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Discussion Starter #1 (Edited)
After a few laps at a track (thread here: http://mazda3revolution.com/forums/auto-x/34738-4-laps-buttonwillow.html) it became clear that the 3i needed bigger rotors. Modding the 3 is difficult since it mostly functions as a daily driver, with the occasional mountain road scouting run and possible few laps at the track when I'm there as a support car. Also, I don't want to spend much money on the 3 as car money generally goes towards feeding my ever growing Lotus addiction.

Though the Mazdaspeed 3 front rotor swap is a popular mod, it wasn't for me since the MS3 setup would require more money and effort. I ended up deciding on swapping to 3s (2.5) rotors to gain 0.9 inches of rotor diameter in front and about a half inch of rotor diameter in the rear. With the lesser power of the SkyActiv compared to the 2.5, hopefully the increased sizes will help cure brake issues.

In order to do this swap, you'll need a set of 2.5 rotors. I purchased a set of Centric Premiums, though cheaper blanks are available. You'll also need rotor brackets from a 2.3-2.5, any year 3s model between 2004 to 2013 will do. I got mine from a junkyard. In my opinion, here are the pros and cons of the 2.5 setup as compared to the Mazdaspeed 3 setup:

Pros:
1. Cheaper. The 2.5 only requires new rotors and brackets, versus requiring a new caliper and rotors. Also, the 2.5 rotors are generally cheaper. I got my setup for about $250. If you hunt around, I think you can go cheaper. Since I upgraded the brakes on my Lotus and ended up selling the stock Lotus parts for around that amount, this is the ultimate hand me down brake upgrade.

2. Easier to install. The Mazdaspeed 3 install requires installing a caliper and the attendant mess of brake fluid while this just needs some brackets to be swapped and so is a total bolt on.

3. Can fit inside 16" wheels. Since I've been procrastinating on installing my NC wheels, this is a big deal.

4. No master cylinder sizing issues. This is a debatable pro, but we know that at least there is no master cylinder misplace as compared to factory specs.

5. Less unsprung weight.

Cons:
1. Less mass so less heat capacity. I don't think a SkyActiv with Mazdaspeed 3 front brakes will ever fade the front brakes. Interestingly though, my fade on track was from the rear rotors.

2. Less leverage. Of course there's the debate about tire grip versus braking power, but I won't get into that here.

3. Less cool. Quite simply, huge Mazdaspeed 3 brakes are cool. This is sort of a dorky old school 90s Honda mod.

The process:
If you've ever done a rotor change, then this is easy. I won't get into details here since there are plenty of good tutorials on this forum. The only additional item is that the brackets of each rotor needs to be swapped to the 2.5 brackets. This is easily done and only requires undoing four bolts per corner. A helpful hint is to be sure to remove the safety wires so that bracket removal is easier.

So on to the pictures!

Front rotors stock and new (an easy way to compare sizes is to look at the dust shield):
IMAG0906.jpg
IMAG0908.jpg

Rear rotors stock and new:
IMAG0910.jpg
IMAG0909.jpg

Pictures of new rotors installed. Note how close to the front is to the inner barrel of the wheel:
IMAG0916.jpg
IMAG0919.jpg
IMAG0923.jpg

Bonus picture from the shakedown run:
IMAG0915.jpg
 

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3. Can fit inside 16" wheels. Since I've been procrastinating on installing my NC wheels, this is a big deal.
At least some 15's fit fine too (just barely) providing even further weight and gearing advantages. Here are 15x7.5" Rota Slipstreams on my 2.3L S. With 225/45/15 BFG Rivals the wheel/tire combo only weighs 33 lbs. The gear reduction and weight reduction really woke the car up.

 

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Discussion Starter #3
At least some 15's fit fine too (just barely) providing even further weight and gearing advantages. Here are 15x7.5" Rota Slipstreams on my 2.3L S. With 225/45/15 BFG Rivals the wheel/tire combo only weighs 33 lbs. The gear reduction and weight reduction really woke the car up.

Damn that is close!
 

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Discussion Starter #5
LOL, yeah few mm to spare :D Several people said it wouldn't work. I took a gamble and ordered the wheels anyway. Glad I did, it's been one of the best performance upgrades I've done to the car.
I went 1" smaller in diameter for my Lotus as well. It really does help acceleration. An unexpected problem is that with a 27 pound total front assembly (9 pound wheel + 18 pound tire), tire balancing is tough to get right. I have a vibration at high speeds.

