How Harley-Davidson Suspension Works: Improvements

The suspension's mission is to absorb terrain imperfections, allowing the tires to remain in contact with the ground most of the time, while maintaining chassis stability while optimally transferring traction and steering.

Components
The first "component" of damping when we hit a pothole is the tire, which deforms to absorb the unevenness; the force then passes through the rim to the fork legs, which are compressed by the springs installed inside, absorbing the energy. Once over the pothole, the springs extend again, ensuring the tire maintains as much contact with the ground as possible.
With this system alone, the spring would begin to oscillate, continuously releasing and absorbing energy, making driving more difficult. This is why suspensions (springs) often incorporate a damping system, which, through friction, diverts some of the accumulated energy, smoothing the travel and the compression and extension speeds.

In conventional motorcycle forks, this element is oil, which passes through a series of valves (holes) both when raising and lowering the fork leg, allowing for smooth and controlled movements.

System improvements
Although there are many ways to improve a motorcycle's front suspension, perhaps the simplest and most economical is to improve the quality of the fork oil and springs.
Oil degrades over time and absorbs residue and moisture, losing its properties. Depending on our driving style, we can replace it with a slightly thicker (but not too thick) oil that contains better additives.
Springs "fatigue" with prolonged use, becoming less firm. Replacing them with high-performance, progressively acting components (soft and "fast" at first, then firmer and harder at higher compressions) will provide greater safety and ride quality.

Case study
To illustrate the above, we will take advantage of the fork oil change on a Sportster model (which must be done every 32,000 km) and install a kit from the Dutch manufacturers Hyperpro , known for their high-tech damping systems for competition.

The kit consists of firmer progressive springs, with the specific length and hardness for each Harley-Davidson year and model, and higher density, high-performance oil.
We'll start as always by following the procedure indicated in the service manual to drain the oil, loosening the lower bolts on the fork legs (these bolts must be replaced with new ones).

Once the oil has been drained, raise the front wheel with a stand or jack, releasing the compression from the forks. Next, remove the upper fork caps using a special tool .

With the plugs out we can now remove the spacers

and the original dock.

If we compare both springs we can see the variation in thickness and "density" between the original and the high-performance springs.

After reinstalling the drain plugs, we'll place the new springs inside the legs (the part with the most spring turns facing down), followed by the original spacers. We'll fill the forks with the recommended amount of oil supplied (in this case, 342 ml per leg) and reinstall the top plugs, tightening them to 30 to 70 Nm.

Final tests
We can now lower the motorcycle from the lift to verify that everything is OK.

We will apply the front brake while simultaneously pressing the forks several times, allowing the oil to lubricate the entire system.
We'll perform a road test, verifying the improvements in suspension and braking, listening for any unusual noises. If upon returning, we see no excess oil in the upper fork legs, it means there are no leaks in the fork seals, so we can consider the job complete.

Frank Burguera

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