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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 |
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