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Confused by Geometry

Old 06-27-17, 10:16 PM
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Confused by Geometry

Hi all,
Have been considering a new endurance or light gravel bike for a while. I took my old Fuji World 2004 bike to my physio who did a bike fit for me so I could use his info to inform my future purchase. I have osteoarthritis in my hips and also a little in my thumbs, though neither are a big issue at the moment but are driving me to be more cautious in choosing the right geometry. My physio gasped when he saw me on the bike and straight away said I would need a setup that was about 50mm or 2 inches shorter!!!

I am about 5'8" and the Fuji is a size 54 (seat tube, or 56 top tube). I was a little gobsmacked at first as I'd looked at the geometry of a number of bikes and thought, whilst the next size down on any bike might be better for me, it's not going to reduce the reach by 50mm (OK could use a shorter stem, but still)! Later I had a go at measuring the reach on my Fuji bike. I worked it out at about 420mm. Not 100% accurate, but only 5mm out at most. That got me thinking, has bike geometry changed in recent years where reach has gotten shorter? The current Fuji Touring bike in that size only has a reach of 374mm. It seems racing bikes in that size only have a reach of about 390mm!
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Old 06-27-17, 11:17 PM
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You may have a problem with your measurement of 420. The reach on my 61c endurance frame is 410...
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Old 06-28-17, 03:24 AM
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For you: https://www.competitivecyclist.com/S...latorBike.jsp?

For your bike: "Reach" usually refers to a very specific measurement---which I might not understand or describe properly. I believe that it you draw a horizontal like from the top center of the head tube (the almost vertical tube with the fork sticking out the bottom and the stem/bars on top)and a vertical line rising up from the center of the bottom bracket (where the pedals attach) the length from the center of the top of the head tube along that horizontal line to the intersection with the vertical line is "reach."

The stem is not included.

Best way to measure it might involve something like a light piece of lumber or something, which you can stand up from the bottom bracket and tie to the frame Make sure it is perpendicular to the ground ... take another piece, or a tape measure, and run it from the center bolt on top of the stem to the upright board, and use the protractor or triangle to ensure another right angle.

I have a bunch of different sized frames and even on my 61 cm the reach is under 400 mm. For my two 56s the reach is 385-395. I don't see an accurate reach of 420 mm on qa 56-cm frame (nominal 56-cm top tube.) Make sure you measure vertically from the BB and horizontally-Not along the possibly sloping top tube--to the center of the stem-bolt at the head-tube end, not the bar end.

I have a bunch of fit diagrams and could attach one but I am late for work already. Will try to get back later.

Last edited by Maelochs; 06-28-17 at 03:53 AM.
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Old 06-28-17, 04:44 AM
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You don't need to do all that with boards and protractors, etc. You just put the bike upright with the inflated rear wheel touching and perpendicular to a wall. Then measure all distances from the wall and the floor. So reach is the difference between the distance from the wall to the bottom bracket and the distance from the wall to the top center of the head tube. And so on. Stack is the difference between the height of the bottom bracket center and the height of the top center of the head tube. Easy.
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Old 06-28-17, 04:49 AM
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Maybe OP is confusing frame stack and reach with finished bike drop and reach. Whenever we talk stack and reach, we mean of the frame. But you could define a bike drop and reach that takes into account saddle placement, stem length and angle, bar design, spacers, etc. That would be personal to the rider No matter what frame is used, the finished drop from saddle to bars and reach from saddle to bars should be the same for the same style of riding.
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Old 06-28-17, 07:09 AM
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Originally Posted by Maelochs
For you: https://www.competitivecyclist.com/S...latorBike.jsp?

For your bike: "Reach" usually refers to a very specific measurement---which I might not understand or describe properly. I believe that it you draw a horizontal like from the top center of the head tube (the almost vertical tube with the fork sticking out the bottom and the stem/bars on top)and a vertical line rising up from the center of the bottom bracket (where the pedals attach) the length from the center of the top of the head tube along that horizontal line to the intersection with the vertical line is "reach."

The stem is not included.

Best way to measure it might involve something like a light piece of lumber or something, which you can stand up from the bottom bracket and tie to the frame Make sure it is perpendicular to the ground ... take another piece, or a tape measure, and run it from the center bolt on top of the stem to the upright board, and use the protractor or triangle to ensure another right angle.

I have a bunch of different sized frames and even on my 61 cm the reach is under 400 mm. For my two 56s the reach is 385-395. I don't see an accurate reach of 420 mm on qa 56-cm frame (nominal 56-cm top tube.) Make sure you measure vertically from the BB and horizontally-Not along the possibly sloping top tube--to the center of the stem-bolt at the head-tube end, not the bar end.

