Summary

Don’t just rely on mountain bike reach! Effective top tube(ETT) is also an important number for bike size fit. Recently, I was looking at Airdrop Bikes and became interested in the Airdrop Filter, but sizes were limited. This blog is my research on figuring out ETT and if an in stock size would fit.

I have ridden a Commencal Meta 29 with a reach of 460 and seat tube angle(STA) of 76.5.

I have ridden a Chromag Stylus with a reach of 465 and STA of 76.0.

Question, will an Airdrop Filter with a reach of 475 and STA of 78.2 fit? (This is size large and medium is out of stock).

Geometry Numbers

How to calculate

% Functions
DegreesToRadian = fun(Deg) -> math:pi()/180 * Deg end.
Hypotenuse = fun(B, Beta) -> B / math:sin(DegreesToRadian(Beta)) end.

% Parameters
Stack = 614.0.
HeightLow = 600.0.
STALow = 78.1.
HeightHigh = 700.0.
STAHigh = 78.7.
Reach = 475.0.

% seat tube angle(STA) difference
STADiff = abs(STALow - STAHigh).
% percentage difference between high and low STA
PercentHeightDiff = abs(HeightLow - Stack) / abs(HeightLow - HeightHigh).
% Effective STA
ESTA = STADiff * PercentHeightDiff + STALow.

% Answer - effective top tube(ETT)
ETT = Hypotenuse(Stack, ESTA) * math:cos(DegreesToRadian(ESTA)) + Reach.

Conclusion

The Airdrop Filter ETT is shorter than expected. Comparing to a reach of 460 or 465, the 475 reach Filter has the shortest ETT of 603. This is due to the steep STA.