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      10-19-2011, 07:42 PM   #1
dapopa9
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BBK - front and rear?

I've noticed some are running the BBK in the front only. What are the cons of only do front and not rear if any? Is that bad or unsafe in anyway? thanks for the help with this!
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      10-19-2011, 08:15 PM   #2
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Not unsafe in anyway...maybe after a full day at the track and the rear pads/disc's have finally had it but not unsafe on the street. Won't really make any difference in stopping distance either, that's a function of the tires gripping. The stock brakes are powerfull enough on the street.

Although it'll look like crap or you ran out of money or something. It'll look stupid IMHO. Just do all four wheels. Your choice of BBK though, totaly up to you on color & brand/cost, etc.
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      10-19-2011, 08:19 PM   #3
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And guess I should also ask is there a large benefit to upgrading the back to BBK in addition to the front? My understanding is the fronts do the majority of the braking.
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      10-19-2011, 09:47 PM   #4
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Looks like I may have found some answers,,,article on StopTechs website says basically NO need for Rears. Funny that they also sale them! haha. kinda long but good read:

"
Rear Brake Upgrades ... Is Bigger Really Better?
by James Walker, Jr. of scR motorsports

February 18, 2001

One of the most common questions received from new owners of our front brake upgrade kits is "Do I now need to upgrade my rear brakes too?" To answer this, we need to look at the role of the rear braking system from a few different perspectives. The answer may surprise you, especially hearing this from a company that sells big brake upgrades!
Rear Brake 101

One of the many design factors that goes into the development of a base braking system is the mysterious "bias" or "balance." Truth be told, it's a pretty simple concept to grasp: for vehicle stability under braking, it is required that the rear brakes do NOT lock before the front brakes. Simple, right? Most of you probably knew that already.

OK, so what governs the 'lock up' point of the rear brakes? Drum roll, please:

1. tire tractive capability (friction)
2. tire normal force (weight on the tire)

This can be proven from looking that the fundamental relationship for maximum sustainable tire force: F=N, where:

F = the lock up point, or peak force
= tire-road coefficient of friction
N = normal load sitting on the tire

So, when the OEM is designing a brake system, they 'size' the system components (calipers, master cylinder, rotor OD, etc.) to generate the proper amount of torque at both ends of the vehicle so that the front brake force ('F' above) exceeds its peak traction first. At this point, the front brakes lock and the car slides in a nice, stable straight line. (Note that at STOPTECH, we design our front brake upgrade kits in the very same fashion for the very same reasons.)
Potential Impacts of Big Front Brakes

Fortunately (from a safety standpoint anyway), when most big-brake suppliers adapt a mondo rotor and caliper package to a vehicle, they end up actually increasing the FRONT bias. How? By increasing the effective caliper piston area and the rotor effective radius, these two factors work together to increase the 'mechanical gain' of the front brakes, building more torque for the same pressure, everything else being equal. So, from a bias perspective we are not pushing the vehicle toward instability, but rather just the opposite - we are underbraking the rear axle! The obvious impact would be an increase in stopping distance - probably the one thing the new owner was actually hoping to reduce. Ironic. So, say you chose to install these big brakes on the front axle but want to maintain the OEM bias. What's the answer? Well, one way would be to invest in big rear brakes too which increase the rear mechanical gain to the point that the system is balanced once again.
So, What's The Harm In Doing That?

Well, let's look at why we upgraded the front brakes in the first place. Contrary to popular belief, the real reason sports- and racing cars use big brakes is to deal with heat. Period. There has been a bunch of stuff published which will disclaim this, but when you look at the braking system from a design standpoint, making them 'bigger' doesn't fundamentally do anything for stopping distance. It's all about the heat. So, you upgraded the front brakes because of thermal concerns but as a hidden surprise got a shift in brake bias. As a band-aid to this condition, you now spend thousands more on a rear brake upgrade because the front system was not sized correctly in the first place. Sure, it looks great, but there is another option...
Which Is?

When upgrading your front brakes, it is possible to size the caliper pistons and rotor effective radius to maintain the original brake system's pressure-torque relationship. Yea, it takes more engineering know-how and you can't sell the same part to everyone anymore, but you are not altering the base brake balance from what the OEM intended. This design philosophy stands behind every brake upgrade kit STOPTECH manufactures. Now, if you sized the front brakes correctly, why would you need to change the rear brakes? Good question. If there are no thermal concerns with the rear brakes (and on a front-engine street car there rarely are) then by installing a rear big-brake kit all you are doing is (a) spending money and (b) adding unsprung weight. This is not usually viewed as favorable, unless you like driving a heavy, expensive car.
Oh Yea - One More Thing…

Finally, under an OEM bias condition, the rear brakes only contribute about 15-20% of all the braking force the vehicle generates, and when you install sticky tires you actually DECREASE the amount of work they need to do. Why? Because at the higher deceleration levels afforded by race tires, there is more weight transfer taking place, reducing the normal force on the rear tires and increasing it on the front (remember F=N from above?). If anything, we now want to decrease the rear effectiveness. Ironic once again.
Of course, if you decide to upsize your rear brake system components you can also impact the front-rear torque relationship, and consequently you can "bias" the "balance" more toward the rear. Go too far, and the rear brakes could lock before the fronts. Again, not the end result you were expecting, right?
It has been said that "The folks at STOPTECH should consider developing a rear kit to match their front setup. They'll be very happy with the performance improvement if done properly." Well, since our FRONT systems are designed properly, we save you the need to spend your money on the back axle.

