First, although the comparison with tyres might be interesting, I think it might be clouding the issue a bit. The original question was about grooves in brake rotors which expanded to a discussion of putting grooves or holes in rotors and their theoretical and practical effects, which has then moved onto pads and brake performance in general. It's producing some interesting discussion, so can we keep it focussed there please chaps (and chapesses if there are any reading), thanks....
Takes "mod" hat off and puts "techy" hat on....
Quote:
|
Originally Posted by Dave Brand
When you press the brake pedal you apply a force to the brake pads. The resultant force, the force that produces the braking effort on the disc, is that force multiplied by the coefficient of friction. Pad area has no significant real-world effect on the braking force.
Reduced to basics, a brake is a device to convert kinetic energy into heat. Heat is generated at the pad-disc interface. If the pad is very small the heat will be concentrated into a very small area, thus for a given rate of energy dissipation the temperarture rise will be much higher than it woould be for a large pad.
Pad wear, measured in volume of material per unit of energy dissipated, is fairly constant. It follows, therefore, that the bigger the pad area the less the reduction in thickness & therefore the longer the life of the pad.
A lot of road car pads have large chamfers to reduce the tendency to squeal. As a result of this, the effective pad area can vary by probably as much as 15% over the life of the pad, with no discernable variation in braking effort.
|
Thanks for that Dave. I understand your logic and can't fault it, so that leaves me with trying to reconcile it with my practical experience. That leads me onto something my brake supplier said about brake pads and selecting the right compound. He said essentially that pad wear is only directly proportional to area when working well within the temperature limit of the pad. But as the limit is approached the wear rate increases dramatically. He says that once the pad reaches a certain temperature it will wear very rapidy indeed and be next to useless.
This would suggest that the argument about braking force vs pad area only holds true while well within the operating parameters of the pads. This obviously falls down when comparisons are made between 1mm sq pads and 500mm sq pads (not getting at you trist) so means that this sort of discussion can't be made by considering extremes.
Anyway, it seems that to get a nice linear relationship of area to braking force we'd need to pick a pad with temperature resistance well above where we want to use it... but (as I'm told, and from my own experience) high temperature resistance = very hard and that I'd get no "feel" from them (an issue we haven't covered yet). My brake supplier says that I should aim for the softest pad I can without reaching the critial high wear temperature. This is supposed to give me the best "feel" (NOT braking force) even though I'll probably get through pads quite quickly. That suggests that I could quite happily buy some really hard pads that would last me all season, but my braking wouldn't be as effective. (Hmm, seems I've been told something similar about tyres, but lets not go there please
)
So, I think what I'm trying to say is that our two arguments aren't actually contradictory, but apply to different operating temperatures. Ie, if you have very hard pads, then grooves, holes etc make no difference, if you're right on the temparture limit of the pads, then they can have beneficial effects, but if you're right on the temperature (or structural) limit of the rotors then they could be bad news. Am I making any sense at all or should I go and hide my head now?