Why do mountain bikes have fat tires?

There are a few reasons why mountain bikes typically have much fatter tires than other types of bikes. One of the most important is that a wider tire running at a lower pressure gives a much larger contact patch with the ground, allowing the aggressive tread pattern to provide fantastic grip on uneven terrain of all types. But that is just one of the benefits.

Another benefit is that a wider tire (typically run on a wider rim) contains a greater air volume. As this is running at a lower pressure, often around 22-28psi, it provides some amount of damping of harsh chatter and impacts over rough ground – for example when riding fast over rocky terrain. While front (and often rear) suspension is the norm on modern mountain bikes, the air volume of the tires certainly helps as well with overall bike feel when mountain biking.

So why are there different mountain bike tire widths?

Different types of mountain bikes usually come equipped with very different types of tires – and the width of these tires is no exception. There is definitely some cross over, but typical ranges are Cross-Country (XC) bikes running 1.9 to 2.25″ tire width, to Trail bikes running 2.25″ to 2.35″ , to burly Enduro/Downhill bikes with 2.4″ to 2.6″.

As I mentioned there are definitely some lines being blurred there – and it is now common for trail bikes to be set up with 2.4″-2.5″ tires as these bikes become more and more capable on the gnarlier terrain, and riders keep pushing the limits further about where they ride regularly.

While there’s always a weight penalty with the wider tires – something that is made even more noticeable by often being run in conjunction with larger more aggressive and tackier tread patterns with taller knobs (and thicker sidewalls) – the additional grip provided is super obvious when you’re riding challenging terrain. These changes also mean that the tire is much more likely to remain intact and on course through high speed or gnarly trails such as downhill tracks and rockgardens.

What is the best tire width?

As eluded to above, it depends very much on the type of riding you’ll be doing. As I’ve progressed in my riding from an entry level XC bike on flat easy trails through intermediate skills and terrain and now into double black diamond downhill trails full of rocks, drops and other technical features, I’ve found that I’ve modified my current full suspension trail bike to more of a burly enduro all mountain spec – complete with tacky 2.5″ wide tires with very aggressive knobbly tread, as it just feels far more confidence inspiring on steep rocky downhill and bike park laps, with all that extra grip. Sure, it does make the bike feel a bit slower and less zippy on flat ground when pedaling, but for me that trade-off is just fine, as the adrenaline pumping descending and technical challenges are what I most enjoy about the sport.

A cross-country mountain bike racer on the other hand is going to want a narrower tire, with thinner sidewalls, low profile tread pattern and usually a less tacky tire compound, to allow them to have less rolling resistance, and allow for faster acceleration when pedaling and climbing, and increase their chances of a podium using a higher power to weight ratio for example.

Can I put wider tires on my mountain bike?

Usually it is absolutely possible to put on wider tires, but just how much wider does depend on a couple of factors. If your type of mountain bike is designed to run much narrower tires than you’re wanting to put on it, you may run into tire clearance issues – especially on the rear, where fatter tire tread can end up rubbing the frame on a bike not intended for such wide tread – though this is not nearly as much of an issue as it was in the past. Even if it does appear to just fit, keep in mind that when the wheel and tire flex and distort under extreme cornering/braking at higher speeds, this can lead to tire rubbing, and in the worst case damage to your carbon frame or wheel binding.

Bike manufacturers will usually publish what the maximum tire size a specific model frame is designed to fit – as do suspension fork manufacturers – so it’s a good idea to check into that if you think you might be pushing the limits.

So what is best? Fat or skinny tires?

This all comes down to what type of trails/terrain and type of mountain bike you’re riding, and what suits your needs the most.

A downhill bike rider is always going to want something very different out of their tires than a cross-country racer

Where the downhiller wants extremely tacky wide and strong durable tires providing tremendous grip at high speeds over typically short intense descents, the XC racer likely values the low rolling resistance and lighter weight of narrower tires with just enough tread to keep them rubber side down, without slowing their pace up hills and over large pedal powered distances. Trail riders usually sit somewhere between the two, wanting the dependable grip of reasonably wide tires, riding some uphill and being able to enjoy the technical challenge and speed of descent wherever possible.

For me, there is no question in my mind that I most enjoy wider tires on wider rims on my trail bike. Typically a 2.5″ tire on a 30mm rim. It’s important to understand that on modern mountain bikes this is actually not nearly the widest of either possible, but I (and a large number of other riders) have found it has transformed the bike feel to provide a ton of confidence on the trail, with a weight penalty that I’m happy to accept. The best front tire I’ve ever run is the Maxxis Minion 2.5″ DHF WT Maxxgrip, and for the rear tire the Maxxis Minion 2.4″ DHR WT DoubleDown is my tire of choice.

While I certainly notice the wide tacky tires slow me down a little when riding long distances on flat terrain, it’s a trade-off I am more than happy to pay – the long days thrashing laps at the bike park on very challenging downhill trails, or big mountain descents are where I get the most enjoyment from my bike – and importantly, doesn’t stop me taking the bike out on all day epic missions in the mountains involving massive elevation changes on both the climbs and the descents.

What are “Wide Trail” tires for?

Essentially this designation from Maxxis or similar from other brands just means that the profile of the tire was designed to be run on rims that have 30mm to 35mm internal width.

This is because prior to this, most mountain bike tires were designed to be run on rims that 21-23mm. Putting a tire designed for narrower rims on a 30-35mm rim results in the profile of the tire being noticeably different than it would be on a wider rim. It also can lead to the tire rolling over on the rim more during high speed cornering.

Wide Trail (or WT) tires get around this by having a wider upper tread carcass and pattern, and consequently have the correct tread profile between middle and side knobs for cornering grip – as well as having increased cornering stability when paired with a wider rim – reducing the amount the tire folds over when cornering quickly.

What are the fattest tires available for bikes anyway?

Generally around 2.5″-2.6″ is considered the widest generally run on modern 27.5″ and 29″ wheelsize bikes, however there are also many bikes which have been brought to market designed with Plus size tires in mind – usually topping out at 2.8″-3.0″ in width, and offering even more air volume, cushioning, and contact patch with the ground (at a weight cost obviously). Some riders do love them, and Plus tires have been retrofitted to a huge number of different bike brands and models.

Beyond this we get into Fat Bike territory. Usually considered as their own specific type of mountain bike, fat bikes emerged out of a desire from people wanting to ride their local terrain in previously unrideable surfaces such as snow and sand. To get around the issue of narrower normal mountain bike tires sinking into the sand or snow and getting the rider stuck almost immediately, much fatter tires were added, typically to a hardtail, allowing the tire to be run at an extremely low pressure, and resulting in a highly malleable and very large contact patch with the grounds surface. This allows the bike to keep rolling over the top of sand and snow, albeit at lower speeds typically, but solving the issue – and allowing bikes to now ride almost every conceivable location on the planet.

Typically Fat bikes run 3.8″ to 4.8″ tires, on 55mm+ rims, and as low as 8psi.

Why run different width tires on the front and rear of the bike?

So as we have covered, wider mountain bike tires generally provide greater grip, at the cost of greater weight and greater exertion required for pedal powered speed on the flat or uphill.

One popular method to get the best of both worlds is to run a wider tire up front, for example a 2.5″ Wide Trail DHF, and perhaps a 2.35″ Aggressor on the rear. This results in maximum grip up front with a more aggressive tackier tread and wider contact patch , where it is arguably most vital, and a faster rolling, lower profile tread pattern with a smaller contact patch with the ground on the rear.