Carbon Belt vs. Steel Chain Final Drive

[Admin]

Looking inside the Gates Carbon Drive MotoX9 belt. The long helix fibers are the carbon elements. Polyurethane rubber encases the carbon and the better the bond, the stronger the belt. The thin layer that contacts the sprockets is a nylon wear layer. “The magic is our chemists,” says a Gates engineer. (Gates/)

In 1980 Gates Industrial Corp. and Harley-Davidson added a third choice of motorcycle final drive—the Gates toothed belt and sprockets on the FXB Sturgis model. The new Gates drive was essentially silent, required no lubrication and little maintenance, and soon established itself as being at least as durable as the steel roller chains it replaced. Gates’ literature describes it as “the only belt drive system designed specifically for bikes.” Many millions more belt-equipped H-D motorcycles have followed.

The 1981 Harley-Davidson FXB Sturgis was the first Gates-cogged-belt motorcycle. In fact, the “B” was for belt. (Cycle World Archives/)

Such toothed belt drives have long been widely used in industry, and are familiar to motorsports fans as the blower drives seen on Top Fuel dragsters.

Riders have eternally debated the final-drive problem: chain or shaft? Shaft-drive advocates point to cleanliness, low noise, and long life/low maintenance. All in favor of chain drive note convenience of ratio changing (important in racing), light weight, low cost, and lack of bulk. After 1971 the option of a roller chain with its lubricant sealed in place by tiny O-rings changed the discussion somewhat. Chain life and cleanliness improved significantly.

The first production Zero electric motorcycles had chains and we discovered this was essentially the only noise heard when riding. Belts are virtually silent and make for an excellent electric moto choice. Belts used on ICE bikes also give engineers more “noise allowance” in meeting regulations. (Gates/)

In 1980 the tensile strength of Gates Poly Chain toothed belts came from the superstrong aramid fiber, DuPont Kevlar, and the body of the belt was polyurethane. When the word “belt” is used, people often think first of the old fabric-reinforced rubber V-belts used to drive automotive accessories (invented by Gates in 1917). V-belts drive by friction alone, but toothed belts are a positive drive, like chains or gears.

Early in this century Gates began to develop a new belt whose strength is based upon superstrong carbon fiber cords, and it came to market in 2006–07. Carbon fiber is based upon long, largely crystalline elements aligned with the fiber direction. A well-publicized application of carbon fiber is in Boeing’s 787 airliner, whose structure weight was reduced by a claimed 20 percent through replacing traditional aluminum structure with carbon-reinforced composite. Carbon fiber reinforced plastic (CFRP) is familiar to motorcyclists as the lightweight material from which the bodywork of high-end sports motorcycles are made.

Tensile Strengths of Steel and Carbon

How can a flexible belt be as strong as or stronger than a roller chain? Typical high-strength steels offer 120,000 psi tensile strength, but carbon fiber routinely achieves 500,000 psi or more. And carbon is light. Steel has a density of 7.8 (meaning that a cubic inch of steel weighs 7.8 times more than a cubic inch of water), whereas carbon fiber’s density is just under 2.0.

Long ago materials researchers understood that practical metals never come close to the strength they display as perfect crystals. That is because metals are polycrystalline: made up of a jumble of tiny, randomly oriented crystals bonded together.

At present both chains and belts are highly engineered products of great reliability. Confidence in them is high because both forms of drive are backed by decades of experience.

Belt Integrity

When I asked Gates engineer Jeff Schubring about the bond of belt teeth to the carbon cord band, he said, “The force required to shear the teeth is better than the modulus of the belt.”

Wear Resistance

Roller chain inevitably wears at its more than 100 joints, slightly increasing in length over time. This length increase requires periodic readjustment of chain slack, for excessive slack can lead to the chain’s jumping on the sprockets. Connecticut rider Richard Schlachter was comfortably leading the 1981 Daytona 200 when his chain began to “ratchet,” to skip on the sprockets. By using less throttle he was able to salvage third place.

A toothed belt drive has no joints to wear, and the carbon fiber material that provides its strength cannot stretch, so adjustment is infrequent.

Schubring said, “We have guidelines for how much (center distance) variation can be tolerated.”

