August 3, 2008

Belt drive project – initial thoughts

I’ll upload a few (not that exciting) pictures later tonight, but I’m icing my knee (my seatpost managed to slip during the race yesterday and I’m pretty sore on the left side) and have time to type this up right now before I get going packing my stuff for Vermont

Frank at Spot has graciously (albeit after quite a few delays, though I doubt that’s his fault) gotten me some belt drive components, specifically:
-118t belt (I think, it might be 122, I didn’t look closely yet)
-46t drive cog
-28t rear cog
-Spacer kit (which, to be honest, I didn’t really need, but whatever)

Here are some initial impressions and thoughts, in no particular order:

-The packaging for the belt is VERY specific about how it must be handled – any kind of kinking, bending backwards, or twisting force will damage it. It seems that the belt, while very strong when used as intended, is also extremely sensitive to any kind of rough handling. Be forewarned!

-Cost is about 3-4 times that of a typical roller chain/chainring/cog setup. Assuming the longevity claims are true, though, this will pay for itself in a season or two. I usually kill a drivetrain every 18 months or so, I think. Some folks who ride in bad/wet conditions more may find the belt more economical than me.

-The drive cog/chainring is going to be a bit tricky to fit alongside the driveside chainstay – this is going to require some serious manipulation of tubes (ie, yoke) or else very long chainstays to fit big 2.3″+ tires, which is not necessarily a good thing for the 29er crowd. I’m not sure how a conventional chainstay setup (ie, no yoke, no elevated stays) could work out very well unless the chainstays were quite long – probably in the 460mm (ie 18″) range. This is probably not going to make the singlespeeder crowd too happy. Using wide chainline Shimano XT M770 cranks, I can’t put the drive cog on my current singlespeed at all, despite fairly long stay length (445mm) and smallish tire clearance (decent 2.1″, no bigger). The Spot “Longboard” 29er frames that I’ve inspected that are set up for belts appear to use ~18.5″ chainstays to solve this problem, as well as some aggressive dimpling of the driveside chainstay. Great for big folks, maybe not so hot for smaller humans.

-Gates’ weight claims are a little dubious. The entire system (belt, both cogs, no spacers) weighs about 260 grams – nice and light. But I believe the site is currently claiming “Typical chain weights vary around 300 grams, and singlespeed systems are commonly upwards of 400g. The Gates Carbon Drive™ belt weighs just 70 grams and the entire system weighs a mere 220 grams.” This is stretching the truth quite a bit – my scale reveals that my chain (long enough for a 34×20 29er drivetrain) weighs about 240 grams (it’s a generic PC951) and my 34t downhill chainring weighs about 50g. Throw in 30g worth of steel rear cog and you’ve got a total system weight of 320 grams – using parts that are far from trick. A lightweight (ie, aluminum) 18t cog and lighter (ie, not downhill) chainring would probably take 25 or 30 grams off of that, if weight is your big concern. So the weight savings (not counting dropouts and chain tugs and such, which will almost certainly add quite a bit to the belt setup) is going to be about 60g, or two ounces. Picking lightweight parts for a chaindrive could easily cut that in half, so this really isn’t as much of a weightweenies dream setup as it’s been presented. Of course, if you’re running a ginormous BMX chain, you’ll save some weight.

It’s a little annoying to read this kind of statement, too (again, from the CDS website) : “The entire system weighs in at over 226 grams lighter than an equivalent chain drive.” Note the lack of the crucial comma between “grams” and “lighter”. 226 grams, while not a completely accurate weight, is at least close, but the sentence as written seems intentionally ambiguous, so that the reader will assume the entire system is lighter than an equivalent chain drive by 226 grams (or half a pound). Is it lighter? Yes. Is it enough lighter to matter at all? No. In fact, I’m guessing the need for really beefy chain tugs to keep the tension high eliminate the weight advantage entirely.

-My plan, as of now, is to build a bike with these parts (I’ve got an idea for a dropout design that should be much better than what Gates is selling and that will allow easy wheel removal, no caliper realignments, etc) and test ride it over the fall/winter. I’ve got access to a powertap hub, so I’ll also test the efficiency claims (though those seem to have disappeared from the CDS site) head to head with a roller chain setup.

-I’m bummed that I can’t run my favorite setup – the 2×2 (34/38 rings in the front, 20/16 cogs in the rear), which is great for Boulder riding where there can be 8–10 miles of flat pavement between you and the trailhead – I just put the bike into the tall gear (swapping the chain by hand) and then swap back at the trailhead. There’s no way to do that with the CDS setup – the beltline is crucial, so there’s no way to even put 2 cogs on the rear and swap between them.

-There will be no retrofitting of existing frames. No freakin’ way. The spacing requirements for the chainring make it totally impossible to fit onto any existing frame save perhaps an old Alpinestars or something else with elevated chainstays. So don’t even ask me.

More info as things progress, but not until I get back from VT.