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Discussion Starter #1
I was reading in the minitruck section about Bobby testing some rear outputs. It appears that they break at around 2 or 3K ft lbs. Pretty minor. This got me thinkin..

You always hear about torque multiplication. so lower gearing essentially adds to the torque correct? What I am having a hard time with is this.

If you dyno your truck and get a max reading of 200 ft lbs on the dyno. Most dyno runs are done at 1:1 so does that mean that if you have 5.13 gears you are only developing 39 ft pounds before the axle???

If not, how do these numbers start to realistically change as you start gearing down???

:confused:
 

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Multiply the other way.

Engine torque 200
Trans ratio 2:1
Torque at Trans Output 400

Axle ratio 5:1
Torque at Axle out 2000 (or 10x the engine torque, which is the trans gearing (2) mulitplied by the axle ratio (5))
 

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As for How the dyno comes up with a reading of 200 HP or Ft lbs of torque... the unit is calibrated to adjust the readings it gets from the tires and calculate power output - that is the purpose of the machine. Since Newton's laws of physics state that energy must always be conserved - we know that work is always the same for a given task. If you make something 3 times easier to move, you have to move it 3 times as far. That is what the gear reduction in the driveline is doing, making it easier to move the vehicle but making the componensts move slower, therefore covering less distance. Torque is a measurement of twisting force, which is what is needed to turn the tires (In this case they adjust the force to be measured at the end of a foot long lever). Since horsepower is in essence a measurement of how quickly an object of a given weight can be accelerated (and a function of torque), The dyno can take the input readings from the tires, and mathematically adjusts those readings and converts them into an estimation of HP & torque output of the engine. The figures you are seeing on the dyno is the approximate output of the engine minus the parasitic drag of the driveline.

Now for how the multiplication works:

The peak torque number your engine produces is measured on a pony brake. That's a device that simply tries to slow down the crankshaft causing the engine to stall. So, the torque listing of your engine is taken at the crank.

Now, to figure the max static torque load of another part of the driveline, you need to work down the driveline until you find the stress that that given part is under.

Say you want to find the torque at the output shaft of the tranny in 1st gear in your TH400. You take your engine's peak torque (say 300 ft lbs) and multiply it by the first gear of the tranny (2.48), then again by 2 to account for the converter (In manuals you eliminate this extra multiplication). The number you get (1488 ft lbs) is the static load seen by output shaft in 1st gear at the engine's torque peak. These numbers aren't perfect, because the converter multiplies force on a curve and there is some parasitic drag that reduces the load (generally at least 15% by the time you get to the axleshafts), but this is a decent baseline number to work with.

For an axleshaft, you have to mutiply in the t-case low range, and the ring & pinion into the equation as well. Doing this you can easily see why the lower gearing is hard on axles.

Then there's shock load, which can easily multiply these forces by 3-4 times. You can see that no axleshaft in the world is gonna be break proof, unless you are using a 5HP Briggs lawn mower engine to power your truck. Of course, common sense will tell you you want mimimum weight and maximum stength to avoid breakage. What the math tells you is that if you keep the gearing and power output of your truck at a reasonable level, it will help keep things together. The good news is you can control the torque of the engine. If you're not hard into the throttle near the engine's torque peak, you're generating significantly less power and everything down the line is seeing a much lighter load.

Then again, where's the fun in that? Build it right & run it hard!

Hans
 

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The output measured on a chassis dyno is calculated as engine output. They attach a lead to the engine ignition so they know the rpm. It measures the roller rpm and torque then calculates the rpm difference to get engine torque.
 

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Discussion Starter #6
So if I am making 250 ft lbs (measured on a dyno) and the output shaft breaks at a measly 2000 ft lbs, I better not use full throttle with a 2.3:1 tcase and a 7:1 1st gear...

Guess that second tcase of 2.3:1 is a bad idea too then lol
 

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Jason M said:
So if I am making 250 ft lbs (measured on a dyno) and the output shaft breaks at a measly 2000 ft lbs, I better not use full throttle with a 2.3:1 tcase and a 7:1 1st gear...

Guess that second tcase of 2.3:1 is a bad idea too then lol
If you use full throttle at the rpm where you make that 250ftlbs, somethings gonna give, potentially the output shaft could break, but more likely the vehicle would accelerate, or a tire would spin.

The second case is a good idea because it gives you more control, not because it gives you more torque. As you are just figuring out, you already have more torque than you need for true crawling.
 

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Discussion Starter #8
The second tcase is when I really need to just go slow. I never get into the power band at 200:1...

It is nice to crawl around then switch to the single case and flog things ;)
 

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Jason M said:
So if I am making 250 ft lbs (measured on a dyno) and the output shaft breaks at a measly 2000 ft lbs, I better not use full throttle with a 2.3:1 tcase and a 7:1 1st gear...

Guess that second tcase of 2.3:1 is a bad idea too then lol
And thats only if you hook up all the way, or are tranfering all the power to the ground. Usually you not as a tire will spin before then but say if you wedge a tire into an undercut or a crack and you gas it, then yes something will break if your applying all your engine torque.
 
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