AMB: Press fit bottom brackets - why?

[Bulk]

http://australianmountainbike.blogspot.com/2011/03/press-fit-bottom-brackets-why.html


<div><span class="Apple-style-span" style="font-family: Times; -webkit-tap-highlight-color: rgba(26, 26, 26, 0.296875); -webkit-composition-fill-color: rgba(175, 192, 227, 0.230469); -webkit-composition-frame-color: rgba(77, 128, 180, 0.230469); font-size: medium; "><div style="font-size: 18px; -webkit-text-size-adjust: none; background-image: initial; background-attachment: initial; background-origin: initial; background-clip: initial; background-color: rgb(255, 255, 255); background-position: initial initial; background-repeat: initial initial; ">AMB's Oli K caught up with Shimano's Greggy Chalberg to press him (get it?!) for answers about press fit bottom brackets.




Why did shimano go to a Press fit BB?

Customers (frame manufacturers) were asking for it.




Why is that?

Having a bottom bracket shell with thread and having to design a frame to accommodate an external cup on each side was restricting.




Why did Shimano go with their current system (over say a BB30)?

When Shimano originally released the external bearing two piece crankset back in '02 they determined that a 24mm diameter axle was the optimal for strength to weight ratio, stiffness and also minimising friction from the rotating BB bearings. It also allows them to use steel as an axle material without a weight penalty. A BB30 system requires an oversize axle to give stiffness but axle made of steel would be bring a weight penalty as it wouldn't be possible to make the steel thin enough to keep the weight down for this application. Shimano feels aluminium is an inferior BB axle material compared to steel. Shimano chooses to use steel on all its high end axles for durability and peace of mind.




Testing at Shimano Japan reveals that the oversize bearings in the BB30 system cause a measurable increase in friction compared with Shimano's press fit bearings.




It allows us give people the option (manufacturers and riders) to use press fit without having to change to a new system of cranksets. Perhaps one day we'll see Shimano produce BB30 bottom brackets/cranks as well, because people are asking for it. At least there are manufacturers out there making adapters for running our two piece 24mm diameter axle cranks on a BB30 equipped bike.






Advantages of the system

More scope for frame design.

Compatible with current 2 piece cranks.

Lighter by 30g (30%) over XT external BB.

Less pedalling friction compared with BB30.

Potentially more weatherproof than the external bearing BB system, not because of the quality of the bearings but because they are less exposed to water when tucked inside the bb shell (note - this is Oli's theory and has yet to be proven – STAY TUNED!)






Disadvantages


Specific tools required for installation and removal (retail about $300 for both)<span style="line-height: 18px; padding-left: 36px; ">

More...

 

[SummitFever]

...Having a bottom bracket shell with thread and having to design a frame to accommodate an external cup on each side was restricting...

That doesn't make a lot of sense. The OD of the external cup hardly seems larger than the OD of a press fit bottom bracket. Same for the width of the BB+cups and press fit BB.

Its complete crap to go from having a threaded system to a press fit system that requires expensive tools. No doubt the manufacturers like it because its far less work for them. No thread maching required and you can press fit much quicker than threading cups in.

 

[Landon]

BB30 Friction

It's not entirely true to say a BB30 system has more friction than a press fit or threaded external BB. The only seals on a BB30 system (except for the SRAM Pressfit30 BB) are the actual 6806 cartridge bearing seals. It's not really a system designed for install and forget MTB riding. But, nothing will spin faster and smoother than a BB30 setup like this. Maybe they're comparing a hypothetical Shimano designed BB30 system with the same type of seals as the 24mm BB's. In that case, maybe there would be an increase in friction.

 

[Josh Seksy]

That doesn't make a lot of sense. The OD of the external cup hardly seems larger than the OD of a press fit bottom bracket. Same for the width of the BB+cups and press fit BB.

Its complete crap to go from having a threaded system to a press fit system that requires expensive tools. No doubt the manufacturers like it because its far less work for them. No thread maching required and you can press fit much quicker than threading cups in.

Actually it does, they're talking about the actual width of the BB shell, with a threaded BB, the largest width you can make it is a 73mm, without needing to run special cranksets etc.
With pressfit, you can actually run a larger shell, and therefore the frame can have a fatter downtube & seat tube - which improves stiffness.

Its actually less expensive tools for press fit.
BB30 requires special tools, but pressfit does not.

You can install a pressfit BB with a rubber mallet, and remove it with a tyrelever/spoon/screwdriver.

Pressfit will revelotionise bikes - no need to machine a BB so perfectly, thicker tubes = stiffer frames, thicker spindles = stiffer cranksets, etc.

 

[SummitFever]

...
You can install a pressfit BB with a rubber mallet, and remove it with a tyrelever/spoon/screwdriver...

Sure you "can" but press fitting bearings into alu can only be done so many times before the interference fit is out of spec - particularly if you remove the bearings by prying them out - so no tyre lever, pry bar, screwdriver antics unless you want to flog out your BB shell in quick time.

Most press fit bearing into alu techniques involve freezing the bearing and heating the alu to stop the interface getting flogged out. No-one is going to do that with their frame and just freezing the bearings isn't enough because you can't get them cool enough to make a real difference.

What do you do when you flog out the interference fit? Creaky BB or new frame - great choice.

 

[Josh Seksy]

Sure you "can" but press fitting bearings into alu can only be done so many times before the interference fit is out of spec - particularly if you remove the bearings by prying them out - so no tyre lever, pry bar, screwdriver antics unless you want to flog out your BB shell in quick time.

Most press fit bearing into alu techniques involve freezing the bearing and heating the alu to stop the interface getting flogged out. No-one is going to do that with their frame and just freezing the bearings isn't enough because you can't get them cool enough to make a real difference.

What do you do when you flog out the interference fit? Creaky BB or new frame - great choice.

The idea of press fit was so manufactures could use carbon BB shells - the plastic can flex to fill the gaps/intollerance. Alloy bb shell's on carbon frames are well and truely on the way out.

Hell, at work I put in and pull out pressfit BB's with my hands - They're not ment to be 'tight' fit

 

[ja_har]

Chain Guide

And has anyone solved the problem of how to install a chain guide on a bike that has no ISCG tabs and you want to use a front deraillaur (ie for a double guide).

You cannot use BB mounted chainguides on a pressfit BB that I know of?

 

[Josh Seksy]

E13 have something in the works from what I hear.