X-drilled & Slotted Rotors CAN be cut!

[CJ_Boxster]

Well as some of you know, i bought some x-drilled & slotted rotors by Zimmerman and about 2-3 months after installing them on my daily driver boxster, they warped. I torqued my wheels way too tight so they warped... stupid me, i went way over the 90 something odd lbs of torque that i should have had.

I asked on several forum if anyone knew if they can be cut and many mentioned that shops will not want to cause the holes and slots could break there rotor cutting tool

So, I went to my fathers shop to try and have the Rotors cut, Rather than cutting 5000th of an inch into the rotors like on any normal rotor, my father recommended going at 1000th of an inch at a time. We made about 4-5 passes on the rotor and the cut was perfect and smooth again!

So, if anyone wants to have there warped x-drilled & slotted rotors cut... you can on any run of the mill rotor cutting machine just as long as you make cuts at 1000th of an inch at a time till the entire surface is cut clean.

[Lil bastard]

It not a question of if you can machine the rotors, but whether you should. Porsche says you should not.

These rotors, because of their slots and holes require a greater thickness to maintain their overall strength than a solid rotor would. Cutting them down weakens them and makes them more likely to crack or shatter. Also, a rotor is basically a heat sink and the less material you have, the less efficient it is at absorbing and dissapating the heat energy from the system. It is exactly this heat, or more appropriately, energy transfer which stops the car.

On a warped rotor, the maximum allowable runout is 0.3mm (0.118"). And the max Peak to Valley roughness is 0.006mm (0.0002"). That means you don't get many passes to get it right and still maintain the required amount of material for strength and efficiency.

The other thing will be to find a shop willing to turn them on a lathe. This is not only because of worries about lessening their strength and making them prone to cracking and shattering, but also because the slots and holes really mess up the cutting bit on the lathe in a hurry. The shop will not make a profit replacing the cutting bits so quickly, unless they charge you quite a bit extra.

I wouldn't do it. Brake rotors are a consumable item just like the pads are. Gotta pay to play.

[CJ_Boxster]

I see your point and that infact if one of the reasons shops dont like to cut them, I went ahead and did it myself so if they fail, its my fault and ill let you all know but i did notice the Zimmerman rotors i purchased were alot thicker than my original OEM rotor. I dont know if i cut too much of material off the rotor by your measurments but I do know i cut 4000th of an inch from the surface of the rotor... is that to much or am i in the safe zone still ?

[Lil bastard]

Well, there's no way to tell if you did not measure the lateral runout or put a micrometer on the disc before you started.

The thickness of an OEM Boxster S disc is:

• front / rear
  28 mm / 24mm

The minimum allowable thickness of an OEM Boxster S disc is:

• front / rear
  26.0 mm / 22.0 mm

But, the OEM discs, while sometimes supplied by Zimmerman to Porsche for the 'S' model, are not the same discs that Zimmerman offers the public as aftermarket discs, so the allowances listed above may not be the same. But, the Zimmerman aftermarket x-drilled & slotted rotors have been reported to crack more readily by several owners who have installed them.

I'd measure your discs now w/ a micrometer and if inside the minimum allowance for the OEM disc, keep an eye on them for cracking, such as when you wash the wheels etc. Replace as soon as you see any cracks between holes forming. If outside the minimum allowance, I'd think about replacing them for safety's sake.

[CJ_Boxster]

Do you have thickness's for NON-S boxster, which is what i have?

[Lil bastard]

Base Boxster OEM non-drilled solid discs new:
front rear
24mm 20 mm

Brake discs Minimum thickness after machining*
front rear
22.6 mm 18.6 mm

* (from Porsche Workshop Manual) The brake disc may be reworked only symmetrically. ie. from both sides evenly - important point - don't just turn one side, take the material evenly from both sides!

Wear Limit
front rear
22.0 mm 18.0 mm

These are the specs for the Base solid discs. But, IMO, you should go by the 'S' specs as these are the ones the Zimmerman aftermarket discs are imitating.

[CJ_Boxster]

Originally Posted by Lil bastard

These are the specs for the Base solid discs. But, IMO, you should go by the 'S' specs as these are the ones the Zimmerman aftermarket discs are imitating.

Ah ok, Well on Thursday ill be at there shop working on there PC so ill have them measure the rotor thickness... OH BTW we did cut both sides evenly. If the outter section needed another 1000th of an inch cut, we cut the inner side another 1000th of an inch.

Ill post my measurements thursday nite.

[SinnerDC2]

slightly off topic, do you have a non-s ? If so how much where the drilled and slotted brakes? Next time I replace my rotors, I'd like to go slotted.

