Threadless stack height limits

[AndrewS]

Has anyone ever heard the precise reason that threadless steerer tubes have a fairly low maximum stack height?

I just wonder if it is because the spacers are essentially passive and can move. If some or all of the spacers also clamped to the steerer, would that allow more steerer to stick out of the frame.

It could just be a leverage problem, but I'd be surprised if a steel or alloy steerer would be that fragile. As long as you had something clamped near the headset to stabilize the upper bearings, should stack height be an issue? And if it is a strength issue, can a clamp on sleeve/spacer reinforce a steerer sufficiently to allow taller stacks?

[Pete Serotta]

From what i understand it is because of what the steerer tube is made of and how it is joined. with steel I do not think, within reason, there is a structural restriction but for carbon and aluminum there is.

Lastly the fashion police have a say :) :D :no:

[Marcusaurelius]

Hmm, I second that a tall head tube looks so much better than a lot of spacers within reason of course. :)

[93legendti]

The max stack height on the 1" steel steerer tube of my 650c Wound Up fork is 44mm. It looks a little funky with that many spacers.

[hookookadoo]

Lastly the fashion police have a say :) :D :no:

Agree. Keep the fashion police happy. Also the more steerer tube sticking out above the top of the headtube/ headset the less support it is receiving from the headtube/headset and thus will have more flex and generally be weaker all else being equal.

[RPS]

Has anyone ever heard the precise reason that threadless steerer tubes have a fairly low maximum stack height?I haven't. Normally it is assumed it has to do with higher stresses, but that doesn't make much sense to me because if it was it would be adjusted to rider weight and strength.

I just wonder if it is because the spacers are essentially passive and can move. If some or all of the spacers also clamped to the steerer, would that allow more steerer to stick out of the frame.Spacers are in compression so not really passive and shouldn't be moving very much unless the head set needs to be adjusted. In my opinion clamping wouldn't do much because it wouldn't change stresses.

It could just be a leverage problem, but I'd be surprised if a steel or alloy steerer would be that fragile. As long as you had something clamped near the headset to stabilize the upper bearings, should stack height be an issue? And if it is a strength issue, can a clamp on sleeve/spacer reinforce a steerer sufficiently to allow taller stacks? Again, I haven't seen anything in writing, but will assume it is to keep stresses low enough to keep the steerer extension from breaking off. The problem I see is that placing a tall stem extension or using a stem with a very high angle turned up also increases steerer bending stresses by a similar amount; and riders keep using those anyway. If the goal is to limit height, they would also limit extensions or too-tall stems. And even more so for large riders.

The only other reason I can see for a limitation would be additional bending of the steerer above the head tube and bearing which would place an eccentric load on the bearing. However, I don't see this being much of an issue unless the steerer was flexible, the rider large, and/or the stem very long. Some of that bending happens anyway inside the head tube.

[AndrewS]

Thank you for your comments.

RPS, your post got me thinking about the mechanics of the situation, and I think it must have to do more with the length of unsupported steerer tube exposed rather than the total leverage (as in the case of a high angle stem).

The reason I say that is that the important lever arm would be measured from the top cup to the drops (where the rider would put the most torque on the whole assembly). Even with 5 inches of additional stack height, that lever arm has only gotten gone from about 12 to 13 inches - not very significant at 8 percent when you compare it to the range of forces that act on it.

So I'm back to believing that it has more to do with the length of unsupported steerer rising above the headset. The question is then - is there a way to support it better than just a tensioned stack of spacers?

So is it possible reinforce the exposed steerer better than loose spacers? I can only speculate that clamping something directly to the steerer might cause the two pieces to bear stresses more like one.

A modest proposal:
Make a threadless stem that has a steerer clamp area that is 4 inches long, rather than standard 1.5" or so. The additional 2.5" of clamp area would be on the bottom, raising the stem. The fork steerer tube would run all the way to the top of the stem clamp, and be supported by either 3 or 4 evenly spaced allen bolts. Since it is one piece, this extended stem at least has the possibility of not offending the Fashionistas while offering options to someone who's back is aging but wants to keep riding that beautiful Ti Serotta he already paid for when he was more flexible.

[Ken Robb]

my LBS told me to bring in a length of aluminum tubing with an OD the same as the standard spacers for my 1" steerer. The owner/tech reamed the interior to that of a spacer and cut the tube to be one long spacer with much less likelihood of shifting than a stack of shorter spacers. It worked very well allowing me to get the bars high enough without adding noticeable flexing when I rode the bike. This was on an Ouzo Comp with aluminum steerer.

[AndrewS]

That's interesting. Did he cut it on a lathe?

[Dave]

A high rise stem does not increase the bending moment on the steering tube. It's better to use a high rise stem than a huge stack of spacer, as far as strength and rigidity are concerned.

IMO, if you need more than the 30-40mm of spacer generally recommended for a carbon steering tube, then you bought the wrong frame or you've chosen the wrong stem angle.

If additional support is really desired, the simple answer is to do something like Alpha-Q and epoxy bond an internal support tube that extends below the top headset bearing and all the way to the top of the steerer. One of Alpha-Q's mistakes with their design was inadequate instructions that did not make it clear that the support tube should extend below the top headset bearing. That, plus supplying tubes that were too short and not including enough epoxy to totally fill the gap between the steerer and support tube.

Easton takes a different approach, using a high strength epoxy foam in the steering tube that has threads formed into it, to reinforce the steerer and make use of a threaded plug support plug. They allow 50mm of spacer, which should be enough for anybody.

http://www.eastonbike.com/PRODUCTS/FORKS/fork__top.html

[Ken Robb]

That's interesting. Did he cut it on a lathe?

