27.5 Rear & 29er Front wheel-size combo | All you need to know
At the end of the 2018 season, the UCI buckled to the pressure applied from some of the trade teams and changed the rule book on wheel size.
The new rules came into effect on the 1st of January 2019 and basically state that for Enduro and DH categories riders can now use mixed wheel sizes. Almost immediately many professional racers began secret test sessions experimenting with the options available to them. Then, when the 2019 international racing season kicked off the mixed wheel size experiments turned to race winning formulae as Loic Bruni [Specialized] won the opening round of the World Cup and Martin Maes [GT Bicycles] won the first two rounds of the Enduro World Series on 27.5″ rear and 29″ front wheeled machines. Clearly, there are advantages to this setup so with the assistance of Revolution Cycles, Rush Sports and cSixx we did some testing to discover where the marginal gains and losses are.
Hucking Hellsend Compound’s Flow Line on the 279er Levo
Who should bother with this 279 wheel-size gig?
Trail riders, enduro and DH racers who are under about 1.8m tall could certainly be ready for something like this but it’s dependent on riding style and the terrain in concern. The UCI rule change doesn’t extend into the
The lowdown….
The disadvantages..
There is less BB drop which is something that contributes immensely to the high speed stability of a bike. You will also be sacrificing rollover on square edges and will notice the smaller wheel hangs up a little more. There is less traction which is not a massive deal unless it’s an EBike and you enjoy the thrill and challenge of taking on technical climbs. The conversion simply won’t work on every bike.
The advantages…..
We used two bikes for the test, a Commencal Supreme 29 DH bike and a Specialized Levo which are both designed and engineered specifically around the 29″ wheel. On the Levo we noticed the bike performed a lot better in flowy trails which have a lot of turns. It was far easier to lay the bike over into the turns, there was more room to move around on the bike and the acceleration out of slow speed turns and situations is great. The smaller rear wheel is marginally more agile and precise on technical trails. On bigger
On the Commencal Supreme 29 the smaller rear wheel simply compromised the geometry of the bike and the ride dynamic was negatively impacted. The BB was lowered too much which risked pedal strikes and the slackened head angle became floppy and washy. It was clear from the first few hundred meters that the geometry was not optimal. The bike did feel more nimble in tight turns though so it’s very evident there are gains here.
What bikes can be converted to a 279 split wheel size?
If you own a 27.5″ bike you could look at fitting 29″ lowers to your fork, or a completely new 29″ fork to try this out. The impact on your head angle from doing this will be about 1.5 degrees slacker and depending on your wheelbase the BB will be raised by as much as 10mm. If your bike has a geometry adjust flip chip then you have some adjustment available to minimize the lift on the BB particularly if you were running it in the high setting prior to changing out the fork.
If you own a 29″ bike then you obviously won’t have any issues fitting the 27.5″ wheel into the bike, but your BB might be lowered to a level that does not work for your riding terrain, style and in fact it may compromise the performance of the bike as it did with the Commencal we tried.
Optimal BB height is a subjective matter. Some riders will love a slammed BB and others just can’t ride a bike with a slammed BB so it’s difficult in this article to give you a comprehensive list of numbers that will work for you. Additionally, we all ride different sag levels which also impacts the dynamic geometry. So what we can say is for bikes between 130mm and 170mm of rear travel you would want to be wary of a BB height that is lower than 340mm as the chance of pedal strikes is quite high. For bikes with between 170mm and 200mm rear travel the BB height shouldn’t be much lower than 350mm, if at all.
In terms of head angles, if your short travel shredder ends up with a head angle between 64 and 66 degrees you will be fine; it will be rad. On 170mm plus
Remember there are exceptions to all of this and everyone is different. Moreover, if you become very accustomed to a funky setup then that might just be the best setup for you.
Takeout
We have reached out to a few brands and can confirm there is work being done on split wheel size specific bikes for new seasons, but they are a ways off from retail. In the interim, if you are super curious and fit the rider profile we have described here then why not give it a go yourself. Feel free to send us any questions or comments — we will help you out with any advice needed. Remember, it’s not necessarily a faster setup but is a very different feel on the trail which will likely make shorter riders more comfortable, and comfort can often translate to more speed…..