At 23.75 pounds, the stock 5.0 flywheel was a monster...
Track Time
Cordova Raceway near Moline, Illinois, was the venue for our track testing, and thanks must go to track owner/manager Scott Gardener and his management crew for catering so well to our needs. The main focus of our trackside attention was on the street tire-equipped factory-stock machine because: a) The higher percentage of released energy at the shift, and b) It was equipped with an Auto Meter data-acquisition system.
Experience shows that with the heavier of the clutch and flywheel assemblies being tested, best 60-foot times were achieved by leaving with 3,000 rpm on the tach. As the Auto Meter data-acquisition system showed, any more than 3,000 rpm, the tires would momentarily break loose; any less, and the motor would go into an almost imperceptible bog. After making five consistent runs, the numbers were averaged out to show the factory stocker was running 12.390 seconds at 109.757 mph.
...compared to the svelte 10.8 pounds of this Fidanza flywheel.
Less Mass: The Results
After the lighter clutch/flywheel assembly was installed into the factory stocker, it was evident that launch tactics had to be revised. The e.t. slip told us the average 60-foot time had increased. The Auto Meter data-acquisition system showed engine rpm was dropping far more during the clutch engagement period. The fix was obviously more rpm at the launch. Working this up to an optimum figure produced a 3,500-rpm launch instead of the original 3,000. The results of these tests are shown in Fig. 7. As you can see, the lighter clutch/flywheel with a higher launch rpm won out everywhere except, for some unknown reason, the eighth-mile speed. Over a tenth was shaved off the e.t. and the trap speed was up by a little over six tenths of a mph, proving (in this case) lighter is better!
When baselined with the heavy flywheel, Craig Baldwin's Vortech-blown machine's best start-line rpm proved to be 4,600. The average of three close runs produced a 10.863 e.t. With the lighter flywheel installed and a new optimum launch rpm of 5,000, the three-run e.t. average dropped to 10.725. That was more than significant by itself, but Fig. 8 reveals more. These more-comprehensive numbers showed the trap speed did not follow in step with the e.t. improvements, but actually dropped almost 1.5 mph. Although the time for testing was over, the cause for the car to slow past the 1,000-foot mark was investigated. Heat buildup from so many runs was suspected. After cooling for about an hour, a blistering 132-mph pass was made, indicating that our tests, though very much positive, had not pulled the best possible numbers from the lightweight clutch/flywheel setup.
FIG. 5Here are the dyno tests of the factory-stock 5.0. This shows the light flywheel as superior everywhere other than during the shift phase (arrowed green). For about a tenth of a second during the shift phase, the heavy flywheel delivered some 40 hp more than the light one.
Before we wrap up this section on flywheels, it's worth looking at just how much weight we would have to remove from the car itself to equal the effect of the 20-pound-lighter flywheel. If the heavy flywheel was retained, then the car--to equal the First-gear acceleration given by the light flywheel--would have to be reduced by 272 pounds. The equivalency figure for Second gear would be 187 pounds; Third, 127 pounds; and Fourth, 98 pounds. At this point, we think we can safely say lighter is better!
Item 5: The Other Flywheel
No matter what rules seem to apply to a given situation, there always seems to be an exception. The moment of inertia of an object up at the front end of the engine is just such an exception. It's called a crankshaft tortional vibration damper, or crank damper for short. Back in the '80s, even top drag racers were substituting heavy crank dampers for lightweight aluminum "hubs" in the belief they would go faster. The reality is these hubs, although having a vastly lower moment of inertia, actually cut power, even under high-acceleration conditions. The reason they did so is that they did not suppress crank tortional flexure. Such vibrations were transmitted directly to the cam via the timing chain and, as a result, the valve event timing was adversely affected and valve bounce was aggravated. All this led to a reduction in power, and I'm not talking .5 to 1 hp here, but sometimes 12 to 14 hp on a 400-odd-hp engine. The bottom line here is, use a functional damper first and worry about its moment of inertia last. That way your motor will make more horsepower, longer.
FIG. 6Shown here are the results from the Vortech-blown car. Note that due to a slightly higher shift point, the advantage of the heavy flywheel over the light one is diminished as shown by the area indicated by the green arrow.]
