Here's something I wrote a while back regarding transmissions, but it largely applies to rearends as well...
I'm curious just how many stick shift drag racers subscribe to the conventional wisdom that says adding weight makes a transmission more likely to break when the engine torque side of the equation remains the same?
My view is the only way weight makes a difference is if wheelspeed is part of your setup. Adding weight typically equals more weight on the tire, requiring you to hit that tire harder with inertia to get the wheelspeed you need, which in-turn means the transmission will then see a higher peak load as a result. But all that changes when you install a clutch hit controller, as the goal with a clutch hit controller is not controlled wheelspin, but instead controlling the rate that the clutch draws the engine down against WOT. Since a heavier car accelerates at a slower rate, it's clutch will also need to draw the engine down at a slower rate to keep rpm in it's optimum range.
Here's a crude theoretical comparison based on Wallace calculator "ideal" numbers. One 3500lbs and the other 2500lbs, both with the same 620whp. Wallace says the 3500lb car runs 10.00 @ 132.5 with a 1.39 60', while the 2500lb car runs 8.95 @ 148.04 with a 1.24 60'. Both have same tires, same 1st gear, and both geared for 7500 at the stripe- 4.72 gear for the 3500lb car and 4.22 gear for the 2500lb car. Both are also using clutch hit controllers tuned for a 7500 hit that draws down to 5500 before the clutch locks up. Based on averaged acceleration rates over the first 60' of each car, the 3500lb car takes 0.81sec to draw 2000rpm worth of inertia, while the 2500lb car accelerates quicker and takes only 0.755sec to draw the same 2000rpm of inertia. The heavier car spreads the same inertia draw over a longer time period, which in-turn effectively reduces the peak impact value of the inertia that gets passed along to the input shaft.
A key thing to note is that although both cars in the above example have the same 620whp and transmission, rear gears are different to achieve the same 7500 at the stripe-
4.72 for the 3500lb car that goes 132mph
4.22 for the 2500lb car that goes 148mph
If one were simply increasing weight with the same rear gearing, then conventional wisdom would apply. But when you gear for the stripe as drag racers do, the heavier/slower car gets more rear gear which in-turn decreases load on the transmission. When you remove wheelspeed from the stick shift launch equation, in-turn allowing both clutches to be tuned to draw the same amount of inertia, that conventional wisdom goes right out the window.
Grant
