Builder's Spotlight The Paceline Forum Builder's Spotlight


Go Back   The Paceline Forum > Bike Fit

Reply
 
Thread Tools Display Modes
  #1  
Old 01-02-2020, 08:23 PM
velobaba velobaba is offline
Senior Member
 
Join Date: Jun 2012
Posts: 232
Educate me on Stack height vs saddle to bar drop measurement

Let's say I have two almost identical bikes: Bike A and Bike B.
Bike A has a BB drop of 80
Bike B has a BB drop of 70

If everything else is identical on both the bikes then in order to achieve the same saddle to bar drop measurement I'll have to use about 10mm of spacers on Bike B to achieve the same saddle to bar drop that is on Bike A. Correct?

But in reality I am able to achieve the same saddle to bar drop for Bike B WITHOUT the 10mm of spacers. Why? I know for sure that the BB drop measurements are correct on both the bikes (Bike A: Serotta Fierte, Bike B: custom Seven).

Maybe I am confusing the stack height with bar height?

Thanks,
Jalal
Reply With Quote
  #2  
Old 01-02-2020, 09:51 PM
Patb095 Patb095 is offline
Member
 
Join Date: Nov 2016
Posts: 36
You have 2 “almost” identical bike

Almost by 10mm. Geometry angle might be slightly different?
Reply With Quote
  #3  
Old 01-03-2020, 01:41 PM
BdaGhisallo's Avatar
BdaGhisallo BdaGhisallo is offline
Senior Member
 
Join Date: Jan 2004
Location: Bermuda
Posts: 2,045
Is the seat tube length the same between the two frames? If it is, then the frame with the BB drop of 80mm should have a 10mm shorter headtube to keep the contact points the same relative to each other, assuming the same stem and HS spacer spec and seatpost extension.

Picture it as the top tube moving closer to the ground (or the wheel axles) by the size of the difference in BB drop between the two frames.
__________________
"Progress is made by lazy men looking for easier ways to do things." - Robert Heinlein
Reply With Quote
  #4  
Old 01-09-2020, 09:52 AM
Dave Dave is offline
Senior Member
 
Join Date: Nov 2004
Location: Loveland, CO
Posts: 3,693
Fork length may be different. The new method of posting stack and reach for new frames eliminates the need to consider BB drop or fork length.

When stack and reach values are available, comparing two frames with different stack heights still needs a correction before reach can be compared. If one frame has a 20mm lower stack height, subtract 6mm from the reach, assuming that 20mm of spacer would be used to create the same stack on both frames.

This is the reason that reach values may not change much between sizes - they are measured at different heights.

Last edited by Dave; 02-17-2020 at 10:10 AM.
Reply With Quote
  #5  
Old 01-10-2020, 02:27 PM
sparky33's Avatar
sparky33 sparky33 is offline
Senior Member
 
Join Date: Feb 2013
Location: Wellesley, MA
Posts: 2,738
bar height is different from stack height.

stack height is the vertical distance between the center of the bb and the top of the headtube (usually).

in your comparison, the height of the bars relative to the top of the headtube can vary based on height of the upper headset cup, stem angle, stem length, spacers and, almost insignificantly, head tube angle.

so if you have two bikes with the same stack height, where stack height is measured at the top of the head tube, and you control all of the above factors too, then you would have have matching bar drop measurements.

This assumes saddle height relative to the bb is held constant also.

Quote:
Originally Posted by velobaba View Post
Let's say I have two almost identical bikes: Bike A and Bike B.
Bike A has a BB drop of 80
Bike B has a BB drop of 70

If everything else is identical on both the bikes then in order to achieve the same saddle to bar drop measurement I'll have to use about 10mm of spacers on Bike B to achieve the same saddle to bar drop that is on Bike A. Correct?

But in reality I am able to achieve the same saddle to bar drop for Bike B WITHOUT the 10mm of spacers. Why? I know for sure that the BB drop measurements are correct on both the bikes (Bike A: Serotta Fierte, Bike B: custom Seven).

Maybe I am confusing the stack height with bar height?

Thanks,
Jalal
__________________
Steve Park

Instagram
Reply With Quote
  #6  
Old 01-10-2020, 02:32 PM
velobaba velobaba is offline
Senior Member
 
Join Date: Jun 2012
Posts: 232
Yep I had the stack height confused with the saddle to bar drop. Thanks for all the explanation. It really helped.

Thanks,
Jalal
Reply With Quote
Reply

Thread Tools
Display Modes

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

Forum Jump


All times are GMT -5. The time now is 11:10 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.