Items 6 and 7: Drive Shafts and Axles
This section will be short and sweet. The driveshaft is such a small diameter that if a steel one was replaced with one having zero mass, the increase in driving-wheel horsepower in First gear would be barely perceptible. As a performance-enhancing device, it has near-zero value. Where it does score is the fact that rear unsprung weight is reduced by about half the weight reduction of the driveshaft itself. The same applies to lightweight gun-drilled axles, except all the weight saving is realized as unsprung mass.
Item 8: Lightweight Wheels and the Big Payoff
Let's start with wheel moment of inertia first, because the tire moment of inertia situation is one where we have less room to maneuver, thereby making our choice much simpler.
Driver consistency is the key to good testing. Run after run, Craig Baldwin would catch 6 inches of air at the launch.
The mass of a live rear axle is one of the most offensive aspects of a production-based, Detroit-built, high-performance street or race car. Using a 5.0 Mustang as an example, the axle weighs in at about 220 pounds with brakes, but less wheels and tires. A set of wider wheels and tires bought without due consideration to how much they may weigh can add a further 100 pounds to this already-burdensome figure. This totals some 320 pounds of unsprung weight at the back. If we add to this about 80 pounds a side for the front suspension, we can see that of the car's 3,200 pounds stock weight, some 480 pounds, or 15 percent, is unsprung. One of the first steps many will take toward improving the car's track-day performance or race worthiness is to lighten it. Sticking with our Mustang, we find that by the time the heater and A/C are removed, along with the 5-mph crash bumpers, door bars, sound deadening, and back seat, about 400 pounds have come out of the car. If no attention has been paid to the unsprung weight, the ratio of one to the other has deteriorated to well over 17 percent. This means we need to give some serious attention to unsprung weight. The greater the unsprung weight, the greater the amount of energy transmitted to the chassis when riding an uneven surface. This reduces grip during acceleration, braking, and cornering modes. It also cuts the effectiveness of any anti-squat that may be built into the rear suspension system. This make the 60-ft times longer at the dragstrip.
There is more to lightweight wheels than just the simple, but nonetheless important, reduction of moment of inertia due to rotation. Most PHR readers drive cars with a lot of unsprung weight in the form of a heavy rear axle. This can have a lateral moment of inertia that can, under certain commonly occurring conditions, significantly affect road holding and handling. Here is a quick rundown: The worst kind of bump a live axle has to deal with is the type where both wheels hit at once, as per a speed bump. This is bad because the entire mass of the axle/wheel assembly is involved. A reduction of unsprung weight here is worthwhile, but to far less of an extent than if just one wheel (as is more commonly the case) of the axle pair hits the bump. When this is the case (as per Fig. 9), the axle looks like a long arm with its center of rotation at the contact patch of the opposite wheel. Although most of the axle mass is in and around the diff housing, this is only half the distance from the center the axle is rotating about. The wheel and tire assembly, though, is right out as far as it can be from the rotation center. What this means in such a bump scenario, is that taking a couple of pounds out of the wheel does far more than just the percentage the unsprung weight is dropped by. Just how much effect wheel-mass reduction has in this instance is dependent on the severity of the bump. But on average, you can safely assume that with a typical live axle, a 10-pound-weight savings at the wheels will be about equal to 30 pounds off the axle.
JC Beattie of ATI is seen here using the company's portable equipment (crank pick-up arrowed) to run a damper test at Dale Earnhardt Inc. By manipulating rubber characteristics and inertia mass of the ATI damper, JC reduced crank deflection to less than 0.2 degrees double amplitude. This was worth more power than an otherwise lower inertia damper.
Now let's get down to wheel weights for a typical Detroit-based g-Machine. A typical set of 17-inch- diameter, 9-inch-wide cast-aluminum wheels with a set of tires can weigh in at as much as 54 pounds, with 50 being relatively common. By selecting a set of cast/forged one-piece wheels and suitably grippy tires on the basis of weight, it is possible to get that figure down to 42 pounds. If a modular three-piece competition-style wheel is used, the weight can drop even further to some 34 pounds. Assuming 50 pounds apiece to start with, the weight reduction could amount to 16 pounds per corner, for a total of 64 pounds off the vehicle, and more importantly, 64 pounds off the unsprung weight. Looking at the percentages, and including the reduction in overall weight, we now find that the ratio of unsprung to sprung weight has dropped to 14.5 percent. Finally, a number less than was seen with the stock machine.
