New Math (Frame Geometry)

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06-28-12 | 05:49 PM

#26

kaliayev

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Joined: Nov 2008

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From: BFOH

Quote:

Originally Posted by oldbobcat

Actual top tube and actual seat tube are virtually useless unless you like to ride bikes that are too large for you.

It is also important if you have a body build with a long torso and shot inseam.

06-28-12 | 06:02 PM

#27

mmmdonuts

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Joined: Dec 2007

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From: Raleigh, NC

Bikes: Yes

Back to the OP, since I didn't see it amongst the arguing, I use a spreadsheet to calculate stack and reach using the geometry given by the makers. It is within 1mm of those who also provide stack and reach along with their HTL, TTL, STA, HTA measurements. Usually it's off because I have to guess the fork length. The spreadsheet uses trig lifted from sites where stack and reach are discussed with adjustments to account for fork offset. It's come in very handy when I want to compare frames with surprising results every now and then.

Assuming the TT length is correct this is your Habanero:

(HTL includes the bottom headset cup)
[TABLE]
[TR]
[TD]Stack
[/TD]
[TD]622
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]Reach
[/TD]
[TD]363
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]

[/TD]
[TD]

[/TD]
[TD]

[/TD]
[/TR]
[TR]
[TD]ETT
[/TD]
[TD]547
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]HTL
[/TD]
[TD]221
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]FL
[/TD]
[TD]370
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]FO
[/TD]
[TD]45
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]HTA
[/TD]
[TD]73.5
[/TD]
[TD]deg
[/TD]
[/TR]
[TR]
[TD]STA
[/TD]
[TD]73.5
[/TD]
[TD]deg
[/TD]
[/TR]
[TR]
[TD]BBD
[/TD]
[TD]68
[/TD]
[TD]mm
[/TD]
[/TR]
[/TABLE]

And this is the integrated headset Nashbar frame:

[TABLE]
[TR]
[TD]Stack
[/TD]
[TD]603
[/TD]
[TD="width: 65"]mm
[/TD]
[/TR]
[TR]
[TD]Reach
[/TD]
[TD]392
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]

[/TD]
[TD]

[/TD]
[TD]

[/TD]
[/TR]
[TR]
[TD]ETT
[/TD]
[TD]565
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]HTL
[/TD]
[TD]205
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]FL
[/TD]
[TD]370
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]FO
[/TD]
[TD]45
[/TD]
[TD]mm
[/TD]
[/TR]
[TR]
[TD]HTA
[/TD]
[TD]72.5
[/TD]
[TD]deg
[/TD]
[/TR]
[TR]
[TD]STA
[/TD]
[TD]74
[/TD]
[TD]deg
[/TD]
[/TR]
[TR]
[TD]BBD
[/TD]
[TD]68
[/TD]
[TD]mm
[/TD]
[/TR]
[/TABLE]

Last edited by mmmdonuts; 06-28-12 at 06:48 PM.

06-28-12 | 07:32 PM

#28

oldbobcat

Senior Member

Joined: Jul 2005

Posts: 4,987

Likes: 709

From: Boulder County, CO

Bikes: '80 Masi Gran Criterium, '12 Trek Madone, early '60s Frejus track

Quote:

Originally Posted by kaliayev

It is also important if you have a body build with a long torso and shot inseam.

If your legs are extremely short, or you're not looking at a compact frame. Chances are, if your legs are so short that actual seat tube on a compact frame becomes a factor, the head tube is going to be too tall anyway.

OK, if your legs are super-short, you like sitting upright, and you have an aversion to long stems.

06-29-12 | 04:22 AM

#29

Campag4life

Voice of the Industry

Joined: May 2007

Posts: 12,572

Likes: 8

Quote:

Originally Posted by Bacciagalupe

Given the situation, I don't think anything really has changed. Again, you just have to look for "effective top tube" with the compacts.

It is, but my recollection is that road bikes have largely settled on the 72º-74º range for seat tube angles. (E.g. the seat tube angle on the Roubaix and Tarmac are nearly identical.) And a 0.5º difference over 580mm only results in a 5mm difference.

