I wear through brake pads quickly on my electric-assist cargo bike as I'm often hauling 2 kids around San Francisco hills. I recently checked my rotors and discovered they're well past the recommended replacement thickness (Shimano recommends a minimum 1.5mm, these are reading about 1.3mm). I've replaced them, of course - not taking chances with safety, especially when it involves my kids - but the brakes were still working fine and the rotors were true. They're solid steel, SM-RT64 rotors. I can imagine that if the rotors get down to the thickness of a soda can then I'd have problems, but there's a long way to go before I'd get any where near that point. What sort of failures do disc rotors experience when they're still thick enough to be rigid? Is there some warning, or there's sudden catastrophic failure?

On a related note, the bike has TRP 4-piston brakes, and came with TRP pads, which only lasted about 700 miles. I replaced them with another pair of TRP pads and got another 700 miles out of those. I've since switched to Shimano pads - the D03s, which are resin and e-bike rated and cost the same as the TRP pads - and they're lasting 2000 miles, but perhaps at the cost of the rotors. Any opinions on whether I'm better off with the apparently-softer TRP pads to save my rotors? (The TRP pads are hard to find at a LBS, whereas the Shimano ones are readily available; haven't tried other options, and I like to support a LBS when I can.)

Lot to unpack. I doubt there is a large population (on BF) of riders that have experienced catastrophic disc failure. I doubt your calipers are any better than mine and I wouldn't trust mine to determine 1.5mm vs 1.3mm. I don't ride my disc equipped bikes as much as the ones with v-brakes so wear has not been top of mind but measuring rotor thickness was not how I plan to determine when, or if, the rotors need replacement. The chances of both rotors failing simultaneously I can't even imagine. Let's go with that. Cross the bridge when you come to it.

Someone may come in with better info, but I think the issue is that if the rotor gets too thin, it may not be able to dissipate heat effectively, which may slightly affect braking performance (but would more likely just cause the rotor to warp a bit).

Easy solution is to buy some calipers (even cheap ones) and replace the rotors when they get somewhere close to the lower limit.

Everything manufactured product has a safety margin, because the engineers get a vote, marketing gets a vote, and then the legal team gets a vote. Use common sense.

The only somewhat catastrophic failure of the rotor I can think of that I've heard about is when someone went down a long steep hill keeping constant pressure on the brakes to stay slow. I think they overheated and the top layer of a different material started peeling off the base material of the rotor. Not to mention I think the pads and brake body were probably toast too.

You have a somewhat unique situation. Being in San Francisco with heavy cargo bike and two kids to haul around. So probably brake pads and rotors will wear out fast for you. But since you don't seem to be squawking about other brake issues than wear, you must be braking correctly down those hills they call streets in San Francisco.

Just keep checking for wear and make sure the center lock or rotor bolts are secure. You must be doing it right not to have any other issues but wear.

Cost analysis of 3 sets of TRP pads to 1 Shimano rotor and 1 Shimano pad set. That is how I would determine which way to go.

There are so many variables here....Metallic brake pads will be harder on the rotors than resin brake pads. Also your riding style and the conditions which you ride under will have some effect on the rotors. I am still using rotors which I have purchased between 15-18 years ago. I use resin pads because they are cheaper and most common in the bike shops around here. I ride all year round which includes snow and road salt and grit. I don't live in the mountains so no downhill rides and no hard braking for me, mostly flat and rolling terrain. I also take it easy on the brakes, I don't slam my brakes hard all the time.

Check with your local shop and see if you can fit larger rotors, I would also look at different rotors like IceTech rotors or if you have the space MDR-P rotors from Magura those are really stiff rotors perfect for electric bikes. I have them on my Magura MT5 brakes and they are excellent I also went to a 203mm from a 180mm. I would also get Kool Stop or SwissStop pads and get a couple sets of them. I have loads of spares for my e-bikes for the very reason I want to be able to brake confidently all the time and don't want to be without pads ever.

Bigger rotors and adapters would be a worthwhile upgrade. They give you more braking force, and more surface area means they last longer, and dissipate heat better. Pad performance varies greatly. I prefer SwissStop pads and rotors, they are longer lasting than the Shimano components I formerly used, and are easily obtainable on Amazon.

No such lame performance with my SA XL drum brakes. 33,000 miles and still the pads not worn anywhere near thin. I did have to make a different pull arm to extend use. Used on my tour bike at 290 lbs total. Any 4 finger lever will work.
Might be hard to stuff in a 16" wheel. The smaller 70 mm drum is 1/4 as good.

I would imagine any rotor could fail if worn down to paper thin - but the likely hood is remote. Far more likely is boiling the brake fluid and losing your brakes entirely.

I guess you are the comic relief this thread needed. I trust the manufacturers of production rolling stock to know what works for a given use case. The o.p. bike is more likely to have 24" or 20" wheels than 16".

There is a maximum distance that the posts can push from the caliper body. It's possible that if the rotor is too thin and the pads are also worn, that you can can squish all the way to the bar and not stop. I've never had that happen even on worn rotors and pads but I generally maintain my stuff and it never gets too extreme.

