fastest tyre size for mountain biking?

Updated: Mar 16, 2019

2.3in, 2.6in and 2.8in tyres tested against the clock on a 27.5


Hunt sponsored this test and provided three sets of their Endurowide wheelset so tyres could be quickly swapped when testing

Plus-size tyres (2.8–3.0in wide) were hailed by many as the future of mountain biking. They were claimed to have more grip, a smoother ride, and faster rolling-speeds on bumpy terrain — because less energy was lost to vibration.

The odd thing is, I found the bold claims made by “big tyre” advocates to be largely true. When I tested 3in plus tyres on 650b wheels against 2.3in tyres on 29in wheels, the fatter tyres were faster everywhere I tested aside from tarmac.


Are 27.5+ bikes faster that 29ers?:

Nevertheless, plus was a flop. The few available options punctured easily, were expensive to replace and could feel vague if the pressures weren’t right. Racers didn’t adopt them and they were perceived as a beginner’s option. People just didn’t buy them.


Less is more which is why the 27.5 Plus Standard is doomed:

These days, many in the industry are claiming 2.6in tyres are the new hot thing. They say they’re the best of both worlds: offering many of the benefits of plus, without as much sidewall-squirm and weight.

But is this in-between tyre size the goldilocks solution, or is fatter still faster?


To find out, I’ve extensively tested similar tyres in the three most relevant sizes: 2.3in, 2.6in and 2.8in. This involved over 100 timed runs over technical descents, as well as rolling-speed and climbing tests.



The equipment

A Specialized Enduro Comp 27.5 was chosen for its low BB height and clearance to accommodate 2.8in tyres

I chose Specialized Butcher Grid tyres for this test because they are available with comparable tread pattern, casing thickness and compound in all three sizes, making a fair comparison possible.

The tyres were tested on a Specialized Enduro Comp 27.5. This bike was chosen because it’s sold with 2.6in tyres, but has room for 2.8in rubber.

I used Hunt’s Enduro Wide wheels. Their 33mm internal width is a good compromise for all the tyres tested; not too wide for the 2.3s, but not too narrow for the 2.8s.

There is logic to using wider rims for wider tyres, but this would introduce other variables such as wheel stiffness and wheel weight.

Hunt sponsored this test and provided three sets of wheels (one for each tyre size) so that tyres could be swapped quickly. Regular wheel swaps were key to testing the tyres fairly.


How does tyre size affect rolling resistance?

We’ve seen that bigger tyres offer more grip, better comfort and allowed me to ride consistently faster on all three downhill test tracks. But does that grip come at a cost in terms of rolling speed?

To answer this question, I performed roll-down tests. These consisted of riding downhill on a slight gradient without pedalling or braking, and timing how long it takes to roll between two marker points. I set off just above the first marker at the same point every run, and adopted the same position (seated with straight arms).

I did six timed runs for each tyre size, so 18 in total. I did this on two surfaces: a smooth fire road and a rough track.

The rough track had a similar surface to a cobbled road, with large enough bumps to activate the suspension, but not so rough as to make riding seated uncomfortable.

Six timed runs were completed for each tyre size and the average time taken to complete the course is shown below, along with the percentage difference in time relative to the 2.3in tyres.


How does tyre size affect geometry?

Fatter tyres raise the bottom bracket height, but not by as much as you might expect. The table below shows the bottom bracket (BB) height of the Specialized Enduro, measured with each tyre size fitted at riding pressures.

A change of 5mm across the range of tyres is noticeable when riding, but small enough that the tyres can reasonably be compared on the same bike without modifying the frame to preserve the geometry.


The test bike fitted with 2.3in tyres


…with 2.8in tyres

It’s worth noting that the 2019 Specialized Enduro 27.5 has a bottom bracket height roughly 12mm lower than claimed on Specialized’s geometry table with the stock 2.6in tyres fitted.


Finding the right tyre pressures

The width of the tyre's casing from bead to bead was measured in this way. For ease, we call this measurement the tyre's circumference

Tyres of different width will perform differently at the same pressure. That’s because a tyre supports the weight of the rider and resists deforming through the tension in the casing.

