Originally Posted by
hueyhoolihan
i was surprised when my integrated headset bearings just dropped into the reamed but raw carbon headtube on my recently purchased carbon frame. thought it wouldn't last, but has... so far.
anyway i'm shocked they are using the same technique on the BB. i assumed the new large spindle, press fit BB bearings (a a la threadless headsets) were pressed into an alloy insert, considering the forces at play. wow!
huey....not how it works with a Foil or one of the many Press Fit 30 BB's on the market. A 46mm ID virgin carbon hole comprises the carbon shell.
46mm OD and 42mm ID Delrin (acetyl plastic) bushings are pressed into (sometimes epoxied) into the carbon shell. Basically what you have now is a composite of carbon and hard plastic...hard plastic bushing protect the carbon and when they wear out are replaced as other types of bushings are. 30 mm ID bearings are pressed into the Delrin bushings. Now you can run a 30mm spindle crank or a 24mm spindle diameter as in the case of Shimano with spacers. Also there are smaller diameter press fit BB's on the market that directly bolt to Shimano's 24mm crank....a huge array of different Press Fit BB's now on the market...some with 90mm shell width and others with 68 or even narrower shell width like Specialized carbon OSBB which measures 61mm without press fit bushings in place.
PS: if what you write is true...would love to see a picture and have you tell us what frame you have where the bearings are resting right inside the carbon head tube and not sleeved for wear resistance...and I have no reason to doubt you, I can comment on why you will never see bearings resting right inside a carbon BB shell without either plastic or metal bushings to protect the carbon. Although there are many analogies between a head tube and BB...on most carbon race bikes now, both are integrated...the principle difference is load path and stresses....they are completely the opposite.
In the case of a head tube, stresses are axial, i.e. the bearings are in compression and along the centerline of steerer tube. This is hugely friendly to any radial bearing stress which is proportional to the cosine of the HTA and relatively small compared to principle forces due to weight of rider + bike. By contrast the force on a BB are 90 degrees or opposing this directon...most stress it radial or parallel to the bearings seats. This completely changes the bearing stresses and the needed environment to support the bearings. There is a much greater lateral component of stress to the BB than there is compared to lateral stress on a headset where the bearings are subject mostly to compressive forces. So a BB has to have a more rigorous bearing system to support the magnitude and direction of the stresses upon it.