Welcome to the North American Subaru Impreza Owners Club Wednesday July 23, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
Click here to visit TireRack
Brakes & Suspension Forum sponsored by The Tire Rack

Losing traction? Need new tires?
Click here to visit the NASIOC Upgrade Garage...
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Brakes, Steering & Suspension

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 01-10-2003, 06:04 PM   #1
ciper
Scooby Guru
 
Member#: 15543
Join Date: Feb 2002
Location: san francisco, ca
Vehicle:
90BJ Legacy LS ABS
AWD 946 Rio Red Jpn built

Arrow I wonder if you do more harm than good swapping from drum to disc in the rear

This has been bothering me for a while. Im not going to write the longest post possible at first, I want to see what others think. Hear me out.

First, drum brakes get an advantage over discs since the friction material is Wedged into the drum. This means for the same force you get additional braking power with drums (to a point).

Second is the amount of fluid sent to the drums. If you had a car setup with drums and an equal car setup with discs the drum brakes would get FAR less fluid for the same braking power.

Third is a valve that is present on disc/drum combo cars. At the moment I forgot the name(Im not referring adjustment of bias). In drum brake vehicles the pads are drawn away from the drum by a spring. Discs are basically in constant contact. Becuase of this a valve is installed that will send brake fluid to the rear FIRST, activating the rear brakes a short amount of time before the fronts. Since the rears need to take up the slack this pause usually means both brakes are activated at the same time.


Now the problems with the drum to disc conversion:

Brake power to the rear is GREATLY reduce for two reasons, the large increase in piston size compared to the drums and the reduced force needed to normally activate drums.

It seems to me that you would need to change the entire "chunk" of components that include the bias/proportion/timing valve in order for it to function correctly.

I would think that some of you have been driving around with basically no rear brakes.

Lets just say for a moment that is not as large an issue as I think. You still have the problem of the rear brakes activating way too early.

Comments?
ciper is offline   Reply With Quote
Old 01-10-2003, 06:09 PM   #2
sszyma
Scooby Newbie
 
Member#: 31399
Join Date: Jan 2003
Location: Chicago
Vehicle:
2007 WRX
2014 Forester 6MT

Default

I see where you are coming fom and am interested to see comments also.
sszyma is online now   Reply With Quote
Old 01-10-2003, 07:35 PM   #3
Gonz
Scooby Specialist
 
Member#: 18642
Join Date: May 2002
Chapter/Region: MAIC
Location: MAIC
Vehicle:
2005 Impreza WRX STi

Default

Have you improved your front brakes yet? Improving your front brakes will go a long way to improving braking, prior to doing the rear swap.

Quote:
First, drum brakes get an advantage over discs since the friction material is Wedged into the drum. This means for the same force you get additional braking power with drums (to a point).
A couple of points on this. Rear brakes only provide abou t20% of the braking force, and front brakes about 80%, so I think it's not an issue in that sense.

They don't need much force unless you are trying to lock up your rear tires. Nevertheless, you could tune their effectiveness with an adjustable brake bias valve, or by using different brake pads. The rear disks may not give you any improvement in braking distance, but will give more control or feel.

One thing that makes drum brakes bad is that they tend to stick, or not release braking force the instant you let off the brake. As you say they are wedged into the drum. Sometimes, I believe this is called " they are self energized".

As for the " rear brakes are applied first " problem. You can probably gut or remove that valve from the system. If your rear drums are properly adjusted, they should be dragging slightly on the shoes.

So you might be better off just upgrading your front calipers and rotors to a newer version, if you're primarily concerned with stopping distances rather than feel, control, repeatability.
Gonz is offline   Reply With Quote
Old 01-10-2003, 08:37 PM   #4
Legacy777
Scooby Guru
 
Member#: 4800
Join Date: Mar 2001
Chapter/Region: TXIC
Location: Houston, Tx
Vehicle:
1990 Legacy & 97 OBS
AWD 6MT EJ22T AWIC Swap

Default

Some points....

