Gates Poly-Chain

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svejkovat

10 mW
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Gates Poly-Chain.

I was reading their literature about all the exciting reasons for substituting this NEW technology for roller chain applications. Longer lived, less adjustment, no lube req'd, lighterweight, lower noise, yadda dadda. It started to sound pretty exciting I'll admit. Then, while reading up on one project that was incorporating this NEW urethane carbon fiber synchronous belt i noticed the published date at the top of the article. 1996!

engineer.jpg

The belt at right is claimed to meet or exceed the tensile loads, torque, shock, performance, etc of the chain at left. In numerous places in the literature there are tables for adapting this GT-2 Carbon belt to previous chain driven applications.

I realize that one of the greatest obstacles to this, particularly for the experimenter, is the inability to break, shorten, or lengthen the drive chain as you go. You'd end up with a very very expensive wall of these belts in short order. Also, the sprockets are proprietary -- therefore expensive and difficult to adapt to diy designs.

But I've seen very little actual use of these belts now for having been available for more than a decade. Was the product itself really found to be that lacking?
 
There are bikes that use belts rather than chains. Heck, I use them on my electric drives. However, belts have there place and chains have their place. Here is my understanding of it;

Belts are great for high RPM. Chains are great for low RPM. Also, belts have a torque load rating far below a chain of similar size. So, even though our legs aren't all that strong in actual power output, they have huge torque especially when standing on the forward pedal. Belts are not really best for that type of load.

Again, higher RPM is best run with a belt, low RPM and high torque is best with a chain.

Matt
 
Also, there is only a very limited historic range of the PolyChain GT belts available in 5mm pitch. So, for 5mm pitch the difference between PolyChain GT and Powergrip GT3 isn't that great. 8M Carbon PolyChain could replace a chain drive but with a bit more width needed.
 
I can see having a toothed belt on bike if I lived in a very flat place (for high-end customers) like Manhattan, Holland, or Iowa. I would have a Nexus 7-speed internally geared hub in the back, which is plenty if there's no hills. Schlumpf has a 2-speed planetary for the BB, but I think its $600+ just to turn your belted 7-speed into a 14-sp. I think wealthy soccer-moms would like that it has no chain-oil.

Remember the VHS/Betamax wars? Like trying to put a Ford wheel on a Chevy...they both have 5-lugs and look similar, but they just don't quite fit. I remember new video stores stocking movies with half the store being VHS and the other half Betamax. It my undestanding that Betamax was patented by Sony, and although its arguable that they had a slightly better picture and sound, they wanted to exert a lot of control over licensed manufacturing partners and charge a fairly high rate on each machine produced for acquiring the license.

The patent-holder of VHS licensed it to anybody who wanted it for a very low fee, and all over Asia, small electronics manufacturers began cranking out cheap VHS machines with a variety of innovative features. Betamax died away (my sister paid $800 for one). I'm certain the belts and their pulleys could easily be less expensive, but clearly they have a small market niche. If they are happy selling very few units, more power to them, they are not asking for my advice...
 
there just seems to be something missing in the narrative and I'm not sure what it is yet.

"...belts have a torque load rating far below a chain of similar size. So, even though our legs aren't all that strong in actual power output, they have huge torque especially when standing on the forward pedal. Belts are not really best for that type of load."

This has been, of course, the conventional stepping-off point for any design desision. But all sales, technical, and even anecdotal (rare tho it is) comment on this "GT2 Carbon Poly Chain" suggests that it is equal or superior in torque rating to similarly dimensioned chain (see photo). Check out youtube and there are a few videos of mountain bike racers using it. Trek just last year came out with a model using it. I've seen a few comments from 250lb+ riders of some of these belt drives that mentioned tooth skipping under extreme hill conditions, but not one instance of actual damage, breakage, or stretch of the belt.

