[Closed] Facing BB's?

The only thing missing is a liberal sprinkle of marketing bollocks and there you have it.

When you facing a BB, all you arre doing is make the face nice and square so when you put a BB in it will sit flush and not on the piss.
If you dont face the BB square then you will damage the BB, and all so if you take to much meat off you risk losing thread length and there for the bottom bracket is more likly to strip the threads.

Don't the threads align how the bearing cups sit in the frame? Or am I missing something?

no it's the mating faces which align it.

I understand the reasoning and concept of facing the bottom bracket shell. What I don't get is why I should need to do it when it presumably has already been done at the factory if it is so important?

DrC, won't the BB align with the threads rather than the face which it basically sits / stops against?

Toys, that was my interpretation of it!

No, when cutting the thread in to the BB it follows axis of the BB, so aslong as you are using the correct facing tool (not a disc grinder, bad times) you will face square to the axis, therefor square to the thread.
You only need to face it, if you are using a top end BB, such as XT, or XTR.

Most facing tools have a thread tapping attatchment too, so why not do both at the same time?
Unless there's something very wrong with the shell, it's not really necassary in my experience. My XT hollowtech 2 was in my Sunn for two years before I got round to facing the shell and was fine - still is another year down the line.

You are thinking that the bottom bracket is infinitely rigid so that it stops going into the frame as soon as it touches anywhere on the shell face. However it is not, it will continue to screw in but flex slightly so although the threaded section is in straight the bearings can be skewed.

konaboy2275 - Member

DrC, won't the BB align with the threads rather than the face which it basically sits / stops against?

Exactly

Toys, that was my interpretation of it!

Exactly

Mister P yes you are correct. I suppose really you are only going to need to face the bottom bracket if you start with a cheap bike such as one from Halfords, as they would not give the time to De-bur all faces in manuctoring process. This is because they use the square drive BB which dont rely on a square face unlike hollow tech BBs.

You only need to face it, if you are using a top end BB, such as XT, or XTR.

Eh??? Cheaper bearings don't need alignment?

won't the BB align with the threads rather than the face which it basically sits / stops against?

No, it won't!

But maybe the threads and faces from driveside and opposite aren't aligned with each other from the factory, since the thread cutter there doesn't cut both at the same time. None of the aftermarket cutters or chasers would correct that, and I don't believe anything you screw into the shell will take on any alignment other than the threads once it's passed the first couple of turns.

won't the BB align with the threads rather than the face which it basically sits / stops against?

No, it won't!

Yes it will.

Sorry when i say cheap bearings i mean Cassett/square drive bearings as the bearing is in the BB, rather then on the face of the BB like Hollow Tecs.

Square taper, hollowtech and isis.

OK toys so if the bb cup is screwing into the bb nice and straight, it can obviously move around, and the shell-face is not true to the threads, you say the cup will rest true to the threads rather than the shell-face? i.e. whether the shell-face is true or not doesn't matter?

Would like to see you justify that if I understand you right.

Midlifecrashes, when bike are made they use tube for the bottom bracket , so it will never be not alligned unless they use a bent bit of tube.

Right, didn't want a fight, gonna ring my mate in Houston who designs gearboxes, maybe he will know! 😀

cynic-al - Member

OK toys so if the bb cup is screwing into the bb nice and straight, it can obviously move around, and the shell-face is not true to the threads, you say the cup will rest true to the threads rather than the shell-face? i.e. whether the shell-face is true or not doesn't matter?

Would like to see you justify that if I understand you right.

Firstly[/b] Yes sort of. it depends on the clearance stack up difference in the threads and the face. But this is why facing makes no difference as the clearance on the threads when you use the facing tool will influence how true the face is to the axial centre line of the BB, ie it wont be true to the axial centre line of the bb..

[b]Secondly[/b] if you defer to the big S, they do not anywhere in their literature recommend facing.

[b]Thirdly[/b] the torque on a bb recommended by the big S 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.
So to that end I've just made a contact model with a 1 degree offset face with a 60 newton preload. I haven't modelled the threads yet for brevity so the assumption is that the bb threads perfectly align and we want to see if the BB cup will flex. I've been mean to the bb cup to as I havent put in a bearing for any support.

Result?

at the outermost edge of the cup on the side with the 1 degree offset, the deflection is 0.4x 10-4 mm - ie three fifths of eff all. The deflection decreases as you get closer to the cup/bb interface face. The deflection on the other The gap has not been closed and the cup does not deform to fit the face.

Pics later I'm too busy just now to upload them.

When I get round to it I'll fiddle with some threaded models, anyone know what the clearances might be between the threads?
[s]
[b]Fourthly[/b]. Ball Roller bearings can take pretty big axial misalignment, that's what they are designed for.[/s] edit scratch that, I remembered these are normally deep groove bearings so they are crummy at misalignment...

[b]I remain convinced this bollocks invented by park tools et al to sell tools and process work for workshops.[/b]

^^ I agree, toys19.

Never faced any of my frames, and the BBs still go on for years and years (if serviced properly).

You know what they say about a fool and his money...

Symptoms indicating need
of facing
[b]There is only one symptom that indicates the need
for facing the bottom-bracket shell[/b]. When attempting to
adjust a high-quality adjustable-cup bottom bracket with
new parts, the spindle feels [b]smooth through a portion of
its rotation and tight in another portion of its rotation.[/b]
This is called a tight/loose pattern.

Barnetts manual of Bicycle Repair

Ah OK toys so slightly different to your original statement. 1mm out on one side of the face would amount to 5 degrees - 1 degree is **** all as you say.

How about re-modelling on say 1mm?

