Pat Olsen
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Old 01-05-2002, 08:16 AM   #5
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Patrick, good points, sometimes the obvious is hard to see
I've been looking for road test numbers, seems they are all WRX tests (no suprise there), I hadn't thought about the ABS triggering sooner with drums.

HAs anyone seen a road test on the '02 OBS?
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Old 01-05-2002, 03:10 PM   #6
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The next question is, If you put the rex 11.4" rotors up front in place of the OBS, TS 10.7", will there be a noticable difference in braking and how much more noticable. The calipers can be the same although you would need to get the rex caliper bracket to fit the larger rotors.
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Old 01-05-2002, 03:38 PM   #7
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I would think you will notice a differene. Larger rotors = more suface area for friction. Best thing to do; head to a dealer and drive both cars back to back.
I'll think the difference will be noticible.

Paul G.
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Old 01-05-2002, 07:21 PM   #8
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ttoversteer put the WRX rotors on his 2.5RS and was very pleased with the difference. I'm not sure if he had swapped pads at the same time or not - if so the better pads would account for some of the improvement.

Pat
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Old 01-05-2002, 08:50 PM   #9
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Quote:
Originally posted by Patrick Olsen


I'm not sure on the master cylinder question, but I would guess it is different on the cars with rear discs. Drums generally use more fluid, so the master cylinder on the cars with drums probably is designed to account for that. I'm basing that on my knowledge of Mustangs w/drums vs. Mustangs w/discs, though, so don't quote me on it.

Pat Olsen
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heh... I quoted you on it anyway. Actually, the amount of fluid used in a drum brake system wouldn't matter as much as the surface area of the wheel cylinders compared to the area of the pistons in the new calipers. That would be the make or break in that scenario.
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Old 01-05-2002, 09:02 PM   #10
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first the less nit-picking:
Quote:
I would think you will notice a differene. Larger rotors = more suface area for friction.
i would think (if every bit of pad is against the small rotor) that there'd be no increase in friction per se because the actual surface area that the pad is touching is the exact same. it seems the bigger rotor is only the greater amount of metal provides for greater heat dissipation which will in effect lead to better (or is it only longer duration-wise?) braking performance (less fade?)

and then
Quote:
This might not be a factor in a single hard stop from 60-0, but you do a few in a row and the car with rear discs will definitely stop shorter.
you mean longer instead of shorter, correct? (i told you it was nit-picking)
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Old 01-05-2002, 09:21 PM   #11
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larger rotors increase the heat dissipation, but also provide a slight amount of leverage too. It's only an inch, but think of it as using a large wrench on a bolt compared to a short stubby one. Is it easier to stop a merry-go-round at it's center, or out on the edge? (I know, bad analogy)
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Old 01-05-2002, 10:03 PM   #12
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oh, you're right.

i was forgetting about the radial speed.

edit: and radius, obviously.

Last edited by elemental; 01-05-2002 at 11:02 PM.
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Old 01-05-2002, 11:09 PM   #13
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Quote:
i would think (if every bit of pad is against the small rotor) that there'd be no increase in friction per se because the actual surface area that the pad is touching is the exact same. it seems the bigger rotor is only the greater amount of metal provides for greater heat dissipation which will in effect lead to better (or is it only longer duration-wise?) braking performance (less fade?)
Nit-pickers unit! Your correct, and I know the larger rotor allows for better heat dissipation. Your assuming the pads for the WRX and the OBS/TS are the same though, and I am not sure if that is the case or not.

Pat:
ttoversteer also added Portifeild R4S pads, and stainless steel lines, and a single stage brake booster at the same time. I drove the car, and it stopped on a dime! It was really nice, especially the booster.

Paul G.
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Old 01-06-2002, 12:54 AM   #14
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Actually, Paul, the single stage booster and SS lines were done after the rotors. I was at Drew's shop swapping out my suspension (again) while Tom was doing the booster and lines, and I know the rotors were already on the car. And I think, but I could be wrong on this, that he already had the R4S pads even with the normal RS rotors.

