Math Help / Engine placement

[TrentNell]

Need some math help I am debating on moving the engine / trans / tcase back 6" or so in the chassis to get some more weight on the rear tires and hopefully loose some more weight up front also.

I can transfer a bunch with the chassis setup but that just makes the front end have to have a bunch of rise, plan is to get the truck as close as i can to having 50% of the weight on the rear tires , instead of like 20-30% need to decide on how much I would have to move it back and if it would be doable and what kind of increase of weight to the rear tires that moving back 6" would do as a start .

details full truck weight 5450-5510 lbs hopefully less by next year.

estimated drive train weight that would be moved back as a whole ( engine , trans , and t-case is around 1500 lbs maybe more , each Inch it moves back should make a decent difference I would think .
currently the front of the motor is 13' in front of the rear axle , and can reasonable move 1' at a maximum

any thoughts ???

Also another idea is to buy a single cab Fiberglas body and move the rear tires forward 2 full feet . and is by far the easier choice but more $$

 

[Evan@InglewoodTrans]

do you already know the weight difference front and rear ?

 

[Evan@InglewoodTrans]

They teach this stuff in basic aviation ground school and call it "weight and balance". The location of the CG is rather critical on an airplane and you have to make sure you don't screw it up too far. If you do it can quite easily be a life ending lesson. Now it has been a few years since ground school, but let me see if I remember how to do this...

The first thing you have to do is calculate just where your CG is now. If we assume 100" wheelbase, then the CG is 40.3" back from the front wheels.

Now we pick a semi-arbitrary reference point in front of the car to reference "moments" around. These moments are really rotational torques caused by the weight of an item multiplied by it's distance from the reference point. Let's use 40" ahead of the front wheel center for fun.

Your Chevelle has a total weight of 3554 lbs and since the CG is 80.3" back from our reference point for a total moment of 285,386. That is your base moment.

Now let's move the battery. First we need it's weight and distance from the reference point. I am going to guess 50 lbs and 6" for a moment of 300. We remove the battery so we can subtract 300 from our total moment which becomes 285,086. We move the battery back 24" behind the back wheels so it's 164" behind the reference point and it's moment is now 8,200. This we need to add to our total which becomes 293,286.

That is nice but what happened to the CG? Well if we take that moment and divide by the weight, which did not change (used those new weightless battery cables ), we get a CG at 82.5" behind the reference point. Which works out to a 2.2" rearward movement of the CG.

So there you go. Now you too can figure out what difference it will make when you move parts around
found that Maybe that will help

 

[TrentNell]

do you already know the weight difference front and rear ?

Not really but need to find out , 4-wheel scales are expensive , even to rent them , I check on it in the spring when building the 4-link .

They teach this stuff in basic aviation ground school and call it "weight and balance". The location of the CG is rather critical on an airplane and you have to make sure you don't screw it up too far. If you do it can quite easily be a life ending lesson. Now it has been a few years since ground school, but let me see if I remember how to do this...

The first thing you have to do is calculate just where your CG is now. If we assume 100" wheelbase, then the CG is 40.3" back from the front wheels.

Now we pick a semi-arbitrary reference point in front of the car to reference "moments" around. These moments are really rotational torques caused by the weight of an item multiplied by it's distance from the reference point. Let's use 40" ahead of the front wheel center for fun.

Your Chevelle has a total weight of 3554 lbs and since the CG is 80.3" back from our reference point for a total moment of 285,386. That is your base moment.

Now let's move the battery. First we need it's weight and distance from the reference point. I am going to guess 50 lbs and 6" for a moment of 300. We remove the battery so we can subtract 300 from our total moment which becomes 285,086. We move the battery back 24" behind the back wheels so it's 164" behind the reference point and it's moment is now 8,200. This we need to add to our total which becomes 293,286.

That is nice but what happened to the CG? Well if we take that moment and divide by the weight, which did not change (used those new weightless battery cables ), we get a CG at 82.5" behind the reference point. Which works out to a 2.2" rearward movement of the CG.

