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Beta Editors’ Choice: Crankbrothers Synthesis Enduro Alloy i9 Wheelset

Alloy wheels to write home about

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There’s not much competition in the alloy wheel game. Sure, there are plenty of players, but I wouldn’t exactly say they’re “competing.” It seems like it’s mainly the carbon rim brands who are making all the bold claims about being the strongest, lightest, most compliant, or most eco-friendly. And that’s on top of the steady proliferation of no-questions-asked lifetime warranties. Meanwhile, a nice set of alloy wheels may get a patronizing nod from us professional gear snobs, but too few of those wheelsets have any compelling story to tell. The Crankbrothers Synthesis Enduro Alloy i9 happens to be one that does.

Crankbrothers Mountain Bike Wheels

I chose to run alloy Synthesis wheels on my Guerrilla Gravity Dream Build because they suit the workhorse motif I was going for, just like its SLX component mix and Marzocchi fork. But more importantly, they suit the way I wanted this bike to ride. “This bike,” for those of you who weren’t in the mood to click through to a 40-minute bike-building ASMR video, is a mixed-wheel take on the Guerrilla Gravity Shred Dogg. At 130 millimeters of travel, front and rear, my Shred Dogg is the yin to my 170mm-travel Scott Ransom’s yang. It’s a quicker-handling, fun-first, BMX-inspired trail bike that I knew would be a little undergunned on much of my local terrain. That’s going to play a big role in how I explain why I love these wheels so much. Honestly, I’m surprised the combination worked out as well as it did, but it makes sense. Wherever travel and geometry wouldn’t save me, I tried to find technology that would. 

In the case of Crankbrothers’ wheel lineup, technology meant taking a novel approach to stiffness. That story is almost four years old by now, but briefly, Crankbrothers learned through blind testing that riders performed best with a stiff rear wheel and a compliant front. That’s why the Enduro class of Synthesis wheels (both alloy and carbon) run 32 spokes in the rear and 28 up front. You also get a flatter, wider front rim profile to accommodate wider, lower-pressure front tires. Crankbrothers even built the front wheel around a slightly lower spoke tension than the rear. That means they don’t just have more vertical compliance, but also horizontal. This would seem like sacrilege until you consider how much chaos truly happens between our tires and our touchpoints. 

Thanks to all that attention we give to carbon wheels, it’s become common knowledge that too much lateral stiffness can make it hard to hold a line. If your bike has some natural waggle, your body will have an easier time heading straight down the trail. It’s the sort of thing that a lot of us might think would be too subtle to notice, much less pinpoint as a result of something as nerdy as rim material and spoke tension. But now that I’ve been spending most of my time on an aggressive short-travel bike, I’ve become a lot more sensitive to wheel feel.

One variable here is that, again, I’m talking about a mixed-wheel setup. That naturally will make the rear wheel even more laterally stiff, thanks to it being smaller and giving the spokes a wider bracing angle relative to the wheel’s radius. I built this bike specifically because I wanted to treat the rear of the bike differently than I treat the front. I often want to lose traction on the back tire. But that doesn’t mean gratuitous, guitar-wailing skids. It means letting the rear end skip across loose rocks while I’m leaning on the front end to hopefully carry the correct arch through a turn. It’s why I eventually sized down to a 2.4-inch tire in the rear, which works perfectly with the Synthesis Alloy rear rim’s 29.5mm inner width. It also pairs well with the higher pressure I run on this bike. Usually 32 PSI in the rear compared to 29 on my more practicality-focused enduro bike. To be honest, though, that choice was only partly because I tend to get a little more wild on this bike. I suffer significantly more pinch flats here than I do on my ENVE M635-equipped Scott Ransom. 

Something we don’t like to admit about modern, well-designed (read: expensive) carbon rims is that they actually have some practical benefits. The wider bead on my ENVE rims aren’t as sharp as those on my Synthesis Alloys, which is probably to blame for the higher frequency of pinch flats. Those beads are also, by nature, easier to bend. Of course, you hit a carbon rim hard enough and the show’s over, but the evidence is telling me that the margin between bending an alloy bead and breaking a carbon one is exactly where many of my hard impacts happen. So, although this rim will still seal a tire just fine, I acknowledge that it’s not going to last forever. The good news is, I can buy a rim-only for $130 when the time comes. On that note, all Synthesis wheels are sold individually, which made my mixed-wheel setup easy to pull off. 

The front rim, naturally, doesn’t have any significant dings in it, but not for lack of trying. Given that these wheels live on a short-travel bike, I get a little squirrely sometimes. And that’s exactly why these wheels are so perfect. They add some noticeable calmness when I’m trying to thread the needle through a rock garden. And threading the needle is key on this bike. While on my enduro bike, I’m more likely to just straightline over stuff, this bike is better suited to looking for any notch between boulders that’ll keep me from getting bucked. I found that I could usually maintain traction and focus on what was coming next. Now, I admitted earlier that wheel compliance tends to be subtle, and it’s easy to fall victim to a sort of emperor’s-new-clothes confirmation bias when praising an alloy wheel for its natural deflection. But after swapping in my Enve front wheel for a ride, I immediately felt a difference. It’s not like my entire body mass was getting redirected in rough sections, but those sections’ tendency to jerk my bars a few degrees left or right was far more pronounced. That just put me a little more on edge and forced me to slow down which, in turn, caused those bar-jerking impacts to last a fraction of a second longer and throw me even further off course. 

This effect does not put my short-travel bike into the long-travel category. If I’ve spent a week on my Ransom before getting back on my Guerrilla Gravity, the first thing I notice is the rougher ride at speed. But the point of having two different bikes is for the experience to be different. I’m more likely to let the trail dictate my lines on my short-travel bike, and almost ride defensively. There’s that yin vs. yang dynamic again.

The only thing I wish these wheels were a little better at is the one thing we all expect out of alloy rims. They’re not exactly a jaw-dropping value. These are the Industry Nine 1/1 versions, which go for $900 a pair. This is about as nice a hub as I’d ever want, but still, there are a growing number of modern $1,200 carbon wheels out there, many of which include one of those fancy lifetime rim warranties. And the house-brand-hub builds of the Synthesis Alloy wheels are a still-hefty $600 per set. Though we’re getting into apples-and-oranges territory here, the Hunt Enduro Wide wheels are $500, and Bontrager Line Comp 30s are $400. I’ve ridden both, and they’re fine wheels, but there’s just nothing that special about them. Crankbrothers could have easily dumbed down the tech when the Synthesis went alloy. It would have been forgivable, especially since they’ve since gone after the lucrative OEM market and landed spec on most YT complete bikes in 2022. But nope. They sought to bring some fancy nerdiness to the often boring world of alloy rims. As a bonus, they’ve brought my personal bike a little bit of blue-collar swagger. Just pay no attention to my titanium cranks.