I think you'll see a lot more variation with head tube height, trail, wheelbase and the like than seat tube angle, and you'd only see something like a 77º ST on a dedicated TT bike.

I'd think that if he needs a radically non-standard seat tube angle, that's going to alter his weight distribution on the bike, and you're almost certainly back in Custom Geometry Land to really get everything right.

Probably silly to debate frame geometry on the web. Fit can be poor or perfect for a given rider. Lance Armstrong is said to be able to feel 1mm change in his difference bikes. I am particular about fit and I can feel 2mm in change to my fit fore/aft, reach and saddle height. But you may not be so it doesn't matter.

06-29-12 | 04:41 AM

#30

Campag4life

Voice of the Industry

Joined: May 2007

Posts: 12,572

Likes: 8

Quote:

Originally Posted by rruff

What this tells me is that bikes have a lot of adjustability. The puzzling thing is why you think ETT is important.

If you want to get the "right" frame size... which means you have standard parts in "aesthetic" adjustment ranges... the most important aspect to look at is head tube length. This analogous to the old frame sizing method of looking at seat tube length when top tubes were horizontal.

In this sea of madness, good to hear from somebody who understands the subject. Since you do and I am not speaking in a complete vacuum, you may appreciate an example of a name rider in a box when trying to ride a stock BMC frame....George Hincappie. Big George, as do many racers like to ride a small frame for their size because of shorter head tube to lower the handlebar. The problem with BMC...the mfr he was paid to ride is....a BMC has both an upright sta and a proprietary seatpost that can't be swapped out with an aftermarket seat post with more setback. Big George needs his setback however to put down his big power and balance his long body over the BB so have a look at how his saddle was set up initially....pushed all the way back on its rails. Of course if he rode that way, the rails would have likely bent or broken as he is no small guy. The result is a customization or kluge to the stock post to support his seat. So George's needs supercede the aesthetic as you put it....into the realm of what is considered functionally necessary. The aesthetic in this case is awful. And of course, if not paid to ride a BMC, he would choose a different frame with more laid back sta to satisfy the setback he needs. Modern sta's with stock posts don't work for everybody. As a sidebar, this is the reason I never choose a frame with a proprietary seat post. The average guy can't change to a post with either a better clamp design or one with more offset to best calibrate fit. Anyway...even the pros with their vast resources are pressed into a compromise by sta being ill suited for the frame they deem 'best dimensionally otherwise'. Figured you may appreciate this anecdote since you are one of the few who responded to this thread that understand frame geometry. Indeed in the hierarchy of importance, head tube height is every bit as important as top tube. STA is hugely important as well....above being an example of why.

Attached Images

Hincapie Seat Post BMC.jpg (89.0 KB, 16 views)

George Hincapie's Seat Post.jpg (87.4 KB, 18 views)

Last edited by Campag4life; 06-29-12 at 04:49 AM.

06-29-12 | 06:03 AM

#31

datlas

Thread Starter

Should Be More Popular

Joined: Dec 2007

Posts: 46,194

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From: Malvern, PA (20 miles West of Philly)

Bikes: 1986 Alpine (steel road bike), 2009 Ti Habenero, 2013 Specialized Roubaix

Quote:

Originally Posted by mmmdonuts

Back to the OP, since I didn't see it amongst the arguing, I use a spreadsheet to calculate stack and reach using the geometry given by the makers. It is within 1mm of those who also provide stack and reach along with their HTL, TTL, STA, HTA measurements. Usually it's off because I have to guess the fork length. The spreadsheet uses trig lifted from sites where stack and reach are discussed with adjustments to account for fork offset. It's come in very handy when I want to compare frames with surprising results every now and then.

