Originally Posted by
repechage
if you want to compare mod to classic - ya start
with the numbers that are reasonably easy to reference. Reach takes out setback, we will assume that the seat on either bike will have the same position in relation to the crank axis.
from there, one would want to know can they get the bars in the same or near same location. Plenty of ways to achieve it. With mod bikes as things are almost always sloping, was suggesting a way to get to a common measure to point.
I agree that the saddle will be in the same position in relation to the crank axis which is why comparing the top tube length is as valid a measurement as reach. It’s also an easier measurement to make since you don’t have to drop a plumb line to the crank from a horizontal line. Even with sloping top tube it’s fairly easy to get a consistent horizontal line to the seat post.
not knowing the type of mod bike, the more recent are extending the headtube, that might guide one to a larger level top tube frame, let the numbers tell the tale.
The idea of measuring the “stack” to the top of the head tube makes no sense to me. The top of the head tube can be higher than the top of the top tube which would make putting people on a smaller frame unless that measurement is taken into account. Head tube lengths vary a whole lot (I’ve checked) so the stack will vary a lot from bike model to bike model. The stack is also another measurement that is difficult to make…probably even more difficult than doing reach. You have to project a line from the center of the crank to the head tube then
raise a plumb line to the top of the head tube. That’s a complicated measurement to make. Standover accomplishes the same thing with less effort and can be done on the fly.
Originally Posted by
70sSanO
Interesting discussion. I’ve wondered from purely a road bike geometry perspective, how much things have changed. Not materials, aerodynamics, stiffness, etc., just the numbers.
Modern mountain bikes share little with their “golden age” counterparts. You can’t lay one over the other and have any semblance.
But for road bikes are we still around 73/73 or 74/73? Bottom bracket height about the same. Similar chainstay and trail.
However the dots are connected, sloped or horizontal, can I replicate an older road bike setup with a modern road bike. Not necessarily stated size to size.
John
I would agree. Road bikes haven’t changed all that much, even with the advent of the sloping top tube. For direct comparison consider a 1985 Cannondale touring to a 2010 Cannondale touring. Here’s the geometry chart for the 1985
And the geometry chart for the 2010 (and guide for the letters). The only differences are in fork rake and seat tube length. Even with the sloping top tube, the difference between the old seat tube and the newer one is 2” which ain’t much
Granted there aren’t too many road bikes where the same model was made for 30+ years but road bikes haven’t really changed all that much. Look at the 1985 Trek 760 compared to a 2021 Émonda ALR 5. There are just minor differences.
As you said, that doesn’t hold with mountain bikes. They are radically different from the first mountain bikes. Modern mountain bikes are even radically different from 90s mountain bikes but have come back just a little towards older mountain bikes.