The Robert Axle Project – How to Mod a BoB Trailer to Work with your Fatty

I might have a slight bicycle trailer obsession. Maybe it’s because I spent a good long while, pulling my kids in a burley, back there, in the last century, and I associate pulling a trailer with good times. I remember…back in the day, just me and my boys, out on the Palatine Bike Trail, dodging puddles, riding up to Deer Grove for some dirt. After my boys grew out of the burley, I borrowed a BoB trailer from my amigo Kenny (Thanks Dude!) to do some gravel touring and eventually bought a BoB from one of my dearest friends (on this planet) Adam Blake, and have pulled that BoB on bike tours. picnic rides and trail maintenance missions, attached to any number of bikes with 135mm QR rear spacing.

Ever since I’ve had a fat-bike, I’ve wanted to be able to hitch up my BoB Trailer, but with wider/changing axle configurations, which created a fast moving target……I just never quite caught up….until just recently. I started at this project by seeking a custom axle to replace the 190mm Q/R on my Fatback and that led me to a commercially available thru-axle solution made by the Robert Axle Project. Both of these solutions, came from my amigo Patrick (Thanks Patrick!). I also started, knowing that my BoB had the longer extended yoke/fork that allows the trailer to work with 29r’s and my Krampus 29+ (important detail to ensure fat bike compatibility)

What is a BoB Trailer and How Does it Attach?

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A BoB Trailer is a single wheeled cargo trailer that is comprised of two sections. The sections are connected at a vertical pivot point. The cargo section of the BoB and the rear wheel are behind the pivot point and forward of the pivot, is a tubular steel yoke that attaches to the rear axle of the bike. The yoke has two vertical dropouts that slide onto special ‘Bobbins’ (pictures below) that are held in place with a special spring loaded pin. Because of the BoB’s ability to articulate, it is amazingly singletrack capable and that’s what drives this quest to make BoB work with my fat-bike.

A standard 135mm BoB Trailer Q/R and a BoB Nut with the Bobbin shown removed
A standard 135mm BoB Trailer Q/R and a BoB Nut with the Bobbin shown removed


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The Yolk Mounted on the Bobbin and secured with the locking pin.

The wider rear spacing on fat-bikes require an axle that is wider than the QR that BoB sells and BoB doesn’t have any solution for bikes with thru axles. Below we’ll show you solutions to both a fat-bike thru axle and Q/R question of “How do I get BoB to work with my Fat-bike or How to fatten up BoB?”

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Fat BoB all hitched up and ready to rip


Robert Axle Project Thru Axle installed without the wheel in place to show how the bobbins protrude to accept the BoB trailer yoke dropouts
Robert Axle Project Thru Axle installed without the wheel in place to show how the bobbins protrude to accept the BoB trailer yoke dropouts

177/197 Thru Axle

If your fat-bike is relatively new, you probably have a 12 mm thru axle that attaches your rear hub to the frame. Thru axles are the way to go. They’re light and strong and stiff. The really lock your hubs into place and that adds to the overall lateral stiffness of the bike under load. With a few notable exceptions (Surly) Fat-Bike rear spacing is, for the most part, 177 0r 197 mm wide and most rear thru axles are 12 mm in diameter. There are enough variations, in the exact length and thread patterns that make choosing the correct Robert Axle Project Thru Axle important. The folks at RAP have a template that you can print with clear instructions on how to determine the correct item to order.

Robert Axle Project Thru Axle (top) and 197 x 12 mm Thru Axle (bottom)
Robert Axle Project Thru Axle (top) and 197 x 12 mm Thru Axle (bottom)

The Robert Axle Prpject (RAP)  thru axle does not come with the bobbins attached. RAP sells Bobbins, but I had multiple sets of BoB Q/R’s and BoB Nuts with Bobbins, so after I installed my used Bobbins, the RAP thru axle simply replaces the stock thru axle. There’s a ~6mm hole drilled into the end that allows a 5mm allen wrench the be used to snug up the thru axle. Once the T/A is installed, then the other end cap (with bobbin) is threaded into the other end, on the drive side. It’s a little crowded on that side with the rear derailleur cable but it works (see above). You are then ready to drop those vertical drop outs on the BoB Trailer yoke and secure the locking pins….but wait…..the yoke is still too narrow to fit on your new, wider axle.

Cold Forging the Yoke

I guess, that I should have probably told you that you would have to bend the yoke to make the drop out spacing wider and then bend the dropouts a bit the keep them aligned to fit squarely onto the bobbins. And I bet that voids the warranty on your BoB Trailer and it also means that that yoke won’t work with your other 135mm spaced bikes (unless you bend it back). If that makes you squeamish, you probably shouldn’t watch this video. If you’re rock’n a twenty year old used BoB, like mine, I watched the video and said, “yeah, I can do that.” It was the perfect excuse to go a buy a vice for the shop. MSRP on a new replacement BoB trailer yoke (they call it a fork) is $55.

The bending of the yoke is the most intimidating part of the conversion, but it turned out to be easier than I had imagined. Once the yoke is modified, the trailer installs, just like it did before the conversion. After a few short test rides, I towed my new fatter BoB,  loaded with a full load across Lake Winnebago. It performed flawlessly. We packed a grill and some brats along with refreshments and had a little picnic, before heading back across the lake into a strong quartering headwind (good times).


170mm or 190mm Q/R

If you have a fat bike that has a Q/R rear axle, you are faced with having to fabricate a new rear axle. I had a friend (who probably aced every shop class that he ever took) make the threaded axle that’s pictured below. This set-up requires that I carry two 15mm wrenches to remove the rear tire, but I have a big ole trailer to carry what ever the hell I want now, so that doesn’t seem like a very big deal to gain the ability to pull a trailer load of firewood deep into the snowy woods! The cat that made this for me, wanted me to make sure that people know, that he’s not looking to make any more of them. But maybe you know a guy that know’s his way around the old machine shop or maybe you ARE a guy that knows his way around the old machine shop.

190mm Paul Components Q/R (top) - Fabricated Threaded Axle With Bobbins (below)
190mm Paul Components Q/R (top) – Fabricated Threaded Axle With Bobbins (below)

It’s the same deal with the cold forging of the yoke for this set-up, as in the Robert Axle conversion. I’m pretty sure that I’m the first kid on my block to have a fat-BoB, but I hope that this information allows anyone that wants one, to eventually get there. The Robert Axle Project web-site has more helpful information and they make thru axles that work with other brands of trailers as well.  Check them out at Happy Trailering Amigos!

For more information about BoB Trailers visit –


About Gomez 2576 Articles
Just an old cat that rides bikes, herds pixels, ropes gnomes and sometimes writes stories. I love a good story.


    • hi, thanks for your article. I also have a 2016 krampus. I did not understand if the axis for this bicycle is produced by the guys from Robert Axel Project or if you had to have it done by your friend. can you tell me the model suitable for me by robert AP? thanks a lot 🙂

  1. I have a fatbike with a QR rear wheel (2019 Norco Bigfoot 1). Any potential BOB trailer solutions? Is there any way to replace the actual skewer in the QR with a longer one? Would love to use it for family bikepacking.

    • Look at the last photo in the post. There is no commercial product made for retail that will work, but you could machine one of your own.

  2. Been looking at these and wonder if I could pull them behind my dualsport motorcycle. They do make them for motorcycles, just visit a lot. I think these would work. If safe at 60+ mph. I would buy one today.

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