View Full Version : How to figure out Angles?

11-23-2011, 02:57 PM
I am looking to purchase a frame and the seller does not know the Fork Rake or HT and ST angles. It is local so I have easy access to the frame.

How can I figure them out accurately at home?

11-23-2011, 03:49 PM
Wixey Digital Angle Finder (http://www.craftsmanstudio.com/html_p/C019003.htm)

It's easier if the TT is horizontal, but it doesn't have to be.

I have one of these, and they work very well. Also, look around, but I think the price at the link above is a very good deal.


11-23-2011, 04:25 PM
I've used the same angle finder and agree it works well. I glued a string bubble level to the top of it as a visual aid when I zero it out. I would say it has decent consistent readings to about 0.5 degrees. Sometimes it hunts a little to much to get a more precise reading.
The best thing about it for bikes(well steel frames anyway) is that it has a magnetic base so you can stick it on the HT or DT.

11-23-2011, 04:36 PM
I am looking to purchase a frame and the seller does not know the Fork Rake or HT and ST angles. It is local so I have easy access to the frame.

How can I figure them out accurately at home?

Put a headset and wheels on it and use the iphone ap for an angle finder.

11-23-2011, 04:44 PM
You can get pretty close with rake by measuring from the dropouts to a spot on the BB with the fork normal and turned backwards, then dividing by 2. This requires a headset, though. You could try the same thing with the steerer on a table top and measuring to the floor.

11-23-2011, 04:50 PM
On a post somewhere on the forum Dave Kirk describes a clever way to measure rake with the fork off the frame. You may want to search for that.

Having said, that, unless you think that a given fork is way wrong for a frame, I'm not so sure that that's one of things someone would need to measure when considering a purchase. If the frame fits, then great. If it handled well for the previous owner, I would think that it would also handle well for you.

11-23-2011, 05:31 PM
Digital or analog inclinometers are the best way, but if you really need to know your seat tube angle, you can get there by using this table. You have to have the measurements though. This doesn't translate well when I publish it as all the columns left align. They are correct in the preview. That's strange. But, you should still be able to figure it out.


Seat tube angle is the angle between the seat tube and top tube. Setback is the distance the seat tube is behind the bottom bracket, measured along the top tube. Setback will influence weight distribution, bike fit, and handling.
With a normal frame, where the top tube is parallel to the ground, setback can be determined by dropping a string with a weight from the top tube so that it intersects the center of the bottom bracket. Measure along the top tube from the string to the center of the seat tube.
To use the table, first measure the setback and seat tube, center to center, in centimeters. Find your frame size (seat tube measurement) on the left, then follow that line across into the table until you find the closest setback that matches your measurement. Now, go to the top and you will find the seat tube angle. If the setback measurement is between the numbers in the table, then the seat tube angle will fall between the two angles at the top.
Record this measurement for a frame that fits really well. Then, in the future, when you shop for a frame, try to find one with a similar setback, given a specific frame size.

70 71 72 73 74 75 76 77
S 48 16.4 15.6 14.8 14.0 13.2 12.4 11.6 10.8
E 49 16.8 16.0 15.1 14.3 13.5 12.7 11.9 11.0
A 50 17.1 16.3 15.5 14.6 13.8 12.9 12.1 11.2
T 51 17.4 16.6 15.8 14.9 14.1 13.2 12.3 11.5
52 17.8 16.9 16.1 15.2 14.3 13.5 12.6 11.7
T 53 18.1 17.3 16.4 15.5 14.6 13.7 12.8 11.9
U 54 18.5 17.6 16.7 15.8 14.9 14.0 13.1 12.1
B 55 18.8 17.9 17.0 16.1 15.2 14.2 13.3 12.4
E 56 19.2 18.2 17.3 16.4 15.4 14.5 13.5 12.6
57 19.5 18.6 17.6 16.7 15.7 14.8 13.8 12.8
I 58 19.8 18.9 17.9 17.0 16.0 15.0 14.0 13.0
N 59 20.2 19.2 18.2 17.2 16.3 15.3 14.3 13.3
60 20.5 19.5 18.5 17.5 16.5 15.5 14.5 13.5
C 61 20.9 19.9 18.9 17.8 16.8 15.8 14.8 13.7
M 62 21.2 20.2 19.2 18.1 17.1 16.0 15.0 13.9
63 21.5 20.5 19.5 18.4 17.4 16.3 15.2 14.2