ACR-X Brakes

[treesnake]

Are there any other experience with the ACR-X Brembo discs?
Easy to install or must be rebuilt?

The rotors are easy to install. When the rotors need to be replaced, you can order just the rings and reuse the center hat.

I used them for the entire track season last year. Other than typical track wear, I liked them a lot. Obviously the bigger pad and rotor
with more contact surface area, resulted in improved braking.

 

[ViperGeorge]

Were you using the same pad compound before and after the upgrade to ACR-X brakes? I guess I'm wondering whether the change you saw was more a result of a compound change or the size of the rotor/pad. The bigger pad would have less pressure per sq. inch when the brakes are applied as compared to the stock size pads given the force applied by the pistons is unchanged. A change in pressure would also require a change in master cylinder, booster, or caliper.

I have been told by an SRT engineer that the ACR-X brake package will reduce fade (if that is an issue) due to the larger rotor mass and greater surface area. Otherwise no real braking advantage would be realized. Now if there were a compound change that would have a resulting braking impact. The PFC 01 compound is quite agressive. I've used it before, but it is also pretty tough on rotors.

 

[LikeACR]

Hello,
thanks for the answers. I want to order the Brembo rotor for my 08 ACR.
Where I would like to order the Brembo rotors it nor know:

I need to know what hardware you have (whith or without springs?)


What pads would be a good alternative to the PFC? My dealer can not order the PFC. It should be a pad for Racetrack.

Excuse my English is not good and the translation makes google for me

 

[treesnake]

Were you using the same pad compound before and after the upgrade to ACR-X brakes? I guess I'm wondering whether the change you saw was more a result of a compound change or the size of the rotor/pad. The bigger pad would have less pressure per sq. inch when the brakes are applied as compared to the stock size pads given the force applied by the pistons is unchanged. A change in pressure would also require a change in master cylinder, booster, or caliper.

I have been told by an SRT engineer that the ACR-X brake package will reduce fade (if that is an issue) due to the larger rotor mass and greater surface area. Otherwise no real braking advantage would be realized. Now if there were a compound change that would have a resulting braking impact. The PFC 01 compound is quite agressive. I've used it before, but it is also pretty tough on rotors.

I understand the principle but, I change the brake pressure with the pedal to the left of my "traction control"
pedal...

For example, say I was to use the same compound on two different size pads. One being a large pad the other small.
Now you are on track and flying into the corner. Which size pad will be the most effective? The pad with larger pad/rotorcontact, or the pad with less contact? Remember, YOU are in control of pedal/brake pressure
My point is that you will physically apply as much brake pressure as needed.
It has been my experience that a larger contact area would get you there faster.

Now if you we're inhibitted in some way and could only apply literally 25# of pedal pressure on the brakes, I would see your point of the pressure being spread out with a larger pad....

I think a fair comparison would be tires. Which tire would give you better straight traction, a 275/18 or
345/18?
Both tires being the same brand, compound etc.
The 345 has a lighter footprint but more contact...?

I do realize you are speaking more to the scientific side where I am talking real life.

And as far as the X rotors.... We really don't have a choice of pad brands as PFC is the only one making
the large annulus pads. And I agree, they are very ******* rotors...

 

[lmcgrew79]

Mike i think the mitsu evo uses the same size pad and since when did you start using brakes?

 

[treesnake]

Mike i think the mitsu evo uses the same size pad and since when did you start using brakes?

I don't. I just like the way the rotors look through the wheel spokes......



BTW... I've got a couple track dates to go over with you...

 

[Viper X]

"And as far as the X rotors.... We really don't have a choice of pad brands as PFC is the only one making
the large annulus pads. And I agree, they are very ******* rotors..."


Treesnake,

Most of what you are saying is dead on, real world stuff. The driver controls the pressure on the brake pedal. I've run the same compound on the smaller rotors and on the larger rotors. The larger rotor / pad combination stops the car better, period.