Assuming the TT length is correct this is your Habanero:

(HTL includes the bottom headset cup)
[TABLE]
[TR]
[TD]Stack[/TD]
[TD]622[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]Reach[/TD]
[TD]363[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]
[/TD]
[TD]
[/TD]
[TD]
[/TD]
[/TR]
[TR]
[TD]ETT[/TD]
[TD]547[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]HTL[/TD]
[TD]221[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]FL[/TD]
[TD]370[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]FO[/TD]
[TD]45[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]HTA[/TD]
[TD]73.5[/TD]
[TD]deg[/TD]
[/TR]
[TR]
[TD]STA[/TD]
[TD]73.5[/TD]
[TD]deg[/TD]
[/TR]
[TR]
[TD]BBD[/TD]
[TD]68[/TD]
[TD]mm[/TD]
[/TR]
[/TABLE]

And this is the integrated headset Nashbar frame:

[TABLE]
[TR]
[TD]Stack[/TD]
[TD]603[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]Reach[/TD]
[TD]392[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]
[/TD]
[TD]
[/TD]
[TD]
[/TD]
[/TR]
[TR]
[TD]ETT[/TD]
[TD]565[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]HTL[/TD]
[TD]205[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]FL[/TD]
[TD]370[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]FO[/TD]
[TD]45[/TD]
[TD]mm[/TD]
[/TR]
[TR]
[TD]HTA[/TD]
[TD]72.5[/TD]
[TD]deg[/TD]
[/TR]
[TR]
[TD]STA[/TD]
[TD]74[/TD]
[TD]deg[/TD]
[/TR]
[TR]
[TD]BBD[/TD]
[TD]68[/TD]
[TD]mm[/TD]
[/TR]
[/TABLE]

Thanks for the numbers.

I am aware that the Nashbar does not exactly fit me, but it's within a couple cm and with my goofy body that's not bad.

Actually, if the Habanero were to fit me PERFECTLY it should have a HT and ST approx 2 cm longer....but then my choices in forks would be severely limited. That's why I have to resort to a slightly flipped-up (or unflipped, in 41 parlance) stem.

__________________

Quote:

Originally Posted by rjones28

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06-29-12 | 06:14 AM

#32

guadzilla

Pointy Helmet Tribe

Joined: Feb 2009

Posts: 4,338

Likes: 629

From: Offthebackistan

Bikes: R5, Allez Sprint, Shiv

Quote:

Originally Posted by Campag4life

STA however is a pretty big deal. The reason is...indeed reach has to be satisfied as you believe...and stack...beauty of why we like our Roubaixs....but STA is very important for fore/aft weight distribution which is really the starting place or cornerstone of fit.

STA in turn affects net reach...especially if deviating from a stock seatpost which some need, including me for optimal balance on the bike. For the Roubaix...Spesh sells two varieties of seat post...a single bolt with 21mm of setback and a 2 bolt post which is 25mm. So depending on which post your bike came with...Sworks has been sold with either, on clamp center, your net reach will be affected by 3mm. Neither are enough for me to balance my weight however. Many amateur rides including pros like Hincappie, Schleck, Boonen etc prefer in excess of 100mm of setback...saddle tip to BB center. I am long legged and ride with my Speedplay cleats a bit rearward therefore I ride with a slightly lower saddle height for my leg length. This reduces setback even fruther with the stock post that came on my bike of 21mm. My 58 Roubaix has a sta of 73 deg...and I believe Colin your 61 has a sta of 72.5 deg. Ideally I have the leg length which works better with a 72.5 deg STA like you have....but a 61 Roubaix is a hair big for my overall size...you are a bit taller. So...I run a 32mm setback post...see pic below. I ride it just forward...about 3-4mm of center for a net setback of 28-29mm or so...or about 5-8mm over stock. This dramatically changes the weight distribution on the bike and for the better. But by having a stock 73 deg STA and increasing setback, this effectively elongates my effective top tube length. This btw is OK in my case as I ride with a 120mm stem...I have long arms to match my long legs and choose to ride with a higher handlebar which effectively shortens reach. Reach should always be served independent of what bar height you ride. Most importantly over and above reach...is fore/aft weight distribution must be served for any chance of a good fit. This is a function of STA and saddle setback as it relates to net reach and why so vitally important to answer your question.
The thing about fit is...it is much more complex than just 'top tube' or head tube height. STA really affects overall reach but most importantly, it affects fore/aft position on the bike AND effective reach to the handlebars. So STA is significant.
Hope that makes sense.

