We know a tandem couple in the Miami area who have replaced their timing chainrings and chain on one of their tandems with a notched rubber belt and pulley system. I have not seen it yet because they live about 4 hours away, though I plan to see them and the bike within the next month or so. It sounds like a good idea. Should be lighter and quieter and not need as much maintenance (clean and lube). As far as replacement life goes, I don't know.

My question is: Why haven't I seen any discussion of this technology on this forum, Tandem@Hobbes, Bicycling Magazine, tandem vendor catalogs or web sites? Is it that little known? I haven't seen ads for it either. People are always discussing the latest carbon parts, shock posts, disc brakes, yadda, yadda, ad infinitum. Yet if I hadn't known this couple, I would have never heard of this timing belt system. I even attend tandem specific rallies and other large events - never saw one of these set ups. Is it such a limited supply item that it has eluded the radar of the tandem enthusiast community, or has it been tried and dismissed as impractical by the tandem community at large? I've only been into tandeming for the last two years or so.

Do any of our knowledgeable forum members know any more about this? If so, what are the opinions about it? Where can I find more information on it so I can evaluate whether it is the right thing for me?

I would have posed this on T@H but I just read the digest version and I don't want to get started as a full contibuting member. Thanks.

Like shaft drive, belt drive is a viable alternative to chains and you can find bikes being made today that use both belts and shafts. There have even been some bikes built with planetary pulley systems.

The problems with belt systems -- even as a timing chain -- is the loss of efficiency, noting that even with a notched belt (ala Harley) you must still achieve a lot of tension between the pulleys to prevent slippage under loads and in all weather conditions. On a motorcycle or snowmobile, the power loss is no big deal. Even on Chinese commuter bikes -- which is where you'll often times find belt drive systems -- efficiency isn't a big deal since the bikes aren't being used for "performance oriented" riding.

Back to tandems, even if you could minimize the loss of efficiency you're faced with unique tooling costs, compatibility/standardization issues (belt length = boom tube / stoker compartment length), supply chain (no pun intended), etc... and most of all, consumer resistance. It all begs the question, just how bad is the chain? Efficiency wise, it's pretty incredible -- something like >98%. As for maintenance, if you can resist the shiny ones, chains are pretty cheap. And then there is the learning curve and economies of scale; after all, if 99.9% of the bikes produced use chains, converting buyers to a tensioned pulley and belt system could (and has) proven to be a challenge. Again, single speed or bikes with internally geared rear hubs are available with both drive shafts and belt drive. But, chances are, if you need a belt your LBS won't have one.

Anyway, it will be interesting to hear how it works out. It's certainly a viable technology; the question remains how practical and economically feasible is it for the broader market?

To add to Mark's post, not only would a timing belt be less efficient, I would think it would contribute negatively to bottom bracket wear as the bottom brackets would be under tension at all times to keep the belt from slipping, in addition to the tension of pedalling.

As far as efficiency of a timing belt, I think a properly engineered belt would be as or more efficient than a chain. You may be assuming that the belt would transfer force by means of friction between the belt and pulley, in which case I would agree with your assertions of efficiency losses and potential BB damage due to excessive tension. However belts similar to those employed in many internal combustion engines today, have teeth which engage recesses in the pulleys to totally eliminate slippage and make belt tension less critical. These belts last 60 to 100K miles in a grueling environment of high temps and much higher torques than any bicycle would ever give it. The only criteria in which a chain might be more efficient is the resistance to flex, since a chain has lubricated bushings on pins vs. rubber flexing over layers of cord, which could cause internal friction to convert some power into heat.

Mark, you are probably right about the need for various sizes vs. removeable links on a chain. I also agree that it would be difficult to find parts, at least until the new system were to catch on and become a standard. Also true that chain is a mature product and automated machines can spit out chains for unbelievably low cost. The cost of a chain is amazingly cheap, when you consider how many precision parts are assembled to work so well under load.

However, isn't (or wasn't) it also true that disc brakes parts are expensive and hard to find in stock at most LBS, especially compared to the cheap and easily available brake shoes and pads that essentially do the same job? Yet that has not stopped the tandem community from embracing the new technology. I am actually not surprised that belts are not taking the tandem market by storm. What I am surprised about, is that I have seen literally NO discussion or promotion about the technology by the enthusiast fringe.

Steve,

You started the discussion!!!!!!!

I know belt drives are used all over the auto and motorcyle industry. I have not followed motorcycle racing recently. Back when I did follow it, every bike on the course had chain drive.

I think Harley uses a belt for reasons other than performance.

I am not saying it could not happen, I just don't see a problem with chain drive as it is so effecient.

If a belt drive came along that was better, I am sure it would reach the market eventually.

...belts similar to those employed in many internal combustion engines today, have teeth which engage recesses in the pulleys to totally eliminate slippage and make belt tension less critical.

Again, there's no doubt in my mind that someone could design a belt drive to efficiently transmit power -- perhaps approaching the efficiency of a chain -- from the captain to the stoker even in adverse weather conditions (cold/wet/hot/dry) but, at what cost and added complexity? While spline-drive belt & pulley systems require less tension and are more efficient, there still needs to be a lot more tension than what is required for a chain drive which despite it's shortcomings deals with all of the environmental rigors and current bicycle frame design constraints quite well.

