Helmet Review: Bontrager Rally MIPS vs Specialized Ambush

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For the past six months I’ve been riding in a load of different helmets from a variety of brands, including Giro, Bell, Leatt, MET, Scott, Troy Lee Designs and Kali. There are two brain buckets that I’ve been wearing a little more of recently, and it turns out they actually share some rather similar specs on paper. Those two helmets are the Specialized Ambush, and the Bontrager Rally MIPS.

Both of these are what I’d refer to as ‘trail’ helmets. They’re heavier than a XC race lid, though they do come with more coverage to add protection around your skull. They’re designed to be goggle friendly, but neither offers a convertible chin-bar, so they’re not specifically suited to enduro racing or park riding.

specialized ambush helmet
The Specialized Ambush helmet.
bontrager rally mips helmet
And the Bontrager Rally MIPS helmet.

Each helmet comes from a big American bike brand; Specialized and Trek (which owns Bontrager). Both brands offer a huge range of components and accessories that are available through a worldwide dealer network, which means their helmets are readily available in most corners of the globe. They’re also basically the same price at just on a hundred squid each.

As it turns out though, these two helmets offer two very distinct styles with some unique features that set them apart. Which one gets our pick? Read on…

bontrager rally mips helmet specialized ambush
tête-à-tête

Fit & Adjustability

How a helmet fits is entirely down to what shape your head is. Just like shoes, not every helmet is going to fit every head. That said, there are a few key differences that separate these two lids.

In terms of the sizing range, Bontrager has Specialized beat here. The Rally is available in four sizes (compared to three in the Ambush), including an XL that claims to fit boofheads up to 66cm in circumference. That’s one of, if not the biggest helmet on the market. Those with large craniums take note.

bontrager rally mips helmet
Easy to use Boa dial.

With a less impressive 55cm head circumference, I’ve been testing a Medium in both helmets. The Rally covers a broad range of 54-60cm, and with a more rounded shape, it’s arguably a touch big for me. It’s easy to adjust the fit though, with a clicky Boa dial at the back that can be easily accessed one-handed while riding. The Boa dial is located on the rear basket, and as you tighten it to draw in the laces, the whole basket snugs down around the back of your head. You can also adjust the basket vertically via a 3-position anchor.

bontrager rally mips helmet
3-position vertical adjustment for rear basket.

In comparison, the Ambush has a slightly narrower range of 55-59cm for the Medium size. That means it’s a little more cosy, and I like the slightly oval shape that gives it an overall narrower profile. The Mindset 360 adjustment system integrates the tension dial into the back of the helmet shell for a very neat look, and it offers up to five vertical positions for the rear basket. There’s significantly less material used around the rear of the spindly basket though, and that contributes to a slightly flimsy feeling even if you have the dial overtightened.

specialized ambush helmet
Specialized integrates the adjuster dial into the helmet shell.
specialized ambush helmet
Tug on these straps to alter the basket’s vertical position.

Both helmets feature thin profile straps and plastic dividers that sit underneath your ears. I had no issues with getting the straps in the right location with either helmet, but I did find I had to retighten the main buckle strap on the Rally each time I’d go to wear it. It never loosened while riding (the strap locks in the buckle at that angle), but if you’re moving it about when you go to put it on, the thin strap can slide enough that you’ll need to retighten after putting it on. Thicker straps please.

bontrager rally mips helmet
The plastic strap dividers allow for plenty of clearance around the ears.
specialized ambush helmet
There’s less adjustment with these fixed dividers, but I didn’t find that to be an issue.

While the Ambush does initially feel a better fit when first worn, the Rally is more secure in use. A big reason for this is the way the straps anchor to the helmet.

On the Ambush, the straps attach quite far out on the underside of the helmet rim, which creates a much wider approach angle. And because the rear basket doesn’t hug the back of your head as securely, you need to tighten the strap more to stop the helmet from wobbling, which creates an uncomfortable pinch point right underneath your chin.

specialized ambush helmet
The straps on the Ambush anchor to the outside of the helmet.
bontrager rally mips helmet
For the Rally, the straps anchor up much higher inside the helmet.