The same treatment can be done on the front suspension. Such things as tubular A-arms and lightweight brakes plus an intelligent choice of wheels can make a significant reduction in the unsprung mass.
The savings delivered by brakes and other suspension components are easy to see, but wheels deliver far more than a reduction in unsprung weight. Not only do they have mass that is moved up and down with the suspension and forward with the car as a whole, but also rotational inertia. Saving weight on a wheel not only means a total weight savings four times greater, but also a reduction in moment of inertia. This in itself adds up to a whole lot more in terms of performance gains than just the weight saving itself.
FIG. 9If only one wheel hits a bump, as is most often the case, weight saved at the wheel is equal to about three times the amount of the axle.
Demonstrating Rotational Moment of Inertia
To demonstrate what lightweight wheels are worth on the dragstrip, some tests done years ago are worth recounting. To remove tire differences from the equation, a set of steel disks were attached to the lightweight wheels being tested to bring them to the weight (and moment of inertia) of the wheels they were replacing. Also, removable ballast was bolted to the car's rollcage at about its center of gravity. This meant adding or removing ballast had no effect on the weight transfer or the front-to-rear weight distribution.
To establish a baseline, the car was ballasted to bring the weight, with fuel and driver, to 2,000 pounds. Also for baseline purposes, the wheels were equipped with the steel disks to bring the wheel and tire weight up from 36 pounds to the original 44 pounds. After the baseline passes were completed, the car was run with the steel disks removed from the wheels and the 32-pound reduction this brought about added as ballast in the car. For the next test, the 32 pounds of extra internal ballast was removed. For the last test, the disks were replaced on the wheels and internal ballast was removed until the car went as fast as was produced by the lightweight wheels. Fig. 10 shows the results.
If you are building on a budget, then a damper, such as this big-block Chevy item from Professional Products, could well be what you are looking for.
Even on this basically low-powered test car, lightweight wheels had a measurable effect. Fitting them and restoring the weight by means of internal ballast showed that the reduced wheel moment of inertia alone was worth 1/3-tenth on the quarter-mile. At the end of the quarter, this translated into a lead of 4.4 feet over the baseline-spec vehicle. The reduced wheel moment of inertia plus the reduced chassis weight dropped the e.t. by a solid tenth, resulting in a 13.1-foot lead over the baseline vehicle. The last test showed it was necessary to reduce the internal ballast by 60 pounds to replicate the performance given by wheels only 36 pounds lighter. What we can say here is that weight removed from the wheels has at least 70 percent more positive effect on performance than weight removed from the sprung part of the vehicle. Extrapolating these results and applying them to a typical Detroit pony car of some 450 hp, we can say a set of three-piece Weld wheels such as shown would be equivalent to adding some 15 hp and 12.5 lb-ft to the engine's output.
This is Maximum Motor Sport's three-link/torque-arm axle for a road-race 5.0 Mustang. It is equipped with the new forged-alloy version of the Cragar SS by Weld Racing. These wheels, which are 1.5-pounds lighter than Ford's cast Cobra wheels, are tough enough to deal with high-speed curbing during cornering. Shod with Toyo tires 1.5 inches wider than what they replaced, the wheel/tire combo is 6 pounds lighter for a 24-pound overall weight reduction.
The laws of physics that apply to acceleration also apply to deceleration. The less the unsprung mass is, the better chance the tires have of gripping the road. Also, any reduction in the moment of inertia of the rotating parts, that being principally the wheels and tires, the less energy the brakes have to dissipate.
Just before wrapping this piece up, let me say a few words on wheel choice and safety. Wheels are purpose-built. A wheel designed for drag racing can be very light because it does not have to take the kind of abuse a road-racing wheel would be subjected to. Do not under any circumstances shortchange wheel strength for the job just to get a lower moment of inertia.
Lastly, tires: Because these are at a larger radius than the wheel, they can have a bigger influence for a given weight reduction. The way to check to see if the tire you intend to purchase is heavier than some of its direct competitors is to go online at www.tirerack.com and check out the tire specs. All the weights are listed. Yes, it really is that simple.