chain line and bmx cranks....
 

I've got a pair of answer alumilite cranks, on a 113 bottom bracket, as recommended by answer. this width was obtained when I asked them what size bottom bracket would give me a 42cm chain line. well this weekend I finally got a chance to put my new wheels on, put the cogs on the wheel, slapped the chain on, spun the cranks, only to release some of the nasty noises ever, creeks and cracks. I pulled out the tape measure to check the front chain ring, to find it 50cm off center. 8 ding dong centimeters off. So here's where I show my stupidity....

Knowing that I need to pull the chainring in 8cm, what width bottom bracket do I need? I remember seeing the way of figuring it out on sheldon brown's site, but after searching through there for a few hours last night I came up with nothing. my assumption is to double the amount off, and subtract it from what I currently have, 113 - (8 x 2) = 97. But I don't beleive there to be a 97cm bottom bracket. or do I only subtract 8 from 113, leaving me with 105, which would allow me to go with an easily found 107.

damn bottom brackets, one of those things I just can't figure out by just fidling with it...

50cm? you mean 50mm right?

double check your measurements because that seems way off. are you sure you have a 113mm BB?

Then again, a 50mm chainline is about what you want for a SS MTB, so uhhhhh?

whoops, yeah mm, I use cm at work all day. Yep it's 113, or well should be, that's what I remember buying, I think I'll pull out the bottom bracket tonight and check.

I still swear that I saw saw how to adjust chainline on sheldon browns page a while ago.... grr...

oh, and track bike, not mtb. using IRO hubs...

oh, yeah, your original calculation makes sense 113-8*2=97.
this assumes that your 113 axle is symmetrical though. (shimano 113's are symmetrical)

so a 107 would only move things in 3 mm. so would a 110mm.
AFAIK a 97 mm BB doesn't exist.

oh and moving the chainring to the inside of the spider should get you another ~5mm in.

Are you sure your cranks are on the tapers all the way? If they aren't, your chainline will be way out there, like it is. If you aren't sure about your crank installation, show it to somebody who knows what they're doing.

I rock Bulletproof cranks (I think they're pretty much the same) with a 108 (or 9, I forget) BB, perfect chainline for me (Surly double fixed). Just fyi.

I had my dad (ex-bike mechanic) look at it when I was putting it together, and he seemed pretty confident that it was on as far as it would go. After I posted last night I was looking at my bike and I think I will end up just flipping the chainring to the inside of the spider...

I guess another question would be, what is the "standard" chainline for a bmx?

A quick google reveals 43 to 43.5 mm as standard BMX chainline. So, pretty close to the 42mm track standard.

hmm, I talked to answer, and a 113 bottom bracket is what they recomended, so I'm gonna assume that I've got 7mm more that I need to draw that crank in with. when i pull off the crank arm it seems like it's not on to far, I've greased the spindle with Parks grease, and I've tightenned the crank arm down as far as I was comfortable. Ever had to smack a crank arm with a rubber mallet? I guess it's also possible that the taper in the crank arm isn't right, a deffective maybe..

check "as far as I was comfortable" with a torque wrench
rubber mallet won't do anything. you're pushing it on with the bolt already.

are you sure you are using the correct taper bb for your crank?

that was something I was thinking about, it was described to work with a standard taper bottom bracket, which leads me to beleive a standard shimano taper. It may be worth calling answer again about this.

yeah I think I need to get a torque wrench, it's one of those tools that when I'm in sears I don't buy and always wonder why...

bmx cranks use an ISO taper. trying to fit an ISO crank on a JIS taper is going to cause the cranks to sit about 4.5mm farther out than an ISO crank on an ISO taper.

well ain't that neat, so my assumption is that a standard shimano is JIS?

ok, so a quick google search turned up shimano's as being iso....

this just get's more confusing. I think this bike may see a bike shop soon.

nononononono. Shimano are JIS and have always been.
you're most likely seeing ISO *threading* (aka British)

I don't quite believe the BMX/ISO link.

Horn swaggled!

Ok, just because I'm really in the dark here, BMX cranks use an ISO taper, where as Shimano uses a JIS taper? So the *possible* solution to my problem would be to get a bottom bracket with an ISO taper?

Seriously, thank you guys. I probably would have shot myself in the head by now trying to find this information.

yes that is a likely solution

I have some Primo SGT Euro BMX cranks that are JIS square taper.

http://harriscyclery.net/page.cfm?Pa...ils&sku=CR1307

So, not all BMX cranks are ISO taper.

Just FYI, I have a 122mm BB which gives a low 40'ish chainline with the chainwheel on the inside of the crank arms.

Maybe you can check for defective taper by comparing the left and right side cranks. Are the pedals the same distance from the chainstay on both sides? If the right side is way farther from the chainstay, that might be a clue. It could mean the right side isn't fully seated or maybe you don't have the bottom bracket you think you have. Could it be one of those weird asymetric ones.

bulletproofs also use JIS taper.

who says BMX canks are all ISO? is someone getting confused with the "european BB" nomenclature?

Harris sells sells an Answer Alumilite crank.

http://harriscyclery.net/page.cfm?Pa...ils&sku=CR9595

They say it's JIS.

I called answer today, yep JIS.

crazy crazy crazy....