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Understanding Clutch Tuning

Clutch tuning, although potentially complex, is easy once you recognize a few simple realities of nitro cars:
  • Nitro clutches will deliver progressively more torque to the clutch bell as RPM rises.
  • Generally, higher spring tension will cause initial engagement to begin at a relatively higher RPM.
  • A clutch is fully engaged when the motor RPM equals the clutch bell RPM.
  • Nitro motors produce very little if any usable power at idle and low rpm levels.
  • Power output rises with RPM (to the limits that clutch tuning is significant).
  • Nitro motors under throttle will always rise in RPM as quickly and as much as they are able, given the current load.
  • A nitro motor that is overloaded will stall, stumble, or rise very slowly in RPM.
  • Nitro cars can use only so much power at any one time for acceleration.
  • The amount of power that can be used for acceleration rises with speed.
  • Exceeding this amount of power will cause a loss of traction and effectively decrease the amount of power that the tires can handle, hurting acceleration and control.
  • Using too little power will produce less than optimal acceleration.
The best acceleration will occur when the maximum possible USABLE power is transferred to the tires from the very first moment and at every successive point during acceleration.

The goal of clutch tuning is to adjust the torque delivery to accomplish this goal.

An optimally tuned clutch will:
  • Respond quickly to the throttle.
  • Begin to engage at an RPM where the motor produces sufficient power to deliver usable torque.
  • Allow the motor to progressively increase in RPM and power output to match the car's acceleration.
  • Progressively deliver more torque to the drive-train as the car's increasing speed allows it to handle additional power.
  • Not deliver too much torque so as to disrupt traction and reduce acceleration.
  • Fully engage at the earliest appropriate RPM level (considering available power and traction).
For tuning purposes we can break down clutch setups into just 2 conditions:

Delivering too much power and Delivering too little power at the immediate moment.

Recognizing the symptoms of each condition is an important skill for proper clutch tuning. The visibility of the symptoms, and the related loss of acceleration and control, will increase with their severity.

Too much power:

Too much power causes High RPM LAG: A LAG occurs when excessive power causes a break in traction but the motor continues to rise in RPM. This break in traction reduces the car's ability to accelerate while the motor produces even more power and the clutch delivers still more torque (and may become fully engaged). The RPM gap between the motor and the vehicle's speed widens until enough of the excess power is wasted through friction that the RPM stabilizes. The amount of time that the motor remains at this static RPM level will indicate the severity of the LAG. The condition is detected by listening to the sound or tone of the motor, which will remain constant even as the car continues to accelerate, although at a diminished rate. The most severe lag will be evidenced by ballooning tires and very poor acceleration.

Too little power (overloading the motor):

Too little power causes Low RPM STUMBLE: The motor fails to smoothly increase in RPM from the initial application of the throttle. It may be very brief, or in severe cases can cause the motor to stall. This condition typically results from too great a draw on the motor in the lower RPM range and is most likely to be experienced when traction is very good or the vehicle is heavy. There are two kinds of Low RPM Stumble: Static and Dynamic. A STATIC STUMBLE occurs when the clutch is fully engaged at a low RPM and the motor has insufficient power to accelerate quickly. This condition can be identified by a slow acceleration accompanied by a matched rise in the RPM. A DYNAMIC STUMBLE occurs when the clutch has a comparatively higher engagement RPM, but as it starts to engage, the power absorbed between acceleration and the friction of the clutch is enough to prevent the RPM from rising for a noticeable period of time. This can be identified by a rise to a relatively low but static RPM level with the vehicle accelerating under positive traction (little or no tire slippage). This condition is similar to a High RPM Lag but in this scenario it is the clutch, not the tires, that is slipping and absorbing the wasted power.

Tuning Guidelines

To Correct a LAG:

A lag can be substantially diminished or eliminated by drawing more power from the motor at a lower RPM. This accomplishes 2 important goals:

1. Power draw at a lower RPM will slow the motors rise to higher power levels.
2. The power drawn will begin accelerating the car earlier, which increases its speed and increases the cars ability to use additional power.

These 2 factors will work together to reduce the probability of spontaneously breaking traction and experiencing a LAG.

To draw more power at a lower RPM level you must allow one or more shoes to begin engaging at a lower RPM. You can begin by reducing the tension (turning screw out) of one shoe out by ½ to 1 turn and observing the effects. Continue to reduce the tension to further improve the LAG. Try to balance the reduction in spring tension between the three shoes.

The following table shows a typical adjustment scenario to correct a substantial LAG. Actual change is in parentheses.

 Initial Level
(turns in)
1st Adj.2nd Adj.3rd Adj.4th Adj.5th Adj.
Spring 11 (-½)½ (-½)½ (0)½ (0)½ (0)
Spring 22 (-½)1½ (-½)1 (-½)1 (0)1 (0)
Spring 333 (0)2½ (-½)2½ (0)2 (-½)1¾ (-¼)

It only takes a few moments to make adjustments, so you should be running the car after every change to re-evaluate the performance.

As you lower the engagement RPM it should gradually decrease the time and severity of the LAG, which will be evident in the SOUND of the motor. You should also see acceleration improving as you move towards power levels that maximize acceleration without disrupting traction.

