Tell Me About Helmets | How Shells, Liners, Straps, Impact Protection, Fit and Helmet Design Work

Tell me about helmets. A helmet is a wearable protective system designed to reduce forces transmitted to the head during impacts, falls or contact with objects. Most helmets combine a hard or semi-rigid outer shell, an energy-absorbing liner, comfort padding, a retention system and ventilation or visibility features. Different helmet types are engineered for different hazards, so a bicycle helmet, motorcycle helmet, climbing helmet and industrial hard hat do not solve exactly the same impact problem.

When people ask how helmets work, the central idea is managing energy and time. During a collision, the head’s velocity changes rapidly. A helmet increases the distance and time over which that change occurs by crushing foam, flexing a shell, sliding against the surface or distributing force across a larger area. Reducing peak acceleration and preventing direct penetration can lower the risk of serious head injury, although no helmet can prevent every injury.

Modern helmet design therefore combines mechanics, materials science, anatomy, ergonomics and testing standards. Shell stiffness, foam density, liner thickness, strap geometry, fit, coverage and ventilation all trade against weight and comfort. Good protection depends not only on laboratory performance but also on correct size, secure retention, replacement after damaging impacts and using a helmet designed for the activity.

The 50-Second Answer

A helmet protects by managing impact energy before that energy reaches the skull and brain. The outer shell spreads load and resists penetration, while the liner deforms or crushes to lengthen the stopping time.

A secure retention system keeps the helmet in position during the event. The exact design varies because different sports and jobs involve different impact speeds, shapes, repetitions and environmental demands.

Impact Energy

A moving head has kinetic energy proportional to mass and the square of speed.

Because speed matters quadratically, a modest increase in impact velocity can produce a much larger energy challenge for the helmet.

Stopping Time

Injury risk is related partly to how quickly the head’s velocity changes.

A deforming liner increases stopping time and distance, reducing peak acceleration compared with a direct hard impact over a very short distance.

Force Distribution

A shell spreads a local impact over a wider liner area.

This reduces extreme pressure at one point and lets more energy-absorbing material participate.

Penetration Resistance

Sharp or concentrated hazards can puncture weak materials.

Hard shells resist penetration so objects are less likely to reach the skull directly.

Outer Shells

Helmet shells can be made from polycarbonate, ABS, fibreglass, carbon fibre or other composites.

Material choice balances stiffness, toughness, weight, cost and manufacturing method.

Polycarbonate

Polycarbonate is tough and impact resistant.

Thin shells can flex and distribute load while remaining lightweight, making the material common in bicycle and sports helmets.

ABS

Acrylonitrile butadiene styrene is tough, inexpensive and easy to mould.

It is common in hard hats, skate helmets and some protective shells where durability matters.

Fibreglass

Fibreglass composites combine glass fibres with polymer resin.

They can create stiff strong shells that crack progressively under severe load, dissipating some energy while maintaining structure.

Carbon Fibre

Carbon-fibre composites offer high stiffness-to-weight ratio.

They are used in premium motorsport and motorcycle designs but require careful laminate engineering because local impact behaviour differs from simple metal or plastic.

Composite Layups

Composite helmet shells use several fibre orientations and resin layers.

The layup is engineered so cracking, delamination and fibre fracture manage loads while preserving coverage.

Energy-Absorbing Liners

The liner is the main crushable layer in many helmets.

Expanded polystyrene, expanded polypropylene and specialised foams are common because controlled cellular collapse absorbs energy.

EPS Foam

Expanded polystyrene consists of fused closed-cell beads.

It crushes under impact and is lightweight, but much of its deformation is permanent, so significant impacts usually require helmet replacement.

EPP Foam

Expanded polypropylene can recover more after moderate compression than EPS.

It is useful in helmets designed for repeated lower-energy impacts, although severe impacts can still damage it.

Foam Density

Density changes how easily liner cells crush.

Soft foam activates under lower loads, while denser foam handles higher energy before bottoming out. Multi-density liners combine ranges.

Multi-Density Liners

Different foam densities can be placed in layers or zones.

This broadens the impact range the helmet can manage rather than optimising for one single energy level.

Crush Distance

Liner thickness provides room for controlled deformation.

A very thin liner has less distance over which to slow the head, while thicker helmets trade compactness for energy-management capacity.

