Mountain Gear

Tech FAQ: Lennard Zinn on rotors and drilling frame drain holes

Lennard Zinn answers your questions on rotors and carbon frames.

Regarding thickness of the rotor.

Dear Lennard,

Is there a magic number for disc brake rotor replacement – the thickness of the rotor?  I happened to caliper my old rotors when I made a recent change (I didn’t care for the sound my Avid clean sweep rotors make, so I went to a smoother style Avid).  They all seem to be .073 thick.  The front rotor was still close to original, while the rear had lost .005 inches.  It just got me wondering if there was a minimum recommended thickness, and I was curious if all disc brake brands use the same thickness?

— Steve

Dear Steve,

Yes there is a minimum recommended rotor thickness, and it varies from brand to brand.

Magura’s minimum recommended rotor thickness is 1.7mm. At that thickness, Magura recommends replacing them for several reasons, one being reduced heat stability. Stefan Pahl, Magura’s product manager for its bicycle division says, “In the past we had the rotor thickness limit at 1.7mm. With the Storm and Storm SL rotors we even have increased that limit to 1.8mm (which is also laser printed on the rotor, on one of its spokes).” So you can wear the rotors down by 0.2mm down from 2.0mm. The reason for the high recommended thickness is not only heat stability, but also structural stability and reduced squealing.

For Hope’s 140mm diameter and 160mm diameter floating discs, the minimum recommended rotor thickness is 1.4mm (they are 1.60mm +0.05mm when new). For all the rest of Hope’s discs, floating or plain, minimum thickness is 1.5mm (they are 1.80mm +0.05mm when new).

Hayes rotors start out life at 1.75mm to 1.78mm thick, depending on rotor diameter. Hayes’s minimum thickness call out is 1.52mm and is marked on all Hayes rotors.

Shimano rotors are made 1.8mm thick and should be replaced when the thickness has been reduced to 1.5mm. Since 2010, the 1.5mm minimum recommended rotor thickness has been printed on Shimano rotors.

As for your specific rotors, that’s the least easy information to find. By my measurement, SRAM/Avid rotors seem to begin life at 1.85mm thickness, but I have not been able to find it in SRAM/Avid technical documents, and it is not printed on any of the SRAM/Avid rotors I’ve checked around here. I’m awaiting a response from the SRAM/Avid tech department.

— Lennard


Regarding drilling a drain hole in a carbon frame

Dear Lennard,

I have two questions regarding my Orbea Alma Carbon Frame. This year has been particularly rainy around my parts, and although I have tried to keep water out of the frame by using a piece of Lizard Skins neoprene chain stay protector around the slit in the upper part of the seat tube, I keep collecting up to an ounce of water inside the frame anyways. Having always owned metal frames in the past, I almost drilled a drain hole in the inferior part of the bottom bracket, but then I thought twice. Is this advisable on a carbon fiber frame? If it is, then what kind and size of drill bit should I use, carbide tip, metal? I certainly do not want to ruin the whole frame for the convenience of keeping water from collecting within.

The second question is, can I use nail polish to cover up clear coat damage from pebble impacts or do I risk damaging the frame’s integrity?

BTW, thanks to you and Morningstar I ride with true disc rotors.

— Manuel in Guatemala

Dear Manuel,

I would drill the hole, and any drill bit for metal will work; carbon is soft and requires no carbide tip or anything like that. I would drill a 1/8″ (3mm) hole straight under the center of the bottom bracket shell so it points straight down when the bike is standing up on its wheels.

I can’t imagine that there would be a significant weakening of the frame and cause for any concern, since that’s a low-stress area, but I suppose it’s possible it would void your warranty. If you’re still within the warranty period and want to ensure that the warranty is still valid, then I’d recommend you contact Orbea before drilling the hole.

And yes, you can use nail polish on your carbon frame. It does not have a solvent in it capable of dissolving your clear coat or damaging the laminate structure.

-Lennard