By providing a shock-absorbing layer of crushable material around your head, a cycling helmet is intended to reduce the damage if you are unfortunate enough to come off your bike and hit your head. Cycling helmets have come a long way in weight, fit and ventilation since the heavy brain buckets of the 1970s, and we've racked up tens of thousands of miles in a huge range of helmets to bring you the best helmets you can buy.
A cycling helmet holds a layer of shock-absorbing material against your head so that the force transmitted to your head if you crash is reduced. Holes in the structure (the posh term is 'vents') allow air through to reduce overheating; channels inside the shell give it somewhere to flow and produce comedy helmet hai.
To provide any protection a helmet needs to to fit snugly; take the time to fit it properly. Modern helmets make this fairly straightforward with dial adjusters at the back.
In general, the more you pay, the less you get. To make helmets lighter manfufacturers use built-in reinforcing skeletons and lightweight components, which all pushes up the cost. A recent innovation, Mips, is claimed to improve impact absorption in certain crashes and is becoming fairly common.
Tester Liam writes: "An update of Specialized's previous top-end helmet, the S-Works Prevail II Vent is cool, comfortable and fits very well, plus you still get the safety benefits of Mips and the optional ANGi technology should you have an off. As well as plenty of vents at the front and back, there are now three huge vents at the top and sides instead of ten large ones, so with my hair calibrated for waft, I headed out to see if I could tell the difference. The results? A marginally more frozen head on a January morning with the Prevail II Vent than with the standard Prevail II.
"The Vent keeps the Prevail II's comfort and is an easy lid to wear on longer rides. On my head the Vent sits quite low, with the front rim dropping to just above my eyebrows. How it sits on you will depend on your head shape, but if you're looking for a bit of extra skull coverage, it could be a good option. The retention system provides plenty of adjustment at the rear."
The ANGi crash sensor is now a £45 optional extra. When he tested it Mat Brett said "I set myself up as my own emergency contact just to test out ANGi, and I've been receiving all the start/finish ride alerts, while a 'test mode' function on the Ride app allows you to check that the device is detecting the various directional forces when you give it a shake. It's a neat system that offers some peace of mind."
Tester Dave writes: “If you asked me how much I'd be prepared to pay for a comfortable, well-ventilated helmet, with a good dial retention system and modern construction techniques, that weighs about 250g, I'd probably say that was about sixty quid's worth of hat, or more. So the fact that the Oxford Raven is just £39.99 makes it a bargain, if the performance lives up to the spec on paper. Which it does.
“Mostly you want to put a helmet on your head and forget about it until it's time to take it off again. The Raven is EN1078 rated so it should do the same amount of protecting your head as any other helmet with the same rating, should the worst happen. And once you're off and running it's an easy helmet to forget. It's comfortable and it's well ventilated, and the retention system is secure without exerting undue pressure. I've completed some long rides in this lid (including this 280km all-dayer ) and I've had no major issues with it. It doesn't play particularly nicely with my favourite Tifosi sunglasses; the thick legs get knocked about by the helmet and straps a bit. It's much better with glasses that have thinner legs, and ones that bow out from your temples a bit more. The only other issue is that the pads at the front have fairly rigid spines that press into your forehead: it's not at all uncomfortable, but it does make you look like an extra from Star Trek when you walk into the cafe.”
“The only other issue with the yellow one is the colour: it's a bit 'cheap fluoro' and the black/blue/red/white ones are better, especially black and red. If it was my money I'd probably plump for one of those two. But if it was my money, or a friend's, I'd certainly be happy to lay it out on this, or recommend that they did, if they were looking for an inexpensive road helmet that put in a solid performance. For forty quid you're getting what would normally cost you significantly more.”
Tester Neil writes: “Your tester's new favourite helmet: the Giro Agilis Mips is a great fit for narrow craniums. It's also high quality, well vented and decent value. It's a rare and happy day for a hard-working product tester when he can plonk on a new arrival and everything feels just right from the off. I didn't even need to adjust the straps; though our charming model must have done this at the office before it came to me so I can hardly thank Giro for that. What I can applaud is a helmet that fits my narrow head so well. (I always maintain the midwife delivered me by trapping my head between a couple of house bricks.)
