belt drive timing chain
 

i'm here at interbike at the demo days, co-motion has a new belt drive to replace the timing chain. the belt drive system is exclusive to co-mo at this time. for you weight weenies they claim it to be 10 ounces ligther than the traditional chain setup. they expect it to be a high performance system as they have it as an upgrade on their top three tandem models. i forget the difference in price, i think it's an extra $300 dollars. sorry no pics.

before you get your hopes too high, as i was demo'ing a mountain bike riding up the road to the trail a riding came coasting down the trail on a belt drive mountain bike yelling "hail to the chain, long live the chain, belt drive sucks!" as he raised his arm aloft carrying the belt

You're...????


...at Interbike and you've seen something new and exciting???

Yes, I know; pretty cool, eh?

doesn't actually happen very often... maybe i'm just that jaded...

Belts + Off-Road have some real issues, particularly when rocks and what not get sucked into the belt and pulley... which is why you won't find them on anything but motorcycles that stick to asphalt.

However, for the road-going machines my only benchmark is a friend from Florida who has been using his own belt-drive system on his Seven Axiom 007 for several years now without any issues.... to include using the same Gates kevlar reinforced belt that I believe Co-Motion is using.

It will be interesting to see how well it does both from a performance and consumer acceptance standpoint.

Belt drives are nothing new . . .
Photo of a belt drive on a prototype 'unnamed tandem' that I rest rode 2 years ago.
Production tandems with belt drive are something new.

I've often thought that a belt drive system would work well with an internally geared hub. No lube, no maintenance. I'm not sure why they wouldn't work in an off road application.

I design machines that use timing belt drives. The applications are pretty clean but the belt drives themselves are virtually maintenance free.

Now, if somone can just figure out how to make a derailer for belts. The other issue is that for a given amount of tension, a chain will likely be narrower than a belt.

Other than weight savings I do not understand the benefits of a belt drive, seems to me that it would require more energy to "bend" a belt than a chain?

They are supposed to be about as efficient as a chain.

Benefits that I see: less maintenence, cleaner, quieter.

You don't have to oil them and they don't stretch.

Rocks, sticks, mud and other debris are more readily trapped between the belt and the ramps of the splined drive gears which can cause belts to bind, break, or derail.

Some of the gear-drive makers have developed splined drives with open voids between the ramps to help with shedding mud and smaller rocks; however, it doesn't completely solve the problem.

Chain and sprokets don't have this problem with rocks or mud and, while chain drives will occassionally get bound-up by a stick that can't be chewed-up and spit out by the cutting action of the chain and narrow sprocket interface, that's the exception and not the rule.

The benefits to roadies have already been mentioned:

- Low maintenance
- Longer service life
- No lubrication / fairly clean
- Belts are lighter than chains

I'm not sure about the reduced noise in that, belts don't eliminate noise... they just change the pitch as they have their own unique noises.

Belts also seem to lock tandem builders into a fixed stoker compartment length, that is unless there will be a range of belt lengths and/or different size splined gears to allow for shorter or longer boom tubes.

Again, the pros and cons have apparently been weighed by Co-Motion and debates based on theory and hearsay will hopefully give way to first hand impressions as the first gear-drive Robustas, Machiattos, and Supremos make their way into true consumers hands.

That's interesting; something I hadn't thought of.

If you're Co-Motion and use the same 28.5" long stoker compartment on all of your stock size tandems -- regardless of frame size -- this isn't an issue, as 'one size fits all'.

However, if a client wants a custom-sized tandem with an extra 1" or 1.5" in the stoker compartment I'm guessing the belt wouldn't be an option.

Belt molds are available in a limited number of teeth. As the belts get longer, the jumps in length get bigger. A belt mold can be custom made to any integer number of teeth but the larger the mold, the higher the cost. So, it would be cheaper if tandem makers could agree on a limited number of lengths to amortize the cost of the mold.

How about belt drive for both sides with an internally geared hub? A couple we met were touring with internal hub transmissions. He said that high gear was about 10:1. I think these are 5 or 8 speed hubs. If these hubs could take tandem loads, this could make for a pretty low maintenance drivetrain.

Rohloff does a 14 speed internal hub, which could be adapted for belt drive. However, Rohloff's a bit hefty, both in weight and price. Hear they may also be working on a higher count internal hub.
A whole new market could open up for this type chainless system.

If timing belt folks don't standardize lengths, then a rather good sized investment in proprietary belts in custom length/tooth profiles and pulleys would be required for the specific losads needed on a tandem.
Co-Mo sells far more ready-made models than customs.
No doubt they've done their market research before taking the leap from chain to belt drive.

Has anyone ridden one yet?
 

I'm assuming that the belt must be less stiff than a chain, so I wonder if you can feel any difference as a captain.

Webcyclery has them on their website as a retro kit for $525. If you are a weight weenie, that's a pretty good $/gram payback.

Does anyone have hard numbers on efficiency difference between the belt and the chain?

