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geometry help

Dr MDr M Posts: 171
edited August 2009 in Road beginners
Hi can someone tell me what the measurement A1 is on the bianchi chart for the C2C 928 carbon?

AS is the measurement to the top of the seat tube... which looking at the dotted line seems to be what the centreline of the seattube would be if it was horizontal?

So what is A1?

I'm trying to work out how some cross bike frames compare to my road bike.

Also, is there anyway to work out standover height from frame geometries? Why dont manufacturers give this, as i'm struggling to work out what size bikes i should be looking at. I know the TT length + stem length on my bianchi is correct as i had a bike fitting for this, and i can measure its standover height, but not sure how to work out what size i need in other frames... and unfortunately the 2 i am interested in are mail order - Planet x uncle john and a Focus Mares from wiggle - so i cant really try them for size until i order one :-(


  • neebneeb Posts: 4,386
    Hmm, not sure what A1 is, although it seems to be consistently 30cm more than AS, which as you say is the seat tube length (center of bottom bracket to top of seat tube). It might represent where the top of the seat tube would be if the top tube was horizontal?? In any case I can't see why it would be particularly important, unless perhaps you are wanting to transfer a short seatpost from a bike with a horizontal TT.

    Standover height is unlikely to be an issue in a road bike with a sloping top tube if the frame is anything near the correct size, although I guess on a cross bike it will be more important. The most important measurements are the effective TT length (B1) and the headtube length - the latter is very important as you don't want to find that you can't get the bars low enough or that you need lots of spacers to get them high enough.

    To work out standover height you would need to take into account the height of the bottom bracket (determined by D, i.e. bottom bracket drop, in the diagram) as well as the tube lengths and angles (using standard school geometry; yes, there is a use for it after all! :wink: ). The BB drop might differ significantly between a road bike and a cross bike. Also, fatter tyres on a cross bike will further increase bottom bracket height and thus standover height.

    Don't forget that seattube angle will change the reach to the bars for the same effective TT length (steeper seat tube = less reach for the same effective TT length).

    If you trawl the web you might be able to find a program that will work out standover height from frame geometry - it's the sort of thing that someone is bound to have written!
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