Unless you’ve been living under a rock for the past year or so, you’ll probably have heard a lot of speculation about plus-size tyres. Everything from far-fetched marketing nonsense to vitriolic condemnation has been plastered all over the internet: often in all capital letters, so you know it must be true.
At BikeRadar, we like to find out for ourselves what really works and what doesn’t, so we set out to try and answer a simple sounding question: could plus-tyres ever be faster than normal ones?
Since 27+ wheels have a similar outer diameter to 29ers, it’s possible to install 29in wheels in a 27+ bike (or in some cases, vice-versa). As if to underline the point, Scott’s Rémy Absalon used a pair of 29in wheels in a plus bike to win the 2015 Megavalanche.
This gave us an idea. Why not use a 27+ bike to compare both 27+ and 29in hoops? That way, everything other than the wheels can be kept the same. By comparing times over seven different tracks, totalling no less than 56 timed runs, we’ve learned a lot about those funny-looking tyres. It’s fair to say it was an eye-opening test – read on for all the details, and check out the video below.
A Specialized Stumpjumper FSR Comp 6Fattie was the test subject of this experiment. With 150mm/135mm travel and based on the original Stumpy 29er, it represents a fairly typical trail bike, just with fat-tyre compatibility.
Geometry wise, the 437mm chainstay, 51mm fork offset, and 67-degree head angle would not look out of place on a 29er.
Our Specialized Stumpy 6Fattie test rig, fitted with 3in tyres on 40mm DT Swiss rims
One inevitable difference in geometry between the two wheel sizes was the BB height. With 27.5 x 3in tyres, the BB is super-low at 329mm, but swapping to 29×2.3in rubber raised it to 333mm: a small difference, but the Stumpy’s low BB means it was far from high even with the 29er wheels.
In order to keep things fair I got hold of the most comparable wheels possible. Both spin on DT Swiss 350 hubs and the spokes and nipples are identical too. We went for DT’s 40mm wide XM 551 rim for the plus-tyres, and the 30mm wide XM 481 for the 29er option. We then fitted identical rotors and cassettes on each wheelset.
We stuck with Specialized’s 3.0in Purgatory / Ground Control tyres for the plus option, and tried out three combinations of ‘normal tyres’ to compare against. These were the 2.3in version of Specialized’s Purgatory Control / Ground control to give the most direct comparison to the big rubber; a Specialized Butcher / Slaughter Grid combination for more comparable weight and grip levels, and a Maxxis Minion DHF for the ultimate race-proven tyre.
Specialized’s 3in Purgatory / Ground Control tyres (left) were used throughout our experiment
Tyre pressure was critical to getting the most out of each tyre and making the comparison fair. To keep it consistent, I hit a section of hardpack berms as hard as I could on each tyre, and lowered the pressures until I could start to feel the tyres squirming. This defined the pressure used throughout testing. The pressures are shown in the following table:
The weight problem
The unfurnished wheels weighed 1937g for the 27+ and 1864g for the 29ers. The 3in tyres weighed 1045g and 985g, whereas their 29×2.3in equivalents weighed 766g and 760g. Overall then, the plus setup worked out 577g heavier.
That sounds like a lot, but I subsequently found that much beefier conventional tyres were needed to deliver similar grip and low-pressure characteristics to the plus tyres. We spent a lot of time testing a Butcher/Slaughter Grid combination, weighing 960g each. That meant the total weight difference was reduced to 183g.
Getting the measure of the 6Fattie Stumpy
Before getting the stopwatch out, I wanted to get familiar with the Stumpy: experimenting with tyre pressures, suspension settings and adjusting my riding style to the big rubber. I headed to some steep and technical Welsh trails – trails that I’ve become very familiar with.
I soon discovered that I required at least 18psi in the front and 19psi in the rear to prevent the tyres from rolling and squirming on the 40mm rims. I’m very particular about my rebound settings, and found that the extra bounce from the big tyres was bucking me on fly-offs, so I set the rebound one click slower than I would normally run to address this.
Our test rig fitted out with 29in wheels and 2.3in tyres
The Stumpy’s rear suspension is not nearly progressive enough, so I added Fox’s largest volume spacer to the shock to get it close to balancing the Fox 34 fork, which is not overly progressive itself. I left the shock in the medium compression mode throughout, and experimented with doing the same to the fork to add support on the descents. This worked fine with the plus tyres, but gave an intolerably rough ride with 2.3in rubber, so I ended up leaving the fork open for both wheelsets to keep things fair.
At first, the front tyre was prone to floating and surfing over muddy ground, but this could be remedied somewhat by weighting the front wheel extra hard and leaning the edges of the bike into corners exaggeratedly.
Once my technique and bike setup were tuned in to the demands of the big tyres, I was impressed with the extra confidence, stability and traction they offered. Riding with several properly quick riding buddies, I noticed I was riding rooty corners clipped in when they were dabbing; I seemed to carry more speed through rough, flat straights; and felt faster and more planted on rocky lines. This could all be subjective though, so it was time to break out the timer!
The first thing I was keen to establish was whether the bigger tyres rolled significantly more slowly. This meant doing a roll-down test. I marked out a section of slightly downhill fire road, with a third marker point 10m further up the hill. Starting at the highest marker at a standstill, I clipped in, steadied myself and rolled down to the second marker without pedalling, where I started the timer. This allowed me to get my balance before the timer was started, reducing the effect of little wobbles when setting off.
We timed each run using a simple DRC moto timer
Keeping my body position fixed so it could be easily repeated on each run, I rolled down to the third marker where I stopped the clock. This was repeated on each wheelset five times. Throughout the experiment, I always made sure that the brakes weren’t rubbing at all after swapping wheels.
The table below shows the time (in seconds) taken for each tyre to complete the same distance. The pressures used here were the same as those used on the trail, and are shown in the table.