Technical Free body diagrams and motor mounts

Free body diagrams and motor mounts question?
Suppose the engine stays put but the frame mount moves back towards rear axle. That change moves the frame mount for engine from Just a head of the front axle to just behind.
Will that net any change in weight distribution? Small as it may be,,,
Looking for advantage in +/- 4% of 1650 lbs window.

 

draw a sketch of before, and another sketch of after. I can't envision what you mean....it sounds like you want to move the engine back a few inches, but you also say the engine stays put.

 

No.

 

So for example a Corvette or Hurst style mount vs side mounts? How the engine is mounted does not change weight distribution. Is that the question?

 

x2 No. Engine is still in the same place.

 

I’m no engineer but yes it should, if I have it correct, you are talking leave the engine in the same spot just move the frame side of the mount towards the rear?

It’ll change where the weight acts on the frame which will change weight distribution. How much I have no idea!


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Doing that will not change the weight distribution. The weight has not moved in the car. The attaching point does not affect where the weight is.

 

It’ll be a little bit before I can get to pen and paper.
But the engine will not move back.
Only The mounting point of the engines weight moves. Currently the motor mount is 2” forward of the Front axle centerline. Moving the mount 4” back new position would be 2” behind the front axle center line.

Again 4% of 1650lbs is only 66 lbs.

The general consensus amongst my circle of influence is that it must change something. 1/3 of them say pissing in the ocean changes the amount of water too. 2/3 say a real world live experiment with scales is the only way to say for sure how much.

 

Yes. It's all leverage.
The real advantage won't be weight distribution overall. There will be some.
The real advantage will be removing weight from in front of and on top of the front crossmember. That will improve steering, ride quality, and weight transfer.

 

If the wheelbase is unchanged, which I think is a safe assumption, are we talking about changing the length of the drive shaft? Or just using a different style motor mount?
If the drive shaft gets shortened, weight is transferred off the front and toward the rear.
Weight transfer has to do with engine position between the front and rear axle. Think of your bedroom scale under each wheel.
Am I way off base?

 

I guess none of them know how to draw a free body diagram, either?

The position of the mounts will change where on the frame the load is applied, but it will not affect where the center of gravity of the complete chassis is. If you want more weight on the rear tires, you have to actually move weight back in the car.

 

There you go! Can’t argue with an actual engineer...I stand corrected.


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You can argue with an engineer, but it helps to understand the subject. I make plenty of mistakes....

 

btw, if you want to see a free body diagram that might explain what's going on here....



The 700 lbs down arrow shows the mass of the engine/transmission, and it's location fore/aft. The load on the front mounts F and rear mounts R are what we're going to calculate, based on the changing front mount position, moving from 45 inches ahead of the rear mount, to only 35 inches ahead of the rear mount.

The sum of the moments about any point equals zero. We'll take the moment about the rear mount, in this case.

F1 = 700 (30/45) = 467 lbs

R1 = 700 - 467 = 233 lbs

When we move the engine mount back ten inches...

F2 = 700 (30/35) = 600 lbs

R2 = 700 - 600 = 100 lbs

So, you can see that moving the motor mount back does indeed change something...it takes a lot of weight off the rear mount, which is closer to the wheels, which you're trying to add weight to!

The net effect is that the center of mass of the car is still the same, but the load on the frame, and on the motor and transmission mounts, is redistributed. The load on the wheels does not change.

 

I understand your diagram and it is technically correct.
But two important factors have been omitted. If we assume that the rear axle is beyond R1 and we know that the front crossmember in this scenario is in between F1 and F2, we have removed 467lbs worth of weight from in front of the crossmember and applied 600lbs worth of weight behind it. The amount of weight stays the same true but moving it behind the front crossmember takes stress off of the front suspension to be able to perform work. Braking ability increases, steering is MORE responsive, there is less amplified rotating mass, body sway on deacceleration is removed because it is not nose heavy.
Weight transfer is also improved because because you have moved the applied weight closer to the fulcrum point wich is the rear axle if we assume it is beyond R.

 

For those that think yes, if you screw a floor flange on one side of a seesaw, add a 12" long pipe up,an elbow, and a 6' pipe across, if you stood on the end of the pipe you should go up. There is moment involved so mount location does not matter.

