Threadless headset: how high can you go?

A good analysis. Summary: torque (f x d) is the key independent variable that will determine
probability of breakage.

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Andre Charlebois BPE, MCSE4.0, CNA, A+ webmaster for Triathlon New Brunswick www.TriNB.com "Jason
Cortell" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Hi,

I know this topic has come up before (I clipped the below discussion from a thread a few years
ago), but I haven't seen any clear explanation of the reasoning behind the limits often cited. For
example, limits on headset spacers for a 1" carbon steerer I've seen range from about 15 to 40 mm,
with a bit more allowed for 1-1/8" steerers.

Can anybody give me an idea of the engineering (if any) that went into those numbers? I have a new
Look carbon fork which I installed without cutting it, using about 70 mm of spacers. It turns out
that this gives me about the right height, using my 17 degree stem in the horizontal position, and
I kind of like the way it looks, more like a traditional road bike than a threadless setup. It is
certainly not very stiff compared to my previous threaded steel steerer, but I haven't found any
rideability problems related to that (yet), and I've tried gravel, grass, road shoulders, steep
hills, and a few hard corners. But it's only been a hundred miles or so (a very nice hundred
miles, though).

My informal engineering analysis goes as follows:

Three things to worry about - breakage, stiffness, and reliability (other than breakage).

Starting with breakage - if it's going to break at the steerer because of excessive length, the
most logical point is just above the top headset race (bottom of the spacer stack), since this is
where the most bending stress will be. Yet the bending stress at that point is dependent not on
the length of the steerer per se, but on the position of the handlebars. In other words, the
stresses will be the same if the handlebar position is the same, whether the steerer goes up
vertically a long way and then is attached to a horizonal stem, or if the steerer is just a stub
and the stem is almost vertical.

So, for breakage, it seems that the critical issue is not the spacer height, but how high (and
extended) the handlebars are (specifically, the hand position). Yet the warning is for spacer
height, not overall handlebar position. If I flip the stem over so it sticks up, I can get the
same bar position with less than half the spacers, but I don't see why it would be less likely to
break (and I lose some of my height adjustment options).

As for stiffness, I think a longer steerer will give some moderate loss of stiffness, both in
bending and torsion. Still, there's a lot of fork besides the top couple inches to flex, so I
don't think the change would be dramatic.

With more spacer and more steerer, I expect the reliability would be slightly reduced also, if for
no other reason that there are more things that could break.

So, if you see something I missed, or why there's a spacer limit, let me know. One aside - unlike
my Look fork, the Profile carbon fork has an aluminum reinforcing insert that you glue in after
you cut it. So it makes sense that they would limit the steerer height to the portion reinforced
by the insert. Most forks don't do that, they have a little expansion plug to allow the headset to
be tightened.

--Jason

"ReidRik_Von" <[email hidden]> wrote in message news:OsO9O39u$GA.306@cpmsnbbsa04...

Quoted message said:

Quoted message said:

Q1 How high can you safely build up the height on a threadless

setup?

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My late spring 2000 CO Cyclist catalog as a picture of a Tommasini frame on page 9 with 6
spacers below the Deda stem. This looks to be add close to 4-6 cm in height. Is this a safe
setup?

Quoted message said:

I have a Cannondale tandem with threadless headset. I have about 2"

of

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spacers (using a Zoom Headsup, which is a complicated spacer system)

and

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then use a high rise (90cm, 130 degree) stem to get the bars where I

want

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them. I have had no reliability problems with this setup. The Zoom

Headsup

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has been on the market for quite a while and haven't heard of

failures.

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alex

most shops use the following rule of thumb:

25mm max for 1" steerer 35mm max for 1 1/8" steerer

deviate from that at your own risk because it may affect warranty.

my personal view is that if you have a shorter stem, you can use more spacers, longer, less so. it's
the total /leverage/ that's important, and that's simple trig for the distance from the center at
the top of the bearing and the center of the handlebar [assuming straight bars].

jb

jim beam said:

most shops use the following rule of thumb:

25mm max for 1" steerer 35mm max for 1 1/8" steerer

deviate from that at your own risk because it may affect warranty.

That, like most "rules of thumb" is a gross oversimplification.

In the case of forks with steel steerers, especially 1 1/8 ones, there's no need to cut the steerer
at all, if you want it that high.

For aluminum or plastic steerers, strength is an issue, and those "rules of thumb" might have some
merit with such steerers.

Sheldon "Materials Matter" Brown +------------------------------------------------+
| You can get more with a kind word and a gun | than you can with a kind word alone. | -- Al
| Capone |
+------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
617-244-9772 FAX 617-244-1041 harriscyclery.comharriscyclery.comOpen ↗ Hard-to-find parts shipped Worldwide
captainbike.comcaptainbike.comOpen ↗ sheldonbrown.comsheldonbrown.comOpen ↗

Quoted message said:

In the case of forks with steel steerers, especially 1 1/8 ones, there's no need to cut the
steerer at all, if you want it that high.

no argument from me on that one - i ran a steel-steerered psylo with 70mm of spacers on it for over
2 years without any mechanical problems.

got endless grief from my friends though - it looked /way/ ugly.

jb