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Discussion Starter · #1 ·
See this link for an explanation and diagrams of the standards http://problemsolversbike.com/files/tech/Bottom_Bracket_Standards_Reference.pdf

After searching the web, there has been much talk over the stiffness of these various standards.
BB30
The BB30 shell is a straight through bore with grooves for a retaining ring on each side. A bearing is pressed against the retaining ring. If the bore is completely machined through from one side, the concentricity (and axial alignment) of the installed bearings is as good as it could be.
PF30
PF30 uses nylon cups, which are pressed into the BB bore. Bearings are then pressed into the cups.
My first concern here is the tolerances on the nylon cups. If the cups are not molded/machined and inspected well enough, then the installed bearings will not have good axial alignment. Do you see that?
Nylon isn’t that stiff though… so maybe any minor molding/machining inaccuracies are alleviated though the compliance of the cups. This brings up my most important concern… How is a BB with nylon cups stiff? You don’t need to be an engineer and/or scientist to see this. Just visually wrap some soft stuff (a polymer such as nylon) around some bearings and you will see that the polymer absorbs some of the energy when you are out of the saddle, pushing down on the pedal. I can’t tell you how much energy is lost, as that would take some time for me to derive
It seems to me that PF30 was created to alleviate some creaking/noise issues. Or maybe it was created just to be different. Both are bad reasons to alleviate a relatively easy problem (noises). As anecdotal evidence, my Felt came with BB30 and I have never heard a creak. I do hear a squeal when the cranks are at a specific position every now and then, though. It doesn’t bother me too much.
BB86/92
The BB bore is now stepped. This means that the steps on each side of the bore are machined by different setups. In other words, in the first setup, the through bore is machined and the step on this side is made. Then the part (the BB shell) needs to be repositioned in a fixture to create the step on the other side. The runout between the two steps will not be as good as a machined completely straight through bore. Nylon cups are then pressed into the steps. Again the compliance in the cups will likely alleviate minor runout/alignment issues in the machining. My concern here once again is the nylon cups… nylon isn’t anywhere near as stiff as steel. Look up the modulus/stiffness of these 2 materials.
BB90/95
The BB shell is stepped once again. However, there are no cups. If you haven’t noticed, I’m not a fan of the cups J. The bearings are pressed into the “steps.” The steps need to have a good runout to create good alignment for the bearings.
386 EVO
Same idea as PF30, only the BB is wider. Spacing the bearings a little further apart will increase stiffness negligibly. The added benefit comes from being about to add more “frame” around the BB to increase stiffness. How much of that stiffness is lost in the nylon cups?

There are engineering advantages and disadvantages in every design. I could go into more detail but I feel like I am writing too much and likely boring you.
The manufactures have done their stiffness analyses and I’m sure they are comfortable with their decisions. I don’t have the time to do the same but I can assure you that PF30 is not as stiff as BB30. Maybe the difference is negligible in the real world but nonetheless, nylon is more compliant.
My top picks right now are BB30 and Trek’s BB90/95. Trek’s standard requires more QC/inspection due to the steps…. no biggie for a decent manufacturer.
Some people are worried about damaging the bore (shell) when removing/installing bearings. In a proper press-fit, the material is “moved out of the way.” When you remove the bearing, the bore will have the same dimension as it previously did. Damage only occurs when the bore and bearing are not the correct dimensions or when the bearing isn't aligned well when it is initially being pressed in. All you need is some care when installing the bearing and everything will be fine. Furthermore, how many times (realistically) is someone going to remove and install bearings on a frame? A couple? No sweat.
I love healthy discussions, so please comment and/or ask away. I have plenty of experience with bearings/materials and their properties, so feel free to ask.
 

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My first concern here is the tolerances on the nylon cups. If the cups are not molded/machined and inspected well enough, then the installed bearings will not have good axial alignment. Do you see that?
Nylon isn’t that stiff though… so maybe any minor molding/machining inaccuracies are alleviated though the compliance of the cups.
This seems to be the real motivation behind PF30 though - the soft nylon cups allow the BB shell to have looser tolerances. This makes manufacture much easier.

How is a BB with nylon cups stiff? You don’t need to be an engineer and/or scientist to see this. Just visually wrap some soft stuff (a polymer such as nylon) around some bearings and you will see that the polymer absorbs some of the energy when you are out of the saddle, pushing down on the pedal. I can’t tell you how much energy is lost, as that would take some time for me to derive
I doubt this is going to be significant. Even though nylon is more compressible than aluminium (about 10x I think), the cups are very thin, so the amount of compression is going to be tiny. And if that's too much for you, you can get ones with Al cups.
 

