nez dans le guidon wrote: Mick F wrote:First car I owned with disc brakes was a Triumph Herald 12/50, though I've never owned a car with four disc brakes.I wonder, what is the benefit of rear discs?
I have a Saab with all round discs (actually it has drums for the hand brake too, the mechanic tells me). It's got a lot of stopping power. I have absolutely no idea if it offers any advantage because of course in a heavy front wheel drive car most of the braking is on the front.
SAABs (like many other cars) are usually rather front heavy when empty and rather tail heavy when full of people and stuff. They normally have a pretty good brake specification. Because of weight transfer under braking, even an inherently rear heavy vehicle usually needs bigger brakes at the front than the back. Very few road-going cars have vented rear discs; the rear brakes just don't have to do enough work to need it.
I've had cars with drum brakes all round and they were OK for about the first half of the first stop, and after that the brakes went away. I had one car where you couldn't pull up from 70mph (even once) and have any brakes left by the time you were doing 10mph. Downhill, it was just rather dangerous. I'm sure that modern brake lining materials would improve greatly on that, but that is how they were back in the day. You couldn't easily brake hard enough to boil the brake fluid, because the brake linings stopped working well before that. Drums on cars have their plus points (they are kinder to wheels; far less hot brake dust gets stuck in the finish) but discs normally offer more braking power.
Crucially (with the exception of being completely soaked - think fording or the very heaviest rain) disc brakes can be made to modulate more consistently. This is good for confidence and it also allows the ABS system to work at its best.
I guess I've owned about a dozen different cars with discs all round. Some have had drum brake shoes inside the bell of the rear disc as well, thus giving a handbrake, others have had the handbrake mechanically applied via the rear brake caliper.
The common feature of all these cars is that the rear brakes have been operated by a bias valving arrangement. There are various ways of doing it, (for example the rear ride height can be sensed and used to modulate the rear brake power, using quite a simple mechanical arrangement) but on many passenger cars there is simply a bias valve that establishes a fixed pressure differential between the front and rear brake circuits. This protocol means you can't instantly lock the rear wheels on the slippiest surfaces which (without ABS) causes people to crash otherwise. It also helps avoid rear wheel lockups under heavy braking.
But there is a problem; in light braking, the rear brakes hardly get used. So if light braking is normal braking, the rear discs can easily start to rust and unlike the front brakes, normal use won't necessarily clean the discs up. At one time my regular car commute could be (and often was, courtesy of uphill exit slips ) executed with nothing more than a couple of episodes of light braking; not enough to keep the discs clean. So I've worn out front discs many times, but never rear discs. However I've bought as many rear discs as fronts; the rears have simply corroded. I used to use pad compounds that were not inherently abrasive (some of these work by depositing a film of the pad material on the disc, thus reducing disc wear to near zero) but these days I use a more abrasive compound; this wears the front discs but it also keeps the rear discs cleaner, even though I don't do as many miles these days.
Needless to say it is the winter road salt that causes the corrosion to be as bad as it is, and in extremis the front brakes can rust too. Normally this happens on the inside face of the disc more than the outside, because of the airflow direction under the car. I've seen front discs that looked fine on the outside, but were 50% rusted on the inside face.
Bicycle (and motorcycle) discs have a problem which is that they are more likely to be completely wet through than car brakes; this is simply because the discs are more exposed. Very many disc brake pad compounds are completely useless when the brakes are wet through (ever tried your car brakes after fording? -like that) so what you can get away with on a car is not necessarily applicable to a bicycle or motorcycle.
Pad compounds that (for bicycle use) are variously described as 'organic' through to 'semi metallic' can be broadly similar to the kinds of compounds that are used on cars. Many of these can offer zero braking efficiency (for a few heart-stopping seconds when the brakes are wet right through) on first application; to my mind this makes them completely unsuitable for all weather urban commuting, because in an emergency, you might as well have wet chrome steel rims, (or no brakes at all, even), the brakes can be that crap to start with (certainly a lot worse than even quite mediocre rim brakes).
So if I had to choose a disc setup for commuting it would be one with sintered pads (yes despite the ghastly noise in the dry) because that is one of the few setups that has reasonable bite in the wet on first application. That in turn means mechanical discs, not hydraulic, because sintered pads conduct the heat too well, into the vulnerable parts of a hydraulic caliper. Thus one of my favourite MTBs was spared commuting duties in wet weather, because the (otherwise fantastic, for
MTBing) hydraulic discs were next to useless; if some cretin stepped off the pavement in front of me, there was no way I was going to stop.
For utility bikes, discs have their problems in other ways too; the workings can corrode in winter (the otherwise pretty good BB5 and BB7 calipers suffer from this in the FPA as do many others) or leak (any hydraulic system, really) if the bike is neglected in the usual way. If any maintenance is done, the rear disc usually gets 'oiled' along with the chain. Both discs are vulnerable to contamination via road film (water, oil, diesel, all mixed in together) and are also very vulnerable to knocks in bike racks etc.
So for utility riding, I prefer drums. I suppose shimano roller brakes are a kind of drum but I have concluded that these are not my favourites for a variety of reasons. Conventional drums (as we have them eg SA ones) are good because they are consistent in a way that other brakes are not, in most flavours of wet weather riding. Many of the troubles that afflict drums in motorbikes and cars don't cause problems in bicycles in the same way. Drums ain't perfect though; the modulation isn't as good as with decent disc brakes and 70mm drums in 27" wheels can be weak unless in good condition (and ideally given a floating brake plate). Also, if you leave the bike outside in the rain overnight leaning the wrong way the linings can eventually get wet and this can make for a variety of exciting things including;
- rusty drums making the brakes unusually sharp on first application
- much wetness (very rare this; the heaviest driving rain or submersion required) making the brakes ineffective as per discs with organic pads. NB unlike with discs I have never known this to happen to drums when riding in the rain.
- slightly damp brake linings that have ( I think) swollen a little and therefore can cause the brakes to jam on a little ( or a lot) until the brake linings have dried out.
But despite that you can have (I think) the most reliable brakes with the least maintenance if you choose drums. For example in my utility uses, 90mm drums offer more power than I need, modulation that is good enough to use it, and near-zero maintenance. In a year's riding (about 3000 miles, I think) you might want to use the barrel adjuster just the once to adjust for wear. Maybe two or three times if you live somewhere hilly.
For 'performance cycling' discs at first sight have attractions. But not all of these hold up under close scrutiny;
- overheating (tandems, tourists who drag their brakes, potentially any hydraulic system in road use)
- weight (all)
- knock-on effect on ride quality through stiffer fork (tourists and racers)
- maintenance (some brakes worse than others)
- initial cost
- non-availability of spare parts in LBSs (cf caliper brakes)
- initial wet weather performance (some)
- vulnerability of discs during transit/maintenance
- potential for burns on hot discs
- increased loading on the hub, hub bearings, spokes
are variously issues that have not been solved or can cause trouble in some (rather than all) disc brake applications. In any event these things can blunt the attractions of good power and modulation (that you might have
most of the time), and a perception that your rims might last longer. I know some folk love their disc brakes, and they can suit some uses just fine, but IMHO it isn't a done deal for everyone, not by a long chalk.
cheers
.
