29+ Carbon Mountain Bike

How many of us know someone that would always have gone faster if only they had been using that brand new thing that was in all the media. They would have been faster if they had fewer knobs on their tires. They would have been, but they forgot to lower their dropper.

There is always that one person who will have a range of excuses for why they were slower than everyone. It will never be themselves. What we want to know is, are they right or should we always follow the adage of a good workman never blames his tools?

So we decided to start with mountain bike descending. Will tires affect my descending speed or should I perhaps just lay off the brakes and ride downhill correctly?

Are Bigger Tires Better?

We have seen a steady stream of data coming in from road bikes that wider tires can be faster than old school narrow tires. It is a lot harder to quantify on mountain bikes though. Trails can change from one run to the next, but we can apply a little bit of research and see what happens. 


Plus bikes tires (tires that are 2.8” to 3” wide) were billed as the best thing since sliced bread when they started to appear on the scene a few years ago. We were told that they rolled faster over rocky terrain as there was less tire deflection. They provided more grip, which was confidence inspiring on tight berms and corners.

The funny thing is that this is all true. All the marketing hyperbole for plus size tires was actually correct. The problem was people never really bought them. The issue seems to have been that many people felt they were cheating or they should only be on beginner’s bikes.

It was as if the plus size was too good for its own good.

New Tire Sizes

The world started to move on.

Manufacturers started to pay more attention to tires. We began to see tires that kept some of the features of plus tires but started to make them in minus size. We found that the manufacturers tried to keep the high volume, that is why you want wide rims and taller sidewalls.

The taller sidewalls were reinforced to add more stiffness to the tire casing, tires started to get rounder (yes, we used to ride tires with square profiles) and the knobs began to get more compliant.  This is what all the tire manufacturer’s marketing departments are telling us.


As this happened, we saw tires go from 3” to 2.8” to now 2.6”. 2.6” is now being paraded through all the media as the new size to jump on. According to the manufacturers, it offers all the benefits of plus-sized tires minus the squishy feeling of 2.8” or 3” tires.

The question though is this right or are we just being sold a new “standard”?

Testing the Width

The best way to find out is to go and ride tires on trails. As we said, it is hard to quantify the data in the same way as we can with road bike tires. The way forward is to take a lot of runs and average out the data and see what happens.

When you get out and test a 2.6” tire against a 2.8” tire one of the first things you might notice is that the 2.8” will let you away with more mistakes. It also feels more stable, so you tend to leave braking a little bit later. This is especially true when the trail gets a little bit loose. The wider tire will find more to grip into and propel you onwards, whereas with the 2.6” tire you might feel more likely to dab and lose time.

There is a simple way to test this hypothesis. Put your wheels and tires on the same bike and perform a roll down test. A roll-down test is exactly what it sounds like, and you roll your bike down a hill and time between two markers, you are not allowed to pedal or brake doing this, you will also want to take the same seating position.

We took various runs and found the 2.8” tires to be faster, roughly a second faster over a 200m roll. We then went back to school and used some statistics training. We wanted to make sure that this extra time was due to the tires and not due to something out with our testing.

When we use statistics we want to work out a P-value, apologies if you’ve had to study statistics and just had flashbacks to the lessons. A P-value allows us to work out how statistics to work out significant a result is.

How Fast are Plus Tires?

Your P-value will come out as a number between 0 and 1. You need a result of under 0.05 for a null hypothesis. In the real world, we call this under 5%. The hypothesis in our test is that the tires make no difference to speed. So if we have a lower than 5% result, then we can say that the tires made a difference.

In our roll down tests, we found that the 2.8” tires rolled down our course faster and more importantly the P-value for our results worked out at 2%. What this stills us is that the tires made a difference. If the number had been above 5%, the tires would have made no difference to the time.

For instance, if the P-value had come back at 6%, we would have assumed that there could have been 6% chance of a difference in speed even if the tires did not affect the outcome. The fact that we get such a low P-value means that it is highly likely that the tires made this difference.

We then decide to see what happens if we compared these tires to a more standard 2.35” tire. What we found was that the 2.35” was almost the same speed as the 2.6” and slower than the 2.8” tire.  So for us, the 2.8” tire is still a tire that you want to be shod on your bike if you want speed.

29+ Carbon Mountain Bike

So if your friend is riding 2.35” tires, they may be right to blame their tires for their lack of speed. If they are blaming 2.8” tires, then they may just have to face up to the fact that it is themselves that is slow.