The human head has been cracked, smacked, slashed, set on fire, whipped around like a Skip-It—and made to survive.

Motorcycles will bounce your skull off pavement without a spark of consideration for riders who think they can hold their heads up in a fall. Racecars will pinball your head around a steel roll cage while a harness holds your body tight.

Helmets for both have been evolving since they were leather caps whose only purpose was to make sure you still had ears at your wake. They’ve also been diverging, as they become increasingly dedicated to their tasks. Beneath the similar-looking shells and visors of motorcycle and auto racing helmets are two very specialized ways to armor up.

The voice of authority in all this is the Snell Memorial Foundation, a nonprofit group of scientists, physicians, and engineers who create a new safety standard for auto racing (SA) and motorcycle (M) helmets every five years. This month, we’re due for fresh rules.

The certifications are voluntary, but almost all North American racing series require Snell-rated helmets, and full-face motorcycle helmets that aren’t Snell don’t sell.

Helmet design is about more than what happens when you bonk your head. “I doubt whether there’s any real difference in impact protection between Snell M2015 [for motorcycles] and SA2015 [for cars] helmets,” says Ed Becker, Snell’s executive director.

But motorcycle helmets are downright unsafe for auto racing and auto racing helmets are unsafe for riding.

Motorcycle helmets don’t have auto helmets’ fire-resistant Nomex liner because a rider usually leaves the bike immediately, conscious or not. Getting out of a complete and caged race car safety system takes longer than flying off a motorcycle, and 15 extra seconds of fire resistance could save your life.

“Nomex and other flame-resistant materials generally have no problems (with Snell’s fire test), but other materials standard in motorcycle helmets can’t handle it,” Becker says. Auto helmets are also HANS-ready. A Head And Neck Support (HANS) device tethers the helmet to the shoulders and keeps the head from whipping forward in a high-force impact; without it the spine can fatally break where the neck meets the back of the skull. It’s up to the buyer to decide whether to buy a HANS, but every SA helmet comes with anchor posts to use it. They mount to a shell insert that motorcycle helmets don’t have. Manufacturers won’t mount HANS posts to a motorcycle helmet, even if you ask real nicely, because the posts don’t anchor anything without the shell.

But they do trump auto helmets in visibility, with at least 210 degrees of lateral vision to an auto helmet’s 180 degrees. That’s crucial: Motorcycle riders crouch, sit upright, look sideways, and look back, so their helmets need bigger visors to let them see in all positions. If you’ve ever nestled into a car’s racing seat and strapped into a harness, you’ll be thrilled to have sideview mirrors—you can hardly move your head. You can only turn it a few degrees in any direction, so extra visor space would be a waste.

There are many more motorcyclists than racing drivers, says Becker, so manufacturers can justify the expensive tooling to produce cheaper polycarbonate-shell motorcycle helmets. Auto helmets are Kevlar or fiberglass because although they’re more labor-intensive to make, the tooling is cheaper. Carbon fiber is top-of-the-line for both auto and motorcycle helmets, but it’s years away from trickling down the price range.

This year, though, testing diversifies. SA2015 will test for low-velocity impacts and lateral impacts to the side of the helmet. “The ‘low-velocity’ tests came about because of concern at [international motorsports governing body] FIA and elsewhere that helmets built to withstand very severe impacts might also transmit high levels of shock in less serious impacts,” says Becker. “So we were urged to include some low-velocity impact tests.”

SA2015’s general test impacts at 8.5 m/sec and allows a peak of 300 G’s. The new, additional low-velocity test impacts at 5 m/sec and allows a peak of 200 G’s.

Since 2010, standards allow slight variation based on helmet size, but Becker says the force transmitted to the head is the same regardless. “We performed a series of impact tests at velocity levels from 3 to 10 m/sec and found that peak G is generally proportional to impact velocity.” There was (also) concern that head strikes against lateral edges might fall below the protective zones of the helmets. In fact, though, results to date indicate that this is not a real issue either.

The result will be helmets more different than ever—and safer because of it.