This post is a little more useful than your earlier attempts at being snarky - stick to content next time: you do better there. Witty put-downs aren't exactly your forte.

There are 2 things with replicating a fit - getting the saddle where it needs to be relative to the bottom bracket, and putting the handlebars where they should be.

So the first question becomes, can you get the saddle where it needs to be on a 72.5 STA or a 74 STA bike? The answer is yes.

The answer to my earlier question was - there is a 2cm difference between a 72.5 and 74 STA for an 82cm BB-saddle length (for most riders, this number will be less, as they'll have a shorter saddle height and possible less extra variance in STA). This is easy enough to achieve with by sliding the saddle back/forward or by swapping the seatpost.

Now on to the second part of the fit - putting the handlebars where they need to be.

You are correct - sliding the saddle back/forth to compensate for the difference in STA will affect reach, as the reach to the head tube changes. A 565 ETT on a 72.5 STA frame isn't the same reach as a 565 ETT on a 74 STA frame. However, unless you are riding in an extreme position (stem maxxed out), you can offset this by choosing a different stem.

So the answer to the second part is: yes, you can get the handlebars where you need them as well, by changing the stem length.

In short, you can get your bike set up exactly the same way, regardless of the STA and HTA.

Yes, it will affect weight distribution - no one is saying that a bike with a 72.5 STA is going to handle exactly the same as a bike with a 74 STA. Duh. But we were talking about replicating a particular FIT.

As long as you pick a frame based on your preferred ETT and HT, and you don't ride in an extreme position on this bike, then you can get the frame to be built up to replicate your desired fit. That isn't "BS", as you had claimed in your initial post.

Last edited by guadzilla; 06-29-12 at 06:29 AM.

06-29-12 | 06:25 AM

#33

guadzilla

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From: Offthebackistan

Bikes: R5, Allez Sprint, Shiv

Quote:

Originally Posted by rruff

What this tells me is that bikes have a lot of adjustability. The puzzling thing is why you think ETT is important.

Not sure I understand the question.

The ETT figure gives me approx reach - yes, this is going to be affected by the STA and, to a lesser degree, the HTA. But as mentioned in my post earlier, the variation caused by STA is typically manageable in terms of setting up the bike to replicate a particular riding position.

Taken in conjunction with the HT length, the ETT lets me know if the dimensions of the frame are within the range that will fit me or not.

I know that if I have a bike in the 565-570mm ETT and a 175-185mm HT, I can get it set up exactly the way I want, b/c these numbers give me a fit that is right in the middle.

OTOH, if I have a bike with a 575-585mm ETT, or with a much lower HT length, then I know that the fit will require extreme measures to get it to work - and that is where STA will make a difference (a 72.5 STA bike with a 575-580 ETT may work, a 74 STA bike will definitely not).

06-29-12 | 06:40 AM

#34

Garfield Cat

Senior Member

Joined: Oct 2004

Posts: 7,124

Likes: 111

From: Huntington Beach, CA

Bikes: Cervelo Prodigy

Quote:

Originally Posted by datlas

I have been an avid road cyclist for more than 25 years.

I would like to think that I have a good basic understanding of frame geometry, but with the movement towards compact frames I feel like my knowledge is moldy. I still think in terms of Top Tube, Seat Tube, Head Tube lengths...and HT angle/ST angle.

However I have been hearing more lately that since manufacturers are lying about their geometries it makes more sense to look at "stack" and "reach."

I want to learn a bit more about this but I suspect it will require a change in my thinking.

Anyone know a good website to help me convert my old math (geometry) thinking into the 21st century??

the Cervelo website explains their thinking on stack and reach. www.cervelo.com

There on the left side is an index. Try "Engineering" then click onto "Geometry & Fit"

https://cervelo.com/en_us/engineering...presentations/

06-29-12 | 06:47 AM

#35

datlas

Thread Starter

Should Be More Popular

Joined: Dec 2007

Posts: 46,194

Likes: 11,755

From: Malvern, PA (20 miles West of Philly)

Bikes: 1986 Alpine (steel road bike), 2009 Ti Habenero, 2013 Specialized Roubaix

Quote:

Originally Posted by Garfield Cat

Thanks for the link. It does not quite load properly on my browser but I was able to finagle it...very helpful.

