Go Back  Bike Forums > Bike Forums > Road Cycling
Reload this Page >

Understanding geometry...

Notices
Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Understanding geometry...

Old 11-15-07, 01:27 PM
  #1  
HJR
Senior Member
Thread Starter
 
Join Date: Nov 2003
Location: Pasadena, CA
Posts: 241

Bikes: Cervelo Soloist Team, Cervelo SuperProdigy, Colnago C50

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Understanding geometry...

Hello All,

As I have learned more and more over the years about how my bike fits, I have studied various geometry charts and think I've realized a major omission from various manufacturers' charts. Correct me if I am wrong, but isn't horizontal offset from the BB more important than top tube dimensions. Colnago and Pinarello include it from the seattube and Cervelo from the headtube, but Bianchi doesn't, Giant doesn't, Cannondale doesn't, Specialized doesn't, Merlin doesn't, Litespeed doesn't, Scott doesn't to name a few.

I have bikes that have varying top tubes from 55 to 56.5 and their "cockpit", how Cervelo describes the BB to head tube dimension, differ by only .6.

For example, Colnago 56, 57 and 58 sized bikes have a top tube dimension of 55, 55.6 and 56.3 respectively. But their cockpit dimension, i.e. the top tube dimension minus the published setback dimension, are 39.7, 39.5 and 39.9. I take this to mean that the 57 is actually the "smaller" bike. It would require a further setback saddle or a slightly longer stem to maintain the same position.

I just thought it was intereseting, because I used to look at top tube dimension for my purchases, but no realize that may vary your relationship to the BB.

Is my understanding right, and is it incorrect to size based on top tube dimensions?
HJR is offline  
Old 11-15-07, 02:17 PM
  #2  
DocRay
Guest
 
Posts: n/a
Mentioned: Post(s)
Tagged: Thread(s)
Quoted: Post(s)
Top tube dimensions will put you in the ballpark, but there are a few different seat tube angles out there.
This will affect setback.
Some manufacturers vary the seat tube angle for size, others, like Cervelo, doesn't. It's important to know all this because you could end up with too much setback, and it will influence the stem and post choice.
Colnagos vary their geometry between models to give very different rides for different purposes.
 
Old 11-15-07, 02:30 PM
  #3  
Erstwhile Trogon
 
terry b's Avatar
 
Join Date: Mar 2006
Posts: 1,032
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
You really can't look at it the way you are suggesting. Setback does not reduce the length of the TT or the reach for a given rider, it is simply a portion of the TTL on the Colnago charts. It is a arbitrary reference point in space that simply reflects the difference in the seat tube angles between the three frame sizes.

Setback = COS(STA)*STL.

Looking at the Colnagos for the sake of argument, the 55STA is 74, The 56STA is 73.3 and the 57STA is 73. The setbacks calculate as 14.6, 15.3 and 16.1 when using the seat tube CTC measurement. Just as it says in the chart.

To get the same fit on a 55 that you'd have on a 57, a rider would have to move the saddle back by almost 1CM, which means that the "virtual TT" would be 55.3 on the 55 and 55.6 on the 57. Virtually identical. The comparison between the 56 and 57 is similar, a .6CM slide back yields 55.6 and 55.6, respectively. Colnago bases their fit on the assumption that a taller rider has a longer femur and that change results in the increase in height. They are assuming that reach (as affected by torso and arm length) is about the same for the riders of those three bikes. They are also assuming that since they build their bikes in 1CM increments that you are going to buy the correct bike for your femur length. If you adhere to their fit philosophy, the 57 will never be "smaller" because a 57 rider would never push their saddle far enough forward to make it fit like a 55. Their leg length would prohibit that.

As I recall, Cervelo does some funky number crunching yielding a value called "reach" which is the TT - Setback.

