really it's for chainstay/chainline clearance. every bike has some variation in frame design

Hambini Time

https://www.youtube.com/c/Hambini/videos

 

really it's for chainstay/chainline clearance. every bike has some variation in frame design

That makes sense, but then why are user manuals and instructional videos always careful to show you exactly where to put the BB spacers? .

If you followed the user manuals (like most mechs do), the BB spacers would always be in the same place and, as a result, provide no flexibility in terms of chainstay/chainline clearance.

 

So it started with having two BB widths, 68/73. You need a 2.5 mm spacer on each side of the BB to space out a 68 to 73. Then Shimano came out with BB mounted FD and that was another 2.5 mm in width that they had to adjust for. That is why you have a 2.5 mm spacer on the drive side of a 73 mm. Not sure why they continued to space it like that, but that is where they came from originally.

 

So it started with having two BB widths, 68/73. You need a 2.5 mm spacer on each side of the BB to space out a 68 to 73.

Was 68mm more common on road bikes and they wanted wider shells for MTBs? The BB spacers would make sense in that case because it would allow the same BB to be used.

Then Shimano came out with BB mounted FD and that was another 2.5 mm in width that they had to adjust for. That is why you have a 2.5 mm spacer on the drive side of a 73 mm. Not sure why they continued to space it like that, but that is where they came from originally.

There are also chain guards that can mount where the 2.5mm BB spacer goes. Maybe that's why they kept it.

I assume the BB shell in a frame is always centered (within some tolerance). So does this mean that a single DS spacer actually puts the cranks slightly off center? Not that anyone would notice, but it's something I'm curious about.

 

it's a plot by the nefarious spacer manufacturer's secret cabal.

 

I guess it's going to create yet another shell standard and there is enough of them already over the different disciplines and bb standards out there.

Spacers are needed to allow the same standard spindle lengths and overall bb widths to match up properly.

Cranks might not be off-centre enough to notice but I couldn't handle knowing they weren't.

I don't see a problem with the way things are. Easy enough to fit a bb as recomended and treat tweaking chainlines as a seperate job.

 

I don't see a problem with the way things are. Easy enough to fit a bb as recomended and treat tweaking chainlines as a seperate job.

You probably don't fatbike, where in one direction or the other you will find yourself without an OEM ring or spider that will get you the chainline you need

just my most recent two examples:

1. RSD mayor. Even with the widest tires made, a standard 6mm offset ring has enough space to move in several mm with no tire rub - and the chainline is poor as is, so this will be an improvement
2. surly BFD ... well, yes this is a unique bike. But non the less .. i needed to swap spacers to get the additional offset i needed.

 

Offset drain hole syndrome


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it's all a cluster of why it is the way it is, and honestly, there's not much consistency.

it started with old standards and then little tweaks here and there to adjust things for new standards. some things got kept as legacy standards, some did not, so to get the whole story, you'd have to follow a pretty full history of bike development. AFAIK, e-type derailleurs and ISCG bash guard adapters replace your drive side bb spacer, they don't add to it. They exist now BECAUSE that spacer is there and replacing it with one of these other parts gives you some extra options that you wouldn't otherwise have.

These days, enough frame manufacturers are pushing the limits of chainstay width that I would not say that you can universally adjust chainline by messing with bb spacers. Maybe you can do that on some frames with some cranks, but there are enough combinations where there's not enough room for that, that I don't consider it an option.

I consider the ability to adjust chainline and the amount of flexibility you have to be largely a function of the cranks you choose for a bike. Cranks like RF CINCH cranks (with CINCH spindles) will give you the most flexibility and the most options. Everybody else gives you a lesser ability to adjust chainline.

 

are there any other reasons why a frame-Cup spacer is helpful? What comes to my mind is that it might serve as a mitigation against creaking? The plastic spacers are obviously rigid enough to withstand whatever loads a high pedal force can cause BB / cup strain ... obviously good prep (clean, face, antiseize, etc) is imperative, as is torque. But does the non-metalic spacer help?

I ask because i want to remove a spacer, making it metal cup to steel frame ... and am just wondering if this is making things slightly worse for a bike i hammer on but would like to last a long, long time (no longer made, no remotely similar alternatives)

 

All this makes me wonder why there isn't a standard shell width that doesn't require any BB spacers.

This is the case with Shimano BB and cranks on road bikes.