“The strap system is Giro's Roc Loc 5 which worked well for me, adjustment being by a single wheel at the back of the head. There's also some scope for reducing the volume of the helmet with the three-position adjuster inside the rear. That's a bit fiddly but you only need to do it once.
“There's lots of ventilation here, too. It's been warm enough lately to ride without any headwear under the helmet and there's a definite cooling breeze blowing through. The Mips insert is custom-cut so that it doesn't cover any of the vents.
The Met Manta Mips Aero Road helmet offers a good blend of aero performance and ventilation while integrating Mips protection nicely. It's a good all-round helmet with a strong leaning towards fast club riders and racers interested in aero performance.
Tester Leon writes: “Out on the road I've found the new Manta Mips to be fast (well, fast-feeling) compared to my usual airy and lightweight climber's helmet – a Kask Valegro at the moment – and surprisingly airy too. The front six channels in combination with the seventh top vent provide just enough ventilation to regulate the temperature around your head on a warm British summer's day. It's only overwhelmed by a particularly humid warmth or when temperatures on the road start edging towards the high 20s.
“The Mips technology has been nicely integrated, and the safety benefit of that – which no one hopes to ever have to test – puts you in the best position possible if you do have a spill. The Manta Mips costs a fair amount of money, but in exchange you get a very accomplished helmet well-attuned to fast riding and racing.”
The Ekoi Corsa Light is a well-vented featherweight helmet that can be switched to an aero lid in seconds. The buckle system is genius, and the straps comfy, though the aeroshell seems a little flimsy – Ekoi itself says 'requires careful handling to avoid damage'.
Tester Liam writes: “The Corsa Light features 24 vents and 10 exhaust vents. These do a fantastic job of keeping the head cool, with noticeable airflow across the top and temple areas. Covering these vents with the aeroshell decreases that airflow, but does not lead to a hot head. During normal riding, around 16mph, my head was cooled efficiently by the exhaust vents.
“For a base package of just the helmet, the RRP of £93 is not overly expensive for a pro-spec helmet (and at the time of publishing, it's reduced to £68ish, depending on exchange rate). Adding on £25 for the aeroshell is still significantly cheaper than the majority of performance helmets, with the added benefit of two helmet options. Ekoi has kept the ratchet simple and the magnetic closure system works really well.”
Giro's Aether is a fabulous helmet that offers an excellent level of ventilation, plenty of comfort and new Mips Spherical technology.
Tester Mat writes: “Let's start with the Mips Spherical tech because that's the most important feature. The idea is that in the event of an angled impact, the inner layer slides beneath the outer layer, reducing rotational motion transferred to the brain. The advantage of Mips Spherical compared with a Mips liner is that there's no obstruction to ventilation. You get loads of air flowing over your head here, the three central vents being particularly large.
“At £259.99 RRP, the Aether is far from a bargain bucket option, but if you want a load of ventilation and comfort, and you're convinced of the value of Mips, this helmet is exceptionally good.”
Tester Niel writes: “The Bell Stratus Mips Helmet is quality, comfortable with a great fit, and comes in a wide range of colours. The first thing I noticed on opening the box, apart from the obviously lurid colour, was the great quality finish. All the expanded polystyrene is covered by the outer shell (with the minor exception of some aero channels). It's super-shiny and slippery and, along with the overall shape of the thing, looks like it has been designed with smooth airflow in mind.
“The Mips insert is closely matched to slots in the helmet shell so there's no restriction of airflow over the scalp, with space between the helmet and the insert allowing plenty of room for air to get in (and out). This is further aided by the grooves cut into the foam shell itself, at the brow, which channel air around this crucial area too. While March and April have failed to deliver any of the 30°C days that Bell anticipates you may be riding in, I found the temperature comfort level good on both mild and cold days, with no disagreeable draughts on cold descents. In fact, the helmet has a very slippery feel about it and didn't generate any sense of turbulence on fast downhills.
