Adjustment Tips For Harley Drive Belts

Adjustment Tips For Harley Drive BeltsIf you have a 3″ belt open primary and you find tranny oil on the pavement where you stop your bike it could mean that you are loading the tranny’s bearings. Your belt may be too tight.

When properly adjusted, the primary belt should have nearly 3/4″ free play when cold. As the motor warms up and you use the clutch the aluminum pulleys will expand and the belt will tighten. When fully hot the belt should still have some free play, but not as much as when cold.

If the pulleys are not properly aligned, the belt will tend to crawl off or rub against the sleeve on one side or the other. Fortunately, when the clutch basket pulley is properly square with the drive pulley the wide belt will easily keep its position when loose or tight.

The important thing to remember is that if the belt is as tight as a drum it will overload the bearings and you will eventually have a fluid leak that won’t stop, and certain damage to the bearings.

Final Drive Ratio

Find that you just can’t get good gas mileage no matter how you adjust your carb? Does your stroker need to be revved up to 4000 rpm just to go 75 mph? You may need to take a look at your Final Drive Ratio!

The Final Drive Ratio (FDR) is the PRODUCT of the ratios of your primary drive sprockets (or pulleys) and the ratio of your secondary (or final) drive sprockets (or pulleys). In a formula, this can be expressed as (T/E)*(W/C), where T is Transmission sprocket circumference, E is Engine sprocket circumference, W is rear Wheel circumference and C is Countershaft circumference. Each circumference is measured in sprocket or pulley teeth.

Given a typical setup, the stock configuration is usually very near 1.5 for the primary, and around 2.2 for the secondary. The FDR for a stock bike might be around 3.3. But wait a minute, you’re running a stroker motor. You should know that you don’t want to run anywhere near redline, which happens to be at 4,500 rpm.

You want to stay below 4,000 rpm to avoid unusual wear and tear (like leaving aluminum on the walls of your cylinders). A stroker wants to run with about a 2 ratio for the secondary. This means a FDR of about 3. This way you will keep the revs down and make use of all that torque. For a typical setup you will be revving at about 2500 rpm when going 60 mph. Now, these numbers are highly general, and depend on lots of factors, not least of which is the diameter of your rear wheel.

Be CAREFUL not to get the FDR much below 3.0, as this will put too much stress on the drive train components.

A 10% decrease in the FDR may translate into about a 10% increase in your gas mileage, but not necessarily. Turns out that the energy required to move your bike is the same regardless of what your drive ratio may be. What this means is that the fuel that is burned will be about the same to go a given distance, no matter how much mechanical advantage you obtain. However, there will may be SOME change in your gas consumption. It really depends on the changes you make to your riding style based on the new sprocket ratio.

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  1. Rob Fleming - August 31, 2011

    New blog post: Adjustment Tips For Harley Drive Belts

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