My rears are a 35 pound total assembly (12 pound wheel + 23 pound tire).
 

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Good writeup. I have a 3i also and will probably replace the original brakes soon and am considering this swap for a little braking improvement.
 

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Great writeup. My only comment is that if you have to go out and find the brackets for the 2.3/2.5 model, they might end up costing the same as if you went out to find Speed3/Mazda5 calipers. The places I went to didn't want to sell the brackets alone and I ended up buying the entire Mazda5 caliper just to get the bracket.

Speed3 brakes will probably fade on the Skyactiv if you've got the tire grip to really punish them. Just my guess, I'll be finding out for you soon anyways lol.
 

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Discussion Starter #8
Great writeup. My only comment is that if you have to go out and find the brackets for the 2.3/2.5 model, they might end up costing the same as if you went out to find Speed3/Mazda5 calipers. The places I went to didn't want to sell the brackets alone and I ended up buying the entire Mazda5 caliper just to get the bracket.

Speed3 brakes will probably fade on the Skyactiv if you've got the tire grip to really punish them. Just my guess, I'll be finding out for you soon anyways lol.
Interesting. I got my brackets from a junkyard in Irwindale. They wanted a bit more than what I would've preferred to pay, but some negotiating helped bring the cost down. I'm considering selling them my 3i brackets so they can have full working calipers again. I guess you just have to search to find someone willing to sell you the brackets, or convince them to end up with 3i calipers instead.

As for more tire grip, I really can't see that having an effect on brake fade. Sure you'll have higher corner exit speeds due to the extra grip, but you'll have also higher entry speeds because of the grip. The amount of energy dissipated by the brakes should be the same. If anything, I'd expect that extra grip will reduce the strain on the brakes as it allows higher cornering speeds and speeds in general. The higher the speed, the more the aerodynamic drag, more linear energy is expended to overcome the drag, and less energy required for the brakes to scrub off the speed (since the drag has already done part of that for you).
 

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You do save the brakes in a holistic, averaged-across-time sense. But instantaneously you'll be able to put a lot more heat into the brakes with more tire, so in a worst-case scenario you could shoot straight past 900F in just a few rapid stops.
 

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Interesting. I got my brackets from a junkyard in Irwindale. They wanted a bit more than what I would've preferred to pay, but some negotiating helped bring the cost down. I'm considering selling them my 3i brackets so they can have full working calipers again. I guess you just have to search to find someone willing to sell you the brackets, or convince them to end up with 3i calipers instead.

As for more tire grip, I really can't see that having an effect on brake fade. Sure you'll have higher corner exit speeds due to the extra grip, but you'll have also higher entry speeds because of the grip. The amount of energy dissipated by the brakes should be the same. If anything, I'd expect that extra grip will reduce the strain on the brakes as it allows higher cornering speeds and speeds in general. The higher the speed, the more the aerodynamic drag, more linear energy is expended to overcome the drag, and less energy required for the brakes to scrub off the speed (since the drag has already done part of that for you).

I'm sorry to point you out, but this up above is just plain wrong. It takes more energy to slow the car from 120mph to 100mph than it does from say 80 mph to 60 mph for example. In both cases you are dropping 20 mph but the former takes more energy to slow and a lot of that energy is converted into brake heat. Aerodynamic drag does play a role in slowing the vehicle but there is simply much more stored energy in a car moving 120 mph as opposed to 80, too much for wind drag alone to compensate for. 70 mph is approximately 15% faster than 60 mph. If we were to believe you then a 70-0 stop should be only 15% longer than a 60-0 stop because of wind drag and that is simply not the case. The distance differential is much more than 15%. The energy curve is NOT linear as you suggested.

Additionally, stickier rubber shortens stopping distances. When you shorten the elapsed time it takes to reduce speed that in turn produces more brake heat. A simple illustration of this is spinning a bike rim. If you lightly touch the braking surface of the rim it doesn't feel that hot to the touch but it takes a long time to stop it. Now apply much more pressure. What happens? The wheel stops spinning much faster and you said "OUCH" because you just burned your finger. Stickier rubber most certainly produces more brake heat and if you were to go into the clubroom at Buttonwillow for instance and proclaim that my brand X All Seasons can create as much brake heat, or even "strain" the brakes more than a set of Hooiser R6's they would get a good laugh, then tell you that you are wrong.
 

