Engine setback/weight change

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TEROL
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Engine setback/weight change

Post by TEROL » Sun Aug 25, 2013 3:35 pm

For a given weight and wheelbase is there a formula to calculate the weight dictribution change? I think I have it figured but would like input from others.

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MadBill
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Re: Engine setback/weight change

Post by MadBill » Sun Aug 25, 2013 9:16 pm

Easier to explain with a diagram, but essentially, you calculate 'moments' about either the front or rear axle and determine how they change. Let's say the car weighs 3,000# and the engine/trans is 600#, wheelbase is 100" and the center of gravity (balance point) of the engine/trans assembly is originally 80" forward of the rear axle. The moment of the assembly around the axle then is 80" x 600 #, or 48,000 inch-pounds. The reaction (support) at the front axle will be in proportion to its distance from the 'pivot point' of the rear axle, so 48000/100 = 480#. Obviously the other 120# is carried by the rear axle.

Now, let's move the engine/trans back 10". The new moment is 70 x 600 = 42000 so the reaction at the front axle is now 420# leaving the rear to supporting 180#, thus the front is now 60# lighter and the rear 60# heavier. If the original weight distribution was 50% rear (1500#), the new one is 1560/3000 or 52%.

Many people are surprised at how much an engine must be shifted to get a substantial result, but try calculating the effect of moving a 40# battery from say 20" ahead of the front wheel to 30" behind the rear!
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