It is important to remember that tuning your clutch does not eliminate the need for throttle control, especially in low traction conditions. In these cases a properly tuned clutch will be far more forgiving of excessive power and lead to more consistent acceleration and superior control.

Be certain to balance the power delivery and not draw too much power at a low RPM level or you will begin to see a STUMBLE.

To Correct a STUMBLE:

A stumble can be improved or eliminated by drawing less power from the motor at low RPM. Reducing the load on the motor will allow it to rise to a higher RPM where additional power is available for acceleration.

To draw less power you need to raise the engagement RPM by increasing the spring tension. Just as in the process of correcting a LAG, this should be done gradually and in a balanced manner. The following table shows a typical adjustment scenario to correct a substantial STUMBLE. Actual change is in parentheses.

 Initial Level
(turns in)
1st Adj.2nd Adj.3rd Adj.4th Adj.5th Adj.
Spring 1¾1 (+¼)1 (0)1 (0)1¼ (+¼)1¼ (0)
Spring 21½ (0)1¾ (+¼)1¾ (0)1¾ (0)2 (+¼)
Spring 311½ (+½)2 (+½)2¼ (+¼)2½ (+¼)2¾ (+¼)

It only takes a few moments to make adjustments, so you should be running the car after every change to re-evaluate the performance.

It's important to remember that as you increase the RPM where the clutch delivers power to the car you are also increasing the RPM gap that exists between the clutch shoes and the clutch bell. This gap must be closed by the action of the clutch.

In the case of a heavy car or a motor that is relatively less powerful, pushing the engagement RPM too high into the RPM range may improve acceleration, but at the expense of excessive clutch wear. Excessive clutch slippage also wastes power which may otherwise be used for acceleration. This condition will present itself as a DYNAMIC STUMBLE. Care should also be taken to avoid pushing the RPM levels to a point where traction is exceeded and a LAG begins to develop.

Keep in mind that these scenarios are only illustrations to show the methodology of adjustment. In most cases you will not make such a large change to the overall engagement profile, unless you are completely changing the terrain, like moving from dirt to asphalt or the overall conditions are quite different from the last time you drove.

Throttle Lag:

Throttle lag is the period of time between the carburetor physically opening and the point when the tires begin turning. Throttle lag results from a clutch setup where the RPM must rise to greater a degree before any torque is transferred to the tires. This condition is quite common when traction conditions are very good, and drivers respond by using stiffer springs in their clutch setup. Although usually brief, lasting just a few tenths of a second, throttle lag can have a substantial cumulative effect on lap times. It also makes driving more difficult. With the recognition of the delay between throttle application and actual acceleration, a driver must either accept the delay or adapt by anticipating the need for power and applying the throttle slightly early. Because the throttle is the opposite of the brake, this interferes with breaking. The driver is faced with the choice of either aggressively entering turns with unencumbered breaking or quickly accelerating out of corners, but not both.

The easiest way to reduce throttle lag, and also improve acceleration overall, is to stagger the engagement of the shoes. By having one shoe engage as early as possible, without overloading the motor, you can start the car moving quickly, which also increases the cars ability to use power as you continue accelerating. Even though a motor may not produce a substantial amount of power at the lowest RPM ranges, it's beneficial to begin drawing power for acceleration as soon as possible without overloading the motor.

Acceleration is an Ongoing Process.

Keep in mind that acceleration is an ongoing process and it's possible to see a STUMBLE and a LAG in the same car. It's entirely possible that a clutch which positively engages at a low RPM will cause an immediate STUMBLE. Because of the lack of power and poor acceleration, the driver will maintain full throttle. As the RPM rises, the power output may abruptly increase, breaking traction and causing a LAG. A controlled delivery of torque would prevent the motor from being overloaded, allowing the car to accelerate quickly from the start. The increased speed would improve the tires' capacity to handle the added power as the motor's RPM increased.

The most effective way to tune your clutch is to observe your car throughout the entire acceleration process, watching and listening for points where you could use more power, or have too much.

For most if not all situations, a staggered engagement of the shoes will provide the best performance. The exact balance of engagement can ultimately only be determined by the driver, and will be dependent upon the current setup and conditions. Still, some generalizations can be made to guide tuning:

"Typical" low traction spring tensions:

Spring 1: +½ turn
Spring 2: +1 turn
Spring 3: +1½ turns

Low traction conditions will benefit from a tight and relatively early engagement that gets the car moving to speeds where more power can be used and also prevents the motor from revving up to a high RPM too quickly.

"Typical" high traction spring tensions:

Spring 1: +1¼ turns
Spring 2: +2½ turns
Spring 3: +3½ turns

For high traction conditions the motor must be allowed to move through the rpm range more easily so it's capable of delivering the additional power. Notice that the lowest shoe in this scenario still engages relatively early, at only 1¼ turns, decreasing the likelihood of experiencing throttle lag.

Proper engagement will result in smooth acceleration under positive traction with a progressive increase in the motor RPM.

Remember that these setups are examples. In reality, the only way to know what profile is best for your particular vehicle is to test and tune. The spring tension may be widely spaced, narrowly spaced or some other combination. As you gain experience you will also most likely make adjustments of less than ¼ turn.

If you have any questions regarding tuning you can always contact Buku at 877-285-8769 or email techsupport@bukupower.com.
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