__________________

Quote:

Originally Posted by rjones28

Addiction is all about class.

06-29-12 | 07:05 AM

#36

ColinL

Two-Wheeled Aficionado

Joined: Jul 2011

Posts: 4,903

Likes: 5

From: Wichita

Bikes: Santa Cruz Blur TR, Cannondale Quick CX dropbar conversion & others

Quote:

Originally Posted by Garfield Cat

thank you for that link. I had not previously seen it, but C4L needs to read it (maybe he will) and understand it (

) and agree with it (

Quote:

Originally Posted by guadzilla

In short, you can get your bike set up exactly the same way, regardless of the STA and HTA.
Yes, it will affect weight distribution - no one is saying that a bike with a 72.5 STA is going to handle exactly the same as a bike with a 74 STA. Duh. But we were talking about replicating a particular FIT.
As long as you pick a frame based on your preferred ETT and HT, and you don't ride in an extreme position on this bike, then you can get the frame to be built up to replicate your desired fit. That isn't "BS", as you had claimed in your initial post.

+infinity
it's so simple. the STA, HTA and other things C4L is hung up on do matter for handling, and they're critical for framebuilders. but when we're talking fit, they matter not.... assuming that you intelligently and properly adjust your saddle (including seatpost setback) and stem to achieve proper bar height and saddle position relative to the bottom bracket.

it isn't "rocket surgery".

06-29-12 | 07:27 AM

#37

ColinL

Two-Wheeled Aficionado

Joined: Jul 2011

Posts: 4,903

Likes: 5

From: Wichita

Bikes: Santa Cruz Blur TR, Cannondale Quick CX dropbar conversion & others

The OP and others curious might find this stack & reach calculator quite helpful:
https://bb2stem.blogspot.com/

06-29-12 | 07:28 AM

#38

rruff

Senior Member

Joined: Jan 2006

Posts: 1,359

Likes: 2

From: Ruidoso, NM

Quote:

Originally Posted by guadzilla

The ETT figure gives me approx reach - yes, this is going to be affected by the STA and, to a lesser degree, the HTA. But as mentioned in my post earlier, the variation caused by STA is typically manageable in terms of setting up the bike to replicate a particular riding position.

ETT doesn't give you meaningful reach, since the STA effects it quite a lot... in addition to HTA and HT. Most people don't realize that HT length effects reach on two frames that otherwise have the same STA and ETT. That's one reason why "stack" and "reach" are not so great for sizing. A 30mm HT difference is ~10mm in reach. And as you mentioned there is a huge amount of lateral adjustment possible in seatposts, saddles, and stems. So long as you can achieve your fit with normal components, all is well.

There really is no substitute for figuring out where you want your saddle and bars relative to the bottom bracket, and then breaking out the HS geometry book and seeing what it will take to connect the dots.

For instance, I'm 6ft with average proportions, but like a lot of drop from the saddle to bar. Because this is "unusual" it is the controlling factor in sizing. I ride a 54cm frame with a 145mm head tube, 130mm -10 stem (no spacers), and 20mm offset post with the saddle back about as far as it will go. If I didn't want so much drop I could ride a 56, 58, or even a 60 quite easily.

06-29-12 | 09:00 AM

#39

Campag4life

Voice of the Industry

Joined: May 2007

Posts: 12,572

Likes: 8

Quote:

Originally Posted by guadzilla

This post is a little more useful than your earlier attempts at being snarky - stick to content next time: you do better there. Witty put-downs aren't exactly your forte.

There are 2 things with replicating a fit - getting the saddle where it needs to be relative to the bottom bracket, and putting the handlebars where they should be.

So the first question becomes, can you get the saddle where it needs to be on a 72.5 STA or a 74 STA bike? The answer is yes.

