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Guerrilla Gravity BFC Cog (discontinued)

Vital Rating: (Good)
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Tested: Guerrilla Gravity Big F-ing Cog

Rating: Vital Review

by Kevin Shiramizu // Photos by Dave Trumpore

When I started riding, everything was 8-speed and it worked pretty well. Then came 9-speed. Then came 10-speed. And yet all this time there was still a pesky thing in the way called the front derailleur. When I was younger and more spry, I would just hammer through on 1x8, 1x9, and 1x10 set ups. Now I’m a little older, a little more tired, and have to spend my money on things like paying off student loans for a useless degree and rent so the current crop of 1x11 options that work amazingly well are just out of my practical grasp. So I had high hopes for solutions like the BFC from Guerrilla Gravity to open up the range found in the expensive 1x11 systems and give my legs a fighting chance on the steep climbs in my area.


BFC Highlights

  • 42-Tooth Cog
  • 7075-T6 Heat Treated Aerospace Grade Aluminum
  • Works With Most 11-36 SRAM and Shimano 10-Speed Cassettes,Displacing One Sprocket From Main Cluster
  • Extra Wide Spline Interface Designed to Work with Aluminum Freehub Bodies
  • Short/Tall Tooth Profile for 21 Shift Points and Immediate Gear Changes
  • Additional 7 Chain Ramps for Extra Shift Boosting Every 0.14 Revolution
  • Weight: 2.9-ounces (83 grams)
  • MSRP: $90

Setup and Initial Impressions

Installing the BFC was a breeze after the stars aligned. Getting the stars to line up was another story. My previous set up was a hand-me-down SRAM XX cassette and a short cage X0 10-speed derailleur. Problems. The XX cassette is one piece through the spread so removal of the 17t cog is not an option to accommodate the BFC at the top of the cluster. After tracking down a SRAM 1050 cassette, I thought maybe I’d be all set. I threw the BFC and cassette on with the 17-tooth removed and then ran face first into the issue of the short cage derailleur not being able to clear the BFC. Big F-ing Cog indeed. So Guerrilla Gravity ever so kindly sent out a medium cage X9 to borrow and once that was in play, everything cleared. Sort of. Now that I had added a longer cage and a bigger cog, my chain was, like certain whistle loving rappers, too short. Fresh chain. So with a mid-range cassette, longer derailleur, and new chain, the BFC was installed and ran pretty much perfectly as expected in the bike stand. On this topic, it's worth pointing out that Guerrilla Gravity actually sells drivetrain upgrade kits that include the BFC, cassette, derailleur, shifter, chainring, and chain, or any combination thereof, at advantageous prices.


On The Trail

The BFC does open up a lot more gear range on the trail and for that, my legs are thankful. A lot of my rides greet me fresh off the couch with 10-13% grades, and try as I have, there doesn’t seem to be a pleasant way up miles and miles of that kind of pitch. Bailouts tend to be big and the BFC lives up to its name. Jumping from a 36 to a 42-tooth means a lot more spinning and a lot less hammering to get me to the part of the ride that I actually care about, which is of course where things point back down those steep grades. I wasn’t magically flying up previously impossible climbs with the BFC but I was picking my way up stuff with less blown out legs at the top so it did the trick. As a flat-pedal rider, every little helps on the climbs.


Shifting up into the BFC was surprisingly smooth thanks to the alternating short/tall tooth profile and 7 shift ramps, and I never had sloppy shifts under heavier pedaling forces which is a must since your bailout gear has to be ready at a moment’s notice when you’re really slamming into a climb. Shifting back down out of the cog was a bit more abrupt of a shift. The chain climbs up into the BFC more gracefully than it falls off it in my experience. Luckily, it’s not like you’re doing gate starts at this end of your gears so you’re probably not pedaling hard for that shift.


Things That Could Be Improved

As noted in our recent review of the similar OneUp cog, the shifting performance in the rest of your cassette leaves something to be desired with the addition of the big 42-tooth in the mix. The removal of the 17-tooth cog meant there was a huge jump in the gear ratios in the middle of the cassette. As it turns out, that’s right where I spend a lot of my time during more mellow descents for pedaling out of corners, and jumping from a spun out 19-tooth to a bogged down 15-tooth was annoying. The shift performance in that jump was also finicky and lacking compared to the rest of the single gear jumps. I also experienced an issue with the maxed out B-tension screw holding the top pulley so far away from the smaller cogs that shifting performance at that end of the cassette was not as good as it could be. Taking out the 15 and replacing it with a 16-tooth cog would probably have taken care of a lot of my gripes as the jumps would at least be more consistent both on the derailleur and on my legs. This comes at added cost, however, and still isn't a perfect solution.