Bottoming Out

Bottoming out occurs when the liner is fully compressed and can no longer absorb significant additional energy.

Designs aim to avoid bottoming out within the test impacts the helmet is intended to manage.

Comfort Padding

Soft pads improve fit and skin comfort but usually provide little major impact protection compared with the structural liner.

They can be removable and washable without changing the main energy-management layer.

Fit

A helmet must match head size and shape closely enough to stay stable.

A helmet that slides excessively can expose parts of the head or move before the main impact.

Head Shapes

Human heads differ not only in circumference but also front-to-back and side-to-side proportions.

Brands and models therefore fit different head shapes even at the same labelled size.

Sizing

Helmet sizes usually correspond to head-circumference ranges.

Actual fit should be checked because padding thickness, shell shape and adjustment systems alter effective dimensions.

Retention Systems

Straps or harnesses keep the helmet on the head.

They must resist pulling loads without stretching excessively or detaching from the shell.

Chin Straps

Chin straps prevent helmets from lifting or rolling off during movement and impact.

Correct tension keeps the system secure without restricting breathing or causing significant discomfort.

Buckles

Buckles allow rapid fastening and release.

They are tested for retention strength, accidental release resistance and usability under expected conditions.

Y-Straps

Bicycle helmet side straps often form a Y around the ear.

Proper adjustment keeps the chin strap aligned and limits forward or backward helmet roll.

Harnesses

Hard hats often suspend a shell above the head with an internal harness.

The suspension spreads loads and creates clearance between shell and skull.

Rotational Motion

Many head impacts include rotation as well as straight-line acceleration.

Rapid rotational acceleration can strain brain tissue, so some helmet systems are designed to permit limited sliding or rotation between layers.

Slip-Plane Systems

Low-friction layers can allow small relative motion between helmet and head.

The goal is to reduce some rotational acceleration during angled impacts without compromising normal fit.

Angled Impacts

Real-world impacts often occur obliquely rather than straight on.

Tangential contact creates both linear and rotational components, complicating protection compared with simple vertical drop tests.

Bicycle Helmets

Bicycle helmets are designed mainly for single severe impacts against road or objects.

They prioritise low weight, ventilation and broad coverage while using crushable foam liners.

Road Cycling Helmets

Road helmets use large vents and aerodynamic shaping.

Low mass and cooling are valuable during long exertion, but vent geometry still must preserve structural performance.

Mountain Bike Helmets

Mountain bike helmets often extend farther around the rear and sides of the head.

Some include visors and stronger retention because off-road falls can involve varied impact directions.

Full-Face Bicycle Helmets

Downhill and BMX helmets add chin bars and greater facial coverage.

The additional structure increases weight and reduces airflow but addresses higher-speed and obstacle-rich riding.

Motorcycle Helmets

Motorcycle helmets are engineered for higher-energy road crashes and abrasion.

They use robust shells, thick liners, secure retention and often face shields.

Full-Face Motorcycle Helmets

Full-face helmets cover the chin and face more completely.

They offer broad impact and abrasion protection but require effective ventilation and visor management.

Open-Face Helmets

Open-face designs leave the chin exposed.

They provide less facial coverage but may improve airflow and communication.

Visors

Visors protect eyes from wind, debris and insects.

Optical clarity, scratch resistance, anti-fog performance and impact resistance all matter.

Anti-Fog Systems

Fog forms when moist breath or ambient air condenses on a cool visor.

Coatings, ventilation and double-pane visor inserts reduce condensation.

Pinlock-Style Inserts

A secondary inner lens can create an insulating air gap.

Keeping the interior surface warmer reduces the chance of water vapour reaching dew point.

Skate Helmets

Skate and BMX helmets often use tough shells and liners designed for repeated lower-energy impacts depending on certification.

Their smooth round shape also reduces snag points.

Climbing Helmets

Climbing helmets protect against falling rocks and impacts against the wall.

Some use suspended shells, foam, or hybrid constructions depending on design.

Industrial Hard Hats

Hard hats prioritise falling-object protection, penetration resistance and electrical or environmental requirements depending on class.

Suspension systems create clearance and distribute impact.

Sports Helmets

Football, hockey, lacrosse and equestrian helmets manage sport-specific impacts.

Face protection, repeated impacts, ventilation and field of view differ across sports.

Equestrian Helmets

Riding helmets are designed for falls from height and contact with hard ground or obstacles.