The fact that the designers are aiming at penetrating the bmx/offroad crowd forces their hand for another reason. The front chainring of a bmx bike needs to be as small as can be practially accomplished. Ground clearance here is a selling point. That means that the belt designer has to work beyond the luxury of simply putting more driving teeth against the belt by specifying a larger sprocket. It's got to be a pretty damn strong belt.

Personally, Once I had all other considerations nailed down in a design, I'd be very interested in giving this a try. It's the smoothness and quietness that turns me on. It's always a premium factor in anything I try to build. The prices are still up there, but the product has been around long enough now that there are limited quantities of belts and sprockets showing up at surplus outlets for half price or better, which puts them right in the range of higher quality chains and sprockets.
 
Harleys use belt drive to rear tire. Automobiles use belt drive valve timing. These are "toothed" belts. Don't think they would work on a derailer.
 
I think these belts are a great product, but If they were asking me I'd say they are too expensive and marketed to the wrong demographic.

Their benefit is no chain-maintenance and no lube required. They are advertised as quiet, but a properly lubed and adjusted chain is not loud.

Since a belt won't be using a deraillier, I've seen them advertised for "Fixies" (single-speed, no freewheel) which is a trendy phenomenon for flat-land upscale cities where young adults carry their bikes up stairs to lock their bike inside an apartment. (they're not buying a lot of belts, they still use chains. Current trend is using wax lube instead of a belt to eliminate oil)

So Gates is trying road and mountain bikes. Since a chain can't deflect too far out of line, most 21-speed bikes (3 X 7) actually only have 15 useable evenly-spaced gears. A Nexus 8-speed hub is about $240 http://www.sheldonbrown.com/harris/shimano-nexus.html so if someone was happy with 8 gears, and was willing to pay the extra price to get no oil or chain-maintenance by using a belt, that would work.

The Schlumpf planetary 2-speed pedal-axle (Bottom Bracket/BB) could accept a belt. It is around $700, and if using the Nexus 7-speed (no benefit to 8-sp due to gear overlap) it has two overlaps and 12 distinct gears. $700 is a lot of money to get 4 more gears, so I don't see this ever being a big seller just to save cleaning and lubing a chain on occasion. http://www.schlumpf.ch/hsd_engl.htm. Most Schlumps are sold to bikes with a conventional rear deraillier having external gears.

At that point its easier to pay $1,000 for a Rohloff 14-speed hub, instead of $940 for the 12-speed Nexus/Schlumpf combo.

So, if we start with a single-speed (SS) and a belt-drive, its $240 to get 8-gears, but an extra $700 to get 4 more gears (or an extra $760 for 6 more gears). Who would do that? I can only see wealthy moms with babies in flat-land cities willing to pay the 8-speed Nexus/belt/pulley price to eliminate dirty oil-stains and chain maintenance (cleaning, oiling, adjusting, replacing). Just an opinion...
 
Isn't there a fundamental problem with using a belt to replace the chain on a bike? The chain goes through one of the rear diamonds and therefore has to be able to be joined in the field. It would require a redesign of the frame.

Nick
 
That's a minor change - they just "cut through" the rear dropout and bolt it closed.

Personally, for all the cost and bother, I'd just as soon go for a shaft drive. They have all the advantages of a belt with less chance of breakage. The best shaft drives are negligibly less efficient than a chain drive.
 
I am told that the difference between Carbon Poly-Chain and other toothed belts is that Poly-Chain operates well with much less tension preload than most toothed belts. The required pretension on many toothed belts would impose undesirable friction and bearing wear issues, with possible axle breakage and other complications in the long term. The tensile stiffness of the carbon fiber backing is probably what allows lower operating tension.

One reason chains historically have been such a good match for cycles is that they can be run with no pretension at all-- some early alternatives to the derailleurs we know allowed the chain to droop loosely from the sprockets. One-speed drivetrains can be practically slack, with only the excess length taken up so that the chain doesn't climb off the sprockets. Toothed belts must have some amount of tension applied.
 
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