Linear vs. Rotational eBike & Bicycle Helmet Impacts: Biomechanical Analysis of Head Trauma Types and Protective Strategies
Why This Matters
Most bicycle and eBike crashes don’t involve a straight-on hit — they involve angled, oblique impacts. That means the head experiences both linear and rotational forces simultaneously. And those two forces cause completely different types of injuries.
Linear impacts primarily cause skull fractures and focal injuries.
Rotational impacts primarily cause concussions, subdural hematomas, and diffuse axonal injury (DAI).
For eBike riders traveling at up to 28 mph, the difference matters even more — because crash energy increases exponentially with speed.
This article explains the biomechanics behind these forces, how helmets protect against them, where traditional standards fall short, and what modern technologies (MIPS, WaveCel, SPIN, etc.) actually do — along with the growing industry debate about whether slip-plane systems genuinely outperform high-quality shell/liner designs.
Understanding Linear Impacts: Direct Hits and Focal Injuries
A linear impact occurs when the head is struck squarely, pushing it straight backward or forward in a straight line. This causes linear acceleration and high force at the impact site.
Biomechanically, linear impacts create:
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Coup pressure (high pressure at impact site)
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Contrecoup pressure (negative pressure on opposite side)
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Rapid skull deformation that compresses underlying brain tissue
Research shows:
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Skull fracture risk begins around 180 g of linear acceleration
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At 250 g, fracture risk rises to ~40%
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High linear loads often cause contusions and epidural hematomas
EPS foam — the material used in nearly all helmets — is specifically engineered to crush under linear force and prevent these injuries.
Pure linear impacts are rare. Most real crashes involve oblique contact that introduces significant rotational motion as well.
Understanding Rotational Impacts: The True Driver of Concussions and DAI
Rotational impacts occur when the head hits at an angle, causing it to twist around its center of mass.
Rotational forces create shear strain throughout the brain. This is profoundly damaging because neural tissue is weakest in shear — not compression.
Rotational motion is the primary cause of:
1. Subdural Hematoma (SDH)
Bridging veins stretch and tear during head rotation. Even moderate angular acceleration can rupture them.
2. Diffuse Axonal Injury (DAI)
Rapid twisting stretches white-matter axons beyond their mechanical tolerance, causing:
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Axonal tearing
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Disrupted microtubules
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Loss of cognitive and motor function
3. Concussion
A rotational injury at its core. Linear acceleration contributes, but rotational kinematics correlate far more strongly with concussive brain strain.
Finite element models consistently show rotational impacts produce magnitudes of brain strain far exceeding linear impacts at the same velocity.
Real-World Crash Data Confirms It: Cycling Impacts Are Mostly Oblique
Analysis of 23 real-world damaged helmets found:
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Normal velocity: 3.5 m/s
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Tangential velocity: 2.5 m/s
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Peak linear acceleration: ~108 g
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Peak rotational velocity: ~15.7 rad/s
Even concussive impacts didn’t require extreme force — just the right rotational conditions.
This is why concussions can occur even when linear g-forces seem “moderate.”
For eBike riders (Class 3, 28 mph), kinetic energy is 5.4× higher than at recreational cycling speeds. Higher speed → more off-axis impacts → more rotational trauma.
Where Traditional Helmet Standards Fall Short
Legacy standards such as CPSC (U.S.) and EN 1078 (EU) focus almost entirely on linear acceleration, because they were created decades ago to prevent skull fractures.
They do not:
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Test rotational acceleration
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Measure brain strain
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Simulate oblique impacts
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Use realistic scalp or hair friction coefficients
Helmets can pass these tests while offering little protection against the most common and dangerous injury mechanisms in modern crashes — especially for faster eBike riders.
Modern Helmet Technologies for Rotational Protection (Neutral & Evidence-Based Overview)
Rotational-Mitigation Systems: What They Aim to Do
Rotational protection technologies—whether slip-plane layers, deformable liners, or elastomeric structures—share one goal:
Reduce the twisting motion of the head during oblique impacts, which is linked to concussions and diffuse brain injuries.
Different systems attempt this in different ways.
MIPS (Multi-Directional Impact Protection System)
MIPS uses a low-friction slip liner designed to allow the helmet shell to move slightly relative to the head—typically around 10–15 mm.
Independent laboratory studies using Hybrid III headforms have reported:
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↓ Rotational velocity: 22–26%
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↓ Rotational acceleration: 22–37%
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↓ Brain strain indicators: ~20%
These results show meaningful reductions in certain controlled test configurations.
However, effectiveness can vary depending on:
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Helmet model
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Headform friction settings
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Impact angle
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Integration quality
In other words: MIPS is a tool—not a guarantee—and its real-world performance depends heavily on the specific helmet design.
