Rear Wheel Drive aka Secondary Drive: Sprockets and Chains, Pulleys and Belts for Harley-Davidsons

Since 1915 Harley-Davidson use 5/8” x 3/8” chains. This code means these chains measure 5/8” from pin center to pin center and fit onto sprockets with (nominal) width of 3/8”. According to modern terminology one would call it a 530 chain. Chains of this type were used on all Harley-davidson motorcycles form 1915-1984, and on Sportster models even including to 1990. Afterwards the maintenance- and lubrication-intensive chans were replaced by the more or less maintenance-free drive belts. But when it comes to stubble fields, gravely and muddy terrain, the W&W Cycles Wrecking Crew still opts for Tsubaki chains and PBI sprockets — and Dynamit belt-to-chain conversion kits.

Belts

When Harley-Davidson riders say belts these days, they mean drive belts. With the FX Sturgis model, Harley started the belt drive experiment in 1980 with full power, both for the primary drive and to the rear wheel. The belt in the primary has not caught on, as we all know. However, on the rear wheel, the belt was virtually “the best thing since sliced bread”. The drive belt runs quietly at the right tension, it is low-maintenance and does not require much care. So it's the right drive type for thousands of carefree miles. When it comes to belts, pulleys, belt drives and belt wheels, we rely on well-known brands such as BDL, Primo and S&S. They have the right stuff to make your own miles as carefree as those planned by the factory.

How to Speed Up a Harley: Altering the Secondary Transmission Ratio

Let's do a bit of physics! The smaller sprocket turns on the gearbox mainshaft, the larger sprocket is mounted on the rear wheel. We can determine the transmission ratio here with a simple calculation: rear sprocket / front sprocket = transmission ratio. So for a 1961 FLH the formula is: 51 teeth / 22 teeth = 2.31818..., rounded = 2.3. First conclusion, 2.3 drvie ratio means that the rear wheel turns 2.3 times slower than the transmission shaft. The faster your Harley goes, the more turns per second (minute, hour …) the rear wheel has to undergo. For that extra bit of top speed, the rear wheel has to spin even faster (= less slowly). So far, so logic, isn'it? Further logic tells us that the sprocket ratio must be smaller (=less). So let's convert the 2.3 ratio into a 2.0 ratio. The calculation is easy enough: 22 front sprocket teet * 2.0 = 44 rear sprocket teeth. Smaller rear sprockets result in higher top speed — purely in term of figures.

Why then some Harleys won't top up a couple of mph with less rear wheel sprocket teeth?

The reason therefore is a second phenomenon that is governed by transmission ratio, besides mere mathematical relations. Any tansmission ratio is not only a term for the difference of rotational speeds, but also of rotational torque. Torque is — in this equation — inversely proportional to speed. Means: if you increase rotation speed by lowering the ratio you will inevitably lessen the transmitted torque. Physics can be a pain in the a…, yes.

With a 51-tooth rear sprocket, the rear wheel turns more slowly but with more power. When comparing 51 teeth to 44 teeth, the torque increases by almost 16% at the same wheel speed. Conversely, if a smaller rear sprocket is installed, the power at the rear wheel decreases by these 16%. But your heavy-weight Harley needs a certain amount of power to reach a certain speed.

So it's quite possible that the bike will literally run out of power if the rear sprocket is excnanged for a significantly smaller one — and then the calculated higher top speed will never be realized.

Technical support

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