Earlier this year at the Sea Otter Classic, we launched Bucksaw, our full-suspension fatbike featuring Split Pivot™ suspension technology and RockShox’s new Bluto suspension fork. In case you missed the information we put out about Bucksaw then, you may wish to check out these articles:
In this post, I’m going to dig a little deeper into the design of Bucksaw and why we made some of the choices we did.
2015 Bucksaw 1...
First and foremost, it is important to know that Bucksaw is not just a Spearfish or Horsethief frame that has been modified to fit fatbike tires. Bucksaw was designed from the ground up…in the most literal sense. Together with Split Pivot inventor Dave Weagle, we began by looking at how 26 x 4.0” fatbike tires at 8 to 12 psi interact with the ground. Once we had a handle on the range of characteristics of the tire, we were then able to begin designing the frame. The suspension layout, leverage ratio, frame geometry and shock tunes were all designed with the unique characteristics of the four-inch tire as the driving factor; every aspect of Bucksaw is influenced and tailored around that component.
There is a lot of subtlety and detail here that can’t be seen by the rider, and for now we are keeping those secrets to ourselves. What is important to note is that all these design choices manifest themselves in what you feel on the trail as you ride Bucksaw. Traction and stability are the two key defining traits. Bucksaw allows you to change the way you approach obstacles and which lines you choose out on the trail.
The most outwardly noticeable design feature of Bucksaw (and one we’ve already received many questions about) is tire size. Bucksaw is designed specifically for and around 26 x 4” tires on up-to 82mm rims. There is no compatibility with larger (4.6 - 4.8”) tires and/or 90/100mm rim combinations, nor does the frame work with 29 x 3.0” (29+) tires. This was intentional. Designing to accommodate these different tire sizes was possible, but it would have meant compromising on several aspects of Buckaw’s design, and ultimately, its performance on the trail.
Both of these alternative tire sizes have very different tire characteristics on the dyno, which would have affected the suspension layout, leverage ratio and shock tune. These other tire sizes also both have significantly larger diameters, which would require making compromises to BB height, chainstay length, headtube angle, Q-factor and overall chassis weight. In the end, we decided that the benefits of designing to fit these alternative tires sizes did not justify the compromises we would have to make on the suspension design and overall frame geometry. As with all our other bicycles, we aim to design the best product possible, and frankly, don’t believe making one frame to handle all of these very different tires sizes a valid product design solution, and that is why Bucksaw is designed specifically around 26 x 4” tires.
2015 Bucksaw 2...
It is also important to consider rim width. Bucksaw can use the 4” tire on up-to a 82mm-wide rim, but we spec both Bucksaw 1 and Bucksaw 2 with 65mm-wide Surly Marge Lite rims. There are two reasons for this:
First off, wider rims increase the footprint width and square off the tire profile. For fatbikes, this greatly increases flotation over soft surfaces; however, it also creates more auto-steer. Auto-steer is caused by the tire’s contact patch shifting laterally to the point where the steering axis intersects the ground. It manifests itself in the wheel (felt through your handlebars) wanting to turn into a corner by itself; the rider will feel the inside grip push into their hand and the outside grip pull away from their other hand. This happens with all sizes of bike tires, but due to relatively narrow widths and higher pressures of most tires auto-steer is hardly ever perceptible and therefore largely ignored. The extreme width and low pressures of fatbike tires however, make this phenomenon a very noticeable reality. To mitigate auto-steer in Bucksaw, the formula was simple: Use the narrowest and smallest fatbike tire size to keep the tire profile as round as possible, and at as high a pressure as possible, while still maintaining the benefits and feel of the fatbike concept. We find this sweet spot to be a 4” tire mounted on a 50 to 82mm-width rim.
Secondly, weight is the other reason for our Surly Marge Lite rim choice. A 65mm-wide Surly Marge Lite rim with rim strip is 220g lighter than a 82mm-wide Surly Rolling Darryl rim and rim strip. That’s almost a full pound of weight saved on the complete bike, all of which is in rotating weight. With flotation taking a backseat to traction and stability in Bucksaw, this choice made the most sense.
I hope this article adds a bit of clarity into our design effort that went into this new bike. And I hope you get a chance to experience Bucksaw in the near future. Many of you will find it to be the best of both worlds: a Split Pivot suspension bike and a fatbike rolled into one, redefining the boundaries of what a mountain bike is and where or how it can be ridden.
2015 Bucksaw Spec
2015 Bucksaw Geometry
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Hi, I'm Pete and I am a product development engineer for Salsa. I like all kinds of riding from commuting to dirt jumping. I think flat pedals make you a better bike handler, that the thru-axle is vastly superior to the quick-release for off-road applications, and that moving through the world on bicycle allows one to see things they might not otherwise. I suffer daily from hunger-induced anger, also known as [i]hanger[/i]. Outside of work and riding, I enjoy kiteboarding, traveling, and watching hockey.