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aka bOb
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Discussion Starter · #1 · (Edited)
Alright, I have discussed this a little in another thread and now about to jump in but have a few questions from my trusted Fat Friends. I'm planning to have a Shimano 2009 SLX crank with external BB modified by a local machine shop to fit a 100mm BB. Now originally the spindle is for 68-73mm BB and my question is how much should I have it extended? Should I make up the difference between 68mm and 100mm or 73mm and 100mm (i.e. add 32 or 27mm) to the shaft? I also need to leave enough room for the e-type front dérailleur.

Thanks....Bob
 

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aka bOb
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Discussion Starter · #3 ·
Borgschulze said:
I would say 27mm.

Measure it up yourself?
Yeah I don't have the frame right now but as long as the BB tube is exactly 100mm on a 907 I should be able to do a mock up.
 

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bdundee said:
Yeah I don't have the frame right now but as long as the BB tube is exactly 100mm on a 907 I should be able to do a mock up.
You might want to wait for the frame since one of the critical points is the crank arms clearing the chainstays. Without knowing what you need to clear you could make it too short. I had a Kooka crank on a Fatback that only cleared the left chainstay by about a millimeter. It worked but barely. On the other hand the newer cranks seem to be wider (look at the Q factor thread) so maybe you will have tons of room and you'll want to go as narrow as possible. Either way the old adage measure twice cut once is good advice.
 

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The BB shell will be close to 100mm within about 1mm of tolerance if they have even a half assed factory making them.

I measured my Fatback at around 100.27mm I believe.

My Raceface Atlas cranks were a pain in the ass to install, so I did some measuring before installing to make the process a little quicker.
 

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I agree with Borgschulze - 27mm is the theoretical figure for a 100mm shell. If the spindle is for a 68/73 it means it is actually a 73 and is adjusted to fit a 68 with a spacer.

If you wait until you have the frame you can measure the shell and get an exact size for the +/- of the actual shell measurement from 100mm. Personally, I wouldn't bother waiting because any variation could be sorted with shims or a facing tool.
 

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aka bOb
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Discussion Starter · #9 ·
dRjOn said:
how are they going to modify it?
Cut the spindle in the middle and add an internal spacer and weld it up. Pretty easy job if it holds up, we will see.
 

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bdundee said:
Cut the spindle in the middle and add an internal spacer and weld it up. Pretty easy job if it holds up, we will see.
It will be interesting to see if/how they deal with warpage from the welding, or how bad the warpage is.
Andy
 

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I would have to agree with Sryanak that chainstay clearance does not necessarily follow the assumed increase from 68/73 to 100. You might see if Chain Reaction can provide you with a Q-factor number, then measure the existing Q-factor and do the math from there.
 

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How much does it weigh?
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If you would like, I could measure the distance between the crank arms on the SLX crankset.

My girlfriend has the SLX crankset on her Ragley, so it would be no issue to measure the distance between the arms to see if it will clear.
 

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aka bOb
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Discussion Starter · #13 · (Edited)
Borgschulze said:
If you would like, I could measure the distance between the crank arms on the SLX crankset.

My girlfriend has the SLX crankset on her Ragley, so it would be no issue to measure the distance between the arms to see if it will clear.
Thanks man that would be great! I emailed Chain Reaction earlier for the Q factor on the 907.

Edit: Thanks anyways the crank came in today and from what I can figure it is about the same as a Mr. Whirley as far as Q-factor. It's off to the machine shop and should have it back on Friday.
 

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aka bOb
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Discussion Starter · #14 ·
Here we go it actually turned out very straight but I told the machine shop to make it as light as possible and they didn't. Well we will see how it holds up, I have my doubts! Oh and it added about 100 grams of weight to it so its at 990 with complete with 3 rings and bb. They easily could've shaved about 50 grams off with a little love! Oh well it's a fat bike not a racer.
 

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is buachail foighneach me
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Sweet! For a first attempt at something like this, I would err on the side of overbuilding, as the machine shop did, to ensure it holds up. Looking forward to seeing it mounted up.

Would you be willing to say how much they charged? You might have a small side business in the works here....
 

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aka bOb
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Discussion Starter · #16 ·
sean salach said:
Sweet! For a first attempt at something like this, I would err on the side of overbuilding, as the machine shop did, to ensure it holds up. Looking forward to seeing it mounted up.

Would you be willing to say how much they charged? You might have a small side business in the works here....
They charged $50 which I didn't think it was to bad if it holds up. I would think any machine shop would do it. I sold my Pugs and now waiting on my 907 so set up will be a little out yet.
 

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Silver bullet
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I was thinking about doing the same thing myself so I was interested to see this thread. How do you plan to extend the inner cover to fit between the two bearing cups?

Bummer about the delivery date! Never sell a bike until you have one to replace it!!:nono:
 

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Inner cover should be no big deal... Raceface makes them.

Or you could just not run one and seal the holes in the BB shell.
 

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aka bOb
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Discussion Starter · #20 ·
I think I will just extend the original one. A little epoxy and some plastic and I should be golden!

p.s. I still have a steed just not a fat one at this time
 
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