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Bikes. Parts. Chaos.

Extra : added to an existing or usual amount or number

Terrestrial: of or relating to land as distinct from air or water

By now you’ve probably seen or heard about our newest tire, the ExtraTerrestrial touring tire.  In a very literal way the name is indicative of what the tire is designed to take on.  The ET tire was designed specifically for use with our original off-road touring bike, the Troll.  In a typical off-road tour you are going to experience everything from paved roads, dirt roads, double track, to single track or hike-a-bike sections.  The biggest design challenge was how to make a tire that can perform in all of these situations, and not be terrible at any one of them.  An ideal off-road touring tire needs to roll fast when you are on smooth or packed surfaces and provide traction when things head off the beaten path.  Sounds pretty simple, right?  Well not so much.

In addition to these performance characteristics we wanted to pack in the tech features that most road touring tires have – puncture protection and cut protection.  There are certain tires out there that offer excellent cut and flat protection, but they sacrifice suppleness.  Another consideration was to try and offer additional comfort through larger volume tires.  Suspension forks can be unreliable when you are trekking across South America and can leave you high and dry if they fail.  For this reason, a lot of people tour on rigid forks.  Steel can provide some vibration damping, but high volume rubber will offer additional damping and added traction.

The first couple iterations of the ET tire featured a pretty similar tread pattern to what is now available, but were only 2” wide.  The first iteration had the tread pattern spread out a little too far for our liking, the gap between pattern instances can increase your rolling resistance when there isn’t a constant contact between the ground and tread.

Downward view of a white bike tire cut-out, with the tread facing up, laying across a yellow legal pad - 1st

The second one tightened up the pattern, but there was still something missing

Downward view of a white bike tire cut-out, with the tread facing up, laying across a yellow legal pad - 2nd

We’ve always believed in large volume tires, I mean it says it on the chainstays of all Surly bikes “Fatties Fit Fine” so we decided to pack in as much meat as the frame would legally allow.  We beefed the tire width up to 2.5 inches, wrapped the tread a little further around the casing, added a more directional characteristic to the tread, and made the side knobs a little more aggressive.

Downward view of a white bike tire cut-out, with the tread facing up, laying across a yellow legal pad - 3rd

Some further tweaking produced the file tread and alternating the transition knobs aided in a smoother transition as you lay the tire over.  We added a Kevlar cap underneath the tread for cut protection and placed the knobs to allow flexibility of the tire and casing.  The file tread adds additional material on the main contact patch (more puncture protection) without just laying down unnecessary amounts of rubber (this would have stiffened up the tire too much and added weight).  This led us to the final 3D print (excuse the quality of the file tread, we had some humidity issues in the prototype lab that day).

Downward view of a white bike tire cut-out, with the tread facing up, laying across a yellow legal pad - 4th

In addition to the previously mentioned features, we added a nylon breaker in the sidewall for cut prevention, and a molded pattern to stop the propagation of a cut if it does happen.  The final icing on the rubber cake was a tubeless ready bead.  Running a tire tubeless allows you to push the tire pressure limits and removes any additional stiffness the tube may impart on the system.

Additional specs

  • 60 Tpi casing
  • 62A durometer rubber
  • 64mm max tread width*
  • 63.5mm max casing width*
  • 690mm OD*
  • Max pressure 65 Psi.

We have been riding these tires for several months now and they make an excellent commuting tire, especially if your commute takes you on mixed surfaces.  We sent a couple sets to our friend and bike packer extraordinaire Cass Gilbert who put some time on them, and they are being put through the wringer on an expedition across S. America as you read this novel by another bike adventurer.  I’m happy to say that I haven’t heard of any flats yet, and I’ve made believers out of skeptics that you can, in fact, comfortably and efficiently tour on 2.5” tires.  Throw them on your stock Troll, or get crazy and mount them up on Rabbit Holes if you want and add some volume to your 1x1 or Troll.  The tires fit in a Big Dummy fork, but there are some tricks that can help to prevent any chain-tire rub...look for an additional tech blog on that in the future. The tires are now in stock at our US Distributor, QBP. If they aren’t at your local shop, they can order them up for you.

*Rubber manufacturing doesn’t hold tolerances like manufacturing other materials.  For this reason, there is some variability to different manufactured lots and your tires may measure slightly larger or smaller than what the design dimensions are.  In addition different rims can cause the casing and tread width to vary, however, OD is a fairly constant dimension regardless of rim width.

For any other nerds out there, yes I know my greek letters are in the wrong place/strain is the wrong letter in the stress-strain plot sketched in my notebook.  It's been a while since I took materials science and I was trying to explain it to someone in a meeting. 

In somewhat related news, ET was definitely a Jedi

About Slippers Cortez

Ben Jungbauer a.k.a. Slippers Cortez

Well hello there, meet Ben, aka Slippers Cortez. Ben is one of the Engineers here at Surly. When Ben isn’t doing bike math and drawings, he’s likely enjoying a beer and playing the mandolin with his brothers in their folk/bluegrass band, The Thirsty River. If you don’t know what a mandolin is, there’s a google for that. If you meet Ben, you’ll notice that he’s the most Minnesotan person you know. We mean that in the best of ways of course. So the next time you're straddling your Surly, think of Ben, because he just might be thinking about you too.