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MIPS and Sliding Resistance of Bicycle Helmets

Summary: The MIPS company holds a patent on one means of using a slip plane in a helmet. It may or may not help you avoid brain injury in a crash. Testing by two officers of the Snell Foundation showed no performance advantage. Others are bringing alternatives to the market. MIPS bought the Fluid Inside brand and patents in 2019.

Lab tests demonstrated decades ago that you want the outside of your helmet to slide when you hit the pavement, not stick and jerk your head and neck. Rounder, slicker helmets are proven to do that better.

We have only one report of scalp damage from a MIPS liner.

Testing by two Snell officers shows no improvement

In 2018 the Snell Foundation's Bill Muzzy presented to ASTM's F08.53 subcommittee the results of testing by two Snell officers of MIPS performance using a linear impactor and offset (oblique) impacts. In the course of working on a test method for rotational injury they tested a MIPS and non-MIPS version of the same Specialized helmet. Their results with full details will be published in a journal soon.

Two of Snell's officers, working in a University of Washington test lab, dropped a 5kg guided impactor onto a helmeted Hybrid III headform and neck, impacting the helmet sides to achieve an oblique transmission of energy. The MIPS layer activated and moved. They used both flat and hemispheric impactors, and measured both linear and rotational acceleration. They hit each location twice. Helmet straps were tight. They chose the locations based on a Harborview study of the most likely impact locations on bicycle helmets.

The resultant data showed no significant improvement in the MIPS helmet's performance over the non-MIPS model. In some cases the non-MIPS model performed better.

The MIPS representative present at the ASTM meeting, Peter Halldin, said that he was not surprised, since MIPS' own testing with linear impactors had the same result. (MIPS normally tests with vertical drops on a slanted anvil. Most importantly, the headform is always free to move in any direction.)

Our conclusion is that this testing by officers of a respected organization with a distinguished history of consumer-oriented helmet activism, shows that MIPS is not likely to help you in a crash configuration similar to the one tested by the Snell officers. Other lab testing using an unrestrained moving headform with a sticky rubber covering and no neck attached impacting a very rough 45 degree slanted anvil with the straps tight over an inflexible jaw (the configuration MIPS uses) has shown that MIPS does reduce rotational acceleration. But when the head is constrained--as by a neck--MIPS does not perform well. That does not happen in the field, where heads are attached to the body. We still think your helmet, with a normal scalp under it, will move anyway.

In March of 2021 the Snell officers provided this clarification. We have revised the paragraph above to reflect the points they raised.

And here is the MIPS response from Peter Halldin

"You are welcome to send out my comments. For all of you I think that the understanding of the forces to the human head can be divided into the radial and the tangential force. In a bike accident as I see it there is a dominant tangential force. In the test presented by Snell there is a very small tangential force. This is also seen in the linear impact test machine designed by Biokinetics and used by NOCSAE. So, as I said in the meeting and that for some reason Randy missunderstood, I agreed in that the test showed by SNELL showed no reduction with MIPS. We have as I show in the attached PDF seen that the test method similar to the test by SNELL will not show a reduction for helmets like MIPS. We do have more than 17000 tests done in Sweden showing that all helmets with MIPS are significantly better than helmets without MIPS. We do have scientific evidence that a helmet with a low friction layer will make a difference in a test including a tangential force. So, as I told Bill Muzzy at the ASTM meeting I am willing to help out to design the test to mimic a realistic bike accident. With best regards, PETER"

In May of 2019 Consumer Reports published an article on their web page recommending rotational energy management as their first buying criterion for new helmet buyers, mentioning only MIPS and the WaveCel liners in Bontrager's helmets.

In May of 2019 the MIPS company purchased the brand Fluid Inside with its patents for another, completely different, type of rotational energy management.

Background: MIPS and slip planes

A Swedish company called Multi-directional Impact Protection System - MIPS - has revived and patented the slip plane concept, using two layers in the helmet to help the head rotate slightly on impact. The hope is to reduce the rotational component of an impact, thought to be a prime brain injury mechanism and related to concussion.

