Disc brakes

[ChrisH]

Resin is ***!

 

I would like to take this opportunity to point out that TitusTi agrees with me.

 

That is all.

[dirtrider]

sorry Chris my mistake .

[Johan Bornman]

Heat buildup is only a problem if you plan on resting your leg on the rotor   (Don't do it).  Sintered pads last long and when you need to brake hard' date=' the noise doesn't seem like such a big deal anymore.  On one of the SA websites I saw it mentioned that sintered pads were "overkill for SA conditions", but all it takes is one very rainy and/or muddy day and your pads are screwed.  I also assume he limited SA to Gauteng rather than somewhat wetter and muddier coastal towns and cities.[/quote']

 

Whoever wrote that on the website, has never ridden in wet conditions - coastal or inland. Grit carried by road splash greatly reduced a pad's life, to the point as someone pointed out here, where they're history in 40 minutes.

 

The heat build-up story also needs some clarification. First, lets distinguish between build-up and heat creation.

 

In a friction material such as a bicycle's brake, the heat is generated in the softer of the two materials. In all bicycle cases, whether we're talking rubber pads or disk pads from whatever material, it is thus the pad that generates the heat. This heat is generated when the molecules of the friction material are stretched and snapped.

 

The pad itself is such a poor conductor of heat that it can't transfer much of the heat backwards, through the pad material, to the brake calliper.

 

Therefore, the heat is transferred elsewhere by conduction, to the disk, which is a better conductor of heat. Heat thus moves from the pad to the disk or in the case of rim brakes, to the rim. With rim brakes (with the exception of carbon rims that brake very very poorly), there is a large surface area to dissipate the heat. The air washes over the rim and cools it quite effectively.

 

With a disk brake, the surface area is small and the heat is dissipated less effectively. However, the disk can take more heat because we don't worry about it melting a tube and causing punctures. Also, the disk is designed to work effectively until it glows a dull red - after than it may warp and fold.

 

Now to look at the heat build-up. For a given disk size at a given rate of air cooling, all equal-sized disks will build up and shed an equal amount of heat. A smaller disk will build up more heat than a large one simply because the input is the same but the output (shedding of heat) much lower. A disk with fancy cut-outs and a wavy outer edge is worse than a plain disk. Cut-outs do help shed water and grit through but does nothing for heat dissipation.

 

It is at the input side where things differ. Different pads will generate more or less heat which has to be transferred to the disk. Given that the disk can only shed X amount of heat under certain conditions, it is the input side that makes the big difference. In other words, if reports of "heat b uild-up" with sintered pads are true, these pads generate more heat than resin pads.

 

Disk material also makes a big difference. Stainless stell is a very poor conductor of heat and cast iron an excellent conductor. However, cast iron is too brittle to be used on thin disks like that of a bicycle (but perfect enough for motorcycles and cars). Therefore the bicycle's stainless steel disks don't do as good a job as other metals would have done. This comes at the expense of braking, since the pad can then transfer heat slower and the interface surface gets far hotter, to the point where it could form a gas bearing or continuously melting surface, both situations that are quite common on carbon wheels with rim brakes. Needless to say, under these conditions the brakes suddenly fail.

 

Some on The Hub wrote about such an incident on his road bike in the Suikerbosrand reserve recently.

 

JB 

[Quicksilv3r]

Johan thanks thats you explained everything!