MIPS and Sliding Resistance of Bicycle Helmets
Summary: MIPS has a patent on using a concept called the slip plane in a helmet. It may or may not help you avoid brain injury in a crash.
Lab tests demonstrate that you want the outside of your helmet to slide when you hit the pavement, not stick and jerk your neck. Rounder, slicker helmets are proven to do that better.
MIPS and slip planesA 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 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 manufacturers are scrambling to put MIPS helmets on the US market. In mid-2016 there are 200 models from 58 brands. Does it work? Do you need it? We can not answer those questions below, but here is the story.
The first POC helmet with MIPS has 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 point. We saw MIPS models from nine manufacturers at the September 2014 Interbike show. None of them had a full layer of sliding enabler fabric, and we found spots on all of them where the polycarbonate MIPS layer contacted 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 liners we have seen leave a large void in the back for the rear stabilizer, with a quarter or more of the helmet unlined and no MIPS 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.
We do not like the fact that the liner takes up space inside the helmet that could be devoted to more 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 faster, increasing g's. So unless the helmet is made larger, we are skeptical that the thin inside MIPS layer will perform better than a 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 is very round and smooth on the surface as well, so they have minimal sliding resistance to begin with and the slip plane is 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. Helmets for 2015 will have details on the newer designs.
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 (nature's MIPS) 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.
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 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 if the helmet is tightly strapped on the headform, particularly if the surface of the headform is relatively sticky 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 are 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 scrambling or considering how to treat the issue. Some are working on alternative solutions to address the rotation question. One company has a European grant to develop a technology that changes the structure of an EPS liner to permit movement on impact. Both 6D's technology and Smith's Koroyd are possible alternatives. Leatt and Kali have helmets with liner "doughnuts" that may permit head motion on impact. Any spur to innovation in what has been a long stagnant period for new helmet technology is very welcome.
In the meantime, do you need MIPS? Using careful evaluation, we can't answer that. 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 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.
Sliding ResistanceIn 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 viewThis appeared in a study by the Swedish consumer 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 Hövding 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.
StandardsWe hope to amend the ASTM standard some day to add a requirement to measure sliding resistance of the shell, but that will take time, and will probably be done when we develop 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.
This page was updated or partially revised on: February 6, 2017.