When do you change your brake rotors?

There is usually a minimum thickness specified by the manufacturer in the rotor specs, I just change when it gets close to that,as measured with some cheap amazon vernier calipers.

Current discs have a minimum thickness of 1.8mm, they're currently at 1.9mm.

This is a good question. I have never even thought about it, am I now going to have to buy yet another tool that is used once a year?

The min thickness on these is 1.5mm, (Min. TH=1.5 printed on the side). They are down to 1.2mm in places so I'm going to replace them, especially as it's a front brake

I'd love to know thin the rotor that collapsed was, I didn't measure it. It felt a bit thin between the fingertips though!

no wonder they collapsed if they're down to 1.2mm, and the minimum is supposed to be 1.5mm

When I can feel too much of a step on the rotor, or when  I start thinking my brakes are getting wandering bite point but it's actually the rotors are so thin the pads can't reach them properly.

😀

"no wonder they collapsed if they’re down to 1.2mm, and the minimum is supposed to be 1.5mm"

I would say that the one that collapsed was almost certainly thinner, which obviously explains a lot.

I'm also, rightly or wrongly, a bit sceptical of manufacturer recommendations as they

A) have nothing to lose by playing it safe

B) have everything to gain, including selling more of their product (eg that ridiculous 3 year limit on helmets, which has at best a wet rizla's worth of evidence behind it)

"When I can feel too much of a step on the rotor, or when  I start thinking my brakes are getting wandering bite point but it’s actually the rotors are so thin the pads can’t reach them properly."

This is me for sure! I just felt the rotor that is at 1.2mm, you can feel a definite step, but it is also not the largest I've felt lol

I used to run them for years, never worried about it. Then I put some new rotors on and was amazed at how much better they were than the worn ones (using the same partly worn pads). Now I replace them as soon as I can feel wear by running a finger over them.

Intuitively, it seems like a rough rotor should brake better than a perfectly smooth one but that's just not how they work. The flatter and smoother the rotor, the better the pads can bed to the rotor surface and the more effective the brakes. If the rotors are noticeably worn, you won't be getting optimal braking. They aren't expensive so replacing the rotors along with the pads makes sense (then put the partly worn rotors on your spare bike and the rotors from your spare bike on your commuter bike and the rotors from your commuter bike on your spare commuter bike, and the rotors from your spare commuter bike on your shopping bike, and the rotors from your shopping bike on your spare shopping bike, etc. - this is why my shopping bike runs Saint brakes with 8" rotors front and rear.)

"This is a good question. I have never even thought about it, am I now going to have to buy yet another tool that is used once a year?"

The one I bought is really cheap, but you might be alright with just a fingertip test if you can check a few rotors with a known amount of wear on them, you'll probably get a reasonably good feel for it pretty quickly. I'm quite the tight arse though, so I like a number to hang my purchasing decisions on lol

Depends on your pad alignment and the cooling hole pattern but I've round they tend to thin in the middle first. That might give you a false reading with callipers. You can usually feel the dip with fingers or look closely when you have the callipers on to see if the surface curves away

I've had one collapse before, wasn't too bad, so I run mine pretty thin. Depends a bit if the wear is even - a vernier caliper will only measure the widest point

3 times a year...

When the new bike arrives 😀

3 times a year…

When the new bike arrives

This is a good question. I have never even thought about it, am I now going to have to buy yet another tool that is used once a year?

A vernier is useful for all sorts of stuff. I probably use mine once a week.

You can probably get a good enough one for brake discs and general bike stuff for 10-12 quid. (i spent more because i use mine for all sorts of other stuff, car, toy car, hobby electronics, CAD modelling)

I have chucked one away because it was badly worn - obvious scalloping..  I think I might go and check the others now.  I'll report back 🙂

edit:  NOne of the discs I am using have any measurable or visible wear.

You can get a decent idea running a thumbnail over the disc - is there a lip between the worn and unworn parts?

A vernier is useful for all sorts of stuff. I probably use mine once a week.

You can probably get a good enough one for brake discs and general bike stuff for 10-12 quid.