So there you go. Now you too can figure out what difference it will make when you move parts around
found that Maybe that will help

That just made my head hurt :rofl: thanks for the info and will read it a few times , and my 4 link software will help me find C/G so it might help me a bit .

 

[SteveFord]

Trent I have an idea....after you make your changes come down about 3/4 of a mile or so in altitude so we can see what it can do:thumb:!

 

[TrentNell]

Trent I have an idea....after you make your changes come down about 3/4 of a mile or so in altitude so we can see what it can do:thumb:!

Good idea but i simply race what i have where i have it , the best thing about running at RMR is no one ever has to question its legitimacy and if i can do it there i can do it anywhere, any time !!!!

 

[Evan@InglewoodTrans]

Not really but need to find out , 4-wheel scales are expensive , even to rent them , I check on it in the spring when building the 4-link .


That just made my head hurt :rofl: thanks for the info and will read it a few times , and my 4 link software will help me find C/G so it might help me a bit .

Made my head hurt too but after reading it a few times it starts to make sense

 

[SteveFord]

Good idea but i simply race what i have where i have it , the best thing about running at RMR is no one ever has to question its legitimacy and if i can do it there i can do it anywhere, any time !!!!

Thats the truth there! You wont get guys saying..." you know what I want to see how fast I can really go, lets go to RMR!:rofl:

 

[Osubeaver]

I think that Chevelle answer quoted above is fine, but it's way too complicated.

Instead of picking an "arbitrary" datum. Pick something that makes sense and you can actually measure from, like the front axles or something.

If you do that it looks something like this:
Front axle = 0 (Datum)
Rear axle = 100"
Truck weight 5000lbs
weight dist = 70/30 so front weighs 3500 rear weighs 1500

A moment is weight x distance (a torque).

Total moment is:
3500 x 0 inches + 1500 x 100 inches = 150,000
Total moment/weight = 150,000/5000 = 30

So that means the center of gravity is 30 inches back from the front axle.

Now assume the engine and trans is 1000 lbs and it's "center" is 10 inches from the front axle (20" in front of CG).

So now "remove it" and recalcuate.
Moment of engine and stuff is 10 x 1000lbs = 10,000
Now total moment is 150,000 - 10,000 = 140,000

Move the engine back 2 inches.
Now it's moment is 12 x 1000 = 12,000
Add that back in to the system so total moment is now 152,000.

152,000 / 5000 = 30.4 is our new distance of the center of gravity from the front axle.

Clear as mud.

 

[TrentNell]

Thanks Jerry that helps a bunch and can use that formula with my own measurements easy enough :hug:

 

[Sledhead]

I love this $hit! :thumb:

 

[JoshH]

Just to add a little more confusion to the subject, here is another way to go about it.

First thing you need to do is measure your wheelbase.

Next, with the truck fully dressed out how you plan to race it, you need to take it to a scale where you can get axle weights (truck stops are great for this).

Now, pull the engine/trans/t-case combo out of the truck and go reweigh.

Now you should have everything you need to do the calculations to find out exactly where you need to place your motor.

Here are some arbitrary numbers to start out with using a wheelbase of 132" (11'), a total weight of 5400 lbs, and an initial weight distribution of 70% front and 30% rear.

This is not going to be 100% accurate unless you figure unsprung weight, but it should get you pretty close. You need to sum the moments to find the optimal placement of the motor (I will say it will probably have to move a long way to get 50/50 distribution).

Now, we will calculate the moment at the front axle.

You have a positive moment of 1620 lbs * 11 ft at the rear axle and a negative moment of 5400 lbs * X (X=the distance from the front axle to the center of gravity of the truck). Since truck is not rotating around the front axle, we know the sum of the moments is 0. So we can set them equal.

5400 lbs * X = 1620 lbs * 11 ft

5400 lbs * X = 17820 lb-ft

X = 17820 lb-ft / 5400 lbs

X = 3.3 ft

So now we know the center of gravity of the truck is 3.3' back from the front axle.

