Upgrading rear brakes - a theory !!
13 July 2002, 02:45 PM
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Bit of a long one 
I’ve quite often seen on here in the past people saying not to upgrade rear brakes because it will upset the brake balance, this I understand if you were upgrading the rears but not touching the fronts.
However my little mind has been thinking that if you upgrade the fronts, in my case from 2 pots to Subaru 4 pots with 335mm disks, then you drastically increasing brake power at the front and so are effectively moving the brake balance forward. Say for example you had 25% rear to 75% front braking effect as standard (these are made up figures but could be about right), the front upgrade could change that to 90% front. So my thought is that if you fitted bigger callipers and disks to the rear, say a standard 2 pot front set-up then not only would you increase overall stopping power even more but you would also re-balance the braking effect to the original 25%/75%.
Obviously this is based on my very limited (read none!!) understanding of braking dynamics, but it makes sense to me. So what do you guys think? am I daft? Or is there something in what I am thinking?
I’ve quite often seen on here in the past people saying not to upgrade rear brakes because it will upset the brake balance, this I understand if you were upgrading the rears but not touching the fronts.
However my little mind has been thinking that if you upgrade the fronts, in my case from 2 pots to Subaru 4 pots with 335mm disks, then you drastically increasing brake power at the front and so are effectively moving the brake balance forward. Say for example you had 25% rear to 75% front braking effect as standard (these are made up figures but could be about right), the front upgrade could change that to 90% front. So my thought is that if you fitted bigger callipers and disks to the rear, say a standard 2 pot front set-up then not only would you increase overall stopping power even more but you would also re-balance the braking effect to the original 25%/75%.
Obviously this is based on my very limited (read none!!) understanding of braking dynamics, but it makes sense to me. So what do you guys think? am I daft? Or is there something in what I am thinking?
13 July 2002, 03:11 PM
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Yes, if you do a major upgrade to the front then the back will be weak, this is offset to an extent by the fact that you will brake a bit harder (more G) and therefore transfer more weight forwards, needing more front brake.
Look at touring car brakes, massive 14"+ vents with 6 pots up front, unvented 10" 2 pots rear.
There is a good thread running on www.i-club.com on this topic.
Look at touring car brakes, massive 14"+ vents with 6 pots up front, unvented 10" 2 pots rear.
There is a good thread running on www.i-club.com on this topic.
14 July 2002, 06:54 PM
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Simmy,
what you say is 100% true. I am, however, a little surprised that you think you came up with that! LOL!
Most people may not upgrade their rear brakes when upgrading the front, but that doesnt mean that they are right (most people used to believe that the earth is a disc until not long ago
)
I believe that the fact people do not upgrade the rear brake discs and calipers is due to 3 main reasons:
- budget
- little education and availability from manufacturers
- lack of cognition of car owner
what you say is 100% true. I am, however, a little surprised that you think you came up with that!
LOL!Most people may not upgrade their rear brakes when upgrading the front, but that doesnt mean that they are right (most people used to believe that the earth is a disc until not long ago
)I believe that the fact people do not upgrade the rear brake discs and calipers is due to 3 main reasons:
- budget
- little education and availability from manufacturers
- lack of cognition of car owner
14 July 2002, 10:49 PM
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Didn't say I was the only one that's ever thought about it 
just wanted to start a discussion about it, cos i've seen the "don't do it" replies many many times and don't understand their logic !!
Who says that the earth aint a disk, this theory about it being round
SS
just wanted to start a discussion about it, cos i've seen the "don't do it" replies many many times and don't understand their logic !!Who says that the earth aint a disk, this theory about it being round
SS
14 July 2002, 10:56 PM
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Check out the brake section of www.turnfast.com. This confirms what I have heard that brake balance is the key to faster stopping once you have eliminated fade under high heat bulid-up. Everyone gets distracted by "monster" front kits (note - all of the knowledgeable manufacturers sell proportioning valves). I am about to invest in a pyrometer to measure rotor temp differentials so that I can bring the rears more into play.
Good thread.
Good thread.
Trending Topics
15 July 2002, 11:09 AM
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Too much brake torque up front can cause a loss of traction at the rear if not balanced correctly. For your 335mm fronts an ideal rear would be the MY01/02 290mm vented rear with STi 2 piston rear callipers. This would help lower the centre of gravity and improve the performance both front and rear.
