BB Size (length) When Switching from Triple to Single Chain Ring

[Hollywoodguy99]

Hello,

Long time lurker, first time poster.

I have a 7 year old gary fischer mtb with an XL frame I have converted to a city bike,

I am changing out the triple chain ring to a single, it will be a 1 x 9 when complete (originally 3x7).

My current bottom bracket I am replacing is 122.5 MM in length, should I downsize to 118 MM because I am losing two chain rings?

I searched and did not see this particular question addressed.

Regards,

Tim

 

[daveryanwyoming]

The trick in the end is to achieve a good straight chainline from your chainring to your single speed cog. Whether that requires a new BB spindle depends on exactly what your final system will look like:

- Are you planning to reuse your existing cranks with two chainrings removed?
- Have you considered using the middle ring mounting position for your single ring?
- How are you managing the single rear cog, singulator and spacer stack on your freehub, normal cassette and just choose which cog to wrap the chain around? retain your current rear derailleur and adjust limit stops to select a single cog? Lot's of options here which may influence chainline.

But in the end you want a relatively straight chainline for your single speed setup. Lot's of good information on single speed conversions here: http://sheldonbrown.com/singlespeed.html

-Dave

 

[Hollywoodguy99]

Great answer with great questions.

New cranks (Dimension 175mm Cycle Cross)
New Tangent 41 T Chain Ring
Starting with 7spd cassete and spacer
changing to Shimano 9spd Casettte (hub designed for 8-9 spd)
New Medium Swing Derailleur (Sram x7 or x9 series most likely) pairing with matching shifter.

The sheldon article is great, I guess I am trying to have to avoid reassembling my front drive train and measuring.

Thanks

 

[daveryanwyoming]

Sorry, I misread your original post and got it in my head you were doing a single speed, not just single ring conversion.

The trick will be figuring out what chainline you get from the new crankset and what the manufacturer of that crankset recommends for BB spindle length (assuming something like square taper or octalink if it's an integrated spindle style replacement crank then you take what you get) and that's just to ensure chainstay clearance and not ridiculous Q factor. But the exact chainline isn't a big deal on a multi-speed but single chainring installation. Be sure to either use a chainguard system (bash ring, outer ring, whatever you want to call it), a good chaincatcher or keep your front derailleur on the bike with no front shifter and use the limit stops so that it keeps the chain from jumping off your front ring when you hit bumps and get instantaneous chain slack.

Single ring conversions are very easy and I've gone back and forth on my cross bike. Single speed conversions are also easy but with a bit more to pay attention to in terms of chainline.

-Dave

 

[Hollywoodguy99]

Super Thanks! I think I will keep 122.5 MM Stock Length to avoid any inteference issues. Thanks for the heads up on the chain guard.

 

[alfeng]

Originally Posted by Hollywoodguy99 .

Great answer with great questions.

New cranks (Dimension 175mm Cycle Cross)
New Tangent 41 T Chain Ring
Starting with 7spd cassete and spacer
changing to Shimano 9spd Casettte (hub designed for 8-9 spd)
New Medium Swing Derailleur (Sram x7 or x9 series most likely) pairing with matching shifter.

The sheldon article is great, I guess I am trying to have to avoid reassembling my front drive train and measuring.

If your current crankset and your planned new crankset BOTH have square taper spindles & crankarms, then you need to know that there is a great deal of variety -- or, to put it another way, little continuity -- in which spindle will work with which crank to yield a consistent chainring offset ...

One thing about spindle length (regardless of type) is that it affects the chainring clearance, or lack of ...

AND/OR, crankarm clearance relative to the chainstays.

Without seeing your bike it is difficult to say what spindle length (as you can see, I am presuming your bike currently has a square taper BB spindle) you should use, but if I had to guess I would say that you want a Bottom Bracket with a 113mm asymmetric JIS spindle.

If you will [COLOR= rgb(24, 24, 24)]eventually be using a different 4-arm 104BCD crank or other MTB crank, then your only option will probably be to mount the chainring on the inner shoulder -- THAT IS, if you were NOT going to replace your current crankset, then you would simply mount the chainring on the inner shoulder of the spider with some BMX chainring bolts ([/COLOR]or, use spacers in lieu of the absent chainring[COLOR= rgb(24, 24, 24)]), or equivalent -- that is, as delivered from the factory, THE MIDDLE RING IS YOUR IDEAL CHAINRING OFFSET FOR YOUR FRAME ([/COLOR]presuming a geared drivetrain[COLOR= rgb(24, 24, 24)]).[/COLOR]

In theory, to continue to use your current crankset, you would want a spindle that is ~10mm shorter ...

