How Do I Convert My Single Speed Belt Drive to a Fixed Gear?

This is a question I get a lot.

Many companies make some great belt driven bikes, but I haven't found many that are sold exclusively as fixed gear belt driven bikes.  Belts are still relatively new in the bike market, so I can see the reason why companies want to sell something that appeals to as many people as possible and not just one niche, but lately I hear from more and more people that would like to run their bikes with either a fixed option or a flip flop hub.

Unfortunately, there's no kit you can go out and buy to do everything magically, though if you're lucky enough to have a good LBS you could probably drop it off there and have them do the entire conversion.  When I did the write up on the parts I swapped out on Nine, I included a lot of information and part sources, but it's by no means a step by step guide.

So, I decided to write a step by step guide.

I do have a few disclaimers though: 
- This is not a cheap or fast upgrade.  Be sure you're willing to put in the time and the money before you commit to anything.
- It might not work for all belt driven bikes.  Most likely if you have a single speed and follow the instructions, it *will* - but I haven't dealt with converting geared bikes, and certainly not all brands of bikes.
- If you are unsure how to do something, get help.  You need some bike knowledge to do this.  You don't need to be Sheldon Brown, but you do need to know your bike and how to dismantle it before diving into this.
- If you have any doubts, do research.  Think you know your chainline?  Double check.  Has someone converted your type of bike before?  To Google!
- I am not responsible if things go wrong.  I'm writing this because many people have asked me to, and there is not a comprehensive guide online to do so yet.  If you have questions, I'm more than happy to answer them - please make sure you know what you're doing!
- If you finish and are unsure if your bike is safe to ride, GET IT CHECKED.  

Additional note: I am writing this to convert to the CDX system, due to the lack of components for fixed gear CDC systems and the tension debate.  This is also a pretty quick and dirty step by step guide, and I assume that the reader knows enough about bikes that I can skip a play by play on how to remove a wheel, pull a crank, tension a belt, etc.  

Specs:

You will need to know:
- Chainline
- Original belt length (number of teeth)
- Dropout width
- What belt system you're running (CDC or CDX)
- Wheel size
- Bottom bracket axle width 
- Current belt ring and cog size
- Current BCD 
- Chainstay length

Tools:

You will need (and need to be able to/have friends to bribe to)
- Pull the crank*
- Remove the rear wheel
- Lace a wheel*
- Install a cog and lockring
- Replace the belt
- Tension the belt

*Depending on your current setup, this might not be necessary.

Components:

This is going to be dependent on your current belt system, current rear wheel, and what you would like to upgrade to.  If you are going for a full upgrade (like I did), here's what you'll need.

- New belt
- New belt ring and cog
- Rear wheel with the correct spacing

First, we're going to determine belt components.

1) Check which system you have.  If you are running a CDX system, you're lucky.  If you're running a CDC system, the entire drivetrain needs to be replaced.  The first thing you need to figure out is what you want your current gearing to be.  Currently, the only available option for a thread on fixed cog is 21T.  Most SS belt bikes I have encountered come with a 22T rear cog and a 50-55T belt ring.  If you're running the CDX system and are happy with your current gearing, I suggest keeping your belt ring and belt to save on cost.  If you're running a CDC system, you can either buy components for CDX in the same gearing you have now, or you can experiment a bit.  Keep in mind that changing the gearing more than one or two teeth will require a new belt, which can be calculated here at the bottom of the page. 

In total, you will need three parts for this section: a CDX belt ring, a CDX fixed gear thread on cog, and the appropriate CDX belt.  Once you determine what you need, you can get most things here.  Select the correct belt ring that matches your tooth count and BCD, and select the 21T threaded on fixed cog.  Belts can be found here too, or order these parts from your LBS if you can!!  Keep in mind you will need a lockring!

2) Once you know which belt components you need, assess your rear wheel.  My District used a rear wheel with road spacing (130mm) and a belt freewheel that fit on a Shimano hub body.  If your dropouts are standard 120mm fixed gear, then congrats!  All you need is a standard 120mm rear wheel with either one side threaded for a fixed cog or a flip/flop hub (more on that later).  If not, you have a few options here:

- Surly Fixxer: This one is tough, because the Fixxer is dead and compatibility is a huge issue.  If your wheel is compatible, this will save you the most money...however, if you can avoid it, I'd go with one of the alternate routes to avoid problems in the future.
- Special Order: Unfortunately I don't know of any pre-built fixed gear wheels that are spaced to 130mm or 135mm (or whatever your bike is), but your LBS or places like Wheelbuilder can make one for you.  
- DIY: If you know how to lace a wheel, you can do it yourself.  Surly has some amazing options for hubs and doing it yourself gives you complete control over what you want.  I laced a Velocity B43 rim to a Surly hub for my rear wheel, and it worked beautifully.  If you are lacing a wheel, be certain that the hub you're getting will match your current chainline.  If there is zero chance of this happening, you will have to change your bottom bracket axle to compensate.  

If your frame is steel and you're thinking of adjusting the dropouts to fit a better wheel size, only adjust to 5mm or less total!  Do NOT go from 130mm to 120 - that's too much stress on the frame, and eventually you're gonna have a bad time.  If you're going to do this , do it right!

3) The last component you might want to consider is the crank.  I always recommend shorter cranks on a fixed gear to avoid pedal strike and help with spinning, so if your cranks are longer than 170mm you can opt to go down to a shorter length.  This certainly isn't necessary and can always be done later, but if you can do everything in one pass it's kinda nice.  Keep in mind that belt rings are not made in 144BCD, so many track cranks won't work.  Also check the chainline specs of the crank you are upgrading to and make sure they match!!