“Out on the road, this felt light and unobtrusive. I don't want to be reminded that I'm wearing a helmet and this sat securely and low on the brow but without getting in my line of vision or causing any obvious shift of balance in my head weight. If comfort is very important to you, and if you've had problems finding a helmet that sits well, the Stratus will be worth a test fitting.”
Tester Stu writes: “The Scott Centric Plus is one of the most comfortable helmets I've worn in a long time. It's very light too, and the ventilation works brilliantly at transferring cool air over your head. While it's not cheap, it is very well finished.
“At 222g for this medium (55-59cm) size, it is certainly a very light helmet and feels that way when you are wearing it. With 18 vents it is also one of the most airy I have used. Catching the tail end of the warm weather of the summer during testing and then heading into autumn, it certainly keeps you cool when you're riding hard or the temperatures are high.
“Overall, this is one of my favourite helmets. That's largely down to the fact that it fits me so well, but also because of the lightweight construction and extra airflow. Yes, you can get quality helmets with Mips for less than £100, but compared to the competition the Scott does well.”
The Abus StormChaser is an exceptionally comfortable helmet with great ventilation. It's light, offers lots of adjustment, and all pads and straps can be stripped out for washing – in fact, it gives some of Abus' own more expensive helmets a serious run for their money.
Tester Matt writes: “I found the fit excellent and the helmet very comfortable despite minimal padding. The amount of adjustment is enough that it should fit most head shapes. The dial adjuster is easy to find and use, even with gloves on, and the steps are small enough to really fine-tune the fit. You can also adjust the height by almost 4cm, and even the mounting points for the cradle itself can be moved between three options for the most secure, comfortable fit.
“The 23 vents (7 in, 16 out) have a noticeable effect, and the cooling is very welcome on warm days. The ventilation seems at least on a par with the more expensive AirBreaker model. Sunglasses fit neatly into the vents and sit securely, too, which is a bonus.”
The Laser Z1 Mips helmet brings together the already impressive strengths of the original Z1 and adds the extra protection promise of the Mips – Multi-directional Impact Protection System – layer.
Tester George writes: “The original Z1 offered impressive comfort thanks to the innovative closure system, with the dial on the top of the helmet tightening the system around the entire head rather than just at the back, so exerting pressure around the entire head too, not just the back. The system holds it in place well even when you're particularly sweaty, and whatever your head shape. The Mips has been integrated really well so it doesn't impact this, which is something I was initially worried about having used the original model for a couple of years prior to this one.”
“Ventilation is also a real strength of the helmet, with the Mips layer having negligible impact compared with the original version. You can still feel the wind blowing through your hair as you ride, with just a slight reduction that I only really notice because I regularly wear the Mips and original version on consecutive rides.”
Met's Strale helmet strikes a balance between aerodynamics, cooling and comfort that makes you wonder whether you really need to spend any more on a polystyrene lid.
Tester Stu writes: “With eight vents up front, the Strale has a lot fewer than most traditional helmets but you certainly don't overheat when you're wearing it. A few weeks ago the temperature in this part of the country was nudging 30°C and I spent the week riding in the Met without issue. Met says that it uses the Venturi effect, scooping air in at the front, and with the small gap between your head and the inside of the helmet, the hot air from your head is forced out of the rear to keep you cool. If it is working then you can't physically feel it, but I certainly never felt like I was baking in this matt black helmet.
“Met's shape and fitting system suits the shape of my head so I find its helmets really comfortable as I can get them fitting snug without any pressure points anywhere. That's pretty good from a helmet that only adjusts the rear section for size rather than the full circumference, like the cable system used on my Poc. The fit is controlled by the Safe-T Duo Fit System, which is adjusted circumference-wise by a small dial with close increments to get everything just right. You can also adjust the position vertically by four settings – and flop your ponytail through, too, if necessary.”
The Persist replaces the Portal in Smith's range – and gets some decent upgrades, yet the price remains basically the same. There are now three more strategically-placed vents across front and rear. These are an integral part of the AirEvac design used in Smith Optics' helmets, and designed to drive warm air away and keep glasses steam-free.