Let's reword that quote to reflect the STOPTECH philosophy: "Our competitors should consider developing a FRONT kit to match their stock bias condition. They'll be very happy with the performance improvement if done properly, AND will save their customers the cost of a rear brake upgrade in the process."

<< Learn More

<< StopTech Home


COPYRIGHT 2004 STOPTECH LLC"
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      10-19-2011, 09:48 PM   #5
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And from a thread here also - http://www.m3post.com/forums/showthread.php?t=323391
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      10-19-2011, 09:53 PM   #6
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intersting.
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      10-28-2011, 11:49 PM   #7
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Great info i thought when you want your brakes to be "balance" when upgrading to BBK is to get both front and rear setup. Then again go big or go home even just for looks.
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      10-29-2011, 12:58 AM   #8
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There are advantages to having a rear kit, but as long as you can keep an appropriate braking bias front-to-rear, the best bang for the buck is the front kit, as it does most of the braking on a front engine/transmission car like ours and therefore will benefit the most from better thermal management of big brake kits.

Oh and StopTech's article is not widely known for an obvious reason... Why sell half a burger when you can sell a whole burger?
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      10-29-2011, 01:30 AM   #9
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Nice article.

I believe that certain classes in the Continental Grand Am series, upgrading the rear brakes is not allowed. So likely there is no safety concern considering those are fully prepped and sponsored race cars running just upgraded fronts.
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      10-29-2011, 01:16 PM   #10
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It's about a lot of different variables that aren't always obvious. The M3 works its rear brakes pretty hard, and with stiff springs (less weight transfer) and Rcomps you load them up even more. Other cars, like my new Mustang, are quite different in that regard, but that's too OT for this thread.

If I had to pick the single most challenging issue with the stock rear brakes on an M3, it would be that of finding good track pads to match with the front. Decent BBK pads are available from lots of vendors. Finding matching rears that retain good balance and don't overload the thermal capacity of the rear brake assembly is harder than it looks.

For street use this is not an issue - you don't need a BBK in the first place.
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      11-01-2011, 09:55 AM   #11
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Like others have mentioned the front does most of the braking. You need to keep the correct brake biased. A simple way of putting it is if you are coming into a braking zone and 80% of the weight transfers to the front, you will need 80% brake bias for the front of the car and 20% for the rear. This isn't an exact explanation just might help clear it up some.
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      11-01-2011, 11:24 PM   #12
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Quote:
Originally Posted by JAJ View Post
It's about a lot of different variables that aren't always obvious. The M3 works its rear brakes pretty hard, and with stiff springs (less weight transfer) and Rcomps you load them up even more. Other cars, like my new Mustang, are quite different in that regard, but that's too OT for this thread.

If I had to pick the single most challenging issue with the stock rear brakes on an M3, it would be that of finding good track pads to match with the front. Decent BBK pads are available from lots of vendors. Finding matching rears that retain good balance and don't overload the thermal capacity of the rear brake assembly is harder than it looks.

For street use this is not an issue - you don't need a BBK in the first place.
You are spot on JAJ .
I've never experienced any fade from the rear brakes (race pads), but have gotten them hot enough to melt the dust boots, so I know they're working. My biggest complaint was consistency throughout a 20 minute session. I could feel the bias changing as a session wore on and was loosing precious time in the brake zones because I never had the confidence in the brakes. I'm hoping my soon to arrive 4 wheel kit will resolve this.
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      11-01-2011, 11:54 PM   #13
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Quote:
Originally Posted by JAJ View Post
It's about a lot of different variables that aren't always obvious. The M3 works its rear brakes pretty hard, and with stiff springs (less weight transfer) and Rcomps you load them up even more. Other cars, like my new Mustang, are quite different in that regard, but that's too OT for this thread.

If I had to pick the single most challenging issue with the stock rear brakes on an M3, it would be that of finding good track pads to match with the front. Decent BBK pads are available from lots of vendors. Finding matching rears that retain good balance and don't overload the thermal capacity of the rear brake assembly is harder than it looks.