Steve
Steve
15 July 2002, 03:40 PM
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Sorry for being thick,
I have absolutely no technical knowledge also, if you take off the front 2Pots, could these replace the 1Pot on the rears? Probably not I expect, but wanted to ask.
Chris
I have absolutely no technical knowledge also, if you take off the front 2Pots, could these replace the 1Pot on the rears? Probably not I expect, but wanted to ask.
Chris
15 July 2002, 08:41 PM
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The piston area for a front calliper is too large for the rear and would upset the brake bias. This would probably cause the rear wheels to lock up prematurely. Thats of course if you could get them to fit.
Steve
Steve
15 July 2002, 08:43 PM
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Didn't say I was the only one that's ever thought about it just wanted to start a discussion about it, cos i've seen the "don't do it" replies many many times and don't understand their logic !!
16 July 2002, 01:43 AM
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Read what AndyF says.
The stronger your front brakes are, the less you want on the rear. Optimum brake bias is not a fixed setting, but is variable according to weight tranfer. On a major full-on stop with serious front brakes, the rears do virtually nothing other than stabilise the car. ("Virtually nothing" doesn't mean nothing at all, and the rears can get very hot, and they have crap cooling. Brake temperature is not a very good way to measure their stopping efficiency. In fact, it's useless.)
The last thing you want is powerful rear brakes that trigger the ABS before the fronts have reached their maximum. You'll run in to the back of things.
If you want to see how front/rear brakes work in a full-on stop, watch some bike racing. They use the back brake (operated separately on a bike) to maintain straight-line stability and as a trail-brake to aid turning. It's not for stopping and when the going gets hot, the back wheel is actually off the ground.
Regards,
Richard.
[Edited by Hoppy - 7/16/2002 1:55:08 AM]
The stronger your front brakes are, the less you want on the rear. Optimum brake bias is not a fixed setting, but is variable according to weight tranfer. On a major full-on stop with serious front brakes, the rears do virtually nothing other than stabilise the car. ("Virtually nothing" doesn't mean nothing at all, and the rears can get very hot, and they have crap cooling. Brake temperature is not a very good way to measure their stopping efficiency. In fact, it's useless.)
The last thing you want is powerful rear brakes that trigger the ABS before the fronts have reached their maximum. You'll run in to the back of things.
If you want to see how front/rear brakes work in a full-on stop, watch some bike racing. They use the back brake (operated separately on a bike) to maintain straight-line stability and as a trail-brake to aid turning. It's not for stopping and when the going gets hot, the back wheel is actually off the ground.
Regards,
Richard.
[Edited by Hoppy - 7/16/2002 1:55:08 AM]
16 July 2002, 02:24 AM
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There is a little window between locking up the rears and lifting the rears off the ground.
Its called the correct balance.
A serious race vehicle will have an adjustable bias control on the dash where the front and rear brakes can be adjusted to suit the weight changes from fuel usage. A typical V8 supercar will have 343mm ventilated rears and 375mm ventilated fronts. 343mm is a lot of nothing on the rear.
Steve
Its called the correct balance.
A serious race vehicle will have an adjustable bias control on the dash where the front and rear brakes can be adjusted to suit the weight changes from fuel usage. A typical V8 supercar will have 343mm ventilated rears and 375mm ventilated fronts. 343mm is a lot of nothing on the rear.
Steve
16 July 2002, 12:33 PM
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I don't claim to be a expert, but most of the braking is done by the front and the rears keep the back end under control and provide very little retardation.
When Subaru initially fitted the 4 pots as std they only vented the rear disks.
The 2 pots originally fitted to the front will not fit (without a lot of fabrication work) and (as stated earlier) the piston area is to big.
Fitting a balance controller would indeed seem the simple solution if you were to fit better rear brakes, but who would be confident they had set it up perfectly?
A disk and pad upgrade is suficiant for most of us. Though if you were to fit say AP 6's and wanted maximum braking from the rears (again as stated earlier) you could fit the rear 2 pots from the late STI/RA's and 01/02 WRX's, though you would have to change the rear hubs to get them to fit.