Last edited by terry b; 11-15-07 at 02:40 PM.
terry b is offline  
Old 11-15-07, 03:40 PM
  #4  
HJR
Senior Member
Thread Starter
 
Join Date: Nov 2003
Location: Pasadena, CA
Posts: 241

Bikes: Cervelo Soloist Team, Cervelo SuperProdigy, Colnago C50

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by terry b
You really can't look at it the way you are suggesting. Setback does not reduce the length of the TT or the reach for a given rider, it is simply a portion of the TTL on the Colnago charts. It is a arbitrary reference point in space that simply reflects the difference in the seat tube angles between the three frame sizes.

Setback = COS(STA)*STL.

Looking at the Colnagos for the sake of argument, the 55STA is 74, The 56STA is 73.3 and the 57STA is 73. The setbacks calculate as 14.6, 15.3 and 16.1 when using the seat tube CTC measurement. Just as it says in the chart.

To get the same fit on a 55 that you'd have on a 57, a rider would have to move the saddle back by almost 1CM, which means that the "virtual TT" would be 55.3 on the 55 and 55.6 on the 57. Virtually identical. The comparison between the 56 and 57 is similar, a .6CM slide back yields 55.6 and 55.6, respectively. Colnago bases their fit on the assumption that a taller rider has a longer femur and that change results in the increase in height. They are assuming that reach (as affected by torso and arm length) is about the same for the riders of those three bikes. They are also assuming that since they build their bikes in 1CM increments that you are going to buy the correct bike for your femur length. If you adhere to their fit philosophy, the 57 will never be "smaller" because a 57 rider would never push their saddle far enough forward to make it fit like a 55. Their leg length would prohibit that.

As I recall, Cervelo does some funky number crunching yielding a value called "reach" which is the TT - Setback.
Hi Terry, I see what you are saying, but aren't the three relationships of equal importance. If you have a 56 with a 15.3 setback and then a 57 with the 16.1 setback you'll adjust the seat the same on both. On the 56 it may be neutral but on the 57 forward .8cm. That would then leave you with less TT length on the 57 which would require a slightly shorter stem or .2cm which they don't make. So then on the 57 you would be further forward on the bike7.
HJR is offline  
Old 11-15-07, 04:35 PM
  #5  
Erstwhile Trogon
 
terry b's Avatar
 
Join Date: Mar 2006
Posts: 1,032
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
I think that Colnago approaches the problem from the assumption that you're going to buy the correct bike for your height.

I own two 57 Colnagos as well as one 58. The reach on them is about .7cm different, and that's driven entirely by the increased TTL. Reason - the saddle is in precisely the same place horizontally and in relation to the BB. The setback between those two frames is different only because the calculation is done with the longer seat tube on the 58. Because the seat tube angle is the same, the saddle is positioned the same relative to the BB and it simply boils down to more seatpost showing on one bike, vs. the other.

The reason I am suggesting that you ignore published setback values is because in the case of Colnago, the seat tube angles change between some sizes and because they are calculated with different seat tube lengths which do not contribute any effect on reach (my example above.) The picture gets confused by that fact and the example you are giving just happens to be exactly where they change.

I think it's easier to just consider the seat tube angle impact on the position of the saddle.

Let's say you're riding a 56 and you're used to the leg position. Now a good 57 frame comes along. To make that 57 fit the same way as your 56 you will have to move the saddle forward by .5CM. The result would be .5cm less reach, effectively reducing the TTL by that amount. If you were happy with the reach on your 56, that's good, because that .5CM forward reduces the longer 57 TTL from 55.6 to 55.1. Close enough.

Going the other way, let's say you're riding a 57 and a nice 56 comes along. To maintain your leg positon on the 56, you have to move the saddle back by .5CM which is good, because you increase the TTL from 55.0 to 55.6. Again, close enough.

Same stem on both bikes, some adjustments needed to maintain bar height due to different head tube lengths.

