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    Professional Fuss-Budget Bacciagalupe's Avatar
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    Figuring out an older bike's geometry?

    Hopefully, this should be an easy one for the local experts. Basically, I picked up an old steel road bike. The fit and geometry work pretty well, but a lot of components need updating. I'm thinking I can keep it as a beater / backup bike until it falls apart.

    What I'd like to do is figure out the geometry of it, in order to compare to other bikes. Any idea on how to properly figure this out? Most specs are pretty easy (e.g. chainstay, wheelbase). The main things I'm stuck on are seat tube angle, head tube angle, and fork specs like rake / trail.

  2. #2
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    plumb-bob and a large protractor?
    Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
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    If you have a digital camera, take a picture of your bike from the side. Then import the picture into a CAD program and take measurements.

    I was wondering about my Miyata 210 and found an old catalog online with a very good side view, so I saved it and imported it into Visio. I put the endpoints of a line at the axles and checked what angle the line was at, then rotated the picture so that the line between the axles were exactly 90 degrees from vertical. I measured the diameter of the tire on the picture and compared it to the actual tire diameter to get a scaling factor so I could take lengths from the picture and convert to real lengths. Then I put a line going down the center of the headtube and found that it was at 73 degrees. I extended that line to the ground, dropped a vertical line from the front axle, and drew a horizontal line between the bottoms of the tires and was able to measure my trail. I also drew a line 90 degrees from the headtube line that went through the front axle and measured that to get the fork rake. I measured the seattuibe angle the same way as the headtube angle.

    Here's the picture:
    Keith Hearn

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    You Know!? For Kids! jsharr's Avatar
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    aka Tom Reingold noglider's Avatar
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    I don't understand your goal well. Why do you think components need replacement? Maybe they don't. Old components often do the job just fine.

    Also, the thing you need to know about the geometry is really whether the bike suits its purpose. If it's comfortable and you like the way it rides, the geometry is good, whether you know its numeric values or not.

    Lately, I'm riding a 38-year-old steel bike. The geometry would probably make people cringe. I even had doubts as I renovated it. But I find I'm impressed with the way it rides. The angles are probably lax, and the fork rake is huge. I thought it would handle like a dog, but it doesn't. It also climbs well. The brakes are ultra-long reach, so I thought they wouldn't work well, but they do. What a pleasant surprise this bike has been! For what it's worth, it's a 1971 Raleigh Super Course.

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    Quote Originally Posted by noglider View Post
    I The angles are probably lax, and the fork rake is huge.
    These two factors compensate for each other. In general a slack headtube angle gives more trail and slows down handling but a lot of fork rake reduces trail and the end result can be quite satisfactory.

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    Professional Fuss-Budget Bacciagalupe's Avatar
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    Ok, I'm trying the protractor approach first, and coming up with:

    73 head tube angle
    74 seat tube angle
    50mm fork rake

    Calculations indicate the bike has a trail of 51, though it doesn't feel particularly twitchy. Any of this sound typical for an 80s road bike?


    noglider: I haven't ruled out replacements and upgrades, but: It was an entry-level bike with crap components. Eventually, the plan is to go from downtube to STI shifters; standard to compact cranks; standard handlebars to ergo; and steel frame to CF. It wouldn't hurt to have rims that can use folding tires, too. Even if I didn't want to change the frame material, that's a pretty hefty set of upgrades to a bike that didn't cost much to begin with.

    Ergo, replacement is at least a rational option here. Plus, if something happens (e.g. bike gets stolen, frame breaks etc) I'd like to have that info at the ready.

  8. #8
    Quirky Grifter LesterOfPuppets's Avatar
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    A $12 magnetic angler finder helps, if you like gadgets

    Probably too big to figure out the head angle without taking off the fork and headset.

    They make fancier ones if you'd like to get more precise.
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    Decrepit Member Scooper's Avatar
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    khearn has the right idea, but to measure accurately I take everything off of the the frame except the headset and fork, and lay the frame on a sheet of butcher paper, taking care to ensure the fork is positioned so it's facing directly forward. The idea is to transfer key points from the frame onto two dimensional paper. Admittedly, this approach requires significant effort, but it's quite accurate.

    I start out marking the centers of the fork dropouts and rear dropouts, and drawing a straight line between these two points. This is the wheelbase.

