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Comparing the geometry of different bike frames
Recently, I was comparing the geometries of two frames and, after a while, found myself in a dilemma that went something like this: Normally when comparing two frames, I focus primarily on the Effective Top Tube (ETT) measurement. My problem arose when I noticed that the two frames I was comparing each had different Seat Tube Angles (STA). Did the STA have any effect on the ETT and, in turn, affect the reach to the bars? I hope this article helps answer that question, and I hope it can be useful if you’re ever comparing different frames or trying to figure out what length stem to get for a new frame.
Some basics about frame geometry
There’s an excellent article in Dirtrag’s Issue # 118 about transferring your bike’s ’setup’ from one frame to another. Also, there is some great information in there about your bike’s basic geometry dimensions.
The first place you should start when setting up your bike is with saddle placement or, more specifically, with saddle height and fore / aft placement. There’s lots of debate on the how to determine the proper fore / aft position, which I won’t get into here, but you can read more at these sites:
Regardless of your chosen fore/aft position, it’s important to note what the saddle setback (see diagram below) is. Note that this dimension is specific to a particular saddle.
Once your saddle is in place you can measure your reach, which is composed of the effective top tube length (ETT) and your stem reach. Typically, reach is measured from saddle tip to the center of the handlebar. Note that the back sweep of your handlebars will affect the more important dimension of the saddle tip to the grip.
Seat tube angle (STA)
For every saddle, your setback dimension should be the same, regardless of the STA of the frame. This is because you want to keep your position of your knee relative to the BB / pedal spindle the same between the two frames.
If you are moving to a frame with a relatively steep seat tube angle (ie. a higher seat tube angle than your first frame), you’ll need to slide your saddle farther back on the rails in order to keep the same setback.
If you’re moving to a frame with a slacker seat tube angle, you’ll have to push the saddle forward. From the above picture you can see that my saddle has been pushed forward because the new frame has a relatively slack seat tube angle (71.4 degrees).
To put it simply, sliding your saddle forward has the effect of reducing the ETT of a frame while sliding it backwards increases the ETT of the same frame. When comparing two frames, you need to make an adjustment to the length of the ETT in order to compare the two frames properly.
But how do you figure out how much the ETT changes between different STAs?
Geometry 101 - A quick introduction to bike geometry
I bet you were hoping never to think about geometry again following your sophomore year of high school. Right? Well me too, until I found the basics of sin, cos, tan rearing their ugly heads when thinking about bicycles.
One way to determine the effect of STA on ETT is to create your own ‘rule of thumb’. I say your own rule because it is dependent on your own, particular saddle height. Using the diagram below you can calculate a rule of ETT change /degree STA where H in the diagram is your saddle height.
Using this technique, you now have your own rule of x cm (or inches) of effective top tube change for each degree difference in Seat tube angle.
Note: Isn’t it interesting how most ETT dimensions are listed in inches, but most stems are listed in centimeters? I convert everything to cm to make it easy.
Example: Measuring a real world bike
Here is an example using two different frames: the first frame belongs to my current bike, a 2004 15″ Iron Horse Hollowpoint, and the next is a 2005 / 2006 15″ Iron Horse Azure. I wanted to compare the ETT of the Hollowpoint to see how it compared with the 15″ Azure.
The Hollowpoint has a STA of 71.4 and an ETT of 22.7″ (57.66cm) and the Azure has a STA of 73 and an ETT of 21.5″ (54.61cm). My saddle height is 69cm.
My saddle height is 69cm
My rule of thumb is 69cm * sin (1degree) = 1.2cm ETT change / degree for every STA change. The difference in seat tube angles between the two frames is 1.6 degrees. Using my rule of thumb (1.6 degrees x 1.2 cm / degree), I get an adjustment in the ETT of 1.9cm.
Because the Hollowpoint has a slacker seat tube than the Azure, I need to shorten the ETT of the Hollowpoint to properly compare apples to apples when looking at the Azure. The ETT of the Hollowpoint after the adjustment is 55.76cm (57.66 cm ETT - 1.9 cm adjustment).
After adjustments, the Hollowpoint has an ETT that is 1.15 cm longer than the one for the Azure. If I’d failed to account for the different seat tube angles, my measurements on the Hollowpoint would be incorrect (ie. 3.05 cm instead of 1.15 cm). That is a fairly significant difference when sizing up a new frame.
Conclusion
So what is the point of doing all of this math? FIT.
I’m a huge believer in fitting yourself on a bike that matches your personal biomechanics. Once you find that fit — either through a professional fitter or through trial and error — it’s important to transfer that fit from one bike to another. The excercise I describe in this article helps you determine if a new frame’s effective top tube is going to work, and helps determine what stem length you’ll need to dial in the fit prior to having the frame in hand.
I hope this made sense and I hope it helps you get on the right frame for your next ride (and I hope I did my math right!). Feel free to add your thoughts below.
1. Posted by Week 4 » The Biking Hub | 8:14 am, 3 February 2006
[…] This week’s most popular post was Dan’s 10 questions with Jim Wannamaker from Kenda USA. We also had two pieces by Ashwin this week — a review of the Avid Juicy 7 disc brake on Monday and a discussion about comparing the geometry of two different frames on Tuesday. On Wednesday, Greg posted up the third in his series of four articles on the mountain biker’s road trip. […]