Redline Motive

Spring vs. Solid Center Discs – And the Verdict is…

We hear all kinds of ideas about what makes a spring center disc or solid center disc better. In this article, we will clear up some of the myths and mystery so you can better understand the differences and what clutch is best suited for you or your customers.

The primary purpose for the springs in the clutch disc is to dampen out these engine’s torsional vibrations in order to reduce these annoying gear noises within the transmission. But there is no free lunch and the explanation doesn’t stop there. After discussing some of the common myths, allow us to detail the specific benefits and tradeoffs to both spring center discs and solid center discs and what to expect from your next clutch. When an engine is running, especially a high performance engine, the crankshaft and rotating assembly experiences violent torsional vibration (rotational pulses) due to the combustion firing and other factors. These torsional vibrations will transmit through everything downstream of the engine unless properly dampened out. Since there is backlash between the transmission gears, resulting gear rattle noise becomes a main area of concern to the vehicle manufacture. Through careful consideration and testing of the torsional dampening system car makers achieve what is considered acceptable for each vehicle.

Some common myths:

Myth – Solid center discs wear out faster than spring center discs
Truth – The friction material will last the same on either disc. The splines of a solid center disc may wear faster on some applications due to the absence of torsional dampening. It is possible to reduce the wear on the splines by increasing the hardness of the hub. But since something has to give, the input shaft would wear instead, which is unacceptable. On the other hand, a spring center disc has many moving parts that a solid center disc does not have, which are all prone to wear.

Myth – A spring center disc will prevent transmission damage.
Truth – Transmission damage comes in many forms. The spring center may help prevent or contribute to transmission damage, depending on what form of damage you are dealing with. For instance, if you dump the clutch such as in a drag race launch, a spring center hub assembly will quickly bottom out due to the inertia far exceeding the torque capacity of the spring center, which results in full impact on the driveline. If transmission breakage is occurring due to the extreme torque and inertia, the springs will not reduce breakage. Such as the case of this twisted input shaft below taken out of our clutch testing Camaro.

Although a spring center may provide some cushioning, it is also heavier, adding to the inertia of the disc. With this additional inertia you can expect more wear on synchronizers, especially if you are demanding quick shifting. On the other hand, we have seen the torsional dampening increase transmission life on some Mazda road race cars but in other cars it made no difference at all. So in our judgment the truth of this myth depends on the inherent weakness of the particular transmission for your application. If the transmission for your potential drag car has strong heavy gears but weak synchronizers and shift forks, then a spring center disc is only going to cause trouble. In contrast, if you are building a high powered road race car with a very light under-designed transmission, a spring centered disc may help you finish the race season without rebuilding the transmission.

Myth – A solid center disc will cause clutch chatter (harsh engagement)
Truth – Clutch chatter (harsh engagement) is when the car shakes or shudders violently upon engagement. A solid center disc will not increase clutch chatter. When everything is working correctly, clutch chatter is most often caused by the use of an aggressive friction material, such as ceramic, cerametallic, bronze, or sintered iron. For this reason we warn our customers that our puck racing discs engage harshly. In our testing we did notice that a solid center disc was less forgiving in the gear changes. In other words, if you smoothly shift as you would in normal driving, you probably wouldn’t notice any difference, but if you are WOT (wide open throttle) shifting from one gear to the next, engagement is more immediate and violent with a solid center disc. Engagement quickness should not be confused with clutch chatter. Clutch chatter varies on every application and can also be caused by one of several malfunctions within the clutch system. The design of engine mounts and suspension components, improper flywheel surfacing all can contribute to clutch chatter just to name three. Stay tuned for more discussion in later newsletters.

Myth – A spring center disc will eliminate gear rattle while a solid center disc is always noisy.
Truth – It is true that a spring center disc will reduce gear rattle noise but is a mistake to use terms like “eliminate” or “always” since there is such a broad spectrum of conditions that affect the outcome. Gear rattle comes in many different forms and severities. Expect a more in-depth discussion about torsional vibration and gear rattle in later articles. In some cases a solid center disc will be acceptable and it other cases even a spring center disc may be very loud.

Here are two recent examples we encountered: When testing our new twin disc clutch for the Corvette Z06 which features two solid center discs and a light solid flywheel, we expected some gear rattle but we were pleasantly surprised at the extremely quiet operation. We only really encountered any significant gear rattle while lugging the engine at very low rpm in first gear.

In contrast, recent testing on a BMW E46 produced severe gear rattle whenever we replaced the dual mass flywheel with a lightened solid flywheel. Of course there was an increase in throttle response due to the light flywheel, but even when using a spring center disc we found the resulting gear noise objectionable. Further testing on these cars is ongoing to find a good alternative to the dual mass flywheel without suffering from gear rattle noise. In some cases, extensive testing and proper tuning of the torsional dampening under various conditions is the only way to bring gear noise to desirable levels.

To sum it up, here are the main benefits and tradeoffs to a spring center disc: Benefits to spring center disc:
• Quieter operation – Reduces or possibly eliminates different types of gear noise.
• Longer spline life of the clutch disc
• Potentially less violent engagement during gear changes
• Absorbs some of the shock load from the drivetrain

Tradeoffs to a spring center disc:
• Harder on synchronizers when attempting fast shifting due to added weight
• More moving parts and more complicated = More to go wrong
• Slightly less of a positive feel between the driver and the road
• More expensive

[Source: Advanced Clutch Technology]


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