Bob Saget- Fixed Gear MTB Build Log

Here goes a build log for the work in progress “Bob Saget”

The goal was to replace a broken Karate Monkey and make the ultimate 27.5 +rear/29 front fixie.

The first priority is getting the BB as high as possible. I designed the bike to have 25MM BB drop in 29" mode which translates to 20 MM in 27.5+ mode. My area is very rocky and I hope this is a big improvement over the KMs 68 MM of BB drop.

The second priority is tire clearance. This is a silly bike and I want it to fit everything regardless how bad it rides :rofl:. The frame will fit up to 29X3.

The third is to make it work with the Niner carbon fork from the KM. This is a problem because those only have 100MM HTs… This means a super low stack height but it will be made up for with some goofy steel bars.

Anyway here we go…

Bob’s head badge to be printed.

Send Cut Send Package. The Yoke is @ben.land101 $12 chainstay yoke. The drop outs are my design using Surly 12MM Gnot Boost. They will fit 135, 142 and 148 with some clever adapters.

The yoke is hand filed after welding so I needed to get clever with fixturing. Some 80/20 and spare shims did the trick.

As for bending…

Welded yoke. The BB shel edges were tough to not put to much heat into the BB.

This is my fist chain stay assembly. I always have started at the HT and worked backwards. I really like this style of building

This is also my first bike with miter templates. They are great!

Frame tacked together.

And welded awaiting for a bunch of braze removal with the belt sander

Alignment is better than I am :rofl:


I sometimes use my granite kitchen countertop, too! :smiley:


It’s remarkably flat! I move the frame around and it provides the same result anywhere in the area.


Looks great!

One of my favorite parts of this forum is seeing that there is no “right” geometry. Everyone has such different ideas and priorities.

I’m a fan of big BB drop, and was surprised that the KM has a 68mm drop, so I looked up the geo. The Surley website says the BB drop is 55mm for a rigid, and 44mm with a sagged 140mm fork. Has that changed over the years?

Does the Gnot Boost system hold-up to SS mashing without slipping?


Yeah the first/second gen were way lower. I’ve had some hard falls thanks to that BB height…

I’ve had no trouble with bolted axles in the past but you can also add a tensioner as needed.


So good! I have the same era KM and know that low BB well. You should add crank length and BB height to your drawing to make easier to understand the ground clearance. I now have a SS hardtail with a 300 mm sagged BB height (320 unsagged) and love that pedal through anything feeling. Oh yeah and post pics of the finished build!

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Does the surly axle with a standard thru axle hub hold as well as a bolt-on hub? I’ve always been concerned that the typical aluminum end cap of a thru axle hub wouldn’t hold well in a horizontal dropout. My bolt-on hubs all have knurled steel end caps.

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We will cross that bridge when we get there :laughing:

It will likely spend its life with 135 bolt on hubs with surly’s steel adapters. If I go TA I’m going to make washers that are knurled on one side to protect the hub and give extra bite.

They run this system on a bunch of their bikes and people I know don’t have issues. They even come factory spec in TA configuration. There is also a conversation of tolerances because theirs are machined and mine are laser cut.

Worst case scenario, I hate it and melt them out and replace with 10 MM drops or maybe I have an excuse to refine the design and build another frame :star_struck:


Rattle cad has limited dimensions but we are looking at around 350 MM BB height. It has 165 cranks so we are looking at 185 MM of crank to ground clearance! There is another guy in my area running a fixed gear mtb with 0MM BB drop. He says its to high. My single speed has 50MM drop with a 140 fork and it does pretty good pedaling though our rocks but I never feel like it couldn’t be higher, the bike handles amazing. Basically 0 drop is not enough, 50 is totally fine (single speed) so we are going to find out how 25 is.

Fixed gear riding is much more similar to trials than regular riding. This will be low speed balance riding.


Cheers. Seems like we have similar feelings on the SS hartails so interested to find out how you feel about the handling on this one. I’ve never ridden fixed off-road though!

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Fixed gear in rocky terrain must be quite the challenge! I can see why you’d want a super high BB.


That is a very well-engineered chainstay subassembly: custom yoke, custom dropouts, slotted stays, etc…


Bob lives!

So far the Surly Gnot boost adapters are holding! This is a big deal because it was my biggest concern.

The high BB works great avoiding pedal strikes and I cannot notice the difference although there is no high speed leaning into corners on a fixie MTB… It honestly feels like a Jones SWB but I think that’s due to the steel high rise bars.

3 bolt Sram style cranks don’t work very well on a fixed gear. The chainring shifts between pedaling and braking.

Bob was painted with Spray Max paint. This product is interesting because it has an activator and is like automotive paint. Hugh’s improvement over or regular aerosol paint.