S-Volve Sizing — CP Platform
Size Evolution — how Dirtlab matches frame dimensions to rider body proportions for optimal fit and performance.
S-Volve is Dirtlab's sizing methodology. Where most brands assign a size by height alone, S-Volve starts with reach — the horizontal distance from the bottom bracket to the head tube — and builds outward from there. The result is a sizing system where every rider, regardless of torso length, arm span, or inseam, can identify the frame that fits their body rather than just their height.
Why Standard Sizing Fails
Conventional mountain bike sizing — S, M, L assigned by rider height — is a blunt instrument. Two riders of identical height may have very different torso-to-leg ratios, arm lengths, or riding style preferences. A rider with a long torso and short legs who buys a Medium by height may find the reach too short. A rider with a short torso who buys the same Medium may find it too long. Height is a proxy, not a measurement.
S-Volve replaces height-based sizing with reach-based sizing, cross-referenced with rider proportions and intended riding style. The three Dirtlab sizes — S, M, L — are defined by reach ranges, and the RA Reach Adjust headset extends each size by an additional ±5mm to close the gaps between them.
The Three Dirtlab Sizes
Small
Designed for riders who prefer a shorter reach for quick, responsive handling. Typically suited to riders in the shorter height range or those with shorter torso proportions who prefer a more upright, agile riding position. S pairs well with a shorter stem and wide bars.
Medium
The benchmark size — designed around the most common combination of rider proportions and trail riding demands. M delivers a balanced reach that works equally well on technical descents and sustained climbs. The most versatile of the three sizes.
Large
Extended reach for taller riders or those who prefer a more stretched, low and long riding position. L gives more stability at speed and suits riders with longer torsos. Works best with a short to zero-rise stem and bars set at a comfortable width.
RA Reach Adjust — Closing the Size Gaps
Every Dirtlab frame is compatible with the RA Reach Adjust headset system. RA shifts the front axle path by ±5mm in the horizontal plane without affecting head angle, stack, or any other geometry variable. This gives you two positions on any frame size — effectively creating six fitting positions across the three sizes (S-, S+, M-, M+, L-, L+).
RA is a mechanical headset — it requires no spacers, no stem angle changes, and no bar adjustments. It is set once during build and does not move. See the RA Reach Adjust page for full technical details.
How to Choose Your Size
| Measurement to Take | How to Use It |
|---|---|
| Torso Length | Sit upright on a flat surface, measure from the seat to the top of the shoulder. Longer torso = more reach needed. |
| Arm Length | Measure from shoulder to wrist. Longer arms allow a longer reach without overstretching. |
| Inseam | Used to verify stack compatibility — taller stack suits longer inseam riders who need more standover clearance. |
| Riding Style | Aggressive/descending preference = lean toward longer reach. XC/climbing preference = lean toward shorter reach. |
| Current Bike | If comfortable on your current bike, note its reach. Use that as your baseline for comparison. |
S-Volve and G-Volve Together
S-Volve sizing is designed to work with G-Volve geometry. G-Volve sets the angles and proportions of the frame; S-Volve matches you to the right frame size within that geometry. A correctly sized frame running G-Volve geometry will feel balanced, planted, and confidence-inspiring at both ends of the speed range. An incorrectly sized frame — even with perfect geometry — will not.
If you are unsure which size is right for you, contact us. We can walk through your measurements, riding style, and goals before you order. Getting the size right is the most important decision you will make about your frame.
Not sure which size is right for you? Email us with your measurements and we will advise.
Contact sales@bikelab-inc.com