So, that brings us back to the average steel/alum/ti/carbon tandem frame which as presently designed and executed, deflects under pedal loads between the captain & stoker, both at the axle and in the connecting structure/boom tube. For evidence one only needs to take a look at any tandem under way and note that even a timing chain that was tight when static will droop along the lower return once pedal loads from the captain are applied (remembering that only 1/2 of the tandem's power is carried by the sync chain). To eliminate that deflection you'd have to move the sync belt to the drive side, beef up the bottom brackets, and beef up the frame and/or add a tensioning device. In either case, does the potential benefit outweigh the added cost/weight/complexity? Probably not... at least for the masses for the reasons already stated.


...However, isn't (or wasn't) it also true that disc brakes parts are expensive and hard to find in stock at most LBS, especially compared to the cheap and easily available brake shoes and pads that essentially do the same job? Yet that has not stopped the tandem community from embracing the new technology.

It's an interesting observation but I think there are a couple differences. First, it's worthwhile to note that the disc brake and brakes in general don't put unusual demands on the core frame (fork drop-outs are turning out to be a different story). Second, brake performance has always been a problem for tandems and ever since the first tandem team flew down a hill out of control folks have been searching for a better brake solution. The Arai drum purportedly was developed as a brake for a moped and then adopted for use on tandems and remains the standard for heavy-duty performance. In the rim brake department, self-energizing brakes have come and gone (awful things), canti's and calipers have always been around but continue to get better, and then there are the disc brakes. Phil Wood developed the first bicycle specific disc brake for tandems back in '75; it ultimately provided to be a flawed design and eventually led Phil to retire and sell his company the to current owner. However, before the design and reliability issues were fully understood the darn things were showing up on all kinds of tandems, to include becoming OEM on Schwinn's Paramount tandems. Mountain bikes created the new seed money and provided the ideal application for disc brakes which, again, were an easy adaptation given the scaled-down versions that were already being produced for kid-size dirt bikes and what not. Like V-brakes -- which were also designed ostensibly for the mountain bike market -- discs have been adopted for use on tandems as a trickle down in search of a solution to a problem, not a tandem innovation. Adopting drive belts, on the other hand, would suggest that timing chains are perceived to be a problem which, at least from my perspective, they aren't. However, that said, if drive belts ever show up on the bikes used by pros in cycling competition you can certainly expect that they will quickly find their ways into the mainstream market and perhaps on tandems.

Let us know how the system being used by your friends works out...

From Whitt and Wilson, Bicycling Science, p.281:

"The best roller-chain drives appear to have an efficiency of about 98.5%..."

My expectation would be that roller-chain drive will be with us for a very long time yet, though perhaps the width and pitch and materials and construction may change.

You can find some discussion of toothed belt-drives in Bicycling Science.

Belt or shaft drive bicycles seem to show up at Interbike every couple years.
None seem to get beyond a limited production stage.
A better mousetrap will sell itself, and only time will tell.

Belt or shaft drive bicycles seem to show up at Interbike every couple years.
None seem to get beyond a limited production stage.
A better mousetrap will sell itself, and only time will tell.

I used to be in Motor sport, 100cc Karting to be precise, and the big problem we had was with chains. I used to do endurance racing, 6, 12, 24 hour racing and one bright spark came up with a belt system. It worked absolutely fine, no belt breakage, wheresas we had to run a duplex chain to keep it together for a 6 hour race. The belt drive system worked fine. On the weight side it was fine as although the drive cogs were heavier, the belt was lighter so comparison weights were good. However, what did happen was that the engine knocked out crankshaft bearings and the rear axle bearings that I never changed, had to be replaced every meeting. Tension on the belt was also another problem as things had to be retensioned quite frequently to stop snatching over the pulleys and hence stripping the belt.

There is a long history swimming against the current with belt drives, Porsche built a belt drive commuter bike in the mid 90's around the same time BMW got into building MTB's.

If your looking for how to approach it from an engineering point that would be my first stop, I know they didn't split the stays to do it.

I'm sure it's as much marketing as engineering, how else do you explain people riding on perfectly good bike paths with 2" wide tyres.

Well, I finally got a chance to look at the timing belt system and I've got to admit, it is slick. The reason that word of this new idea has not spread, is now clear. It is not a commercial product. It is an application that has been custom built by one tandem owner for his own use. Although he farmed out the actual machining, he did all the research, engineering and assembly work for himself.

Bottom line, it is impressive. Very quiet. Over 98% efficient. Over a pound lighter than a chain, and never needs lubrication. Based on similar belts used in industrial applications at much higher torques and speeds, indications are that these belts should last many tens of thousands of miles. And replacement belt cost (if a stock size can be adapted) is cheaper than that of a premium chain. Of course time will tell. I'd like to see if it is still working perfectly after 10K, 20K and 30K miles. And how often belt adjusting will have to be performed, due to stretch will be another thing to watch. Will bottom bracket life be foreshortened? Too early to tell.

I won't get into the design parameters at this time, because my friend may want to apply for a patent. In the end, it is too early to determine whether this will ever become a commercially viable product available to tandemists, due to the potentially limited amount of demand never translating into enough quantity to bring the price to a popular selling point. It may be of interest to some though. I can visualize a tandem team that forked out for a Santana Vision, a Calfee, a Zona or other expensive ultra lightweight bike being pretty interested in a way to cut another pound to pound and a half off their 25 to 28 pound tandems.

Steel timing chains on Harleys are max rated by the chain maker for 20mph!The "rubber" belts are better and more efficent,and Harley didn't start using them till their customers starting getting them other places.Maybe on Tandems they will also take the place of the steel timing chain---time will tell.