For most of us this is a little over the top!!!!!
Quote
I believe that the fact people do not upgrade the rear brake discs and calipers is due to 3 main reasons:
- budget
- little education and availability from manufacturers
- lack of cognition of car owner
End Quote
Sorry but BOLLOX!!!!
1-Budget - to an extent. I don't have money to throw at the car
2-Education - (surly this comes under cognition (knowledge)
2.5-Availability - If someone made a direct replacement at reasonable money, that had been proved to work! I'd be interested as would a lot of people
3-Well I've think that's down to the indervidual and why this BBS is so popular ('cos we don't all know everything)
When Subaru initially fitted the 4 pots as std they only vented the rear disks.
The 2 pots originally fitted to the front will not fit (without a lot of fabrication work) and (as stated earlier) the piston area is to big.
Fitting a balance controller would indeed seem the simple solution if you were to fit better rear brakes, but who would be confident they had set it up perfectly?
A disk and pad upgrade is suficiant for most of us. Though if you were to fit say AP 6's and wanted maximum braking from the rears (again as stated earlier) you could fit the rear 2 pots from the late STI/RA's and 01/02 WRX's, though you would have to change the rear hubs to get them to fit.
For most of us this is a little over the top!!!!!
Quote
I believe that the fact people do not upgrade the rear brake discs and calipers is due to 3 main reasons:
- budget
- little education and availability from manufacturers
- lack of cognition of car owner
End Quote
Sorry but BOLLOX!!!!
1-Budget - to an extent. I don't have money to throw at the car
2-Education - (surly this comes under cognition (knowledge)
2.5-Availability - If someone made a direct replacement at reasonable money, that had been proved to work! I'd be interested as would a lot of people
3-Well I've think that's down to the indervidual and why this BBS is so popular ('cos we don't all know everything
)
16 July 2002, 03:35 PM
#15
The Impreza Chassis is poor at using the rears eficiently. It has a tendency to rear wheel lifting under heavy braking, which not all cars do due to their centre of gravity location. An escort Cosworth has a much more usefull platform for utilising front and rear brakes by comparison.
All cars are diferent, the race esprit i help run for example does about 40% of its braking on the rear, due to the fact its got very wide rear tyres and is mid engined.
Unfortunately, with the impreza, big brakes on the rear on tarmac doesnt really work. In the forests you can use a much closer 50/50 bias, but the car setup is masively diferent over any road cars out there.
A lot of brake bias can be built into the cars just by selecting the correct pad material. What the bigger AP brakes etc give you is much more consistent results each aplication and a much more capable system when it comes to heat management. On the road the standard 4 pots can be made to work extremely well, on track is where they suffer due to heat soak issues.
All cars are diferent, the race esprit i help run for example does about 40% of its braking on the rear, due to the fact its got very wide rear tyres and is mid engined.
Unfortunately, with the impreza, big brakes on the rear on tarmac doesnt really work. In the forests you can use a much closer 50/50 bias, but the car setup is masively diferent over any road cars out there.
A lot of brake bias can be built into the cars just by selecting the correct pad material. What the bigger AP brakes etc give you is much more consistent results each aplication and a much more capable system when it comes to heat management. On the road the standard 4 pots can be made to work extremely well, on track is where they suffer due to heat soak issues.
16 July 2002, 06:16 PM
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Please at least TRY to keep this friendly and on track guys, there is some very usefull info surfacing here, so lets not spoil yet another thread by throwing about personal insults.
Stuart
Stuart
16 July 2002, 07:25 PM
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John,
Im not trying to put words into your mouth but would you say the standard solids with single pot on the rear are sufficient, or given the opportunity would you change to vented rears with 2pots?
At the moment I have 4pots and braided lines all around, but possibly will change to something... depending on available wheels..
How do you go about choosing what pad to use where, without trying them all, are they rated in someway like tyres??
