Member Question: Bike Frame Differences

author : FitWerx

comments : 0

Does a degree or two difference between two frames(head and/or seat tube) really make that much of a difference when it comes to fit?

Member Question from GregKL

"Does a degree or two difference between two frames(head and/or seat tube) really make that much of a difference when it comes to fit?  Say I am riding a Specialized Tarmac and I get fitted and it works great for me.  Then I go out and buy a Felt with similar though not exactly the same specs as the Tarmac and I get fitted to the Felt.  Is it probable that I may not be able to get a good fit on the Felt?"

Answer by Dean Phillips
Lead Fitter FitWerx2

A degree or two can certainly make a difference between the way two frames fit, but that difference can sometimes can be accounted for with a different stem length/angle, spacers underneath the stem, handlebar reach, or seatpost setback position. It’s possible that you will fit well on two different frames, but the selection of these key components will be different in order to match the bike to your ideal position.

We discussed the differences between frame sizes and how they fit different riders earlier in Getting a Good Bike Fit . This time, let’s consider how two different frame models of the same size will impact your position. Let’s take the Specialized Tarmac size 54. You were fitted on it, and are very happy. Now you decide to purchase a Felt F1 size 54 and swap all the components over. The seat tube angle on the Felt Z4 is 74.5 degrees versus the 73.5 degree seat tube angle on your Tarmac. You’ll need to adjust your saddle about 1 cm aft in the Felt F1 seat post clamp in order to maintain the same setback relative to the bottom bracket you had on the Tarmac. Depending on your position, you may need to substitute a different seatpost with more setback to allow this to happen.

The effective top tube length is similar on these two bikes taking into account the different seat tube angles. The Felt F1 is slightly longer, but the difference is small so it may or may not work with the same length stem. It may be necessary to shorten the stem or use a handlebar with a shorter reach in order to maintain the same reach as your Tarmac.

There is a significant difference between the head tube lengths. The Specialized Tarmac has a head tube length of 14.5cm, compared to 12.0cm on the Felt F1. If you’re happy with your front end on the Tarmac, and simply used the same stem and number of spacers underneath the stem, you’d end up in a position 2.5cm lower on the Felt F1. In this case you’d need to either add an additional 2.5cm of spacers underneath the stem, or use a stem that’s angled up more to make up that difference and keep your position the same. There’s a limit to the number of spacers you can place underneath the stem before the  safety of the bike is compromised, and there’s also a limit to how much upward angle on a stem is possible before the handling of the bike gets dicey. If you can’t match your position on the Felt F1 with 4cm or fewer spacers underneath the stem and a stem angled upward 17 degrees or less, then the Felt F1 is not a good bike for you. In this case you’d be better off on the Felt Z4 which has a head tube of 16.0cm and is a far better fit for you than the same size Felt F1.

There are plenty of bikes with similar geometry, but in this example you can see how a small difference in geometry of a couple centimeters can make one bike fit far better than another.

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date: January 9, 2010