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Dan,
You let the cat out of the bag. I have been designing Tri bikes with a 71.5 degree head tube angle and the Ouzo Pro 5.0 rake for the past three years. The 6.1cm trail, created by those parameters make for an extremely stable ride along with a “softer” front end. The increased front/center dimension also helps eliminate toe overlap problems when you start building shorter frames. I have always wondered why Tri manufacturers use steep headtube angles and short rakes. It is counterintuitive for the desired purpose of the bike.
It only makes sense to have a bike designed to be overly stable when the position you need to maintain while riding it is inherently unstable. Having your elbows resting on the arm pads close to the center line of your body’s mass makes for a narrow footing for lateral support. And as you mentioned, with the additional weight on the bars, it becomes a more twitchy ride. My athletes have had great success with your “experiment”.
Keep up the good work in your research.
Paul Levine
I've posted on here a few times about my conversion to a forward geometry frame after years of riding road geometry. I finally bit the bullet when I realized how much steering I was doing on the road frame at race speed. It seemed that the longer front-center dimension in tri-geometry would restore the handling.
My Caliente handles as I had hoped; however, the front-center dimension was not the only change. QR is building that frame with a 72.5 head tube as opposed to the 73 - 73.5 found on most frames. I have been wondering how much difference that makes in the overall equation - evidently quite a bit.
"You let the cat out of the bag. I have been designing Tri bikes with a 71.5 degree head tube angle and the Ouzo Pro 5.0 rake for the past three years."
the bugaboo with this sort of bike geometry has always been front/center. maybe i related this before on slowtwitch, but a dozen years ago, when kestrel first built its km40, it was a straight copy of our 57cm superform. except they put their own EMS fork on it, which had 3mm more rake than ours used in that size. so the km40's front/center was 65.2cm, which was 2mm illegal. and there it was, printed right in their catalog: "front/center: 65.2cm." pretty funny, really. not that it mattered. how many USAT officials take a tape measure to the races?
and the whole thing was, every one of our bikes above 55cm was 64.9cm of front/center. even back then, 12 and 14 years ago, we knew we wanted long front/centers, but the rules got in the way. and they still do. now, there's the morphology exception, which allows me to build what i want (for me). but that didn't use to be there.
USAT ought to either:
1. just lengthen the front/center to 68 or 70cm;
2. make the morphological exception start at 76cm of seat height;
3. best yet, make the front/center a multiplier, like 85% of the seat height. this would really be the most fair, i think. in this case, my 80cm seat height x .85 = 68cm of front/center. 76cm of seat height yields a max front/center of 64.6cm. and so forth.
Dan Empfield
aka Slowman
Cervelo has a 72 degree head tube on their tri bikes on the 54 cm and smaller models. Goes to 72.5 on anything bigger.
Is Cervelo's 72 degree the shallowest on the current run of production tri bikes?
I guess the problem I see with this geometry is that you are limiting yourself on the selection of forks. There are zillions available in the 40-43 mm rakes, but how many are available with 50 mm rakes?
And is triathlon bike stability really a problem? I can descend on my aerobars at 42 mph with no problems, so why should I switch steering geometries?
Good point Dan. The way that Paul and I deal with this begins in the design phase of the frame. We both use a software template that coverts our fits to points in space and allows us to draft a frame around those points. In making a frame match those points we can try various configurations of stem lengths, head tube angles, fork rakes, etc, and know ahead of time how the fit lines up, but also all pertinent frame variables including front/center. If I've drafted a frame whose front center is just out of spec, I can slightly alter the draft to bring it into spec and keep the position the same by doing something such as changing the stem and top tube lengths (assuming this doesn't lead to a negative change in frame design). It's a round-a-bout process, but allows for the right outcome in most cases.
Christopher Kautz
Christopher Kautz
Director of Technology, Product Development, and Education
GURU Sports, a division of Cannondale Sports Unlimited
Paul -
What BB drop do you use? Pedaling through corners isn't really an issue and it seems to me lower BBs would produce slightly smaller frontal area, but it seems most production bikes are 7.0cm or less BB drop like a crit bike.
I use 8.0 cm. It doesn't reduce your frontal area, it lowers your center of gravity into the frame making the bike more stable.
Having a short bottom bracket drop makes the frame more tippy. That is one of the reasons why I don't recommend 650 wheels for tri bikes. It uses a very short BB drop because of the smaller wheel size, thus putting the athlete's center of mass relatively higher above the axles.