As mentioned drum brakes produce more brake torque then disc brakes, with the same amount of piston pressure.

The wedging effect is called self activation. The leading shoe is self activating.....the rear is not.

I'm really not too familar with the valve you speak of which kicks the rears in first.

Some other comments.

There is no aftermarket adjustable brake proportioning valves that will "swap in" without redoing the brake lines.

Also....one of the drawbacks of drum brakes is the cooling. They don't cool as well as discs so brake fade can be a rather serious issue.

I've got some stuff from my vehicle dynamics course that that shows the different brake setups and info on each. I can scan and post if people are interested.

Josh

BTW ciper, I got the ECU today.......I'll PM ya....
Legacy777 is offline   Reply With Quote
Old 01-10-2003, 08:41 PM   #5
ciper
Scooby Guru
 
Member#: 15543
Join Date: Feb 2002
Location: san francisco, ca
Vehicle:
90BJ Legacy LS ABS
AWD 946 Rio Red Jpn built

Default

Gonz: All of my subaru came with 4 wheel disc, I was just commenting for the sake of others.

Im currently running 11.6 inch rotors in the front and 11.4 inch in the rear on my "main" legacy, two pot fronts single pot rears.
ciper is offline   Reply With Quote
Old 01-10-2003, 09:52 PM   #6
Portly
Scooby Specialist
 
Member#: 1197
Join Date: Apr 2000
Chapter/Region: MWSOC
Location: Cedar Rapids, Iowa
Vehicle:
1997 Impreza Wagon
Mystic Blue Pearl

Default

Hmmm - Well, the first real mod I did to my Impreza was to swap out the drum brakes for a set of discs taken from a Legacy wagon. The improvement in braking feel was pretty tremendous, and the overall stopping power seemed to go up as well. I would not want to trade back to the drums under any circumstances - I hated everything about them, quite frankly.

I can't say anything technical to discount your theory, but my personal experience with the swap was quite different from what you're suggesting. It's possible Subaru doesn't use any of the special tricks you mentioned with their drum brakes, which would explain why they felt so awful to me and why the discs worked well.

Jeff
Portly is offline   Reply With Quote
Old 03-28-2003, 08:16 PM   #7
ciper
Scooby Guru
 
Member#: 15543
Join Date: Feb 2002
Location: san francisco, ca
Vehicle:
90BJ Legacy LS ABS
AWD 946 Rio Red Jpn built

Default

Id like to bring this thread back to life. Please post your comments.

I still feel the size of the "actuator" of the drum compared to the disc creates a great inbalace. In effect everone who has done this conversion might have disabled the rear brakes.
ciper is offline   Reply With Quote
Old 04-02-2003, 12:33 AM   #8
codger
Scooby Specialist
 
Member#: 13486
Join Date: Dec 2001
Chapter/Region: W. Canada
Location: Calgary
Vehicle:
06 Legacy GT wagon
Black

Default

Based on results from people who have actually done this, there seems to be no major problem - and actual improvements instead.


Quote:
First, drum brakes get an advantage over discs since the friction material is Wedged into the drum. This means for the same force you get additional braking power with drums (to a point).

Quote:
Second is the amount of fluid sent to the drums. If you had a car setup with drums and an equal car setup with discs the drum brakes would get FAR less fluid for the same braking power.
It's possible that Subaru uses the same proportioning valve, or keeps a very similar crack point (split point) and reducing ratio, on drums and disc models, but just sizes the drum piston accordingly, so as to get the same amount and pressure of fluid going to the rear brakes, and gets the same amount of brake torque. The WRX has split point of 285psi, ratio of 0.3. Anybody know the TS/OBS numbers?

