Originally Posted by
trail_monkey
When a manufacturer steepens the seat tube angle as the frame size of a the same model gets smaller, do they compensate for the CG balance point by shifting the BB shell further forward or is the rider of the smaller size frame literally that much more over the BB than the larger size with a relaxed STA? ...............................................snip................................................. ........ Why do manufacturers do this as I have read it is common? Wouldn't the smaller riders want the same CG balance point of the larger riders?
You have put your finger on it, and you have asked the right question.
The answer?
It's a fudge.
Simple as that. The name of the game in mass manufacturing bikes is to put as many people as possible on the smallest variety of sizes with the smallest variety of part sizes possible.
Small people don't want to or benefit from sitting further forwards in relation to the BB than larger people do but there is only so far that the front/centre distance can be reduced for a given wheel size.
So what do they do?
Rather than using a smaller size wheel, which is a different part and therefore more costly, they move the saddle forwards instead. Not in the riders interest, just in the manufacturers interest.
It gets more complicated (WAY more complicated) than this but that's it in a nut shell.
I'm a short guy who rides custom made small bikes and also a bit of a crank on this subject.
What's the RIGHT answer for short people. Well it starts with smaller wheels, shorter cranks, low bottom brackets and RELAXED seat tube angles. NOT steeper seat tube angles.
This whole subject is a rabbit hole but slowly things are getting better. Children's and juvenile bikes are getting MUCH better than they were 5-10 years ago as they slowly evolve back to shorter cranks, smaller wheels and relaxed seat tube angles.
End Rant/