In comparison, the straps on the Rally tuck underneath the harness and anchor much higher up inside the helmet shell. This puts the straps closer to your skin, so you’ll have to wear your glasses over the top of the straps, but it does lead to a much more comfortable fit when you have the main strap tightened up. Even with the MIPS liner that creates a bit of wobble to the helmet, the total range of wobble is less than that of the Ambush. And for that reason, the secure fit of the Rally sees it edge out the Ambush on fit & adjustability.

Winner: Rally

Weight

This one’s pretty simple really. I weighed both helmets on a set of Park Tools digital scales, and the Rally came out at 391g, and the Ambush measured 288g. That’s a difference of nearly 30%, which is considerable and noticeable. At that weight, the Ambush is one of the lightest trail lids on the market. As you’ll see though, there are reasons for this.

Winner: Ambush

specialized ambush helmet
The vents on the Ambush are large and well placed to draw out warm air.

Ventilation

One of those reasons is ventilation. The Ambush actually has less vents than the Rally (20 vs 22), but each vent is considerably larger, and that helps to bring fresh air into the helmet at the front, while the six big vents at the rear of the helmet draw hot air out of the back. On some of the piping summer days I’ve had riding around Italy the past month, the Ambush has been my choice for minimising heat and sweat buildup. In particular, those two horizontal vents around the forehead are really effective at evaporating sweat that’s soaked into the brow pads.

specialized ambush helmet
The brow ventilation works exceptionally well.

I wouldn’t say the Rally has poor ventilation – it’s still decent for a trail helmet, and the internal channels moulded into the foam core do help with keeping air moving inside. But there is simply less negative space in its structure, and that has an effect on air flow.

As it stands, not only is the Ambush the winner here, it’s also one of the breeziest trail lids I’ve used.

Winner: Ambush

Protection

As you’d expect, both helmets are rated to the same ‘EN 1078: 2012’ helmet safety standard – like any bicycle helmet worth wearing should be. Without our own test lab though, it’s a little hard to validate those ratings. However, there are some differences in the construction and spec of each helmet that are worth your consideration.

specialized ambush helmet
Being trail-oriented helmets, each features a dropped rear section and more coverage around the temples.

Specialized has utilised what it calls ‘Energy Optimised Multi-Density EPS Foam’, which is a fancy way of saying some of the foam is a bit softer than the other bits. This is good – you want softer foam closer to your skull where it will deform more easily to absorb impact energy. Inside the EPS shell is an aramid skeleton that reinforces the entire structure so that the helmet is less likely to shatter into pieces when shit goes really south.

The Rally also gets a composite skeleton inside the EPS shell, and like the Ambush it’s shaped to provide deep coverage around the temples and rear of the head. But while the coverage of the two helmets is pretty similar, it only takes a quick side-by-side comparison to see that there is significantly more meat to the Rally. As well as having thicker EPS walls, the overall coverage is also broader, which comes back to the ventilation tradeoff – whereas the Ambush has more of its surface area hollowed by large vents, the Rally has more solid space, particularly on each side and at the back of the helmet.

bontrager rally mips helmet
The Rally uses quite a lot more EPS in its construction – note the thickness of the walls around the forehead.

Of course the thicker EPS shell contributes to the Rally’s weight. But when it comes to impact protection, the equation is pretty simple: the more (soft) material you have between your skull and the ground/rock/tree, the better the helmet’s ability to absorb the impact, increase deceleration time, and reduce the chance of damage to your noggin.

As well as the thicker EPS foam shell, the Rally is also equipped with a MIPS liner. Standing for ‘Multi-Impact Protection System’, MIPS is a low-friction liner that sits inside the helmet shell, attaching to it via four elastomer straps. The MIPS liner allows the helmet to rotate around the rider’s head in the event of an angled impact, in order to lessen the rotational forces being passed on to the brain. While it does lead to the helmet feeling a little more wobbly on the head, it’s regarded across the industry as one of the most effective methods at absorbing rotational impacts. It doesn’t guarantee a helmet’s safety, but in the case of the Rally, it no doubt adds to it.

