I reckon it's worth talking about the Stack and Reach measurement that'll be very invaluable when it come to figuring out what size bicycle you need.
The Stack and Reach measurement is a fairly new way of measuring the frame size, the horizontal distance from the bottom bracket to the top centre of the head tube is always consisted (Reach), so the measurement is the most accurate way of measuring the frame;
I'm going to talk about the Reach as this is the most important part.
The big problem with uncommon frame sizes is that they're never always what they claim to be in term of sizing, I'll put down an example;
Let look at two different size, the 51cm (S) and 54cm (M), the 51cm should have a shorter reach than the 54cm, especially with the top tube being 1.8cm shorter on the 51cm, sound about right doesn't it?
51cm - 51.8cm top tube.
54cm - 53.7cm top tube
Except it doesn't, they both have exactly the same reach, if you look at the Stack and Reach measurement, this is what you get;
It starting to get confusing, how did the reach stay the same, when the top tube is 1.8cm shorter?
The main reason is due to the seat tube angle, the 54cm have a 74 degrees seat angle (standard is 73), however the 51cm have a staggering 75.5 degree seat angle, the steeper seat tube shorten the top tube making it felt a lots longer than it claim to be.
When you slacken the seat angle from 74 to 73 degree, you add an approx 1cm in length from 54cm to 55cm, this is how I figure out the correct fit for customer's bicycle before I heard of the Stack and Reach measurement which made light work of fitting people properly.
the 51cm Specialized is 1.5 degrees steeper, so approx 1.5cm shorter, which is why the geometry said that the top tube is 1.8cm shorter.
This is very important, as when it come to fitting, you'd need to ensure that your saddle to bottom bracket is set at an idea distance that you find comfortable;
With a steeper seat tube, you end up setting the saddle further back to get the idea distance from the bottom bracket that you find comfortable, worse, you may have to invest in a new seatpost that have enough setback to fix this (Velo Orange is the cheapest you can get), Vesalius found out the hard way when his new bike have a 76 degrees seat angle, he end up buying not only a new seatpost that have a big layback, but also a new saddle that have a longer rail to allow it to be set further back, you can see that the saddle is set far back as possible so he'll be able to comfortably sit on the saddle without putting too much weight on the handlebar, very important in audax ride where you spend all days, even a week on a bicycle as shown below.
If this sound confusing, please let me know, so I can edit to my heart content to make it clearer.
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