mschwett 

most of the really cheap ones i looked at claimed to be only one tenth that "precise," e.g. ±0.2mm/0.01” for several highly reviewed <$20 units on amazon. if the range of a rotor is 1.8mm to 1.5mm, that seems almost useless, no? it could say 1.4mm when it's 1.6, or 1.7 when it's 1.5. i guess i'll find out

cyccommute 

I’ll see your $3 caliper and raise you a a $2 caliper. The best part is that you can amaze your friends with your ability to read the vernier. Kind of like knowing how to use a slide rule.

But a caliper really is the wrong tool for the job. You should be using an outside micrometer.

mschwett 

AHA. that's probably the answer i was looking for, given then we're measuring something so very thin. i see quite a few of those in the <$40 category that claim much, much higher precision than similarly priced digital calipers.

this $30 one looks promising. https://www.amazon.com/dp/B07KC7YNR8...kZXRhaWwy&th=1

cyccommute 

Well if you insist on making multiple measurement, expect multiple results

But, that said, for most bike work, even those $2 calipers I listed above are quite serviceable and accurate enough. But for disc rotors, the arms of a caliper aren’t going to measure the wear track of the rotor. A micrometer is what is needed since it gets past the outer lip of the rotor.

Barry2 

And I'll raise you again $0.92

Barry

And before you find it ....... $0.01

cyccommute 

That one would probably work. This one would also work. It’s not like you are going to be using it all that often.

cyccommute 

Okay that’s cheaper than dirt. But what does it have to do with eyebrows?

Black wallnut 

It depends on your definition of cheap. here is a set I purchased to cut apart and use for a dro on my lathe tailstock, haven't done so yet. I looked at many and many were as precise. Dial calipers also fall into the cheap category and are .001, as are vernier calipers. Spend more to get more.
Wen $11.67 also from Amazon. Look long enough and you will find plenty in the <$25 with .001" accuracy.

sweeks

The majority of the dental students I teach are unable to understand the operation of a vernier caliper (the "Boley Gauge") commonly used in dentistry. If they can estimate to a half a millimeter, they think that's close enough. They don't even look at the vernier scale.

This is the reason for mentioning the Crown Thickness Gauge, though its limit is 0.1mm.

Andrew R Stewart 

"The majority of the dental students I teach are unable to understand the operation of a vernier caliper (the "Boley Gauge") commonly used in dentistry. If they can estimate to a half a millimeter, they think that's close enough. They don't even look at the vernier scale." sweeks

I completely agree. I have felt as we go further into the digital world the value of an analog approach is lost. This is sad as real life and our interactions are far more shades of grey then black and white. Andy (who still has the slide rule from his grandfather in the desk)

SoSmellyAir

For disc brake rotors, why are we futzing with calipers? Why isn't there a Shimano or a Park tool with a slot in which, if the rotor fits, you should replace the rotor? Like those 3 prong chain checker tools?

Barry2 

Because the brake pad does not come to the edge of the rotor.
Meaning the very edge of the rotor will not wear. This stops the use of a Go / No-Go gauge.

Barry

cyccommute 

Most of the ones I see are ±0.02mm or better. The 1¢ ones that Barry2 listed have that kind of precision. Many of the digital ones only read out to 0.1mm. They are probably more accurate than that but the display is limited. I’d avoid those. There are others that have a 0.01mm read out which would be a better choice at the same price.

SoSmellyAir

Good explanation; I should have thought of that. Which means my calipers also would not work.

sweeks

Heck, I still have *MY* slide rule! (Though I don't keep it in my desk...)

Garthr

My only suggestion for digital calipers is to remove battery when not in use as "some" of the calipers can drain the battery even when not in use. At least until you can determine if it drains the battery or not.

esasjl

Pads on motor vehicles tend not to contact the disc right to the edge. I think with bicycles the pads contact over the edge (to clear debris?) making thickness measurement easier. I'm surprised there are not marker areas that show when the disc is in need of replacement.

bunnyberg

I have been using a plastic 3d printed caliper some time. Not great, but works in my partner's house, where I don't have everything. You can find the design file in thingiverse... I think the material cost is below 1$.

mschwett 

this one was $29 delivered. i try and avoid amazon whenever possible but for things like this, it's certainly convenient. i also have a similarly inexpensive outside micrometer on the way, will keep one or the other.

the big news is that it's either WAY OFF, or one of my LBS was a little overzealous in their statements about my rotors. they claimed them to be at 1.52 and in need of replacement. 2,000 miles of moderately hard use. last one lasted 6,000 in similar conditions.

per this tool, the front is at 1.72 and the rear is 1.62-1.75 depending on where it's measured. repeated measurements in different spots on the front are all ±.01, whereas the rear does seem to vary quite a bit. will be interesting to see what the micrometer says, which should be able to be positioned on a particular spot in the interior of the rotor more accurately.

