S-Volve Sizing: Finding Your Perfect Fit

Traditional bike sizing uses vague labels — Small, Medium, Large — that mean different things from brand to brand. A "Medium" from one manufacturer might have a reach of 450 mm while another's is 475 mm. At DIRTLAB, we developed S-Volve, our sizing philosophy that puts measurable numbers ahead of arbitrary letters.

Why Reach and Stack Are Everything

The two most important dimensions on any bike frame are reach (the horizontal distance from the bottom bracket to the head tube center) and stack (the vertical distance from the bottom bracket to the top of the head tube). Together, these define where your hands sit relative to your pedals — which determines your weight distribution, climbing position, descending stance, and overall comfort.

Seat tube length, which traditional sizing is based on, only affects standover height — something that matters much less with modern dropper posts. A rider who measures "Medium" by seat tube length might actually ride better on a "Large" reach. S-Volve eliminates this confusion.

How S-Volve Sizing Works

Instead of S/M/L, our S-Volve sizing uses reach ranges as the primary selection criterion:

• S1: 420-435 mm reach — riders 155-165 cm
• S2: 440-455 mm reach — riders 165-173 cm
• S3: 460-475 mm reach — riders 173-181 cm
• S4: 480-495 mm reach — riders 181-190 cm
• S5: 500-515 mm reach — riders 190+ cm

These height ranges are guidelines — arm length, torso proportion, and riding style all influence the best fit. A rider with long arms relative to their height might size up; a rider who prefers a more upright position might size down.

How to Measure Yourself

You'll need a tape measure and a wall:

• Height: Stand barefoot against a wall, measure to the top of your head
• Inseam: Stand with a book between your legs (pressed firmly upward), measure from the top of the book to the floor
• Arm Span (Ape Index): Stand against a wall with arms outstretched, measure fingertip to fingertip. Subtract your height — positive means long arms, negative means short arms

Your ape index is the key modifier. Riders with a positive ape index (arms longer than height) often prefer a longer reach, while riders with a negative ape index prefer shorter reach for the same height.

Fine-Tuning with Stem and Spacers

Once you have the right frame size, small adjustments are made with stem length and headset spacers. We recommend staying within 40-60 mm stem length for mountain bikes — shorter stems give quicker steering response, while longer stems add stability. If you need a stem shorter than 35 mm or longer than 70 mm, you're likely on the wrong frame size.

Headset spacers adjust stack height in 5-10 mm increments. Running spacers under the stem raises your bars; removing them lowers the front end for a more aggressive position.

Still Unsure?

Email us your measurements and riding style at sales@bikelab-inc.com and we'll recommend a size. If you're between sizes, we'll explain the tradeoffs so you can make an informed decision. Getting the right size is the single most impactful thing you can do for your riding — it's worth getting right.