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Help regarding TT Bike sizes/geometry and difference between bikes
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Hey there STers,

this post may be rather long, apologies.
Im cheap. Im not the kind of cyclist to spend big money on bikes and Ive spent many years riding the same aluminium TT bike (Canyon Speedmax AL 2014).
Plus, I always tend to spend money on things like UFO chain and such, instead of frames. What I really like about this frame (apart from the fact that its aluminium which gives me more bragging rights beating friends) is that its long/low geometry which suits the bike fit that Ive eventually gotten used to ride. I am REALLY stretched out on the bike.
This is me on aboard the bike during a 112 mi ride (in 04:24).


Now, the things is that me and my missus are having a baby in a few months time and I realized that training for triathlons is not possible in the couple years ahead. I'd like to focus more on a pure cycling and time trial events.
This made me realize that my bike fit is nowhere near UCI regulations. I need to move my extensions back 11 centimeter (4 1/3 in) to comply! Making these changes Im toying with the idea of finally upgrading frameset to something with more "bling" and more up to date (less cables in the wind, possibly brakes hidden etc).
Not that I need to change the bike, but since I will be doing a whole lot of wrenching and recabling, why not use the opportunity and switch frames?
I know most frames today are not long/low, but I do have 4 cm of risers under my extensions with the current frame and since I will be moving extensions backwards Im thinking that using a more of a "short/high" kind of TT frame geometry would be just fine. And this is something that I need to validate by someone knowledgable. That I am not:)

Where I live theres not many offers to choose from (remember, Im cheap:)). Guy nearby in Austria is selling new Isaac Photon frameset in size 55 and I think it would fit me well. Whats troubling my mind is that size 55 is the largest frameset Isaac is producing. But my Canyon Speedmax is size M and they are making those up to XL!
But looking at the frame geometry charts I think it is okay for me. Could someone here possibly validate this for me?

THIS is a geometry chart for my current frame (Im riding size M)
THIS is a Isaac Photon geometry chart (looking at size 55)

If I understand it correctly, then despite size 55 being the largest Isaac frame, it is still a few mm shorter than my Canyon in size M? The only difference would be that Isaac is 3 cm higher? If right now I swapped the frames and only difference I made was replacing 4 cm of riser for 1 cm I would be almost entirely where I am right now (just a few mills less stretched)?

This I need to validate. I do now know how much to trust these geometry tables and if it really works like that regarding stack and reach. I would think that it does but just the fact that size 55 is the largest they make and Im not that tall (5'11" actually) makes me doubt my way of thinking about this.

Even if I wont buy this particular frame, it would still help me deciding about other ones as well. What to look for in those sizing tables:)

Looking at used bikes I only found Trek Speed Concept Gen 1 in size M nearby and even though I love trek, gen 1 version is pretty old and Id like to be able to buy spare parts in the future.

Thank you very much for advice!
Last edited by: Rachela: Feb 1, 21 3:22
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Re: Help regarding TT Bike sizes/geometry and difference between bikes [Rachela] [ In reply to ]
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First question is are the TT events you plan to race UCI governed? Appreciate this is likely a yes if you are at the pointy end and depends on where you live. Before committing to a new frame might be best to see if you can get UCI legal on your current set up and adapt? The hip angle will be closed off but you don't need to run afterwards!

Congrats on the baby. When we had our first I also ditched Tri for two years and just cycled. Whatever you do though don't stop running. Even if it's just a couple of short easy runs a week. I learnt the hard way and had over 12 months without any running at all, and wow did I regret it when I started it up again. It's the hardest of the sports to get back after a long break.

Giant trinity are very popular here in the UK on the TT scene, and would tick all your boxes of integration and cable management.

Good luck!
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