seat angle as size increases-Masi Legacy content

[stackie]

So, I was just perusing the Masi Legacy site after having read the thread on Breaking Away Masi project. In particular, my interest was piqued by the Gran Criterium geometry.

Anyway, I noticed that they are using a 73.5 degree seat angle in a 61cm bike. Also, 60 cm top tube. Oh, and 73 degree head tube. Hey, that's pretty much the basics of my custom Speedvagen. I've only had one stock bike that has fit me as well as the Speedvagen, and that was a 99 Serotta Atlanta 60L. Same geometry.

This set up seems pretty different from the usual slack 72-72.5 degree seat angle and 74 degree head tube combined with 58-58.5 top tube I see on most 61 cm frames.

I get that anthropomorphically, most people get longer legs, particularly femurs, as they get taller, so the slack seat angle helps them get enough setback to get KOPS, if you subscribe to that theory. Also, slack seat and steep head tube helps keep wheelbase short.

So, I guess my question is, why would Masi go out on a limb with this fairly non traditional geometry. I can't believe they are building a bike custom for me. Or, that they are limiting their taller clientele to those with relatively shorter legs and relatively longer torsos. This Masi Legacy Gran Criterium is definitely different from the regular Masi Gran Criterium geometry.

So, please feel free to educate me on this geometry. Are you regular 6'4' guys with 93 cm inseams going to ride this thing with the saddle slammed back and a 100mm stem, or just tall guys with shorter inseams (89.5cm) like me?

I'm really super interested in hearing people's thoughts on this.

Thanks,

Jon

[Steve in SLO]

The CA Masis had relatively steep angles, and even at bigger sizes they had stock 73-73.5 deg ST angles, IIRC. They were crit bikes, afterall.
What they seem to be after is a 'racier' geometry rather than century geometry. Both are actually traditional geometries.

[Mark McM]

I get that anthropomorphically, most people get longer legs, particularly femurs, as they get taller, so the slack seat angle helps them get enough setback to get KOPS, if you subscribe to that theory. Also, slack seat and steep head tube helps keep wheelbase short.

Actually, this is not true. If the dimensions of both the rider and the bike increased in size proportionally, the seat tube angle would remain a constant. In other words, if everything increased to scale, the seat height would increase in the same proportion as the set back, so that angle would remain the same.

But what is often not appreciated, is that there is one component here is not scaled proportionately - specifically, crank length. Despite the wide range in size of cyclists, crank lengths vary only a little. For example, say a 170 cm (6' 7") cyclist with used 170 mm cranks (cranks 10% of height), than if cranks varied in scale to the rider, a 185 cm (6' 1") cyclist would use 185 mm cranks. But usually a cyclist this tall would use only 175 mm cranks, or 10 mm less than what would be perfectly proportional. If this rider rode in the KOPs position, the 10 mm shorter cranks would require that the rider sit 10 mm further back than they would with proportional cranks. Since the taller rider sits proportionally further back than the shorter rider, larger frames typically have shallower seat tube angles.

Now, there are a variety of reasons that cranks are not sized proportional to rider size, but the fact of the matter is that they are not, and this has an affect of bike geometry and fitting.

[stackie]

I get that anthropomorphically, most people get longer legs, particularly femurs, as they get taller,

What I meant by this statement is that as people get taller, proportionally more of their height is in their legs than their torso.

I hear your point on crank length. Zinn's crusade. He may well be correct. One of these days I should try at least 180 cranks. I've tried 177.5 and didn't feel much difference from 175, but then again, it's not much different.

Good point about the crit geometry. But, don't most crit bikes have a shorter top tube?

Jon

[tuscanyswe]

Actually, this is not true. If the dimensions of both the rider and the bike increased in size proportionally, the seat tube angle would remain a constant. In other words, if everything increased to scale, the seat height would increase in the same proportion as the set back, so that angle would remain the same.

But what is often not appreciated, is that there is one component here is not scaled proportionately - specifically, crank length. Despite the wide range in size of cyclists, crank lengths vary only a little. For example, say a 170 cm (6' 7") cyclist with used 170 mm cranks (cranks 10% of height), than if cranks varied in scale to the rider, a 185 cm (6' 1") cyclist would use 185 mm cranks. But usually a cyclist this tall would use only 175 mm cranks, or 10 mm less than what would be perfectly proportional. If this rider rode in the KOPs position, the 10 mm shorter cranks would require that the rider sit 10 mm further back than they would with proportional cranks. Since the taller rider sits proportionally further back than the shorter rider, larger frames typically have shallower seat tube angles.

Now, there are a variety of reasons that cranks are not sized proportional to rider size, but the fact of the matter is that they are not, and this has an affect of bike geometry and fitting.

You always write very well written and thought out responses that is full of knowledge!