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Discussion Starter · #1 ·
I currently ride a 56cm Spesh Roubaix, and it fits pretty well. Very soon I am purchasing a C-Dale Synapse and recently at the bike shop I was told that based on my height a size 56 is the right size. But he wasn't on to tell me I also have a longer than average femur length for my height and that he might want to put me on a size 54.

Can someone explain the logic behind this?
 

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I too have longer than average femur length per height and shopping for the proper size frame has been more difficult than average. I use seatpost (FSA) with 32mm setback to make it work.

Have you tried this site for determining what size frame may be right for you?
 

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Discussion Starter · #3 ·
So you size down and use a setback post?

I Have used that calculator in the past and the results are only as good as the measurements, and I am not to confident in my measurements.
 

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Femur length has nothing to do with frame size.

Some fitters and bike shops maybe/possibly/might argue about Rider X needing a particular seat angle to accommodate a long femur, but to me even that's a reach.

Traditional thought is, as frame size goes up, the seat angle gets slacker (say, goes from 73 to 72 degrees as the frame size increases) to accommodate the longer legs of a taller rider.

However, there is other thought that as the hip angle gets more acute as the frame size goes up with that slacker seat angle, physical problems can arise such as back pain. This other school of thought suggests that riders with longer femurs (or just plain longer legs) use longer crankarms, even if it means going custom. Framebuilder Lennard Zinn espouses this theory and thinks a 73 degree seat angle will fit virtually all cyclists and any adjustment be made in crank length.

I personally am suspect of your local shop's suggestion and think they have it backwards, if at all. Typically a 56cm frame will have a slacker seat angle than a 54cm frame and therefore better fit someone with longer femurs. I also tend to agree with Lennard Zinn's point of view.
 

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So you size down and use a setback post?
No, I picked the proper size frame for my height but specifically with lower seat tube angle (not common), and then used seatpost with setback to make further adjustment.

As the above poster said, larger frame size typically has shallower seat tube angle. Downsizing of frame would be the opposite of what you want, unless the reach was so far out that it had to be downsized.
 

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This other school of thought suggests that riders with longer femurs (or just plain longer legs) use longer crankarms, even if it means going custom. Framebuilder Lennard Zinn espouses this theory and thinks a 73 degree seat angle will fit virtually all cyclists and any adjustment be made in crank length.
There are so many different methods and theories of choosing crank arm length with different results that it's hard to pick one. :mad2:
 

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From How to Fit a Bicycle

And perhaps even two riders with identical skeletal proportions would find after testing that they required different crank lengths to achieve maximum performance due simply to differences in their muscles.

Trying different cranks to find the optimum length would be time consuming and expensive, but I believe it is the only way to determine the correct length for any individual, assuming there is a correct length.
It took me a few weeks and some pain to get used to my mtb's cranks vs the road bike's, even with the seat set at ostensibly the correct height. Now, I wouldn't change them.
 

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I currently ride a 56cm Spesh Roubaix, and it fits pretty well. Very soon I am purchasing a C-Dale Synapse and recently at the bike shop I was told that based on my height a size 56 is the right size. But he wasn't on to tell me I also have a longer than average femur length for my height and that he might want to put me on a size 54. Can someone explain the logic behind this?


The only possible logic is that you have a shorter upper body and therefore a shorter top tube might provide a better fit. Weak, but since we have no idea what your other measurements are, or how you have set up your current bike, we can't really answer.
 

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I have long legs and femurs (5'9, 33.5 inseam) and own a Synapse. The top tube is the perfect length for my torso, but I had a hard time getting the proper setback because the Synapse seat post only has 15 mm of setback. I ended up buying a Selle SMP saddle because it was the only brand with rails long enough to give me the proper setback. Enve makes a 25.4 mm seat post with 25mm of setback for the synapse but it's 250$.

If you are taller than I am, I would strongly advise against going with a 54cm, as you probably won't be able to fit it properly with what is available on th e market
 

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If you are taller than I am, I would strongly advise against going with a 54cm, as you probably won't be able to fit it properly with what is available on th e market
That depends on the brand. Any given size frames have varying top tube (& other tubes) length depending on who designed them and for which purpose, i.e. road race, tour, CX...
 
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