Tester Janine writes: My husband tried to nick this helmet in the first week thanks to its appealing looks. You can't help but be struck by its dazzling colour; this is the only brave choice of three options that include matte black and matte white. The next thing to note is the high quality finish, with its expanded polystyrene covered completely by the attractive matte polycarbonate shell. Its in-mold construction means the two layers are permanently bonded together. The outer is super-silky, the only downside being that it doesn't hide minor scrapes as easily as a gloss shell.
"Overall, this is a good price for a premium-finish helmet with excellent adjustability, a secure feel, a comfortable inner and generous vents.
The Giro Syntax Mips is a well ventilated and comfortable helmet that deals well with all kinds of road riding. The rotational-force damping Mips system is really well integrated for noticeable gains in cooling and comfort, and the quality is high. If you're looking at mid-market helmets, there's almost nothing to dislike about this one.
Tester George writes: “The channels in this helmet aren't as deep as some, but those 25 vents still keep your head nice and cool regardless. It's very comfortable regardless of temperature, too, with soft and plush pads that keeps the Mips system off the top of your head. They don't become unpleasantly saturated or start smelling over time, either.
“Overall I was really impressed with the Giro Syntax. It looks good, ventilation is impressive, and the level of Mips integration is both impressive and worthwhile. It's a comfortable, stylish and well-priced lid.”
The LEM Gavia helmet offers safety, comfort and style. Its ventilation is excellent and a decent range of colours means you've got a good chance of getting one to match your bike/kit. The only potential drawback is its lower than average profile, which might interfere with certain sunglasses.
Tester Emma writes: “The five-point height adjustment of the cradle is worth taking time to get right – once set, it won't need moving again. The dial pulls the cradle in from the temples and it feels exceptionally secure and comfortable, more so than many low to mid-range helmets that tighten only at the rear. I did find that it sat quite low on my head, with more temple coverage than many helmets I have tested. This is clear if you compare it with photos of, for example, the Giro Isode. My sunglasses sat plush against the Gavia – in fact it was a perfect setup for my slimline glasses; they were held rigidly in place and didn't budge all ride. But it is possible that the lower shell might be a problem with a chunkier pair of sunnies.
“LEM has nailed ventilation with the Gavia. The 27 vents really do prevent a build-up of heat under the lid. The padding at the front includes a bug net for the five forward-facing vents, leaving the largest central vent open. This vent is the main source of airflow through the lid; it's shaped at its rear end to channel the flow through and away. The Gavia is by far the most effective in terms of ventilation in comparison with other helmets I've used.”
The Met Estro Mips Helmet is a great fitting, well ventilated helmet that looks good. It's a bit dearer than some helmets offering similar features, but then it's lighter too.
Tester Neil writes: “Adjustment is via what MET calls a '360 degree Head Belt,' which amounts to a flattened plastic strap that widens at the forehead. The idea is it provides a comfortable fit whatever head shape you have; I sometimes find helmets too round for my narrow head, but I got an excellent result with this. I'm guessing you should still get the same result if your head is rounder. Ventilation is well-attended to with 17 ports directing the air in and nine rear outlets. It's vented right down to the base at the back, and there are five channels inside the forehead protection to really ramp up the airflow. (Being on winter test, overheating was not a problem for me). The Mips insert is carefully cut and arranged to make the most of all that venting, too.
"The Estro is an excellent helmet in many respects: it's very comfortable to wear, gives good coverage, is well ventilated and keeps that potentially intrusive Mips insert carefully tuned to the shell. It's definitely worth a look.”
The Bontrager Velocis Mips road bike helmet is reckoned to pair pro peloton performance with all-day riding comfort, which seems like a strange remark given that stage races commonly last a day. That said, it's a really well made and very airy helmet, which is fast becoming a favourite.
