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Loose Nut Behind d' Wheel
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Discussion Starter · #1 ·
Here's my "rigid singlespeed":


I should say that's my *current* rigid singlespeed. I was at the BMX track on Sunday and a fellow rider let me take his Brew cruiser for a spin around the track. By the first turn, I was smitten. I want one. I need one (well, that's subjective). The handling was worlds apart from the Haro, and it was soooo light. Sigh.

Anyway, I found out I'm getting a bonus at work, so I see a Brew in my future. I can swipe a lot of parts off the Haro, but not the crank. It's an "American" BB (large diameter) and the Brew will have a "Euro" BB (normal mtb size). The Black Widow crank and BB are a matched pair with a hexagonal spindle, so I can't just get a new BB. I'd like to run an mtb crank instead of the heavier BMX units. Since I'm only 125 lbs, and don't do big air, I don't see a problem with this. The problem I do have is that I'm not sure how to figure out what chainline I've got.

The over-locknut spacing on my rear hub is 124mm. The distance between the centerline of the cog and the inside face of the dropout is 12mm. Does this mean I have a 50mm chainline? How do I figure out what crank/BB will give me a match up front?

Kathy :^D
 

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Lucky said:
Here's my "rigid singlespeed":


I should say that's my *current* rigid singlespeed. I was at the BMX track on Sunday and a fellow rider let me take his Brew cruiser for a spin around the track. By the first turn, I was smitten. I want one. I need one (well, that's subjective). The handling was worlds apart from the Haro, and it was soooo light. Sigh.

Anyway, I found out I'm getting a bonus at work, so I see a Brew in my future. I can swipe a lot of parts off the Haro, but not the crank. It's an "American" BB (large diameter) and the Brew will have a "Euro" BB (normal mtb size). The Black Widow crank and BB are a matched pair with a hexagonal spindle, so I can't just get a new BB. I'd like to run an mtb crank instead of the heavier BMX units. Since I'm only 125 lbs, and don't do big air, I don't see a problem with this. The problem I do have is that I'm not sure how to figure out what chainline I've got.

The over-locknut spacing on my rear hub is 124mm. The distance between the centerline of the cog and the inside face of the dropout is 12mm. Does this mean I have a 50mm chainline? How do I figure out what crank/BB will give me a match up front?

Kathy :^D
Howdy Lucky,
So you want to trade in your old Haro for a Brew, eh? Can't say I can blame you, the aluminum Haro is pretty stiff and the Brew sounds like it rides great.

It should be no problem to get any mountain bike crank to work as a BMX crank. Lots of pros use the XTR M952 models. Truvativ also makes BMX race cranks and when used with their bottom bracket should get you the chainline you need.

But back to your original question about chainline, yes, if you take the OLD and half it and then take off the distance to the cog, like you did, you have the chainline for your rear wheel. Ideally, you would want to match this to the front. But 50mm sounds wide for a BMX bike. Most, If I can recall correctly are between 43-45mm. I'll verify later at work and let you know.
 

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Full Tilt Boogie
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I've installed a few Saint cranks and several XT's for use in BMX race bikes. You'll need to run pretty much any chainring on a mtb crank in the middle position, and in the case of a hollowtech 2, you'll need to adjust the spacers on the bottom bracket to make for a better chainline. There are a few lighter weight "racing" BMX cranks. Truvativ makes a cranks that's just called that, racing cranks. With a nice euro bb, (get a ti spindled one!) you can drop pound off of the crank setup you're running now. WIth your weight, you shouldn't be worried about breaking anything you use. Take a lot of weight off your bike where you don't need it, and have fun at the races!
 

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Loose Nut Behind d' Wheel
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Discussion Starter · #5 ·
gatman said:
I would check your LBS to see if they can get you a euro conversion kit for your cranks. There are several different sizes. You keep your spindle, so that should not be a problem.
I could do that but, I'm planing to leave the Black Widow crank on the Haro for a couple reasons - one, I'm going to take this opportunity to shed some more weight, and two, I have enough of the original Haro parts to put back on it that I can sell it as a whole bike. The original crank from that bike isn't worth the effort of putting back on. It and the BB together weighed about 5 lbs - no joke. The arms were solid cast Cro-Mo. The Black Widow is much lighter, but a mtb crank setup is lighter still, especially if I splurge for a Ti BB.