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Discussion Starter #11 (Edited)
I'm sorry to point you out, but this up above is just plain wrong. It takes more energy to slow the car from 120mph to 100mph than it does from say 80 mph to 60 mph for example. In both cases you are dropping 20 mph but the former takes more energy to slow and a lot of that energy is converted into brake heat. Aerodynamic drag does play a role in slowing the vehicle but there is simply much more stored energy in a car moving 120 mph as opposed to 80, too much for wind drag alone to compensate for. 70 mph is approximately 15% faster than 60 mph. If we were to believe you then a 70-0 stop should be only 15% longer than a 60-0 stop because of wind drag and that is simply not the case. The distance differential is much more than 15%. The energy curve is NOT linear as you suggested.

Additionally, stickier rubber shortens stopping distances. When you shorten the elapsed time it takes to reduce speed that in turn produces more brake heat. A simple illustration of this is spinning a bike rim. If you lightly touch the braking surface of the rim it doesn't feel that hot to the touch but it takes a long time to stop it. Now apply much more pressure. What happens? The wheel stops spinning much faster and you said "OUCH" because you just burned your finger. Stickier rubber most certainly produces more brake heat and if you were to go into the clubroom at Buttonwillow for instance and proclaim that my brand X All Seasons can create as much brake heat, or even "strain" the brakes more than a set of Hooiser R6's they would get a good laugh, then tell you that you are wrong.
No problem, a healthy debate is always good. As you correctly state, the energy curve is not linear (kinetic energy has a squared relationship to velocity, namely KE=1/2*m*v^2). However, I must point out that's true for acceleration as well as braking.

In a frictionless environment, it takes as much work, and thus energy, to accelerate from 70 to 80 mph as it does to decelerate from 80 to 70 mph. However, our real world has friction which tends to slow things down and act as parasitic losses. Friction dissipates speed and is a barrier to acceleration. Thus, in the real world it takes more total energy to accelerate a car from 70 to 80 mph as it does to decelerate it from 80 to 70 mph.

So what about the higher exit speed? Doesn't that mean you'll still end up dissipating more energy (and thus generate more heat) by decelerating the car from 100 to 90 mph as you would to decelerate from 80 to 70 mph? Well, that doesn't work either since, according to the work energy theorem, the change in work done equals the change in kinetic energy. The equation for work is essentially the same as kinetic energy [W=1/2*m*(vf^2-vi^2)]. Horsepower is a measure of work over time. Since your car can only generate a finite amount of horsepower and it requires more work (horsepower) to accelerate from 90 to 100 compared to accelerating from 70 to 80, in reality your car isn't going to accelerate from 90 to 100 when it previously accelerated from 70 to 80. Instead, if you raise your exit speed from 70 to 90, you'll expect to be going...

(Fun with equations time)
1/2*m*(80^2-70^2)=1/2*m*(vf^2-90^2)
The 1/2*m cancels...
(6400-4900)=(vf^2-8100)
1500+8100=vf^2
9600=vf^2
vf=~98!

However, the increase in kinetic energy (and thus the energy the brakes are required to dissipate) is the same going from 70 to 80 as it is going from 90 to 98. Therefore, you shouldn't be putting any extra load on your brakes by getting higher cornering speeds.

For this exercise, I did not take into account gearing or energy lost during acceleration due to aerodynamic drag. Drag we've already touched upon; it'll lighten the load on the brakes by decreasing the amount the car is accelerated. As for gearing, higher speeds tend to use higher gears, which has a lower torque multiplier, thus decreasing the amount of work you can put into accelerating the car and lightening the load on the brakes in the braking zone.

You do have a point in that the shorter braking zone should increase the temperature of the brakes by forcing deceleration during a shorter period due to the higher grip of the tires. However, I generally find that single braking applications aren't what causes fade; it's the repeated stomping on the brakes that ends up causing fade. And while higher grip tires will shorten laptimes, I have a feeling the higher heat concentration due to laptime gains will not be able to overcome energy loss due to aerodynamic drag from the higher speeds the car travels at. Obviously, I may be wrong, but I think we may need some numbers for the trade off to be resolved...

(Please let me know if any of the above contains errors. Especially the kinematic calculations.)
 
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