What is BikeSPEZL?
Designing a bike in five layers, and the R&D where they combine
Why BikeSPEZL splits the design process
A bike is not one object. It is a stack of five interacting systems, each with its own discipline.
Designing a bike from scratch — or fitting a rider to one — touches the human body, frame geometry, component selection, the mass the rider carries, and the suspension/drivetrain system that links the wheels to the frame. These five systems have different inputs, different experts, and different failure modes. Treating them as one undifferentiated problem is why so many bikes end up well-geometrered but poorly fit, or well-fitted but poorly suspended.
Body — the rider is the starting point
Every other layer is tuned to a body. So the body comes first.
The Body section captures the rider through Body Dimension Characteristics (BDC): inseam, arm length, torso, femur, shoulder height, and the population distribution those measurements come from. A rider is not a single number — they are a distribution across sizes, ages, and regions. BikeSPEZL models the rider as a population-aware body, not a static measurement.
Why the body comes first:
- 1Reach and stack are meaningless without knowing the rider's arm and torso length — the same reach feels cramped for a long-armed rider and stretched for a short-armed one.
- 2Seat angle is meaningless without knowing femur length — a steep seat angle corrects a long femur, a slack one punishes it.
- 3Crank length, bar width, and stem length all derive from the rider's joint ranges, not from a brand default.
Geometry — the frame in relation to the rider
Once the body is known, the frame can be placed around it.
The Geometry section defines the frame's key parameters — head angle, seat angle, reach, chainstay, BB drop, stack — and, crucially, how they relate to the rider's body dimensions. A geometry that is excellent for one body can be wrong for another, even at the same nominal size.
- 1True_Fit geometry measures at the bar end and saddle at 25% sag — the geometry the rider actually sits in, not the frame's raw numbers.
- 2Multi-size design lets a brand generate a full size run from a single BDC dataset, with each size's geometry auto-derived from its target body.
- 3Category-aware recommendations keep the geometry inside the right range for the bike type (see Geometry Recommendations).
Parts — the components that realise the geometry
Geometry is a target. Parts are what physically deliver it.
The Parts section is the bike build: fork, shock, bar, stem, crank, drivetrain, wheels, brakes. Each part has real geometry (axle-to-crown, offset, bar rise, crank length) and real mass — both of which feed back into the geometry and the centre-of-gravity calculations.
The Parts section also connects to the affiliate retail infrastructure — the build's selected parts can be handed off to retailer checkouts, with no transaction taking place on BikeSPEZL itself.
Gear — the mass the rider carries
A rider is not a naked body. The kit they wear moves the centre of gravity.
The Gear section models the helmet, protector, shoes, clothing, hydration pack — every mass the rider carries above and on the bike. Each has a weight and a centre-of-gravity offset. Together they shift the combined rider+bike centre of gravity, sometimes by tens of millimetres.
- 1A full-face helmet adds mass high on the body, raising the combined CoG and increasing weight transfer under braking.
- 2A hydration pack shifts mass rearward, affecting front-wheel grip on climbs.
- 3Knee and shin guards add low mass, lowering CoG slightly but changing leg inertia.
Kinematics — the suspension and drivetrain system
Geometry is static. Kinematics is geometry in motion.
The Kinematics section defines the suspension linkage — the pivot points that determine the rear axle path, the leverage ratio curve, anti-squat, anti-rise, and pedal kickback. It also defines the drivetrain geometry (crank length, chainring, cassette) that is mechanically linked to the suspension through the chain.
Kinematics is where the frame becomes a ride:
- 1Two frames with identical static geometry can ride completely differently because their linkages produce different anti-squat and leverage curves.
- 2The drivetrain is not separate from the suspension — every gear changes the chain line and therefore the anti-squat and pedal kickback.
- 3Kinematic design is the layer most often invisible to the rider but most decisive to the feel.
R&D — where it all comes together
The five layers are tuned separately. In R&D, they are combined into one simulated ride.
The R&D section is the synthesis layer. It takes the body, the geometry, the parts, the gear, and the kinematics — and simulates them together as a single ride system: the rider's position, the suspension's response, the weight transfer under acceleration and braking, and the dynamic centre of gravity as the bike moves.
Two principles define the R&D synthesis:
- 1Precision — each layer's inputs are measured, not guessed: real BDC, real part masses, real pivot points. The simulation is only as honest as its inputs.
- 2Ride-system combination — no single parameter is optimised in isolation. The goal is a coherent whole: a bike that fits the body, holds the geometry, carries the parts and gear, and suspends and pedals as one system.