I have a bunch of fit diagrams and could attach one but I am late for work already. Will try to get back later.
I actually measured it by using a long spirit level lined up on one edge to the centre of the bottom bracket and ran a tape across to the head tube at the location where the steerer emerges (well, the spacers at least)
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Old 06-28-17, 07:12 AM
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Originally Posted by rpenmanparker
You don't need to do all that with boards and protractors, etc. You just put the bike upright with the inflated rear wheel touching and perpendicular to a wall. Then measure all distances from the wall and the floor. So reach is the difference between the distance from the wall to the bottom bracket and the distance from the wall to the top center of the head tube. And so on. Stack is the difference between the height of the bottom bracket center and the height of the top center of the head tube. Easy.
Thanks for the suggestion. I'll try that tomorrow as a check of what I've done.
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Old 06-28-17, 07:13 AM
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Originally Posted by rpenmanparker
Maybe OP is confusing frame stack and reach with finished bike drop and reach. Whenever we talk stack and reach, we mean of the frame. But you could define a bike drop and reach that takes into account saddle placement, stem length and angle, bar design, spacers, etc. That would be personal to the rider No matter what frame is used, the finished drop from saddle to bars and reach from saddle to bars should be the same for the same style of riding.
No, the saddle was in no way involved
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Old 06-28-17, 07:15 AM
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Originally Posted by Nomad2
I actually measured it by using a long spirit level lined up on one edge to the centre of the bottom bracket and ran a tape across to the head tube at the location where the steerer emerges (well, the spacers at least)
Try the trick of using the wall and floor as reference points for the BB and head tube top center. It makes the measurement very easy. Also that same method can be used for transferring fit from one bike to another. Just prop up the bike perpendicular to the wall and floor with the inflated rear tire touching the wall. Make sure the front wheel is straight. You can use an indoor trainer stand to hold up the bike. As long as you have a shim for the front wheel to make the bike level, all the measurements will cancel out the height of the bike off the floor so you don't have to worry about that.

Sorry I didn't see you acknowledgement of my other post. But the trainer stand is a good way to do it.
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Old 06-28-17, 08:18 AM
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Maybe your physio rides a comfort hybrid.

Consider getting a 2nd opinion.
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Old 06-28-17, 05:09 PM
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Originally Posted by Nomad2
Thanks for the suggestion. I'll try that tomorrow as a check of what I've done.
Well OK, so i wasn't so accurate after all. Taking the bottom bracket and head tube measurements from a wall, the difference was 405mm.
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Old 06-29-17, 05:50 AM
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Originally Posted by Nomad2
Well OK, so i wasn't so accurate after all. Taking the bottom bracket and head tube measurements from a wall, the difference was 405mm.
Just in case it could be helpful, here is the full method that I had written up earlier. The emphasis here is for the three contact points of the finished bike, but the method can be supplemented for stack and reach measurement by adding in the distances vertically and horizontally from the BB to the top center of the head tube. I realize you have already done this, but others may like to see the full writeup. I was away from the computer before and didn't have a copy of the Word file to paste in here:

The way these measurement are done is not in relation to each other, but in relation to a fixed point in space. It is much easier that way as you don't need to use a level, plumb bob, or other tool to make sure you are not making a mistake. Here is how it is done:

You first inflate your tires to normal pressure. Then using supports or a trainer stand or an assistant place the bike with its rear wheel against a wall and perpendicular to it. The bike must be perfectly upright and not slanted to either side.

Then you make six measurements:
A) Wall to center of bottom bracket horizontally.
B) Wall to front tip of saddle horizontally.
C) Wall to back edge of handlebar center horizontally.
D) Floor to center of bottom bracket vertically.
E) Floor to top front of saddle vertically.
F) Floor to top edge of handlebar center vertically.

Now you have everything you need to relate the three contact points on the bike to each other. A-B gives you the saddle setback relative to the bottom bracket. E-D give you the height of the saddle from the bottom bracket. C-B gives you the distance horizontal distance from the saddle to the handlebars. E-F gives you the drop from saddle to bars.

Done and done.
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Old 06-29-17, 08:06 AM
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I did okay with geometry but trig kicked my butt right out of the math program.

Originally Posted by rpenmanparker
Just in case it could be helpful, here is the full method that I had written up earlier.
That si excellent and all, but the issue is making sure the measurements are taking perfectly horizontally and vertically. That is my I mess with boards, and a giant metal right angle (18x12 inches or so) and all that other stuff.

A plum bob (weight on a rope) is the best ... well, simplest---way to determine the vertical ... hold the rope at the top of the ruler and it will always hand perpendicular tot eh center of the Earth. Piece of string and a washer works fine.