Thanks
Nick

[CJ_Boxster]

Originally Posted by SinnerDC2

slightly off topic, do you have a non-s ? If so how much where the drilled and slotted brakes? Next time I replace my rotors, I'd like to go slotted.

Thanks
Nick

Didn't get a chance to measure rotor thickness but sinner, I paid 310 shipped for front and rear x drilled & slotted Zimmerman rotors from an ebay vendor.

[nola911]

Slightly off topic, but do you mean.. you cut 1/1000 of an inch five times to be the same as 5/1000 of an inch?

1000th of an inch (1/1000) us much larger than 5000th of an inch (1/5000), though in wording they sound the same, in writing they are totally different. If you cut were aiming to cut 1/5000 and you cut 1/1000 once, twice, four , or five times, you cut too much.

Is it obvious that I am at work?

[RandallNeighbour]

I had no idea one could warp a rotor by over-torquing wheel bolts, but it sure does make sense!

I bought a torque wrench off ebay a while back and I am now seeing that it may be the single most important tool I own for my boxster.

[Lil bastard]

@Randall - You sure can, very easy to do. This is why you should never use an impact wrench to replace the wheel bolts (and insist that your shop doesn't either, despite what they may say).

These are too prone to over-torque the bolts due to variances in the air pressure and gauges. They will produce significant differences in torque between all 5 wheel bolts.

Impact wrench is OK for removal, but always replace manually, using a torque wrench for final tightening in a cross pattern.

[tholyoak]

Believe what you wish, but there is no way over torquing your wheel lugs is going to warp a rotor.

The wheel stud attaches to the hub, not the rotor, the rotor sits flat against the hub, sandwiched between the wheel and the hub. If you were able to exert enough torque by over torquing, you would distort the hub not the rotor. Secondly if you did warp the rotor in this fashion, you would warp the mounting surface, and turning the disc would not cure the runout problem.

There many reasons to use a proper torque wrench to attach the wheels but warping the rotors in not one of them. Now if you under torque the wheels, the rotor could distort under braking forces if not clamped with the correct force between the wheel and hub, which will lead to damage.

Todd

[CJ_Boxster]

Oh i forgot to post my rotor thickness after the cutting. Both rotors i cut measure at 23.53mm and 23.54mm thickness.

About the warping cause by over torqing the lugs... Zimmerman told me that can happen as well as people at Brembo.

Personally i think it was a combonation of over torqed wheels and too much zinc plating... My shipment of Zimmermans were delayed cause of a defect in the plating so they replated them once more before sending them to me and i noticed that the plating was also on the surface area of the discs where the brake pads touch so i think when my brake pads were trying to rub that zinc off... that it generated more heat & slippage and causes them to warp... But thats just a theory

[CJ_Boxster]

Originally Posted by tholyoak

Believe what you wish, but there is no way over torquing your wheel lugs is going to warp a rotor.

The wheel stud attaches to the hub, not the rotor, the rotor sits flat against the hub, sandwiched between the wheel and the hub. If you were able to exert enough torque by over torquing, you would distort the hub not the rotor. Secondly if you did warp the rotor in this fashion, you would warp the mounting surface, and turning the disc would not cure the runout problem.

There many reasons to use a proper torque wrench to attach the wheels but warping the rotors in not one of them. Now if you under torque the wheels, the rotor could distort under braking forces if not clamped with the correct force between the wheel and hub, which will lead to damage.

Todd

Todd your missing one factor in your theory there... What does the Rotor do when its gets Hot..... It expands and not just the disc, the Hat of the rotor also expands when hot, and if the torq on the lugs is too tight, it will not allow the hat to expand with the heated surface area of the disc and will cause warping.

Ever seen a 2 peice racing Rotor... The Disc/Surface area is separate from the hat and only held together by little nuts and bolts, This is because the heat that will be generated will be so high that a solid 1 peice rotor design's hat will not beable to expand enough to prevent warping. If the Hat cannot expand, it'll warp

[tholyoak]

You are entitled to your opinion and maybe I am wrong but I don't believe the rotor hat is going to expand appreciably when sandwiched between the hub surface and the wheel. Which are also going to expand. And even if it did, the difference in the expansion between being sandwiched with the correct 96ftlbs of torque vs say a mis-torqued wheel at where some lugs are at 120ftlbs of torque is not going to lead to appreciable distortion of the rotor hat.

Secondly on a street car you are never going to experience the types of temperatures you refer to in race cars. I would be pretty concerned if I saw someones rotors glowing red hot at night on the exit ramp as you see at night during Le Mans. The most common way people warp rotors on the street is when they run them too thin or they do some heavy braking and while the rotor is hot they still have their foot on the brake, thus the pads make contact with the rotor both melting some pad material to the disc and/or resulting in differential cooling of the disc leading to warping.