However, PFC pads are not are only choice with the larger Brembo rotors. Call Porterfield Brakes and ask for Tim, the manager. Tell him I sent you. I've been buying the larger and thicker Raybestospads from them for over a year now. They work great, are easier on rotors and no more $$$ than the PFC's.


Dan

 

[treesnake]

"And as far as the X rotors.... We really don't have a choice of pad brands as PFC is the only one making
the large annulus pads. And I agree, they are very ******* rotors..."


Treesnake,

Most of what you are saying is dead on, real world stuff. The driver controls the pressure on the brake pedal. I've run the same compound on the smaller rotors and on the larger rotors. The larger rotor / pad combination stops the car better, period.

However, PFC pads are not are only choice with the larger Brembo rotors. Call Porterfield Brakes and ask for Tim, the manager. Tell him I sent you. I've been buying the larger and thicker Raybestospads from them for over a year now. They work great, are easier on rotors and no more $$$ than the PFC's.


Dan

What...!!!!!.

I have been told on several occasions that PFCs were the only choice...???

Thanks VIPERX, I will be looking into them.....

 

[Viper X]

What...!!!!!.

I have been told on several occasions that PFCs were the only choice...???

Thanks VIPERX, I will be looking into them.....

I've only told a few folks about these.....now anyone who reads this will know.

We need to work together to beat up on ZR1s, Porches, etc., etc.

 

[ViperGeorge]

I understand the principle but, I change the brake pressure with the pedal to the left of my "traction control"
pedal...

For example, say I was to use the same compound on two different size pads. One being a large pad the other small.
Now you are on track and flying into the corner. Which size pad will be the most effective? The pad with larger pad/rotorcontact, or the pad with less contact? Remember, YOU are in control of pedal/brake pressure
My point is that you will physically apply as much brake pressure as needed.
It has been my experience that a larger contact area would get you there faster.

Now if you we're inhibitted in some way and could only apply literally 25# of pedal pressure on the brakes, I would see your point of the pressure being spread out with a larger pad....

I think a fair comparison would be tires. Which tire would give you better straight traction, a 275/18 or
345/18?
Both tires being the same brand, compound etc.
The 345 has a lighter footprint but more contact...?

I do realize you are speaking more to the scientific side where I am talking real life.

And as far as the X rotors.... We really don't have a choice of pad brands as PFC is the only one making
the large annulus pads. And I agree, they are very ******* rotors...

Curious. I hear you on the tire comparo. I'd have to sit down and do some math on that problem. Re the brakes, I literally got my explanation from a senior SRT engineer on Monday. You would know him by name, I've known him for several years and he is quite adept on track. He says he has tested the X brakes on a street car and the stopping power was unaffected although they are less subject to fade. Again this assumes the same compound. He gave me an explanation that included piston pressure vs. rotor radius. Here's exactly what he said "The brake torque on a given corner comes down to the radius of the rotor rubbing surface and the force exerted by the pistons, so long as the pad coefficient it the same. If you want more front brake power, those parts need to be upsized. The ACR-X kit doesnt go there. I have tested the wider annulus rotor and larger pad on the production car. It does improve fade and wear."

When I'm on track I am ******* the brakes with maximum effort when I need to slow down, not much more to give I think. I would therefore be applying the max pressure I could.

 

[Viper X]

The other variable in the instance you quote is front tire size and thus footprint. Most of us run larger sizes up front on the track, so comparing street car response would not be an "apples to apples" comparison.

Dan

 

[TrackAire]

I've only told a few folks about these.....now anyone who reads this will know.

We need to work together to beat up on ZR1s, Porches, etc., etc.

Apparently SRT needs to hire ViperX as a brake consultant for the new Gen 5, at least when running at Laguna Seca!.....lol

Thanks for the info on the pad upgrade.

Cheers,
George

 

[Viper X]

Apparently SRT needs to hire ViperX as a brake consultant for the new Gen 5, at least when running at Laguna Seca!.....lol

Thanks for the info on the pad upgrade.

Cheers,
George

Hey George,

You would need to upgrade to the Brembo "larger" rotors too....

