Sunday, November 22, 2015

Rolling quasi-fatty

Finished sealing up the tires this morning and took my new Carver Gnarvester for a ride today. Pretty much flawless out of the box, but I think one tweak will be needed at some point.

The build starts with a Carver Gnarvester alloy frame, which is a "29er+" rig. The "+" means the frame was designed to accommodate an extra wide 29" tire in back, up to 3" wide. That does not make it a fat bike though. Fat bikes can accommodate 3.7" and wider tires. I owned one for two seasons and had to abandon it after experiencing knee pain each time I rode it. To accommodate 3.7" and wider tires, the bottom bracket is widened so the chain and crank arms can clear the tire. This spreads the pedals out away from each other, the so-called Q-factor. I surmised this was behind my knee pain.  A plus-bike uses a standard bottom bracket width, so the Q-factor is the same as all my other bikes. Thus it should not bother my knees. The downside is you cannot put a proper fat bike tire in back for maximum float on snow.

The front-end is fair game for experimentation. The frame does not dictate what kind of tire, or wheel for that matter, you put up front. The diameter of a 29"+ tire is nearly 30".  Fat bike tires come in 26" rim size. Could I mate a 26" fat tire with a 29"+ tire in back? On the surface, there would seem to be huge mismatch, and that could upset the riding geometry and bike handling greatly. But fat bike tires now come in sizes up to 5". What is the diameter of a 26x5" tire? Almost 30"! So there-in hatched a plan. I'd give up some float in back to save my knees but maximize float up front by going as wide as possible. On paper, it looked great.

Over the past few months I collected parts. This was going to be a budget build. I salvaged a few parts from my previous winter beater bike. No carbon wheels. The difference between carbon and steel or even alloy forks is substantial though, so I did invest in a carbon fork. Plus it was critical that I maximized crown to axle distance to keep geometry where I wanted it. The Salsa Makwa fork could accommodate 26x5" and 29x3" tires, so it had generous crown to axle dimension to clear the big tires. Here's the build spec:

Carver Gnarvester alloy frame with sliding 142mm thru-axle dropouts
Salsa Makwa all-carbon fork, 483mm crown to axle, 142mm thru-axle
Sun Ringle 80mm Mulefut rim laced to 142mm Salsa Fat Conversion hub
Vee Tire Co Snowshoe XL 26" x 4.8" front tire, stud-able
Stan's 52mm Hugo rim laced to Halo 142mm Spin Doctor hub
Vee Tire Co Trax Fatty 29" x 3.0" rear tire
Tires and rims all tubeless-ready and set up with Stan's sealant
Shimano Deore triple crank with Gamut TTr 30t wide-narrow ring
Shimano XT 11-36t 10spd cassette
Shimano XT 10spd shifter
Shimano XT pedals
Shimano XT rear derailleur, medium cage
Bontrager saddle
Cane Creek 10-series integrated headset (no standards here anymore!)
Avid BB7 disk brakes with Avid levers
Thomson Elite post
Thomson Elite stem
Easton EC-70 carbon bar

The built weight came in about the same as my Santa Cruz Tallboy, just under 28 pounds. Surprising really, since there are no suspension components.  But when you consider how heavy the front tire alone is, it isn't so surprising (I think I measured 1500g!). The wheel base is about 1" less than my Tallboy. With a light front-end, it should be easy to loft it up on stuff.

The tires set up tubeless flawlessly except for one caveat. When I first mounted the Snowshoe on the Mulefut rim, I did so without sealant as a dry run, literally. It didn't hold air overnight, but no surprise there. So when I went to pop the bead this morning to pour in a few cups of Stan's, I couldn't do it! I don't mean it was hard to break the bead, I mean I tried everything and I could not break the bead in or out of the rim. I even resorted to screwdrivers! No dice.  The rim has a locking grove for the bead, and it locks alright. There is no way, ever, that tire on that rim will burp, even at 0.01psi! It also means if I ever flat with that tire on the trail, I'm walking out.

The Gnarvester at the new Wasserman bridge built yesterday

So how did it ride? In a word, great! The geometry felt spot-on. I started with tire pressures a little on the high side, as it is easier to adjust down than up on the trail. I hit my lunch stomping grounds in and around Horse Hill. Not the most techy area, but plenty of root and rock for a shake down test.

I met up with Arvid at his house by chance. He had a low pressure gauge. I tuned the tires down to 6.5psi front and 9.5psi rear. Those are essentially winter riding pressures, keeping in mind the rear is not a fat bike tire. Arvid joined me for the second half of the ride on a loaner fat bike he just got yesterday. Some spirited riding ensued. What impressed me the most was how the giant tires just steam roll over anything. Not the same as suspension, but still pretty sweet. I nearly wrecked myself several times over steering. The tires behave very differently than the 2.35" Racing Ralph's on my Tallboy. The shorter wheelbase added to the grippier cornering. I repeatedly came close to clipping trees on inside of corners. The 9.5psi in the rear 3.0" wide tire was probably a bit too low. I felt it bottom pretty hard a couple times.

Arvid leading on the Twister reroute the Bully was working on when
we came through.

On pavement, the reduced pressure tires were painfully sluggish. Pavement is not the intended purpose of this bike though. On the trail, I'd say it rides better than my Superfly hardtail with 80mm front suspension. The head angle is a little more relaxed, and the bigger diameter tire makes smooth work of root mazes.

The front tire is so massive it almost looks bigger than the rear, but the
rear is slightly bigger outside diameter.

The one thing I'll have to tweak is chain line. The triple crank places the ring out too far, such that using the biggest cog is marginal. It is very grindy and rough sounding. If I flip the ring to the inside of the spider, it hits the frame. Thus I should invest in a single ring specific crank that puts the ring around a 51mm chain line. That will just clear the frame and tire and give me a little less offset to the big cog.

So now we wait for snow. None in the foreseeable forecast. I'm not eager for snow so I can ride in/on it. I want to skate ski on it. I built up the quasi-fat bike so I can join others on weekend epics and be less disadvantage than struggling with my skinny hardtail on snow machine trails.


Anonymous said...

Be careful with those Hugo rims. I've found the edges to be easily bent, so much that I couldn't get the tire reseated, although it was able to bend it back after I got home.
I've been running 29x3 Knards at a minimum of about 10 psi, and I'm slightly lighter. It's a fine balance between getting it soft enough to get the most out of those tires and getting too many rim strikes.

Enjoy the new ride.

Paul said...


Why break the bead to add sealant, and not just inject it through the valve? Makes things much faster, easier, and neater - especially when topping off sealant. In this case, it might be a good thing, since you discovered the issue - but in general, it's a lot easier.

Hill Junkie said...

That is how I ended up adding the sealant. The Stan's valves have removable core. Eventually though, the Stan's dries up and you could end up with a big Stan's "booger" in there you'd want to remove. I've since learned this rim is notorious for making bead break almost impossible. Generally though, the Racing Ralph's I normally ride with are very easy to pop bead, remove boogers, add sealant, re-inflate.