Tester Shaun writes: “As I'd expect from Bontrager and this price point, the Velocis is very well finished, with some lovely detailing. The in-mould construction, where EPS liner and polycarbonate shell are fused together simultaneously, saving weight and improving strength, is top notch. Regular readers will know that one of my bugbears is where the EPS liner is exposed, leaving it vulnerable to damage. The Velocis shell looks as if it's been drizzled over and baked on – beautifully executed. Large vents bode well for airflow and the design allows easy exit for pony tails. These will also entertain LEDs, so long as their rubberised straps/O-rings have sufficient give, which comes in handy for winter and night riding.
“There's no doubting the pedigree of this helmet and it would be high on my list were I regularly riding on the Continent or further afield – at home, the NeoVisor is unlikely to spell the end of caps/similar headwear, especially during the cooler months. Aside from being airy and secure, it feels lighter than 279g would suggest, and the detailing is top notch. An excellent helmet deserving of its price tag.”
Game changer? Maybe. Very good helmet? Definitely. The Abus GameChanger might not be supported by any actual aero figures against its competition, but it delivers sleek styling, a low profile and a comfortable fit with a large range of adjustment.
Tester Liam writes: “The fit is excellent, with a low profile and very little bulging on the sides of the lid. It looks good straight out of the box. My poorly shaped face doesn't usually sit well with aero helmets, so to have one that looks good makes a very nice change (it's not me in the photos).
“Is it worth the cash? Well, if you care about free watts, then it's a good option. However, its competitors are currently cheaper and there are no figures to say that the GameChanger is faster than any of them. However, finding a helmet that works well, looks good and feels comfortable is a tricky task, and so for me, the GameChanger wins my money.”
The Kask Valegro is a lightweight and well-ventilated helmet with a fairly low profile.
Tester Mat writes: “It's certainly light, our medium-sized review sample weighing just 206g (Kask claims 180g for a small). I've never seen the point of counting vents, but for the record you get 37 here. More relevant, they do a really good job of keeping your head cool, internal channeling allowing the air to circulate and helping to prevent the buildup of heat and sweat. This is the Valegro's biggest strength – it's something you really notice – making it a helmet to consider if you find that others leave your head too hot in summer conditions.”
“Overall, the Valegro puts in a strong performance if you're after a lightweight and cool helmet. The only real issue I had was that the pressure didn't feel as comfortably distributed on my head as with other helmets in Kask's range, despite an excellent fit system, although other people didn't find the same thing. If you're interested in the Valegro, I'd suggest you try before you buy because if the fit feels good to you this is a high-quality option.”
Giro's Vanquish Mips is a well-designed aero road helmet with a great fit system, and it comes with innovative new technology designed to reduce drag. Rather than a development of the brand's existing Air Attack, Giro says that the Vanquish Mips is an entirely new design resulting from extensive computational fluid dynamics analysis.
Tester Mat writes: “I got on really well with the Vivid eye shield. It offers good eye coverage and excellent vision. I can imagine some people struggling with the sci-fi looks but you can't really argue with the function. I've been using the Vanquish for several weeks and the shield has always stayed firmly in place. It just isn't going to budge accidentally. Plus, being slightly removed from your face, it doesn't get sweaty as easily as a pair of glasses.”
“The Mips design used here is specific to the helmet and is integrated with Giro's Roc Loc Air fit system. It surrounds your head entirely, you get the choice of three different height settings at the back and it's adjusted via a clicky dial. It's comfortable and tuneable.
”All in all, this is a really high-quality aero road helmet. It didn't make my head too hot, it's not ridiculously heavy, and the eye shield system is excellent. Add in a very good fit system and, if Giro's figures are to be believed, impressive aerodynamics and you have a top design for those with their sights locked on speed.”
The Abus AirBreaker is an excellent helmet that provides loads of ventilation in a lightweight and low profile package.
The AirBreaker's venting is its best feature. It's an in-mould design (meaning that the outer shell is permanently connected to the expanded polystyrene inner) with what Abus calls ActiCage structural reinforcement – essentially a plastic framework within the EPS, the idea being that you can have big vents without the helmet's safety being compromised. Complex internal shaping allows loads of air to flow over your head as you ride. You get large openings at the front and top, and deep channels that run front to rear.