SoSmellyAir

I have a similar one from Amazon.com, also a little under $30 -- they are probably all made by the same factory -- but how can you use this to measure surface thickness past (i.e., inside of) the edge of the rotor?

Andrew R Stewart 

Most every caliper I have seen has a gap/cut back portion at the base of the jaws to do just this. But with the jaw length and the pad contact to the pad "width/height" being less that the jaws' length the jaws would need to be angled to only contact the pad wear band. At best I suggest that a caliper measurement with a common caliper will tell the high/thick spots along the line the jaws contact.

Not yet mentioned is that to replace the parts by following minimum pad or disk thicknesses published guidelines is trying to max out lifespan. While there's nothing wrong with that goal my experience with sucking every last mile out of a part can lead to disappointment in performance at the later stages of wear and if not frequently (and more so as the end points are approached) checked can be used longer than should be. But that's the rider's choice, not mine. I am willing to trade off the last miles of a part for a better working system and experience on my rides. Andy

SoSmellyAir

Thank you, you are right; mine has the same gap as the one mschwett bought.

The wear band on a used rotor should be easy to spot.

True. I hope that is good enough. Otherwise, I have to ask a polymer film lab I have worked with to use this: Mitutoyo, Product: Laser Hologauge LGH.

Same here. I plan to measure and replace well before the Shimano recommended minimum threshold, to account for potential inaccurate measurements.

mschwett 

yes, it’s a little awkward but it does seem to clamp true and tight to the braking surface of the rotor, although at various angles the results slightly varied. interested to see how much more targeted the micrometer will be.

as for lifespan, my wear components don’t seem to last very long. 2k for pads, 6k for rotors, 2k for rear tires, 4k for front, 2k for a chain and 5-6k for a cassette. it adds up and so i more or less replace when the manufacturer recommended wear point arrives, and not much sooner. i doubt the manufacturer is aggressive about this, if anything i’d hope they’re slightly conservative.

awac

Spot on Maddog. Reminds me of a woodworking saying. “Measure with a micrometer, mark with a crayon and cut with an axe”. The point is accuracy is required at all stages for it to be any use. 1/10mm is fine for work in bicycle world. If fact you can find yourself fussing over to much when it is not required.

I have micrometer and verniers that go more than this, but I find using an old East German dial vernier that shows 0.1mm my most useful tool, no batteries and I can see it (just lol). If you need real accuracy then just pick up an analogue vernier and magnifying glasses for the odd thing you might have to do.

Electronic are great in a shop where speed is a pay-check. The big advantage is you can reset zero at any point and compare with a second measurement. I like to stress my brain sometimes, so do a mental exercise when I can. It hurts I know, I know.

Hey don’t listen to me, buy what floats your boat, I never need much of an excuse to buy a tool I am lusting after. Do though pick up an analogue vernier and learn how to read it, most people don’t take the time and you will be cool coming in with your old skool ways…..well that’s what I tell myself……lol.

Bike Gremlin

Analog instruments are very practical when you need to see the ballpark value. A quick glance does it (like an analog watch for example).
Digital instruments are more practical when you need to get a very accurate reading very quickly.

A "catch" with digital calipers is that you generally need to pay a lot more for the similar precision compared to analogue calipers. Also, as noted above, a micrometer is a better tool for measuring disc wear.

Some digital calipers are a lot easier to read with imperfect eyesight or with poor lighting, compared to analog ones (though not all, and some displays are crappy in bright sunlight).

Being able to quickly "zero" a digital caliper can be a big plus, depending on what and how you measure (you can one-click zero it to any caliper opening width and just measure the difference from that point).

Relja