Tires: General Truths, Misconceptions, & Subjective Opinions

TRUISM #4: Smaller tires with higher pressure roll faster.

Marshal: Shorter centerline knobs roll faster. Pressure has nothing to do with it. Pressure is related to keeping the tire’s bead on the rim, durability, maintaining traction.

Pressures around 30psi front, 35psi rear seem to roll the fastest for me. Any more pressure than that and I lose pedaling and braking traction. Any less pressure and I bottom the tire on the rim and cut tires.

High level XC racers have grown tire width dramatically in the last ~5 years, and pressures have dropped significantly. WC XC’ers run 52-56mm wide tires. WC DH’ers run 58-63mm tires. That means the average DH rider is only running ~10% wider tires than the average XC rider, generally at comparable pressures.

I believe tires are about matching the terrain, bike, skill set, and riding style to a tire design.

Tom: Rolling speed is entirely about friction and weight. Friction is dependent on the trail surface and the tire surface.

Tire weight doesn’t matter too much once you are up to speed, but dramatically affects acceleration. Lighter tires get up to speed quickly. Because larger tires are often heavier and have large knobs providing more friction, they end up being slower, but it isn’t primarily because of the size.

Studies have actually shown that there is a sweet spot for tire pressure.

Too little tire pressure, and the tire flats too easily. Too much tire pressure, and the tire bounces a little off every bump, slowing things down.

In the sweet spot, tires conform to trail texture enough to prevent bouncing, but not so much as to result in excessive flat tires.

Additionally, the same study shows that wider tires can, in fact, be faster on rough terrain. (http://www.mtbonline.co.za/downloads/Rolling_Resistance_Eng_illustrated.pdf)

Noah: Like Tom said, for maximum rolling speed, there’s a sweet spot for tire pressure that balances a couple different factors, and really depends on the terrain that’s being ridden. Also, studies show that wider tires tend to have less rolling resistance, but then, as Tom mentioned, you end up with a bigger, heavier tire. Again, there’s a sweet spot for tire width vs. weight that will vary depending on the terrain you’re riding (and how you’re riding it). So yeah, I pretty much agree with Marshal’s bottom line.

TRUISM #5: Softer rubber compounds wear more quickly, but provide more grip on hard surfaces: dirt, rocks, roots. (They don’t do much to change grip in loose soil though.)

Marshal: If we’re talking strictly about road riding, then yes. Then again, if you don’t brake like a goon, and have a legit tire with firm corner knobs, it’s not that different, actually, when mountain biking.

I personally replace mtb tires every 60 or so days of riding them — the front tire gets moved to the back around day 30, then I throw it away at day 60 and rotate again. That rarely changes (+/- 5 days or so) unless I cut the casing, which rarely happens now that a good pressure has been settled on, and appropriate casings are selected.

Many firmer tire compounds will wear faster that one might think, since they don’t brake as cleanly and skid more frequently.

Noah: I’d say softer compounds wear more quickly, and they provide more grip in almost all situations. I suppose if you were riding in soft dirt that was extremely consistent and non-chunky, the difference might be reduced a bit, but most of the soft dirt I ride in still has a bunch of crap in it, and soft compounds help out a bit in that situation.

Going back and forth between a sticky tire to a 60a durometer tire, the difference is night and day, regardless of soil conditions.

But soft compounds do wear more quickly. The soft compounds are more inclined to chunk out and tear. How long they last for me really depends on the bike and where I’m riding. Soft compounds on my DH bike usually aren’t long for this world. Similarly, soft compounds typically don’t last long in a place like Moab where you’re riding on rock a lot.

Tom: “Soft” is a rather vague term. It could refer to low-density rubber that compresses easily, but it is also frequently used to refer to rubber compounds that rebound slowly (think 5.10 shoes).

Wear can be better with a firm, slow rebounding compound than a softer compound, but either will still wear quicker than a similar rubber made firmer.

Noah: Yep, there’s more to it than just calling a compound “hard” or “soft.” An “intermediate” compound (and by that I mean, something in between a super hard and super soft compound) from some companies holds up pretty well, while others (*cough* Schwalbe) starts to chunk out after a handful of rides. This is where I mumble something about imperfectly distributed carbon black within the rubber, and then shut up because I’m not a rubber engineer and I don’t really know what I’m talking about.

Marshal: In my opinion, rubber compound is more about slow rebound than soft in compression.

Tom: Continuing from what Noah said, companies are doing more and more interesting things to simultaneously improve traction and wear life. One novel solution is to place firmer compounds down the middle of the tire to allow the most commonly used knobs to last longer.

Another solution is to use stiffer rubber at the core of every knob and then just coating over the top with softer rubber. More and more, the compromise is becoming price rather than tire life vs. traction.

TRUISM #6: Edge / Cornering knobs that stand wider than the casing provide significantly more grip in dirt when cornering.

