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8-3/4 Rear Performance Limits

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
Grant,
I'm interested in hearing more about your system.
 
Grant,
I'm interested in hearing more about your system.

It helps to think about a stick shift car itself, and it's engine's rotating assy, as separate energy storage devices similar to batteries with the engine doing the charging. On the starting line, the car itself is not moving and contains zero charge. On the starting line the engine is running, so it's rotating assy contains at least a partial charge. When the car crosses the finish line, wheels/tires, driveshaft, etc are all going to be charged to capacity (spinning at max rpm/mph). If the car is geared correctly, the engine's rotating assembly will also be charged to capacity as you cross the finish line. Since the engine is making the power to charge both, and the car itself starts out with zero charge (zero mph) prior to launch, there's an advantage to starting a drag strip pass with the rotating assy as charged as possible prior to triggering the clock.

An engine's entire rotating assy (crankshaft/flywheel/pressure plate/balancer/etc) is one big flywheel style energy storage device. You must put energy into it to speed it up, you must take energy out of it to slow it down. While engine rpm is climbing, engine torque alone is accelerating the car. Here's a graph showing evidence of additional torque due to stored inertia being pulled out of an engine's rotating assy by the clutch.
...Red is engine rpm
...Blue is accel G
...Green is driveshaft rpm
Notice that all the highest points of the Accel "G" trace on the graph occur while the engine's rotating assy is losing rpm (discharging energy) against WOT. But then as soon as the clutch stops pulling engine the down and engine rpm begins to climb, the G trace drops like a rock as engine torque alone is now accelerating the car with no help from discharged inertia...

q3graph_fe1224c588808239ace60b3fa1ff1fc17c91a401.png


On the graph above you also see zero wheelspin during launch, but note the sudden loss of 2000+ rpm against WOT after the shifts. When the clutch pulls the engine down too quick, a big portion of that released energy can end up wasted in intense wheelspeed spikes. The clutch is what determines the rate that the engine is pulled down against WOT, in other words how fast the clutch pulls the engine down is what determines the rate that released inertia energy hits the input shaft.

Quick bit of hillbilly physics- the inertia energy contained in an engine's rotating assy increases exponentially with rpm.
...Lets say a rotating assy spinning at 1000rpm contains 1 unit of inertia.
...Double the rpm to 2000, now it contains 4 units of inertia.
...Spin it up to 7174rpm like the launch on the graph, now it contains 51.46 units. The clutch then pulled the engine down to 4630rpm from 7174, that's 30.03 units of inertia energy dumped into the input shaft in 0.589sec.
...After the shift, the engine got pulled down from 7820rpm to 5132rpm. That's 34.82 units of energy dumped into the input shaft in only 0.163sec. More inertia energy dumped after the shifts than during launch, in a far tighter time frame. A spike like that can put a lot of hurt on a car's drivetrain.

Might help to think of it like accelerating a glass by pulling on a tablecloth. Jerk the tablecloth too quick, the glass doesn't move. Pull slow and the glass moves, but not very quick. Happy medium for accelerating the glass as efficiently as possible lies somewhere in between.

Here's a few different ways to adjust your car's inertia release rate...
...1- modulate clutch engagement with your foot. Impossible to do with optimum efficiency, also puts a lot of wear/tear on the clutch when you slip it too much.
...2- select a clutch with just enough clamp pressure to optimize inertia release rate. Mixing/matching clutch components in the search for optimum release rate can get expensive and labor intensive, things also change as the clutch wears. Hitting it with nitrous will also force you to compromise.
...3- use an external controller like my Hitmaster system to temporarily reduce clutch clamp pressure. Makes it possible to adjust your car's inertia release rate by simply turning a knob. Turn the system off for street driving if you want, also makes it easy to switch between NA or nitrous launch settings. Tires last a lot longer, you may find that you no longer need to do burnouts.

It's pretty much impossible to produce this shape of TOB pressure release curve using your foot, the system's use of a timer also limits excessive wear/tear on the clutch...

hitmasterreleasecurve_c1fe6427386d0062cbdac36c5618bb4c7bb4116e.png


My Hitmaster system is in-line hydraulic, and generally limited to cars with hydraulic clutch release systems. For cars with mechanical release linkage, my ClutchTamer is easier to install. For power adder cars, the Hitmaster is generally preferred over the ClutchTamer as it gets to full clutch clamp pressure much quicker.

After you discover that you can consistently control the rate that inertia energy is fed into the chassis, you will then soon realize that there's not much keeping you from raising launch rpm to make more inertia energy available during launch. Raising launch rpm allows you to take advantage of a much longer lasting inertia surge without drawing the engine down below it's torque peak. So why launch the car from 4500rpm with 20.25 units of inertia, if you can dial in a dead hook 7500 hit with 56.25 units on board without breaking anything? Being able to put that extra rpm/inertia energy to work is like adding a small shot of nitrous during launch.