Steady Increase in Belt Capability

Metals typically fail in fatigue (the accumulation of internal material damage during thousands of stress cycles leads to crack formation). To increase their resistance to fatigue, the plates of premium roller chains are shot-peened. Carbon fiber displays exceptionally high inherent fatigue resistance. The late John Britten said to me in the 1990s, “Once you get used to working with directional materials like carbon fiber, you lose trust in metals—they begin to seem like tightly packed sand.”

I asked Schubring how carbon fiber is bonded to the other elements of the belt. He explained that the matrix of the belt is polyurethane rubber, and that “the magic is our chemists.”

They work in an ever-active area—that of developing chemical coupling agents capable of bonding strongly to both the fibers and to the matrix that surrounds them. There is steady evolution in this field, allowing periodic reductions in the width of belt required to transmit a given load.

Because of this steady progress, a Gates belt equivalent to a 530 roller chain is now equal in width to or slightly narrower than that chain. The latest Gates Carbon Drive X9 belt, as compared with the product it replaces, allows use of either a 9 percent narrower belt than before, or of 10 percent smaller sprockets.

Shock Loading of Drives

Because we have for some years seen the primary chains of classic motorcycles replaced by Gates belts (particularly in racing), many assume that belts must have shock-absorbing properties.

No, toothed belts are not shock-absorbing or springy. If they were, their pitch would vary, leading to ratcheting on the sprockets (skipping teeth). Shock-absorbing elements originally supplied with the motorcycle—on crankshaft or clutch or rear wheel—are retained when a belt drive is adopted.

A Gates Mudport rear sprocket on a knobby-shod application. Width of the belt helps with shock load as might be encountered with jump landings or the slip-and-grip found off-road. IC engine power pulses also count as shock load. (Gates/)

Schubring notes that for motorcycle and scooter drives “shock loads in the system are typically much higher than peak motor torque—in the range of two to three times. So the strength of the belt from the tooth to the cord and main body of the belt needs to be able to handle this without breaking.

“With belts this comes down to the width of the product,” Schubring says.

Power transmission textbooks include tables for estimating how much capacity is required to allow for the “lumpiness” of various power sources and loads—from smooth-running electric motors to rock-crushers.

Gates manufactures the belts (technical development and production of Gates carbon drive motorcycle belts take place in the US) in large widths which are slit to the desired width. “If in a given application we encounter an issue, it takes only a 10 percent width increase to double the product life,” Schubring says.

Chordal Rise and Fall of Chain Drives

We have all heard the zhhh sound as a chain-driven motorcycle coasts with its engine declutched and shut off. That sound is generated because the “wrap” of a chain around its sprockets is not truly circular: The chain’s wrap is a polygon with as many sides as it has pins.

As such a polygon rotates, its apices (plural of “apex,” right?) have a slightly greater diameter than do the centers of its sides. The result of this is that especially on sprockets with small numbers of teeth (such as 15) the chain’s tension side rises and falls slightly as it runs onto the sprocket.

To visualize this, imagine riding in a car with 15-sided wheels. This is the so-called “chordal rise and fall” of a chain drive. Part of this process is an audible series of small impacts as the chain’s rollers hit the sprocket—the zhh you hear. Industrial drive catalogs recommend that sprockets with fewer than 20 teeth be avoided if possible.

Toothed belt drive does not generate chordal rise and fall.

Foreign Object Damage

What happens if a pebble is trapped between belt and sprocket? “We get belts returned with stones embedded in them,” Schubring says. “The customer’s issue was the ticking of the stone hitting the sprockets—nothing to do with failure.

The Mudport sprocket is designed with holes to release any material that may come between belt and pulley. (Gates/)

“Sprockets can be made in such a way that they shed stones, just as is done with chain sprockets for dirt bikes.” Gates offers the Mudport sprocket design that allows foreign materials to escape pulleys.

Belts Are Attractive Electric Motorcycles and Bicycles

Light weight and low friction are always of interest to bicycle makers and their customers. “Belts are a game-changer for electric bikes,” Schubring says.

Ebikes have seen a proliferation of belt drives. (Vvolt/)

Because electric motors are essentially silent, other noise sources stand out. The quiet operation of Gates toothed belts is compatible with electric traction. Also, electric drive is much smoother than the pulsed torque of IC engines, so a belt to transmit 100 electric motor horsepower can be narrower for a given service life than one transmitting 100 ICE horsepower.

View the full article