Machining Bottom bracket BB90/95

[vib]

Hi everyone,

I'mbuilding my first MTB frame, based on shan production privee geometry with very few changes.
I just finished machining the HT for ZS44/28.6 top cup and EC44/40 botto, they fit in, I'm happy :) (i'm very very new to this kind of stuff).
This weekend I will work on the bottom bracket but as a true beginner, I'm struggling with standards.
Basically, i would like to simplify everything: one tube (bottom bracket), 2 bearings, one spacer and one crankset.
For crankset, i will put something not too expensive: Shimano slx M675 (24mm axle).
According to my research, what I would do is:
BB of 95.5mm long, (36.97 +-0.02 ID at each end), with one bearing at each end (37OD 24ID) and spacer inside beatween two bearings.
With that, in theory, I should be able to put ma crankset with nothing more needed.

Am I wrong?
Thank you for your help :)

[e-RICHIE]

Nice to see you here vib.
Please add a first and last name to all posts you make in the Frame Forum.
http://www.velocipedesalon.com/forum...rum-36866.html
Thank you.

[Gattonero]

Hi everyone,

I'mbuilding my first MTB frame, based on shan production privee geometry with very few changes.
I just finished machining the HT for ZS44/28.6 top cup and EC44/40 botto, they fit in, I'm happy :) (i'm very very new to this kind of stuff).
This weekend I will work on the bottom bracket but as a true beginner, I'm struggling with standards.
Basically, i would like to simplify everything: one tube (bottom bracket), 2 bearings, one spacer and one crankset.
For crankset, i will put something not too expensive: Shimano slx M675 (24mm axle).
According to my research, what I would do is:
BB of 95.5mm long, (36.97 +-0.02 ID at each end), with one bearing at each end (37OD 24ID) and spacer inside beatween two bearings.
With that, in theory, I should be able to put ma crankset with nothing more needed.

Am I wrong?
Thank you for your help :)

A spacer inside the BB shell would give a bit of trouble and won't cope with lateral loads. You should do like a BB30 -with the obvious scaled down sizes- machining one groove each side for a circlip, or machining the should for the bearing right in the BB shell.
You would still need a cover for the bearings, if you want them to last. It cannot be just "two bearings and one spacer", you will need bearing shields and spacers towards the cranks, I do not see any good in the cranks laying directly over the bearings, they'll just hit the BB shell.
In my view, you better use #6805 bearings (25x37) and reuse the covers that are on Shimano HT2 BB cups, as they will reduce the bearing ID from 25 to 24mm, plus give a better interface being a synthetic material on metal (rather than metal on metal). The advantage of this, will be also to act as spacer to keep you cranks off the BB shell

[vib]

Thank you Gattonero for your reply.
So I forgot to precise that I will do a should inside the BB shell so the bearing will not be able to go further inside the BB shell, the spacer is here just in case of (I saw that they use them on spanish BB or mid BB in BMX).
This is why I'm looking for the largest bb shell depending on my crankset. As you can see for the MID BB standard, there is no plastic or aluminium and this is exactly what I want to avoid. Crankset axle is directly on bearings and everything is fine. And also this is quite straight forward to change: replace bearings and you are done, less than 30euros.
But now my real concern is: is a 95.5mm BB shell good for my crankset ? I see no reason why not but I just want to be sure before doing something
I can maybe put a small plastic spacer in order to avoid the crank hit the bb shell, right.

[Gattonero]

BMX BB's are good for their use, actually "abuse". Their reliability for 50+ miles rides is all TBC.
Also, they are good in BMX because of the heavy gauge of tubing used, not sure if everyone wants that burden on different disciplines

[vib]

What do you mean by TBC?
My mtb will not be focused on the weight (even if I will try to improve it) but more on a funny geometry
As an example BB should be 2.5 mm thick which is... Important
I want this mtb very strong to not be worried when I jump or whatever....
I already have a 29 hardtail (BMC te02) which is more focused on performance
And I'm not building for other people, only for my purpose.
After few more readings, bb95 or bb90 seems to be both good for me. That should work