Tire choices are a hotly contested issue, and there are quite a few generally held “truths” out there—some of which bear more basis in reality than others.
So this week, three of our reviewers weigh in on some of the more commonly held beliefs regarding rubber.
Of course, there’s no such thing as the definitive word in tire choice, but when you’re shopping for a new set of shoes for your ride, we hope this discussion might provide a little food for thought.
TRUISM #1: Higher volume tires provide a smoother ride.
NOAH BODMAN: Generally true, but obviously there’s more to it than that. Tire pressure makes a massive difference, and differences in tire casings are certainly noticeable as well. But all other things being equal, a higher volume tire is more comfy than a low volume one.
MARSHAL OLSON: A bigger tire at the same pressure as a narrower tire of the same weight will provide less feedback to the rider, because the casing is flexing more (the wall of the bigger tire is thinner). That is bad, not good.
There are a lot of ways to tune a bike’s ride: rim choice, spoke choice, suspension setup, frame materials and design, wheel size, tire casing, etc.
TOM COLLIER: At a given air pressure, a higher volume tire has more ability to displace as you roll over roots. That is good news, as it (1) can provide more grip by better conforming to the terrain, and (2) it can take the edge off bumps. This second part doesn’t matter too much on a full suspension bike, but it is nice on a hardtail.
The bad news is that this motion is un-damped and uncontrolled, and it doesn’t just flex up and down, it flexes sideways as well. This can make performance unpredictable if the tire flexes through turns. So it isn’t a full substitute for suspension. Like anything else, it is a balance.
A tire with too little volume at a reasonable pressure for traction is going to bottom out easily on rocks and roots, resulting in rim strikes. But a tire with too much volume can squirm. Tire casing stiffness matters, air pressure matters, rim width matters. None of these things are simple.
Marshal: Just my opinion, but high quality suspension with clean break-away will allow the bike to track over roots/rocks, but not sacrifice the way a big soft tire does in every other scenario.
Noah: This point ^^^ bears repeating. There’s a big trend toward massive tires these days. Big tires might provide a smooth ride, but like Tom said, it’s undamped and uncontrolled.
Engineers and garage tinkerers have spent the last 60 years or so dialing in suspension to maximize traction in variable scenarios. Strapping giant balloons to your rims might be comfy, but it’s not the same thing by a long shot.
TRUISM #2: More, smaller knobs provide better grip on hardpack.
Marshal: No, not at all. 100% disagree.
Small knobs roll faster on hardpack, because they create less friction and therefore have less grip than knobby tires. The point is that you don’t need a ton of grip on hardpack since the surface is inherently grippy (ie. sandpaper).
Tom: Shorter knobs are better on hard surfaces because they generally squirm less and provide more contact area. If you imagine running a spike tire on pavement it would feel awful because the knobs aren’t able to sink in and you would only ride on the small area at the top of each spike.
Noah: Generally speaking, sort of. Big knobs work well when they sink into the dirt, but all they do is squirm around when they can’t sink in. So when dirt is really hardpacked, small knobs squirm less. The more important thing on hardpack is that the knobs are spaced more closely together, effectively making the tire a bit smoother. That means more non-squirmy rubber on the ground, which means more traction. As soon as the dirt gets a little bit soft though, traction takes a dive.
TRUISM #3: More open space between knobs yields better mud clearing abilities and better traction in soft soil.
Noah: As to the mud clearing – yeah, pretty much. There’s some other stuff that factors into a tires mud clearing abilities (knob shape being important), but knob spacing is definitely factor #1.
As for better traction in loose soil, I’d say… sort of. Yes, spaced-out knobs mean that each individual knob can dig into the soil and do its thing. But knob shape, knob height, and knob placement have a huuuge effect on this. A tire with low, poorly placed knobs, even if they’re spaced out, is going to get horrible traction compared to a more tightly packed tire with the knobs in intelligent places.
Another really important factor to keep in mind is that “traction” is really a generalization. A tire is really asked to do 3 things: (1) move the bike forward, (2) slow the bike down, and (3) make the bike go around corners. It’s easy to get caught up in the cornering part of the discussion, but climbing and braking matter too. And for each aspect of traction, the ideal knob placement and shape isn’t necessarily the same. Just because a tire grips like hell in a loose corner doesn’t mean it’s going to climb or brake well.
Marshal: Tread patterns that are more open = better clearing of mud.
Loose soil traction is more about knob depth and knob and tire surface area than knob spacing. A tire that is too big floats and surfs (bad). Narrower tires with pointy knobs can dig in there and grip. Knob height is also important because the face of the knob dictates traction for pedaling and braking traction, rather than the top surface.
Tom: There is definitely more to it than just spacing, and any tire designer needs to know how to design knobs that are sufficiently tall and shaped well enough to provide grip in mud or soft soil. However, this is less critical for riders to worry about. It is pretty rare to find a tire that has widely spaced and short knobs—unless it formerly had tall knobs and they wore down.
I agree with Noah and Marshal that knob spacing is the critical factor for determining how well a tire clears mud. Traction is definitely a more complex issue…
Marshal makes a good point about knob height being critical for braking and pedaling traction. I’d go a bit further, though, and blend in Noah’s point that traction is critical for 3 different activities. All knobs play a role in braking and pedaling traction, but the center knobs are most critical.
We’ve all been told that we are supposed to avoid braking in corners. In theory, that means your bike is usually upright when you hit the brakes
When you are worried about pedaling traction, you are usually going slowly, and at low speeds bikes are usually pretty upright.
When a tire is upright, the center knobs are most prominent, and provide most of the contact with the trail. Therefore, center knob shape is critical to braking and pedaling traction.
Squarer and taller knobs provide more grip. Ramped knobs provide less grip, but therefore they can also role more quickly.
For corners, the same principles of knob shape apply, but the load is primarily on the outside knobs and the forces are at ninety degrees to braking and pedaling forces.
An interesting side point is knob siping. Sipes are thin cuts through knobs.
They add more sharp edges to provide traction, and allow knobs to better conform to terrain—again for more traction. However, they let knobs flex and squirm a bit more and can increase tire wear rate.
NEXT: Tire Pressure, Soft vs. Hard Compounds, & the Tires our Reviewers Typically Run