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Many websites recommend replacing rim brake cartridges (pads) when the grooves on them have worn off. Why would this be reasonable, other than for the hypothetical reasons listed below? I am looking for a scientific/engineering perspective, not personal stories on when you replace your pads.

I understand that once the cartridge wears down to its metal holder, then the braking performance is poor and the metal wears out the rim. The rim is much more expensive, so the brake pad should be changed before metal hits metal. But new Shimano brake pads are 10 mm thick (I just measured), of which 6 mm is outside the metal holder. The grooves are less than 2 mm. Why not use say 3 out of the 4 mm remaining after the grooves have worn out?

I understand that cartridges are cheap, might as well change them every week. I am not looking for a financial or preference-based explanation, only performance-based.

Another reason to change cartridges early may be that the worn cartridges would be too far from the rim. Brakes would have to be moved closer to the rim, which takes a little labour. With new cartridges, the brakes would have to be moved out again. Some people may prefer to change cartridges instead.

It may be that the material of the cartridge is not uniform, e.g. softer rubber on top, harder beneath. When the soft rubber is worn off, the cartridge starts wearing the rim out fast and offers poor braking. I cannot detect any difference of rubber across a Shimano brake pad. It would be more expensive to manufacture 2-compound cartridges than uniform-composition. Is the rubber of a cartridge uniform?

Grit and metal shavings stick in a brake pad, making it wear out the rim. But this seems to happen right from the start, no difference whether the grooves are there or not. Presumably grit sticks in softer rubber better, so if softer rubber is on top, then the brakes gather more grit before the grooves are worn out than after.

  • Just a thought, but; new pads will tend to "point up" slightly towards the rim as they contact. After some wear they will be "pointing down" slightly on contact. As this is happening the angle of the face will shift such that the bottom of the pad is nearer the rim than the top. The more they do this, potentially the more likely it is that under pressure something will flex and the blocks will slip past the rim on the spoke side. This would result in a sudden loss of braking power - or if the blocks reach the spokes a sudden increase. Something emotional, anyway. Plausible? – Grimm The Opiner Sep 28 '17 at 14:31
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    Don't forget pads harden with age. They're terrifying when well past the end of their life. Trying to make pads last for excessive lengths of time is doing your braking distance a disservice. There are multi-layered brake pads like Kool Stop, and they work very well for the cost of a big mac meal. Change your pads every year or three, and get on with riding. – Criggie Sep 28 '17 at 19:34
  • @Criggie Hardening with age is a reasonable answer. As is that Kool Stop is multilayered, so multi-compound pads exist. – Sander Heinsalu Sep 28 '17 at 23:01
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    Multi compound pads are put together with different compounds striped along the surface, not in layers that get worn away or exposed as the pad wears. – Nathan Knutson Sep 28 '17 at 23:15
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    Any source for Kool Stop being multilayered? – ojs Sep 30 '17 at 9:33
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This is a good question. The short answer is that the design of modern rim brake pads is greedy and cynical.

This is a Mathauser pad, and is representative of how a lot of old brake pads were, and some still are.

Scot Mathauser canti pad

The design differences with modern pads are obvious. You get a huge simple block of rubber with no grooves. Nothing about the design of the pad is coaxing you to replace it any time before the metal part starts contacting the rim, a point that most riders will be able to see coming and all but the least observant will hear about the second it starts actually happening, before any serious damage is done.

Pads like these are cheap to produce and last a very, very long time. I'm a heavy rider, and I have a set of these I got NOS on a bike that gets moderate use in all conditions that I've had for maybe 8 years now and the pads are about 1/3 worn. There are some theoretical arguments why grooves might help braking or prolong rim life, but I can endo the brake just fine and my rims aren't getting trashed. Many millions of pads of this design have given totally satisfactory performance.

The reason brake pad design moved away from being cheap, thick blocks of rubber is 100% that you can't make any but the most trivial amount of money producing and selling them. They last too long and are too effective for how cheap they are. Only the most hardcore of cyclists become repeat buyers.

Modern cartridge inserts are dainty and full of conservatively sized wear grooves because it creates a situation that gets more pads sold. Nothing bad happens if you wear them all that way down as long as you stop before you get to the holder, at least as long as you can be bothered to keep the brake adjusted along with the pad wear, and watch out for the worn pad overhanging the brake track. The compound doesn't change, the geometry of the brake doesn't change substantively in most cases, and the grooves don't do anything of note in either direction to braking power or rim wear.

A core question about grooves is whether their presence helps or hinders the pad to clear water and debris from the rim, potentially increasing braking power even as they inherently decrease contact area. Personally I think they don't and because they in effect feed water in to the pad/rim contact area they probably are a detriment when it's wet out, but I don't have any data. I believe the contrary idea would be that as the pad touches down on the wet rim, the leading edge has the job of squeegeeing off water to get more friction between the pad and the brake track, and having grooves essentially gives you another leading edge, getting more area clear of water faster after the pad touches down.

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    Just to say - according to Wikipedia at least - "The rubber can be softer for more braking force with less lever effort, or harder for longer life." I'm sure the financial aspect you talk about comes into it (maybe quite significantly), but it's not the only factor en.wikipedia.org/wiki/Bicycle_brake#Rim_brakes – LangeHaare Sep 28 '17 at 8:28
  • +1 for grooves possibly clearing water and debris. Car tire pattern is for water clearance, so theoretically this makes sense. Data would be better, of course. – Sander Heinsalu Sep 28 '17 at 23:04
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I have checked the worn pads I recently replaced on my bike. The remaining of the grooves were filled with debris from the pad wear.

Therefore I think the grooves serve the purpose of discharging the worn layer of the pad which is removed during the braking action, so that the action itself can be more effective.

Imagine what happens when you brake: the pads push on the rim, and being the rim metal it's not going to wear that much against rubber. Therefore rubber has to wear. The debris will be transported between the pad and the rim surfaces, aggregating with each other as the travelled distance increase, acting as rollers and decreasing the friction coefficient and therefore the braking power.

If you have some grooves in the surface the debris can be captured there without lowering too much the braking effectiveness.

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    A lot of the dirt is also flicked up from the road, especially in the wet. The grooves are often shaped to shed water/wet dirt. Some of this dirt is grit, which if it gets embedded in the pads causes rim wear. – Chris H Sep 28 '17 at 6:26
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L. Dutch's answer explains some of the design decisions. As for why you and I should change our pads when the grooves run out, you really don't want to run down to bare metal on the pads. So when you know your pads are quite worn but not down to bare metal is a good time to replace them.

If you've ever cut open an old pad (quite easy if it's perished) you're likely to find that the metal sticks further into the rubber than it looks.

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