Secure retention matters because falls can involve tumbling.

Ski and Snowboard Helmets

Snow helmets combine impact protection with insulation and ear coverage.

They must also integrate with goggles and manage cold-weather ventilation.

Water Sports Helmets

Kayaking and whitewater helmets resist impacts with rocks and water environments.

Materials must tolerate repeated wetting and maintain secure fit when saturated.

Military Helmets

Military helmets combine ballistic, blunt-impact and equipment-integration requirements.

Their design differs fundamentally from sports helmets because projectile threats and communications equipment matter.

Ballistic Helmets

Ballistic helmets use fibre composites to catch and deform projectiles or fragments within rated threat levels.

They are not designed to make every projectile harmless; performance is defined by specific tests and limits.

Firefighter Helmets

Fire helmets combine impact, heat, water and penetration resistance.

Face shields, neck protection and mounts for lights or communication equipment add further functional layers.

Ventilation

Vents improve comfort by allowing heat and moisture to escape.

Every opening changes shell and liner geometry, so ventilation must be integrated without weakening tested protection.

Air Channels

Internal channels connect front vents to exhaust openings.

Head movement and external pressure differences help move air through the helmet.

Heat Stress

Heavy or poorly ventilated helmets increase thermal discomfort.

In hot environments, heat management influences whether users wear protection consistently.

Aerodynamics

Cycling and motorsport helmets can be shaped to reduce drag or lift.

Aerodynamic gains must not create sharp tails that increase snagging or compromise crash behaviour.

Noise

Motorcycle helmets experience wind noise from turbulent airflow around shell, visor and openings.

Sealing, neck rolls and smooth geometry reduce noise but can also reduce ventilation.

Weight

Lighter helmets reduce neck fatigue.

However, extremely low mass cannot come at the expense of liner thickness, coverage or required structural strength.

Balance

Helmet centre of mass affects how heavy it feels.

A balanced helmet places less sustained torque on the neck than one with equal mass concentrated far forward or backward.

Coverage

More coverage can protect more area but adds weight and heat.

Standards define minimum protected zones, while specialised helmets extend farther for particular risks.

A Worked Example: Foam Crushing

A cyclist falls and the helmet contacts the road.

The EPS liner crushes over several millimetres, converting kinetic energy into foam deformation and increasing stopping time compared with the unprotected skull hitting directly.

A Worked Example: Angled Impact

A helmet strikes pavement at an angle while moving forward.

Normal force slows the head linearly while friction creates torque. Shell sliding and low-friction layers can reduce part of that rotational load.

Common Misconceptions

A helmet does not make the head invulnerable, a thicker shell alone does not guarantee better protection and visible damage is not required for internal liner damage to exist.

Protection depends on shell, liner, fit, retention and impact conditions working together.

How to Learn Helmets Properly

Start with impact energy, stopping time and liner crush.

Then compare shells, straps and fit. Finally add rotational motion, activity-specific designs, standards and replacement.

Frequently Asked Questions

Helmets reduce injury risk by spreading impact and lengthening the stopping process. Most severe-impact bicycle helmets should be replaced after a significant crash because the foam may have crushed permanently.

Correct fit matters because a protective shell that moves off the impact area cannot perform as intended.

The Big Picture

A helmet is an energy-management system shaped around the human head.

The strongest mental model follows impact energy from the outer shell into deforming materials and retention straps while asking how much force and rotation finally reach the head.

Linear Acceleration

Linear acceleration describes how rapidly head velocity changes in a straight-line sense.

Helmet liners reduce peak linear acceleration by extending stopping time and distance. Laboratory drop tests often measure this component because it is repeatable and strongly linked to impact severity.

Rotational Acceleration

Rotational acceleration describes how rapidly the head’s angular velocity changes.

Brain tissue is sensitive to rotational motion because different regions can move relative to one another. Angled impacts therefore require more than one simple acceleration number.

Angular Velocity

Angular velocity measures how quickly the head rotates.

Two impacts can produce similar linear acceleration but very different rotation, which is one reason modern research examines both components.

Impact Duration

A short sharp impact creates a different loading profile from a longer lower-amplitude pulse.

Helmet design aims to spread the event over more time without allowing the head to move so far that the liner bottoms out.

Peak Versus Integrated Loads

Peak acceleration is easy to report, but injury risk can also depend on duration and combined motion.

