6

It's well-known that bottom brackets designed for a 30mm spindle and a BB86/92 press-fit bottom bracket shell (41mm internal diameter) will have reduced lifespans due to the very small bearings that are required to fit between the shell and the spindle. Most people recommend using a crankset with a 24mm spindle in such frames.

SRAM released its "DUB" drivetrain line-up in 2018, which uses a 28.99mm spindle. I've seen speculation in a number of places (e.g., here, here, and here) that this will have much the same problems as 30mm bottom brackets in BB86/92 applications. (And with only 1mm difference I can see why!)

However, try as I might, I can't find any reports of this actually causing problems. Many high-end bikes are even specced by default with BB86/92 DUB bottom brackets (e.g., nearly all SRAM-equipped Canyon road bikes).

Should I worry about this when swapping my Shimano crankset (24mm spindle) for a SRAM DUB crankset on my BB86 frame?

2
  • I wouldn’t be too sure about the longevity issues with 4130 bearings either. I’m using 4130 bearings (I think the Rotor steel ones) and haven’t had any issues so far. They didn’t spin as freely when new and I’m not sure how much resistance they add under load but so far they are holding up nicely.
    – Michael
    Apr 4 at 14:40
  • 1
    Side note: I have a bike with a PF30 BB. Traditional PF30 BBs have the bearings inboard, inside the frame. The SRAM DUB PF30 seats the bearings outboard of the frame, and I would assume that this enables larger bearings to be used - which would mean increased bearing life all else equal. Unfortunately, the BB86 DUB BB doesn't appear to seat the bearings outboard, as far as I can tell.
    – Weiwen Ng
    Apr 4 at 20:50

2 Answers 2

7

You're looking for empirical data and you probably will never get any unless you do the testing yourself.

The problem is that the alignment and tolerances of the frame bores and to a lesser extent the spindle have an enormous effect on the bearing lifespan of all press fit BB systems. To make empirical statements about any of it, you need accurate measurements of all those parameters. That requires sophisticated tooling far beyond what most mechanics have, and certainly riders as well. The limit to what most can readily do is measure the bore ID with a caliper at various points. That is very different from measuring runout from a theoretical center. And, measuring the angular alignment and axial alignment all requires serious inspection type tooling, to say nothing of the time involved.

It seems likely at this point that the real story of press fit BB bearing issues is that, depending on your perspective, either it's usually an out of tolerance frame that causes problems or the standards were not developed to match well with what factories are able or willing to turn out. With all the standards (BB30 is a good example) you can find examples of bikes where they run pretty bulletproof despite problems being so common and severe elsewhere. It's likely that way simply because only a few manufacturers get their tolerances right consistently.

To put it another way: systems like DUB in a BB86/92 shell require really small individual bearing balls because of the spatial constraints. That's not necessarily a dealbreaker for longevity, but the smaller the balls get, the less durable they are individually and the more reliant the system is on all the balls being loaded evenly and correctly, which can go bad fast if the shell bores are poorly aligned to each other or if they're too tight or too lose. In contrast, if the individual balls were bigger, they can soak up some of these sorts of issues, to a degree - see for example Mid shell BBs on BMX bikes, which are frequently seen on frames with production values as poor as anything and installed by angry children with hammers, and yet generally have good bearing lifespan. The balls inside are much bigger.

2

Empirical typically means derived from experiment or other structured data collection, as contrasted to theoretical or to anecdotal. SRAM themselves might have warranty data that apply - however, that data might have many holes in it, as many consumers might assume that a prematurely worn BB is wear and tear and not subject to warranty, or SRAM might deny warranty and not record it, they might not have good data on mileage and conditions, etc. If you want actual data, asking around on forums might be the best you can do, and this would be very noisy data.

In the absence of data or anecdotes, we can reason from theory. To the best of my non-engineering knowledge, the more widely spaced the bearings and the larger they are, the greater their load capacity, and the longer life they should have. As you observed, the BB86 shell (at 41mm, see this link) is smaller than the 46mm shell of the T47 standard. For some BBs, the bearings can be placed outboard, I.e. just outside of the shell, much like the Hollowtech II cups sit outside of BSA shells. This enables bigger bearings than if you stuffed them into the shell. I have a feeling this isn’t possible in a BB86 shell, because that standard is wide compared to others (86.5mm width, compared to e.g. 68mm for BSA, Pf30, T47). Thus, all else equal, the bearing load capacity for a BB86 DUB BB should be lower than for, say, a T47 DUB BB, because you’d have to use smaller bearings in the BB86.

I don’t know how this translates to practical bearing life, however. Bearing engineers have methods to calculate expected bearing life, which I assume is derived from a mix of theory and empirical data. To use these methods, you need the bearing size, but you also need to calculate the bearing effective load, and that has to combine the radial and axial loads (the latter are side to side loads, e.g. when you’re pedaling out of the saddle, you impose some axial load on the bearings). Obtaining the correct parameters and translating the result to practical terms is probably beyond most non-engineering laypeople.

I will note that whatever theory might say, I see some high-quality BB manufacturers, like Kogel, White Industries, and Wheels Manufacturing, offer BB86 DUB BBs. I would assume these firms had engineers who believed that the expected bearing life was acceptable for this application. Based on this, I would probably not worry too much about using a DUB crankset in a BB86 bike.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.