Now you need to pull the motor/trans/t-case and reweigh the truck. Lets assume the combo weighs 1400 lbs and removing it takes 1200 lbs off the front axle and 200 lbs off the back axle. We now have a front axle weight of 2580 lbs, a back axle weight of 1420 lbs, and a total vehicle weight of 4000 lbs (obviously we still have a wheelbase of 11 ft). Now we can recalculate the moments (again, the truck is not rotating around the rear axle, so the sum of the moments is equal to 0).

4000 lbs * X = 1420 lbs * 11 ft

4000 lbs * X = 15620 lb-ft

X = 15620 lb-ft / 4000 lbs

X = 3.905 ft

So now we know the CG of the chassis/body/suspension/etc is at 3.9 ft.

Now we can back calculate the amount we need to move the motor to get the desired weight distribution (or just calculate what the new distribution will be for how much you do move it). First we'll calculate the original CG of the motor and then we'll figure the proper placement for a 50/50 weight distributions (brace yourself ). What we will do is sum the moment of the weight on the rear axle, the moment of the chassis, and calculate the moment of the powertrain.

0 = 1620 lbs * 11 ft - 4000 lbs * 3.905 ft - 1400 lbs * X

1400 lbs * X = 17820 lb-ft -15620 lb-ft

1400 lbs * X = 2200 lb-ft

X = 2200 lb-ft / 1400 lbs

X = 1.571 ft

Now that we know the original CG of the powertrain in relation to the front axle, and we can calculate the new placement of it. Since we're going for a 50/50 distribution, we will use a new rear axle weight of 2700 lbs.

0 = 2700 lbs * 11 ft - 4000 lbs * 3.905 ft - 1400 lbs * X

1400 lbs * X = 29700 lb-ft - 15620 lb-ft

1400 lbs * X = 14080 lb-ft

X = 14080 lb-ft / 1400 lbs

X = 10.057 ft

So to find out how much to move it, we subtract them.

10.057 ft - 1.571 ft = 8.486 ft

(of course these are all just guesses and possibly highly erroneous numbers)

If you have a known max movement, you can plug that in and see how much it will change the weight distribution of the truck. Let's say you can move the motor back a max of 1 ft. The moment arm of the motor will be 2.571 ft.

0 = X * 11 ft - 4000 lbs * 3.905 ft - 1400 lbs * 2.571 ft

X * 11 ft = 15620 lb-ft + 3600 lb-ft

X * 11 ft = 12020 lb-ft

X = 19220 lb-ft / 11 ft

X = 1747 lbs

Now we can calculate the new weight distribution.

1747 lbs / 5400 lbs = 32.35%

Hopefully that didn't just confuse you more :thumb:

Anyone feel free to check over my procedure and make sure I'm not giving bad info.

 

[TrentNell]

Great Info Josh , and much appreciated to put all that time into 1 post :hug: , def gives me some things to kick around and start doing some leg work to see if it would be worth any effort .

 

[JoshH]

If you're wanting to get the most from it, you may want to consider things like radiator and CAC placement, batteries, turbos, windshield, front clip weight, etc. Every bit of weight you can move or remove from the front will help your cause.

 

[TrentNell]

If you're wanting to get the most from it, you may want to consider things like radiator and CAC placement, batteries, turbos, windshield, front clip weight, etc. Every bit of weight you can move or remove from the front will help your cause.

Its all being addressed and considered , also looking into a rack and pinion / light weight steering setup , just deciding what i can afford and what I am willing to tackle , but ideally there is a lot to gain IMOP .

 

[LBZ]

Help! I'm surrounded by nerds!!:book::hail:

 

[JoshH]

Manual rack and pinion? Get rid of that power steering pump too?

 

[TrentNell]

Help! I'm surrounded by nerds!!:book::hail:

I would Rather be a nerd and know all that good sh*t :coolspot:

 

[TrentNell]

Manual rack and pinion? Get rid of that power steering pump too?

I would if I thought I could but I dont think we could go manual steering ( dont think there is a manual R/P is there ? ) with the weight of our trucks , I would have to lift weights before each race just to "warm up" :rofl:

 

[JoshH]

Ah, you're a big guy. You'll be fine as long as you aren't trying to parallel park