If you are trying to minimize your immediate effort, then you should simply buy a set of the fore mentioned BMX chainring bolts (which you will eventually need for ANY double-to-single crank conversion) & mount the current outer chainring (which I presume is a 42t chainring) on the spider's inner shoulder as I have described OR buy the appropriate tooth count chainring (is there a reason that you want a 41t chainring?) & mount it on the inner shoulder of your existing crankset rather than buying a new crankset.

OR, to put it another way, the ONLY reason to change to the referred to CX crankset (whatever THAT is supposed to designate vs. BCD measurement unless it means that the crank comes with 48/38 chainrings) would be for cosmetic reasons (IMO) whereby you will be able to mount the chainring on the outside shoulder OR if you want a 5-arm spider OR ...

et cetera

 

[Hollywoodguy99]

Interesting stuff. I presently have a 122.5 mm square taper bottom bracket, isis. My triple is 28 38 48. The new cranks are 110 bcd, I changed my mind and went with a 40t surly stainless ring, as the tangent is bmx and will give fit problems with the chain for a 9spd. Thanks to someof tbe advice I see that the crank manufacturer requests a 113 or 118 mm spindle length, so I will get a 118 mm and figure out spacers etc if needed. 40t ring because I used the 38t most the time and like the options I get with a 40t in a 1x9 set up. Also aesthetics do play a role, i have got to be cool withe the kids here in los angeles.

 

[alfeng]

Originally Posted by Hollywoodguy99 .

Interesting stuff. I presently have a 122.5 mm square taper bottom bracket, isis. My triple is 28 38 48.

The new cranks are 110 bcd, I changed my mind and went with a 40t surly stainless ring, as the tangent is bmx and will give fit problems with the chain for a 9spd.

Thanks to someof tbe advice I see that the crank manufacturer requests a 113 or 118 mm spindle length, so I will get a 118 mm and figure out spacers etc if needed.

40t ring because I used the 38t most the time and like the options I get with a 40t in a 1x9 set up.

Also aesthetics do play a role, i have got to be cool withe the kids here in los angeles.

If aesthetics matter, then you will PROBABLY (?) want to mount the chainring ON THE SPIDER'S OUTSIDE SHOULDER ...

In which case, you will probably want the BB with a 113mm spindle ...

To ensure your crank arms will clear the chainstays, measure the outside width of the stays approximately 7.5" from the center of your BB spindle ...

• IF that distance is [COLOR= #ff0000]4 [SIZE= 10px]5/8ths[/COLOR] [/SIZE]inches, or less, then get the BB with the 113mm spindle
• IF that distance is greater than [COLOR= #ff0000]4 [SIZE= 10px]5/8ths[/COLOR][/SIZE] inches, then choose the 118mm spindle

BTW. You need to ensure that the SURLY (or, other brand) chainring is not for [SIZE= 14px]1/8" [/SIZE]chains (that's the width of the teeth on the chainrings & cogs of most TRACK & many BMX drivetrains) ... [SIZE= 14px]3/32"[/SIZE] is the tooth wdith on multi-gear cogs & chainrings.

 

[Hollywoodguy99]

That is amazingly helpful insight. After the fiasco of switching out my wheels last year I knew to as questions before diving in with this. The cascading amount of components that get affected is incredible every time you make a change. Thanks again.

 

[alienator]

Wheel switching fiasco?

 

[Hollywoodguy99]

Wheel switching fiasco - After seeing the sweet rides at CicLAvia, I a 38 year old maaried guy and father was bummed that my trusty mtb was so pedestrian. So besides stripping the paint and giving a single color paint job, changing the handle bars, grips and seat, I just had to get me some white wheels/rims. As you may know, white wheels in 26" size are not ultra common. I found. Great deal on some velolite white wheels on ebay. Got some tubes and kenda kwik trax tires (with cool reflective stripe) and shoul be good to go. But... The wheel was designed for 8/9 speed cassette. I had a 7spd freewheel. So spacers and new 7 spd sprockets later, good to go. But the chain broke taking it apart, and now the brake pads did not fit the rim, needed narrow ones. Hurray. I could have went out and purchased a hipster linus for less money and time than I spent changing and modding this bike. But it is one of a kind. And I am semi pro bike mechanic now.

 

[Hollywoodguy99]

118 mm it is! I had a measurement at the stays of 4 7/8th inches. Thanks everybody. The wheels were Vuelta zerolites not velolites btw.