Grant
 
It helps to think about a stick shift car itself, and it's engine's rotating assy, as separate energy storage devices similar to batteries with the engine doing the charging. On the starting line, the car itself is not moving and contains zero charge. On the starting line the engine is running, so it's rotating assy contains at least a partial charge. When the car crosses the finish line, wheels/tires, driveshaft, etc are all going to be charged to capacity (spinning at max rpm/mph). If the car is geared correctly, the engine's rotating assembly will also be charged to capacity as you cross the finish line. Since the engine is making the power to charge both, and the car itself starts out with zero charge (zero mph) prior to launch, there's an advantage to starting a drag strip pass with the rotating assy as charged as possible prior to triggering the clock.

An engine's entire rotating assy (crankshaft/flywheel/pressure plate/balancer/etc) is one big flywheel style energy storage device. You must put energy into it to speed it up, you must take energy out of it to slow it down. While engine rpm is climbing, engine torque alone is accelerating the car. Here's a graph showing evidence of additional torque due to stored inertia being pulled out of an engine's rotating assy by the clutch.
...Red is engine rpm
...Blue is accel G
...Green is driveshaft rpm
Notice that all the highest points of the Accel "G" trace on the graph occur while the engine's rotating assy is losing rpm (discharging energy) against WOT. But then as soon as the clutch stops pulling engine the down and engine rpm begins to climb, the G trace drops like a rock as engine torque alone is now accelerating the car with no help from discharged inertia...

View attachment 1168068

On the graph above you also see zero wheelspin during launch, but note the sudden loss of 2000+ rpm against WOT after the shifts. When the clutch pulls the engine down too quick, a big portion of that released energy can end up wasted in intense wheelspeed spikes. The clutch is what determines the rate that the engine is pulled down against WOT, in other words how fast the clutch pulls the engine down is what determines the rate that released inertia energy hits the input shaft.

Quick bit of hillbilly physics- the inertia energy contained in an engine's rotating assy increases exponentially with rpm.
...Lets say a rotating assy spinning at 1000rpm contains 1 unit of inertia.
...Double the rpm to 2000, now it contains 4 units of inertia.
...Spin it up to 7174rpm like the launch on the graph, now it contains 51.46 units. The clutch then pulled the engine down to 4630rpm from 7174, that's 30.03 units of inertia energy dumped into the input shaft in 0.589sec.
...After the shift, the engine got pulled down from 7820rpm to 5132rpm. That's 34.82 units of energy dumped into the input shaft in only 0.163sec. More inertia energy dumped after the shifts than during launch, in a far tighter time frame. A spike like that can put a lot of hurt on a car's drivetrain.

Might help to think of it like accelerating a glass by pulling on a tablecloth. Jerk the tablecloth too quick, the glass doesn't move. Pull slow and the glass moves, but not very quick. Happy medium for accelerating the glass as efficiently as possible lies somewhere in between.

Here's a few different ways to adjust your car's inertia release rate...
...1- modulate clutch engagement with your foot. Impossible to do with optimum efficiency, also puts a lot of wear/tear on the clutch when you slip it too much.
...2- select a clutch with just enough clamp pressure to optimize inertia release rate. Mixing/matching clutch components in the search for optimum release rate can get expensive and labor intensive, things also change as the clutch wears. Hitting it with nitrous will also force you to compromise.
...3- use an external controller like my Hitmaster system to temporarily reduce clutch clamp pressure. Makes it possible to adjust your car's inertia release rate by simply turning a knob. Turn the system off for street driving if you want, also makes it easy to switch between NA or nitrous launch settings. Tires last a lot longer, you may find that you no longer need to do burnouts.

It's pretty much impossible to produce this shape of TOB pressure release curve using your foot, the system's use of a timer also limits excessive wear/tear on the clutch...

View attachment 1168069

My Hitmaster system is in-line hydraulic, and generally limited to cars with hydraulic clutch release systems. For cars with mechanical release linkage, my ClutchTamer is easier to install. For power adder cars, the Hitmaster is generally preferred over the ClutchTamer as it gets to full clutch clamp pressure much quicker.

After you discover that you can consistently control the rate that inertia energy is fed into the chassis, you will then soon realize that there's not much keeping you from raising launch rpm to make more inertia energy available during launch. Raising launch rpm allows you to take advantage of a much longer lasting inertia surge without drawing the engine down below it's torque peak. So why launch the car from 4500rpm with 20.25 units of inertia, if you can dial in a dead hook 7500 hit with 56.25 units on board without breaking anything? Being able to put that extra rpm/inertia energy to work is like adding a small shot of nitrous during launch.

Grant
Thanks for the physics lesson.
Needless to say you've got me on the hook.
 
Grant,
I'm using mechanical linkage, so the Clutch Tamer sounds like the one I would need.
I would like to know for sure, In your opinion, would the combo in my original post live with the Clutch Tamer installed?
 
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