# Kilo TT Belt Drive Conversion



## roynyc (Aug 7, 2013)

Thought I'd share this as I've seen a couple of folks discussing/asking about the possibility of making a fixie belt-driven bike.

The answer is yes! It took a bit of time, money, patience, and ALOT of research. In the end, I'm pretty happy with how it turned out.

A quick bit of background... I recently got a new bike so I was retiring my old Kilo TT daily commuter. I have no metal working experience (and it shows) and somewhat limited bike assembly experience (I perform maintenance on my bikes and some minor upgrades) so I spent a lot of time researching how to do some pretty basic things (like how to remove a BB). Luckily I didn't make any major mistakes or have to redo anything. I'm also not going to get into nor explain why I wanted a fixed gear belt driven bike... it tends to be a polarizing topic (especially on this forum) so if you're interested, please read on.

As all of you know, the belt in a belt drive system cannot break like a chain, so in or order to slip the belt inside the rear triangle, the frame itself has to "break". After some research, I decided on a coupler made by Paragon Machine Works. The coupler came in the diameter I needed for the seat stay.









With coupler in hand I proceeded to cut the seat stay (right one). I measured from the outside edge of the coupler (the exposed parts that do not slot into the seat stay) and cut a section of the tube a hair shorter so that I can file it down to the exact measurement later. Since the coupler has about 1/4 inch of a smaller diameter section that sits inside the seat stay tube, I knew I had to bend the seat stay at the cut-out to actually fit the coupler in. Given that the Kilo is steel this wasn't much of an issue.

Next comes the joining the coupler to the frame. After learning the difference between brazing and welding and the equipment necessary for it, I decided to braze the coupler onto the seat stay. At first I tried a propane torch but the flame was not hot enough to melt the bronze filler rod I wanted to use, so I bought some MAPP gas and that did the trick. After filling the gaps (and letting the molten metal do it's capillary action work), I was left with this:









At first I was quite worried at how ugly the joins were with the excess filler material bubbling up like that. I took an angle grinder and hand-file and very very very slowly smoothed ground away the excess material. Resulting in this:









I then finished off the joins by using a steel brush mounted on a hand-drill, which resulted in the final joint:









After painting and assembling the various new parts I bought, here are some pics of the finished product:

The joint:









The rear cog:









The front "chainring"... Note about this, I initially bought an installed a 55 tooth chainring (belt teeth count the same as chain teeth) but later upped it to a 60 tooth. The smallest rear cog that Gates makes is the 21 tooth fixed gear cog, due to the belt not being able to bend as much as a chain without affecting the strength of the belt. Since this is going to be my rain/icy roads bike, I wanted a lower ratio than what I normally ride but didn't want the ratio so low as to affect my top-line speed. 









Final product:









Hope you enjoyed reading my post. If you're interested in doing this and have any questions, please feel free to ask! While I'm no expert at any of this stuff, I'm happy to give it a try.

Oh I do want to caveat that this is potentially dangerous if the brazing is not done properly. While I have no idea if I myself did it properly, I'm willing to accept that risk and I stress-tested the finished bike by abusively hitting every pothole I can find and going up and down curbs and then inspecting the bike for damage. I've ridden this bike for about 2 months now with no issues.


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## Gregory Taylor (Mar 29, 2002)

That's pretty cool. Kudos for taking this on.

Is the coupler stainless? That can be a bit tricky to braze. Bronze works, silver works well, and you generally need a flux that is specifically designed to work with stainless.

The good thing here is that the joint is on the seat stay, which works in compression. If one of your joints fails, it won't come completely apart. However, after two months, it sounds like you have it nailed.

Keep at it - building and modifying bikes can be a lot of fun.


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## nsfbr (May 23, 2014)

I have to ask if you have noticed a difference in "drive train losses." Some time ago I did some research, including testing, of various belt and chain drives for high efficiency low powered (but more than human powered) vehicles. Low powered like 1-2 kW. 

What I found was that belts can be as efficient as chains, but with two caveats. First, belt tension and alignment is critical. Really, hugely so. New belts will both seat and stretch just like you'd imagine, so retensioning after the first 100 miles or so (my guess) would be important if efficiency is important to you. 

Second, belts tend to have a much higher no load running loss versus a chain, but slightly lower marginal loss as you add transmitted load. So when Gates claims 98%, it is once the running losses become small and it assumes proper tension and alignment.

Not that it would matter to you, as you are looking to get away from chains, but the clear winner in terms of efficiency is a "Silent Chain". There is no slippage by design, including when it wears. I'm not aware of people ever using them in an environmentally exposed situation successfully, but they are what provides the final drive connection in the second generation Prius (not the current one, but the one that made it ubiquitous.) 

I've not seen that particular Gates belt before. I assume its design is intended to keep it from walking at the cost of some loss of efficiency. Probably a reasonable trade unless you wanted to use flanges on one or both pulleys.

In any case, it looks like you did a really nice job. Don't let anything I've said detract from that, just that you may want to invest in a means to measure the belt tension and take care to maintain both tension and alignment as best you can.