Kathy :^)
 

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Loose Nut Behind d' Wheel
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Discussion Starter · #6 ·
ssmike said:
But back to your original question about chainline, yes, if you take the OLD and half it and then take off the distance to the cog, like you did, you have the chainline for your rear wheel. Ideally, you would want to match this to the front. But 50mm sounds wide for a BMX bike. Most, If I can recall correctly are between 43-45mm. I'll verify later at work and let you know.
Thanks. I measured it quickly this morning with a caliper, and I didn't think my eyeballs were that far off, but it's possible. Are BMX bikes all the same?

How do you know what BB length to use with a given crank to give a certain chainline? Is there data on this somewhere?

Kathy :^)
 

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Full Tilt Boogie
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Most BMX bikes are very similar, but there are exceptions. BMX bikes are notorious for not being standard in almost every respect. They are getting better. One of the biggest things that has come along is the aceptance of a euro bottom bracket. The spindle length that is most common is 73mm by 108mm. usually the problem is getting the chainring as far in towards the frame as possible. On a shimano crank, this is achieved with BB spacers and moving the ring into the middle position. Cranks like the Truvativ racing crank have the ring set up on the inner side, or where the middle ring should be. It's not hard to get a usable chainline for a bmx bike like this, it just takes a little more trial and error. Most bike shops don't know a whole lot about higher end bmx stuff, cause it's just plain not out there as much as other things. Buy your LBS mech.s a six pack, and I bet they'll be willing to help you try out some different options, (I knowI would.)

If you want to go lighter than the Truvativ, get XT's and run a single ring. That's what I've found to be the lightest/fairly stiff. That's the setup I'm going to be running on my new FMF and my new Spicer single. If you want light weight, check out FMF's new Carbon 2 or Carbon five frames. The carbon 2 weighs in at 3lbs 1oz, and has carbon top and down tubes. I have two in my shop and they are just gorgeous! And for light weight? Think 18-19lbs rideable BMX racer! It can be done, easily. you just have to spend a [email protected]#$ load of money!
 

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Loose Nut Behind d' Wheel
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Discussion Starter · #8 ·
benwitt11 said:
I've installed a few Saint cranks and several XT's for use in BMX race bikes. You'll need to run pretty much any chainring on a mtb crank in the middle position, and in the case of a hollowtech 2, you'll need to adjust the spacers on the bottom bracket to make for a better chainline. There are a few lighter weight "racing" BMX cranks. Truvativ makes a cranks that's just called that, racing cranks. With a nice euro bb, (get a ti spindled one!) you can drop pound off of the crank setup you're running now. WIth your weight, you shouldn't be worried about breaking anything you use. Take a lot of weight off your bike where you don't need it, and have fun at the races!
I looked up the Truvative crank, but I was looking to go even a little lighter than that. I'm thinking of looking for a good used Race Face Turbine or something along that line. I'd like to get one with a 110 bolt circle since I have a new 110 BCD chainring I can use hanging around the garage. Ideally, I'd find a 175mm, 110 bolt circle with a square taper BB. I know where I can get a square taper BB with adjustable chainline if it comes to that. Time to check Ebay and the Classified board here.

Kathy :^)
 

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Loose Nut Behind d' Wheel
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Discussion Starter · #9 ·
benwitt11 said:
Most BMX bikes are very similar, but there are exceptions. BMX bikes are notorious for not being standard in almost every respect. They are getting better. One of the biggest things that has come along is the aceptance of a euro bottom bracket. The spindle length that is most common is 73mm by 108mm. usually the problem is getting the chainring as far in towards the frame as possible. On a shimano crank, this is achieved with BB spacers and moving the ring into the middle position. Cranks like the Truvativ racing crank have the ring set up on the inner side, or where the middle ring should be. It's not hard to get a usable chainline for a bmx bike like this, it just takes a little more trial and error. Most bike shops don't know a whole lot about higher end bmx stuff, cause it's just plain not out there as much as other things. Buy your LBS mech.s a six pack, and I bet they'll be willing to help you try out some different options, (I knowI would.)
Thanks for the info. The guy I borrowed the Brew from had a Race Face Turbine crank with the ring installed in the middle position. I wish I'd asked him what BB he used, but I was busy telling myself I didn't need a new bike at the time. He did have to go with a pretty small chainring to get clearance on the chainstay. That much I remember. I thought that might have been something unique to the frame, but from what you're saying, it sounds like it may have been because of a really short BB axle.