Testing therefore uses several metrics rather than assuming one maximum number describes every crash.

Headforms

Helmet laboratories use instrumented headforms with standardised geometry and mass.

Sensors inside the headform measure acceleration while the helmet is dropped, struck or projected against test surfaces.

Drop Towers

A drop tower releases a helmeted headform from a controlled height onto an anvil.

Changing height changes impact speed, while different anvils simulate flat, rounded or edged surfaces.

Flat Anvils

Flat anvils represent broad hard surfaces such as road pavement.

They create large contact areas once the shell deforms and are common in certification testing.

Kerbstone Anvils

Narrower shaped anvils simulate concentrated edges.

They challenge penetration resistance and local load spreading more severely than broad flat impacts.

Hemispherical Anvils

Rounded anvils create different contact geometry and load paths.

A helmet may perform differently depending on whether impact area is broad, curved or edge-like.

Multiple Impact Locations

Standards test several helmet regions such as front, side, rear and crown.

Protection must remain adequate beyond one idealised centre point because real crashes are unpredictable.

Conditioning Before Tests

Helmets may be conditioned at hot, cold or wet temperatures before impact testing.

Materials change stiffness with environment, so certification checks that performance does not collapse outside comfortable room conditions.

Water Conditioning

Some standards soak helmets or expose them to high humidity.

The test is important for sports, cycling and industrial use where rain and sweat are normal.

Cold Conditioning

Cold temperatures can stiffen foams and make polymers brittle.

A winter helmet must still manage energy after prolonged exposure to low temperature.

Hot Conditioning

Heat can soften plastics, adhesives and foams.

Testing at elevated temperature checks whether the liner crushes too easily or shells distort.

UV Ageing

Ultraviolet light slowly degrades many polymers.

Outdoor helmets may receive stabilisers, but long exposure can embrittle shells, fade straps and weaken surface layers.

Sweat Exposure

Sweat contains water, salts and oils that contact straps, pads and liners repeatedly.

Materials are chosen to resist hydrolysis, corrosion and odour during ordinary use.

Cleaning Chemicals

Strong solvents can damage shell polymers or adhesives.

Manufacturers usually specify mild soap and water rather than household chemicals whose compatibility is unknown.

Paint and Stickers

Some paints, adhesives and stickers contain solvents that can attack specific helmet shells.

Users should follow manufacturer guidance before applying custom coatings or decals.

Ageing

Helmet materials change with time even without a crash.

Foams can harden, straps can fray and adhesives can weaken, which is why some manufacturers recommend replacement intervals based on use and age.

Hidden Damage

A shell may look normal after a crash while EPS underneath has crushed.

Because the liner does not fully recover, invisible internal damage can reduce protection in a later impact.

Single-Impact Designs

Many bicycle helmets are optimised for one major impact.

After substantial liner crush they should be replaced rather than reused simply because the outer shell still looks intact.

Multi-Impact Designs

Some EPP or specialised sport helmets are designed for repeated lower-energy impacts.

That does not mean unlimited reuse after severe crashes; inspection and manufacturer criteria still matter.

Shell Cracking

Composite shells can crack, delaminate or whiten under severe load.

Visible cracking is a clear replacement sign because structural load distribution has changed.

Strap Damage

Cut, frayed or heat-damaged straps compromise retention.

Replacing only the buckle may not restore safety if webbing or anchor points are weakened.

Anchor Points

Retention straps attach to shell or liner at reinforced points.

These anchors must survive high pull loads without tearing through the helmet.

Roll-Off Testing

Standards can test whether a helmet can be pulled or rolled off a headform.

A strong shell is useless if the helmet leaves the head before impact is over.

Buckle Strength Testing

Buckles and straps are loaded mechanically to verify retention strength.

Testing also checks that users can release the buckle intentionally after ordinary use.

Field of View

Helmets must protect without blocking critical vision.

Motorcycle and sports designs specify opening geometry so users can see traffic, teammates or hazards.

Peripheral Vision

Side vision is especially important for cycling and motorcycling.

Thick padding or poorly designed visors can create blind areas even when forward vision remains clear.

Hearing

Helmet shells and padding can alter sound direction and loudness.

Designers balance wind-noise reduction with enough environmental hearing for situational awareness.

Communication Systems

Motorcycle, military and industrial helmets may integrate microphones and speakers.

