WHEEL SIZE TESTING
Are big wheels always faster or can a ‘business at the front, party at the back’ set-up with a smaller back wheel give you more speed?
Racer, coach and tech expert Will Soffe sets out to discover if big-wheelers are always fastest, or if ‘mullet’ bikes can bring the speed to match their attitude
Mixing up wheel sizes is nothing new – just look at the penny-farthing! When it comes to mountain bikes, Keith Bontrager created the 69er in 2009 – a Trek hardtail with a 29in front wheel and a 26in rear. Specialized did it too with their Big Hit, which paired a 26in front wheel with a 24in rear. But it wasn’t until 650b wheels appeared that brands really started mixing wheel sizes, with specialist companies like Foes, Liteville and GeoMetron building assymetric bikes. This set-up has been dubbed the ‘mullet’, with a 29in wheel (‘business at the front’) for rolling speed and a 650b (‘party at the back’) for sharp handling.
New rules
In the racing world, the UCI (cycling’s governing body) had ruled that both wheels must be the same size, in order to prevent designers using the aerodynamic advantages of a smaller front wheel in the velodrome. World Cup downhill teams argued that this rule wasn’t relevant to off-road disciplines, and the restriction was lifted earlier this year. This led to pre-season questions about how well the mullet bike would work on the race track. It didn’t take long for Martin Maes to answer, riding his ‘mulletised’ GT Force to the top of the podium at the opening round of the Enduro World Series, at Crankworx Rotorua. He was followed by Finn Iles and George Brannigan, who finished the Crankworx downhill in second and third respectively. Then, at the first round of the DH World Cup in Maribor, Slovenia, previously vocal 29er-hater Loïc Bruni took the win and Danny Hart came second, both on mullet bikes. The doubters were silenced and clearly there was some advantage (or at least, no disadvantage) to mixing wheels.
Asymmetric or symmetric?
The results speak for themselves – at least when it comes to the sharp end of racing. But we wanted to find out if mullet bikes can give everyday riders an advantage too,
with some real-world testing. Before we dive into that, let’s explore some of the factors that might make a difference…
Grip Bigger wheels offer more grip because they have a larger tyre contact patch and the bigger diameter means they roll over bumps and obstacles better than 650b (aka ‘27.5in’) wheels. It’s not a straightforward win for 29ers though, because like for like, a wagon wheel is going to be heavier and flexier than its 650b counterpart (although more flex isn’t necessarily a bad thing).
Drifting The greater grip offered by a 29in front tyre means that if you pair it with a 650b rear the smaller wheel will tend to break traction first, making the bike oversteer, or drift. Oversteer on a bicycle is relatively easily dealt with using counter-steering, and is preferable to understeer, which makes the front lose grip before the back. When this happens it often does so unpredictably and ends up with the rider on the floor. With oversteer, the rider knows where the slide will be coming from and can control it.
Bum clearance Riders running a 29in rear wheel have about 1.5in less clearance above the back tyre (depending on the exact tyre size). It might not sound like much, but for shorter riders, having that much less space to throw the bike around in can make negotiating steep, tight sections of track that bit trickier.
Development time Some internet pundits have speculated that teams might be using a 29er wheel and fork on the front of their bikes as they don’t have a 29in-specific bike yet. But this argument is undermined by the fact that both Finn Iles and Loïc Bruni had a 29er Specialized Demo ready to use at this year’s Fort William World Cup but chose not to ride it, instead turning up on a similar bike built around a smaller rear wheel, with a shorter back end and geometry optimised for a 29in front wheel. With GT’s launch of a 29er Force enduro bike, it’ll be interesting to see what Martin Maes chooses to ride.
Geometry When the 29er revolution came around, some of the early bikes had poor geometry with high bottom brackets and steep head angles. Fitting offset shock hardware can help to overcome this, by lowering and slackening the geometry, but if this isn’t enough, then fitting a 650b rear wheel will drop the BB by roughly 13mm and slacken the head angle by around 1.5 degrees. While this is a great move for the home mechanic on a budget, you can guarantee that top EWS and World Cup DH teams, with access to shortened shocks, angled headsets and custom frames, aren’t doing it for this reason. There must be another explanation for why they’re using the mullet!
travel It’s hard for downhill bike manufacturers to fit a 29in rear wheel into a frame while providing acceptable geometry and over 200mm of travel. For example, the 650b Commencal Supreme V4.2 has 220mm of travel, whereas the 29er version of the bike with the same suspension design has just 200mm.
The nerdy bit
If you want to delve deeper into the pros and cons of the mullet bike, there are also some geeky physics reasons why its rise might be about more than just fashion. First, let’s look at the front axle height in relation to the centre of mass. When the front wheel hits an object that stops the bike, the mass of the bike tries to carry on forward, pivoting around the front axle and flipping the rider over the bars. On a bike with a 29in front wheel, the front axle is roughly 14.5in off the ground (half the wheel diameter, give or take variations in tyre size). If the bike has a 650b rear wheel, the mass of the rear mech, cassette, rear hub and rear brake sits at least 0.75in below the front axle height, at a maximum height of 13.75in (half the diameter of a 650b wheel). In theory, lowering more of the bike’s mass below the front axle creates a bike with greater stability, because more weight is driving the rider’s feet towards the ground under the front wheel, instead of over the front axle. This effect should also allow the suspension to work more effectively.