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You ought to have included BSA in your analysis, which from both engineering and practical perspectives doesn't have any of the problems of the various press fit designs. The stiffness claims for the press fits are really pretty minor and for the cranks themselves, not for the bike as a complete assembly. Possibly a minor weight difference, but whatever.

The discussion of risk on press fits needs to include maintenance realities, such as corrosion and foreign materials, as well as minor tolerance disagreements between the QC or spec of the bearing and frame member. Also, press fits carry a somewhat greater risk of damage in the process, especially if the correct tools aren't used to maintain good alignment during the process. And replacement might be considerably more than 'a couple' of times if it's a bike ridden on something other than joyrides on a sunny Saturday afternoon.
 

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Engineer1,

Thanks for the explanation. This was useful as I am looking at new bikes right now. The salesman at the LBS said this model comes with a BB30. What's that I said and what makes it better? His short answer was it was stiffer and harder to service. My present bike has a Shimano 105 BB that screws into the frame. The bike was made in the 2005 time frame. What is the downside of my present screw in BB vs the BB30?

Ride Safe,

Joe
 

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There was never anything wrong with BSA. It's a bunch of horsecrap that there is significant flex, I don't care how much power you're putting out. I hope Campy never gets rid of their BSA cups because the bearings are super duper easy to get to and lubricate.
 

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imo, based on my previous career in design and manufacturing engineering, which included Delrin components, the biggest reason for BB30 and PF30 is to reduce the number of parts used to make a bike, which will reduce the weight a little bit, reduce manufacturing steps, ultimately to lower piece price for a bike.

I think that the nylon bushings, as stated, remove the need for secondary finishing operations like facing or boring the bottom bracket in the frame. That's not such a huge issue with aluminum, or steel, which depends mostly on fixturing (they're just tubes, with consistent tolerancing), but is a bigger benefit for a carbon frame, which comes out of the mold green (even though the mold is to nominal manufacturing size), but if I'm not mistaken still needs boring operations. Nylon and delrin would absorb those tolerance bands.

I might be wrong, if I am you can correct me, it's been 10 years since I've done that stupid job.
 

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Discussion Starter · #8 · (Edited)
Engineer1,

Thanks for the explanation. This was useful as I am looking at new bikes right now. The salesman at the LBS said this model comes with a BB30. What's that I said and what makes it better? His short answer was it was stiffer and harder to service. My present bike has a Shimano 105 BB that screws into the frame. The bike was made in the 2005 time frame. What is the downside of my present screw in BB vs the BB30?

Ride Safe,

Joe
It is difficult to say which is better. Is the BB30 stiffer? Most likely. BUT, the stiffness of the frame also matters. So if you have BB30 with a compliant frame, the benefit of BB30 isn't going to make a difference.

As .je said, ease of manufacture/assembly is a objective for manufacurers. BB30 and others reduce parts and therefore costs.

As far as bearings go, I have never seen a bearing that came in out of spec. That doesn't mean that it can't happen. The outer diameter is within a handful of microns. The outer diameter typically has a tolerance of +0/-.010 (or something near that lower limit). I'm sure manufacturers aren't specifying anything different.

Contamination is a good concern. I honestly can't comment on whether BB30, PF30, etc. are more prone to contamination than threaded BB. I would have to look at them some more and think about it. I would imagine that splashes of water are no issue for any BB since everything uses sealed bearing. Does anyone have any anecdotal evidence? I'm currently taking apart a threaded BB from the mid 80s. It doesn't spin well but it has been left outside for years, so I'm not sure that is saying much :p.

I wouldn't be jumping onto a new frame just because BB30 is slightly stiffer. In the grand scheme of things, and in the great majority of cases, the faster bike is the one with the better rider. If you are shopping for a new frame, I wouldn't shy away from BB30, PF30, etc. because they are a little more difficult to service.
 

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What is BSA? (I assume it does not stand for Birmingham Small Arms)

Ride Safe,

Joe
Indeed it does.

Sheldon Brown's Bicycle Glossary Bo--Bz
B.S.A.
B.S.A. (Birmingham Small Arms) was a major British manufacturer of firearms, and later of bicycles and motorcycles. It reached its peak in the 1920's. The standard thread sizes that B. S. A developed for its bicycles were ultimately adopted as the standard British (B.S.C.) sizes, which, in turn, were mostly adopted by the I.S.O.

Tim
 
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