Winner: Rally

bontrager rally mips helmet
There may be less ventilation on the Rally, but it covers more of the head with EPS foam.

Finish & Features

I’m not going to rate either of these helmets on style, since that is entirely subjective and I think it detracts from the reasoning you’d choose one over another. For those that care though, each helmet is available in a variety of colours as well as those shown here.

In terms of finish, the Rally gets the nod, due to its use of a full-wrap polycarbonate shell that covers the rim of the helmet all the way around. In comparison, the Ambush leaves part of the foam underneath the forehead exposed, which means it’s more likely to get chipped over time.

specialized ambush helmet
Micro-indexing visor is easy to use.
specialized ambush helmet
It’ll play well with goggles and…
specialized ambush helmet smith glasses
…it stores glasses on the back comfortably.

Each helmet has an adjustable visor, and they’ll go high enough that you can store goggles on the front of the helmet. If you’re not into goggles, you can also store glasses on top of each helmet, which I find really handy for long climbs.

The visor is nicer to use on the Ambush, with a micr0-indexing mechanism that keeps it secure at your desired tilt. The Rally’s visor is smaller and more flexible, and since it only attaches on either side, it can occasionally go a bit wonky – particularly if you’ve had it stowed in a gear bag for transportation.

bontrager rally mips helmet wil
The visor is much stubbier on the Rally.
bontrager rally mips helmet smith glasses
Sunglass arms tuck into the side vents neatly.

There are two other reasons the Rally wins on fit & features. The first is the excellent Blendr mount, which is designed to let you fit a POV camera or a helmet light via a GoPro-style mount. The niftiest part about this mount is that it attaches to the main central vent on the Rally via a clip and a powerful magnet. The mount itself is sturdy, and if you pull backwards on it, it won’t budge. If you push it forward with a bit of force though, it’ll disconnect from the helmet – the idea being that the mount will break free in the event of a crash.

I experienced this first hand when I had a nasty OTB crash in the alps, and the forward acceleration of my head towards the ground was enough to dislodge the mount and the GoPro attached to it. I’d much rather that than have the camera piercing the helmet shell.

bontrager rally mips helmet gopro camera
Bontrager includes the excellent Blendr mount with the helmet.

The second reason is Bontrager’s 1-year Crash Replacement Guarantee. If you crash and wreck your helmet within the first 12 months of purchasing through a Trek dealer, you’ll receive a replacement free of charge. That is pretty incredible really, and in my opinion, adds great peace of mind.

Winner: Rally

bontrager rally mips specialized ambush helmet
Between these two big-hitters, the Rally MIPS gets our vote.

The Verdict

If you’ve added up the category winnings already, you’ll know that the winner of this shootout is the Bontrager Rally MIPS!

Though the Specialized Ambush is both incredibly lightweight and well ventilated given its extensive coverage, in my experience the harness and strap system lets it down when it comes to keeping the helmet stable on your head. Fit will of course vary from rider to rider, but a more robust and secure harness will do doubt be appreciated by all head shapes.

In comparison the Rally isn’t as svelte or as breezy, and it could do with both a more sturdy visor and thicker straps, but the chunky EPS construction and sturdy harness provides its wearer with added reassurance when riding technical, high-risk trails. Add in the MIPS liner, that clever Blendr mount and the Crash Replacement Guarantee, and the Rally MIPS will have you questioning why anyone would spend any more than 100 quid on a trail helmet.

Review Info

Brand:Bontrager & Specialized
Product:Rally MIPS & Ambush
From:trekbikes.com & specialized.com
Price:£99.99 (Rally MIPS) & £100 (Ambush)
Tested:by Wil Barrett for 6 months

Comments (1)

  1. Spesh UK offer lifetime crash replacement with around 40% off if you deal with them directly. I’ve used it many times

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