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BB30 conversion and chain line questions

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BB30 conversion and chain line questions

Old 02-28-21, 02:43 PM
  #1  
Brad Bike
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BB30 conversion and chain line questions

Setup:
2012 Felt FC bought used in 2018 (https://99spokes.com/bikes/felt/2012/fc-frame-kit). BB30 with 2013-era SRAM Force 53/39 crank but rest of components are Shimano 105, likely also 2013. Cassette is 11-32. FYI, This is my only bike, but I'll probably be making it my permanent trainer bike once I get a new one, so I do need it to work well.

Issue: When riding in my big ring, I've always had significant chain rub from the third highest gear to the highest gear, as well as the other end in the second or even third lowest gear. (It's loud enough that other riders occasionally ask me about it.) Moving the FD in either direction just exacerbates the issue on the other end. Best I can figure, this is caused by the SRAM crank not playing well with the Shimano FD. Unfortunately, the problem is even more pronounced on my KIXKR, especially in the higher gears

Question: I'm considering using a Praxis BB30 converter so I can install a Shimano 105 or Ultegra crank. I'm hoping this would help. But if I do, given that the bearings in the converter are outboard of the BB shell, is this going to mess with the chainline? I already (also) get cross chaining interference in my second lowest gear. Wouldn't the converter just make that worse since it would move the crank further outboard? I can't find this question asked/answered anywhere online so I appreciate any advice or alternate solutions you guys can offer. Thanks!
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Old 02-28-21, 03:56 PM
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You really need to dig into the SRAM docs for your current crank and find out what it's stated chain line is. Or just measure yourself, from centerline of seat tube to the mid point between the inner and outer chainring....... in millimeters. Then compare that to the crank you are thinking of getting.

For the most part, if your current crank was meant for a road bike that has the spacing on the rear your frame is made with, then changing to Shimano or any other crank probably isn't going to do much for the noise. Is your FD Shimano or SRAM? Didn't SRAM have some issues with their FD's on road bikes? Also, do you know that your trim is set up correctly? I don't have to trim for noise until I get in the largest rear cog with my 105 5800.
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Old 02-28-21, 05:32 PM
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Originally Posted by Iride01 View Post
You really need to dig into the SRAM docs for your current crank and find out what it's stated chain line is. Or just measure yourself, from centerline of seat tube to the mid point between the inner and outer chainring....... in millimeters. Then compare that to the crank you are thinking of getting.

For the most part, if your current crank was meant for a road bike that has the spacing on the rear your frame is made with, then changing to Shimano or any other crank probably isn't going to do much for the noise. Is your FD Shimano or SRAM? Didn't SRAM have some issues with their FD's on road bikes? Also, do you know that your trim is set up correctly? I don't have to trim for noise until I get in the largest rear cog with my 105 5800.
I can try to measure (not optimistic about finding online) but assuming it's the same as current 105, my main concern is the impact of the Praxis converter pushing said new crank outboard 5-10 mm. Any thoughts on that piece? I would guess this would happen every time with the Praxis, so maybe my question is, how is the chain line not an issue with the Praxis with any crank setup? Is there something I'm missing?
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Old 02-28-21, 06:04 PM
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I've never looked at converters and adapters except for once a long time ago when I was wanting to put a more modern cranks on a bike with an old Ashtabula crank. After going through the confusing to me then spec's and weighing the costs and benefits to a bike that wasn't worth $75 I decided to get another bike.

I'd imagine it's not too different than when I went from a cup and cone square taper BB to a Hollowtech II crank with BB that had external cups. The crank spider makes up for that extra spindle length by curving in and placing the rings in the same position that is necessary for any of the same type bikes with a 130 mm rear spacing.

Dishing of your rear wheel doesn't move the rim and tire off the centerline of the bike. It just moves the spokes and hub over so there is room for the cassette. So the chain line on a bike will generally be the same as any other bike with the same rear spacing, give or take a few mm.

So if your bike has a rear spacing typical of todays road bikes, then any two piece crank that fits your shell with it's recommended BB will give you there-abouts the same chain line. Doesn't matter if it's 130 or 135, because only half the difference is the error and that is only 2.5 mm. And that's less than the center to center spacing of your cogs on the cassette. And chain line isn't usually that finicky.