The answer to my earlier question was - there is a 2cm difference between a 72.5 and 74 STA for an 82cm BB-saddle length (for most riders, this number will be less, as they'll have a shorter saddle height and possible less extra variance in STA). This is easy enough to achieve with by sliding the saddle back/forward or by swapping the seatpost.

Now on to the second part of the fit - putting the handlebars where they need to be.

You are correct - sliding the saddle back/forth to compensate for the difference in STA will affect reach, as the reach to the head tube changes. A 565 ETT on a 72.5 STA frame isn't the same reach as a 565 ETT on a 74 STA frame. However, unless you are riding in an extreme position (stem maxxed out), you can offset this by choosing a different stem.

So the answer to the second part is: yes, you can get the handlebars where you need them as well, by changing the stem length.

In short, you can get your bike set up exactly the same way, regardless of the STA and HTA.

Yes, it will affect weight distribution - no one is saying that a bike with a 72.5 STA is going to handle exactly the same as a bike with a 74 STA. Duh. But we were talking about replicating a particular FIT.

As long as you pick a frame based on your preferred ETT and HT, and you don't ride in an extreme position on this bike, then you can get the frame to be built up to replicate your desired fit. That isn't "BS", as you had claimed in your initial post.

I am sorry not to take you seriously, but you are flat wrong...and why I included the Hincappie example on his BMC which resulted in a custom seatpost. Please check your family tree...you maybe related to Colin...lol. You are wrong in my particular case as well. I need less than a 72.5 deg sta to achieved a balanced position on the bike with a stock seatpost. With proprietary aka areo posts on many bikes, then best position isn't achievable if sticking to your range of 'adjustment'...which also affects net reach.
STA is very important pal and to not consider it in the equation of overall reach once fore/aft postion is established is perhaps the biggest mistake any fitter could make. In summary, all your writing suggests you don't get it. You will never get it I am afraid, so pardon me if I balance my boredom with you with some humor.

06-29-12 | 09:03 AM

#40

ColinL

Two-Wheeled Aficionado

Joined: Jul 2011

Posts: 4,903

Likes: 5

From: Wichita

Bikes: Santa Cruz Blur TR, Cannondale Quick CX dropbar conversion & others

You and rruff clearly are not reading the links we are providing and are clearly not interested in any learning, merely spouting your antiquated and ill-advised positions. You have plenty of time to post on BF (as do I) so I know you have time to read the links.

Apparently even source material from Cervelo's engineering group isn't worth your attention. If it were, you'd learn something.

06-29-12 | 09:22 AM

#41

Campag4life

Voice of the Industry

Joined: May 2007

Posts: 12,572

Likes: 8

Quote:

Originally Posted by ColinL

Honestly Colin, all you do is a disservice here by spouting your ignorance about fit and frame geometry. You keep bringing up handling which is also not even the subject at hand. I don't think you are a bad guy...I have read a lot of your posts on other subjects and believe you have pockets of knowledge to contribute. But you are out of your zone man. Rruff and me basically have forgotten more about fit and frame geometry than you could ever fathom. It honestly isn't very complex to me, but I learned a long time on the 41 that fit as it relates to frame geometry goes well over the head of perhaps 80% that post here.
OK...carry on. As you were

06-29-12 | 09:54 AM

#42

GiantDefyGuy

At least I'm not a poseur

Joined: May 2010

Posts: 794

Likes: 0

From: New Orleans, USA.

Bikes: Giant Defy 3

Jesus Christ, man! Your Smug-Level is off the charts! It's disturbing.

As for the OP, I was fitted on a M/L Giant Defy in the store. This is a compact frame, and it has a 56cm effective top tube. So when I was looking for a cheap single speed from Nashbar, I just bought the standard frame in a size 56 hoping it would work. It did. I have the fit almost exactly the same between both bikes. I believe I have "normal" proportions though. From my own experience so far, I know that I either need a standard frame with a 56cm top tube, or a compact frame with a 56cm effective top tube.

06-29-12 | 10:28 AM

#43

guadzilla

Pointy Helmet Tribe

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From: Offthebackistan

Bikes: R5, Allez Sprint, Shiv

Quote:

Originally Posted by rruff

ETT doesn't give you meaningful reach, since the STA effects it quite a lot...