This tension is proportional to the pressure inside the tyre and to the circumference of a cross-section through the tyre. This circumference is the same as the total unfolded width of the tyre, from bead to bead, as shown below.


This relationship between pressure, circumference and casing tension is based on Laplace's Law, which is more often used to calculate the wall-tension in pressurised pipes or blood vessels.

The bead-to-bead measurements (which we’ll call the tyre’s circumference for ease, even though the tyre forms a C-shape rather than a full circle) of the tyres on test are shown below:



Conclusions

Just like in my 2.3 vs 3.0in tyre test, I found that bigger tyres offered a smoother ride, more grip and allowed me to ride faster over rough and technical terrain.

The difference in both ride feel and descending speed was more pronounced between the 2.6in and 2.8in than between the 2.3in and 2.6in tyres. This is perhaps because the difference in tyre depth (the vertical thickness of the tyre) between 2.3in and 2.6in was far smaller than that between 2.6in and 2.8in.

It’s worth pointing out that UK has been unusually dry this autumn, so I didn’t have a chance to test in muddy conditions, where the bigger tyres might not have fared so well. However, I did test in the mud in my previous tyre-size experiment and found the bigger tyres to be no better and no worse than their narrower counterparts.

When it came to rolling speed, the narrowest tyres were markedly the slowest on the smooth fire-road, both uphill and down. On the rougher surface, the 2.8in tyres were markedly faster than the other two when coasting downhill, but all three tyres were very similar in speed when climbing up the steeper, rough track.

In other words, there was no significant penalty to running the larger tyres when it came to rolling speed and climbing speed when riding off-road. In fact, aside from the steep climbing track, the 2.8in tyres were significantly faster than the 2.3in tyres everywhere.


This won’t be a surprise to those who have seen my 2.3 vs 3.0in tyre test, or Joe's 2.0in vs 2.2in cross-country experiment. In both of those tests, the bigger tyres went faster on this same fire-road.

That’s not to say that big tyres are always faster. This test is focussed on off-road riding, but I carried out a brief roll-down test on tarmac and found the 2.8in tyres were significantly slower than the 2.3in tyres. In my 2.3in vs 3.0in test, I found the same thing: bigger mountain bike tyres are slower on the road.

Road riders are starting to realise that 25mm or 28mm tyres are faster than 23mm tyres even on the smoothest tarmac, but don’t expect to see 2.8in tyres on road bikes any time soon!

The rougher the terrain, the bigger the tyre that offers the least rolling resistance. That’s why the 2.8in was the fastest on our rough surface, the 2.6in was (sort of) the fastest on the fire road, and the 2.3in was the fastest on tarmac.

That’s because the fatter tyres (in the range of mountain bike tyres) have inherently more rolling resistance on smooth terrain, because there’s more material flexing in the carcass as they roll.

However, over rough terrain the bigger tyre absorbs more of the energy from bumps and transmits less of that energy to the suspension and the rider.

The tyre acts almost like an undamped spring, so it returns most of the bump’s energy after it rolls away from the bump and the tyre casing rebounds. Whereas the energy transmitted to the suspension or rider is almost entirely absorbed — very little is converted back into forward momentum.

Punctures can be a problem with plus-size tyres, particularly the thin casing, sub-900g ones, which were popular in the early days of plus. Thicker casings are uncommon in plus tyres, perhaps because they’d be too heavy to sell, but also because a thicker casing would severely increase rolling resistance.

I didn’t suffer any punctures during this test, though, and I have had success using 2.8in Maxxis Minion tyres with inserts in rocky terrain, including racing. With the right pressures, plus tyres are, in my experience, not as puncture-prone as some have claimed.



What’s the bottom line?

All the testing I’ve carried out suggests bigger is usually better when it comes to riding fast off-road.

2.6in is not the “best-of-both-worlds” as some have claimed, but is a compromise, offering some benefit over a 2.3in but not as much as a 2.8in tyre.

However, not everyone will like the bouncier, more isolated feeling that a true plus tyre can provide. So something in-between may be a good option for some riders.


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