For those of you who are full-on nerds, here’s a brief insight into why bikes with asymmetric wheels feel unique to ride... The theory suggests that the inertia of the wheels creates a gyroscopic effect, which riders feel when cornering. Having a larger rotating mass that spins more slowly (a 29in wheel goes through fewer revolutions than a 650b wheel over the same distance) on the front of the bike than the rear creates a bike that’s eager to turn and wants to keep turning tighter once the turn is initiated. This could be the main reason why riders report that running two different-sized wheels takes some getting used to.
the full 29er set-up was slower getting up to top speed but carried momentum noticeably better
Time for testing
Many attempts to investigate the benefits of mullet bikes have simply involved taking a stock 29inwheeled bike and putting a 650b rear wheel on it. While at first this may sound like a fair test, in fact the geometry of the bike has been radically altered (head angle, BB height and more), so the results will not give an accurate comparison. To make our test properly fair we deliberated carefully over bike choice, finally honing in on the GeoMetron G1, designed by Chris Porter of Mojo Rising and made by German manufacturer Nicolai. We chose this bike because it has chips in the seatstays that can be added or removed to change the BB height in precise increments. This allowed us to change the size of the rear wheel but keep the geometry of the bike identical. It also helped that Chris has experimented a lot with mullet bikes – and you won’t find a man more eager to test the unconventional anywhere in the bike industry!
We did the majority of our testing on the Y Mynydd downhill track at Cwmcarn in South Wales. That was because we wanted a track that would test all aspects of the bike’s handling, and it had to be somewhere we could easily ride back-to-back runs. In the world of UK downhill, Y Mynydd is one of the classics – first raced back in the late ’90s as part of the Dragon Downhill series, the track as we now know it was built in 2005 by Rowan Sorrell (the man behind BikePark Wales) and his firm Back on Track, and has terrain that’s perfect for bike testing.
We broke the track into three sections: the first is long rocky straights into sharp, low-speed corners; the second is fast, flowing singletrack with tight tree sections and more elevation drop; and the final section is smooth, wide open and very high-speed, with big jumps and tall berms. Using a timing system with race-level accuracy, we did three runs against the clock on each set-up. Then, on a subsequent day, we timed more runs at Clyne Valley Woods in Swansea, to give an indication of performance on flatter, trailcentre-type terrain. The track we chose there is part of the main loop – a smooth blue/red-graded trail with some small jumps and mellow berms. Without a large elevation drop, the bike’s ability to carry speed would be highlighted here.
Ride impressions
The importance of first impressions can be underestimated. As riders, we quickly adapt the way we ride a bike to compensate for any set-up changes. So I spent two weeks riding the G1 before the test, during which I experimented with both rear wheels. Usually I ride a 650b downhill bike and a 29in trail bike, so don’t have a preference, but I found the mullet was more entertaining to ride. When switching wheels it took me a full run to get comfortable on the new setup. More time dialling in the settings would probably have allowed me to go faster, but rider fatigue was starting to set in after six full-speed runs on the track!
The first thing I noticed with the 650b rear wheel was how eager the bike was to turn in mid-corner. This made it a joy in tight turns, as I could turn the bike in sharply and stand it back up on the exit by counter-steering. I found the 29er pushed on, required more effort and sometimes even a dab of back brake to turn tightly. At the limit of grip on loose gravelly surfaces, the 29er had a tendency to understeer, with the front end threatening to wash out, whereas on the mullet set-up the bike would happily oversteer into a predictable slide. This encouraged me to exploit the available grip and take more risks.
On Cwmcarn’s rough, rocky straights, the full 29er set-up was slower getting up to top speed but carried momentum noticeably better, accelerating over bumps rather than getting caught on square edges. In the braking zones, it felt easier to slow the mullet bike down, possibly due to the lighter rear wheel, but maybe because of the increased mass below the front axle height. On the flowing singletrack, the 29er felt more comfortable at speed but sluggish when changing direction, whereas I could hustle the mullet along with pinpoint accuracy. On the big jumps I could throw the mullet around more easily than the 29er, and extra bum clearance meant I could use my body more.
So was it faster?
The times were very tight, so it’s hard to draw a clear conclusion from the testing. The 29in bike was faster on the DH track but only by around 0.5 per cent, while the mullet set-up was speedier at the trail centre but only by one per cent. Maybe the advantages of the 29er on rough terrain were more noticeable on the DH track and the mullet’s benefits in tighter turns made a difference on the flatter, twisty trail? Taller riders or other riding styles may produce different outcomes, but for me the benefits of the mullet set-up outweighed the ploughing ability of the 29er. At 5ft 10in tall, the extra clearance over the rear tyre was useful when muscling the bike around. Most importantly, it was the confident drifting encouraged by the mullet that’s now made it my chosen set-up. We have to conclude by saying that the fastest set-up depends on the track and riding style – just as in the EWS and World Cup DH, where some riders are running 29in wheels and others are bringing back the mullet.
I could hustle the mullet along with pinpoint accuracy