FWIW I agree facing is highly unlikely to be beneficial unless something has happened to make the face "really" uneven - like a blob of paint or something.

Al - given that it doesn't move to close the gap at 1degree why would it at 5? It is still in contact in only a very small area on on side. But I shall do it to prove a point. Hang on.

- I think the most imp[rotant thing is that facing is likely to make things worse as the distance between the faces is very important with regards to axial loading, every time you face yout ake off god knows how much.

FWIW the blob of paint wont make any difference either, I'll model that for you too shall I?

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

Ah OK I get your sciency stuff now. Shirley folk WILL use greater torque than the shimano limit in order to get the cup to tighten to the face?

You don't need to model the blob but an explanation would be nice rather than saying "I am right".

Al sorry, I've never been subtle...
I've modelled the paint anyway for fun. Hang on

Lol, im going to have a go at this see what my out come is. My reasoning for needing to face the BB was, with the likes of Hollowtec a bearing face is tight up against the shell, therefore any large imperfection on the shell face can put unwanted forces on to the side of the bearing case, making the bearing run tight.

Hmmmmmmmmmmmmmmm.

Ok here is the 5 degree offset, that's about a 4mm gap and under load from the torque it moves by 0.014 of a mm (1.4 hundreths of a mm) on the opening side. This acts like a point support on one side really..

[img] [/img]

This is the 1 degree offset the gap is 0.34 ish of a mm under the same load it closes up by 0.00177mm (or 2thousandths of a mm) an order of magnitude less. The analysis shows that the low angle allows more of the face to come into contact so it supports it more, hence less deformation.

[img] [/img]

Problems with this analysis.

1) The major one for me is that there is no support from the thread inside, so this allows the cup threaded portion to deform down inside the bb tube, I reckon with the threads in there stopping it from moving this deformation will be loads less. I shall work on this one.

2) No support internally from the bearing which would stiffen things up.

3) I've never seen a bb with a 5 degree off face or anything like a 4mm gap, so I think this is moot, if the facing tool took off more than 4mm the bb tube would be useless as the gap between the bearings and the contact faces of the cranks would be ridiculous...
More likley the facing tool takes off 1/4mm, so that would imply that it was 1/4mm out, which would give even less movement based on my 1mm analysis.

Therefore - [b]its bollocks[/b] and that's an official engineering term. (Although DrCalumR will prove me wrong in a bit)

Uh did I get radians and degrees mixed up then?

You HAVE accounted for the fact that it's not a press fit and there is movement in the threads?

I've had two frames where before facing the cranks did not spin, after facing no problems, these were oldish frames both 2004 a rockhopper and a blur. YMMV.

Edit I'm home now and not sure of my measurements of the gap - what dia is a bb tube again I think I did 40mmdia so tan1x40 = 0.69 mm gap for the 1degree case, and tan5x40 =3.49mm gap for the 5 degree case. Is that about right?

Al I did say earlier it was a perfect fit in this analysis as I was testing the cup deforming theory.

So to that end I've just made a contact model with a 1 degree offset face with a 60 newton preload. I haven't modelled the threads yet for brevity so the assumption is that the bb threads perfectly align and we want to see if the BB cup will flex. I've been mean to the bb cup to as I havent put in a bearing for any support.

😯

toys19

How many BB's have you faced and how many have you removed from frames and evaluated ❓

If you're reading this and are not sure, I have researched it thorougly, I've spoken to hundreds of riders about BB life. Have a look at your frame and see if there is paint covering the BB shell.

If there isn't then it's cool, if there is then it hasn't been faced in the factory and should be done.

That said if you live in Edinburgh I will face it for free, you have to strip your own bike though 😀

HAHAHAHAHA!

toys19

Problems with this analysis.

Yes there are, you're mental ❗

Can I throw the (soft) plastic spacers into the mix? I would have thought these would soak up a certain amount of misalignment.

good point. I shall test them tomorrow.

Toys, dia. is 34.75 IIRC, 1/16.875 = .059 ... x 180/pi makes 3.38 degrees?

Ah I see the bit about the threads. I guess there'd be a few degrees of movement there - can you model that if you don't know the tolerances?

Toys, dia. is 34.75 IIRC, 1/16.875 = .059 ... x 180/pi makes 3.38 degrees?

Ah I see the bit about the threads. I guess there'd be a few degrees of movement there - can you model that if you don't know the tolerances?

Yeah sounds right, cant rember what dia I used I'll have a look when I get in tomorrow, . I cannot see the tolerances making "a few degrees"

I dont get your trigononometry. In degrees I'm using tan(theta)= opp/adjacent. Adjacent is the diameter, opp is the gap opened up. I'm assuming the whole face is at an angle so the centre of rotation is not int he middle its at one side. So if you rearrange to give tan(theta) x adjacent = opp, then you can calc gap size from angle and dia. So at 34.75 dia 1 degree offset gives .606mm gap, 2 degs 1.21, 3 degs 1.82, 4 degs gives 2.42 and 5 gives 3.04mm. Have you tried rocking a loose bb? And got 1mm of play? I'm going to try it tonight and measure it with the DTI, then I can estimate the thread clearances.

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 - [url= http://www.parktool.com/blog/repair-help/torque-specifications-and-concepts ]here[/url])

awhiles you are right, just checked here [url= http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/FC/EV-FC-M960-2152D_v1_m56577569830608897.pdf ]the big S hollowtech bb[/url], 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 [i]just[/i] 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 [url= http://www.roymech.co.uk/Useful_Tables/Screws/Thread_tol.html ]roymech[/url] and thread dimensions for bike threads are [url= http://www.gewinde-normen.de/en/cycle-thread.html ]here[/url], 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?