Dan

 

[xr4racer]

Carbotech makes the larger ACR-X pads as well

matt

 

[ViperGeorge]

For a thorough discussion of this topic see the StopTech website - http://stoptech.com/technical-support/technical-white-papers/brake-system-and-upgrade-selection

They specifically state that "Increasing the pad area will not increase the clamping force." and "Braking torque in pounds-feet on a single wheel is the effective disc radius in inches times clamping force times the coefficient of friction of the pad against the disc all divided by 12. " However, they also state that a larger surface area will disipate heat better. This is exactly what the SRT engineer stated. To increase braking force you need to increase clamping force, improve the pad compound, or increase the radius of the rotor. Increasing surface areas by itself does not increase braking, it only improves the brake system's ability to disipate heat and hence reduces fade. The ACR-X brakes increase surface area, they do not increase the clamping force nor the size of the rotor's radius.

I would stand by my previous post that any perceived improvement in braking is most likely from a change in pad compound (PFC 01 is very agressive compared to other compounds you may have run). Now you will notice less fade if that was previously a problem.

Regarding tire width, a question that was posed in an earlier post, after reading several forums on physics the best I can tell is that a wider tire has the same sized contact patch as a narrower tire assuming similar construction, air pressure, and vehicle weight. The difference is that a wider tire has a wider but shorter contact patch while a narrower tire has a longer and narrower patch. The longer/narrower patch actually provides better acceleration in a straight line but the wider patch generates more lateral grip (better cornering). There are apparently a lot of variables so no easy forumula exists to describe this.

 

[treesnake]

There are a few alternatives for track brakes. There's a six pi$ton that works great but isn't cheap.

I got my hopes up with this new 4 piston built for the track. You can order the calipers to use with the 14" X-rotors or you can
upgrade to 15" ACRX type rotors....

Problem right now is that they are over $7K.....

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[ViperGeorge]

I've now had the chance to read the entire link from StopTech - http://stoptech.com/technical-suppor...rade-selection - and I have to say it should be mandatory reading for everyone that tracks their cars or that wants to perform upgrades to their braking system. Really does a great job of explaining the factors that effect braking.

 

[TrackAire]

This has been a great thread, very imformative.

For the Gen 4 Viper, is there a preferred method for bleeding the brakes?...maybe what's the best way in a well equipped shop vs out at the track?

Cheers,
George

 

[ViperGeorge]

In a well equipped shop you can use one of several bleeding devices. Some connect to your compressor and the caliper and **** the fluid out using vacuum or you can use one that connects to the master and pushes the fluid out.

At the track you can just bleed the old fashioned way with a friend - pump, pump, hold. Outbound bleeder first, then inbound. Start at right rear, then left rear, then right front, then left front.

Or use a couple of tight fitting clear plastic lines connected to the caliper bleed screws placed into a bottle. Put a little fluid in bottom of bottle, connect hoses to both bleed screws, open both bleed screws, get in car, pump slowly about 8 times, check master cylinder, refill, repeat until fluid appears clearer in plastic lines, tighten bleed screws, done. Lines must be tight so that no air gets in where they connect to bleeder. I use one way check valves in lines to prevent old fluid from being sucked back into caliper when you let pedal go. Check valves are available at auto parts store.

Real old fashioned way is to connect lines to all bleed screws on all calipers, put all in a bottle, open all bleed screws and let gravity do the work. Have to take off all wheels though.

 

[TrackAire]

Ok, I was concerned if using one of the pressure style power bleeders might damage something within the anti lock brake system if you don't have the right kind of scanner to open or close the correct valves like some vehicles require.

Cheers,
George

 

[ViperGeorge]

Ok, I was concerned if using one of the pressure style power bleeders might damage something within the anti lock brake system if you don't have the right kind of scanner to open or close the correct valves like some vehicles require.

Cheers,
George

My Viper Tech used the one that connects to the caliper to bleed by ACR. (It was in for other service so I had him do it instead of doing it myself.) Watched him do it and he did not use the DRB III or a Star Scan to bleed the brakes. He only attached the gizmo to the caliper and sucked away.

