erbfarm

Hi all

I'm swapping cantis for V brakes on my 1990's mountain bike and need to do some cable re-routing by adding a cable stop to my frame. How do I measure the diameter of the top tube so I know which size to buy? The options for the cable stop are 31.8 or 34.9.

Thanks!

billridesbikes

Um, calipers?

If you don’t have calipers measure around the tube with a tape measure and divide by 3.14, that is also the diameter.

Phil_gretz

Digital calipers. A 1990s mountain bike could be anything from 28.6mm upward. Photo?

dedhed

Are the tubes round?

WizardOfBoz

Hey, Phil, they don't have to be digital! One could use a dial or even an old vernier caliper, some of which were more accurate than the current ChinCom offerings at Harbor Fright.

I own an old Starrett 123 Vernier Caliper. Very accurate. Looks like the one below:

Andrew R Stewart 

If you don't have a caliper then take a piece of paper and cut out a square from one edge. The square might be 1", 1 1/8" or 1 1/4". Then place this gage over the tube and see which fits right. That's the tube's diameter.

Or wrap a piece of paper around the tube and mark on it where it meets back up with the start of the piece. This is the tube's circumference. Measure this with a ruler and divide by Pi (3.1415) So a 1" tube has a circumference of 3,1415", a 1 1/18" tube is 3.53" and a 1 1/14" tube will be about 3.93".

This is simple 8th grade math. Andy.

HillRider

If you don't have a metric caliper or even a metric ruler these are the inch equivalents:

1-1/8" = 28.6 mm
1-1/4" = 31.8 mm
1-3/8" = 34.9 mm

As long as your top tube is round it is very likely to be one of these three diameters.

HillRider

Andy, it was when we were in 8th grade. Maybe not so much any more.

Koyote

And yet, the vast majority of adults would have no idea what you guys are talking about...Which is a sad commentary.

Retro Grouch 

Those are your options so you don't have to be super accurate, you only have to know which (if either) will fit your bike.

If you have a big Crescent wrench, tighten it loosely around your tube like you would a caliper and measure the distance between the jaws. It'll be accurate enough for your needs.

Where did you find V-brakes these days? When I was looking a couple of weeks ago, everybody was out of stock.

dsbrantjr

Who knew math could be so complicated?

Phil_gretz

Yes!! I remember working with these. Unless you're over 60 years old or a trained machinist, it's unlikely that you can read any non-digital device.

dedhed

My problem as a 60 YO who uses calipers regularly, is my eyes don't let me read a vernier anymore, so everything I own is dial or digital.
I can't imagine reading my vernier transit anymore.

dedhed

We use these all the time at work because over 150 years of burying pipe in the water business, diameters have changed and repair parts have to fit.
Mostly the vendors or manufacturer reps give them out as swag.
Come in imperial and metric.

https://www.tigersupplies.com/Produc...217-6653-.aspx

bikeme

Math? Who knew it's so handy?! This is a wonderful application of that formula.

WizardOfBoz

The Turnbull circumference tape is nice. I'm familiar with the (very expensive, machinist-oriented) Pi Tape. The tape is has linear measurements (times pi) etched on it. There's an offset and the witness line has verniers. You can read the diameter directly. Work great, but they cost hundreds of bucks. In use:

dedhed

I only need to get to 1/100 of inch so these are close enough especially when measuring crusty old buried stuff. I have a 20' Lufkin tree tape that will let me get to 60" pipe.
https://www.crescenttool.com/measuri...e-measure.html

fietsbob

General Tool # 723 calipers are under $10.


https://www.grainger.com/product/54G...1102900020694!

vane171

This interesting gizmo I take it you don't need to wrap around a pipe, it is enough to have access to just a part of the pipe's curved body, right?

From the preceding, I like most the spanner wrench method. That's exact enough to make you decide which of the two kinds of diameter you have.

As to calipers, everybody should have one of those cheap plastic ones at minimum that sell for several dollars. I picked up ones with mechanical readout on round dial gradated 1/64th inches, because it was on sale but having grown up in metric world, I still have problems with imperial readouts. Takes me some time squaring it in my head and sometimes I can't make it LOL. Next time I see some good sale on digital, I will get it.

WizardOfBoz

No, it's a tape measure that you wrap around the object to be measured. The tape is offset (shaped like a "Z") and the scale on the main part of the tape and a Vernier scale on one end. This allows you to get a precision circumference. The circumference is marked in pi(3.14159) inches or centimeters, and so reads directly as diameter.

vane171

I can see it would be expensive. If you don't need to be too exact, perhaps there are some devices that estimate the pipe size from a portion of its curved surface. Like if you dig out a pipe only partly and or don't have access to wrap the tape around it.

In past I also resorted to measuring smaller pipe diameter by squeezing two sticks to the sides of it while spreading the free ends with the other hand, so the sticks would be parallel and then measure the distance between the sticks right near the pipe. That give good enough result in most cases but it is handy to have more than two hands... or you can use a rubber band to hold the sticks against the pipe to free one hand for measuring.

madpogue 

Funny how we've lost our 8th grade math, but we seem to have retained or 8th grade humor.

_ForceD_

I have calipers. But in lieu of calipers you could do this...then measure across the wrench opening. (Maybe some masking tape on the wrench jaws so you don't scratch your paint.)

Dan

Fissile

Redneck calipers! I've done this many times. This method works just fine to measure tube diameter.