Good thing I had it in my history...
Again guys, remember that ceramic classified friction material only refers to the reinforcement fiber that typically is potassium titanate, which compromises only a small percentage of the friction material matrix. It is an expensive fiber so in the aftermarket you can come across companies that just put a minimal amount into the matrix so they can call it ceramic for marketing purposes. Often these will still be organic or semi-metallic formulations.
Ceramic fibers have made headway in the industry to get away from the temperature sensitivity of semi-metallic formulations, so if you got good frictional delta with temperature you’ve got a semi-met. The higher the metallic, the larger the frictional delta. The other negative aspect of a true ceramic reinforced friction material is brake dust.
Disc / disc equipped vehicles do not have the old brake bias of the front brake doing a majority of the work. Well that is if the brake pad friction is balanced between front and rear, and the brake sizes are relatively equal. This is the direction the industry has gone during the last decade.
A rotor with holes drilled in was primarily designed to lessen rotational weight during events like autocross. While it has some attributes in wiping gasses from the rubbing surface of friction material, unless the rotor vanes are very well designed it does nothing for cooling. In fact without proper vane design there can be more of a thermal spread across the rotors rubbing surface then with a solid surface rotor as the air is pulled in only from the top holes and not drawing air from the inner holes or open vane section at the rotor’s hat. The other problem as mention is the loss of mass or thermal sink.
A quick sideline here is that from testing we had found that with holes or slots the friction material has to be very stiff and not compliant as compliancy will allow a thin layer of friction to be sheared off every time an edge goes past the friction material's rubbing surface as it stands proud.
During normal driving a rotor’s thermal rejection is done over a relative long time, not instantly. A brake relies on thermal mass of the rotor to absorb the energy as heat. You can do quite a few stops before the heat is no longer absorbed by the rotor mass and transferred to the air.
By drilling a rotor you are reducing the mass of the rotor’s rubbing discs, causing the remaining mass to heat up to a hotter level in a designated amount of time. Think of heating a 1” square of iron to a red-hot temperature. Drop it into a bathtub filled with water at 70°F and the water temperature does not increase that much. There’s a lot of mass. Drop the 1” cube into a cup of water at 70°F and all the water will start to boil. A small amount of mass to absorb the thermal energy. Same with rotors.
What you see in the front rotor picture of the Navigator are thermal shock stress cracks, not crack progression from stress risers when the rotors were bored. The surface is getting very hot in a short amount of time.
There are a couple of contributors that could be occurring here as the Navigator is not known for rotor cracking like the ‘90’s F-250/350 were as the front / rear brake balance of the Navigator is good. One is the loss of mass as mentioned. The metallurgy of the gray cast iron used for these particular rotors may not be up to the level that it should be. The friction material compound (the thermal friction characteristics mentioned tell me this is a semi-met in ceramic clothing) is too hard and not compliant causing thermal banding hot rings. Or the friction material used on the front axle is not matched to what is used on the rear axle shifting more brake bias to the front brakes. After all, the Nav is not a lightweight vehicle so improper frictional balance shifts a larger load.
The use of some means to remove gasses from thermally stressed friction material (holes or slots) is only needed when a friction material is exceeding it’s thermal limitations. Or you like the look. A poorly compounded or manufactured friction material can thermally degrade around 500°F. These are usually $20 pads. A good quality pad from a reputable manufacturer will hold up to 1000° to 1200°F or beyond. A performance racing type pad usually can get up to 1500° to 1800°F before developing fade from degradation, but it’s gold effectiveness will be very low. If you are using a good quality pad there is no reason for the holes or slots. Other then looks.
Again guys, remember that ceramic classified friction material only refers to the reinforcement fiber that typically is potassium titanate, which compromises only a small percentage of the friction material matrix. It is an expensive fiber so in the aftermarket you can come across companies that just put a minimal amount into the matrix so they can call it ceramic for marketing purposes. Often these will still be organic or semi-metallic formulations.
Ceramic fibers have made headway in the industry to get away from the temperature sensitivity of semi-metallic formulations, so if you got good frictional delta with temperature you’ve got a semi-met. The higher the metallic, the larger the frictional delta. The other negative aspect of a true ceramic reinforced friction material is brake dust.
Disc / disc equipped vehicles do not have the old brake bias of the front brake doing a majority of the work. Well that is if the brake pad friction is balanced between front and rear, and the brake sizes are relatively equal. This is the direction the industry has gone during the last decade.
A rotor with holes drilled in was primarily designed to lessen rotational weight during events like autocross. While it has some attributes in wiping gasses from the rubbing surface of friction material, unless the rotor vanes are very well designed it does nothing for cooling. In fact without proper vane design there can be more of a thermal spread across the rotors rubbing surface then with a solid surface rotor as the air is pulled in only from the top holes and not drawing air from the inner holes or open vane section at the rotor’s hat. The other problem as mention is the loss of mass or thermal sink.
A quick sideline here is that from testing we had found that with holes or slots the friction material has to be very stiff and not compliant as compliancy will allow a thin layer of friction to be sheared off every time an edge goes past the friction material's rubbing surface as it stands proud.
During normal driving a rotor’s thermal rejection is done over a relative long time, not instantly. A brake relies on thermal mass of the rotor to absorb the energy as heat. You can do quite a few stops before the heat is no longer absorbed by the rotor mass and transferred to the air.
By drilling a rotor you are reducing the mass of the rotor’s rubbing discs, causing the remaining mass to heat up to a hotter level in a designated amount of time. Think of heating a 1” square of iron to a red-hot temperature. Drop it into a bathtub filled with water at 70°F and the water temperature does not increase that much. There’s a lot of mass. Drop the 1” cube into a cup of water at 70°F and all the water will start to boil. A small amount of mass to absorb the thermal energy. Same with rotors.
What you see in the front rotor picture of the Navigator are thermal shock stress cracks, not crack progression from stress risers when the rotors were bored. The surface is getting very hot in a short amount of time.
There are a couple of contributors that could be occurring here as the Navigator is not known for rotor cracking like the ‘90’s F-250/350 were as the front / rear brake balance of the Navigator is good. One is the loss of mass as mentioned. The metallurgy of the gray cast iron used for these particular rotors may not be up to the level that it should be. The friction material compound (the thermal friction characteristics mentioned tell me this is a semi-met in ceramic clothing) is too hard and not compliant causing thermal banding hot rings. Or the friction material used on the front axle is not matched to what is used on the rear axle shifting more brake bias to the front brakes. After all, the Nav is not a lightweight vehicle so improper frictional balance shifts a larger load.
The use of some means to remove gasses from thermally stressed friction material (holes or slots) is only needed when a friction material is exceeding it’s thermal limitations. Or you like the look. A poorly compounded or manufactured friction material can thermally degrade around 500°F. These are usually $20 pads. A good quality pad from a reputable manufacturer will hold up to 1000° to 1200°F or beyond. A performance racing type pad usually can get up to 1500° to 1800°F before developing fade from degradation, but it’s gold effectiveness will be very low. If you are using a good quality pad there is no reason for the holes or slots. Other then looks.