Anyway, all that is neither here nor there. The fact of the matter is that larger rotors provide more leverage and also a larger heat sink, so everyone who said that above is correct.

Quote:
Originally posted by elemental
and then
quote:
--------------------------------------------------------------------------------
This might not be a factor in a single hard stop from 60-0, but you do a few in a row and the car with rear discs will definitely stop shorter.
--------------------------------------------------------------------------------

you mean longer instead of shorter, correct? (i told you it was nit-picking)
Ummm, no. Perhaps I should have been more clear in what I was trying to say. Take two cars, both with identical front brakes, one with rear drums, the other with rear discs. Do one hard stop from 60-0 - the cars will probably stop in about the same distance (of course it depends on how big the drums are and how big the discs are, blah blah blah, but lets assume the manufacturer balanced them exactly the same). Now do 8 more hard 60-0 stops - the car with rear discs will stop shorter than the car with rear drums. Depending on the pads being used, the car with rear discs will probably not be able to stop as quickly on that 9th stop as it did on the 1st stop, but it will always stop shorter than the car with drums. Does that make sense now?

Oh, and unless you have really good pads and rotors, I wouldn't advise trying to do 9 back-to-back hard stops from 60-0 to test my point.

Pat Olsen
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Old 01-06-2002, 02:14 AM   #15
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i read it incorrectly, my bad

...wrong on both fronts, don't know what's with me today. you suck, all of you!
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Old 01-06-2002, 04:34 AM   #16
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Quote:
I've made my rotors glow red - have you?
I got mine to smoke... but i didn't get out to look at them for color. It was prolly my brake pads smoking anyway...

One thing about drum brakes... they're actually a better design than rotors. Drums mechanically multiply braking force with a sorta lever action as you brake.

http://www.howstuffworks.com/drum-brake.htm

The reason they don't work as well as disk brakes is their lack of cooling. Disks have ton's of airflow (especially ventilated disks) so they stay cool, and can therefore absorbe more heat. And that's the name of the game, turning kenitic energy into heat.
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Old 01-06-2002, 08:42 AM   #17
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I've known several auto-xers and especially drag racers to run drums. Why you ask? less rolling resistance. Unlike with disk brakes, drum shoes don't make contact when the brakes are not applied. Autox & drag racing doesn't need the heat disapation capacity of disks.
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Old 01-06-2002, 10:41 PM   #18
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Quote:
Originally posted by Whit
Ohhh, I didn't realize the fronts were different on the WRX.

What got me thinking was my brother just bought a Forester S with LSD and rear discs, and if I could find a wrecked one I might be able to swap the parts rather easily.

But I also seem to recall the brake master cylinder is different on cars with rear disc, any truth to that?
Anybody know if it would work if they got a pair of WRX disc brakes (maybe from an unfortunate addition to your local junkyard?) to put on the front of their OBS and put the "old fronts" in the rear in place of the stock rear drums? Seems to me like a pretty decent bang-for-buck upgrade if it doesn't have any major complications.

Disclaimer: I'm no car mechanic, which is why I phrased the above as a question.

--jeff
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Old 01-07-2002, 06:00 AM   #19
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The front rotors from a WRX would probably bolt on, but you would been the bracket that holds the caliper also. As for putting the fronts on the rear, I'd say that's a real long shot. You will also need different hardware, brackets etc. If you can find a WRX donor car, just take all the brake bits, or the entire rear sub-assembly, diff, axles etc, and bolt it all on.


Quote:
Originally posted by Avedis


Anybody know if it would work if they got a pair of WRX disc brakes (maybe from an unfortunate addition to your local junkyard?) to put on the front of their OBS and put the "old fronts" in the rear in place of the stock rear drums? Seems to me like a pretty decent bang-for-buck upgrade if it doesn't have any major complications.

Disclaimer: I'm no car mechanic, which is why I phrased the above as a question.

