Secondary drive belt maintenance

In this article, we'll discuss the secondary drive belts found on Harley-Davidson models and some Japanese custom bikes. These belts, which transmit rotation from the transmission output to the rear wheel, remind us of days gone by, when drive chains didn't exist. They're seemingly simple, but they incorporate a wealth of design technology to make them quiet, reliable, and low-maintenance.

A little history
The first secondary drive belt on a Harley-Davidson was used precisely on the first model of 1903,

straight out of the famous log cabin in Milwaukee.

At that time it was a leather strap without teeth, quite rustic.
With the industrialization of production plants came the arrival of drive chains, until in 1980 HD surprised the motorcycle sector by launching the Sturgis model, the only one in the world with two toothed drive belts, both primary and secondary.

From that moment on, timing belts began to be used in more and more models, until 1991, when the Sportster model was the last to say goodbye to the classic chain.

Features and benefits
Since their first appearance, timing belts have always been viewed with some skepticism, especially by sportbike riders, as this type of secondary drive seems more fragile than a steel chain. This is only apparent, as their construction makes them truly strong and durable.
The core of these belts is made up of several aramid cords, a material as hard as steel but as flexible as fibers.

This material is what gives the belt its toughness (resistance to elongation). To further improve its characteristics, aramid is being replaced by carbon and Kevlar fibers.
This string core is covered on both sides with a layer of hardened polyurethane, the material from which the teeth are also made.
Finally, the teeth are coated with another layer of Teflon-based material for low friction.
As an example of its durability, you can lift an Electra Glide Ultra with just one aramid cord! Current belts have about 10 cords...

Maintenance and adjustments
One of the advantages of timing belts is their low maintenance, as they do not require any type of lubrication, are resistant to gasoline, oils, and chemicals, and perform well in extreme temperatures.
These belts do not have a mileage interval for replacement; they are replaced when they become worn. The main causes of belt failure are:
-Incorrect tension adjustment
-Misalignment
-Excessive particle drag

As you can see, except for the issue of particles (stones, sand, etc., depending on the environment you are driving in), everything has to do with proper maintenance.
If the belt is too tight it can break, if it is too loose the teeth and pulleys will break when decelerating.
To properly tension the belt, we must follow the instructions in the service manual for our year and model to the letter.

The belt should be checked and adjusted at room temperature (I have seen belts break due to tensioning them in the workshop with the heating at 20°C and then going out to ride on snowy roads).
It is mandatory to use the voltage testing tool (HD 35381 or similar), since our finger does not have the necessary precision.

The belt is checked by applying pressure from the tool and measuring the deflection or total movement of the belt. If this is not adequate, the wheel axle is loosened and the belt is tensioned by applying the same number of turns to the tensioners, or by applying the same amount of turn to the eccentrics, depending on the model.

We retighten the axle to the recommended torque (this step is very important to measure the tension correctly) and apply the testing tool again to see if everything is correct.

It is important to check the alignment. We will do this with a specific tool , to ensure that the distance between the rear wheel axle and the swingarm axle is the same on both sides of the motorcycle. This way, the belt will run completely parallel with the pulleys.

For cleaning, we will use neutral soap and inspect the belt on both sides to verify that there is no wear or breakage. We will also check the pulley, replacing it if it is excessively worn.

Let's run!
For those of you planning a long or adventurous trip, it's a good idea to know that HD sells an emergency strap kit that can get you out of a tight spot in a remote location.

So from now on, we're going to properly "tighten" our drive belt, precisely so that it stays in good condition and lasts for many years. This way, we'll only have to concentrate on what's really important: enjoying the road and the good vibes.

Frank Burguera

#harleymechanics