Funny that you mention calipers and measurements of rotors. I have Magura, so spec is 2.0-1.8 and replace below that. A day or two ago I noticed the plastic digital caliper from harbor freight on my shelf so I thought I'd measure.

My brand new Magura rotors are around 1.9
My old Magura rotors with 2 hard seasons of mountain biking in lava dust are 1.7-1.8
My nearly new Hope rotors that never quite fit are 1.8

Maybe the caliper is 0.1 low. Maybe it's crappy enough not to use to measure tenths of a millimeter.

In my experience with crappily designed rotors on San Francisco hills with a heavy bike and a kid on the back, the failure mechanism is warping, which results in noisy rubbing when not braking, but the bike still stops. I haven’t experienced this much with quality rotors, like yours, and there is no doubt a LOT of headroom in the engineering before something utterly catastrophic happened, like a rotor buckling or something while braking.

Fixed it for you!

Typical heavy overloaded e-bike with brakes that aren’t capable of providing reasonable service life and functionality = xtracycle.

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Nope. Heat dissipation is about surface area, and the surface area of a thinned disc is effectively the same as the surface area of a new one. The amount of heat capacity of the rotor due to its thickness (mass) is a pretty small part of the overall braking/heat issue.

Thanks, I'll take your word for it since I haven't taken a physics class in about 45 years.

So, I wonder why we have to replace rotors at the specified intervals? Is it just that they get so thin that warping can happen for other reasons, or is it something else entirely?

While the heat dissipation from the rotor total is based on surface area, the cross sectional area (thinness) has a direct effect on the conduction of the heat from the contact area to the rest of the rotor. This is a non-trivial part of the heat dissipation.

The thinning of the rotor will affect both heat dissipation and strength. Which matters most probably depends on how they are being used.,

My general experience with disc brakes are about 3 sets of pads to a rotor. However, the latest Shimano resin pads are lasting much longer for me. So much so I've yet to replace them after almost 2-years of riding. And I have not had to replace the rotors either. And I do check them. I had to replace the previous version pads about 3X per year and the rotors 1X per year. Though I've been doing a little less climbing lately.

For me, I replace rotors when they reach the 1.5 mm thickness which I can easily measure to several significant digits with a micrometer. Even when that was once per year, I had no issue with it. If I were goign through rotors much quicker than that, I might consider cheating the spec a bit. The OP seems to have found no issue goign down to 1.3 mm. Since the normal wear is 0.5 mm, this is 0.2 mm more or 40% more usage. I'd not push it beyond that.

Also, once beyond the spec, I'd be sure to check the brakes very often to catch an issue at home rather than out on the road.

Good brakes are important and more so with a heavily loaded bike with precious cargo. I'd not skimp on the brake maintenance.

Heat dissipation is strictly a function of surface temperature, surface area, and rotational speed. Conduction to the rotor surface not in contact with the pads would change minimally as the rotor thins because only the part in contact with the pads is thinning. And it is not thinning very much before replacement (perhaps 25% at the most, and more commonly 20%).

Except when you have a larger and thicker heat sink you dissipate heat faster this is pretty basic stuff here. Not rocket appliances.

When I've worn rotors down they get noisy easier and get the "turkey warble" sound. Also, a lip forms on the outside diameter which makes it harder to get the wheel back on. I only have discs on my mtb but I have worn rotors way past the minimum to the point where I worry about the front one bending under heavy load.

I've also measured the brake track on a rim brake and found it to be .025 inch lower than when new.

I use a machinist's micrometer to measure thickness but just out of curiosity. It's not super accurate anyway since the micrometer can't reach the low spots on the surface of the rotor. Automotive rotor micrometers and brake drum micrometers have pointed tips for this reason.

I've thought about that. In car systems, at least the ones I'm familiar with, the pistons can come out far enough so that even if the rotor surface is gone and the pads are metal on the vent fins in the rotor, the brake will still work. The pistons only retract a few thousandths of an inch and the fluid reservoir is big enough to compensate.

Obviously a bicycle has a much smaller fluid capacity even though the principle is the same.

And I have noted the "pretty basic stuff." As the pad contact area thins, the cross sectional area of the conductive heat transfer to the rest of the rotor is constricted at the edge of the wear zone. This is the point where there is that roughly 20 reduction. But the VAST majority of the heat dissipation is radiation and convection off the heated surface of the rotor. The "heat sink" effect of a thicker rotor is very small compared to the overall heat transfer to the environment. The rotor is not serving as a heat capacitor to any significant extent. Q = U*A*Delta T for convective heat transfer. Radiative heat transfer is controlled by the Stephan-Boltzman equation and is proportional to the difference between the 4th power of the surface and the ambient temperatures.

The primary cause of rotor problems I've ever had is hangar rash.

I noticed minor and harmless "warp" a lot more with cable disk brakes than hydraulic since the hydraulic ones keep themselves trimmed out by design. The cable ones I was always fiddling with the dials to try to bring out the bite point without the rotors scraping