I built an electric fatbike out of a $499 Bullseye Monster from Bikes Direct several months ago but it took me this long to really get it working right. Initially I had a lot of problems with the drive train skipping and the chain dislodging and getting jammed between the chainwheel and the adapter. After replacing the cassette with a SRAM PG-850 (nickel-plated steel 32-11) and adding a n-gear jump stop chain guide I feel like I finally have a build that is worth duplicating.
The Bullseye Monster is an electric fatbike that feels at home both on singletrack and doubletrack. It can also be used as a commuter, but I would advise that you chose either trailriding or commuting as your primary focus as this will determine the proper chainring sizing you should use. For trails a 32-34T will be best, for commuting a larger chainring (38T+) will allow for higher top speeds (25+mph) and less billy-goat style hill-climbing ability.
The bottom bracket on this bike does not allow for easy modification to fit a stock BBS02 drive unit. You will need to install your own, which can easily be done by anyone with a little shop knowledge. Doug rents the tool kit for only $45 and the modification package is $499 which includes a very high quality chainring adapter. I have done several of these axle swaps and they take about 2 hours from start to finish. You will need grease, Tap Oil, some lock-ring pliers and a little patience.
The only real complaint I have about this bike is the tall stand-over height. When riding single-track in deep powder when your front wheel falls off the trail it can often sink over a foot deep into the soft powder. This means your crotch inevitably ends up getting slammed onto the top-tube as your foot sinks through the powder to try to find some solid ground. I much prefer the hydro-formed frame of the Boris X9 when single-track riding and I have to worry about having to put my foot down through a foot or two of powder.
The stock brakes on the Bullseye Monster are total junk, I replaced them with a pair of Avid BB7’s on ebay from Hong Kong for about $30 each shipped. The cassette is also made out of incredibly weak metal. I chipped and bent several teeth abusing the crap out of it for 2 weeks before I finally broke down and replaced it with an SRAM 850 Nickel plated steel 11-32 cassette. With the BBS02, a steel drivetrain is the only way to go, alloy just will not hold up.
I’ve had several very long emails back and forth with Nick from n-gear about how his chainguard was about 1/2 inch too short for fatbikes. Basically it’s not worth the cost to retool and make a chainguard that will actually work properly. By mounting it at an angle and bending the stainless guard out quite a bit I was able to get it to stop most chain derailments. The ones it doesn’t stop at least get stopped from getting lodged between the housing and the adapter which will damage the plastic cover and can actually bend the chainring adapter.
For going 20+ mph on snowmobile trails you will want chains even though they add almost 3lbs of weight. The 4x100mm chains from Slipnot chains work well on the 4.0 V-mission tires on the 50mm rims. You will need the 4 extra turnbuckle extensions in the repair kit as well to make those chains work with that tire. If you call Justin and sound really pathetic, he’ll probably throw in the turnbuckle extensions for free with your order. He did that for me but I sound much more pathetic than you so then again, maybe not. If you want to save a few bucks then you should just make your own.
The front fork will fit a 4.8 tire like the Surly Bud which is a suitable substitute for chains. Be aware that you will have a hard time getting anything bigger than 4.0 to fit on the rear and still be able to fit chains on the rear tire, even the stock tire is a tight squeeze.
The Thudbuster will save your rump when pumping up and down at 20mph on snowmobile trails (faster if you use a larger chainring in the front). You can do this build in about 6-8 hours from start to finish including the axle swap on the BBS02. For step by step instructions on installing the BBS02 you can look here. There are a lot of options for batteries, if you are planning on riding in powder don’t get anything smaller than 20Ah that can handle a 30Amp continuous load. Deep powder sucks watts and you will run out of power long before you expect to. Em3ev has several nice triangle packs for a few more bucks or you can just build a Lipo or Lifepo4 pack from Hobbyking for cheap.
Figure $499 for the bike shipped, $550 for the BBS02 shipped from em3ev, $600 for the Axle kit and rental of the toolkit and another $65 for the Avid BB7 and $35 for the Cassette. That puts you in about the $1750 range without chains or a battery. If that is too rich for your blood and you just want a singletrack machine that maxes out at 11mph for <$900 check out my Deadeye Monster 42lb conversion which totally knocked my socks off and can be completed from start to finish in about 4 hours (with 3 of the 4 hours spent struggling with the drive side bearing cup).