    Next, I mark the center of the bottom bracket shell, and draw a line at a right angle to the wheelbase through the BB shell center. The length of that line from the wheelbase to the BB shell center is the BB drop.

    Next, I mark the intersections of the centerlines of the seat tube to the top tube, the top tube to the head tube, and the down tube to the head tube. I also mark the intersections of the seat stay centerline to the seat tube centerline, the chainstay centerline to the center of the BB shell, the down tube centerline to the center of the BB shell, and the seat tube centerline to the center of the BB shell.

    Finally, I mark the top and bottom of the head tube (the points where the headset cups fit), and the top of the seat tube.

    After marking these points, remove the frame from the paper and draw in the tubing centerlines with a straightedge. Extend the centerline of the head tube down to the wheelbase. For geometry, you really only need the centerlines.

    Using a goniometer (a sort of large variable compass that's inexpensive and available at drafting equipment stores), you can measure the seat tube angles and head tube angles relative to the wheelbase, as well as center-to-center dimensions for all the tubing intersections.

    For fork rake, draw a line from the extended head tube centerline and at a right angle to it, through the fork dropouts. The length of that line is the fork rake.

    Here's an example. In this case, the top tube was exactly parallel to the wheelbase, and the goniometer shows a 74.5 HTA.





    - Stan

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    I suggest taking a picture of your bike, as level as possible, then importing it into google Sketchup, a free cad program. Set it to 2d mode then you can draw lines and measure between them. Its pretty much like doing it on paper as suggested above but quicker (if you can figure out the program)

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    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by nitropowered View Post
    I suggest taking a picture of your bike, as level as possible, then importing it into google Sketchup, a free cad program. Set it to 2d mode then you can draw lines and measure between them. Its pretty much like doing it on paper as suggested above but quicker (if you can figure out the program)
    This will work as long as the bike is photographed directly from the side (camera lens pointed 90 to the longitudinal axis of the frame) and from as far away from the bike as possible using a long telephoto lens to minimize distortion from parallax.
    - Stan

  12. #12
    Light Makes Right GV27's Avatar
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    Man you can get a little plastic protractor/level from the hardware store for like $5

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    Quote Originally Posted by Scooper View Post
    This will work as long as the bike is photographed directly from the side (camera lens pointed 90 to the longitudinal axis of the frame) and from as far away from the bike as possible using a long telephoto lens to minimize distortion from parallax.
    we're not doing brain surgery here. and you all know that bikes aren't perfect out of the geometry spec sheets either. theres a ~5% tolerance in the bike industry.

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    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by Bacciagalupe View Post
    Hopefully, this should be an easy one for the local experts. Basically, I picked up an old steel road bike. The fit and geometry work pretty well, but a lot of components need updating. I'm thinking I can keep it as a beater / backup bike until it falls apart.

    What I'd like to do is figure out the geometry of it, in order to compare to other bikes. Any idea on how to properly figure this out? Most specs are pretty easy (e.g. chainstay, wheelbase). The main things I'm stuck on are seat tube angle, head tube angle, and fork specs like rake / trail.
    For the head and seat angles:



    For the fork rake, lay the fork of a flat, horizontal surface and measure the distance from the surface to the middle of the dropout slot (the slot should be 9mm diameter). Subtract the distance from the surface to the centerline of the steer tube. That's the rake.

    Plug the numbers into a trail calculator and Bob's yer uncle.

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    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by nitropowered View Post
    we're not doing brain surgery here. and you all know that bikes aren't perfect out of the geometry spec sheets either. theres a ~5% tolerance in the bike industry.
    Let's see... 5% of 73 is 3.6, so a bicycle designed to have a 73 HTA or STA could have them at 69.4 or 76.6 and still be within tolerance?

    Or, a bicycle designed with a 58cm long seat tube or top tube could have a seat tube or top tube that's 55cm or 61cm long and still be in tolerance?

    I'm sorry, but with all due respect, that's ridiculous.
    - Stan

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    Senior Member bigvegan's Avatar
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    "we're not doing brain surgery here."

    No, man, Scooper appears to be doing brain surgery here, or at least the bike equivalent. Did you see post #9 in this thread?

    That's just awesome.

    If you don't mind my asking, what was that for?

