From square taper to hollowtech II BB. Advantages?

[byegad]

Square taper BB on QNT 7000+ miles no play. HTII BB on Kettwiesel, 4500 miles before they were worn out. MBth ride in a dry environment, a recumbent with fairing and out of any wheel spray. However the HTech is easy to replace, with the right tools and cheap enough to simply treat as a wearing part.

[Mick F]

...........as MickF says if the chain is catching on the f/mech cage under load it must be the BB and not the frame thats flexing. ......................... I think its got to be remembered that for most people durablity over a slight,almost undetectable flexing wins out every time,especially for tourists.

Agree.

This flexing I've noticed from the very outset of going to multiple chainwheels. I've had it on doubles, and now on triples with a variety of BBs over the years. It isn't just Campag!

If anyone thinks they don't suffer from this, they ain't pedalling hard enough. Try getting on the big ring in a highish gear and honking hard up a hill.

[PW]

I think the so-called flex is in the chainset or the interface between the crank and the axle, it certainly ain't in the axle itself. I can deflect a B/B shell by pressing on one pedal - more so with 531ST than 953 which is nearly solid by the way - but there's no way I can do it to a chromo axle even with a 3ft extension tube and a vice.
Edit - and XX,000 miles from a UN73 outweighs the "stiffness" argument by a country mile.

[BigG]

I think the so-called flex is in the chainset or the interface between the crank and the axle, it certainly ain't in the axle itself. I can deflect a B/B shell by pressing on one pedal - more so with 531ST than 953 which is nearly solid by the way - but there's no way I can do it to a chromo axle even with a 3ft extension tube and a vice.
Edit - and XX,000 miles from a UN73 outweighs the "stiffness" argument by a country mile.

Agreed I think hubgearfreak must have a quite exceptional supplier of steel rules!

[Mick F]

So if it's the chainset flexing, why doesn't the phenomenon occur with external bearing chainset?

Also, the BB shell cannot flex by itself, it must take much of the frame tubes with it.

[CREPELLO]

Mmm. An interesting concept, but can I ask, what exactly is "bearing flex"? And why would it appear that external BB's are less prone to it than square taper BB's?

I wouldn't think a solid s/taper axle would flex(I could be wrong) so I conclude that if theres any flex/movement in a S/taper BB it would be in the bearings as they are inboard of the BB shell ie; closer together on the axle than external bearings

I can see that might be a factor.

If the s/t axle is responsible for this flexing, then presumably titanium axles would be a bad idea, it being a more springy metal. Yet I don't recall my Mercian (with Stronglight ti s/t bb) having more chain rub than any other bike I ride.

[PW]

If it's the interface between crank and axle then the splined joint presumably has a tiny fraction less give than a taper, and possibly the new chainset designs are better than the old ones in that respect.
B/B deflection, lean over the saddle Mick, then press the opposite pedal down HARD with your hand from about the 2 o'clock position. You'll see the bottom of the seat tube and down tube move sideways. I know Mercian are good but they can only work with the materials they have, and Bike is 531C from memory? My 531ST tourer with its heavier gauge tubes will flex so I'm certain Bike will. The 953 barely moves at all but that's a much more modern tube technology.

[CREPELLO]

I think the so-called flex is in the chainset or the interface between the crank and the axle, it certainly ain't in the axle itself. I can deflect a B/B shell by pressing on one pedal - more so with 531ST than 953 which is nearly solid by the way - but there's no way I can do it to a chromo axle even with a 3ft extension tube and a vice.
Edit - and XX,000 miles from a UN73 outweighs the "stiffness" argument by a country mile.

PW, out of interest, does your stiff 953 frame exhibit fewer signs of flexing when pushing hard through the big ring (eg, any chain rub in the big ring), compared to other steel bikes you've ridden (and I'm guessing from what you've said that the 953 equipped with a ST BB).

[The Mechanic]

In the haste to compare the relative stiffness of bottom brackets you have mostly ignored the fact the manufacturers have, for a number of years now, stressed the increasing stiffness of their new chainsets compared to their previous offerings. The significant changes in design of bottom bracket have coincided with similar changes in chainset design, certainly with Shimano. Is it then the chainset that is stiffer or the bottom bracket, or a combination of both? I suggest the last option.

[irc]

In the haste to compare the relative stiffness of bottom brackets you have mostly ignored the fact the manufacturers have, for a number of years now, stressed the increasing stiffness of their new chainsets compared to their previous offerings. The significant changes in design of bottom bracket have coincided with similar changes in chainset design, certainly with Shimano. Is it then the chainset that is stiffer or the bottom bracket, or a combination of both? I suggest the last option.

Sounds reasonable to me.

As a matter of interest has anyone here switched from a sq taper to a Hollowtech BB and not noticed clearly increased stiffness under load?

[Mick F]

...... B/B deflection, lean over the saddle Mick, then press the opposite pedal down HARD with your hand from about the 2 o'clock position. You'll see the bottom of the seat tube and down tube move sideways. I know Mercian are good but they can only work with the materials they have ........

I know about flexing, even with any 531c frame.

You know - and many other people know - that I haven't had my Mercian forever. I've had other bikes before. They all exhibited the same phenomenon.

If the BB and the seat tube and down tube move - and I agree that they do - the front mech would also move, and as it's only a few inches up the seat tube from the BB, the flex difference would be minimal - not far enough to give chain rub.

So what is moving with respect to the front mech and the chain ring? It cannot be the BB shell alone, it must be the chainwheel or the BB axle.

Therefore, if the newer chainsets are the same as the older chainsets, the difference must be the BB axle and outside bearings.

So what is it that makes people report the extra stiffness?

Better designed cranks and chainsets?

[BigG]

So if it's the chainset flexing, why doesn't the phenomenon occur with external bearing chainset?

Also, the BB shell cannot flex by itself, it must take much of the frame tubes with it.

It is effectively only the tubes that flex. The seat tube adopts a slight curve and even though the front mech is mounted close to the bb, the chainring alignment with the front mech also changes. If this is right, there should be no problem with bb mounted front mechs. (Any experience, anyone?) Although the cranks and chainrings do flex, very little of this flex is sideways unless the chain alignment is way out. Even large rear sprockets flex in these conditions which is why Shimano include bigger effective diameter spacers with their larger sprockets when they are not carried on a sub-frame.

EDIT The lateral torque on the bottom bracket should be the same regardless of axle length and bearing position. It is only the force applied mutplied by the horizontal offset of the pressure centre on the pedal from the bike centre line. If I am right, then the amount of flex should therefore only depend in practice on the stiffness of the tubing, the stiffness of the tube/bb joint and the stiffness of the bb itself at this point. Try standing on one pedal with the bike stationary and leaning against a wall. The bottom bracket movement is very noticeable and only partly due to tyre flex.

[mattsccm]

Big buggers might notice some flex. I don't even on my overgeared SS cross bike.
I do find that those external BB's don't last very long in crappy conditions. If you buy the right one, not Shimano, they do allow you to replace the bearings without buying new cups. God diea that. Shimano do as well, its just that they say they don't. They may suit you id you buy one C/S and stick with it. I wouldn't as I am swapping all the time.

[meic]

There is little point in replacing the bearings without the cups as the results of all my searching shows the cheapest way to get hold of these bearings is to buy some Shimano bearing cups and break them open.

[Sailing7]

+ 1 for Hope s/s BB

mine's got 20,000 commuting miles and is still as smooth as when it was new
Getting back to the OP Hollowtech cranksets and external BBs are lighter than square taper

That's an advantage