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It appears I am missing (yet) another tool: a caliper. It simply makes no sense to haul a bike to the LBS to measure a seatpost.

How important is a caliper for a home-based bike workshop?

There is far less variability in handlebars, and handlebar dimensions are anyway more frequently documented in brochures. If seatposts are basically the only thing I'd need to measure, it would make more sense to just take all my seatposts to a friend/neighbor and note the diameter just once, or to borrow/rent a caliper.

In terms of categories, there is a wide choice:

  • very inexpensive (<USD10) no-name digital caliper,
  • very expensive (>USD200)—yet still from an unrecognizable brand—digital caliper, and nothing in-between,
  • stainless steel calipers,
  • brass caliper—either with a rotary needle or with a slide-rule-like fractional measuring scale,
  • (non-scratching) plastic caliper.

My first thought is that I'd use it sporadically, and hence a digital tool would be a bad idea (battery will be depleted between uses). Am I on the right track?

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    This is one of several good videos about using a caliper: youtu.be/vkPlzmalvN4 I hope everyone's ok with this link going in a comment to the question rather than repeated under multiple answers.
    – pateksan
    Mar 12 at 20:36
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    Calipers are good to have in a home-period.
    – whatsisname
    Mar 13 at 5:59
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    Stainless Mitutoyo vernier👌. Lifelong tool, you'll start using it more and more. Probably enrol in engineering 2 years later like I did. Calipers changed my life 😆
    – Lamar Latrell
    Mar 13 at 8:53
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    Terminology: It sounds like you're specifically talking about Vernier, dial, or digital calipers, rather than the sort used more for transferring measurement (similar to drawing compasses). I bought my Verniers years before I did any bike maintenance, but use them as much for bike stuff as everything else put together
    – Chris H
    Mar 13 at 9:23
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    BTW the batteries on the digital calipers in work last ages so long as you remember to turn them off (some of the cheap ones don't turn themselves off). Cheap digital are pretty decent, though I've used fancy mitutoyo as well. A big advantage of digital is you can switch to inches - but you still need a fractions to decimals table
    – Chris H
    Mar 13 at 19:41

7 Answers 7

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Personally I find a caliper invaluable. Aside from seat posts and tubes, handlebar diameters are sometimes not quite as advertised and a caliper quickly reveals the truth!

A small 4" brass non-digital caliper runs about $10 in US home improvement stores (at least) as of 2023. This is about double what they cost a few years ago pre-Covid, but is still cheap enough that I can toss it around or carry it in my shirt pocket without worrying too much if I lose it.

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    Can brass non-digital caliper reveal a difference of 0.2 mm? I know that if you get a reading of 27.3 mm, you can guess that the caliper is not properly calibrated (or else it's a problem since the bike frame is flawed), but a (small) fraction of a millimeter accuracy is still important.
    – Sam
    Mar 12 at 16:11
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    @Sam Yeah, it should be fine. Even the cheap plastic ones can read down to 0.05mm. I think the limitation for those is accuracy, not precision.
    – MaplePanda
    Mar 12 at 16:45
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    People who know the difference between accuracy and precision are 🔥🔥🔥
    – Lamar Latrell
    Mar 13 at 8:54
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    @Sam You can get calipers with 0.05 or 0.02 mm nonius. There are "clock" calipers without the nonius, if you are not comfortable with it. The advantage is simple: They will never run out of the juice.
    – Crowley
    Mar 13 at 16:00
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    I'd advise against cheap brass or plastic calipers: they tend to be poorly-made, are too soft to last well, and are generally poor value when compared with (stainless) steel ones of a similar price. I'd suggest instead getting a steel non-digital one, and investing the couple of minutes it takes to learn to use it accurately: that way you'll never be caught short because the battery's run out.
    – Mark Morgan Lloyd
    Mar 15 at 10:40
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By far, caliper is the tool that I use the most in my toolbox, whether working with bikes or something else.

If you don't have a caliper, you can't make small-distance measurements. With bikes, such measurements are probably more common than large-distance measurements that you would do with a tape measure. Setting constant bike fit obviously requires a tape measure.

Calipers can measure hole depths, inner diameters, outer diameters, thread pitches etc. If you don't have a caliper, you don't know the important dimensions of your parts.

I have a stainless steel caliper. It has good enough accuracy. Just ensure you buy a tool that can be used for all of hole depths, inner diameters and outer diameters. There may be some calipers that can only measure outer diameters.

If you want more accuracy, a micrometer screw would offer that but it usually is restricted to a very narrow range of measurements and can only measure outer diameters, never hole depths or inner diameters. I have never found micrometer screw to be useful when working with bikes.