Long Term Durability

There is no reason for me to expect the BFC to have any long term reliability issues. The wide base of the cog and the spread out chain loads across so many additional teeth lead me to think this part will last a long time, and should there ever be an issue, Guerrilla Gravity backs its products with excellent customer service and I have little doubt any shortcomings would be swiftly remedied. The rest of your cassette may wear out faster with less chain wrap on the smaller cogs due to the maxed out B-screw, however. That being said, I could buy several more SRAM 1050 cassettes for what even my second-hand XX cassette cost.

What's The Bottom Line?

I hate the front derailleur and anything that can put another nail in the coffin of that infernal contraption is a good thing in my books. I’m tired of frames having to be designed to clear that thing. I’m sick of seeing shifters on the left side of bars - that’s where a dropper post remote goes. So the Guerrilla Gravity BFC does its job of offering you a wider gear range, and while it’s a compromise compared to XX1 or XO1, it’s a lot less of a compromise than a stupid front derailleur and it will cost you a whole lot less to get running than a whole new parts group. With thick-thin chainrings, clutch derailleurs, and aftermarket patches like the BFC, you should be all set and front-derailleur-free for a while until we see that sweet 11-speed tech trickle down to mere mortal price ranges. So until there’s a more convenient X7-11 or a patriotic X9-11-never-forget parts group, take a look at options like the Guerrilla Gravity BFC that could save you a bunch of money. But be ready for the reality that this is a compromise and can’t be expected to perform at the same level as a whole group designed to work together.

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About The Reviewer

Kevin Shiramizu has been riding mountain bikes for over 15 years. During that time he accumulated multiple state championships in Colorado for XC and trials riding, a junior national champ title in trials, and went to Worlds to get his ass kicked by euros in 2003. His riding favors flat corners and sneaky lines. After a doozy of a head injury, he hung up the downhill bike for good in early 2010 and now foolishly rides a very capable trail bike with less protection and crashes just as hard as ever. He likes rough, technical trails at high elevation, but usually settles for dry, dusty, and blown out. He spent five good years of his youth working in bike shops and pitched in efforts over the years with Decline, LitterMag, Dirt, and Vital MTB. He also helped develop frames and tires during his time as a guy who occasionally gets paid to ride his bike in a fancy way in front of big crowds of people.

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14 comments newest first

Why was there an issue with the short cage but not with the medium in terms of clearing the big cog? Why did it matter? I thought that the derailleur body and upper pulley were all the same regardless of cage length. Wouldn't the longer cage only eat of more of the chain slack?

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Correct, medium cage length is required to handle the chain slack from the 31t spread (42-11). The B-screw setting is what determines whether the derailleur physically clears the BFC or not.

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Right, the intention of the 40t options that you see quite a few brands offering in addition to their 42t is that a 29t spread can be handled by a short cage.

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Exactly, I've had my eye on one of those 40T options since I have an X9 short cage and don't want to buy a new one. I think the X9 short has a capacity of 30T so it should work with the 11-36 I'm currently running.

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Am I in the minority (I think I am), but what is so annoying about a front derailer? I have been using one for 25 years and they work well. In the Front Range, you put it in the granny gear for the climb, and then switch it to the bigger ring for the descent? I am confused on why this 1 x 11 thing is such a great improvement to mtn biking, especially when you must get a new chain and derailer... Maybe someone can enlighten me..

