Clutch Tuning 101

Some racers tend to forget that the clutch is a vital component of a cars/truck setup. In this article we will brush up on the workings of a basic clutch system and the process by which you tune it. No R/C Cars or engines were hurt in the testing of these clutch setups. For the remainder of the article we will be referring to the clutch diagram just below this paragraph.

The Basics

The first step is to orient yourself with the clutch on your engine, lets start by removing the clutch bell from the engine. Remove the e-clip or the 3mm screw that holds the clutch bell in place. Once you remove the clutch bell from the pilot shaft you will have to remove the bearings. You might need to blast the front bearing with WD40 to help ease it over the shaft. Now you will probably be staring at the clutch shoes and the pilot bushing with the rearmost bearing still attached to the end of the motor. You can work on the clutch without removing the bearing but I suggest you slowly work the bearing out a little at a time, if the bearing is worn this might be a good time to replace it. A typical stock clutch shoe arrangement is depicted over item (1) in the picture. Note: This particular one happens to be the one from a Traxxas engine HPI clutches are similar. Some other clutch configuration use three or four small clutch shoes. These have a small spring that retracts each shoe toward the center of the pilot shaft. Last on the list is the CENTAX clutch: This clutch uses steel balls and an adjustable pressure plate. Only a few racecars have these types of highly adjustable clutches.

Clutch Tech. Talk

Clutch Operation:

R/C car engines rotate CCW (counter clock-wise) As the engine increases revolutions the leading edge of the clutch (picture 1) catches on the clutch bell and aggressively transmits the power from the engine to the clutch bell. This power is then distributed through out the car's drive train and eventually to the tires...

The stock clutch setup loads the engine with the strain of trying to accelerate 5+ pounds of car and at the same time the engine attempts to quickly accelerate to reach it's higher power band. 2-Stroke engines increase power output as the RPM’s increase. So if you want to use more of your engines available power when you need it the most (quick acceleration on the corners). You need to let the engine engage at a latter time when the RPM's are higher and when the engine has more available power to accelerate the vehicle.

For the clutch setup that is showed on picture (2) you pull the clutch shoes off the flywheel and flip them around and re-install. This mod basically changes the power transfer when accelerating because now the trailing edges of the shoes engage the clutch bell. This allows the clutch to slip a bit for the engine RPM’s to build and jump starts the torque level when accelerating from the corners. It is very important when you flip the shoes that you don't pinch the clutch spring with the flywheel pins. You can push a small allen wrench through the pin holes to push the spring out of the way as you press the shoes against the flywheel pins. You can push a small allen wrench through the pin holes to push the spring out of the way as you press the shoes against the flywheel. Reinstall the clutch bell parts in the order removed. Don't forget to use blue Loctite on the screw (if used) that holds the clutch bell assembly together. Only use Loctite on the screw it's not meant to be used as a bearing lubricant..

Clutch Tuning Theory:

There are two ways of doing this. The first is to change the stiffness of the clutch spring. The stiffer the spring the more force the clutch shoes will have to exert on the springs before the clutch shoes can swing open and engage the clutch bell. The second way of altering clutch engaging RPM is to make the clutch shoes lighter. By decreasing the mass of the clutch shoes the RPM need to be higher to equal the same force as before. This can be clearly be shown by the formula F=m*a (Force = Mass x Acceleration). The acceleration is dependent on change in RPM and if we hold the Force constant, we need to increase acceleration to compensate for a loss in mass. There is a third element in tuning the clutch and that is measured in slippage of the clutch. Among the vehicles that need this extra slippage are off-road trucks, which almost always have a slipper clutch mounted on the spur gear. If you don't have a slipper clutch and you force the clutch to slip excessively then you might over heat and melt the clutch shoes.

This brings us back to how to reduce the mass of the clutch shoes. The easiest way of reducing the mass of the clutch shoe is by removing material at the end opposite to the pivot point. In this case the pivot point is the pin on the fly wheel. so to lighten up a clutch shoe all we need is to drill 1/8 holes on the swinging end of the shoe. This is illustrated by the white holes on the pics(2-4). You could cut the entire tip of the clutch shoe but this may cause the clutch to slip even when it should be fully engaged. I don't recommend this technique for the beginner because it's easier to mess up. The good thing is a new factory clutch and spring kit will typically go for less than $10.00 that's not bad for experimentation expenses.

Clutch Performance

Optimizing the variables:

How do you know which setup is right for you? Read on. The variables that affect the clutch tuning are Traction, Gearing, Motor HP and Driving style. If you want the car to accelerate aggressively you need to probably get a taller gear ratio(more bottom end) and have one or more extra holes on your clutch shoes. If you have a 1 HP engine you might do one but not both or you could go with no extra holes on the clutch shoes. What I'm trying to say is that you need to experiment. Make one change and try the car out. If you need more punch out of the corners, lighten the clutch if the clutch is slipping then you've gone to far. What I can say is that by tuning the clutch you can increase acceleration dramatically. On my on-road car with the clutch tuned the engine accelerates the car with so much authority that I can't hardly pickup the change to 2nd speed and there is no lag in power delivery. Tuning the clutch has to be close to # 1 on the performance for the money list.

Aftermarket Clutches:

On an added note, MIP offers aftermarket clutches for most nitro r/c vehicles. Most of the time just swapping to the MIP clutch can make a noticeable difference in performance. Some of the MIP clutch kits have extra pivot holes closer to the middle of the clutch shoe. This lets you change the pivot point of the shoes and alter the way the shoes contact the clutch bell. You basically can tune the take-off on the fly without having to purchase or add other components to the clutch system. Most of the time you can modify the stock shoes when applying the tuning aids shown on this article and come out with good results. There will be times when you will need the stronger spring and different shoe compound offered by the MIP kits. We can't finish the article without mentioning that other companies(Duratrax,Kyosho,Ofna and others...) offer 3 and 4 shoe clutch conversions and a few offer Centax clutch upgrades. More expensive doesn’t mean that it will be better for your type of racing. I sure haven’t seen a lot of Centax clutches on wining Off-road trucks. You will have to do some testing to find out which one's best for you.

Good Luck!

Have fun and whoop some RC car butt!!!

Eric Perez

Team NitroRC.com