Disclaimer... this is stuff I've conjured up in my mind over the years to help me imagine how things work together. I don't know for certain any of this is true. But it's helped me pick out what works on my bikes. So if I'm complete bonkers, let me know easily.
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Old 02-28-21, 10:21 PM
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Thanks Iride01, this makes me feel more confident about pulling the trigger on the Praxis converter and giving it a shot. I appreciate the replies!
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Old 03-01-21, 11:53 AM
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Originally Posted by Brad Bike View Post
Thanks Iride01, this makes me feel more confident about pulling the trigger on the Praxis converter and giving it a shot. I appreciate the replies!
Okay, but that is a lot of money for something that might not be quite what your issue is.

I guess I should have stated it without all the superfluous information I wrote about.

Are you certain the current FD is not the issue? Are you certain it's trim function is working correctly?

I'm not really certain going to another crank and BB will help anything unless you can truly determine the chain line of the current one is off significantly.

Visually, you can sometimes tell if chain line is off. It's easier if your bike is in a stand so it's more at eye level. From the rear, sight from the middle of the rear stack to the middle of the front stack. If that is within about 2 maybe 4 mm then you probably are okay on chain line. If it helps, shift the rear to the middle cog and take a look how the chain bends or not.

Last edited by Iride01; 03-01-21 at 11:59 AM.
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Old 03-01-21, 10:34 PM
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Originally Posted by Brad Bike View Post
Setup:
2012 Felt FC bought used in 2018 (https://99spokes.com/bikes/felt/2012/fc-frame-kit). BB30 with 2013-era SRAM Force 53/39 crank but rest of components are Shimano 105, likely also 2013. Cassette is 11-32. FYI, This is my only bike, but I'll probably be making it my permanent trainer bike once I get a new one, so I do need it to work well.

Issue: When riding in my big ring, I've always had significant chain rub from the third highest gear to the highest gear, as well as the other end in the second or even third lowest gear. (It's loud enough that other riders occasionally ask me about it.) Moving the FD in either direction just exacerbates the issue on the other end. Best I can figure, this is caused by the SRAM crank not playing well with the Shimano FD. Unfortunately, the problem is even more pronounced on my KIXKR, especially in the higher gears

Question: I'm considering using a Praxis BB30 converter so I can install a Shimano 105 or Ultegra crank. I'm hoping this would help. But if I do, given that the bearings in the converter are outboard of the BB shell, is this going to mess with the chainline? I already (also) get cross chaining interference in my second lowest gear. Wouldn't the converter just make that worse since it would move the crank further outboard? I can't find this question asked/answered anywhere online so I appreciate any advice or alternate solutions you guys can offer. Thanks!
No, a HTII crankset installed in a conversion BB30 BB should have the same (intended) chainline as if it were installed on a ISO threaded BB. BB30 has a narrow shell width of 68mm, which is the same as the shell width on a threaded BB, so it's fine to have the bearings external. BB30 as a system has a very narrow bearing stance that was intended to be able to lower Q factors, which frankly hasn't been consistently put into use. I have no idea if it will improve your other issues.
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Old 05-09-21, 11:03 AM
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Keep in mind that the single point of truth in crankset positioning is to have even aperture between crank arm and chain stay in both sides. If you fail to be true with this constraint, you will meddle with the forces of nature and there will be consequences. I have used Wheels MFG converter for BB30A to HTII outer and all went well but only because trim function of 105 allowed it.
Proper chain line cannot always be achieved and you shouldn't feel bad about it. At the end of the day, you should be consistent with cross chain rules of your setup and never go to largest/smallest cog with small/large chainring respectively. Also, there are other factors that might lead to success or fail like shifter quality and whether they have trim function e.g. Shimano x700, x800, Rx000 series (105 and above) which gives you 2 extra positions of the front derailleur (low, low-high, high-low, high) to address those problems. Shimano did a really good job on that by providing the ultimate compatibility feature for the shifters.

Again, as stated in all manuals of Shimano as a disclaimer (I am referencing Shimano but it applies to all companies), "There can always be rubbing on extreme combinations".
You shouldn't blame your bike and there is nothing to do about it. It is the same for me with stock Cannondale SISL2 crank and Stock R7000 derailleurs.

Last edited by AngelPedaller; 05-09-21 at 11:07 AM.
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