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 Post subject: Stack & Reach
PostPosted: Sat Jan 17, 2015 10:49 pm 
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Joined: Sat Nov 24, 2012 8:43 pm
Posts: 44
OK, clearly being able to ride the bike gives the best sense of fit. However, how close are stack & reach charts?

My current bike has:
Reach 394mm (I ride it with 120mm stem)
Stack 603mm (I ride the stem slammed & would go lower if I could)

S Works Tarmac 58cm has:
Reach 402mm (comes with 110mm stem)
Stack 591mm

So does the reach comparison mean that I will be no more or less stretched out on Tarmac vs current?
On Stack I can go 12mm lower--this doesn't sound a lot.

Perhaps I am not fully understanding things as I am coming from a Scott CR1 SL 2012 which is supposed to be of a relaxed geometry & I thought the S Works Tarmac was supposed to have an aggressive fit i.e low & long.


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 Post subject: Re: Stack & Reach
PostPosted: Sat Jan 17, 2015 11:38 pm 
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Joined: Wed Jun 01, 2011 7:53 am
Posts: 516
The specialized measurements are accurate in my experience, so using this method to pick a new frame should be fine. Just consider that differences in the seat tube angle, chainstay/wheelbase, and bb drop can effect the position as well. Assuming all other parts (pedals, saddle, cleats, crank arms) stay the same you should be able to figure it out. Stare at it longer and it will become clear.


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 Post subject: Re: Stack & Reach
Posted: Sat Jan 17, 2015 11:38 pm 


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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 7:36 am 
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Joined: Tue Dec 23, 2008 2:57 pm
Posts: 745
Location: NYC
Below is some required reading on STACK and REACH.

http://velonews.competitor.com/2012/05/ ... lly_216035" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

http://www.cervelo.com/en/engineering/t ... d-fit.html" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

viewtopic.php?f=3&t=29177&start=0" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

EM3

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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 12:43 pm 
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Joined: Sun Oct 05, 2014 11:51 pm
Posts: 238
tinozee wrote:
The specialized measurements are accurate in my experience, so using this method to pick a new frame should be fine. Just consider that differences in the seat tube angle, chainstay/wheelbase, and bb drop can effect the position as well. Assuming all other parts (pedals, saddle, cleats, crank arms) stay the same you should be able to figure it out. Stare at it longer and it will become clear.


I assume that shorter chainstays move the position forward, as the shorter chainstays move the centre of gravity back in comparison to longer chainstays.

Eg. the centre of gravity (relative to BB) on a bike with 42.5 chainstays will be further back compared to a bike with 40.5 chainstays.


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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 1:18 pm 
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Joined: Sat Dec 06, 2008 10:14 am
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Location: Sweden
COG is not relative to BB spindle, COG is relative to fore/aft between wheel axles. Longer CS moves COG relatively closer to front wheel. I.e shorter CS puts less weight on front wheel and as such inflicts upon steering.

Stack and reach is not dependent on BB-drop or STA. But actual reach when comparing bike to bike is actually dictated by stack as stack dictates where the reach measurement is taken. I.e a 40mm stack difference makes for a ~10mm actual reach difference when making the necessary adjustments for how to compare bikes. You can't find numbers for "actual reach", it's a relative and virtual number that you have to figure out yourself. Ofcourse only relevant when the stack numbers differ.

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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 7:48 pm 
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Thanks for the help!

So are you saying from the stack & reach numbers I have provided are not sufficient in themselves to indicate how different the two bikes will be in terms of fit?


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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 9:15 pm 
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tinozee wrote:
The specialized measurements are accurate in my experience, so using this method to pick a new frame should be fine. Just consider that differences in the seat tube angle, chainstay/wheelbase, and bb drop can effect the position as well.


Differences in the seattube angle should not dictate your saddle position, you should use the rail of the saddle or even a different setback post to get the right saddle position. Only extreme changes in seatpost angle or significantly different than average saddle positions will have an effect. Differences in the chainstay length and wheelbase will not affect the position because it's completely irrelevant. Differences in the BB drop will not affect the position because the BB is the reference point for stack and reach. Drop the BB by 10mm on an otherwise identical bike and hey presto, the stack measurement gets 10mm bigger.


