Wednesday, January 11, 2012

Fat Bike Full Suspension?

Salsa Cycles Fat Bike prototype: (Image courtesy of Salsa Cycles blog)
Unless you were off the grid yesterday, this Salsa Cycles fat bike full suspension prototype is no news to you. The reaction to this development was wide ranging, enthusiastic, and rather surprising to me.

Of course, I love riding my fat bike, I think the idea of year-round fat biking is already proven, and I think it definitely has a place in off road/mountain biking. A full suspension fat bike? Sure! Why not? That said, there are several things that would need to be addressed before a full suspension fat bike becomes a reality from any company.

First and foremost is the question, "What sort of suspension does a fat bike need?" Obviously, Salsa is well on their way to answering that for themselves, and need no advice from the likes of me. My way of thinking about this question is as follows though...

A fat bike tire is a big, voluminous air spring. Race car drivers have been wrestling with this aspect of large volume tires for years. Altering air pressures causes the spring/tire to react differently, much like the spring in a traditional shock. Traditional mountain bike tires don't really have enough volume to affect overall suspension performance, but fat bike tires would. How that figures into overall suspension kinematics and design is not well known at this point for bicycles. My guess is that the suspension design would necessarily have to be more focused on damping the tire/spring than anything we've seen for a suspension fork for a mountain bike thus far.

There is a big air spring in this image!
"Where do you get the parts?" Then you have to source a fat-bike compatible suspension fork. Fat-bikers have long been employing Maverick SC-32 forks and more recently, the German Answer "Flame" fork. Trouble is that the Mav is flexy and the Flame fork is limited to using narrower rims. Wide rims and fat bike tires don't clear very well, according to reports I've read, between the stanchions on the Flame.

Added to this are the difficulties with small productions, costs, and distributions that are inherent with smaller manufacturers. Don't forget wider crowns require more down tube clearances as well. That complicates matters a bit. Oh yeah.....fat bikes with front suspension will also suffer from tall front ends, just like 29"ers do.

One other thing not talked about much is weight. Fat bikers are somewhat weight obsessed, and I see guys trumpeting their "under 30lb" bikes all the time. Obviously, adding suspension puts you back up there a ways again in weight. Whether most riders would find that worth the trade off for the suspension is not known.

"How Much Travel?" Just what works best for travel on a fat bike is another unknown quantity. Some feel more is better always. But there are limitations. Pointing at the difficulties within the 29"er suspension designs, one can see where fat bikes, whose tires are 29-30" in diameter depending upon tire model and rim combination, are going to quickly run up against the same issues. Front derailleur clearance, tire clearances, and chain stay lengths are also going to figure heavily here. Ideally, I think four inches or less, (most probably less), is going to be the target. Talk about unsprung weight too! Holy cow! Designing a rear damper to deal with all that mass getting thrown around back there is going to be a big challenge.

That doesn't even touch upon through axles, pivots that may need to be beefed up, and swing arm design to contain it all.

I have to hand it to Salsa Cycles and the Crew there. They bit off a "big one" with this project. Now we'll all get to see if they can chew it up, and spit out something worthy. Whether they can or not, I'm betting it will take a while to come to fruition, if it does at all.

Update: Some more information regarding the prototype fat bike FS from Salsa has been posted by "Dirt Rag". See here. 


9 comments:

mw said...

i can see a dropper post being useful too...how many years for that inovation?

JR. Z. said...

The reason I could come up with for putting suspension on a Fatbike would be big drops. Other than that, those tires are going to be nearly uncontrollable. So, if you add it all up: heavy bike, with lots of grip, and can absorb big drops, you're basically left with a "29er" sized Downhill bike. I can't see this being viable, though, as the control issue creeps back in...

Bruce Brown said...

I could see some suspension on the Fat Bikes. I tried one again this past Sunday (my 2nd test) and felt pretty beat up compared to a 29"er with suspension (going over wooden bridges, bit roots, etc...). I told the owner I would probably wait for at least a nice front suspension fork to be developed. Front suspension and a Thudbuster would probably do it for me, but the idea of a fully intrigues me even more.

MG said...

I love it... I agree there are a lot of issues to work out, but our friends at Salsa are the perfect ones to do it. And in five years, watch all the carbon copies come along...

But I agree that provisions for a drop post would make this an awesome Rocky Mountain (read: Colorado/Wyoming) adventure bike, and not just for winter adventures. I'd ride such a bike all the time. It would quite literally, redefine what we would be riding. I 100% believe that to be true.

I'm very, very excited.

Guitar Ted said...

@mw: I made mention of dropper post compatibility for On One's up coming fat-bike, which is being designed as a rigid rock crawler type bike. Makes a lot of sense for any FS fat bike too.

@JR. Z: The idea that a fat bike FS would be like a 29"er DH bike is a bit of a stretch, as far as use goes. I think more in terms of a trail bike that happens to have ginormous tires.But if you are meaning that the physical properties of such a bike are akin to a 29"er DH rig, that I can see. There is a definite comparison there.

@Bruce Brown: I would think that - at best- we're speaking of a 35lb FS bike, probably more in the range of 38-40lbs. (Depends on what rims and tires get used here) I don't know that this would have a wide appeal when you can get a 2.4"er tire, 29 inch FS rig which would weigh considerably less.

Even a front suspended fat bike will be 30 plus pounds. I have to wonder how many would actually get sold, but I think the front susser type has more appeal for a wider range of users.

@MG: And speaking of user groups, you would be the ideal candidate, I think. However; I have to wonder how many folks would be like you out there? Maybe more than I think.

At any rate, it is an awesome concept, and I also feel confident that Salsa's mind trust has the abilities to pull something like this off. I just have to wonder if the Q will go for it.

Erik said...

+1 on the unsprung weight issue.

I see lots of comments asking for LM Lites, and other wide single wall rims. Seems like a dual suspendered bike would invite the kind of abuse/speed/drops that these rims are ill-equipped to handle. I think that going away from "narrow" double wall eyeletted hoops would be a risky move.

The undamped springy behavior of big boingy tires is another concern.

I dunno, perhaps a 40lb downhiller with fragile rims and "ejection seat" landings is not what I'm looking for. Different strokes, I guess.

It does look pretty boss.

MG said...

Well... If they don't go for it, hopefully at least I'll be able to get my hands on a well-worn prototype. One of my forks/hubs went to the cause, so I will keep my fingers crossed. At one point a few years back, lots of folks questioned the need for suspension and 29-inch wheels, and I think we've largely proven that an invalid argument. Horses for courses... And so it will be with fatbikes.

Bruce Brown said...

I suppose 35-40 pounds is getting heavy, but isn't that why we have a 20/36T combo? ;-]

Turns out the one I tested on Sunday had a psi of 12 in the tires which I was told probably accounted for some harshness.

Guitar Ted said...

@MG: Maybe....it might. I'm not against this idea, but it does present a lot of technological challenges and may find limited potential. It will all depend upon choices Salsa makes. It's in their hands right now to pretty much define what this will end up looking like for the rest of us riders.

@Bruce Brown: Yeah- that's too much air. Folks sometimes have a hard time believing that these big tires are really sensitive to minute changes in air pressure. Drop that by 3psi and I bet you would have had an entirely different experience there. I never run mine above 10psi, and that is for commuting!