Threadless headset: how high can you go?

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" wrote in message
news:OsO9O39u$GA.306@cpmsnbbsa04...
Q1 How high can you safely build up the height on a threadless
setup?
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?

I have a Cannondale tandem with threadless headset. I have about 2"
of
spacers (using a Zoom Headsup, which is a complicated spacer system)
and
then use a high rise (90cm, 130 degree) stem to get the bars where I
want
them. I have had no reliability problems with this setup. The Zoom
Headsup
has been on the market for quite a while and haven't heard of
failures.

alex