Last edited by terry b; 11-15-07 at 04:51 PM.
terry b is offline  
Old 11-15-07, 05:51 PM
  #6  
HJR
Senior Member
Thread Starter
 
Join Date: Nov 2003
Location: Pasadena, CA
Posts: 241

Bikes: Cervelo Soloist Team, Cervelo SuperProdigy, Colnago C50

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by terry b
I think that Colnago approaches the problem from the assumption that you're going to buy the correct bike for your height.

I own two 57 Colnagos as well as one 58. The reach on them is about .7cm different, and that's driven entirely by the increased TTL. Reason - the saddle is in precisely the same place horizontally and in relation to the BB. The setback between those two frames is different only because the calculation is done with the longer seat tube on the 58. Because the seat tube angle is the same, the saddle is positioned the same relative to the BB and it simply boils down to more seatpost showing on one bike, vs. the other.

The reason I am suggesting that you ignore published setback values is because in the case of Colnago, the seat tube angles change between some sizes and because they are calculated with different seat tube lengths which do not contribute any effect on reach (my example above.) The picture gets confused by that fact and the example you are giving just happens to be exactly where they change.

I think it's easier to just consider the seat tube angle impact on the position of the saddle.

Let's say you're riding a 56 and you're used to the leg position. Now a good 57 frame comes along. To make that 57 fit the same way as your 56 you will have to move the saddle forward by .5CM. The result would be .5cm less reach, effectively reducing the TTL by that amount. If you were happy with the reach on your 56, that's good, because that .5CM forward reduces the longer 57 TTL from 55.6 to 55.1. Close enough.

Going the other way, let's say you're riding a 57 and a nice 56 comes along. To maintain your leg positon on the 56, you have to move the saddle back by .5CM which is good, because you increase the TTL from 55.0 to 55.6. Again, close enough.

Same stem on both bikes, some adjustments needed to maintain bar height due to different head tube lengths.
That makes sense. So if the angles are the same then TT dimension should rule the day because the other points are fixed.

But that would then make a case for more manufacturers printing that measurement, because seat angles vary between manufacturers. The only way to know the relationship to the BB is to provide a number. Or for someone to break out with their sine/cosine calculations.
HJR is offline  
Old 11-15-07, 06:04 PM
  #7  
Erstwhile Trogon
 
terry b's Avatar
 
Join Date: Mar 2006
Posts: 1,032
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by HJR
That makes sense. So if the angles are the same then TT dimension should rule the day because the other points are fixed.
Exactly!

Originally Posted by HJR
But that would then make a case for more manufacturers printing that measurement, because seat angles vary between manufacturers. The only way to know the relationship to the BB is to provide a number. Or for someone to break out with their sine/cosine calculations.
The best case for using setback is that it indicates a change in seat tube angle. If the MFGR is not going to provide STA, then yes, they ought to publish setback. One or the other is necessary to make a good decision about fit. You can back calculate STA if you have the setback and the STL. But if you get STA, it's really the easiest way to approach the differences between different design styles.

The rule of thumb is really pretty simple, for each degree of angle, you need to move the saddle about 1CM forward or backward. An easy rule of thumb to remember. But I also keep an excel spreadsheet with a table for calculating it, just for grins.
terry b is offline  
Old 11-15-07, 06:12 PM
  #8  
Senior Member
 
Join Date: Apr 2005
Location: West Palm Beach, FL
Posts: 492

Bikes: Giant OCR1

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Any chances of passing along the spreadsheet? I love Excel.
pharmboyrx is offline  
Old 11-15-07, 06:57 PM
  #9  
Erstwhile Trogon
 
terry b's Avatar
 
Join Date: Mar 2006
Posts: 1,032
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Here you go -

Put the letters "STA" in cell A1
Put the letters "STL" in cell A2
Put the word "Setback" in cell A3
Enter this formula in cell B3: =COS(B1*PI()/180)*B2

Enter your bike's Seat tube angle (integer with decimal fraction like 73.5 instead of 73:30) into cell B1
Enter your bike's Seat tube length (in centimeters) into cell B2
Setback will appear in centimeters in cell B3
terry b is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.