Engine Setback

[exwestracer]

Starting to feel like a thread hog here, but you guys are bringing up some really interesting stuff lately!

Doug, first I would define "advantage". You asked a VERY broad question... What is it you are trying to achieve? Quick turn in? Neutral handling at the traction limit? Controllable oversteer? If you are designing a chassis from scratch, each of those would dictate a very different F/R weight distribution. I am a firm believer in low polar moment of inertia; that is, centering most of the vehicle mass between the axle centerlines. Setback is obviously an easy way to achieve this.



As Kenny pointed out above, the math is pretty simple; IF you have accurate weights for the total vehicle and engine/trans combo. Let's say we have a 3000lb total weight, and a 600lb engine/trans package that we are moving around. We'll use a 100" wheelbase, just to keep the math simple. So every inch we move the engine is 1% of the wheelbase. The engine package is 20% of the total weight.

I'm going to make 1 more assumption...let's say that if we took the engine and trans OUT and weighed the car we would have 50/50 distribution. 1200lb on the front axle, and 1200lb on the rear axle. If we start out with the engine center of mass at the front axle centerline (not realistic, but bear with me), we would have 1800 front /1200 rear, or 60/40 front distribution. The MOST we could achieve is a 20% change if we moved it all the way to the rear axle C/L... 1200 front /1800 rear. Still with me? Now, let's take that 20% swing and divide it by the wheelbase, 100". That means every inch we move the engine back changes the F/R distribution 0.2%.

Again, you have to KNOW the exact weights and distances you are dealing with. Making a wrong "guess" at any of it makes the calculation worthless...