There seems to be a significant difference in head tube length between similarly sized triathlon bikes. For example, a 55 cm Guru TriLite (seat tube angle = 76 deg, top tube length = 55 cm) has a head tube length of 13.6 cm, while a 56 cm Felt (seat tube angle = 76 deg, top tube length = 54 cm) has a head tube length of 11 cm. Although one could argue that the 2.6 cm difference in head tube length is inconsequential, how often have you seen people riding around on their tri bikes with 2 to 3 inches of spacers below their stem?
A couple questions:
Assuming you know your preferred seat tube angle and top tube length, how do you determine an appropriate head tube length? It seems that flexibility (or the potential to gain flexibility) should have a lot to do with that, but is it possible to predict during a 1-2 hour fitting session how your lower back is going to feel about your position after 3-4 hours on the bike? Is the appropriate sized head tube determined solely through trial and error?
Why is it that a significant number of people riding tri bikes seem to have the wrong head tube length? Is it the end consumer or bike fitters (or both) who don’t understand the importance of head tube length?
The problem with the threadless headsets is that you are limited a bit more than the quill stems in your range of adjustibility. Most stems come in 0 degrees or 6 degree plus or minus now. So you have to play with your washers prior to cutting the fork or risk a very expensive cut. As I age my front setup seems to get about a cm higher every few years.
My different aerobar combinations have 3-4 cm elevation difference in the bars and pads.
People that ride in really hilly/mountain areas some times ride with a higher front end to facilitate climbing.
Yea I do think that people with a half dozen honking spacers probably aren’t an ideal fit on that particular frame. That is why buying a bike from a pro fitter can be a good thing if you don’t know what length to buy.
Head tube length is determined by the frame designer…most will try for about 13cm’s for good bearing load/life…anything smaller tends to wear out headsets to fast…If you look at your Guru frame in different sizes you would probably find the headtube length is relatively the same…
The amount of spacers under the stem is more to do with rider position preference…not frame design
Now if you were to order a custom frame the fitter would set you up with the ideal headtube length for your body’s riding position but that may give you a frame that looks odd…let’s say you need a 16cm headtube and your top tube is 54cm and the seat tube is 54cm…not to visually pleasing …like it or not visual appeal some times rules…
It is a gray area. A lot of it has to do with how tall you are and how long your legs and torso are compared to each other. I am 6’ 1/2" and rode a 58cm P3 w/ a 140mm head tube (integrated). It was too short for me. I have long legs and the corresponding high seat post. That head tube was just way down there. Now, I was also riding with Oval A700 bars, which don’t have a huge amount of rise on them (I used the 10mm spacers). If I was using Syntace C2’s or similar, I would have probably been better off. I do think that you can determine a good ball park figure for head tube length in a 1-2 hour fitting. It is best if you can have your preferred aero bars with you (pad height is huge). You also need to take into account whether the headset is integrated or now. My P3 had a 140 integrated, which is actually 145 when you include the bearing top cap. My new bike has a 131mm non-integrated. Yes, it seems shorter at first, but then add in 30mm of stack for a Chris King headset- all of a sudden you’re at 161. Add to that the fact that the bottom bracket on the new bike is 5mm lower than my P3- this makes the head tube a “virtual” 5mm longer because my saddle will be that much lower. All in all, I wanted to use fewer spacers but still get low when I want. I had a custom bike built with the help of a professional fitter. My ‘long course’ setup is a -6 stem and 2 cm of spacers. My TT/short course setup can be up to 4 cm lower by way of a -17 stem and no spacers. I can also bring the arm pads closer together on my Vision Tech TT clip-ons.
The best way to determine HT length may be after you’ve already had one tri bike for a while and are ready for a new one. When you upgrade/trade, the make the new bike a step in the right direction with the head tube length (but don’t forget to take into account the bottom bracket drop and headset stack). If you are happy with the HT length, then go with the same on the new bike.
There is no formula for this and different bike makers seem to have approached this differently. Slowman had a good article about this in the tech section. Boils down to your own flexibility and whether or not you’re racing sprints or IM, assuming that unless you’re a Bjorn, the average AG’er would ride higher in an IM than a sprint for comfort reasons.
You don’t necessarily have to use a lot of spacers. Get a stem that is angled up/down for your requirements. Also aero bar/pad combinations have a good deal of variation in height.
I’d say 3 cm is about the limit. You can also upturn the stem. Which would you rather- an upturned stem or spacers? If you’ve got both, I think you’re on the wrong bike. I started with 3 cm on my road bike (Trek Madone, which has a short head tube for a 60cm bike- it’s only 160mm). I am slowly lowering it as my flexibility/preferences change. I’m at 2.5 cm of spacers right now and will soon try 2 cm.