When the UCI scrapped the rule banning mixed wheel sizes for racing, a lot of teams rushed to experiment with mixed wheel size bikes, which have now become known as mullets. In the early days, a mullet usually meant either squeezing a 29" front wheel and fork into a 27.5" bike, or a 29er bike with a 27.5" rear wheel.
A couple of years ago, Mike Kazimer tested
out a 29er bike with 27.5” and 29” rear wheels. The results were … inconclusive - neither setup was outright faster.
But with these setups, you're compromising the geometry to accommodate the mixed wheel sizes. If you fit a smaller rear wheel to a 29er, the bottom bracket might be a little bit too low; if you fit a 29er front wheel to a 27.5” bike, the BB is probably going to be too high. And either way, the head and seat angles might be slacker than what you want, or at least slacker than the bike was designed for.
Now, though, when people talk about a mullet bike, they usually mean a bike that's purpose built for mixed wheel sizes, and these are everywhere now. They allow you to have the mixed wheel sizes without any compromise in geometry.
There's an increasing number of bikes which are designed to fit either wheel sizes in the rear, such as this Geometron G1. By using interchangeable flip-chips, it can accommodate either wheel size in the back without changing the BB height or frame angles. Because the flip chips are between the link and the rear axle, instead of between the link and the shock, the leverage ratio and the suspension kinematics are pretty similar with both wheel sizes too.
I've tested this bike against the clock with both the 29er and the mullet set up on a couple of different tracks. The first track has a series of fast corners with not much gradient, which is a little awkward to carry speed through, then it drops into a fast section with a load of big roots at the bottom before it drops onto a fire road.
I know this track well, but I did a handful of scoping runs to get fully up to speed before getting the stopwatch out. Then I did two runs on the 29er, then four runs on the mullet, then swapped back and did two more runs on 29". This was to further minimize the effect of getting faster as I became more familiar with the track.Results
My average time was very slightly faster on the 29er, at 49.5s vs 49.9s on the mullet setup. That's just under half a second or 1% difference. It's not statistically significant, so we can't say the 29er was "the winner" here because there just isn't enough data to go on.
Track 1 lap times
Lap 1 49.9 (29")
Lap 2 49.5 (29")
Lap 3 50.5 (27.5")
Lap 4 50.1 (27.5")
Lap 5 49.9 (27.5")
Lap 6 49.1 (27.5")
Lap 7 49.3 (29")
Lap 8 49.1 (29")
Average: 49.45 (29"), 49.9 (27.5")
Subjectively, what surprised me most is how similar the bikes felt in the turns.
When swapping from the 29 to the 27.5” rear wheel, I did feel that the bike tipped into the corner a little bit more willingly. I'm not going to say quicker, but it seems to take less effort to lean into the turn. I don't think this is necessarily “better”; it's just different. And I only noticed this on the first couple of turns so I think that if there is a difference, you get used to it pretty quickly. The 29er is a bit like having a car with slightly heavier steering. It doesn't mean you can’t go round corners just as quickly, it just takes a bit more effort to initiate the turn. But really, the main takeaway is that the difference is very, very subtle. In fact, if it was a blind test I’m not 100% sure if I could tell the difference. But either way, it didn't feel like the mullet was faster through the turns, and that's certainly what the stopwatch suggested.
However, I did notice a difference at the bottom of the track where there's a section of big chunky roots followed by a flat straight. Here, the 29er didn't hang up quite as much and felt a little smoother through the rough. This might have allowed the 29er to edge ahead on the following straight.
Unless you're on the brakes, about 60% of your weight is on the rear wheel, and the rear is more likely to fall into the worst holes and bumps than the front wheel too (this is why we run higher tire pressures at the back). So, the size of the rear wheel probably makes more difference to carrying speed than the front, although the size of the front wheel makes more difference to bike's tendency to "trip up" on large bumps.
For my second test track, I looked for somewhere the benefits of the mullet were most likely to shine. I picked a short section with a steeper gradient to it, which is essentially a series of tight, alternating corners. This is where I thought a mullet was most likely to come into its own. And because the track is so short, I could do lots of runs in quick succession and get very familiar with the track, which is important for noticing differences in handling. But to be honest, both setups felt pretty much indistinguishable. I don't think in a blind test I would have been able to tell the difference and the times back this up.
Track 2 lap times
Lap 1 19.38 (29")
Lap 2 18.70 (29")
Lap 3 19.40 (29")
Lap 4 19.15 (27.5")
Lap 5 18.28 (27.5")
Lap 6 19.29 (27.5")
Lap 7 18.61 (27.5")
Lap 8 (crashed) (27.5")
Lap 9 18.36 (27.5")
Lap 10 18.39 (29")
Lap 11 18.46 (29")
Lap12 18.31 (29")
Average: 18.77 (29"), 18.74 (27.5")
I attempted six runs on each wheel size, though on one run with the mullet setup I had a small crash so I discounted that time. The average time was almost identical with both wheel sizes. The difference in the average was much smaller than the range of times posted on either wheel setup, so there’s essentially no difference.Conclusion
To sum up, in this test I found no real benefit to the mullet setup over the 29er, whether subjectively or against the clock. The main difference I can feel is that the 29er is slightly smoother through very rough terrain. It's not a huge difference, but it's the biggest one I noticed when descending.
But that's not to say that they don’t have a place. I'm 190 centimetres tall (six foot three) so I never really buzz my ass on the rear tire, but for some people this is a problem. Very short riders may still benefit from a full 27.5" setup so that they can get the bar height
low enough. But if you're in that in-between size where you can get the bar height in the right place with a 29er front end, but you want the extra clearance on the rear, then a mullet makes a lot of sense.
A brand manager told me that in their internal testing riders over six foot tall (183cm) didn't feel much benefit with a mullet, but many riders under that height did. So perhaps there are benefits for smaller riders, but if you're tall like me you may as well enjoy the roll-over benefits of a 29er.
From manufacturer's perspective, smaller wheels are easier for designers to package into the back of a bike while preserving tire clearance, seat tube shape, chain-line, stiffness and other considerations. Plus, a mullet configuration can work well with a broad range of riders, even if those at the extremes of the height spectrum might be better off with a full 29er or 27.5" bike. Smaller wheels are tougher too, which compensates for the additional punishment a rear wheel receives. After all, this was a major reason behind the OG mullets with 24"/26" wheels like the Specialized Big Hit.
When it comes to the claims of better cornering agility compared to a 29er, I think there was a very subtle difference on the faster turns on the first track, but otherwise both bikes felt almost identical to me, and whatever differences exist are easy to adapt to and not necessarily bad. I know that some people swear there is a benefit here, but with the riding style that I have I can't feel it, nor can I tell a difference with a stopwatch. And that’s despite going out of my way to find terrain likely to benefit the smaller back wheel.
It’s worth saying that I've done similar tests on both of these tracks before, comparing different fork offsets, tire sizes, suspension settings etc..., and I noticed bigger differences with many of these tests. I would say the difference between available fork offsets is quite subtle, but even that made more of a difference to handling than the size of the rear wheel.
What about climbing? Well, there I really did notice a big difference. In theory, the 29er rear wheel helps when getting up and over bumps on technical climbs, but the main difference I noticed is that the smaller back wheel makes the gearing easier. That’s by far the most tangible difference I felt throughout this test.