should the BB and headtube be faced on a new steel frame?

Dan_xz

satanas wrote:

This is a pointless discussion. If you won't listen to experience based on dealing with this stuff, there's no point in talking about it. Please stop trolling now.
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The fact that you refuse to understand or believe this (or are incapable of doing so) does not change the facts.

I am not going to waste any more time on this, or you.

He can have his opinion, whatever it's based on and has as much right to post here as you. How is that trolling? Anyone who doesn't agree with you is a troll?

satanas wrote:

Of course, since you are American it is to be expected that you will be patronising, whatever your level of knowledge (or ignorance).

He didn't insult you - he was only trying to explain his point with an example and not knowing your background. I'd hardly call that patronising, I'd call it a discussion and attempt to be helpful. To drag in such insulting generalisation based on nationality only makes yourself look ignorant.

It's a shame as you are obviously knowledgeable but you come across as having a big chip on your shoulder about it. Not trying to offend, just pointing things out.


Back on topic, I'd be interested to know how much your efforts to align things correctly have made a noticeable difference. Granted, you could feel the BB screw in easier but did this translate to a noticeable extension of BB life?

Lots of things are sound in principle but tolerances being dealt with are too small to actually manifest in real terms. I had the impression that external BB's (at least shimano ones) have fast wear rates anyway. If facing only adds a day or so riding to the overall life span would it really be worth the effort?

Also, with the load bearing on the outside of the BB then surely a close fit to the frame is needed for maximum strength? Or again, would this not translate into a real world effect?

I'm on my first external BB after 15 years of sealed internal BB's and, theory aside, I'd like to know if this kind of work would result in actual rider noticeable (not mathematically measurable) longevity.

satanas

As far as I know few, if any, double blind studies have been done on anything related to bicycle mechanics. Therefore, virtually anything said here or elsewhere will be anecdotal. I cannot "prove" - in the scientific/statistical sense - whether doing various things helps or not, sorry.

The only potentially rigorous study I know of happened several years ago, when Mavic neutral support tested a number of spoking patterns on wheels carried on their vehicles in professional races. Based on what took place, Mavic concluded that a particular pattern was more durable. Almost every other conclusion (or assertion, if you like) that people offer either comes from their experience or is made up from whole cloth.

To answer the original question:

1. There is no doubt in my mind (which may, indeed, be addled) that headsets and fork crowns should be faced/machined unless there is clear evidence that this has already been done. This ensures that the headset components will not be misaligned, and thus makes riding the bike easier, plus it helps headset longevity. It must be done if a needle bearing headset is used, IME.

2. Re-tapping the BB threads with a "proper" tool and then facing the shell gives the best chance possible that components will last as long as possible. There is (anecdotal) evidenmce that this helps, but unfortunately I cannot quote any meaningful statistics to you to prove it. Caveat emptor and all that...

proto

For what it is worth, no decent engineer would ever rely on a thread to align bearings. Threads are for fixing things together, nothing else. So, even at its best, the BB design is flawed from the outset.

Also note that there is a relatively large tolerance on the tread cutting in a BB shell. The cups can be sloppy or tight and still be to the correct spec. The bearings need to be in line with each other, and if you are only relying on the threads, there there is a chance that this will not happen. Poorly manufactured shells, heat distortion or even paint build up can keep the bearings from aligning themselves properly.

Note this applies pretty well the same to the head tube too, heat, thin wall sections potentially causing problems. The Headset cups need to be a very precise interference fit and reaming will ensure this. The squareness of head tube ends is equally important, not square and the headset will not align, and only some of the bearings will be doing their job (I'm assuming cup/cone type)

In my experience BB threads never need rethreading, but always need facing. Headsets always need facing and reaming.

Just read back a bit and Satanas is right in what he says, and makes my post superfluous.

BTW, the bottom bracket shells are screwcut not tapped.

dennisn

satanas wrote:

2. Re-tapping the BB threads with a "proper" tool and then facing the shell gives the best chance possible that components will last as long as possible. There is (anecdotal) evidenmce that this helps, but unfortunately I cannot quote any meaningful statistics to you to prove it. Caveat emptor and all that...

You need to explain this "re-tapping" to me. Are you talking about simply "chasing" the already tapped threads? This doesn't change anything except make screwing the BB in a bit easier. Any misalignment is still there. You've just sort of "cleaned out" what's there already.
As to "re-tapping" I need you to explain just how you get rid of the factory threading and put in new threads. How do you get rid of the old threads and still have material left to cut new threads? If this is in fact what you're saying, although I doubt you actually remove the old and put new in, then it makes no sense to me. You don't just stick a tap in the shell and the old threads "go away and new ones appear". Well that can sort of happen but I would doubt they would last long before stripping out.
I short I have no real idea what you're accomplishing. Fctory threads are there when you buy the frame They are either aligned or not and short of completely ripping out the old and putting in new(which I would like to know how you do it) you haven't explained how
facing helps put the factory threads back in line.

proto

Re-threading doesn't realign the threads. As you say, it can't. It merely cleans out the thread and removes any out or roundness caused during the frame fabrication process.

The facing ensures that the end faces are square to the thread, parallel to each other, and further, ensures that when seated against the BB shell, the cups are sitting with 360° shoulder contact, no high points, lumps, bumps, which could skew the cup relative to a sloppy thread.

Er ........ that's it.


PS when BB shells are threaded I suspect that they use cut pieces of tube, loaded into a lathe, faced, bored and screwcut. The part is then reversed in the chuck and the process is repeated for the other end (different thread direction, I know). If the part does not run true in the lathe chuck then the two ends will be eccentric and this will cause problems.

It is possible that both threads are cut from one end, in one operation, but I doubt that that is how it is done. With cast steel BB shells, because of the nature of the external shape, there is great potential for errors if not done carefully.

dennisn

proto wrote:

Re-threading doesn't realign the threads. As you say, it can't. It merely cleans out the thread and removes any out or roundness caused during the frame fabrication process.

The facing ensures that the end faces are square to the thread, parallel to each other, and further, ensures that when seated against the BB shell, the cups are sitting with 360° shoulder contact, no high points, lumps, bumps, which could skew the cup relative to a sloppy thread.

Er ........ that's it.


PS when BB shells are threaded I suspect that they use cut pieces of tube, loaded into a lathe, faced, bored and screwcut. The part is then reversed in the chuck and the process is repeated for the other end (different thread direction, I know). If the part does not run true in the lathe chuck then the two ends will be eccentric and this will cause problems.

It is possible that both threads are cut from one end, in one operation, but I doubt that that is how it is done. With cast steel BB shells, because of the nature of the external shape, there is great potential for errors if not done carefully.

I tend to agree with you about the inside of the bearing resting completely flat against
the BB shell face. It's the idea situation, but as you say it doesn't necessarily mean everything is in line. All we can hope for is that the factory did a good job threading and I would guess that the BB area gets a pretty good quality control look see before it's shipped to shops.