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    Calipers measure sub-millimeter distances by showing several lines, 0.0mm, 0.1mm, ..., 1.0mm. You see which of the lines are in the same position. Some calipers may have 0.00mm, 0.05mm, 0.10mm, ..., 1.00mm but I'm not sure if my eyesight is good enough for seeing if 0.05mm or 0.10mm is a better match. This works surprisingly well, it's easy to see if two lines are a continuation of each other, in the same position, or if one of the lines is a bit off.
    – juhist
    Mar 12 at 16:22
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    @juhist yup, that's describing a Vernier caliper which is a good accuracy upgrade over a plain caliper (which is really just a ruler with pins) There's also Dial and Digital calipers too. And they all come in metric or imperial versions.
    – Criggie ♦
    Mar 12 at 18:08
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    @Criggie I see. So the logarithmic-numbers trick that was so helpful for devising slide rules in another era remains in use in calipers, but it is now identified through the use of the Vernier qualifier; is that about right?
    – Sam
    Mar 12 at 19:10
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    @sam logarithms and slide rules have little to do with verniers which are more of an optical thing. But the idea of matching a pair of unequal scales to get a reading, is similar across both.
    – Criggie ♦
    Mar 12 at 21:57
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    @Sam Measure over multiple threads and divide length by number of threads.
    – MaplePanda
    Mar 13 at 3:29
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It depends on what you call "home-bike workshop". If you speak about helping to maintain "unknown" bikes (yours or those of your friend): yes, it's a relatively cheap (universal) tool that can help you to know what to look for when ordering parts.

But you are doing non-critical maintenance of your own bikes, that have known specs, the spec sheet is sufficient, and the caliper is optional. Or it may not matter so much to have the exact answer (example: the inner width of the rim: a 1mm error is sufficient).

Personally, the only time where I regretted to not have one was to measure the disc rotors thickness to check if they were worn out - because rotors don't have wear indicators, the only way to know with certainty is by measuring. For handle bars and seat posts, I could get along with the spec sheet (in one case, the size was written on the part).

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    Getting seatpost and handlebar diameters from spec sheets already means a certain level of quality. Dealing with cheap, kids, or 2nd hand bikes, measuring is often essential.
    – Chris H
    Mar 13 at 9:32
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    I have found that it's usually quicker to measure than try to look up the spec sheet
    – ojs
    Mar 13 at 9:48
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    @ojs me too, but I do keep downloaded copies of the spec sheets for my bikes in my Dropbox, allowing ordering parts from work.
    – Chris H
    Mar 13 at 9:57
  • @ojs True, if you have a caliper and the bikes are "within reach". For the people living in appartments and storing their bikes outside their appartment (in the basement or some condos also have a dedicated bike room), it's faster to search in the spec sheets rather than reaching the bike to measure the part.
    – Renaud
    Mar 13 at 15:43
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In addition to the other answers, I found an additional a use for my caliper on my bike, and that is to measure actual tire width. The width of the tire is a critical dimension when dialing in a optimum tire pressure for riding comfort and traction and a millimeter can make a difference in that pressure (I use a calculator that uses tire width as one of the inputs). The tire width printed/cast into the sidewall of the tire is only a guideline as the width of the rim affects the actual width which is what the calculator uses. I can estimate the width using a tape measure and "eye-balling" it and getting pretty close, but with a caliper I can eliminate that "eye-balling" error and get a repeatable, accurate measurement.

Granted, if one is not THAT concerned with the minor differences in pressure associated with minor changes in tire width, it is irrelevant. But that is one of the things I use my caliper for.

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    This is the first answer that works for calipers in general, not just Vernier or other precision calipers. For tyres you only need about 1mm precision, maybe 0.5 if you're really picky about pressures. I've been known to use a bench vice for measuring mounted tyres - open it until the tyre just drops in, then measure the opening with a ruler (it was quicker than going indoor for the Vernier calipers).
    – Chris H
    Mar 13 at 9:36
  • @ChrisH I tend to be the "picky" about those sort of nuances (a perfectionist, just not a chronic one), which is why I qualified my answer at the end. I like the use of a vise as a backup caliper. Not very portable, but it does work equally as well (I will have to remember that). Another personal example relates to the effect of rim width. I run a 25c tire on my road bike which caliper (measure) out to 28mm. I cannot run 28's because they are truly 31's which don't fit my frame. And the ideal pressure difference between 25 and 28 is pretty significant (in my subjective opinion)!
    – Ted Hohl
    Mar 13 at 19:33
  • My 32s are probably more forgiving and I encounter such a range of surfaces in one ride that any pressure is a compromise. But I did keep fiddling with the pressure in the back tyre of my mtb yesterday
    – Chris H
    Mar 13 at 19:36
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Caliper is useful for applications where you need to know the width of round objects. As it comes, bikes are usually made of round objects.
The major benefit is that caliper 'locks' on measurement, so you can walk around the shop and try what fits instead of reading the measure for every thing separately. A basic quality tool from hardware store will suffice - a steel ones have scale of 1/10th of milimeter, which is enough for anything you would do around bike or a car (maybe except spark gaps, but you use the pile of iron strips for that).
You do not need electronic one (unless you have problems reading the scale on analog ones).