Overall, the Abus AirBreaker is a great helmet that offers superb ventilation at a very light weight. It's priced towards the top end of the market but the quality is excellent.
Lazer's Genesis has evolved from a mid-range helmet into a lightweight, breathable lid that performs well on hot days and during races. The ventilation is among the best that I've tried and it works brilliantly when climbing at threshold. The fit is very comfortable and the Rollsys retention system works well to spread pressure. The price is the only slight issue.
Tester Liam writes: “Lazer's latest version of the Genesis helmet sees it go on a diet, losing 65g and quite a bit of material too. Vents have increased in size and number, from 19 up to 22, but the Rollsys retention system is unchanged and still works very well. While I'm impressed by the low weight – it's the lightest ever Lazer helmet – I'd pick the heavier Mips-equipped version(link is external) (£189.99) for the increased crash protection.
“The new Genesis is an impressively well-ventilated road helmet. The weight is low and it fits nicely, resulting in plenty of comfort on longer rides. The slightly bulkier silhouette means I still favour my Z1, but for rides in hot weather, you'll be well served by the Genesis.”
The Met Trenta 3K Carbon helmet is lightweight, it feels cool in use and, if you accept Met's claims, it offers an aero advantage over a traditional lid, but you're going to have to dig deep for this one.
“Our medium sized Met Trenta 3K Carbon helmet was 220g on our scales, 5g heavier than Met's claimed figure but still very lightweight. The weight saving over most other helmets will have a negligible effect on your speed, obviously, but you might find it more comfortable than one that's even 50g heavier. I barely noticed it in use. Met has managed to keep the weight down by using EPS (expanded polystyrene) foam that's 20% less dense than normal, it says, without affecting the helmet's capacity to absorb energy in the case of impact. How come? It's down to the elastic modulus of the carbon cage that's embedded into it, says Met. You can see the black woven carbon beneath the white in-moulded polycarbonate shell. Met insists that this carbon is structural rather than cosmetic.
“Overall, I've got loads of good things to say about the Met Trenta 3K carbon. It's lightweight and comfortable and we've found that most people get on with the fit (there are always exceptions so try before you buy). Plus, if you accept Met's figures, it offers an aero advantage. The only real sticking point is the price.”
The Giro Helios Spherical helmet benefits from the trickling down of the company's highly praised Mips Spherical technology – which was first introduced on its top-end Aether – to this less race-focused lid. The protection of the Mips (Multi-directional Impact Protection System) comes on a comfortable lid with ample adjustability, at a cheaper price.
Tester Anna writes: “With its understated looks, the Helios Spherical impressed as an incredibly comfortable lid with a low-profile design that still delivers well on aero and cooling performance. Although still not cheap at £229, Giro brings its innovative and well-integrated Spherical Mips technology to a more affordable lid, and leading brain protection technology now doesn't come with compromises. It's comfortable, well ventilated and lightweight enough.”
Tester George writes: “Looks are subjective, but I think the Ekoi Legende is beautiful, and brings to mind those helmets worn in the 1970s but with the modern details you would expect from a current high-quality helmet. It's pretty light and ventilation is excellent, though it's not cheap.
“On the head, the first thing you notice about the helmet is that it is small compared with most modern lids. It looks closer to the kind of thing you'd have seen Eddy Merckx wearing in the 70s than the sort Chris Froome wears today. One of its big benefits is excellent ventilation. Throughout the test rides I could feel a stream of air blowing across the top of my head, removing excess heat perfectly.
“Overall, I was impressed with the Ekoi Legende. It is a high-end helmet with great tech specs, good looks and plenty of choice in terms of design. Ventilation is superb and it's also lightweight. The price is high, but there are others out there that cost more.”
A cycling helmet comprises a thick layer that provides shock absorption, with soft cushioning for comfort where it rests on the head. The main shock-absorbing layer is almost always made from polystyrene foam, though there have been attempts to use polyurethane foam and treated cardboard.
A thin plastic shell covers the foam to protect it from everyday nicks and scrapes. This shell is usually 'co-moulded' or 'in-moulded', that is, the outer shell is placed in the mould and then the polystyrene layer is formed into it.
