We use a stopwatch to test to see if the latest tire size can have a place on the track
Between the end of last year and now, especially with the Sea Otter Classic, you've surely heard about the wide tires. The internet is full of far-fetched marketing copy and vitriolic critical denunciations: and all in all caps, too, and you know it's always coming.
The editors at Bicycle Rim tried to discern for themselves what was available, so they tried to answer a simple question: are wide tires really faster than regular-sized ones? Here's a series of tests and results, which are long and probably boring for some people, so you can just pull up to the end for the conclusion. It starts with a serious editor talking about how he controlled the variables from all sides to make the test look as simple as possible, and then this guy with money and time to spare starts running seconds on all kinds of roads to let time prove which one is the winner, 27.5+ or 29er.
What is 27.5+?
Because the outside diameter of a 27.5+ carbon wheelset is comparable to a 29er wheelset, a 27.5+ bike can fit a 29" wheelset (and in some cases, vice versa). If we had to give an example, we could go back and see that Scott's Remy Absalon won the 2015 Megavalanche with a 27.5+ bike with a 29" wheelset.
This started to make us think, why not use a 27.5+ bike with both 27.5+ and 29" wheelsets? After testing the bike no less than 56 times on different tracks, we came up with some very interesting results.
The Bicycle used for this test was the SPECIALIZED Lightning Stumpjumper FSR Comp 6Fattie with 150mm/135mm travel, a classic Lindau bike and compatible with wide tires.
The 437mm rear lower fork, 51mm fork offset and 67° head tube lean make it out of place in the 29er carbon wheel model.
Our test bike, the Specialized Stumpy 6Fattie, was equipped with 40mm DT Swiss wheels and 3in tires.
The most noticeable difference in the bike's geometry due to the two different wheelsets is the height of the BB. The height of the BB is 329mm with 27.5X3in tires, and can be raised to 333mm with 29X3in tires, even though the Stumpy's BB is already very low, it is still much higher with 29" tires.
In order to ensure the fairness of the test, we used the same DT Swiss 350 hubs, spokes, spokes caps and flywheel, wide tires with DT's 40mm XM 510 rims and 29" with 30mm XM 481.
For the wide tire group, we used Specialized 3.0in Purgatory / Ground Control, and for the control group, Specialized Purgatory Control / Ground Control (to compare the effect of tire width); Specialized Butcher / Slaughter Grid (to compare the effect of tire width). Slaughter Grid (to compare weight and grip); and Maxxis Minion DHF (the ultimate comparison group).
Tire pressure is considered to be one of the most important factors that affects the fairness of the experiment, so I have also slightly tampered with the tire pressure in order to maintain consistency, as follows.
The wheelsets weigh 1937g (27.5+) and 1864g (29er) respectively, and the 3in tires weigh 1045g and 985g respectively, while the corresponding series of 29X2.3in weighs 766g and 760g.
As complicated as it sounds, I then found out that strong conventional tires also need to publish grip as well as low pressure characteristics as well as wide tires. We spent another long time testing the Butcher/Slaughter Grid combination at 960g each, which meant that the overall weight difference dropped to 183g.
Getting to know the model
Before pressing the stopwatch, I wanted to get acquainted with the bike: to try out tire pressures, shocks, and adjust my riding style to accommodate the wide tires, as well as test it on the forest roads that I already knew so well.
I found I needed to maintain at least 18psi at the front and 19psi at the rear. I was particularly concerned about the rebound setting, and during the test ride I also found that the wide tires bounced significantly and could be bounced off if I wasn't careful, so I slightly reset the rebound setting again.