 

The only weight redistribution would be the one pound that the engine mounts total being set back those amount of inches. You could mount the engine to a pair of 12' poles with the far ends attached behind the rear axle and the front contact patch will weigh virtually the same on a scale as with the normal motor mounts. The center of mass isn't changing so the weight distribution doesn't either.
A shopping cart at the grocery store is a good example. The front of the basket floats unattached to the front "axle" and is only attached to the rear "axle". That doesn't direct any more weight to the rear. It's not any easier to lift a full basket at the front than it is at the back. If 2 guys lift and suspend the cart, they both feel about the same force of the weight. By setting the cart back on the floor, the front pair of wheels "feel" and resits the same force as the front dude felt and resisted when lifting against the force.

 

Weight X Arm = moment. This is the formula for center of gravity.
As far as moving your center of gravity, moving the mounts will not change it. You have to move the weight in order to move the CG.
The only difference in CG would be the very small amount of the weight of the motor mounts moved back. Not the full weight of the motor mounts, just the difference of the distance you moved them back.

 

Absolutely , hence my posed question on here.

 

Here’s the sketch,
The engine sits much lower, for clarity its shown higher.

1 is currently the frame mount location
2 is the proposed new frame mount location.
Wheel base is 108”
There’s 4 scales, 1 under each wheel.


What’s spurred this discussion is the act of adding and moving ballast weight around.
For example, when we add 100 lbs the total of 4 scales goes up 100lbs. Placement of the 100lb weight really changes the distribution detectable on those scales. Adding weight behind the rear axle removes weight (few lbs) from the front wheels,,, and if the weight goes far to one side on the rear it’s unloads more on the opposite front.
I’ve watched/helped these guys move small amounts of weight all over to get the scales reading correctly. I was amazed at how much it changes. Again it’s small windows of distribution they are chasing.
That’s just static.
Moving the weights vertically throws centrifugal forces into the mix

 

Looks to me like all you have changed is the weight of the mount itself. Negligible. Lippy

 

It changes the weight distribution of the engine and trans on their mounts but won't affect front axle rear axle weight distribution. Simply meaning more weight will be in front of the spot where the mount connects to the frame on the sides of the engine. The actual weight of the engine/trans on that mount of the frame isn't going to change noticeably.

 

If you look down at the two frame rails and the engine, are you talking about sliding the engine and its mounts rearward, or moving the motor mounts rearward without moving the engine?

The former changes the center of gravity and the distribution of weight on the axles. You can see that if you pretend to slide the engine and its mounts all the way back to the rear end.

The latter does not change the center of gravity, but does shift more of the engine weight toward the rear wheels. You can see that if you pretend that your engine is 10 feet long and you move the engine mounts from the front of the engine to the back.

 

Thanks squirrel, this makes perfect sense.


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im almost done with this subject....
but?

How can it be the frame sees a redistribution of load yet the wheels can seen no change?
Even minimal?
I’m not kidding when I say I watch these guys move a 20 lb weight around for the changes in percentage of weight distribution on the scales.

 

they are MOVING WEIGHT AROUND. They are not just making different brackets, and leaving the weight in the same place.

 

Could you make up something like this with two bathroom digital scales?
Not the best artist, wondering if it would work to verify?

Maybe under the “frame” put two piece of pipe on each scale to mimic contact patch/point.

And hey, off the the right you’ll notice I remembered how to read micrometers

 

you can do the math, or the experiment, you'll end up with the same results, as long as you did the math right, and you didn't cheat on the experiment.

 

Oh, if you’re referring to me, I fully agree with your math, may not understand it, but I agree with it.
But sometimes experimenting with something hands on, can make a person see what’s happening better.

 

Definitely. I encourage both experiments, and learning how to do the math.

This is what the FBD for the chassis might look like, with the engine mounts in two different locations. You can probably see from this that the weight on the front and rear wheels will be the same, even though the frame is loaded differently in each case. (the 1500 down arrow in the middle is the weight of the car, minus the engine/transmission).

make up some dimensions for the wheel locations and center of mass of the chassis, using the same numbers I did for the engine/transmission, do the math, see what happens.