They are fine for measuring things like seatpost diameters but you can't measure the wear in a brake rotor with them because...

a vernier caliper will only measure the widest point

... so you need a micrometer that can take very fine readings at a single point to measure the wear. Cheaper to just run your finger over it, if you can feel the surface is worn, the rotor needs replacing.

"tjagain

I just changed mine after 5 years and 11k miles - they were pretty worn…

New Sram Centerlines for £17.99 each at Decathlon in case anyone’s interested

Iwanson gauge. Cheap as...

You can also lie a straight edge across the disc and look for daylight

I used to run them for years, never worried about it. Then I put some new rotors on and was amazed at how much better they were than the worn ones (using the same partly worn pads). Now I replace them as soon as I can feel wear by running a finger over them.

That, basically. Didnt realise how bad it was till I changed them. Funny, as im absolutely meticulous about disc rotors on the cars and generally never run new pads on old discs.

I say this as a mechanic - folks really should inspect their rotors more often. Same goes for their brake pads as well.

I can't count the times I have had to replace brakes where the pistons themselves were doing the braking. And at one old place we had a display of rotors that customers had ridden in with, sharp as razors and half as thin.

Plus the other display of "snapped in half while I was doing 60 downhill, just all of a sudden" and the disc is a rainbow of colours from repeatedly overheating past any safe bounds...

The guys who do snap a brake disk into bits while braking downhill usually pay more attention to their disks later on...

I can’t count the times I have had to replace brakes where the pistons themselves were doing the braking.

One of my favourites was when a mate's brakes started billowing smoke on a descent. He'd worn through the pads until the pistons popped out and sprayed brake fluid onto the rotor.

I have this to attach to verniers, not sure how accurate it is for bike brake rotor variances.

Park do this https://www.parktool.com/en-int/product/digital-caliper-accessory-dca-1

Also maybe it'd be useful to have a rule of thumb like we have for drivetrain, e.g. rotors typically last X many pads

Ive never worn a rotor down enough to need to replace them. Not due to lack of riding either.

I usually change my rotors after reading a thread like this and getting paranoid. I've tried using calipers to measure a used rotor, random number generator.

Braking is for wimps, real mtbers use a tree to slow down.

I've still got a pair of 2008 rotors in use, rear is definitely a bit stepped.

This thread has reminded me I should really buy some new ones...

I went pretty much 20 years on a pair of Hopes the bike came with.

I change my rotors pretty regularly.
I've also stopped using Shimano IceTech rotors as this happens to them at a pretty rapid rate if most of your riding is gravity focused.
[url= https://live.staticflickr.com/65535/52292428380_2a6a07a459_c.jp g" target="_blank">https://live.staticflickr.com/65535/52292428380_2a6a07a459_c.jp g"/> [/img][/url][url= https://flic.kr/p/2nEUn7L ]A little worn.😲[/url] by [url= https://www.flickr.com/photos/stu-b/ ]StuartBrettle[/url], on Flickr

https://www.flickr.com/photos/stu-b/52290964357/in/photostream/

In fairness I am averaging around 4000 metres a week and during the summer had some 10000 meter weeks.

… so you need a micrometer that can take very fine readings at a single point to measure the wear.

Or just add a sprinkling of common sense.

"The guys who do snap a brake disk into bits while braking downhill usually pay more attention to their disks later on…"

Proof, if proof was needed, that the system works

Still have an original 185mm Enduro 4 lightning rotor, must be 20yo now.

Also maybe it’d be useful to have a rule of thumb like we have for drivetrain, e.g. rotors typically last X many pads

I measured my Hayes rotors, which are claimed to be 1.95mm new, and marked minimum 1.7mm. They've been through 1.5 sets of pads and now the front is around 1.90-1.93, the rear 1.85-1.90 in various places. Depends how straight I can hold the calipers. I used the gadget posted earlier, so not as accurate as a micrometer.

I wear through pads at near enough the same rate to replace them at the same time, but perhaps the rear rotor wears quicker due to having more mud thrown around it.

I bought a hope wheel yonks ago and the rotor on it was the older 5 bolt really deep ones. It had worn so thin, you could have used it to slice bacon. I think that must have been half a mm on the leading edge.