The 1987 Cannondale SM800 was available with a 24" rear wheel. Martin Maes' 29/27.5" GT Force.

The mixed-up Rocky Mountain Instinct BC Edition.





LAP TIMES

Lap 1: 1:41.12 (29)

Lap 2: 1:42.01 (29)

Lap 3: 1:41.06 (Mix)

Lap 4: 1:37.92 (Mix)

Lap 5: 1:37.55 (29)

Lap 6: 1:37.50 (Mix)



Mullet bikes. 97.5ers. Frankenwheelers. Whatever you want to call them, bikes with mixed wheel sizes have made their mark in the racing world this season. Martin Maes took home the win at the first two stops of the Enduro World series aboard a modified GT Force, and Loic Bruni rode his way to victory at the first round of the DH World Cup on a prototype Specialized.Now, this isn't the first time we've seen mountain bikes with two different wheel sizes. Specialized's Big Hit from the early 2000s is a memorable one, with a 26” / 24” wheel combo. Travel even further back in time and you'll find the 1987 Cannondale SM800, which was available with a 24” rear wheel that was claimed to deliver “superb traction.” Don't forget Trek's 69er, a 29” / 26” singlespeed with a dual crown Maverick fork. More recently, Liteville and Foes have both produced mixed wheel bikes. Needless to say, it's not a new idea, but the fact that they're winning races this year makes it a good time to revisit the concept.Is there a reason we're seeing so much experimentation all of a sudden? The recent UCI rule change is one possible answer – riders no longer need to have matching wheel sizes in competition. That means shorter riders who can't quite fit on a 29" downhill bike can run a 27.5" wheel in the back, thereby preventing uncomfortable tire-to-pants contact, while still getting the rollover advantage of the bigger front wheel. What about Martin Maes? He may genuinely prefer the ride of a bike with two different sized wheels, or it could be that GT doesn't currently have a long travel 29er in their lineup, and creating a mullet bike was the next best solution until they come out with one.In any case, we decided to head out and put the concept to the test against the clock. A Rocky Mountain Instinct BC Edition served as the test platform. In its stock 29” configuration it has a relatively high bottom bracket height and a not-so-slack head angle, which made it a prime candidate for a smaller rear wheel. That change dropped the BB height by 12mm, and slackened the head angle by approximately 1.5-degrees.It's worth mentioning that not all wheel swaps will be this easy – if you're starting with a 29er it's possible that running a 27.5” rear wheel will lower the bottom bracket heightmuch, and you'll be smacking pedals on even the tiniest pebble. The inverse is true when it comes to 27.5” bikes – the bottom bracket and front end height will get higher when a bigger wheel and fork are installed, which isn't always a good thing.Once the test bike was ready it was time to knock out some laps. A section of trail was chosen that had a good mix of terrain – tighter turns, short, steep chutes, along with some flatter, more rolling bits. Each lap was timed with a Freelap system and recorded on a GoPro for backup. I started on the Instinct BC in its 29” form, and put down two timed laps. Next, I switched out the rear wheel and did two more laps on the mixed wheeler. To finish things off, I did one more lap on the 29er, then a final lap with the 27.5” rear wheel in place.If you've watched the video, you'll know the result... There wasn't a definitive answer. All of my lap times were extremely close, and my two fastest laps were only separated by .05 seconds – it takes longer to blink than it does to make up that difference. To me, that reinforces the fact that it's possible to win with either wheel configuration. I do think that running a 29” wheel up front is superior to a bike sporting two 27.5” wheels, at least when it comes to racing.The times may not have revealed much, but it was interesting to go back and forth between the two setups. The difference in handling is noticeable, but it didn't take long at all to get used to whatever setup I was on. It felt like I could maintain speed better in the flats and while pumping through sections of trail on the 29er, while the mixed setup felt better in the steeper sections of trail. It was also easier to place the smaller rear wheel where I wanted, whether that was in a tight turn or wiggling through a jumble of roots on a climb. Of course, the slacker head angle and lower BB undoubtedly played a role here too.Is the 29” / 27.5” combo the future, or is it just another trend, a blip on the mountain bike radar that's going to fade away like Plus tires? It's hard to say, but I do think the concept has merit, especially for shorter riders, or riders in search of different handling characteristics. 27.5” wheels aren't dead, but their days of getting to enjoy the view from the front could be numbered.