Yes, it does - and I have quantified by how much and shown how those differences can be accounted for. With all due respect to you (and none whatsoever to Campyfanboi), I think you misunderstand my point, and that at the crux of it, we don't really disagree that much.My point is simple: if you have a preferred ETT and HT that doesn't require any extreme saddle positioning or stem size, then you can get any typical road bike (i.e., with a STA range of 72.5 to 74) to fit. I hve never said that the reach is going to be exactly the same on different bikes with different STAs - I realize that (my example earlier shows a 2cm difference in reach between a 72.5 and 74 STA bike with a 82cm saddle-BB length). The only reason I've brought this point up is to counter Campyboi's exceedingly sweeping statement that there is no value in using ETT and HT to size a bike. Precisely BECAUSE bikes have a pretty largish "fit zone", ETT and HT are accurate enough (even if they aren't precise) to get the right fit for a lot of people.

Quote:

in addition to HTA and HT. Most people don't realize that HT length effects reach on two frames that otherwise have the same STA and ETT. That's one reason why "stack" and "reach" are not so great for sizing. A 30mm HT difference is ~10mm in reach.

Not sure I follow your your point that stack and reach are not great for sizing - do you mean taken individually, i.e., 2 bikes with the same reach but a different stack will have a different effective reach? If so, yes, agreed.

Quote:

For instance, I'm 6ft with average proportions, but like a lot of drop from the saddle to bar. Because this is "unusual" it is the controlling factor in sizing. I ride a 54cm frame with a 145mm head tube, 130mm -10 stem (no spacers), and 20mm offset post with the saddle back about as far as it will go. If I didn't want so much drop I could ride a 56, 58, or even a 60 quite easily.

Ha, I've had to learn geometry lessons b/c I have freakishly long legs and an absurdly short torso. I need a sportive-style head tube to get an aggressive (5" saddle to bar drop) geometry. I need a "short and tall" bike - the Roubaix, with a 185mm HT on size 56 would be an aggressive fit for me.

06-29-12 | 10:35 AM

#44

guadzilla

Pointy Helmet Tribe

Joined: Feb 2009

Posts: 4,338

Likes: 629

From: Offthebackistan

Bikes: R5, Allez Sprint, Shiv

Quote:

Originally Posted by Campag4life

I am sorry not to take you seriously

I am shattered. It's gonna keep me up all night. Seriously, 12-year olds have better repartee than you, so stop trying to be snarky. I feel embarrassed for you.

Quote:

I need less than a 72.5 deg sta to achieved a balanced position on the bike with a stock seatpost.

Stop moving the target around. My initial post talks about using a different seatpost to achieve correct fit, in case the saddle rails do not have enough range to compensate for STA variance. Are you telling me you cannot get the proper saddle-to-BB position on a 74 STA bike with ANY seatpost? If so, you are full of crap. If not, then you are agreeing with what I wrote.

Quote:

With proprietary aka areo posts on many bikes, then best position isn't achievable if sticking to your range of 'adjustment'...which also affects net reach.

This is a good point, yes.

Quote:

STA is very important pal and to not consider it in the equation of overall reach once fore/aft postion is established is perhaps the biggest mistake any fitter could make. In summary, all your writing suggests you don't get it. You will never get it I am afraid, so pardon me if I balance my boredom with you with some humor.

I think you must be having difficulties reading my text up there from Mt Olympus, b/c your lack of comprehension of what I wrote is staggering.

If you ease up on the self-fellating self-congratulations, you'll realize that you are not actually addressing anything I wrote. Re-read my post to rruf.

But somehow, I doubt if you'll let facts come in the way of your one-man circle jerk, so carry on.

Last edited by guadzilla; 06-29-12 at 10:39 AM.

06-29-12 | 02:01 PM

#45

Campag4life

Voice of the Industry

Joined: May 2007

Posts: 12,572

Likes: 8

Quote:

Originally Posted by guadzilla

I know my comprehension on the subject is limited and I might add you are more than worthy of your mantle on Mt Olympus. Please be tolerant as I am trying to learn.

