T47 to BSA bottom bracket adapter - good idea?

Question

I have just started building up a new frame and just noticed that it has a wider hole for the bottom bracket to screw in to than any other bike I have worked on - I think it is for a T47 bottom bracket.

The frame is still 68mm wide and I run SRAM GXP cranks.

I have mostly used BSA bottom brackets in the past and still have many new ones at home.

All the T47 bottom brackets I can find are 2-3 times more expensive than BSA bottom brackets.

I have found an adapter than I can install into the T47 shell that will let me run a standard BSA bottom bracket inside - but is this a bad idea?

Are there any real performance/reliability differences between T47 and BSA?

Thanks

Answer 1

I would nuance the question to answer it: the quality of a bottom bracket depends ultimately on the quality of the bearings and manufacturing tolerances, rather than the format. T47 has been introduced to use wider spindles than the classical SRAM/Shimano ones, but it has indeed a limited benefit if you use one of these cranks.

But from what I've seen, T47 bottom brackets are comparable in price to their BSA equivalent (for Praxis Works for example, the RRP is the same), but the difference is that they are not available from "mass-producing manufacturers". The entry ticket is more expensive, but that doesn't mean that the quality is equivalent. If these "exotic bottom" brackets last longer than the ones from the big brands, you would in fact have an interest in taking them even in BSA. But that turns the question on a comparison between brands rather than a comparison of formats.

Personally I would anyway avoid adding an interface/adapter in an area that is subject to as much stresses as the bottom bracket. Good quality bottom brackets last a long time, so a 20-30€ difference in purchase price for an equivalent quality has a limited impact on the cost of ownership.

Answer 2

OK,

So generally:

Adding adapters seems like a bad idea (2 possible points of failure).

There are not many T47 68mm wide GXP bottom brackets for sale and they are quite expensive compared to normal T47 brackets.

So I think the best option is to abandon the GXP cranks, buy SRAM Dub cranks so that I can use a normal T47 bottom bracket (as they are more available and more affordable than the T47 GXP BBs)

Answer 3

I would try the adapters, but be aware that they may raise the chance of something creaking. BB creaks are usually due to something about the BB interface being out of tolerance, e.g. the BB seat is a bit too large or it's a bit out of round. Each component has a tolerance (i.e. the amount of allowable variance in dimensions). The total tolerance of two components, e.g. a BB shell and its adapter, is the sum of the tolerances of each one¹. Thus, it would be better to just get a T47 BB, except that there aren't a lot of T47 to GXP BBs out there - one exception is, I think, the Chris King T47 BB, which comes with delrin spacers anyway, and they have one kit for GXP spindles (it's kit 6). It's expensive. If you specifically got a T47 to GXP BB, it may not be adaptable to a standard Shimano crank.

Most creaks on the bike are not due to the BB.

I can see a few cheap third party, likely Chinese, adapters. In theory, cheaper items may have poorer tolerances than more expensive components, if everything else is held equal. There are a few T47 to GXP BBs out there as well. Given the low cost of adapters, it might be worth a shot to experiment.

Footnote 1: Technically, the total variance of two random variables is Var(A) + Var(B) + 2 Covariance(A, B). For two independently produced items, we can probably assume 0 covariance. Covariance and correlation measure the same concept (e.g. as one variable increases, to what extent does the other variable tend to increase or decrease), but the calculation is different. Anyway, tolerance is allowed variances. If you've heard someone say that tolerances stack, they mean that if you need to consider multiple components, their individual tolerances add up, so you could have a BB shell plus adapter that individually are within tolerance, but are jointly out of tolerance.