Stupid frame size question

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frankspencer1979
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Stupid frame size question

Postby frankspencer1979 » Tue Dec 18, 2012 11:56 am

My current bike is a 20" frame with a 600mm top tube. If I can get an extra 2" out my seat tube (easy done) there's no reason why an 18" frame with 613mm top tube wouldn't fit me, is there? :? :?:

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cooldad
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Re: Stupid frame size question

Postby cooldad » Tue Dec 18, 2012 12:02 pm

Depends. Different manufacturers measure in different ways. Even different frames within the same manufacturer can be measured differently.
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The Rookie
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Re: Stupid frame size question

Postby The Rookie » Tue Dec 18, 2012 12:19 pm

In terms of the frame size, you need the effective top tube length (horizontal from top of headtube back to seat tube/post) different fixing points at each end can mean the same 'top tube' length can give very different length bikes.

frankspencer1979
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Re: Stupid frame size question

Postby frankspencer1979 » Tue Dec 18, 2012 12:59 pm

Things are never simple are they! I'm not too concerned with the top tube length really, I'm tall so I'm used to quite an upright position already. I just wonder about me leg length and the pedal stroke. It seems logical to me that pulling the seat tube out 2" more would put me in the same position on the smaller frame?

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The Rookie
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Re: Stupid frame size question

Postby The Rookie » Tue Dec 18, 2012 14:52 pm

It would, yes.....which is why leg length is the least importnat measurement when it comes to sizing a frame, you have massive adjustment from the seatpost, far more than you can affect cockpit length by.

querhoch
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Re: Stupid frame size question

Postby querhoch » Tue Dec 18, 2012 15:00 pm

it might have the effect of sitting you at the same hieght, but if you have a shorter seat tube then you might have a shorter top tube and that will change your riding position. if youre a tall rider then you need the correct length top tube or you'll end up with back problems.

frankspencer1979
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Re: Stupid frame size question

Postby frankspencer1979 » Tue Dec 18, 2012 15:33 pm

Well depending on how the manufacturers measure, i think the top tube will actually be longer. It's the difference between a ragley and a vitus, which were designed by the same person so should use the same measurements.

querhoch
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Re: Stupid frame size question

Postby querhoch » Tue Dec 18, 2012 16:14 pm

why should they use the same measurements simply because they were designed by the same person? theyre different bikes and unless they have exactly the same geometry then top tube lengths will be different. it might be longer or it might be shorter, but changing the length of your seatpost wont automatically give you the same riding position.

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cooldad
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Re: Stupid frame size question

Postby cooldad » Tue Dec 18, 2012 17:05 pm

The seat tube is angled, so how far the saddle will move backwards depends on the angle.
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frankspencer1979
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Re: Stupid frame size question

Postby frankspencer1979 » Tue Dec 18, 2012 18:39 pm

The top tube measurements are both 'effective' so should be comparable. My primary concern is that the drop from 20" to 18" will be too cramped for my lanky legs, but as there's 4" of seat post between my current ride position to the minimum insert point, I think I should be ok.

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passout
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Re: Stupid frame size question

Postby passout » Wed Dec 19, 2012 15:04 pm

It is complicated but remember that the most important measurement is 'effective' top tube length. Also try for a test ride if possible.
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frankspencer1979
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Re: Stupid frame size question

Postby frankspencer1979 » Wed Dec 19, 2012 15:26 pm

Test ride not really an option due to the nature of the deal i'm looking at, it's a piglet frame for £190. I could wait for next years models, but I'm not fussed about new colours, and can survive without ISCG tabs. It seems to me the top tube on the piglet will be longer than my current one, and by poking more seat post out combined with the angle of the seat tube, that will create even more length allowing for quite a bit of adjustment. I'm already used to being quite uppright so this may potentially be more comfortable.


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