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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 9:28 pm 
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The actual reach is about 6mm longer on the S-Works when accounting for the different stack heights you've provided. They're very similar for fit, (not touching how fit interacts with handling, that's where FC, CS, BB, etc numbers come in.)

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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 10:07 pm 
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DMF wrote:
COG is not relative to BB spindle, COG is relative to fore/aft between wheel axles. Longer CS moves COG relatively closer to front wheel. I.e shorter CS puts less weight on front wheel and as such inflicts upon steering.


the understanding i was trying to confirm was that with shorter chainstays to achieve the same COG
you need to move your position forward albeit minimally (eg. saddle position relative to BB)?


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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 10:25 pm 
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Yes, that's a correct understanding of the physics as shorter chainstays unweighs the load on the front wheel. Moving your body weight forward of the rear wheel axle will in itself unweigh the rear wheel at the same time as it further weighs the front wheel. Ofcourse pivoting your body forward around the BB-centerline (vertical or thru your body, any way you care to look at it) has its own implications concerning handling as well as fit. I would rather make adjustments to the frame rather than my preferred body position... But in reality, frame geometry can be heavily intertwined with the fit in relation to handling... if you wish to "be one" with your machine at least.

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 Post subject: Re: Stack & Reach
PostPosted: Sun Jan 18, 2015 10:38 pm 
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Location: Cambridge, New Zealand
Once you correct for same bar height (bar y is stack to centre of bar) you can see that the bar reach (bar x) will still be slightly different even with trying to accommodate by different stem lengths.
Attachment:
Barters.PNG
Barters.PNG [ 29.27 KiB | Viewed 804 times ]

I would strongly advise against moving your saddle forward to compensate for different chainstays, likely to compromise your comfort. Especially as the Tarmac has a marginally longer front centre to partially counteract the longer CS.

The CR1 is not really a 'relaxed' geometry and the Tarmac is all over the place.

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 Post subject: Re: Stack & Reach
PostPosted: Mon Jan 19, 2015 12:00 am 
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wingguy wrote:
tinozee wrote:
The specialized measurements are accurate in my experience, so using this method to pick a new frame should be fine. Just consider that differences in the seat tube angle, chainstay/wheelbase, and bb drop can effect the position as well.


Differences in the seattube angle should not dictate your saddle position, you should use the rail of the saddle or even a different setback post to get the right saddle position. Only extreme changes in seatpost angle or significantly different than average saddle positions will have an effect. Differences in the chainstay length and wheelbase will not affect the position because it's completely irrelevant. Differences in the BB drop will not affect the position because the BB is the reference point for stack and reach. Drop the BB by 10mm on an otherwise identical bike and hey presto, the stack measurement gets 10mm bigger.



Hi yeah, I didn't say anything about saddle position, just pointing out that there is more to consider than just stack and reach. I don't want to get into a whole fit discussion. It's not safe to assume anything at all about position or body proportions. Someone with a short torso might not be right for a 72 sta, and may need the saddle jammed all the way forward.

It's too much to discuss all of that, but I do think that it's safe to say that if you have two frames with same stack and reach, identical sta/hta/wheelbase/chain stay you can be pretty sure they will have a similar fit.


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 Post subject: Re: Stack & Reach
PostPosted: Tue Jan 20, 2015 3:34 pm 
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If you add 12/13mm spacer to get the same stack, then the Spesh has slightly more reach by about 4mm (in the frame)

If you have the stems on top of a 15mm cone top-cap, then (given the stem lengths) horizontal reach to the bars is less than 2mm shorter on the Spesh, and you have ~15mm more drop.

Given this extra drop, your position will change slightly, rotating your weight slightly further forward. You may need to make some other very slight adjustments to compensate for this, depending on the handling characteristics of the bikes, and whether it causes you any discomfort.

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 Post subject: Re: Stack & Reach
Posted: Tue Jan 20, 2015 3:34 pm 


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 Post subject: Re: Stack & Reach
PostPosted: Tue Jan 20, 2015 10:09 pm 
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Joined: Sat Nov 24, 2012 8:43 pm
Posts: 44
Greatly appreciate you creating that table for me. To be honest I did not understand much of it! However, it does give me confidence that the Tarmac will give me a more aggressive long & low position vs what I am used to yet will not be a shocking change to the system.


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