From one Swedish manufacturer in 2009, MIPS developed marketing momentum after a 2013 article in Bicycling magazine praised it as the only new helmet technology available. We found six manufacturers in September 2013. In 2014 Bell bought a substantial part of the MIPS company, and other manufacturersbegan scrambling to put MIPS helmets on the US market. In mid-2016 there were 200 models from 58 brands. Does it work? Do you need it? We can not answer all of those questions below, but here is the story.

The first POC helmet with MIPS had two concentric layers, held in place by a pin that breaks and lets the shells slip for about 15 mm upon impact. The layer interface is coated with Teflon. All subsequent helmets we have seen have just a thin layer of uncoated polycarbonate plastic inside the normal helmet liner. It slips, but hold it down hard with your thumb and you can hear it creak against the EPS liner, indicating friction. Your skin does not do that. MIPS says that the helmet is supposed to have a layer of slippery fabric between the foam and the polycarbonate insert, but that turns out to be just small fabric pads on some points. Not many models have a full layer of sliding enabler fabric, and we find many spots where the polycarbonate MIPS layer contacts bare EPS. In addition, the inserts are sliced up to avoid blocking vents in most helmets. The MIPS layer cuts down on ventilation where it impinges on vents.

Almost all of the early liners we saw left a large void in the back for the rear stabilizer, with a quarter or more of the helmet unlined and no slip plane effect if you hit at the rear. Although front and sides are more frequent impact sites, many impacts occur in the rear. We regard that as poor engineering, or hasty marketing at best. The Smith implementation has a grid of MIPS liner material, but with sharp Koroyd "straws" between that might dig into your skin and perhaps prevent the MIPS layer from working, depending on the crash angle and sequence. A few more recent liners have managed to cover the rear void.

We do not like the fact that the liner takes up space inside the helmet that could be devoted to a thicker crushable liner. MIPS says the liner is 0.5 to 0.8mm, reducing the helmet size by 1mm to 1.6mm, so the manufacturer would have to adjust the size in some way, either selling the consumer a larger helmet or reducing the thickness of the helmet liner. Slip planes do not repeal the laws of physics, and if you reduce the distance for stopping the head, it must be stopped more rapidly, increasing g's. So unless the helmet is made larger, we are skeptical that the thin inside MIPS layer will perform better than a thicker conventional helmet.

If it works, the effect of a slip plane is to reduce rotational energy momentarily for the critical first milliseconds of the impact sequence. The first two-layer POC helmet incorporating MIPS was very round and smooth on the surface as well, so they had minimal sliding resistance to begin with and the slip plane was available no matter where you hit. You can find more on POC in our Helmets for the current year page. In 2012 Lazer incorporated the first inner fit cage that was licensed by MIPS as well, on two of their child helmets. In short, the external configuration of a helmet is important in avoiding rotational force. Adding MIPS does not change that.

The helmet community has been discussing slip planes for years, and has been cautiously examining the MIPS data to evaluate the advantages if any. Everyone agrees that mitigating rotational force is important for injury protection, particularly for anti-concussion effects. But there are questions about how much a slip plane actually helps. Helmets are not coupled closely to the head, and will slip anyway. The scalp ensures that, and skin does not stick to EPS much, given sweat, hair, hair products and sunscreen. (The Koroyd "straws" pioneered by Smith Optics helmets might be a different story, given their known ability to abrade skin in a crash.) So the tendency for the helmet to slide on the user's head and to slide on pavement or other impact surfaces is substantial. The Snell testing reported on above seems to confirm that.

MEA Forensic has published a paper on their research showing that when a helmet slips on the head it reduces peak acceleration and peak angular acceleration. You can access the abstract on the ResearchGate site. The article is titled The effect of hair and football helmet fit on headform kinematics. It is very likely that football helmets are more closely coupled to the head than a bicycle helmet due to their construction and additional coverage.

