Measuring ST and HT angles?
 

Hi!

Just wonder if there is an accurate way of measuring Seat-Tube and Head-Tube angles on a built up bike?

I'm not an engineer and don't even have a plastic school protractor, but can always buy one. I do understood basic geometry, Pi, Pythagora's formula and so forth. A plumbline can easily be made... but I'm not sure if I'm thinking in the right direction or if there's a simpler method?

It's an old bike and I've searched in vain for it's geometry.

One thought I had was to draw lines on a photo in Illustrator and take measurements from that, but am uncertain as to how accurate that would be.

Thanks!

I would take a straight side on pic of the bike using a med focal length lens and take measurements off of that (with a protractor) using the straight line running between the front and real axles as the base line.

I don't know if something like this: http://www.sears.com/shc/s/p_10153_1...d=angle+finder is easily available to you in Sweden. Maybe you could find a comparable product there.

Think I just cracked it... A picture in Illustrator aligned horizontally... Then just draw a straight line on a new layer, rotate it in different degrees (72, 73, 74...) and move it across the Tubes until one fits dead centre :)

I'll let you know if I get a realistic result...

What bike is it. If it is a factory built bike you could probably find the information online. If not id suggest the picture way. Iphones have an app for it as well ;-) but im not sure if it is accurate.

Those angle finders are not the most accurate (+/- 1.5 degrees or so) but it will get you in the ball park. I like the idea of taking a photo and then blowing it up and using a protractor.

It's a Miyata 100 (not the 1000 for which there are online geometry charts)

Done deal! 72 x 72 :)

Thanks guys!

Another tool that will do it but runs around $40 though deals can be found, is a Digital Angle Gauge like this one http://www.wixey.com/anglegauge/index.html. Measures accurately to 0.1 degree. If you have a woodworker friend who may have one you might be able to use it for your measurements. I have one which I use as pictured on the link above to set my table saw angle and other such things where angle 'setting' or 'knowing' are important.

I've done the picture method and it works fine. If you have a flat top tube (non sloping) you can measure right from the top tube.

For the picture method to work the camera needs to be set as close to perpendicular to the frame as possible and then set at quite a distance and the zoom used to get well into the telephoto end of the zoom range. That'll flatten the perspective and make the tubes actually look straight. Something that won't happen at a wide or near wide angle setting. If this angle is off by much then the tubes won't be seen from dead on the side and the angles will alter based on the vanishing point lines. All in all you may find that you're getting numbers from the pictures but the picture may well not be as accurate as you think it is. Then there's our ability to judge the center of the tubes by eye to set the line over it at the correct angle. All in all by the time you introduce these sources of error you're just as well off with a hardware store angle guage or a cheap school protractor and a small plumb bob. The only truly accurate gizmo I'm seeing so far is the digital angle finder. Turn it on, zero it to the ground under the bike, assuming it's decently flat, and then take the angles of the tubes. Everything else mentioned here would do no better than + or - one degree. And the photo method could be way out depending on how well the camera is lined up to the bike and how well the bike is lined up to the camera. For example if you just lean the bike against a wall then the angles are going to be off by some amount. Unless that is compensated for by raising the camera so that it is a dead on 90 degrees in both axis then you may as well get one of the super cheap angle finders since it'll be easily as accurate or more so.

Using a digital angle finder I found that my tubes are not really straight, so it matters where you place the little Wixey angle finder.

The most accurate method I've found overall is by Scooper. He lays the stripped-down frame on a big piece of paper and transfers the tube positions near the ends to the paper. Then he measures between the two edges of the tube trace at all the joints to define the center of the tube at each joint. Draw a straight line between the center points for each tube and you will have a model of the frame tube axial directions. Measure those angles with a large protractor, to minimize error. Should be good and accurate to 1/2 the distance between the smallest graduations on the protractor, perhaps better if you are skilled at visual interpolation (like I learned when they taught us how to use slide rules!). Obviously you need to draw the marks that define the tube locations near the lugs very carefully.

Use Search to find posts by Scooper about frame angles. Either Frame Forum, C&V, or maybe Long Distance.