  17. #17
    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by bigvegan View Post
    If you don't mind my asking, what was that for?
    A Waterford track bike.
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    Quote Originally Posted by Bacciagalupe View Post
    Ok, I'm trying the protractor approach first, and coming up with:

    73 head tube angle
    74 seat tube angle
    50mm fork rake

    Calculations indicate the bike has a trail of 51, though it doesn't feel particularly twitchy. Any of this sound typical for an 80s road bike?


    noglider: I haven't ruled out replacements and upgrades, but: It was an entry-level bike with crap components. Eventually, the plan is to go from downtube to STI shifters; standard to compact cranks; standard handlebars to ergo; and steel frame to CF. It wouldn't hurt to have rims that can use folding tires, too. Even if I didn't want to change the frame material, that's a pretty hefty set of upgrades to a bike that didn't cost much to begin with.

    Ergo, replacement is at least a rational option here. Plus, if something happens (e.g. bike gets stolen, frame breaks etc) I'd like to have that info at the ready.
    The angles are actually about what criterium bikes had in the 80s.

    As far as measuring the angles, just take a photo of the head tube/top tube intersection and a photo of the seat tube/top tube intersection, make a BW print of it, draw a line directly through the center of the tubes and use a high school protractor to measure the angles. Good for 1/2 degree.
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  19. #19
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    You can measure lengths much more accurately than you can measure angles, so set up your problem using some simple trig to solve for angles.

    I've done this to record the fit on my bike so it can be transferred/adjusted to a custom frame.

    Setting up the bike in a perfectly vertical position over a dead-level, smooth surface, like a lino floor is essential. You will need to use a plumb bob and a good level along with some props to keep the bike upright.

    Enter the data in BikeCAD, to see if there are any errors. Make sure that your triangles close! Do several iterations to improve accuracy. See: http://www.bikeforest.com/CAD/bcad.php

    I have been able to do this accurately enough to determine that I had 1 mm more wear on the rear tire than on the front.
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  20. #20
    Professional Fuss-Budget Bacciagalupe's Avatar
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    Thanks for the input. I will try the photo method next, as a way to compare.

    By the way, exactly how critical are the angle measurements? Is there really a big difference between a head tube angle of 73% and 74% (assuming all other specs are unchanged)?

  21. #21
    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by Bacciagalupe View Post
    Thanks for the input. I will try the photo method next, as a way to compare.

    By the way, exactly how critical are the angle measurements? Is there really a big difference between a head tube angle of 73% and 74% (assuming all other specs are unchanged)?
    Yes. All other things being equal, a head angle one degree steeper (74 instead of 73) will make steering noticeably more sensitive. This can be tiring on a long ride, but a benefit in manuevering quickly.

    Dave Moulton on Head Angles and Steering
    - Stan

  22. #22
    Full Clout Y'all Skylar's Avatar
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    Quote Originally Posted by bigvegan View Post
    "we're not doing brain surgery here."

    No, man, Scooper appears to be doing brain surgery here, or at least the bike equivalent. Did you see post #9 in this thread?

    That's just awesome.
    I too liked picture #1 with the fullframe sketch. Nice.
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  23. #23
    My bikes became Vintage OLDYELLR's Avatar
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    I've used the digital camera and CAD method and it will get you the angles within a degree; certainly better than those little plastic angle gauges. However, a digital angle gauge like this can be had for as little as $40 and is extremely accurate and will work on the head tube of a small frame.
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  24. #24
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by nitropowered View Post
    we're not doing brain surgery here. and you all know that bikes aren't perfect out of the geometry spec sheets either. theres a ~5% tolerance in the bike industry.
    Yea, I've measured actual specs that are quite a bit different than what the manufacturer quoted.

  25. #25
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by OLDYELLR View Post
    I've used the digital camera and CAD method and it will get you the angles within a degree; certainly better than those little plastic angle gauges. However, a digital angle gauge like this can be had for as little as $40 and is extremely accurate and will work on the head tube of a small frame.
    Lately I've been using the Craftsman digital levels/angle guage to measure frames. barring head tubes. When you have 0.1 degree resolution and accuracy is to reference frame tube angles to level - the floor and the frame tube surfaces become part of the problem.

    This little Wixey looks great for head tubes, thanks! For the head tube I've been using the level software available for my Iphone. I zero it against a Fat Max bubble level, then I think I can depend on the 0.1 degree accuracy.

    Another problem I've noticed is that the angle of a tube varies with where I clamp on the angle sensor. The thickness of decals can become a source of noise, as well.

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