I dont like the way my car brakes, as I feel it is unstable when on the brakes with the rear being light just before you come off the brake and into the corner... (I have found this handy for arsing about.. but thats not the issue)
to contemplate this and my premature abs issues (I believe I could stop quicker without it (i know it doesnt make you stop quicker) i just mean that I could keep applying pressure to the brakes without the wheels locking in my opinion... ie abs shouldnt be doing anything) Anyway I was going to try and counteract this by say lowering the car 45 - 60mm at the back and 30mm at the front (94 5dr) using coilovers to try and counteract some of the weight transfer... this all depends on what it does to the standard brake bias thingy..
edited to add or should it be a higher rated spring (rated in lbs?)
Ideas Anybody..
David
[Edited by David_Wallis - 7/16/2002 7:26:53 PM]
Im not trying to put words into your mouth but would you say the standard solids with single pot on the rear are sufficient, or given the opportunity would you change to vented rears with 2pots?
At the moment I have 4pots and braided lines all around, but possibly will change to something... depending on available wheels..
How do you go about choosing what pad to use where, without trying them all, are they rated in someway like tyres??
I dont like the way my car brakes, as I feel it is unstable when on the brakes with the rear being light just before you come off the brake and into the corner... (I have found this handy for arsing about.. but thats not the issue)
to contemplate this and my premature abs issues (I believe I could stop quicker without it (i know it doesnt make you stop quicker) i just mean that I could keep applying pressure to the brakes without the wheels locking in my opinion... ie abs shouldnt be doing anything) Anyway I was going to try and counteract this by say lowering the car 45 - 60mm at the back and 30mm at the front (94 5dr) using coilovers to try and counteract some of the weight transfer... this all depends on what it does to the standard brake bias thingy..
edited to add or should it be a higher rated spring (rated in lbs?)
Ideas Anybody..
David
[Edited by David_Wallis - 7/16/2002 7:26:53 PM]
16 July 2002, 08:36 PM
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David, it sounds like you may have some suspension issues and it's not just a brake problem. Maybe tyres/pressures? I see where you're coming from with the lower springs but stiffer/newer struts might be a good idea.
JF's comments will be interesting
Richard.
JF's comments will be interesting
Richard.
16 July 2002, 09:13 PM
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You can do a lot with rear pads to balance the brake system. In fact there are pad compounds designed for this purpose.
One example of a balanced system which is relevant to Subaru is the MY02 STi
Front 326mm x 30mm.
Rear 316mm x 20mm.
Steve
[Edited by dbasteve - 7/16/2002 11:09:44 PM]
One example of a balanced system which is relevant to Subaru is the MY02 STi
Front 326mm x 30mm.
Rear 316mm x 20mm.
Steve
[Edited by dbasteve - 7/16/2002 11:09:44 PM]
16 July 2002, 11:04 PM
#22
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sasim,
Sorry if anyone thought I was having a go, I wasn't!! Hence&
and the last comment about nobody knowing everything (that includes me).
If anyone can prove to me that bigger rear brakes will improve the braking and handling performance considerably I'll be first in the que
JF,
I'd be very interested in your thoughts too.
Iain.
Sorry if anyone thought I was having a go, I wasn't!! Hence
&and the last comment about nobody knowing everything (that includes me).
If anyone can prove to me that bigger rear brakes will improve the braking and handling performance considerably I'll be first in the que
JF,
I'd be very interested in your thoughts too.
Iain.
16 July 2002, 11:09 PM
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Damn right I have some suspension / brake / tyres issues... the combination is ****e... and before you all say it I know the car was crash damaged...
I repaired it, and it is straight... and apart from the back door sitting proud by 2/3mm I welcome any inspection.. (the door can be altered when I can be r sed)
by inspection I mean car'o'liner, tape measure, piece of string between wheels... camber, castor, anything...
I straightened the car, and adjusted using tape measure and a hammer, a piece of string etc... and when I had geom. done he said it was pretty damn good considering... was some optiflex thing i had done... + other check just to make sure...
Wheels are MY00 5/6 spokes (never counted em!) with bridgestones on them... I hate the tyres!!
Suspension is probably std.. I replaced a rear strut as it was bent when I fixed the car... but seem pretty matched when bouncing the car and cornering..
Brakes are Brand new 4 pots + disks + pads + goodridge hoses with Dot 5.1
David
I repaired it, and it is straight... and apart from the back door sitting proud by 2/3mm I welcome any inspection.. (the door can be altered when I can be r sed)
by inspection I mean car'o'liner, tape measure, piece of string between wheels... camber, castor, anything...