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Do you need a caliper? I'd say no straight-off. Comparative measurements will often get you far enough. Like any tool, you can always find other solutions.

There are two types of measurement and which one you need depends on your personality, what you're trying to achieve, and how your mind learns.

  • ABSOLUTE which results in numbers. This is rulers and calipers and tapes and protractors.
  • RELATIVE or Comparative, where you're checking two things against each other. Gauges and existing parts would go here.

However as your tool collection grows over your life, each subsequent tool generally does a more specialised task and has a narrower range of uses. We start with a hammer and two screwdrivers and a knife, and that set can probably do 30% of all tasks you face.

Comparative measurements (ie holding a piece of wood up to another) can do a lot, but a ruler should be an early purchase.

Calipers can be had cheaply, which is a great thing for expanding tool collections. Try ebay and garage sales and so on. You do not need new tools. If one comes up at a good price, snag it.

For measuring seatpost holes in frames I have a go/no-go gauge which measures standard sizes.

enter image description here

https://www.tinkercad.com/things/dez4SQ1NoiA-seatpost-stick-small-v2

But once printed I measured its accuracy using both a set of calipers and a micrometer and I test-fitted it to a couple of frames where I know the seatpost size already.

This gauge is no-use for measuring seatposts themselves though, and also can't do handlebars.

I stumbled across this today - the documentation archive at wolf tooth has some phenomenally-useful PDFs at https://www.wolftoothcomponents.com/pages/tech-docs including

enter image description here This seatpost specific page-click for full sized PDF

Print that at 100% scaling, check with a ruler that the dimensions are right, and then cut out the ribbon. Figure where it overlaps and you have an answer. Note not all the sizes are listed.

To measure seat tubes, put the printout over the top of the hole in the frame and press down with a thumb. You should be able to get a visible crease matching one of the common sizes shown.

Personally, yeah I own multiple calipers and micrometers, and I use them at least weekly. As funds permit, more-specialised tools become achievable.

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    That insertion gauge needs a pretty well calibrated 3d printer, which is a nice thing to have in its own right. You could make an external version for seatposts but calipers are easier
    – Chris H
    Mar 13 at 9:33
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    I think this is the exact reason to get a vernier caliper: it can do the same job as a huge number of special gauges even if those gauges would be better for that very specific job
    – ojs
    Mar 13 at 9:46
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    @Chris H Calibrating a 3D printer is easy, you just need a caliper. It's also needed to accurately measure dimensions when 3D-printing replacement parts. Learnt how to use calipers 45 years ago, as a little kid. Also useful to figure drill diameters when the markings have become invisible, or there had never been markings in the first place. Tapping an M3 hole may require a 2.5mm or a 2.4mm drill, depending on the material. Something I can't differentiate with my naked eye.
    – Klaws
    Mar 13 at 15:00
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    Fair enough - I was trying to make the point that "comparing" is a valid way of measuring that still doesn't result in a number, just a "too big/small" or "equal" I kept a bunch of old seatposts around for this kind of thing before making the above gauge.
    – Criggie ♦
    Mar 13 at 17:34
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Summarizing

Concretely, with a caliper (just like handlebar, the singular is accepted—and more common—though the plural is also correct for a single unit) one can:

  • Not wonder whether a red, green, or blue spoke wrench is needed. Measure the width of the spoke nipple and know for sure.
  • Order a seatpost knowing that it will fit, starting from either the present seatpost or from the bike itself.
  • Determine the size of the stem needed for a given handlebar (but not the grips, as they are always 22.2 mm).
  • Measure width of disk rotors and confirm against manufacturer specs whether they need to be replaced (thanks, Renaud).
  • Measure actual tire width—after installation on a rim—and then either 1- calculate pressure accordingly (thanks, Ted Hohl), or 2- know whether you are legal for a given competition (<=33 mm UCI cyclocross, >=1.5" MTBO) (thanks, Vladimir).
  • Measure ten rotations of a screw/bolt (then divide) to determine its pitch (thanks, juhist).
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  • * Order a front derailleur knowing whether it will fit
    – mander
    Mar 15 at 20:34
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    Measure actual tire width—after installation on a rim—and know whether you are legal for a given competition (<=33 mm UCI cyclocross, >=1.5" MTBO).
    – Vladimir F Героям слава
    Mar 17 at 9:00

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