I was just wondring about this after another BB has turned to mush. Surely if you are facing the frame at the BB then it will need re tapping? Don’t the threads align how the bearing cups sit in the frame? Or am I missing something?

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. [b]

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..

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

Thirdly 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?

Fourthly. Ball Roller bearings can take pretty big axial misalignment, that’s what they are designed for. edit scratch that, I remembered these are normally deep groove bearings so they are crummy at misalignment…

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

^^ 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
There is only one symptom that indicates the need
for facing the bottom-bracket shell. When attempting to
adjust a high-quality adjustable-cup bottom bracket with
new parts, the spindle feels smooth through a portion of
its rotation and tight in another portion of its rotation.
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..

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.

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 – its bollocks 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.

I’ll look at thread clearances tomorrow if I have the time.

😯

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.