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1x11, specifically X01 or XX1, has the following advantages over 2x10 and it's those advantages that are encouraging other companies to develop products that convert 10-speed setups to one-by to try and help mimic SRAM's 1x11.
- A little lighter. You pick up some weight at the rear of the bike, but you save even more by eliminating the FD, shifter, cabling, or in the case of old-school one-by, a chain guide.
- Chain retention. A narrow-wide chainring plus clutch RD holds your chain on the front ring far better than the best FD can, and close to what a chain guide can. Even with a guide, I have had the uncommon, but usually catastrophic, chain retention failure. At least with 1x11 or 1x10, if the chain does drop, which it seldom does, you just put the chain back on and keep going; no tools needed and nothing is jammed or out of alignment.
- Simplification. Without a FD, you only need to setup, tune, and replace (at end of life) one derailleur. The front ring is a set-and-forget and changing swapping is easier without the FD in the way. Also, there is less cabling to manage and route.
- Seatpost remotes work really well in place of the front shifter, especially on the Reverb, my go-to seatpost.
- Cross-chaining and its increased chain wear isn't an issue since the front ring [should be] centered relative to the cassette.
- IMO, one-by setups make you a stronger rider if you gear them to eliminate the granny gear. I used to run a 28T with a 1x9 setup, now I run a 34T with XX1. Not having a granny ring to fall back on forces me to sack up and keep pedaling. Although, this philosophy is not required with one-by setups - it is a personal choice - I still see it as a positive.
- One-by gives frame manufacturers the opportunity to design frames with strength and functionality in mind and not have to accommodate the FD. The pivot that receives the highest load on a bike is the drive-side main pivot. Unfortunately, frame manufacturers have to move the bearing(s) that support that load way inward to provide room for the FD. The same is also true for chainstays. In many cases, the chainstay length of a bike is dictated by tire and CS yoke clearance with the FD. Without an FD, CS length can be shortened some, CS yokes can be made stronger and/or allow larger rear tires.

I am sure there are more reasons, but those are the ones off the top of my head. It should help explain why people are so excited about one-by setups.

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Well that makes it sound appealing..however, I don't feel I am strong enough to live without the granny gear, I tend to be more of a spinner, you know, higher cadence, less mashing. That has been my style from the beginning and at 45 I don't think its going to change.

Thanks for the write up, well said..

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I hear ya. I tend to be a bit of a masher. If you go one-by, start with something small knowing you are a bit of a spinner. If that doesn't work, try a ring of another size and sell your original while it is unworn; it should fetch some value.

One-by setups have an advantage over two-by in that the entire gear range can be moved to match your terrain and cadence by just swapping one ring. You don't need to worry about whether or not two rings will shift well together, unlike two-by setups. Hey, that's another bullet point for the list above. smile

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It's like going from bad suspension to good suspension. You don't know what you're missing until you try it. Even I was pretty skeptical of the 1xWide system until riding it. With a bike that climbs well, it's enough gear range, and your bike gets a dose of simplification.

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Its just way more efficient. The front derailleur worked fine but getting a similar gearing range with one ring up front is just better. Its one less part to maintain or that can break while riding. You get rid of the whole mech and the extra shifter and cable cluttering your bars, its more simple to use not having to worry about what 'combination' you are in and just having one set of gears up and down, its very very very quiet when run with a clutch derailleur.....

The list of benefits is endless.

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"a similar gearing range with one ring up front" that is not an accurate statement. Assume you use all your gears on a 2x system, which I do in the front range, there is no such thing as getting the same range with a 1x10 system. You either have to go smaller up front to get the same granny-granny gearing, and then you lose at the top end, or the other way around. I'm on 2x9 22-34 front and 11-34 rear and 1x10 won't get me close enough, and 1x11 is closer but $$$. I think I'll try this 1x10 with 42 rear a try, and it serves as a drive-train refresh at the same time, with a clutch, but I mentally prepare to later change to 2x again... I just got weak legs ;-)

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Sure, if you need the real wide spread of gears then you will need the multiple rings up front. My point was really that with these wide range rear cogs, it provides enough of a range that many riders can get by with. So if the choice is a 1x or a multi ring set up and front mech, for those who can get by with the single ring, its without question the way to go.

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Thanks.. thats a good list, but honestly, I don't think I have ever broken a front derailer or shifter in 25 years of riding..

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There's also chain retention. I've never met the rider who hasn't had a FD drop the chain on them due to poor adjustment, alignment, compromises in frame design that don't allow it to function perfectly, or just plain chance: even road riders run chain catchers on stages that require large amounts dropping to their smallest front cog.

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Product Guerrilla Gravity BFC Cog
Riding Type Trail
Material 7075-T6 Heat Treated Aerospace Grade Aluminum
Speeds 10-Speed: Works with SRAM and Shimano Cassettes
Tooth Options 42 Tooth Cog
Driver Type Shimano, SRAM XD
Weight 0 lb 2.9 oz (83 g)
Miscellaneous - Extra Wide Spline Interface Designed to Work with Aluminum Freehub Bodies
- Short/Tall Tooth Profile for 21 Shift Points and Immediate Gear Changes
- Additional 7 Chain Ramps for Extra Shift Boosting Every 0.14 Revolution
- 10-Speed: Works with SRAM and Shimano Cassettes
Price $90
More Info

Guerrilla Gravity Website

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