Todd

[Lil bastard]

Sorry to say you are misinformed.

I had a shop mount some tires for me and within 50 mi. all 4 rotors were warped. They readily admitted they accidentally over-torqued the bolts due to a malfunctioning air regulator in the shop and paid $850 out-of-pocket to replace them.

There are a number of articles out there on this subject. Even AutoZone warns of this in their DIY Tire Change guide - page down to the WARNING box - http://www.autozone.com/az/cds/en_us/0900823d/80/1b/d6/9f/0900823d801bd69f/repairInfoPages.htm

[tholyoak]

This is the last I will say about this, and go ahead and believe what you want.

When your tech, tells you that over tightening of wheels can warp rotors you are either not understanding him or he doesn't understand what he was originally told.

Over tightening of the wheel will not causes warping the rotor, what it can do is distort the hub flange on which the rotor sits. This distortion causes rotor runout problems, that when you measure with a dial gauge on the rotor surface will appear to be warped rotors. If left alone this problem will result in further braking issues.

There is no physical way that over torquing of the wheel to the hub will cause physical warping of the rotor. That is my opinion from working on Porsches for almost 20 years, not on what I read on the Autozone website or my local garage mechanic told me.

Todd

[Lil bastard]

@tholyoak - I'm sorry, normally I wouldn't pursue an arguement. And I don't mean to belittle or diminish your mechanical abilities in the least. I've read many of your posts and they contain valuable information, I don't think there's any question about your skill level. But in this case, you're simply wrong.

It's not a matter of what one believes, this is fact. I too have been repairing and maintaining cars of all sorts for 20 years and then some, incl. porsches, so I can assure you there was no misunderstanding of what my Tire shop told me.

The reason I'm persisting is that this is one problem which is easy to avoid, but has expensive consequences if the procedure's not done correctly ($800+ for a set of rotors). I don't want readers to get the impression that this is a matter of belief because it isn't.

Over torqueing of lug nuts and wheel bolts is a major cause of rotor warping. Full stop.

If you won't accept an article from AutoZone as proof, here are a dozen more that you can look over, and I hope, learn something new from.

• http://www.articleblast.com/Auto_and_Trucks/General/Warping_Automobile_Disk_Brake_Rotors_are_Preventab le/
  
  http://www.automedia.com/Replacement_Rotors/ccr20050201rr/2
  
  http://www.alldata.com/techtips/2001/20010430b.html
  
  http://www.thecarconnection.com/Enthusiasts/Ask_a_Mechanic/On_Site_Mechanic_for_Dec_27_1999.S209.A1474.html?p g=2
  
  http://www.mpt.org/motorweek/goss/21008.shtml
  
  http://en.wikipedia.org/wiki/Rotor_(brake)
  
  http://autorepair.about.com/library/faqs/bl815.htm
  
  http://www.ctbrakes.com/faqs.html#16
  
  http://www.thevolvosite.com/volvo-brake-rotors-and-warping.htm
  
  http://www.brakewarehouse.com/brkewrhsefaqs.asp#top61
  
  http://www.bestbrakes.com/how_to_install_brakes.shtml
  
  http://www.mpt.org/motorweek/goss/2436.shtml

[Lil bastard]

It occurs to me that there is one more worry in turning cross-drilled rotors. That is, these rotors have holes which are beveled.

These bevels are a design element that serves three important functions:

They prevent the build-up of pad material at the holes (a source of Brake pulsation).

They also prevent the holes from becoming clogged with friction material from the pad, or mud, dirt, etc.

Finally, and perhaps most important, these bevels act to eliminate, or at least delay, the onset of stress cracks which could lead to rotor failure.

This is a very real concern and probably the biggest reason that cross-drilled rotors are not widely used anymore in racing - racers use slotted discs that better prevent rotor failure and warping. Given Porsche's vast racing experience, I'm a little surprised that they decided to offer X-drilled rather than slotted rotors. I supose that it's because the aftermarket is flooded with x-drilled rotors (they're cheaper to manufacture than slotted ones) and so they enjoy a greater popularity. They also cool slightly better than slotted discs, but I'm not sure that it's a good trade-off vs reliability.

By turning these rotors, you eliminate or reduce this beveling. To do the job correctly, it would be necessary to re-establish this beveling by using a drill press and beveling out each hole in the rotor (on each side). By the time you went to all this trouble, it'd probably less expensive and more expedient to simply replace the rotors.