Quote:
Third is a valve that is present on disc/drum combo cars. At the moment I forgot the name(Im not referring adjustment of bias). In drum brake vehicles the pads are drawn away from the drum by a spring. Discs are basically in constant contact. Becuase of this a valve is installed that will send brake fluid to the rear FIRST, activating the rear brakes a short amount of time before the fronts. Since the rears need to take up the slack this pause usually means both brakes are activated at the same time.
This may very well be an issue. Under very light braking, the rear discs would activate first. Not good in really slippery conditions like iceracing. the valve is called the holdback valve: link

Jim

Last edited by codger; 04-02-2003 at 01:19 AM.
codger is offline   Reply With Quote
Old 04-02-2003, 09:22 AM   #9
NotAnRS
Scooby Specialist
 
Member#: 1004
Join Date: Mar 2000
Chapter/Region: MAIC
Location: Elizabethtown, PA
Vehicle:
2002 Forester L
Scratch-n-Dent Blue

Post

I'll add my $1.95.
Anyone who thinks drum brakes are better than discs needs to drive my '65 Ford pickup (4 wheel drums). This thing is a deathtrap, and is getting a disc swap as soon as I get the parts.

My '86 Audi Coupe, which I run in hill climbs and track events, came stock with rear drums. I swapped the rear axle for one with disc brakes, and the difference in braking is tremendous. Per the factory parts book, the proportioning valve and master cylinder are exactly the same between the two braking types for this car, and the system works perfectly.

The above post is on the right track. The wheel cylinder bore sizes are designed to apply the proper amount of pressure vs. the front (discounting the proportioning valve, of course). Master cylinder bore can also vary if a different pedal effort is desired.

I have researched the Subaru parts microfiche pretty heavily (though it was a year ago) as I want to *****can the rear drums on the Impreza, and the master cylinder is the same regardless of braking system on all of them. I don't recall finding any other parts differing downstream of the m/c either.

While I understand you wanting to question the theory of the brake types, I go by seat-of-pants examination. From my experience, the rusty, ugly, too-damn-many-stupid-little-parts drum brakes on the back of the Impreza have numbered days. They are heavy, complicated and outdated, plus they don't work as well and are hideous viewed through the wheels.

Final point of interest: I was at a school put on by a brake parts mfr and they showed us using a welding rod inside a drum just how much drums flex (you can flex them with your hands!).
NotAnRS is offline   Reply With Quote
Old 04-02-2003, 10:03 AM   #10
Safir
Scooby Guru
 
Member#: 3873
Join Date: Jan 2001
Location: Tampa, FL
Vehicle:
. Paradise is
like Disney with hookers

Default

One thing i havent seen adressed in this thread yet are the deficiencies inherent in a drum setup - mainly poor performance due to the inability to dissapate heat. In a competition environment (even in a panic stop) you will quickly overheat the drum brakes and they will prove ineffective to help slow the car at all. In a car that never sees any type of competition the disc/drum combination is adequate 99% of the time. a car that ever ses elevated brake temperatures will find little or no use of drum brakes.

Even if they are engaging earlier than tehy should, or doing less work than they should, I still prefer some braking power to none...
Safir is offline   Reply With Quote
Old 04-02-2003, 10:45 AM   #11
Jaxx
NASIOC Supporter
 
Member#: 177
Join Date: Aug 1999
Chapter/Region: NWIC
Location: Boise,Idaho,USA
Vehicle:
The 93 Imp W/EJ20K
flat black

Default

yeah and drum brakes look lame

i have done the conversion
i added rs brakes the front while i was at it .. brakeing is really good .. noticably bettter than my WRX is perhaps the bias is wrong ..

if you do it .. try and get legacy turbo calipers and rotors .. vented is better than solid
Jaxx is offline   Reply With Quote
Old 04-02-2003, 10:56 AM   #12
tora
Scooby Guru
 
Member#: 5851
Join Date: Apr 2001
Chapter/Region: VIC
Location: Vancouver
Vehicle:
09 Forester XT
+ NA Miata

Default Re: I wonder if you do more harm than good swapping from drum to disc in the rear

Ciper I am a perfect candidate to give actual experience testimony to everything you have posted. I just recently completed a drums to disc swap on my '01 Legacy. This was done first (using '00 Legacy GT wagon rear disc set-up). About a week later I installed Sti 4-pots on te front with WRX rotors. For the time being stock pads are being used and stock brakelines in the rear but stainless in front.

