Schwalbe Magic Mary Radial Tires

Radial tires? I originally thought they arrived in the 60s but, aside from a few models, most cars rode on bias plies into the 70s. Radials were originally patented as a design concept in 1914 but weren't actually manufactured until 1946 when Michelin took up the cause. Because the tiremaker also owned the Citroën brand, they spec'ed the first car to roll off the line with radials with the 1948 2CV. Widespread adoption occurred in North America when consumer reports laid out the advantages in terms of safety, wear, handling and fuel consumption in 1968, and we've been there ever since. Except in mountain biking that is. We've been rolling on bias ply tires since the beginning.

But actually, it's not that simple. In motor vehicles, radial tires have plies, essentially cords, that run from one bead to the other in a straight line, at 90º to the direction of travel. In bias ply tires, those cords run at 45º and they go in layers from both sides, crossing each other. The plies of Schwalbe's radials don't encircle the tire at 90º like conventional automotive tires but they don't run at 45º like a bias ply either. Sitting somewhere between the two, you could easily call them more radial but not fully radial, but we'll allow it.

Why Radial?

Schwalbe's goals were "to develop flexible tires with maximum utilization of the contact area that offer even more grip, safety and comfort." Because the cords are aligned more radially, they are shorter which results in a lower density of cords for a given area. That means when your tire is in contact with an object, or simply with the ground, fewer plies will resist the force applied to that portion of the tire. Schwalbe tells us this produces the results they were seeking.

Another factor in the increased traction, according to Schwalbe, is that the tires don't just deform more, they deform more selectively. This means that trail debris will deflect a smaller area of the tire keeping more rubber in contact with the ground.

Tire Pressures Required

Before you get to the trail you may notice some differences between these tires and what you are used to. If you give them a squeeze, you might discover that your pressure seems low. While that may be true, if you check, they could be set at your usual pressure because Schwalbe's radials deform more easily to the pressure applied by your hand. This is true on the trail as well. Your usual pressure might feel too soft at first, particularly if you prefer higher pressures. I generally run 19 front and 21 rear or so and while they felt too soft, they performed pretty well. I didn't have too much squirm and they cornered well but I found it a little unnerving so I added pressure going up to 24/26 psi. At these pressures the trail feel was similar to other tires at my usual pressures, and at first I assumed performance as we similar as well.

On a recent ride, both of my buddies were complaining about the grip. They were slipping in corners and even on straight lines and I hadn't noticed any issues at all. I felt like the conditions were entirely normal, and this was running 5 psi more than I usually do. Schwalbe says that, for a given pressure, you will get a larger contact patch which could explain the added grip I was feeling.

Physics 100 Digression

A larger contact patch for a given pressure doesn't seem to line up with the physics as I understand them, which dictates that tire patch area is determined by air pressure and load. However, the same should be true of the squeeze test; the deflection of the tire should be dependent on tire pressure and the force you are applying. One squeeze will tell you this is not the case which must mean the construction of the tire has an impact as well. This could be because the increased suppleness of the casing allows an impact to deform the tire further because the rest of the carcass swells slightly due to the increased pressure,* which could be what is happening in terms of contact patch as well.

*Or it may be because the deflection is more selective so you can push deeper because you are pushing against fewer casing cords.

On the Trail

A friend told me that her husband, who had been running the tires in the Santa Cruz area, had to go up to 35 psi to get the support he wanted. That sounded a little crazy to me, and I felt like I was getting lots of support at 24/26 but there are other factors involved. The riding in Santa Cruz is generally higher speed than we have on the Shore and berms and jumps like you'd find in a bike park are also more prevalent. I believe this gentleman is a little burlier than I am as well. Still, this got me thinking about what the radials would feel like at pressures significantly higher than I had already tried, so I went up another 4 psi to 28/30 and saddled up for a moist ride.

The day before we'd had a big storm that started with more nasty rain (although much less than our recent flooding event - it came down harder and did lots of trail damage) followed by some very high winds that knocked out power across much of the North Shore. I knew what I was in for so I packed my small folding saw. I should have packed the big one.

On the climb, the tires felt just fine. I had lots of grip and they were still comfortable, without excessive rebound from impacts. I spent about an hour cutting out deadfall that was across the climbing trail and it was long past full dark by the time I got to the trailhead, and I was a little nervous. When we lost power, we'd needed my riding lights in our home to go along with some candles. Between that and the clearing work, I didn't have as much juice as I wanted so my headlight was off and my bar light was on medium. I started off tentatively with my overinflated rubber, particularly on wet woodwork, but my confidence soon grew. I had no grip issues and managed to ride with relative confidence, and the moist wooden structures were no problem at all, at least until I lost my lights completely and had to limp out using my iPhone's flashlight app.

Downsides

Pinch flats are more likely when deflection increases so, for a given pressure, you could expect to have more pinches on Schwalbe's radials. The big but here is that you can ride with higher pressures and still get good trail feel and traction. In terms of punctures from sharp objects, this is what Schwalbe says; "Since we have only adjusted the angle of the carcass threads, but not the number of carcass plies or the material thickness, Schwalbe radial tires are no more susceptible to punctures than bias ply tires. The cut and puncture protection therefore remains positive." I haven't had any flats since I've been riding these and, unusually for me, I haven't been running inserts.

Schwalbe goes on to say that the increased contact patch will increase rolling resistance some but I haven't noticed these tires to be slow rolling at all.

Overall Findings

This is the first time in a very long time that Santa Cruz has spec'ed tires that didn't come from Maxxis, which is a strong recommendation. For the riding we do in southwestern British Columbia, grip is king, particularly until at least April, and these provide heaps of traction without having to lower your tire pressure to the point that your steering becomes vague at speed. You have the choice of running your usual pressure and having more than usual traction and comfort, or ramping it up to have similar traction to what you are used to but with more precise feeling tires, assuming we are comparing other tires with soft rubber. I have decided to split the difference and run a little more pressure than usual to get better traction, good stability and sidewall support and a little extra protection from pinches.

So far so good.

Schwalbe Radial Tires