BikeSPEZL
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Geometry · Sizing

True_Fit Geometry

Measured at the bar and saddle · At 25% sag

The problem with traditional geometry charts

Every chart you've ever read describes a bike that doesn't exist during riding.

Traditional bike geometry charts measure the frame in isolation — unloaded, suspension fully extended, no rider. The numbers describe the physical structure, not the geometry a rider actually sits in.

Diagram — traditional chart measurement points vs True_Fit measurement points
🔑 Key Concept
Two bikes with identical reach and stack can feel completely different once a rider sits on them, the suspension compresses to sag, and the bars and saddle position are factored in.
–15mm
Typical reach change from full extension to 25% sag
–8mm
Typical stack change from full extension to 25% sag
–0.5°
Head angle change at sag vs published
2 bikes
Same chart numbers, different ride experience

What True_Fit measures

Contact-point geometry at the neutral riding position.

Diagram — True_Fit measurement reference points (bar end & saddle at 25% sag)
All measurements projected from the bottom bracket origin

True_Fit redefines the measurement plane: instead of the frame, it measures from the rider's contact points — the bar end and the saddle — at 25% suspension sag.

The True_Fit geometry set includes:

  • 1Bar Reach — horizontal distance from the bottom bracket to the bar end
  • 2Bar Stack — vertical distance from the bottom bracket to the bar end
  • 3Effective Top Tube — horizontal distance from head tube top to seat tube at saddle height
  • 4Effective Head Tube Angle — the actual head angle at sag
  • 5Effective Seat Tube Angle — the actual seat angle at sag position

Why 25% sag?

The universal neutral position — designed-in optimum, not an arbitrary choice.

25% sag is the industry-standard neutral suspension position for trail riding. It represents the average compression of a properly set-up suspension system under a seated rider on flat ground.

Graph — anti-squat, leverage ratio and BB height across the travel range
25% sag is the designed-in optimum for all suspension parameters

At this position, all suspension geometry parameters operate at their intended baseline — anti-squat, anti-dive, and leverage curve characteristics are all at their designed-in optimum.

💡 Insight
Measuring at full extension (as traditional charts do) describes a state the bike is never in during normal riding. Measuring at 25% sag describes the bike the rider actually experiences.

How BikeSPEZL calculates True_Fit

From published geometry to contact-point coordinates in three steps.

Flow diagram — geometry engine calculation pipeline
Manufacturer geometry → kinematics at 25% sag → True_Fit contact-point projection
  • 1Take the manufacturer's published geometry (reach, stack, HTA, STA, fork axle-to-crown, rear center, BB drop).
  • 2Apply the suspension kinematics for 25% sag travel — recalculating the effective geometry at the new wheel and BB positions.
  • 3Project all measurements to the bar end and saddle contact points to produce the True_Fit table.
✅ Tip
The result is a geometry table that is directly comparable across bikes with different suspension designs — from single pivot to horst link to DW link.

True_Fit for brands

From static charts to fully interactive sizing tools.

BikeSPEZL offers True_Fit geometry as a service for brands — from static two-state charts to fully interactive sizing tools with live True_Fit preview.

Screenshot placeholder — interactive True_Fit brand widget
ℹ️ Note
See the Brands page for package options and enquiry form.
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