It does look a bit odd, but I expect you'd get used to it pretty quickly. If it really lasts 10 times as long as a chain then that would mean for all practical purposes you would never have to do anything with it, as I only replace my timing chain every 3 or 4 years.

Plus the captain would NEVER have a chainring tattoo:)

From a different tandem discussion forum where the Gates Carbon Drive belts are being discussed came this report from a friend and the former Editor/Publisher of Tandem & Family Cycling Magazine who was one of the Beta testers...

I'm wondering if belt driven bikes are going to be the new fad. Here's an article on Trek's prototype cross bike. http://www.cyclingnews.com/tech.php?...008/news/10-03

Trek also has a belt driven commuter bike on their website.
http://www.trekbikes.com/us/en/bikes...trict/district

or maybe different size chainrings could make up the difference in pretty small amounts since the teeth are much more tightly spaced than a chain. obviously you couldn't go too far with it but would allow less belt sizes to handle a range of lengths. the next question would be what's easier/cheaper to make? chainrings or belts?

I suspect the marketing success or failure of the basic system on Co-Motion's tandems will be the the gauge by which Gates decides just how many different size pulleys (new word for the tandem bike lexicon here) and/or belts to offer. Moreover, I suspect by limiting the introduction as a Co-Motion exclusive and making the retrofit cost somewhat high ($525) Gates and Co-Motion are trying to put a throttle on demand until they get a feel for how these things will hold up in the hands of consumers who use them and the home mechanics and bike shops that work on them. Remember, the problem with making things fool-proof is that fools can be pretty resourceful when it comes to screwing things up and faulting a design vs. their own shortcomings.

Anyway, as far as compatibility, it's also worth noting Calfee's standard size stoker compartments are pretty darn close (28.35") to the Co-Motion spec. of 28.5" so it will be interesting to see if the Gates belts are a bolt-up option for Calfee owners as well.

A Santana-compatible set of slightly larger pulleys or slightly shorter belt would seem to be the next logical offering given just how much of the market is comprised of Santana-branded tandems.

For folks like us who have unusually long stoker compartments -- 30" on the Calfee and 31" on the Erickson -- unless we back into an existing pulley size spec'd for single bike cranks, a belt drive may just never be an option.

As one final note, I've been corresponding with yet another friend -- one who works at Webcyclery -- who tells me the Gates belt & pulleys actually have a much lighter weight than the one, 295 grams vs. the 363 grams referenced on Co-Motion's Blog entry. Webcyclery.com has now updated their website with this "as weighed" data and I've asked for the individual component weights for the belt and pulleys.

Trek actually has two belt-drive commuter bikes in the works. The second, with an 8-speed Nexus hub, is here: http://www.trekbikes.com/us/en/bikes/urban/soho/soho/

I have ridden a prototype of the above bike and was impressed. Belt drive will be more than a fad if it works as well every day on the road as in theory and in the test lab. With Trek jumping in, there will be much more empirical data before too long.

TG, how much movement does the front eccentric provide? Could it take up enough slack to go from Calfee's 28.35 to 28.5? It is only 1/6". Or do you know if the 28.5 is the minimum, does it matter?

According to Gates they make 4 different sized belts (113, 118, 122,125T), unless the tandem belt is very specific to Co-Motion.

I'm guessing that the "exclusive" part of the deal with Comotion is that the longer belt for a tandem (which would be on the order of 180 teeth - 1.5 times the length of a half bike belt) is only available through Comotion or their dealers). Maybe Comotion paid for part of the mold cost? Just idle speculation on my part - not trying to start any rumours here.

An eccentric easily takes up one full chain link, which is 1" total or 1/2" of eccentric movement so it would seem that if you are within a 1/4" or so of the Comotion spacing you could be ok.

It's early, but it looks like more than a fad to me. There appear to be some solid technical reasons for adopting this technology. Given some time I'm sure prices will drop dramatically too, Gates isn't the only high tech belt company out there (Brecoflex, are you listening?). I just hope they can agree on a standard pitch and tooth profile.

It varies based on the different eccentric designs, but crank axle fore/aft adjustment seems to fall into the 3/8" to 1/2" range... so double that relative to how much play there is in chain / belt length.

Based on the photos of the Periscope with the belt drive, it would appear that the eccentric is rotated to the rear-most position, maybe a tick of the clock forward for fine-tuning of the tension. That being the case, someone would have to try and install it on a Calfee just to see if it would fit without binding and they'd need to check it on both a pre-March '08 and a post-March '08 Calfee with stock-sized stoker compartments given the change in eccentric size that was implemented in March.

As for belt lengths and pulley sizes, I'd guess what Co-Motion has is what Gates has produced in line with the rationale expressed in my earlier comments.

So who's going to be the first kid on the block?

I have to say the anecdotal report of improved acceleration, and the weight savings are both seductive.

I've got a couple of hesitations though: 1) I'd like to see actual efficiency data,

2) the price penalty for early adopters, and

3) aesthetically, I'm thinking it may look kinda cheap (in spite of the fact that it's expensive.)