Knowing your Center of Gravity

[1leglance]

As part of my SEMA build I was working my way through the Ford Transit Builders Guide & I came across the methods and formulas to figure out the Center of Gravity.

For a road going machine this is less of a concern, but with all the off camber fun we have in the dirt it will be great to know once the build is finished just where that imaginary point is at.

Sure playing on a RTI is fun and looks cool (Ramp Travel Index tells you how much flex your suspension gives), but knowing how if your Center of Gravity is high vs low or front vs back has much more value to the Overlander or VanLife world.

Looks like Martín our resident engineer will have to be involved with all this math, and we will need to borrow 4 wheel scales and a forklift, but in the end should be a good time and great knowledge to share! This will be a real hassle to do, and I hope a "once and done" type thing for sure.

I did come across some info in researching that some state DOTs will do individual wheel scales (horizontal of course) if you call and ask....that info alone would we worth having.

 

[shade]

State police / highway patrol often use individual wheel scales for roadside inspections on large trucks. Smaller agencies located near major highways may do the same. Contact your local branch and ask if there's a way to arrange a free weighing somewhere, or if there's another way they can suggest.

 

[Hillbilly Heaven]

Not only do you want to find center of gravity but also roll axis center line. Which is calculated same way by side to side measurements.

Remember CG on drivers side may be different than passengers side CG. This is because weights maybe different from side to side. CG can be skewed front to back if side weights are different. The same is true of roll axis because front to rear weight differences.

 

[aardvarcus]

For Chevy/GM, the upfitter manuals sometimes include COG measurements for the stock vehicle, at least they did for the GMT400 platform. Many racecar drivers/teams (think local track guys) have individual wheel scales, if you know someone they may let you use them but the maximum weight of the scales may be less than needed.

 

[1leglance]

My sons and I do SCCA Solo fun and I was thinking about reaching out to the club to borrow the wheel scales from them
My wife's company has a fork lift and they are crazy enough to help me do this stuff.

Good call on the Roll Axis, I knew that from our parking lot play but didn't think about it on the van.

 

[Metcalf]

Not only do you want to find center of gravity but also roll axis center line. Which is calculated same way by side to side measurements.

Remember CG on drivers side may be different than passengers side CG. This is because weights maybe different from side to side. CG can be skewed front to back if side weights are different. The same is true of roll axis because front to rear weight differences.

Wouldn't the vehicle roll axis be defined by the suspension geometry? Basically connecting the front and rear roll center.

 

[Hillbilly Heaven]

Wouldn't the vehicle roll axis be defined by the suspension geometry? Basically connecting the front and rear roll center.

Yes Just understand the line may or may not be parallel to the body. It maybe skewed.

 

[Metcalf]

Yes Just understand the line may or may not be parallel to the body. It maybe skewed.

Is that bad? It will also be moving around depending on what the suspension is doing too.

On your typical off-road rig I am not sure many people really would notice much difference.
Not having weight up high or rearward are my golden rules, but nobody seems to really care much these days......roof racks, roof top tents, heavy spare tires in bad locations, etc. They only seen to care about how much stuff they can take with them and vehicle performance dynamics pretty much be dammed.

 

[shade]

Where I live State Weighstations are everywhere. Aside from port of entry, they are seldom open during weekends.
I drive over them frequently. With a small vehicle its easy to weigh front & rear, side to side or individual wheels.
One thing, they round to the nearest 100lb, but otherwise consistient.

There's a grain elevator scale left on 24/7 near me I use the same way.

 

[rayra]

Needs more butt in the seat time. You get a good feel for it in non-critical local conditions, before you have to push things far from home.

 

[billiebob]

As part of my SEMA build I was working my way through the Ford Transit Builders Guide & I came across the methods and formulas to figure out the Center of Gravity.

For a road going machine this is less of a concern, but with all the off camber fun we have in the dirt it will be great to know once the build is finished just where that imaginary point is at.

Sure playing on a RTI is fun and looks cool (Ramp Travel Index tells you how much flex your suspension gives), but knowing how if your Center of Gravity is high vs low or front vs back has much more value to the Overlander or VanLife world.

Looks like Martín our resident engineer will have to be involved with all this math, and we will need to borrow 4 wheel scales and a forklift, but in the end should be a good time and great knowledge to share! This will be a real hassle to do, and I hope a "once and done" type thing for sure.

I did come across some info in researching that some state DOTs will do individual wheel scales (horizontal of course) if you call and ask....that info alone would we worth having.

This is way too complicated to do with accuracy in a driveway. And you'll need to make a major donation to the DMV guys benevolent society to get them to hang around while you jack things around.

Front to rear center of gravity on anything with wheelbase is irrelevant. You'll burn out the torque converter before it tips backwards. But roll center, side to side center of gravity shifts is critical. Make sure all the top loads are present, like the RTT, and make sure all the low loads are empty, like gas, water, sewer, propane. You want a worst case senario or you will get bit with over confidence.

That said, the easy way? Get an inclinometer, dash mount it with a camera focused to video the results. Figure a way to rig straps over the roof to restrain it from rolling over. Figure a way to lift one side till you get close to a balance point. You might need to over inflate the tires.

Once you have those left/right numbers, realize things will vary uphill or downhill. AND realize you need a safety factor in real life. Just because static you balanced at 25 degrees does not mean in motion over rough ground you won't roll before 20 degrees.

Math, scales, computers only give theoretical answers. Field results are way more trust worthy.

 

[Metcalf]

Front to rear center of gravity on anything with wheelbase is irrelevant. You'll burn out the torque converter before it tips backwards.

Rolling a vehicle is usually always a dynamic situation. Going over backwards on shorter wheelbase vehicles with lots of rear weight bias is a real risk, especially in certain terrain.

 

[Happy Joe]

Rolling a vehicle is usually always a dynamic situation. Going over backwards on shorter wheelbase vehicles with lots of rear weight bias is a real risk, especially in certain terrain.

Yep! and a good reason why neophytes need to be extra careful until they learn how to drive off road... not a sport for the functionally unconscious (street) driver, IMO.

... some of the best rolls and flops that I have seen still had the factory stickers in the window, and clean under carriages...

Eventually you learn; not to hammer it in an attempt to fly over the obstacles (air time); and to get off the gas when you start to pull the front tires off the ground, as well as. to step on the gas when the rear tires both start to leave the ground... (easy to say, kind of hard to do initially).
... and not to put heavy items on the roof rack.

Enjoy!

 

[billiebob]

Going over backwards on shorter wheelbase vehicles

Most often that is affected by a heavy right foot more than the static center of gravity.... which is the topic of this thread.
Quite right to clue in everyone that static calculations will only give a false sense of security.

The other scary thing is the trend of the industry to just lift, and throw weight on the roof rather than engineer a low roll center.
I love this design


Or this from 30 years ago.

 

[Metcalf]

or this from 30 years ago.
View attachment 545949

JRs "bronco" was more like 20 years ago. ( 2002 TTC if I remember right)