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PostPosted: Sun Dec 06, 2020 9:43 am 
Dirt Disciple

Joined: Sat Oct 31, 2020 7:59 am
Posts: 77
I know exactly my size and preference for fit on my current bikes which are both modern design compact frames. I know where to put my saddle in relation to the BB and the best distance from saddle to bar as well as saddle to bar drop. I'm in the process of building up an 80s frame with horizontal too tube, and threaded forks / headset and am trying myself in knots about fit. The top tube on the classic frame is almost 20mm longer (horizontally) than my modern frames. Couple this with the fact that my modern bars are also compact, I'm panicking that the reach on the classic frame (with classic regular drop bars) is going to be waaaay stretched out for me. From my fag packet calculations it looks like even if I find an 80mm quill stem, I'm still going to have 10-15mm more reach than on my compact frames. So my question is, are non compact frames expected to give a more stretched position or is there some factor of saddle height in relation to top tube that I am not factoring in? I am currently riding a 52cm frame, I have about 150mm of seatpost showing. I've purchased a 54cm classic frame.


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 Post subject: Re:
PostPosted: Sun Dec 06, 2020 9:51 am 
Dirt Disciple

Joined: Tue Apr 28, 2020 9:38 am
Posts: 69
Potentially daft answer, but how about a 60-70mm quill stem then? It will cost you £15 to try out the idea.


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PostPosted: Sun Dec 06, 2020 10:29 am 
Retro Guru
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Joined: Mon Jul 27, 2009 8:21 pm
Posts: 784
and an inline seat post


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PostPosted: Sun Dec 06, 2020 11:04 am 
Dirt Disciple

Joined: Sat Oct 31, 2020 7:59 am
Posts: 77
oaklec wrote:
and an inline seat post
already run an inline seatpost. Damn my weird body proportions. Short torso, long legs. But long legs consist of short thighs and long calves. Basically I'm completely the wrong shape to ride a bike. A shorter stem is an option but I've found that anything shorter than 80mm on my modern bike starts to feel a bit twitchy. Of course I could just be being a tart.


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 Post subject: Re: Re:
PostPosted: Sun Dec 06, 2020 11:05 am 
Dirt Disciple

Joined: Sat Oct 31, 2020 7:59 am
Posts: 77
M_Chavez wrote:
Potentially daft answer, but how about a 60-70mm quill stem then? It will cost you £15 to try out the idea.

Point me to these £15 quill stems pretty please.


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 Post subject: Re:
PostPosted: Sun Dec 06, 2020 12:03 pm 
Retro Guru

Joined: Wed Dec 10, 2008 9:19 am
Posts: 2884
Forget the stem sizing confusion.

Start with the basic geometry of the frame. Your confusion arises in trying to compare the top tube of a horizontal frame with the top tube of a compact frame. They are different concepts, geometry wise.

To try to understand it, if you were to put the 2 types of frame side by side, one has a tube that slopes down towards the back. Now, if you were to get the horizontal frame and measure from the top of the headset to a couple of inches below the seat lug, you would get a shorter length, similar to the compact.

This is where we get to the more useful measurement of virtual top tube length. As it stands, your compact frame's top tube length is meaningless.
Get a spirit level and measure horizontally from the centre of the top of the headset to the centre point at which it intersects with the seatpin. This is the virtual top tube length and what you need to have in mind when chosing a horizontal tubed frame

Hope it makes sense

To give it context, look at the canyon geometry chart. The actual top tube isn't measured, as it's meaningless. The virtual top (c) is.

https://www.canyon.com/en-al/road-bikes ... ection-tab

Hope it makes sense


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 Post subject: Re: Re:
PostPosted: Sun Dec 06, 2020 12:31 pm 
Dirt Disciple

Joined: Tue Apr 28, 2020 9:38 am
Posts: 69
Holiocornolio wrote:
M_Chavez wrote:
Potentially daft answer, but how about a 60-70mm quill stem then? It will cost you £15 to try out the idea.

Point me to these £15 quill stems pretty please.


https://www.decathlon.co.uk/p/1-stem-60 ... F4QAvD_BwE

£8? If it works for you, you can find something with a fancier brand name ;)


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 Post subject: Re:
PostPosted: Sun Dec 06, 2020 12:35 pm 
Dirt Disciple

Joined: Tue Apr 28, 2020 9:38 am
Posts: 69
Not sure about twitchiness. My "crit" bike was twitchy with 100mm stem and it's just as twitchy with 80. Perhaps a very slight increase.
My cx was stable with 110mm and it's just as stable with (an effective) 70-80mm riser.

I wonder how much of the twitchiness is just a marketing myth to "look like a pro".


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 Post subject: Re:
PostPosted: Sun Dec 06, 2020 12:49 pm 
Retro Guru

Joined: Wed Dec 10, 2008 9:19 am
Posts: 2884
Twitchiness and stability is down to a combination of head tube angle, fork trail and top tube length.
I can't explain it, I don't have the knowledge, a good frame builder will.
It's certainly more than suggesting all frames handle the same, the differences are just marketing hype


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 Post subject: Re: Re:
PostPosted: Sun Dec 06, 2020 2:59 pm 
Dirt Disciple

Joined: Sat Oct 31, 2020 7:59 am
Posts: 77
pigman wrote:
Forget the stem sizing confusion.

Start with the basic geometry of the frame. Your confusion arises in trying to compare the top tube of a horizontal frame with the top tube of a compact frame. They are different concepts, geometry wise.

To try to understand it, if you were to put the 2 types of frame side by side, one has a tube that slopes down towards the back. Now, if you were to get the horizontal frame and measure from the top of the headset to a couple of inches below the seat lug, you would get a shorter length, similar to the compact.

This is where we get to the more useful measurement of virtual top tube length. As it stands, your compact frame's top tube length is meaningless.
Get a spirit level and measure horizontally from the centre of the top of the headset to the centre point at which it intersects with the seatpin. This is the virtual top tube length and what you need to have in mind when chosing a horizontal tubed frame

Hope it makes sense

To give it context, look at the canyon geometry chart. The actual top tube isn't measured, as it's meaningless. The virtual top (c) is.

https://www.canyon.com/en-al/road-bikes ... ection-tab

Hope it makes sense


It does make sense and is a very clear explanation but that's what I'm measuring already. The virtual TT of both my compact frames are about 535mm. The top tube on the non compact frame is 550mm. If I were to buy a horizontal top tube frame with similar top tube length to the virtual top tube of my compact frames, of be buying the same frame size a 50-52cm frame and that can't be right can it??


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