--jeff
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Old 01-07-2002, 04:15 PM   #20
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Drums do have a reliability advantage over discs, but one that means nothing to you southern folks. Rear calipers get bombared with slush and salt by the front wheels. This causes the calipers to require overhaul at about 3-5 years.
Drums are more protected from this so they can last many years without any maintenance.
Subaru may have this in mind for their non-performance models as they sell more cars in the snow belt. That's maybe why they continue to use drum, plus they cost less.
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Old 01-07-2002, 04:34 PM   #21
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Quote:
Originally posted by Avedis


Anybody know if it would work if they got a pair of WRX disc brakes (maybe from an unfortunate addition to your local junkyard?) to put on the front of their OBS and put the "old fronts" in the rear in place of the stock rear drums? Seems to me like a pretty decent bang-for-buck upgrade if it doesn't have any major complications.

Disclaimer: I'm no car mechanic, which is why I phrased the above as a question.

--jeff
First of all, which generation of OBS are you talking about? If it's an old OBS, then yes you could put the WRX rotors on the front, but you would also need the caliper brackets and the calipers themselves (and maybe the brake lines, too, I don't know if they're different or not). As for the '02+ OBS, to be honest I don't know what they have in the way of front brakes. If the brakes are the same as the 2.5RS (10.7" rotors with 2 piston calipers) then you would not need new calipers, you would only need the rotors and caliper brackets.

The Subarus that come with rear discs use an integral drum-type parking brake inside the hat of the disc. Since the front rotors have no provisions for such a setup, you would have to figure out some way to make a new parking brake. It just isn't practical, really. You'd be better off doing what Whit suggested - just scavenge the entire rear setup from a 2.5RS or WRX.

Pat
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Old 01-07-2002, 05:19 PM   #22
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i think we have established that, if the car (be that a RS/TS/OBS/etc) already has the subaru 2-pot front brake, upgrading to the WRX setup is a good and cheap upgrade since you only have to source the brackets and the rotors, while you get larger rotor which can dissipate more heat and get more leverage from the caliper being placed further out

now my quesion:

what happens if you are trying to upgrade from, say, an old single-pot front caliper? because now you have twice the number of pistons at the front, while (i assume) still the same pressure going to the front... although the 2-pot's might have smaller-diameter pistons, and i don't know how to calculate the difference

what i was leading to was, if one has upgrade the front caliper (which is not the case about the RS-to-WRX upgrade since you are using the same calipers), would you have to "play" with the biasing/proportioning valve, or whatever controls the amount of brake pressure/fluid that goes to the front and back??

i've been wondering about this question for some time, since how many RS owners upgrade their front brakes to the Subaru 4-pot, but was never able to get a clear answer

-arthur
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Old 01-07-2002, 08:44 PM   #23
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According to Mike Sheilds at spdusa.com,you don't need to worry much about the brake proportioning. He even went on to say that the old disc/drum set up had the best pedal feel after the disc swap.
Quite a few people have swapped out the drums and put in legacy discs with good results. I plan to do it too someday. The hard part is finding the donor discs in good condition for a good price.
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Old 01-07-2002, 09:07 PM   #24
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At carpoint.com, they tested two foresters, one auto and one manual.

It doesn't say if either is the 'S', but one got 141 ft and one got 138 feet.

On the flip side, the RS got 144 ft. and the WRX got 150 (auto) and 147 (manual)

Sounds fishy to me

Maybe the forester comes with better tires than the WRX

RS must get the same size brakes as the WRX this year?
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Old 01-08-2002, 01:03 PM   #25
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To answer the proportioning question. I upgraded my stock single pot front brakes to the WRX two pots. Under extreme conditions the rears will lock slightly before the fronts(no ABS) but this is the same that originals exhibited with the stock tires. I think this is more a result of the soft suspension unloading the rears under hard braking.
Overall I have been extremely pleased with this upgrade. With my original single pot fronts and upgraded tires, lockup was all but out of reach on dry surfaces. With this new setup and better fluid, ss lines etc. I can out-brake my friends 1999 Mustang. The importance of the fluid can not be overstated.
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