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## roynyc (Aug 7, 2013)

Gregory Taylor said:


> That's pretty cool. Kudos for taking this on.
> 
> Is the coupler stainless? That can be a bit tricky to braze. Brass works, silver works well, and you generally need a flux that is specifically designed to work with stainless.
> 
> ...


Thanks! Appreciate the kind words.

The filler I used is bronze (or so it says on the description in Amazon). It comes flux coated already. From my research I've read that it is okay for stainless steel but to be honest there wasn't a definitive trustworthy resource that made me 100% sure. It sounds like you know what you're doing, do you have a better filler suggestion (maybe for next time )?


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## roynyc (Aug 7, 2013)

nsfbr said:


> I have to ask if you have noticed a difference in "drive train losses." Some time ago I did some research, including testing, of various belt and chain drives for high efficiency low powered (but more than human powered) vehicles. Low powered like 1-2 kW.


I'm not a racer by any stretch of the imagination (I do like to go "hard" on my commutes and I'm quite proud that I average 15 to 17 mph through NYC rush-hour traffic) so I don't have a good gauge.

If I were to unscientifically gauge power output based on how my legs feel after the ride I think it's about the same between a chain and the belt.



nsfbr said:


> I've not seen that particular Gates belt before. I assume its design is intended to keep it from walking at the cost of some loss of efficiency. Probably a reasonable trade unless you wanted to use flanges on one or both pulleys.


It's the Gates "CDX" belt which has a "centertrack". On the chainring and cogs, there is a vertical slit that slots into a cutout in the belt, effectively having 2 rows of teeth that holds the belt in line with the chainring and cog.

I'm not a skidder normally but after building up the bike I went on a skidding craze just to make sure the belt didn't slip and it held up fine. 



nsfbr said:


> just that you may want to invest in a means to measure the belt tension and take care to maintain both tension and alignment as best you can.


On this note Gates actually has 2 tools to help with chain tension. They sell a tension gauge tool that sits on top of the belt which you push and an indicator needle tells you the tension. The other tool is an iPhone app that you turn on and listens to the vibration sound after you tug on the belt, and based on the frequency of the sound the app tells you what tension the belt is at. I used both but unfortunately the results vary between the two methods so I split the difference. I also used a chain tensioner from Origin 8 for more precision.


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## Gregory Taylor (Mar 29, 2002)

I'm certainly not an expert - call me a hobbyist. I've brazed up (so far) three steel framesets for my personal use and enjoyment. I'm of the school that you can't call yourself a "framebuilder" until you start selling them. 

Ass Harpoons, Frantic Activity, and a Finished Bike | The Wrecking Crew

Third Time?s The Charm | The Wrecking Crew

It?s Orange | The Wrecking Crew

Eight Tubes, Part Last: it?s Love! | The Wrecking Crew

Anyway, I prefer silver brazing over bronze when working on a bike frame. The brazing temp is lower, the filler generally flows much better (depending on the type of filler I'm using), and the joint usually ends up stronger. The downside is that silver rod/wire is expensive. I too use a MAPP torch to braze with zero issues. 

(WARNING: GROSS OVERSIMPLIFICATION AHEAD) Stainless steel is different because (among other things) it has different types of surface oxides than other steels. That’s why it doesn’t rust. And that's why it is a good idea to use a flux designed for stainless - it goes after the oxides unique to stainless and also allows the filler metal to flow better. If you still have the packaging that came with the rods that you used it should tell you what type of flux they are coated with. (Also, store any extras in a dry place. Moisture can play hell with flux coated rods over time).

I've had excellent luck using the System 48 brazing wire and flux from Cycle Design. It flows really well, has good gap filling qualities, and you can do small fillets. Excellent stuff. 

Cycle Design

If you are interested in how lugged steel bikes are built - and how a dude or dudette in a garage can build one without killing themselves - I collected up some what I think are good resources. The Paterek manual is especially good.

Eight Tubes, Part 2 ? The Postman Cometh | The Wrecking Crew

Keep at it - 

Greg


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## roynyc (Aug 7, 2013)

Wow great stuff! LOL at "Ass Harpoon"!

These are great resources. Thanks for sharing! Looks like I have a lot of reading to do...


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## nsfbr (May 23, 2014)

roynyc said:


> On this note Gates actually has 2 tools to help with chain tension. They sell a tension gauge tool that sits on top of the belt which you push and an indicator needle tells you the tension. The other tool is an iPhone app that you turn on and listens to the vibration sound after you tug on the belt, and based on the frequency of the sound the app tells you what tension the belt is at. I used both but unfortunately the results vary between the two methods so I split the difference. I also used a chain tensioner from Origin 8 for more precision.


I'm familiar with the Gates tool. My belt testing days predates the iPhone by about a decade, lol. 

If you've got the tension right, then really all that is left is making darn sure you've got the pulleys aligned, and from the looks of things you would have paid attention to that as well. 

It sounds to me like you are all over this. Very, very good stuff.


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