For street use this is not an issue - you don't need a BBK in the first place.
A good place to ask would be Turner or Fall Line since they run BBK front and stock rear calipers... but they probably won't tell you a whole lot for competitive reasons.
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      11-02-2011, 12:13 AM   #14
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er.. isn't this kind of a moot point since brake bias at each corner is electronically controlled? you can't really 'screw up' the bias on the rear as the car will let up on the rears if it detects lock-up.
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      11-02-2011, 12:14 PM   #15
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Quote:
Originally Posted by SECRET M View Post
er.. isn't this kind of a moot point since brake bias at each corner is electronically controlled? you can't really 'screw up' the bias on the rear as the car will let up on the rears if it detects lock-up.
It's not a bias issue per se, it's more a question of whether the rears are and remain effective. Too much rear bias might not affect handling, but more likely the brakes will overheat. Too little and the fronts do all the work and they overheat. It's a Goldilocks problem.
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      11-02-2011, 12:44 PM   #16
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Quote:
Originally Posted by SECRET M View Post
er.. isn't this kind of a moot point since brake bias at each corner is electronically controlled? you can't really 'screw up' the bias on the rear as the car will let up on the rears if it detects lock-up.
It is true that when the car detects wheel lock up, ABS will be triggered, however, when ABS is triggered the braking efficiency from that particular wheel/axle will be reduced (at least on dry surfaces) compared to the maximum braking that wheel/axle can generate using proper braking techniques. This adds to the stopping distance, which isn't desirable on the race track.
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      11-04-2011, 01:57 AM   #17
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Quote:
Originally Posted by dapopa9 View Post
making them 'bigger' doesn't fundamentally do anything for stopping distance. It's all about the heat.
Someone needs to sticky this. Finally an authoratative source saying what so many people argue and question. So many people buy these BBK and comment how their stopping distance was sooo much better when this is completely false. Clearly most who buy these packages do not even understand why they are buying them. I believe many do it for looks and then secondarily think they "stop more quickly" HOpefully people will believe stoptech now that its in writing
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      11-04-2011, 03:42 AM   #18
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Quote:
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Someone needs to sticky this. Finally an authoratative source saying what so many people argue and question. So many people buy these BBK and comment how their stopping distance was sooo much better when this is completely false. Clearly most who buy these packages do not even understand why they are buying them. I believe many do it for looks and then secondarily think they "stop more quickly" HOpefully people will believe stoptech now that its in writing
This has been discussed here in the past in many different threads and in gory details.

In short nearly any OEM or BBK can lock up all wheels on a car. In such cases stopping distances are mostly governed by tire and road surface. Minor factors will be ABS algorithm, weight distribution in car, response time of brake system, suspension subtleties (as they relate to weight distribution, brake dive, etc.). Thus single stops are almost never a measure of brakes but of tires and chassis, etc.

The real challenge for brakes is under repeated braking. Higher temperatures generally cause lower friction with the pad and when things get even hotter under the most extreme braking fluid can boil leading to a soft or to the floor pedal without enough braking torque to lock up the wheels.

It should also be obvious but really good tires put more stress, literally and figuratively, mechanical and thermal on one's braking system.
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      11-04-2011, 08:37 AM   #19
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Turner's Continental GS series car runs Stoptech Trophy ST-40's at the front and stock rear calipers by competition rules. I was a little stunned. They use plenty of cooling! I figure if this is good enough for Turner's race car, it is good enough for my occasional tracker.

I am sticking ST-40's on the front in the Spring, and maybe the rears later in the year if my rears start getting hammered. I started eating front pads, especially with Apex ARC-8 10" fronts with 275/35/18 tires. These were NT-05s, so if I get a stickier tire, it is only going to get worse. Next step is cooling ducts...thanks to Apex for turning out ducts for a reasonable price!

An alternative approach was proposed by my instructor buddy when discussing my brake pad consumption - "Brake less!" LOL!

It is also interesting to me how much harder I hammer the brakes at the track now than when I started, without even being conscious of doing it, even when I am taking it "easy".
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      11-04-2011, 10:44 AM   #20
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Quote:
Originally Posted by dapopa9 View Post
Looks like I may have found some answers,,,article on StopTechs website says basically NO need for Rears. Funny that they also sale them! haha.
Actually this kind of article could make any brake manufacturer sell more kits, and I think this was StopTech's intention too. In this upgrade area, price is a huge issue because there are large sums of money involved. If you are thinking about upgrading your brakes, it's one thing to know (for sure, from the manufacturer's words) that you could do it with $3000 and quite another to know that you need at least $6000 for the upgrade.

More people will upgrade their brakes due to this article, even if only half of the braking system. But for StopTech it doesn't matter, they sell more kits and when the customers are thinking to upgrade the rears, of course they will buy StopTech since it should match the front
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      11-05-2011, 10:34 AM   #21
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Quote:
Originally Posted by LiM3y View Post
Turner's Continental GS series car runs Stoptech Trophy ST-40's at the front and stock rear calipers by competition rules. I was a little stunned. They use plenty of cooling! I figure if this is good enough for Turner's race car, it is good enough for my occasional tracker.

I am sticking ST-40's on the front in the Spring, and maybe the rears later in the year if my rears start getting hammered. I started eating front pads, especially with Apex ARC-8 10" fronts with 275/35/18 tires. These were NT-05s, so if I get a stickier tire, it is only going to get worse. Next step is cooling ducts...thanks to Apex for turning out ducts for a reasonable price!

An alternative approach was proposed by my instructor buddy when discussing my brake pad consumption - "Brake less!" LOL!

It is also interesting to me how much harder I hammer the brakes at the track now than when I started, without even being conscious of doing it, even when I am taking it "easy".
Those cars run a huge amount of cooling, so basing our use on what they use is not going to render the same result.
Great pic BTW.
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