No--a hacksaw and the jig he uses to cut steerers.

This was a 1" steerer so the allowable stack height was less than on a 1 1/8. At the same time I swapped my Ouzo Pro carbon steerer for a Ouzo Comp w/alloy steerer.

[RPS]

RPS, your post got me thinking about the mechanics of the situation, and I think it must have to do more with the length of unsupported steerer tube exposed rather than the total leverage (as in the case of a high angle stem).
Andrew, I don’t think part of the steerer being “unsupported” plays an important role as it relates to lowering stresses. The wall thicknesses of steerer tubes relative to the diameter makes them quite sturdy in that regard. From that perspective I wouldn’t hesitate taking a 1-1/8-inch steel steerer and letting it stick up out of the head tube by 10 CM or more. I’m not strong enough or heavy enough to damage it. And even if I was, the bars or stem would likely break first.

If being unsupported was the main limitation, imagine the problems that similar seat posts would have. Their dimensions are very similar and the unsupported length is far greater. And saddle loads can be high for many riders compared to that applied at bars.

Thinking about this thread reminded me that I attached my wife’s stoker stem on our Co-Motion tandem at the top of my saddle’s carbon seat post which isn’t much stronger (if at all) than a steerer. And it not only has to support her bars but my weight as well. In this case the post has over 10 CM of unsupported tube holding up my saddle and her bars. And the same can be said for just about any tandem whether using carbon or aluminum seat posts. In most cases the stoker’s stem is attached fairly up on the seat post.


BTW: The important thing is to follow the manufacturers’ recommendations for their specific products. Just because I don’t understand why they set their limits the way they do or because I may not follow them strictly doesn’t mean I’m suggesting that the rest of you shouldn’t follow them. :beer:

[totally_fixxate]

"With a reinforced 450mm steerer tube and unlimited spacer stack-height,
we've got you covered no matter what your set-up requires".

http://www.truetemper.com/Performance_Tubing/zpro.asp

[AndrewS]

Looks like Alpha has the best answer of all. Thanks, fixxate.

RPS, did you ever wonder what the manufacturer of your carbon post would say about using it for a tandem captain's position when you assembled it? Or was it sold with that in mind? Just curious, but as you point out, that's a big stress riser in the middle of a seat post that isn't even supposed to be lightly clamped in a repair stand.


Ken, I'm surprised he didn't clean up the aluminum riser with a head tube facing tool. I can't see how a hacksaw or file could square the tube enough to put even pressure on the upper headset race. That would be similar to facing the frame by eye.

Maybe if his jig was faced and hardened so the tube could be filed flush?

[Ken Robb]

Looks like Alpha has the best answer of all. Thanks, fixxate.

RPS, did you ever wonder what the manufacturer of your carbon post would say about using it for a tandem captain's position when you assembled it? Or was it sold with that in mind? Just curious, but as you point out, that's a big stress riser in the middle of a seat post that isn't even supposed to be lightly clamped in a repair stand.


Ken, I'm surprised he didn't clean up the aluminum riser with a head tube facing tool. I can't see how a hacksaw or file could square the tube enough to put even pressure on the upper headset race. That would be similar to facing the frame by eye.

Maybe if his jig was faced and hardened so the tube could be filed flush?

:crap: I didn't say he didn't clean it up. You asked if he used a lathe to cut it. The finished product was very well finished and square.

[AndrewS]

Nothing to get upset about. I asked if he used a lathe because that's another way of getting a tube perfectly square. I wasn't being critical - I was asking you how he got it that way. :confused:

That's also why I asked if he used a hardened jig at the bottom of that post.

[RPS]

RPS, did you ever wonder what the manufacturer of your carbon post would say about using it for a tandem captain's position when you assembled it? Or was it sold with that in mind? Just curious, but as you point out, that's a big stress riser in the middle of a seat post that isn't even supposed to be lightly clamped in a repair stand.
Briefly, but what's the point of asking if you already know the answer they will give?

By the way, as an engineer I sometimes read more into technical information than I probably should. In the case of my tandem, the post was rated for 200 pound riders, and at the time my weight was about 145. My wife at about 100 pounds probably rested less than 40 pounds on her bars. I also knew that when we got out of the saddle to crank "hard" :rolleyes: I wouldn't be on the saddle, plus she's not strong enough to stress a 27.2 MM carbon seat post.

Regarding your comment about not clamping a seat post in a repair stand; in my opinion this may have more to do with their concern about the owner damaging the seat post by applying external forces to the bike after clamping than by the "clamping" itself. If an owner clamps a bike by the seat post and then tries to remove a stuck BBKT or something requiring a lot of force he could very easily damage the post.

From the perspective of clamping a stem on the seat post I don't see that being much different than clamping a stem on a steerer.

[AndrewS]

I thought the work stand thing had more to do with directional rather than uniform pressure. The same reason eccentric or wedge threadless stems aren't acceptable on carbon steerers. I don't think there's much that could happen in a workstand that would be more than the forces the bike withstands while riding.

I guess I was just wondering if it was a Santana or Burley, etc. spec'd part.

It makes you wonder if the first threadless setup was a tall fork with the threads cut off, a headset with the threads filed down and a 26.8 (25.4?) stoker stem.

[RPS]

I don't think there's much that could happen in a workstand that would be more than the forces the bike withstands while riding.

I disagree. Your leverage applying forces can be far greater.