Ironically, the shop I'm getting the frame from doesn't seem to know that much about what they are selling, though I've only talked with one person who may not have been one of the mechanics. This is my first time dealing with them, and they're about an hour from home, but not as far from the BMX track. There's a shop much closer to home where I know the mechanics better, but I feel funny bringing in a new frame I bought somewhere else (they're not a Brew dealer). :^/

Kathy
 

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Full Tilt Boogie
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Like I said before, buy them a six pack, and go hang out for a night or something. If they're anything like me, (most good mechanics have similar mentalities,) they'll just like the chance to play with something new and fun. I don't mind at all when someone brings in a fun frame for me to "play" with. Bottom line, we like wierd, fun, differet stuff as much as you do, sometimes quite a bit more.
 

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Lucky said:
Thanks. I measured it quickly this morning with a caliper, and I didn't think my eyeballs were that far off, but it's possible. Are BMX bikes all the same?

How do you know what BB length to use with a given crank to give a certain chainline? Is there data on this somewhere?

Kathy :^)
After posting, I was thinking - 124mm rear hub spacing on a BMX? Is the hub the stock hub or a new hub from your new wheelset? A BMX rear hub should be 110mm spaced which would put the chainline in the 43mm range.

If you go for the Brew, the bb shell will probably be 68mm. To get the crank/bb for BMX, you want to go for a bottom bracket that will get you a 43mm chainline. There is no set data for figuring bb lengths, but if you go by the manufacturer's recommendation it should be easy to figure out. The first thing you want to decide is which crank you want. A square taper crank will have a different axle length than a splined bottom braket / crank combo.

Here's an example, if you went with a RaceFace ISIS crank, you would want a 113mm for a triple MTB. If you wanted to go single BMX, you would use their 108mm bottom bracket. Does that make sense? Go one step narrower for single BMX chainline.

If you are thinking square taper, the RaceFace would be a good crank as would an older Ritchey crank. Ritchey also made some titanium bottom brackets.
 

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Loose Nut Behind d' Wheel
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Discussion Starter · #12 ·
ssmike said:
After posting, I was thinking - 124mm rear hub spacing on a BMX? Is the hub the stock hub or a new hub from your new wheelset? A BMX rear hub should be 110mm spaced which would put the chainline in the 43mm range.

If you go for the Brew, the bb shell will probably be 68mm. To get the crank/bb for BMX, you want to go for a bottom bracket that will get you a 43mm chainline. There is no set data for figuring bb lengths, but if you go by the manufacturer's recommendation it should be easy to figure out. The first thing you want to decide is which crank you want. A square taper crank will have a different axle length than a splined bottom braket / crank combo.

Here's an example, if you went with a RaceFace ISIS crank, you would want a 113mm for a triple MTB. If you wanted to go single BMX, you would use their 108mm bottom bracket. Does that make sense? Go one step narrower for single BMX chainline.

If you are thinking square taper, the RaceFace would be a good crank as would an older Ritchey crank. Ritchey also made some titanium bottom brackets.
I'll go remeasure it tonight, but you're probably right. I did measure it before I had coffee this morning, and I was in a bit of a hurry, so it's possible I read it wrong. The hub is new, but it's the same width as the stock hub. It dropped right in, so it must be 110, if that's the standard.

I understand what you're saying about the BB length. That make it pretty clear, and it sounds pretty simple. Being able to go ISIS opens up more possibilies, but I was afraid of not being able to get the chainline right. Thanks a bunch!

Kathy :^)
 
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