Frame Angles for Everyone!
#1
What??? Only 2 wheels?
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Bikes: 72 Peugeot UO-8, 82 Peugeot TH8, 87 Bianchi Brava, 76? Masi Grand Criterium, 74 Motobecane Champion Team, 86 & 77 Gazelle champion mondial, 81? Grandis, 82? Tommasini, 83 Peugeot PF10
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Frame Angles for Everyone!
Here is a Frame Angle Calculator program for everyone. Enjoy.
A month or two ago someone asked about how to measure angles on a newly-acquired vintage frame. One person's solution was to use a digital level. But I figured, what the hey, it's a matter of geometry, right? With a few key measurements the angles can be calculated.
Or course the measurements have to be accurate.
Anyway, I threw together this little Windows program to do it all for you. You take five key measurements on the frame, plug them in, and click the magic button.
The program can be obtained (in a zip file) from here. Just download it and extract. It is self-contained.
https://www.theworld.com/~muller/Bike...Calculator.zip
I tried it on my bikes and learned a few things. For one thing, the Gran Sport has a steeper seat tube angle than I expected...
A month or two ago someone asked about how to measure angles on a newly-acquired vintage frame. One person's solution was to use a digital level. But I figured, what the hey, it's a matter of geometry, right? With a few key measurements the angles can be calculated.
Or course the measurements have to be accurate.
Anyway, I threw together this little Windows program to do it all for you. You take five key measurements on the frame, plug them in, and click the magic button.
The program can be obtained (in a zip file) from here. Just download it and extract. It is self-contained.
https://www.theworld.com/~muller/Bike...Calculator.zip
I tried it on my bikes and learned a few things. For one thing, the Gran Sport has a steeper seat tube angle than I expected...
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Real cyclists use toe clips.
With great bikes comes great responsibility.
jimmuller
Real cyclists use toe clips.
With great bikes comes great responsibility.
jimmuller
#2
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The mis-measure of bikes is amazing, trash in, trash out, but yes, it's all in the geometry. That and assuming the top tube is level to the ground is a big mistake.
#4
car dodger
I didn't try it yet, but it looks like if you have a level to insure the top tube is level, you can trial-and-error your runs until the headtube solution matches your measurement.
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1989 Schwinn Paramount OS
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1989 Schwinn Paramount OS
1980 Mclean/Silk Hope Sport Touring
1983 Bianchi pista
1976 Fuji Feather track
1979 raleigh track
"I've consulted my sources and I'm pretty sure your derailleur does not exist"
#5
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As I do not use a Windows platform, I will not try it out. The the only known level line for a bike with wheels and tires of the same size is the line described by axle centers.
I have measured many of my bikes, and only about 20% have a level top tube. Sometimes the difference is less than 3 mm, other times it is as much as 9 mm from bikes in the middle size range where one would assume have a level top tube. Almost all that are off rise to the head tube. I place the rear wheel all the way forward in a bike with horizontal dropouts exclusively. Maybe one day I will buy a bike with rod and pinion shifting.
The geometry is straightforward, the measuring is the big effort.
I have measured many of my bikes, and only about 20% have a level top tube. Sometimes the difference is less than 3 mm, other times it is as much as 9 mm from bikes in the middle size range where one would assume have a level top tube. Almost all that are off rise to the head tube. I place the rear wheel all the way forward in a bike with horizontal dropouts exclusively. Maybe one day I will buy a bike with rod and pinion shifting.
The geometry is straightforward, the measuring is the big effort.
#6
What??? Only 2 wheels?
Thread Starter
Join Date: Apr 2010
Location: Boston-ish, MA
Posts: 13,434
Bikes: 72 Peugeot UO-8, 82 Peugeot TH8, 87 Bianchi Brava, 76? Masi Grand Criterium, 74 Motobecane Champion Team, 86 & 77 Gazelle champion mondial, 81? Grandis, 82? Tommasini, 83 Peugeot PF10
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Is your Top Tube not level? The calculator has a correction for that, but it does require two extra measurements.
I found that accuracy of measurement was critical. The best approach is to lay the bike on its side, eye-ball the nominal points on the lugs where lines running down the center of the tubes would intersect, and mark those points with an erasable marker. Then you can measure precisely between those points.
Seat tube angle is a direct calculation via the Law Of Cosines with TT length, ST length, and Diagonal 1. Because it is a large triangle the result should be pretty accurate.
Head tube angle is tougher. I figured measuring the HT length wouldn't be precise enough so I did the trig to use the computed ST angle and Diagonal 2. But it may be just as imprecise that way too. I should work out the imprecision some time. Maybe release Version 1.1!
Suggestions are welcome.
Incidentally, the default values it uses are for the Masi I'm rebuilding. I wasn't especially careful though.
I found that accuracy of measurement was critical. The best approach is to lay the bike on its side, eye-ball the nominal points on the lugs where lines running down the center of the tubes would intersect, and mark those points with an erasable marker. Then you can measure precisely between those points.
Seat tube angle is a direct calculation via the Law Of Cosines with TT length, ST length, and Diagonal 1. Because it is a large triangle the result should be pretty accurate.
Head tube angle is tougher. I figured measuring the HT length wouldn't be precise enough so I did the trig to use the computed ST angle and Diagonal 2. But it may be just as imprecise that way too. I should work out the imprecision some time. Maybe release Version 1.1!
Suggestions are welcome.
Incidentally, the default values it uses are for the Masi I'm rebuilding. I wasn't especially careful though.
__________________
Real cyclists use toe clips.
With great bikes comes great responsibility.
jimmuller
Real cyclists use toe clips.
With great bikes comes great responsibility.
jimmuller
#7
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The extensions beyond the tube intersection centers to the ends of the head tube are not always the same. Not even for lugs from the same supplier throughout their style family. Measuring the tube intersection centers is not that easy. To get data points to match what a CAD program comes up with has always required me to pull off the seat and head angles in relation to the axle centers.
Might make sense to measure a vertical line from the bottom bracket center to the center of the top tube and from there extend to the top tube intersections or offset down a distance below those joints to get a clean dimension.
Might make sense to measure a vertical line from the bottom bracket center to the center of the top tube and from there extend to the top tube intersections or offset down a distance below those joints to get a clean dimension.
Last edited by repechage; 01-28-12 at 07:24 PM.