Safer At Any Speed Part 2:
Clutches, Flywheels and Bellhousings
By Brion Gluck
Everybody knows that harnesses and roll cages are important pieces of safety equipment. They’re hard to miss, since you strap them on or crawl over them every time you drive. It’s easy to miss the things you never see, the things buried inside the other mechanical systems of your race car.
Tops on the list of easily overlooked safety items are probably clutches and flywheels. Think about it for a moment; you’ve got a clutch, pressure plate, and flywheel assembly spinning at several thousand RPM. If any of this rotating mass lets loose, the only thing between you and flying metal is a thin bell housing and an even thinner floor pan.
Take my word for it, I’ve been there. At Road Atlanta in 1994 I was leaving turn 12 when the clutch and lightweight aluminum bellhousing grenaded into shrapnel. One header was cut in half at the collector, the power brake booster received a golf-ball-sized dent, and pieces of pressure plate embedded themselves into the track and a Porsche behind me. I was lucky I didn’t walk away with some of pieces embedded in me, too.
Upgrading the driveline components and adding a containment bellhousing will greatly increase the safety of your car no matter whether you’re a road-racer or drag racer. At a minimum, all these components should be certified by the SFI Foundation for their intended use. Parts intended for a nitro-methane drag car need to meet higher standards than those for a car that will be road-raced, for instance.
The SFI website, http://www.sfifoundation.com, has information on the standards and links or phone numbers for vendors who sell compliant parts.
Containment bellhousings need to be recertified by the manufacturer every five years, or two years for nitro-methane-fueled, alcohol-fueled, supercharged or nitrous-equipped cars.
Single-disc clutches don’t need to be recertified, but multiple disc units have only two years of certified life, and those for the nitro and alcohol cars need a new sticker every year to remain compliant.
What’s the difference between OEM parts and SFI-approved ones? SFI-certified parts have been tested using uniform procedures to meet a set of minimum standards that are typically higher than OEM, and often much higher than aftermarket parts store parts can meet.
“The test standards basically are, every piece of the puzzle has to be tested, ” said David Norton of Star Performance Engineered Clutches (SPEC). “The flywheel has to be tested, the pressure plate has to be tested with the cover attached, and the disc has to be tested.”
Two types of tests are performed. One, called the breaker bar test, uses a tensile test machine that is capable of applying a particular tensile load. The load is increased until the part breaks, and the load and elongation of the material are measured. For example, for a ductile iron component, the minimum tensile strength is somewhere around 55-60,000 psi to get SFI approval.
Another test ensures that the clutch and flywheel can safely be used at 150 percent of the engine’s original rev-limit. If a factory redline is a 6250, the parts must be safe at 9375 RPM to get the SFI sticker.
The clutch, pressure plate and throwout bearing should be treated as a matched set, Norton said.
“Don’t try to mix a performance disc with an OEM pressure plate.”
Many off-the-shelf performance clutches are not designed or tested for racing applications. The materials used can’t withstand the RPMs that are often seen in a racing application and they can’t withstand the heat, according to Norton. Typically, they’re not designed to handle more than 25 percent more torque capacity than an OEM unit.
A real racing clutch will be built to handle the heat and torque of sustained racing, and will typically be made of fewer pieces of metal instead of several pieces riveted together with all their attendant weak points.
Springs are important for street drivability; without them, the clutch disc will enage immediately like an on/off siwtch and make it hard to keep the clutch from chattering. Rigid discs without springs are lighter and give a sharper engagement through the gears, but SPEC’s Norton recommends them for track applications only. If the car will see any time on the street at all, a sprung hub is called for.
When replacing a clutch and pressure plate, don’t forget to replace the pilot bushing and release bearing at the same time. “It’s very important, and a lot of times they go untouched,” Norton said.
Also make sure to replace flywheel and clutch cover bolts with correct shouldered hardware of at least Grade 8 or OEM quality.
A containment bellhousing should be considered a necessity whenever the revs are going to exceed the OEM redline. “A scattershield is the best way to be safe,” Norton said. Even SFI-approved rotating parts can break or be installed improperly, and the thick, heavy bellhousings can keep the explosion confined.
Star Preformance Engineered Clutches
3420 Davey Allison Dlvd
Johns Mustangs and Classics
2277 National Ave
San Diego,CA 92113
Transmission Technologies Corporation
248 471 2421
1600 Sierra Madre Circle
Placentia, CA 92670
465 Memorial Drive SE
Atlanta, GA 30312
Mr. Gasket Co.,
10601 Memphis Ave,. #12
Cleveland, OH 44144