 

[mjorgensen Woodhouse]

The DRB is supposed to be used during bleeding (when possible) to bleed and circulate the ABS unit and lines fluid where remaining contaminants and air can be trapped. We do it on all ABS brake flushes.

 

[Longdaddy]

I have used ACR-X rotors with a few variations on pad compounds (all from Carbotech, they can make these in any compound, the shape is "EVO VIII/IX/X, CTS-V Caliper - .630"/16mm ")

Some observations:
1. I had to go to less aggressive compounds, in both front and back (I run racingbrake rings on original stoptech hats in the back), to dial in initial application/bite, probably due to rotor material/surface and groove shapes
2. The rotors are now moderately warped after about 35-40 heat cycles. I used to warp stoptech rings to where they were unusable in 20-25 heat cycles.
3. They are truly "floating" rotors (unlike stoptech). Once they go through the first few real heat cycles, they are "track only" rotors, low speed noise/vibration are too much for street driving, especially with worn pads. Also they create more wear on the pads, I think J shaped grooves are the main reason. I like to drive the car to the track, and that experience is a lot less nice with these rotors.

Replacement rings will run $450-500 each depending on where you buy. I am looking to see if Racing Brake can make a replacement ring. If not, I will probably go to Racing Brake ring on the stock Stoptech hat in the front.

I would say that if you race for a trophy, like to run longer sessions (30+minutes), and/or have issues with heat buildup/dissipation, these are very nice.
If you just run track days they may or may not be cost effective depending on how much you get out of stoptech stuff
If you drive your car mostly on the street, these rotors are just stupid (cost, noise and pad wear)

 

[Longdaddy]

update: RB does have a replacement ring that seems to match the dimensions of ACR-X Brembo piece close enough to fit Brembo hat. It is much, much cheaper than Brembo part. I will be trying that in the next few days, will update. From the picture, it also looks like the slot geometry is a little less aggressive, which would hopefully help with pad wear and initial application torque. We'll see.

 

[ACRucrazy]

update: RB does have a replacement ring that seems to match the dimensions of ACR-X Brembo piece close enough to fit Brembo hat. It is much, much cheaper than Brembo part. I will be trying that in the next few days, will update. From the picture, it also looks like the slot geometry is a little less aggressive, which would hopefully help with pad wear and initial application torque. We'll see.

Thanks for the update! Good info in this thread!

 

[treesnake]

update: RB does have a replacement ring that seems to match the dimensions of ACR-X Brembo piece close enough to fit Brembo hat. It is much, much cheaper than Brembo part. I will be trying that in the next few days, will update. From the picture, it also looks like the slot geometry is a little less aggressive, which would hopefully help with pad wear and initial application torque. We'll see.

......

 

[Longdaddy]

RB "Mustang" ring fits the brembo racing ACR-X hat. There a couple of spots where the pad ends up overlapping the friction surface of the rotor, creating non-standard wear.

I do like the "feel" of initial application in high-speed braking sections, it is easier to modulate the initial engagement compared to the Brembo ring with both pad compounds I tried. It is also, as expected, much more street-friendly in terms of noise/vibration. I am working with Racing Brake to see if a version of the ring can be made that addresses the issues (relatively minor) with pad/ring contact surface. I am also going to be running more events with these rings to see how they hold up over time. Will update on the progress (most likely in July)

 

[bluesrt]

 

[Viperless]

There a couple of spots where the pad ends up overlapping the friction surface of the rotor, creating non-standard wear.

Can you elaborate on this? Not sure I understand what you're describing.

 

[Longdaddy]

Below is a picture showing pad wear for the "standard" EVO/CVS-V pad shape over RB ring.

I did grind my next set of pads to remove material in that area and that helped with this issue. I put about a dozen of heat cycles on them on a track that's very ******* the brakes - I wore a brand new set of Carbotech XP16 pads down to 3-4mm of material left and my rear calipers actually changed color permanently due to heat, but the rings held up just fine.

Waiting for the new set of rings from RB modified to make pad grinding unnecessary.

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