I straightened the car, and adjusted using tape measure and a hammer
, a piece of string etc... and when I had geom. done he said it was pretty damn good considering... was some optiflex thing i had done... + other check just to make sure...Wheels are MY00 5/6 spokes (never counted em!) with bridgestones on them... I hate the tyres!!
Suspension is probably std.. I replaced a rear strut as it was bent when I fixed the car... but seem pretty matched when bouncing the car and cornering..
Brakes are Brand new 4 pots + disks + pads + goodridge hoses with Dot 5.1
David
16 July 2002, 11:43 PM
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John F
What is it about the Impreza that makes it inferior to an Escort cosworth regards rear braking ? If anything the Impreza has the better weight distribution and I would have thought the cars share a similar C of G and wheelbase ? What am I missing here ?
Andy
What is it about the Impreza that makes it inferior to an Escort cosworth regards rear braking ? If anything the Impreza has the better weight distribution and I would have thought the cars share a similar C of G and wheelbase ? What am I missing here ?
Andy
17 July 2002, 01:25 AM
#25
Oh Gawd, opinion attack
I did a huge amount of playing with the brakes on my westie before i found a good balance that allowed me to feel safe in the car in all conditions. The most important thing when it comes to brakes and safety on the road is ensuring the rears dont start to work more quickly than the fronts. Its not quite so critical on an Impreza as there is a link through the transmition that helps the brakes work more in sync anyway, but its still important.
I'll be honest with you, i'm just starting to play with my Impreza on the braking front (the sport had small brakes and i hardly used them, key to that car was being smooth and carying the speed through the corners), so will probably have a better understanding of the dynamics of the car in a months time. My current opinion on them is that the rears do bugger all in standard trim. Compared to a Cosworth the rears dont seem to be braking much at all. As an example, on my Sapphire cosworth i used to go through rear pads in about 80 miles on track, fronts used to last twice as long, using the same compounds (i drove it very hard on track).
So far in 4 weeks i have tried 4 diferent front compounds and 2 diferent brake setups, so far i have found the standard STi5 4 pots and disks the best for daily use, on standard STi pads, it felt a great balance. For fast track use though it isnt up to it, the main problem isnt brake force, its heat retention. Pushing on, even with EBC greens (which are crap by the way!) in the AP's they offer more consistency, which is half the battle.
The Impreza has a superb weight distribution due to the lightweight boxer engine and longitudinal gearbox, but its has an apauling tendency to dive under heavy braking and its rear roll centre is too high so it tends to want to do wheelies, which is bad for heavy braking stability (good for getting the back end out for power slides though). The Escort Cossie by comparison has a better rear suspension design and anti dive charicteristics under braking, the chasis stays flater. Put the two cars side by side with the same brake config and the cossie will win in stabilty and stopping power under braking. It loses on agility though.
I personally, so far think the key to making the Impreza work on braking, is to not use too agresive a pad up front and try to avoid the nose dive tendency, less braking power up front (these are all relative at the limit things) may actually be faster when you take into acount corner entry control. Making the suspension stiffer should improve things on braking, it could compromise you in other areas though and make the car slower overall, sometimes small braking forces and using trail braking is faster than nailing it on its nose then sorting out the weight transfer.
Dont give the car a negative rake though (**** lower than the front) It will understeer like a pig!
When i have worked my car out better, i'll be able to tell you what i think would be a good combination. To be honest, i dont think the one pot / two pot rear brakes are that significant. I would say the rear mechanical diff in mine is though, it works under braking and acceleration so does change the way the car behaves compared to a UK car, so what i find that works may not suit the UK car, but i think it will.
I would say absolute key to these cars though, is be smooth both in aplying and releasing the brakes, try and avoid that fast nose dive and try and make it a smooth transition, you will be faster in the long term if you learn to control that.
I did a huge amount of playing with the brakes on my westie before i found a good balance that allowed me to feel safe in the car in all conditions. The most important thing when it comes to brakes and safety on the road is ensuring the rears dont start to work more quickly than the fronts. Its not quite so critical on an Impreza as there is a link through the transmition that helps the brakes work more in sync anyway, but its still important.