Marshal: I don’t agree. I like the corner knobs to be the same width as the casing. The taller and stiffer the corner knob, the more grip. Tire knobs that are closer together provide better grip in loose soil (less lateral slide). Knobs that are further apart yield better grip in soft soil (better penetration).

Noah: This depends on a bunch of things. Rubber compound, casing material, knob spacing, knob shape, and how far the knobs are sticking out there all make a difference. If the compound is too soft, the casing is too flimsy, or the knobs are sitting out there too far, the knobs will flex and fold rather than dig in. I can’t say I have a strong preference one way or the other on this one, just because there are so many variables.

Tom: Prominent, tall, cornering knobs that don’t fold easily provide more grip in dirt. These need to be supported by a good casing and enough air pressure to prevent the tire rolling relative to the rim. It is also key that there is enough space between the cornering knobs and the adjacent knobs to allow them to really dig in.

An interesting experience for me last summer was trying the Maxxis Minion DHF 2.3 and 2.4 tires in the same compound, back to back.

The tires look very similar, but the knobs are a bit bigger on the 2.4 and the cornering knobs stand proud of the casing. The difference in cornering grip was dramatic at extreme angles. I’d attribute that to the slightly taller knobs and the additional exposure of the cornering knobs.

Noah: Rim width plays a big factor here. A given tire is going to have a different shape on wider vs. narrower rims.

The side knobs on a 2.4 DHF might be more or less in line with the sidewalls on a wider rim, but if you put the same tire on a narrower rim, the side knobs are going to protrude more. While the tire shape will be rounded out a bit more, you’ll also probably get a bit more grip at high lean angles because of it.

But of course, that increased grip might be offset by having a less supported tire and (potentially) a flexier rim. My point: there are a lot of factors to consider, and there isn’t really any universal truth on this front.

Marshal: Just like skis, skinnier tires carve cleaner since the rider can get on the edge quicker and has more leverage to hold the tire on edge. Wider tires are more stable in a straight line, and can drift more smoothly (everything else being equal), since there is a bigger region between upright and on edge. But you need more leverage to get the thing up on edge, and to hold it there.

QUESTION: What tires do you typically run and for what conditions? What pressures do you run them at?

Tom: On my Santa Cruz Nomad, when I’m focused on rides with great descents and I’m willing to lug slower tires uphill, I’m running the same tire front and rear, a Maxxis High Roller II 27.5 x 2.4” 3C Maxx Terra.

As for pressure, In the front I’m running 28 PSI for just a bit more grip, and in the rear I’m running 30 PSI to increase flat resistance and to encourage the rear tire to break free just a bit more easily so I drift rather than crash if I push too hard through a corner.

When I’m headed out for longer rides or riding in drier / sandier conditions, I run a Maxxis Minion DHF 27.5 x 2.3” 3C Maxx Terra in the front, and a Maxxis Ardent 27.5 x 2.25 Dual Compound tire in the rear. Tire pressure remains the same for me – Front: 28 PSI, Rear: 30 PSI.

In reality though, I don’t swap tires to often to match the riding I’m doing, and instead I just ride the bigger, grippier tires and tell myself I’m making my legs stronger by pushing slower tires around.

Noah: On my hardtail, I’ve been liking the WTB Trail Boss in a 2.2. It has relatively closely spaced, medium-low knobs, which strike a good balance between rolling resistance and traction. It doesn’t roll like an XC race tire, and it doesn’t trench corners like a Minion, but for all-around hardtail duties, it performs pretty well.

On my squishy bike, I generally go for some combination of a Maxxis Minion DHF, DHR 2, or Highroller 2, usually in a 2.3 – 2.4” size, and usually in the “3C Maxx Grip or Maxx Terra” compound. Usually I have a single ply in the front and an EXO casing in the rear (or sometimes EXO on both wheels). Those three tread patterns are all a little different, but I can be happy on any of them.

For the DH bike, again, I can be happy on any of the above mentioned Maxxis options, but most recently I was running DHR 2’s front and rear. I’m always running a DH casing, and either the 3C Maxx Grip or Super Tacky compound. I’ve found other tires that were passable in certain situations, but no others that I liked as completely as the Maxxis lineup.

Marshal: Since the bikes I’m on are all riding the same trail surface (rocky, dry, loose over hard), I run similar or the same tread design on all my bikes: square blocks with a bit of channel between the corner knobs and centerline, and frequently a rear tire with less pronounced knobs.

Last season, it was a Specialized Butcher front, and Specialized Slaughter rear. I am currently riding WTB Trailboss 2.4’s for a review.

I vary the casing according to use. My hardtail typically sees lighter ~750 gram non-reinforced 58mm tubeless-ready tires.

My full suspension trail bike is spec’d with butyl reinforced tubeless-ready tires (Maxxis EXO TR, Specialized GRID), also around 58mm wide.

And if I still owned a big bike, I would run 2-ply DH casings full time, in the 58-60mm tire width.