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Power Transfer to the Rear

by John Hildreth
HILDRETH PERFORMANCE

Let’s assume you have built a well tuned, sweet-sounding motor. Now you can spend some time getting all that positive energy back to the rear wheel. Remember, as horsepower and torque are increased, the efficiency of your clutch goes down. Sooner or later you’ll find that it’s time to enhance your clutch performance.

Stock clutch springs are designed to handle in the neighborhood of 50-60 horsepower and approximately 70-80 pound feet of torque. And they are designed to do it with a soft touch. In other words, they are meant to be user-friendly to all riding skill levels. However, this operational ease can create lag time between shifts and acceleration. If you’re in a hurry, such as at the drag strip, or if you find yourself stuck between two semis on I-65, acceleration is definitely your friend. The faster that clutch can hook-up and transfer the power to the transmission, the better life is going to be.

There are many packages available for clutch improvements, and upgrading your clutch performance can be a fairly inexpensive project or it can really dig into your wallet. Here are some economical ways to accomplish those improvements.

The first logical choice for upgrades is to increase the actual spring pressure on the clutch plates. There are many manufacturers of higher-pressure spring rates out there who offer OEM and aftermarket products for around twenty-five dollars. These spring rate can vary from a step up from stock or go all the way to “heavy-duty” springs. Any choice you make will be an improvement over stock in how fast the clutch is capable of coming to full engagement.

Another inexpensive upgrade for 1990-1997 Harley Big Twins (and 1991 and later Sportsters) is to replace the clutch spring plate with two steel plates and one fiber plate. This will increase clutch performance by more than twelve percent. Not too bad for a small investment of about fifty dollars in parts. Combine this with a higher performance compression spring, and you’ll have a clutch that engages quickly and with a very positive power transfer.

Upgrading your clutch will add a whole new dimension to your riding fun. However, you should expect to feel more resistance in pulling in the clutch lever when you upgrade the spring rate. If this becomes a problem to your riding comfort, there are four basic ways to reduce lever resistance. The first is to reduce the angle of degrees on the disengagement ramps. There are three pockets where the ball bearings rest on the ramp plates. For most bikes, the angle of degree of the sides of these pockets is 18 degrees. In 1990 only, H-D reduced that angle to 15 degrees, which in turn reduced the pressure required to pull in the clutch lever. You can think of this as the difference between walking up a shallow incline versus walking up a steep hill – the elevation change may be the same, but the distance traveled to get from the bottom to the top varies drastically. A problem with these shallow ramps is that full movement of the clutch lever is needed to get proper disengagement of the clutch plates. Running oversized grips, leather wrapped grips, or a leather wrapped clutch lever all reduce this travel. Hard shifting then results because of incomplete clutch disengagement.

The second way to upgrade is to install an Easy-Pull kit for about $15 plus labor. The kit increases the leverage on the disengagement ramp by lengthening the arm at the ramp. This gives similar results to replacing the ramps with a 1990 Big Twin type. With the increased leverage, you end up with less movement on the ramps, so once again you need the full movement of the lever. Be sure to tell your technician that the clutch adjustment should be made per the 1990 service manual specifications.

Another way is to install a spring assist on the clutch lever. The Power Clutch is a little over $100, and is similar to an archery compound bow. It uses a cam and a spring to reduce lever resistance after you make the initial movement of the lever. You will still encounter stiff resistance for the initial part of the lever travel, but after the cam-over assist comes into play, pulling the lever the rest of the way in and keeping the clutch disengaged is very easy. If you install both the Power Clutch and the Easy Pull, you will need almost no strength to use the clutch.

A more expensive way to reduce clutch lever resistance is to install a hydraulic clutch disengagement kit. Conversion kits are available from Harley-Davidson and aftermarket suppliers for a limited number of models. Check with your dealerships for parts and installation costs.

To get the most out of any of these upgrades, it is important that your clutch cable is in good condition, adjusted correctly and lubricated every 5,000 miles. This attention to maintenance will go a long ways to improving lever operation and reducing resistance.

A WORD OF CAUTION

Quick engagement clutches can make riding on wet pavement dangerous. Maintaining control in slippery conditions requires specific clutch lever management skills to keep from breaking the rear tire loose. Some riders enjoy the feeling of having that rear tire come around during acceleration, especially those with a racing background or extensive dirt bike riding experience. However, for many riders, this quick and positive engagement can cause panic and possibly loss of control of their bike. Clutch lever management skills can be learned and safely practiced in a rider’s safety course such as the Rider’s Edge classes offered through many H-D dealerships these days.

If you need more information about transferring power to the rear tire or have other performance questions, you can reach me via email at jhfxr@aol.com.


© 2004 Hildreth Performance

Last Updated on 06/14/2004