You’re staring at a bike spec sheet and there it is: stack 572mm, reach 390mm. Numbers with no context. I ignored these for years, bought bikes based on frame size alone, and ended up with two that never felt right no matter how much I adjusted the stem and seatpost. Once I understood stack and reach, I stopped wasting money on bikes that didn’t fit.
Here’s why these two measurements matter more than the frame size number on the sticker.
Stack: How Tall Is Your Cockpit?
Stack measures the vertical distance from the center of the bottom bracket straight up to the top of the head tube. Think of it as how high your handlebars sit relative to your pedals. Higher stack means a more upright riding position. Lower stack means you’re bent over more — faster and more aero, but harder on your lower back over time.
Most people want more stack than they think. That race bike with the slammed stem looks cool in photos, but you’ll hate it after two hours. Endurance bikes run stack heights 20-40mm taller than race bikes in the same size. Your back will thank you.
Reach: How Far to the Bars?
Reach is the horizontal distance from the bottom bracket center to the top of the head tube. It tells you how far you’ll be reaching forward to grip the bars. Longer reach stretches you out. Shorter reach keeps things more compact and upright.
This is where a lot of bike purchases go wrong. You can find a bike with perfect stack height, but if the reach is too long, you’ll be overextended — arms locked out, weight too far forward, uncomfortable after an hour. The instinct is to swap in a shorter stem to fix it, but that changes the steering geometry and makes the bike feel twitchy and nervous. It’s better to get the reach right from the frame than to try fixing it with parts.
The Ratio Trick
Divide stack by reach and you get a quick shorthand for how aggressive a bike’s position is. Race bikes typically fall around 1.35-1.45. Endurance bikes: 1.5-1.6. The higher the ratio, the more upright you’ll be. This number lets you compare geometry across brands without getting lost in marketing language. A “performance endurance” bike from Brand A and a “sportive” bike from Brand B might have identical stack/reach ratios despite very different names. The numbers don’t lie; the branding does.
Why This Beats Frame Size
Frame size (54cm, 56cm, etc.) means different things at different brands. I’m a 56 on my Specialized but a 58 on certain Trek models. The number is nearly useless for cross-brand comparison. Stack and reach are universal — they measure the same thing regardless of who built the frame. Get a proper bike fit that gives you your ideal stack and reach numbers, and then you can evaluate any bike from any brand using those two dimensions.
My fit numbers are roughly 575mm stack, 385mm reach. Every bike I’ve bought since learning those numbers has felt right from the first ride. Before that, it was guesswork and expensive mistakes. Spend the $150-200 on a professional fit before you spend $2,000+ on a bike. The geometry chart will finally make sense.
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