Added hardware must not create hard pressure points or compromise liner thickness in critical zones.

Camera Mounts

Action-camera mounts add protrusions to helmets.

Rigid mounts can catch on surfaces or concentrate load, so breakaway designs and manufacturer guidance matter.

Visor Mechanisms

Motorcycle visors pivot on side mechanisms and seal against eye-port gaskets.

Detents hold selected positions while allowing quick opening during fogging or communication.

Anti-Scratch Coatings

Visors often receive hard coatings to resist abrasion.

Fine scratches scatter light, especially at night when headlights create glare.

Optical Distortion

A curved visor can distort vision if thickness or shape varies.

High-quality visors control optical power so straight lines and distances appear natural across the field.

Sun Visors

Internal tinted visors reduce glare without replacing the clear external shield.

They must retract fully and remain optically clear enough for road use.

Photochromic Visors

Some visors darken under ultraviolet or visible light.

Response speed, temperature and legal tint limits influence real-world usefulness.

Vent Inlets

Front vents admit air when the wearer moves.

Ducts inside the liner guide that air over the head before it exits at rear pressure zones.

Exhaust Vents

Low-pressure regions behind the helmet help pull warm air out.

Vent placement is therefore aerodynamic, not simply decorative hole cutting.

Rain Management

Helmet vents can admit water in heavy rain.

Channels, covers and internal padding help redirect moisture away from eyes while preserving airflow.

Sweat Pads

Removable pads absorb sweat and improve fit.

Washable pads extend hygiene life without requiring the impact liner itself to be cleaned aggressively.

Odour Control

Helmet odour comes from sweat, skin oils and microbial growth in padding.

Drying thoroughly between uses and washing removable liners reduces buildup.

Hair and Fit

Hair volume changes how a helmet sits.

Fit should be checked with the hairstyle normally used during the activity, especially if thick braids, head coverings or seasonal caps alter circumference.

Head Coverings

Religious or thermal head coverings can affect helmet fit.

Workplaces and sport organisations increasingly consider compatible designs so protective equipment can remain secure without forcing unsafe compromises.

Adjustment Dials

Bicycle helmets often use rear ratchet systems to tighten a cradle around the head.

The dial improves stability but cannot compensate for a shell that is fundamentally the wrong size or shape.

Internal Cradles

Cradles distribute adjustment pressure around the head.

They separate comfort fit from the hard EPS surface and allow small positional changes.

Occipital Support

Rear retention often cups the occipital region at the back of the skull.

This helps resist forward roll and stabilises the helmet during rapid movement.

Chin-Bar Design

Full-face helmets use chin bars to protect jaw and lower face.

The bar must resist direct impact without creating excessive leverage on the neck or collapsing into the face.

Motorcycle Shell Sizes

Some helmet models use several outer-shell sizes across the size range.

Using one huge shell with thick padding for small heads can increase bulk and mass unnecessarily.

Interior Pads

Cheek pads and crown pads fine-tune fit inside motorcycle helmets.

They should hold the helmet firmly without painful pressure points and often compress slightly with use.

Helmet Break-In

Comfort padding can soften with wear.

A new helmet that is slightly snug may become comfortable, but a painful shell-pressure mismatch will not be fixed by padding compression.

Noise and Sealing

Neck rolls and cheek pads influence wind noise by controlling airflow around the opening.

A quieter helmet often feels tighter around the lower edge because leakage paths are reduced.

Low-Friction Layer Systems

Some helmets use low-friction internal layers intended to reduce rotational acceleration in selected angled impacts.

The exact implementation varies by model, so the presence of one technology does not by itself define total helmet performance.

Sliding Shell Concepts

Other designs use shells, pads or elastomers that permit controlled relative motion.

The shared principle is to manage tangential energy as well as direct compression.

Cellular Liners

Some helmets use honeycomb, lattice or tubular structures instead of conventional foam.

Cells can buckle, shear or collapse in designed patterns to absorb energy and control airflow.

3D-Printed Liners

Additive manufacturing can create complex lattice liners with tuned stiffness by region.

The technology enables geometry difficult to mould traditionally, though cost and certification remain major factors.

Inflatable Fit Systems

Some helmets use air bladders for fine fit adjustment.

Air pressure can distribute contact more evenly, but bladders are primarily fitting systems unless specifically engineered and tested for impact management.