I'll be honest with you, i'm just starting to play with my Impreza on the braking front (the sport had small brakes and i hardly used them, key to that car was being smooth and carying the speed through the corners), so will probably have a better understanding of the dynamics of the car in a months time. My current opinion on them is that the rears do bugger all in standard trim. Compared to a Cosworth the rears dont seem to be braking much at all. As an example, on my Sapphire cosworth i used to go through rear pads in about 80 miles on track, fronts used to last twice as long, using the same compounds (i drove it very hard on track).
So far in 4 weeks i have tried 4 diferent front compounds and 2 diferent brake setups, so far i have found the standard STi5 4 pots and disks the best for daily use, on standard STi pads, it felt a great balance. For fast track use though it isnt up to it, the main problem isnt brake force, its heat retention. Pushing on, even with EBC greens (which are crap by the way!) in the AP's they offer more consistency, which is half the battle.
The Impreza has a superb weight distribution due to the lightweight boxer engine and longitudinal gearbox, but its has an apauling tendency to dive under heavy braking and its rear roll centre is too high so it tends to want to do wheelies, which is bad for heavy braking stability (good for getting the back end out for power slides though). The Escort Cossie by comparison has a better rear suspension design and anti dive charicteristics under braking, the chasis stays flater. Put the two cars side by side with the same brake config and the cossie will win in stabilty and stopping power under braking. It loses on agility though.
I personally, so far think the key to making the Impreza work on braking, is to not use too agresive a pad up front and try to avoid the nose dive tendency, less braking power up front (these are all relative at the limit things) may actually be faster when you take into acount corner entry control. Making the suspension stiffer should improve things on braking, it could compromise you in other areas though and make the car slower overall, sometimes small braking forces and using trail braking is faster than nailing it on its nose then sorting out the weight transfer.
Dont give the car a negative rake though (**** lower than the front) It will understeer like a pig!
When i have worked my car out better, i'll be able to tell you what i think would be a good combination. To be honest, i dont think the one pot / two pot rear brakes are that significant. I would say the rear mechanical diff in mine is though, it works under braking and acceleration so does change the way the car behaves compared to a UK car, so what i find that works may not suit the UK car, but i think it will.
I would say absolute key to these cars though, is be smooth both in aplying and releasing the brakes, try and avoid that fast nose dive and try and make it a smooth transition, you will be faster in the long term if you learn to control that.
17 July 2002, 11:45 AM
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Interesting...
You dont rate the ebc's?? they were going to be my next trial... but was going to use red pads...
David
You dont rate the ebc's?? they were going to be my next trial... but was going to use red pads...
David
18 July 2002, 02:23 AM
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John F ......... what crap !
Joking Thanks for that reply, I've been checking out pads myself and to get more braking to the rear I'm fitting DS3000's
their coefficient of friction is 0.62 (IIRC) thats almost double the standard friction Even 1155 mintex are only 0.3 something !
I have fitted big Wilwoods to the front (big pistons) and now have a very light pedal so I need more friction to the rear, then I can fit a bias valve if necessary.
ps Disagree that the Scoob back end is worse than that old VW beetle swing arm design on the Cossie Wouldn't be normal if we agreed would it
Joking
Thanks for that reply, I've been checking out pads myself and to get more braking to the rear I'm fitting DS3000'stheir coefficient of friction is 0.62 (IIRC) thats almost double the standard friction
Even 1155 mintex are only 0.3 something !I have fitted big Wilwoods to the front (big pistons) and now have a very light pedal so I need more friction to the rear, then I can fit a bias valve if necessary.
ps Disagree that the Scoob back end is worse than that old VW beetle swing arm design on the Cossie
Wouldn't be normal if we agreed would it
18 July 2002, 02:03 PM
#30
i am not saying the back end is worse in all conditions, its definately worse under dive. The whole chassis is.
DS3000 will destroy the laquer on your wheels, if they are for the road, dont use them. Coeficient of friction is not the most important factor, the way you can modulate the bite is very important, especially on the rear!
DS3000 will destroy the laquer on your wheels, if they are for the road, dont use them. Coeficient of friction is not the most important factor, the way you can modulate the bite is very important, especially on the rear!