An article in the May/June 2016 issue of Consumers Digest available only behind their pay wall reported that their interviewees did not offer strong support for MIPS. A Snell Memorial Foundation employee was quoted at that time as saying that in her opinion MIPS is "snake oil to get people to spend money."

There is even the question of how much you would want your helmet to slip. This study calculates risk factors for helmets that slip due to poor fit, and how much that increases the risk of head and facial injury. A MIPS helmet has a very small amount of slippage designed in.

A slip plane might help if the impact surface did not permit sliding and the head is rigidly coupled with the helmet. MIPS uses computer brain simulations to support their claim that it performs in a lab test when the helmet is tightly strapped on the headform, particularly if the surface of the headform is relatively grippy rubber. Other labs have not found similar effects. We are more cautious than the patent-holder, and are still looking for test data from other sources and for any field experience that would show that the technology will actually reduce injuries, and in what situations.

Whatever the performance verdict for MIPS, their marketing took off after the Bicycling article. Bell then purchased a substantial stake in the MIPS company, so they were committed to it and wanted direct access to the MIPS expertise. They are still marketing some of the worst examples of missing rear MIPS liners. Other manufacturers are giving in to the marketing and fashion push. A Bell restructuring actually placed the MIPS holdings in a different company, but their direction is set.

MIPS announced at Eurobike in August 2017 two new versions of their product. Both are variations of the basic slip plane, with the slippery plates encased in stretch fabric. One is a cap covering the whole head, suitable for unvented helmets. The other consists of fabric circles with slippery material in the center, to be placed between vents in a helmet with that much liner space between vents. MIPS is obviously trying to keep up with the alternatives below.

Possible Scalp Damage

We have one report so far of a damaged scalp apparently from a MIPS liner. Here is the photo the rider sent us.

Scalp damage apparently from liner

This is of course "only" scalp damage. The rider is alive and well, and did not report any brain damage.

Hair loss

We have one very credible report of a child with long hair whose hair gets caught in the MIPS add-on in his helmet, resulting in hair loss every time he removes it.

Alternatives: Trek, 6D, Leatt, Kali, POC

As concussion awareness has risen, many manufacturers are working on alternative solutions to address the rotation question. Helmet companies say "you have to have a story on concussion protection." One company has a European grant to develop a technology that changes the structure of an EPS liner to permit movement on impact. 6D has demonstrated that its technology performs well in lab testing. Leatt and Kali also have helmets with liner "doughnuts" that may permit head motion on impact. And POC, the original MIPS user, introduced a similar alternative in 2018 that it calls SPIN pads, placed around the interior of the helmet to promote lateral movement of the helmet in a crash. (A MIPS patent-infringement lawsuit against POC has now been settled.) Trek has introduced their WaveCel liner, with a mesh that can collapse at an angle to reduce rotational force.

Although unbiased test results are very hard to find, any spur to innovation after what has been a long, stagnant period for new helmet technology is very welcome.

In the meantime, do you need MIPS? Using careful evaluation, and in light of the Snell testing showing no benefit in their test configuration, we are still not convinced that you do. It probably won't hurt, other than any effect on ventilation, of if your manufacturer has kept the same outer profile and reduced the thickness of the normal liner to accommodate the MIPS layer, or if it lets the helmet slip too much, or if it pulls your hair out, or if the extra cost of the MIPS model makes a difference to you. We do not see compelling evidence that you should trade in your current helmet on a MIPS model unless having the Latest Thing is important to you. Despite Snell's research we think the jury is still out on MIPS. And we think your first concern should be to wear a helmet with a round, smooth exterior that will not snag as you slide on the pavement.

Read on below for the story on rotational crash energy and the outer surface of the helmet. Lab testing has shown that makes a difference.

Sliding Resistance

In a crash you want an interface with the road that is smooth, hard, round and slick. That keeps your head from snagging, adding to the severity of the impact and putting more strain on your neck.

