ModeratedUser24102018

having a problem telling what size this is. its 68mm by xxxx. its gone rusty and i cant see what length it is where the rust has covered. Plus i dont know where to measure to and from. hence the picture with the tape measure.



Thanks

TejanoTrackie 

You measure it end to end. Looking at your tape measure, it is well over 120mm, so it is intended for a road double or triple crank, definitely not meant for a single speed / fixed gear application.

SquidPuppet

Maximum spindle width, from tip to tip. It looks close to 125mm. If you are going to buy a replacement, you'll need to know if the tapers are ISO or JIS.

ModeratedUser24102018

ah thanks for clearing that up.

i would assume that theyre ISO tapers because im from the UK so generally ISO tapers are used over here. i am pretty close to 125 mm on length but im stuck inbetween a size on a new purchase i can either go too short at 118mm or too long at 122mm. does 4mm even matter?

SquidPuppet

That assumption has a good chance of being wrong. It's all about the crank manufacturers and JIS is very common. Probably more common than ISO. Be sure before you buy, it matters.

Your crank will dictate which taper type and which spindle length to use. What crank is it?

ModeratedUser24102018

some crappy run of the mill no branded crank. is there any identification i can look for on the bottom bracket?

SquidPuppet

Is it a single speed specific or track crank? As Tejanotrackie pointed out, that spindle length seems too long, unless it's a road crank being used as a single speed.

The Bottom bracket should be marked, but it is possible that it isn't, or that the markings have been destroyed.

This article is perfect for your needs. It explains the exact differences between JIS and ISO tapers. It also explains their potential interchangeability. You can mix-n-match, but the fit is different and therefore chain line will be affected. Sometimes a desired result can be achieved. Other times it throws things out of wack. The article has big clear color photos too, so identifying what you have should be doable.

https://www.sheldonbrown.com/bbtaper.html

ThermionicScott 

Even more evidence that it's probably not ISO.

You'll want to get a better measure before ordering the new one. Maybe tape it down to a piece of cardboard, and use an upended ruler on each end to make a good mark on the cardboard, and measure between those.

Last I checked, you can get Shimano cartridge bottom brackets in 122mm and 127mm, so it's probably one of those.

ModeratedUser24102018

It was a single speed crank. The bottom bracket was in the bottom of a frame i bought. The guy said he didnt have the tools to take it out. Now ive just took it out its rusty as hell. So my guess is that its probably not a single speed bracket if you are saying it is too long. also thanks for the information, ill defiantly check that out

ModeratedUser24102018

looking at the article that SquidPuppet had left. i could probably say that it is not ISO. i recall the crank arm having a thread with a plastic casing. it also appeared to look alot like the crank arm in the article.

ThermionicScott 

So, I'm back to being confused: are you trying to mate a single-speed crank with a different bike that already had a bottom bracket installed in it? If so, the important thing is to get the right BB for your new crank, not to get the exact same thing your frame had in it, since (as was pointed out earlier) it's probably too long for SS use.

ModeratedUser24102018

okay okay, lets get rid of the confusion. I bought a frame to build my own fixed gear. bottom bracket was already in the frame when i got it. ive just taken it out. all i have, IS the frame. everything else is getting thrown away so i can start fresh. but i wanted to know what size i needed because i cant see cause of the rust and grime.

ThermionicScott 

In that case, quit trying to measure the old bottom bracket and just throw it away. Seriously.

Can you post some pictures of your crank? Most of the low-priced single-speed cranks these days take a 107mm or 110mm bottom bracket, but there are exceptions.

SquidPuppet

Okay, so the two weren't already together?

If you want to use that crank you need to figure out the right spindle length for the crank. The frame has nothing to do with what length spindle you need. Since you have no way to contact the crank manufacturer, your task is a simple one. Install the bottom bracket into the frame. Install the drive side crank arm and fully torque it down. Now take a chain line measurement. It will probably be far from 42mms. Then just do the math to determine how much shorter a spindle you need. Easy, just a little time consuming.

This is assuming that you are running a real single speed freewheel or track hub and cog in the rear. Which would both be at approximately 42mms from center.

ModeratedUser24102018

i just assumed i needed to match the bottom bracket size i will try and get a few photos and upload them soon

SquidPuppet

Nope. Different cranks are built slightly differently from other cranks. Slightly. But it matters. You want the tips of your chain ring teeth to be as close to 42mm from bike center as possible.

ModeratedUser24102018

so if youre saying that they need to be close to 42mm from the centre. would it be better for me to put the bottom bracket back on the frame, with the cranks and measure how long that is? and then just buy a new shorter one? or is that just a bad idea

SquidPuppet

That is a perfect plan. See below.

ModeratedUser24102018

so these arent the cranks im having on, these are what were stuck to it. reason im showing this is because ive just noticed how close it actually is to the frame already and there is a dip in the chain stay, presumably for the chaining/chain. if i go any shorter on a bottom bracket, could i risk grinding it?

SquidPuppet

You need to be a bit more precise when taking a chain line measurement. It is easy though. Use calipers to measure the the diameter of the seat tube (or down tube). Divide that number by 2 and record the result. Then measure from the TIP of a chain ring tooth to the outer edge of the seat tube. Record the result. Then add the two numbers together. You want the front ring and the rear cog to be as equal distance from center as possible. Since rear cogs are pretty much stuck at ~42mms from center, adjusting the front chain ring to match is what's necessary.

So....take that front measurement and see how far inward you'd need to move the chain ring to get it to 42mms. Also, measure the current gap between the chain ring and the stay. Do you have enough room between the stay and the chain ring to move the crank inward the required amount? Let us know.

There are some frame and crank combinations that just wont work. If that is the case for you, one possible solution is to run a smaller chain ring to avoid contact with the stay. You'd then also need to change the rear cog in order to maintain the current ratio (if you like where it is).

ModeratedUser24102018

okay so the dimiter of my downtube is roughly 10cm...so i divide that by 2 = 5. then the distance from the chainring tooth to the seat tube is 3.4cm. so that gives me a total of 8.4cm?

ThermionicScott 

I think your math is off. Just hold up a ruler like this.

ModeratedUser24102018

thats what i did haha ill do it again

SquidPuppet

No because your down tube and seat tube are likely to be different diameters.

And also no because there is no way your down tube is 100mm diameter.