Finding the horizontal is important and more complicated ... but a simple spirit -level (bubble level, carpenter's level) works fine.

Not everyone cars if the measurements are half an inch or an inch off I suppose.

Not saying this way is not good or any other way is better. I know that I made some faulty measurements because what looked vertical to my crooked eye was actually not vertical to the rest of the world, and thus every measurement or calculation made off that measurement was wrong and had to be redone.
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Old 06-29-17, 06:13 PM
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Originally Posted by Maelochs
I did okay with geometry but trig kicked my butt right out of the math program.

That si excellent and all, but the issue is making sure the measurements are taking perfectly horizontally and vertically. That is my I mess with boards, and a giant metal right angle (18x12 inches or so) and all that other stuff.

A plum bob (weight on a rope) is the best ... well, simplest---way to determine the vertical ... hold the rope at the top of the ruler and it will always hand perpendicular tot eh center of the Earth. Piece of string and a washer works fine.

Finding the horizontal is important and more complicated ... but a simple spirit -level (bubble level, carpenter's level) works fine.

Not everyone cars if the measurements are half an inch or an inch off I suppose.

Not saying this way is not good or any other way is better. I know that I made some faulty measurements because what looked vertical to my crooked eye was actually not vertical to the rest of the world, and thus every measurement or calculation made off that measurement was wrong and had to be redone.
If the tape measure or meter stick isn't perfectly vertical or horizontal, the errors will be pretty small. But using the wall and floor make it very easy to measure with a large T-square relying on the wall and floor to be vertical and horizontal, respectively. Or a square taped to a meter stick.
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Old 06-29-17, 07:50 PM
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Originally Posted by rpenmanparker
If the tape measure or meter stick isn't perfectly vertical or horizontal, the errors will be pretty small. But using the wall and floor make it very easy to measure with a large T-square relying on the wall and floor to be vertical and horizontal, respectively. Or a square taped to a meter stick.
Really? Sounds a lot like
Originally Posted by Maelochs
Best way to measure it might involve something like a light piece of lumber or something, which you can stand up from the bottom bracket and tie to the frame Make sure it is perpendicular to the ground ... take another piece, or a tape measure, and run it from the center bolt on top of the stem to the upright board, and use the protractor or triangle to ensure another right angle.


Whatever. I will keep your system handy and try it. Until i have tested it I cannot say if it is always, sometimes, or never better than some other way. It never hurts to have more tools and methods available.
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Old 06-29-17, 10:25 PM
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Originally Posted by Maelochs
I did okay with geometry but trig kicked my butt right out of the math program.
It's about time.

Thought I was gonna hafta get all dad joke in here.
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Old 06-30-17, 07:10 AM
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Originally Posted by Maelochs
Really? Sounds a lot like

Whatever. I will keep your system handy and try it. Until i have tested it I cannot say if it is always, sometimes, or never better than some other way. It never hurts to have more tools and methods available.
Yes, my comment does support yours. I personally don't use a square, but was simply observing that it couldn't hurt. You should be able to eyeball it within 5. At such low angular errors from horizontal and vertical that you might make, the hypotenuse of the very acute right triangle you define is hardly different from the adjacent side that you actually want to measure. Much less than 1%.
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Old 07-01-17, 11:44 AM
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Originally Posted by Nomad2
Hi all,
Have been considering a new endurance or light gravel bike for a while. I took my old Fuji World 2004 bike to my physio who did a bike fit for me so I could use his info to inform my future purchase. I have osteoarthritis in my hips and also a little in my thumbs, though neither are a big issue at the moment but are driving me to be more cautious in choosing the right geometry. My physio gasped when he saw me on the bike and straight away said I would need a setup that was about 50mm or 2 inches shorter!!!

I am about 5'8" and the Fuji is a size 54 (seat tube, or 56 top tube). I was a little gobsmacked at first as I'd looked at the geometry of a number of bikes and thought, whilst the next size down on any bike might be better for me, it's not going to reduce the reach by 50mm (OK could use a shorter stem, but still)! Later I had a go at measuring the reach on my Fuji bike. I worked it out at about 420mm. Not 100% accurate, but only 5mm out at most. That got me thinking, has bike geometry changed in recent years where reach has gotten shorter? The current Fuji Touring bike in that size only has a reach of 374mm. It seems racing bikes in that size only have a reach of about 390mm!
I'm 5'7" - I ride a 54 cm bike and the endurance geometry fits like a glove. Of course the head tube is taller and the angles are slacker and the wheelbase is extended compared to a traditional road bike but that's because the ride is aimed at comfort rather than competition.
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