A few helmets still have thicker hard plastic shells, which provide some shock absorption in addition to the polystyrene layer, but add weight.
The shock-absorbing layer and shell are held on the head with straps, usually made from some sort of synthetic fabric webbing and closed with a buckle. There's almost always an additional cradle at the back of the head to stabilise the helmet by grabbing the occipital protuberance near the base of your skull.
Between the green and black sections of co-moulded outer shell you can see the shock-absorbing polystyrene
Big vents in modern helmets help keep your head cool.
Since polystyrene foam is an insulator, helmets need ventilation to stop the rider's head from overheating. Early helmets simply had holes in the shell. Modern designs use channels inside the shell that allow air to flow over the rider's head.
Many helmets have dial adjusters to tweak the fit
For a helmet to stay in place and be comfortable, it must be adjusted to fit the wearer's head. Most helmets have some sort of mechanism to adjust the fit on the occipital cradle at the rear of the helmet.
Internal reinforcement helps high-end helmets meet standards while shedding weight
Some helmets have internal reinforcing skeletons to hold them together in an impact. This allows for larger vents and air channels, and usually a lighter helmet, but the complication of including extra components in the moulding makes such helmets more expensive.
In the UK and the European Union, helmets must meet the EN 1078 standard, which calls for a deceleration of no more that 250g to be transmitted to the head in an impact at 5.42-5.52 m/s (a little over 12 mph). The standard involves impacts on a flat surface and a kerbstone.
In the US a Consumer Product Safety Commission (CPSC) standard applies. The two are roughly equivalent in terms of impact absorption.
Mips stands for Multi-Directional Impact Protection System. A Mips helmet is claimed to offer additional protection against rotational forces in a crash, by allowing two layers of the helmet to move independently. The idea is that the outer layer moves when you hit your head, absorbing the rotational forces of the impact. Mips was developed by the Karolinska Hospital and the Royal Institute of Technology in Stockholm, Sweden, so it's not proprietary to any one helmet maker. Helmet makers offering Mips helmets include Giro, Scott, Smith, Bern, Bontrager, Giant, and Bell.
In practice, anything under about 300g is light enough that its weight won't be annoying.
The cheapest helmets cost around a tenner, but decently ventilated ones start from around £30. Helmets for general use go up to around £250 while aerodynamic time trial helmets cost from £50 to over £400.
A peak helps keep the sun out of your eyes or the rain off your glasses
Helmets intended for mountain biking often have a peak or visor, which provides a degree of shade for the rider's eyes. The styling comes from motocross, but peaks are useful on the road too. They provide shade in summer, keep the rain off your glasses in winter and annoy people silly enough to take The Rules seriously.
An aero helmet will shave another few seconds off your 25-mile time trial time
Dedicated time trial specialists use helmets shaped to cut through the air, with smooth shells and long tails. Their shape has earned these helmets the nickname 'sperm hats'. They're effective if the tail is near the rider's back, but some find the effort of maintaining the right position fatiguing. Helmets with shorter, rounded tails provide most of the benefits without causing aerodynamic problems if you look down.
Giro's Air Attack was one of the first 'aero road' helmets with fewer vents for better aerodynamics
A recent trend is for helmets with minimal ventilation and smooth outer shells to reduce air resistance for general road racing.
Is this on right? (CC BY-NC-ND 2.0 Arnaud Lancelevee/Flickr)
Helmets for toddlers carried in child seats often have little or no ventilation so they help keep junior's head warm.
For older kids, helmet design is very similar to lids for adults, with the exception that they have buckles that release if the helmet gets caught on something so the straps don't throttle the child.
A radically different approach is the Hövding cycling airbag, which comprises a collar that rapidly expands into a head-enclosing airbag when it senses movement consistent with a crash. At £249 it's not cheap, though some top-of-the-line racing helmets are similarly spendy.
Hövding claims to provide "the world's best shock absorption capacity", an unusual statement for a head protection manufacturer, most of which seem to carefully avoid making any claims at all about their efficacy against impacts, aside from specifying the standards they meet.
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