WaveCel
WaveCel uses a collapsible honeycomb liner that deforms during impact to manage both linear and rotational forces.
Studies have shown:
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↓ Linear acceleration: ~17%
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↓ Rotational velocity: 29–73%
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↓ AIS2+ injury risk: ~22%
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Lower simulated brain strain than traditional EPS-only helmets
In certain controlled lab tests, WaveCel outperformed slip-plane systems.
But again, results vary depending on helmet geometry, fit, headform type, and test configuration.
SPIN, ODS, LDL, and Other Systems
These systems use gels, elastomers, or internal structures to reduce rotational kinematics.
Findings are mixed:
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Some models show significant reductions in rotational acceleration
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Other designs have shown increased rotational forces under specific conditions
Key takeaway:
Technology type matters less than how well it is engineered and integrated into the helmet.
Industry Debate: Why Some Brands Don’t Use Slip-Plane Systems
Not all manufacturers agree that slip-plane systems are the only—or even the best—solution for rotational protection.
KASK’s Position (WG11 Test Standard)
KASK, a premium Italian manufacturer, has publicly stated that MIPS did not improve the performance of their helmets in their internal testing.
Their product director explained:
“It is still not proved that it makes the helmet safer… we realized that it doesn’t really add any value to the security of the helmet, while for sure it adds weight and provides less comfort.”
— VeloNews Interview
KASK’s rationale:
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Their WG11 rotational test uses friction conditions they believe more closely resemble actual human scalps
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Many MIPS studies use high-friction headforms, which can exaggerate rotational-reduction benefits
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They argue that a well-designed shell and liner can achieve excellent rotational performance without a slip layer
This illustrates an important point:
Rotational protection is a design philosophy—not a single product.
Different brands approach it differently, and there is no single universal solution.
The “Hair Is a Natural Slip Plane” Rumor — What the Science Actually Suggests
A common belief among riders is:
“If you have hair, it acts like a natural MIPS.”
There is some biomechanical logic behind this:
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Hair reduces scalp-helmet friction
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Some studies using more realistic scalp coverings or wigs have shown reduced differences between MIPS and non-MIPS helmets
But this effect is highly variable:
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Hair type, length, density
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Sweat/oil
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Helmet fit
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Strap tension
And crucially:
Hair does not provide a predictable, engineered slip mechanism.
So while hair may modestly reduce friction in some scenarios, it is not considered a reliable safety feature.
Why eBike Riders Need Protection From Both Linear and Rotational Forces
Because of the higher speeds involved (20–28 mph), eBike crashes create:
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Higher impact energy
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More oblique (angled) impacts
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Higher rotational accelerations
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Increased risk of DAI and subdural hematoma
This is why the NTA 8776 standard, used in high-speed eBike helmets, requires:
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Higher test velocities
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250 g impact limits (vs. 300 g in CPSC)
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Thicker/densified foam
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Extended head coverage (temples + occipital)
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Testing that better reflects real-world eBike crash conditions
Slip-plane systems can complement these foundations, but NTA 8776 already represents a major safety step beyond traditional bike standards.
How to Choose a Helmet for Real-World Crash Protection
1. Minimum:
CPSC or CE certification for baseline linear impact protection.
2. Better:
A helmet with either rotational mitigation (MIPS, WaveCel, SPIN, etc.) or a brand that uses a validated rotational test protocol (e.g., WG11).
3. Best for eBikes:
An NTA 8776-certified helmet engineered for higher-energy collisions, extended coverage, and real-world oblique impacts.
4. Fit is as important as technology:
Even the best helmet performs poorly if loose.
Conclusion
Linear impacts break bone.
Rotational impacts damage brain tissue.
Real crashes involve both.
Rotational-mitigation technologies like MIPS and WaveCel can reduce rotational forces, but:
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Their performance varies widely
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Not all designs integrate them effectively
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Some brands achieve strong rotational protection without slip planes
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And natural factors (like hair) complicate real-world friction conditions
The most important takeaway for riders is this:
Choose a helmet designed for the impacts you’re actually likely to experience—especially the higher-energy, off-axis impacts common with eBikes.
NTA 8776-certified helmets already address many of these real-world challenges through higher impact thresholds, improved coverage, and more demanding testing standards.