I thought I was.

Last edited by Campag4life; 06-29-12 at 02:06 PM.

06-29-12 | 04:13 PM

#46

rruff

Senior Member

Joined: Jan 2006

Posts: 1,359

Likes: 2

From: Ruidoso, NM

Quote:

Originally Posted by ColinL

You and rruff clearly are not reading the links we are providing and are clearly not interested in any learning

Hey... say that to my face!

I haven't said anything "incorrect". It's as basic and unassailable as math and geometry.

06-29-12 | 04:22 PM

#47

rruff

Senior Member

Joined: Jan 2006

Posts: 1,359

Likes: 2

From: Ruidoso, NM

Quote:

Originally Posted by ColinL

Apparently even source material from Cervelo's engineering group isn't worth your attention. If it were, you'd learn something.

Cervelo's presentation is very good, but they do not mention that stack and reach are dependent on each other. A frame with a higher stack is effectively "longer" than a frame with a lower one, even if the reach value is the same. So stack and reach are not the best values for determining a frame's geometry for sizing.

A better method would be to locate the vertical point for measuring reach at some arbitrary stack value, rather than measuring it at the top of the head tube.

06-29-12 | 08:59 PM

#48

ColinL

Two-Wheeled Aficionado

Joined: Jul 2011

Posts: 4,903

Likes: 5

From: Wichita

Bikes: Santa Cruz Blur TR, Cannondale Quick CX dropbar conversion & others

Quote:

Originally Posted by rruff

Hey... say that to my face!

I haven't said anything "incorrect". It's as basic and unassailable as math and geometry.

sorry to lump you with C4L. at least you read the Cervelo info.

but one of their points is that they design frames so that stack and reach *do* linearly grow and shrink together as you move up and down sizes. other brands often do not decrease reach linearly as you go down, making their bikes smaller than 54cm seriously jacked.

06-30-12 | 06:47 AM

#49

guadzilla

Pointy Helmet Tribe

Joined: Feb 2009

Posts: 4,338

Likes: 629

From: Offthebackistan

Bikes: R5, Allez Sprint, Shiv

Quote:

Originally Posted by rruff

Cervelo's presentation is very good, but they do not mention that stack and reach are dependent on each other.

I think you are using the terms differently. Stack and Reach, as used to describe bikes, simply define specific measurements along the X and Y dimensions. Since reach (not Reach) is a function of both those factors, you are correct, both X and Y will affect it. So "Reach" the measurement is different from "reach", the distance from saddle to bars.

However, I think you are making it a little more complicated than it needs to be. The purpose of Stack and Reach is to standardize bike dimensions.

I am not sure why you keep saying that S/R are not the best values for determining a frame's geometry for sizing. If I know my current bike's S/R and it fits, all I need to do is get another bike with comparable/close enough S/R numbers and I know I'll get it to fit.

My current TT bike has a stack of 523 and a reach of 418. The reach is a little more than I'd prefer, and the stack a little lower than I'd like (I have a mountain of spacers there). The Shiv has a stack of 540 and a reach of 405. This bike is going to put the bars closer and higher - which is perfect for me.

Yes, I will still need to do some measurements to get the fit right, but simply by reading the numbers, I know that the Shiv is going to require less futzing around to make it work.

Quote:

A better method would be to locate the vertical point for measuring reach at some arbitrary stack value, rather than measuring it at the top of the head tube.

Most of us generally have an idea of how many spacers we put on top of our headsets, so using the HT-based methodology is quite apt, I feel. By comparing S/R numbers, I know approx where the handlebars are going to end up, relative to my current bike, all else being equal. Then I can adjust by swapping parts.

06-30-12 | 07:46 AM

#50

mmmdonuts

Gluteus Enormus

Joined: Dec 2007

Posts: 2,245

Likes: 0

From: Raleigh, NC

Bikes: Yes

I completely agree with the above. If you know the location of 3 key points and the adjustment range for 2 of those then any frame within that range can fit. Knowing the stack/reach and STA values will help you fine tune that selection if you are worried about saddle setback, stem length, and stack height.

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