Engineering calculators for motorcycles

Gearing, compression, spring rates, trail, brake hydraulics, charging budgets, dyno correction and more. Every calculator shows the formula and works it through with your own numbers, in metric or imperial, so you can check the result by hand.

Formula shown, step by step
Metric and imperial
Every formula tested twice

Engine

Displacement, compression, big bore kits, cam timing, piston speed, BMEP, airflow and fuel injectors.

Engine displacement calculatorBore, stroke and number of cylinders to cc and cubic inches.Compression ratio calculatorStatic compression ratio from bore, stroke, chamber, gasket, deck height and piston volume.Mean piston speed calculatorAverage piston speed from stroke and rpm, the classic measure of how hard an engine is revving.Bore-stroke and rod ratio calculatorBore/stroke ratio and rod/stroke ratio, the two numbers that shape an engine's character.Power from torque and rpmConvert torque at an rpm into kW and horsepower.BMEP calculator (brake mean effective pressure)How much torque an engine makes for its size, as an average cylinder pressure.Engine airflow calculator (CFM)Air the engine needs at a given rpm and volumetric efficiency, in L/min and CFM.Fuel injector size calculatorInjector flow needed for a power target, in cc/min and lb/hr.Big bore and stroker kit calculatorNew displacement and compression ratio after a bigger bore, a longer stroke or both.Dynamic compression ratio calculatorEffective compression once the intake valve closes, from static ratio, rod length and cam timing.Cam timing calculator (duration, overlap, lobe centres)Duration, overlap, lobe centrelines and lobe separation from the four valve events on a cam card.

Gearing and chain

Road speed in every gear, gear charts, sprocket and belt pulley changes, chain length and chain wear.

Road speed in gear calculatorSpeed at any rpm from primary, gear and final drive ratios and tyre size.Cruising rpm calculatorEngine rpm at a given road speed in any gear.Sprocket and pulley change calculatorWhat a front or rear sprocket (or belt pulley) change does to final drive, speed and rpm.Chain length calculator (links)How many links a chain needs for your sprockets and axle position.Chain wear (stretch) calculatorPercentage elongation from a length measured over a number of pitches.Track gearing calculatorThe rear sprocket that puts the redline exactly at the end of a circuit's longest straight.Speed in every gear calculator (gear chart)Road speed in all six gears at one rpm, from primary, gearbox and final drive ratios.Belt drive pulley change calculatorHow far the rear axle moves, and how the gearing changes, when you fit different belt pulleys.

Tyres and wheels

Tyre dimensions, speedometer error after a size change, and pressure with heat and altitude.

Tyre size calculatorSidewall height, overall diameter and circumference from a size such as 120/70-17.Speedometer error after a tyre size changeHow far off the speedometer reads with a different tyre size.Tyre pressure and temperature calculatorHot pressure from cold pressure, or what a cold morning does to your setting.Tyre pressure at altitude calculatorWhy your gauge reads higher on a mountain pass, and by how much.

Chassis and suspension

Rake and trail, sag, spring rates and lean angle.

Rake and trail calculatorGround trail from steering head angle, fork offset and front wheel size.Suspension sag calculatorStatic and rider sag from three measurements, as millimetres and percent of travel.Rear shock spring rate calculatorSpring rate for extra load (a heavier rider, passenger or luggage) through the linkage.Fork spring rate calculatorFork spring rate from the load on the front wheel and the sag you want.Lean angle calculatorLean angle for a corner at a given speed, and the lateral g it takes.

Brakes

Hydraulic ratio, clamping force and stopping distance.

Brake hydraulic ratio and clamping forceLine pressure, clamping force and hydraulic ratio from master cylinder and caliper pistons.Stopping distance calculatorReaction distance plus braking distance at a given deceleration.

Electrical

Charging budget for accessories, wire voltage drop and fuse sizing.

Motorcycle electrical load calculatorCan your charging system run heated grips, lights and a heated jacket? Spare capacity and battery drain.Wire voltage drop calculatorVoltage lost in a cable run, and the wire size needed to keep it small.Fuse size calculatorCurrent drawn by an accessory and the minimum fuse rating to protect its wiring.

Performance and air

Power-to-weight, top speed from drag, fuel range, the fuel cost of luggage, altitude and dyno correction.

Power-to-weight ratio calculatorkW per tonne, hp per tonne and kg per hp, with or without the rider.Top speed from power and dragThe speed where aerodynamic drag and rolling resistance use up all the power.Luggage drag and fuel range calculatorWhat panniers, a top box or a tall screen cost in power, fuel and range at cruising speed.Altitude and temperature power loss calculatorAir density at altitude and how much power a naturally aspirated engine loses.Dyno correction factor calculator (SAE J1349)Correct measured power to standard conditions from temperature, pressure and humidity.Payload and GVWR calculatorHow much rider, passenger and luggage the bike can legally carry, and what is left.Fuel range calculatorHow far a tank goes, and how far until the reserve, at your fuel consumption.

Workshop

Bolt preload from torque, torque wrench extensions and two-stroke premix.

Bolt torque and preload calculatorThe clamping force a tightening torque produces, using the nut factor.Torque wrench extension (crowfoot) calculatorThe wrench setting when an adapter or crowfoot lengthens the wrench.Two-stroke premix ratio calculatorHow much oil to add to the fuel for a 32:1, 40:1 or 50:1 mix.

How the Lab is built

Each calculator uses a standard engineering relation from a named textbook or standard, listed on its page. The formula runs in your browser and is shown with your numbers substituted, so a mechanic or engineer can follow every step. Before a calculator goes live, its formula is implemented twice, once in the site’s engine and once independently, and both must give the same answers on a set of test cases.

Calculators give engineering estimates. Where a result depends on a figure that makers rarely publish, such as drag area or a bike’s own electrical load, the page says so. For safety-critical work such as brakes, suspension and fastener torque, your service manual’s figures come first.

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