The optimal bicycle helmet for that moment when you hit the pavement has a round shape and a hard, or at least smooth, plastic, shell. The first ones we saw in the US market were skate-style helmets. Most of them have minimal vents and are hot for warm weather bicycle riding. But now road and mountain bike helmets are improving every year. Some are lumpy and have styling ridges, but most manufacturers are moving toward more rounded designs.

For sliding, the rounder the helmet the better. In addition to possibly adding sliding resistance if a rear projection digs into the pavement, the rear of the elongated helmet could shove the helmet aside when you hit, leaving your head unprotected. Although it has been debated ever since the elongated designs appeared, Professor Hugh Hurt raised this question again in 2005, based on both testing problems and field reports of injury from helmets being pushed aside. You probably do not want a helmet that could only be tested by duct taping it onto the test headform. Fortunately, the current trend is to "compact" helmets, and the aero shape looks dated now.

Vents are necessary, but make sure they are smoothly faired into the helmet shell, and avoid any helmet with unnecessary fashion ridges on the outside, or snaps for visors, or little animal ears and noses, or any other feature that could cause the shell to snag. This is an easy issue for a consumer to assess, as long as you keep in mind that you want your head to slide on impact. It should be evident that you don't want to add any accessory or cover to the exterior of a helmet that adds to its sliding resistance. That includes lights and cameras. Many of the ones we see have mounts that are much too strong to break away easily when you need to slide. ASTM is developing a standard for the force that should trigger an accessory to break away. Bell and other manufacturers already have their own internal standard. In the meantime you are on your own to figure it out.

A Swedish view

This appeared in a study by the Swedish insurance company magazine Folksam:

One of the helmets, Yakkay, which is available in a version in which it is possible to fit a cover in the form of a hat/cap, was tested both with and without the cover. A major difference was measured in the oblique tests between the Yakkay with and without the cover, which indicates that this cover provides good protection against rotational forces, similar to MIPS; Figure 17 and Figure 18. The difference is that the sliding shell in the case of the Yakkay is fitted on the outside of the shell of the helmet. It is probably not an intentional rotational protection, but shows that a surface-mounted layer can provide similar protection as a sliding layer fitted on the inside. There is a similar concept among motor cycle helmets, known as SuperSkin, which has been shown to reduce the rotational forces in oblique impact tests (PhillipsHelmets. 2015). Another example is the 6D helmet that consists of two layers of EPS linked with "dampers" that allow energy absorbing shear between the layers (6D Helmet. 2015). The Hovding did also obtain very good results in the rotational tests; Figure 17. When it is inflated, the exterior fabric can slide sideways in relation to the fabric on the inside against the head. Thus two shearing layers are created that considerably reduces the rotational acceleration. The above examples clearly demonstrate that there are several ways to design a helmet to absorb rotational forces.

Brain models and strain measurements

MIPS and others who promote slip plane technologies make use of computer brain models as indicators of the efficacy of their products, showing red areas of tissue strain. But the state of computer brain models is at best unsettled. A study interpreting the same impact dataset with eight different brain models concluded that the results could be highly misleading, with one helmet rated two stars by one brain model and four stars by another. The authors included two early MIPS company founders. They recommended "Therefore, we suggest that the biomechanics community should create a norm or recommendation for future ranking and rating methods." But that misses the main question: is there a brain model today that will produce consistently valid results? We don't know the answer to that.


BHSI would like to see the ASTM standard some day add a requirement to measure sliding resistance of the shell, but that will take time, and will probably be done when ASTM develops a requirement for measuring and limiting rotational acceleration. The European community is working on that and is way ahead of ASTM.

We have put up the lab study that established the value of a round, smooth, slick outer surface if you want to see the scientific data. And we have study on Chin Strap Forces in Bicycle Helmets confirming that a shell that does not slide well increases the jerk on the chin strap.

Whether MIPS helps or not in an impact, you can make sure that the exterior of your helmet slides well on pavement.

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This page was revised on: June 14, 2021. BHSI logo