"First, drum brakes get an advantage over discs since the friction material is Wedged into the drum. This means for the same force you get additional braking power with drums (to a point)."

By feel I'd have to disagree. After putting in me rear discs my braking improved....although it was not a huge improvement it was better.

"Second is the amount of fluid sent to the drums. If you had a car setup with drums and an equal car setup with discs the drum brakes would get FAR less fluid for the same braking power."

I don't think so. You still have the same proprtioning valve so the same amount of fluid goes to the rear. When I installed the rear discs (still using stock single pot fronts) my brake bias transferred towards the rear. Nose dive lessened amazingly.

"Third is a valve that is present on disc/drum combo cars. At the moment I forgot the name(Im not referring adjustment of bias). In drum brake vehicles the pads are drawn away from the drum by a spring. Discs are basically in constant contact. Becuase of this a valve is installed that will send brake fluid to the rear FIRST, activating the rear brakes a short amount of time before the fronts. Since the rears need to take up the slack this pause usually means both brakes are activated at the same time."

The rear brakes are not activated before the fronts. With the disc install I still have consistant brake application. There was no sense of the rears kicking in ahed of the fronts. Even under very hard braking where the abs kicked in there was no notice of that.

"Brake power to the rear is GREATLY reduce for two reasons, the large increase in piston size compared to the drums and the reduced force needed to normally activate drums. "

Absolutely not. As I said my braking power increased as did rear bias both of which discount that.

"It seems to me that you would need to change the entire "chunk" of components that include the bias/proportion/timing valve in order for it to function correctly."

Nope. All I changed was the e-brake cable, backing plate, rotor and caliper. I ended up needing to change the hub as well since my drum hubs were stuck on due to rust. There was no change in master cylinder, or proportioning valve.

"I would think that some of you have been driving around with basically no rear brakes."

Not at all.

"Lets just say for a moment that is not as large an issue as I think. You still have the problem of the rear brakes activating way too early."

Nope, I'm curious what gave you these ideas. Although you did give your reasons there doesn't seem to be any actual experience with this. (Which I suppose is why you are asking).

I now have the Sti's on the front and there is still heaps of rear brake. Except under very hard braking there is little nose dive (and I am on stock suspension at the moment). I had my car on the auto-x course this past weekend and the breaks are great. The car stays far more settled under braking and braking power has improved tremendously.

I can't wait to get some stainless lines and better pads and a master-cylinder brake from MRT.

Hope this helps with the quest for info. It's just one persons opinion but if you ever find yourself in Vancouver you are welcome to test the car out.

Jamie

Comments? [/b][/quote]
tora is offline   Reply With Quote
Old 06-03-2003, 10:40 PM   #13
codger
Scooby Specialist
 
Member#: 13486
Join Date: Dec 2001
Chapter/Region: W. Canada
Location: Calgary
Vehicle:
06 Legacy GT wagon
Black

Default

Well, I finally got the rear disc brakes installed. Noticeable improvement in feel and maybe brake dive, no drawbacks like premature rear lockup. I noticed after several heavy stops, the rear rotors are quite hot to the touch, although not as hot as the fronts, so they are definitly not disabled.

Although it is better now, I still don't have anywhere near the stopping power and feel of my Volvo XC.

Jim
codger is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
DSS Aluminum driveshaft, more harm than good? D Money Transmission (AT/MT) & Driveline 125 03-24-2009 11:00 AM
When safety belts may do more harm than good AVANTI R5 Non-Subaru News & Rumors 5 09-17-2008 04:02 PM
converting from drum to disc brakes in rear outback2.5 Off-Topic 12 12-25-2006 09:55 PM
Does a K & N filter really increase power or does it do more harm than good? macrwrx Factory 2.0L Turbo Powertrain 34 02-27-2004 02:39 PM
Upgrade 02 outback brakes from drum to disc? Marty S Brakes, Steering & Suspension 2 10-21-2001 04:26 PM


All times are GMT -4. The time now is 10:41 PM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.