Helmet Standards

Safety standards define test methods, impact conditions and performance limits.

Different standards target different activities, so a helmet certified for one use should not automatically be assumed suitable for another.

Certification Labels

A certification mark indicates that a helmet model has been tested or assessed to a defined standard.

Users should verify the relevant standard rather than treating any sticker as universal proof of safety.

Test Limitations

Certification tests cannot reproduce every real crash.

They provide a minimum repeatable benchmark, while real-world performance also depends on impact angle, speed, fit and secondary contacts.

Comparative Ratings

Independent programmes sometimes rate helmets beyond pass-fail certification.

Such ratings can add information but use their own protocols, so users should understand what is measured rather than assuming one score captures every hazard.

Counterfeit Helmets

Counterfeit products can copy logos and certification labels without matching tested construction.

Buying from reputable channels reduces the risk of shells, liners or straps that only imitate protective design visually.

Used Helmets

A second-hand helmet may have unknown crash history, storage or chemical exposure.

Without reliable history, hidden liner damage is difficult to rule out.

Storage

Helmets should be stored away from extreme heat, solvents and crushing loads.

A car dashboard or hot enclosed boot can expose materials to temperatures far above ordinary room conditions.

Transport

Helmets can be damaged by heavy items pressing against them during travel.

Protective bags reduce cosmetic scratches, but rigid cases are better when luggage may be crushed.

Crash Reconstruction

Investigators can examine helmet scuffs, cracks and deformation as part of reconstructing a collision.

Damage marks can suggest contact locations and sliding directions, although helmet evidence must be interpreted with scene, vehicle and medical information rather than alone.

Multiple Contacts

A real crash can involve several impacts as the body tumbles or slides.

A helmet that manages the first contact may face a second load before the event ends, which is one reason retention and broad coverage matter.

Secondary Impacts

After an initial collision, the head can strike another object or the ground.

Helmet performance is therefore not only about the first peak but also staying correctly positioned through the entire sequence.

Surface Friction

The friction between shell and impact surface affects rotational loading.

Smooth shells can slide across pavement more readily than shapes with large protrusions, reducing some snagging forces.

Shell Shape

Rounded exterior geometry helps impacts glance rather than catch.

Visors, vents and accessories are designed carefully so they do not create unnecessary hooks against the ground.

Neck Interaction

Helmet mass and centre of gravity influence neck loading during rapid movement.

A heavier helmet is not automatically unsafe, but unnecessary mass far from the head increases inertial torque.

Children’s Helmets

Children’s heads differ in size and proportion from adults.

A properly sized child helmet should sit level, remain stable and use straps that can be adjusted without large loose loops.

Growing Children

Children can outgrow a helmet even when it remains physically undamaged.

Regular fit checks are important because circumference and head shape change over time.

Rental Helmets

Rental operations need inspection, cleaning and retirement procedures.

High turnover makes crash-history tracking and strap maintenance especially important.

Fleet and Workplace Helmets

Employers may manage helmets as safety equipment with inspection records and replacement schedules.

Shared equipment also requires hygiene procedures and size ranges so users are not forced into poor fit.

Custom-Fit Helmets

Scanning and additive manufacturing can produce liners shaped around an individual’s head.

Custom fit can improve comfort and stability, but protective performance still requires validated materials and impact testing.

A Worked Example: Crash Replacement

A bicycle helmet falls with the rider and strikes pavement hard enough to dent the EPS beneath the shell.

Even if the shell springs back visually, the crushed liner has permanently lost part of its energy-absorbing thickness, so replacing the helmet restores full design capacity.

A Worked Example: Poor Strap Fit

A helmet sits loosely with the chin strap several centimetres from the jaw.

During an angled fall, the shell can rotate backward and expose the forehead before the main impact. Tightening and correctly positioning the straps changes real protection without changing the helmet materials at all.

Practical Diagnostic Thinking

If a helmet feels unstable, first check size, head shape, strap geometry and adjustment.

If protection is in doubt after a crash, visible shell inspection is not enough; impact history and liner condition matter more than cosmetic appearance.

The Systems View of Helmets

Helmet protection emerges from shell, liner, retention, coverage, fit and the physics of the specific impact.

The best helmet is not simply the thickest or hardest object; it is a carefully tuned energy-management system that stays correctly positioned when needed.