Sources & References (URLs)
(All mapped to studies and articles referenced in this article)
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Gennarelli et al. “The relationship between head impact characteristics and brain trauma.”
https://www.iomcworld.org/open-access/the-relationship-between-head-impact-characteristics-and-brain-trauma-2155-9562-5-181.pdf -
Kleiven, S. “A review of head injury criteria in relation to automotive crashworthiness.”
https://www.frontiersin.org/articles/10.3389/fbioe.2013.00015/full -
Ratajczak et al. “Computational Modelling and Biomechanical Analysis of Age-Related Craniocerebral Injuries.” Applied Sciences 14(7):2681.
https://www.mdpi.com/2076-3417/14/7/2681 -
Meaney & Smith. “Diffuse axonal injury.”
https://www.ncbi.nlm.nih.gov/books/NBK448102/ -
Giordano & Kleiven. “Evaluation of axonal strain as a predictor for TBI.”
https://pmc.ncbi.nlm.nih.gov/articles/PMC4654450/ -
Harlos & Rowson (2021). “Laboratory Reconstructions of Real-world Bicycle Helmet Impacts.” Annals of Biomedical Engineering.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8452122/ -
Rowson et al. “Concussion biomechanics and brain response.”
https://pmc.ncbi.nlm.nih.gov/articles/PMC4090913/ -
ACT Lab. “NTA 8776 Standard Testing for e-Bike Helmets.”
https://act-lab.com/nta-8776-standard-testing/ -
CPSC Bicycle Helmet Business Guidance.
https://www.cpsc.gov/Business--Manufacturing/Business-Education/Business-Guidance/Bicycle-Helmets -
KTH Thesis: “Helmet optimization for linear and rotational head injury metrics.”
http://kth.diva-portal.org/smash/record.jsf?pid=diva2%3A1780790 -
Bottlang et al. (2019). “Impact Performance Comparison of Advanced Bicycle Helmets with Dedicated Rotation-Damping Systems.” Annals of Biomedical Engineering.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6928098/ -
Design News – “How MIPS Helmet Technology Reduces Concussion Chance.”
https://www.designnews.com/automotive-engineering/how-mips-helmet-technology-reduces-concussion-chance -
MIPS official explanation.
https://mipsprotection.com/helmet-technology/ -
Bliven et al. (2019). “Evaluation of a novel bicycle helmet concept in oblique impact testing.” Accident Analysis & Prevention 124:58–65.
https://doi.org/10.1016/j.aap.2018.12.017 -
Outside Online – “Trek WaveCel Helmet Controversy.”
https://www.outsideonline.com/outdoor-gear/bikes-and-biking/trek-wavecel-helmet-controversy/ -
Hoshizaki et al. (2022). “Evaluation of two rotational helmet technologies to decrease peak rotational acceleration in cycling helmets.” Scientific Reports.
https://www.nature.com/articles/s41598-022-11559-0 -
Kleiven. “Why brain injury measures are complicated.”
https://pmc.ncbi.nlm.nih.gov/articles/PMC4227498/ -
Wu et al. (2022). “Finite element models of the human brain for injury prediction.”
https://www.frontiersin.org/articles/10.3389/fbioe.2022.860112/full -
Zou et al. “Head kinematics and brain response under impact.”
https://ibrc.osu.edu/wp-content/uploads/2014/11/Zou-et-al.pdf -
Frontiers in Cellular Neuroscience – axonal injury mechanisms.
https://www.frontiersin.org/articles/10.3389/fncel.2014.00429/full -
Abayazid et al. (2021). “Brain Injury Mitigation Effects of New Technologies in Oblique Impacts.” Annals of Biomedical Engineering 49:2716–2733.
https://doi.org/10.1007/s10439-021-02785-0 -
Bern Helmets – “E-Bike Helmet Guide: NTA-8776 Certified Safety.”
https://bernhelmets.com/pages/e-bike-helmet-guide -
LeoGuar Bikes – “Electric Cycle Helmet 20–28 mph eBikes.”
https://leoguarbikes.com/blogs/news/electric-cycle-helmet-20-28mph-ebikes -
Virginia Tech Bicycle Helmet Ratings.
https://www.helmet.beam.vt.edu/bicycle-helmet-ratings.html -
Helmets.org – “Rotational Injury and Bicycle Helmets.”
https://helmets.org/rotation.htm -
Dr. Biomechanics – rotational acceleration and helmet testing.
https://drbiomechanics.com/tag/rotational-acceleration/ -
Concussion Alliance – Biomechanics of head trauma.
https://www.concussionalliance.org/blog/2022/1/23/a-review-of-the-biomechanics-data-acquisition-and-head-trauma -
European Transport Safety Council – “Seniors in Traffic and E-bikes.”
https://etsc.eu/wp-content/uploads/Katerina-Bucsuhazy-seniors-in-traffic-and-ebikes.pdf -
Virginia Tech STAR methodology overview.
https://helmets.org/vatechstar.htm -
XNITO – “How Fast Is Too Fast? eBike Speed Limits, Safety Tips, and Risks Over 20 mph.”
https://xnito.com/nl-au/blogs/our-news/how-fast-is-too-fast-ebike-speed-limits-safety-tips-and-risks-over-20-mph - Industry Debate (KASK, WG11, MIPS)
https://velo.outsideonline.com/road/road-racing/kask-wg11-rotational-energy-impact-test-and-mips/