by Archie Bosman:
Most high-performance flywheels are created in billet steel or in billet aluminum. The advantage of the billet steel over the original cast iron unit is that, though they weigh the same, usually around 32lbs, the billet steel unit is significantly stronger. Crucially, it remains free of stress cracks and, therefore, safe at high engine speeds and higher clutch clamping forces. In addition billet steel flywheels operate more effectively with modern high-performance clutch friction materials than do their cast-iron counterparts.
|In comparison, the billet aluminum flywheel has the decisive advantage of carrying minimal weight. Generally it rids itself of half its mass—often tipping the scales at around 16lbs. Lower mass means a lower moment of inertia, which translates to faster response; that is, faster acceleration and deceleration as well as less wheel spin.
Clutch and flywheel maker Ram of Columbia, SC construct their aluminum flywheels from 6061-T6.To avoid distortion they use a 1/4in thick steel insert as a friction surface, which mates with the clutch disc friction surface. To maintain flatness the inserts are fastened to the flywheels by 18 rivets.To overcome clutch snatch or chatter and provide minimal run-out, both steel and aluminum flywheels are Blanchard-ground. The Blanchard grinder contributes a proper friction surface and also ensures the flywheel is ground parallel to the crankshaft mounting flange. In addition, Ram counter-bores the flywheels for button-head cap screws that secure the starter gear ring to the flywheel.Nonetheless, the lightness of the flywheel must be considered in conjunction with the vehicle’s gear ratio and its overall weight. If its gearing is too high and its flywheel too light the engine may be unable to sustain its momentum, causing the car to “bog”. But on a properly geared vehicle, that is one with a low gear that invites the use of a light flywheel, it will perform with consistent brilliance. The formula for matching gearing to flywheel weight is usually around 11 or 12:1. To calculate your drive ratio multiply your lowest transmission gear by your rear gear. For instance, if your low gear is 3.00:1 and your rear gear is 4.10:1 your drive ratio is 12.3:1. Lower drive ratios usually require more flywheel mass to launch the car effectively. Accordingly, if the car weight remains unchanged and the drive ratio is not 12.3:1 but instead 10.0:1 greater flywheel mass might be required.What’s SFI certification?The official sticker tells you that the materials used in the manufacture of the flywheel have been certified by the SFI. Here’s an interesting story about drag racing flywheels before the influence of SFI.Menacing repercussions in earlier times before SFIOne evening in September, 1959, 22-year-old drag racer Tom “Mongoo$e” McEwen rolled into Lions Drag Strip near Los Angeles, CA with his 1948 Fiat Topolino. It was fitted with a 340cu in blown Chrysler Hemi gasser engine. In those days drag racers ran cast aluminum flywheels as billet aluminum flywheels were not readily available and SFI certifications were non-existent.In McEwen’s 3-speed La Salle transmission he discovered that second gear was broken and he didn’t have a spare. The second gear in the LaSalle was the starting gear. So, he determined to leave the line in high gear by drenching his M&H rear slicks with water and inflating them to 30psi.Earlier the Topolino had run almost 130mph. But on this night, though its elapsed time was impeded, the Topolino with its wet M&H slicks and smoking half-way down the track set a record speed of just over 139mph—the fastest ever Gas Altered speed recorded. It had picked-up 10mph! Mongoo$e duly returned to the starting line to back-up his time, “And at about 900 to 1,000ft, says McEwen, “the cast aluminum flywheel comes apart. It came out of there like shrapnel—it almost cut that car in half, exploding all around me. I still don’t know how it missed my legs and ankles.”Source