Ah cool I don’t have a scientific calc atm.

1mm play maybe from threads. Anyway it’s all proving I WAS RIGHT 😉

toys19 – Member

the torque on a bb recommended by (shimano) is about 4-6 inch pounds or a maximum of 8 newton metres, which translates to an axial force of about 60 N. So I think any ideas that the bb deforms under the preload is bunkum.

those numbers are suspicious to say the least.

4-6 inch pounds is the preload torque – which is roughly sod all.

we’re concerned here with the BB cup tightening torqe, which is around 300 inch pounds or more.

which is loads.

i’m giving you a fail, go back and do it again.

(i got my numbers from the park tool website – here)

awhiles you are right, just checked here the big S hollowtech bb, dunno where I got my numbers from. Anyway I shall redo and we shall see.

my apologies! – i forgot to include a 🙂

Ok this morning I haver just measured my shimano BB on my cotic frame with a dti- (which came as far as I can see un faced.)
With the bb finger tight I backed off half a turn, and could not measure any rock whatsoever side to side with my DTI. ZERO NADA. I’m off to work now, tonight I’ll measure the other 5 bikes and see what I find.

the total lack of understanding so regularly demonstrated by this forum is really quite scary.

its not rocket science, its just a bit of metal.

I think you find bike engineering is very close to rocket science, especialy with the complexity of material used in bikes today.

especially aluminium, which is basically a slightly stronger form of cheese.

a bit like pecorino, only not as tasty on a cracker.

(BB cups are made from thin walled aluminium, which is a flexible material, don’t act all surprised if you haven’t modelled the behaviour of an aluminium component, and it turns out to be all wobbly)

I can solve your problem. Get a square taper or an octalink.

forget the facing malacky

😯

OK so if the bearings rotate better, which they will if the bb is faced, will they last longer ❓

kaesae – Member

OK so if the bearings rotate better, which they will if the bb is faced, will they last longer

In theory, yes they would last longer – but the rate at which they degrade would not be noticed in the time frame you’re realistically going to run the BB.

If you had a road bike, which only got ridden in perfect dry weather, serviced regularly, you may see the side effects of ‘not facing’ the frame in what 10 years?? (Yes, it’s theoretical)

BB’s in mountain bikes get put through far more abuse than the above road bike – so it’s not uncommon for them to be replaced as little as 12 months?

So… you would never see the side effects of ‘not facing’ the frame in the life time of the BB.

You got data to back that up xiphon?

The main difference between road and mtb use is mud and cleaning.

cynic-al – Member
You got data to back that up xiphon?

The main difference between road and mtb use is mud and cleaning.

None what so ever 🙂

In many years of riding, I’ve never felt the need to face a frame – and I would not say I go through BB’s any faster rate than the next rider…

Devil’s advocate in my would say it’s another ‘service’ the bike shops can offer to bring in revenue…

toys19 – Member

Ok this morning I haver just measured my shimano BB on my cotic frame with a dti- (which came as far as I can see un faced.)
With the bb finger tight I backed off half a turn, and could not measure any rock whatsoever side to side with my DTI. ZERO NADA. I’m off to work now, tonight I’ll measure the other 5 bikes and see what I find.

Ok I am interested in this. I expected there to be some rock in the bb cup in the frame when not fully tight so I just took my bike apart to check

Teh cup backed off a turn in the bb. ( no fancy measuring thing) no rock felt attempting to move it by hand

Tap it with a hammer you could just feel the tiniest bit of movement Gnats ba’ hair maybe? Not enough to give any significant misalignment if tightened down hard I wouldn’t have thought. Maybe the large diameter fine thread means no play ‘cos you often get play in a loose thread on a nut and bolt.

I am surprised at that.

Despite my gut feel that this is “marketing bollocks” I have tried to approach this with an open mind, and when awhiles pointed out my torque error I was ready to agree with the PArkTool-O-Philes. So despite a ridiculous workload I have managed to re do my analysis and sort out the “first stab” shortcomings. Results below. I also measured my other five bikes last night with the DTI, all at half a turn backed off from finger tight, all tight no movement. TJ I wonder if your hitting with a hammer was probably giving you some deformation rather than actual movement due to gaps/tolerances……

The next thing I want to measure is the deformation of the BB under tightening torque which is tonight’s project. (to check my calcs)

So I think there is a series of questions that I need to answer to see if my assertion that facing BB’s is just marketing speak or if it it is a genuine process that is necessary.

1) Is there any tolerance around the threads to allow the cup to move/settle on the angled face. Based on my measurements – no, there is not. I dunno if I can be arsed to do the tolerance calcs, its all here roymech and thread dimensions for bike threads are here, someone else do it if it takes your fancy. I am expecting them to be small. 24TPI is a fine thread..

2) BB cup deforming under tightening torque. After Awhiles pointed out my error I re did the 1 degree analysis, with some mods that account for the restriction of the threads, although it still assumes there is no thread tolerance. Result; a peak deformation of the high side of the cup of 0.0144mm (human hair is 0.1mm) the side not in contact moves by 0.00018mm..
To compare this I wondered what the tolerance on the shaft between the bearing was:
Locating/Slight interference fit for holes is normally H7 and Shafts is K6
24mm Hole = + 0.033mm tolerance
24mm Shaft = + 0.015mm

So the difference is 0.018mm.
Over approx 70 mm (width of a BB) this gives an offset angle of the shaft of 0.013 degs, which across the face of the BB gives a gap of 0.0077mm. Which is half of the predicted deformation under stress of torquing up the cup.

Or you can look at the surface tolerance of the cup which would be of the same order, if you add these together to get the total tolerance that the cup would be out due to machining and that of the shaft and inner diameter of the bearings, its the same or more than my calc for deformation.

3) I have a logical objection – I don’t understand how the cutter works:
Either:
a) There is wild tolerance on threads such that a BB will not go on straight, then surely the cutter wont go on straight either, it will cut a wonky surface rocking around on the threads, with the movement dependant on what angle the torque bar on the cutter is when the operator turns it?
or
b) the cutter follows the threads exactly because they which means the new face will be 90 degrees to the threads and the cutter removes any paint and offset.

If b) is correct (and I suspect it is) then the only argument for facing is that the cup will deform under torque, and based on my calcs the cup deformation is the same or less than the added maximum tolerances of the cup/shaft combo.

Finally there is one issue here that I think is more important and that is this:

bassspine – Member

with hollowtech II type bottom brackets the most likely cause of early wear is over-tightening the preload cap.

Which I agree with and may be a reason why facing appears to give people more bearing life, because if you reduce the length of the BB shell by facing then as you tighten the preload cap (too tight) it tightens up against the land on the crank shaft and not the bearing as the bearing length across the BB is now too short. Therefore no excessive axial preload and longer life than if you had too much preload. This assumes that life of bearing under correct preload is > than life of bearing under no preload (life affected by side play) > than life under excessive preload.

Basspine again proving that Devon peeps are the smartest 😆

You have to faces that give the through axle it’s alignment, your arguing that it makes no difference because of the miniscule amount of miss alignment.

Do you not understand that it is missalignment x load x impacts or excessive load?

Tandem jeremy, how many frames do you have that could be faced? I will face all the alloy frames you own for free and you can see how your BB’s compare to when they where not faced?