Wait wait wait - how much is this going to cost me?
For the belt, cog, and belt ring, you're looking at about $250, give or take.  For the wheel: with the hub, rim, spokes and nipples it's around $175 - but that is entirely dependent on type of hub, rim, etc and doesn't include labor.  If you're throwing a new crank and the White Industries belt freewheel into the mix, that's another $375.  This is not a minor upgrade, so be absolutely sure you want to do this before proceeding!!  

4) At this point, lets say you have your components: your complete wheel, cog, belt, and belt ring.  It takes some time to install and align everything, so wait until you have a few hours free before you start throwing parts around.  The next few sections involve some basic tasks (changing a chainring, etc) which I'm not going to go into detail for since the internet has a ton of tutorials.

5) If you have a brand spanking new wheel, install the cog and lockring on it.  This is done just like any fixed gear cog, except you need to get a bit creative with the "chain whip."  I used a sheet of rubber to tighten my cog, though now you can get a belt drive chain whip!  Once the cog and lockring are on tight, the rear wheel is good to go.

6) If you're running a CDX system and aren't swapping your belt ring or belt, remove your rear wheel and move on to step 7!  

First, remove your rear wheel.  If you opted for the Fixxer approach and are keeping this wheel, now is the time to remove your belt freewheel, install your Fixxer, and then install your cog and lockring (see step 5).  

Next, you need to pull out the old belt by splitting the frame.  This is going to differ from frame to frame, but my Trek splits at the dropouts.  Some frames split near the seatstays, others near the dropouts - every bike is different.  Gently remove the belt without twisting or bending it, and set it aside.  Install the new belt - never bend, twist, or tweak the belt, since this can weaken it!  You can let it hang out while the belt ring is being switched out.  Now, swap the belt ring - you can pull the crank to do this if it's easier for you (or if you're swapping the crank at the same time).  Just like installing a chainring, follow all instructions for greasing and tightening the bolts and don't overtighten!  

7) Right now you should have the belt and belt ring you'll be using all installed and ready to go.  Gently wrap the belt around the belt ring, and then get your rear wheel ready.  This is going to go on your frame the same way you'd install it if you were running a chain.  Get the belt around the cog and then slide the wheel into the dropouts.  At this point you'll notice pretty quickly if this didn't work, but hopefully things look good!

8) Make sure to close your frame where the belt came out, and then start tightening, aligning, and tensioning your belt.  If your chainline seems way off, double check your current chainline with what you had written down originally - you might have to change the bottom bracket axle or use spacers in your hub to get everything working.  Tension is the same whether your frame uses dropout tensioners, an eccentric bottom bracket, or eccentric dropouts.  If you're unsure how to tension your belt...well, there's a first time for everything!

9) Once the belt is tensioned and aligned, make sure all bolts are tight and everything sounds okay.  The wheel should be straight and tensioned - I usually run my CDX belt at around 30-50 lbs.  If all looks and sounds good, take a test ride.

10) Now that you're running a fixed gear, make sure you've got proper foot retention and a front brake!  If your hub is a flip flop hub, keep that rear brake though - White Industries makes a thread on 22T freewheel!  Order it from your LBS or get it here.  

You can save the old belt parts, or sell them to offset the cost of the upgrade.  I kept my old set in case I come across a belt compatible frame and want to throw a single speed together.  

Troubleshooting/FAQ:

I followed all measurements, but my chainline is off!!
Sometimes components just don't play nice together, or measurements aren't exactly what they say they would be.  The best fix is to swap the bottom bracket axle, but first make sure your wheel is straight in the dropouts and is dished properly!  

I can't find a hub that fits my dropouts and keeps my chainline.
This happens with dropouts that are 135mm sometimes (when paired with road cranks).  Your best option is to change the bottom bracket axle to compensate.  
My new belt is too long/not long enough!
This is why I highly recommend sticking with your current gearing when ordering parts and ordering the identical components in CDX.  You can probably exchange your belt for something longer or shorter.  If you didn't change your belt and just swapped the cog or belt ring, then you dropped more than two teeth and didn't remeasure.  Use the belt calculator at the bottom of this page.

I can't get my frame apart.
Check that all the bolts are removed.  It's possible the frame is stuck together and needs a bit of muscle to be pulled apart.  

This is really expensive...is there a cheaper option?
Yeah, I know.  The only comparable option that I know of is the Base Urban FX 1.0, which isn't cheap but it does get you a whole bike that is similar in price to things like the Trek District, and is already set up fixed.  

What if I want to stick with my CDC system?
The Base Urbans seem to be using a CDC fixed cog, but I don't know where you can get one...unless you got your hands on a Phil Wood one waaay back when they were still making them.  If you can get the parts, go for it.  You have to be especially careful with chainline and tension on CDC systems though, and I highly don't recommend running a CDC fixed system without a front brake because the belts can slip under high tension if not maintained properly.

My belt makes a funny noise.
Your tension is too high or too low, or your chainline is off.  

Can I skid with a belt?
If you're maintaining your bike and regularly check your belt tension, then you will probably be okay.  I enjoy my knees so I avoid skidding and run a front brake, but with a well maintained CDX system you should be okay.  It seems that the consensus on this is that there is not yet enough data to know how well the belt will hold up to frequent skid stops, so I highly suggest running a front brake in case of emergency until there's more data on this subject!!

Will you do this for me? 
I guarantee a good LBS will be much more help to you than I will, though I'm happy to answer any questions by e-mail (aeyoQen<at>gmail<dot>com).  

If you follow through with this, you'll end up with a bombproof ride, a great conversation starter, and most of all a killer bike!  Best of luck, and I'd love to see before and after conversion photos.