How do you find the car's center of gravity in the Z axis?
 

I know how to find it in the X (length) and Y (width) axis, but how do you find the car's center of gravity in the Z (height) axis?

I don't know the answer.

But I need to ask, are you a TC convert?

Fill your boots

section 2 page 3

http://users.pandora.be/elvo/


mmm page 3:drool:

who gives a s+++,this is offroad,

haha this is coming from the guy who balances his wheels:p

:p and a reply from some one who weighs them:p

how do you balance your wheels?

i don't balance them all the time just when i can be bothered,
i use a old wheel hub to mount the wheel on, spin it slowly and use plastercine opposite were it stops on the bottom of the wheel,u can feel when its balanced as the hub doesn't vibrate in your hand, so double bonus,

is there a need for this if you run JC wheels
going on what other people have said about the egg shapes?

never tried jc wheels sorry,but on ae and losi wheels it did help, (massively) especially at place like Italy, (smooth dirt) but you can see the difference at UK tracks.

Place the car on a board and lift one edge of the board until the car tips over, measure the angle of the board at which the car tips. You could work out the C of G height in mm with some geometry calcs, or just compare tipping angles between cars / changes.

Go to your local model shop and ask for 20g of roll center :p

That's awesome! Thanks!
I'll post a simple Excel spreadsheet with automated calculations or chart if I can figure it out.

I think I am picturing what you have said incorrectly, but how would the car tip over before the board goes vertical?

the car would only tip when the board gets to vertical if the car's centre of gravity was at 0mm off the ground, obviously it is higher than this, so the car will tip when the centre of gravity is directly above the outside edge of the wheels that are at the lower end of the slope.

Imagine standing a book up on a board and doing the same, it would obviously tip way before the board got to vertical, the average car would have a much lower C of G / wider stance than the average book, hence the board would be closer to vertical (but not actually vertical) for the car than the book.

I saw it in a C of G comparison between Tamiya Clodbuster and a Kyosho USA1 monster trucks, the magazine put both on a board, lifted it and saw which one tipped first.

My understanding of it would be that at the exact point where the car tips it means that more weight is past the outter edge (center?) of the lower wheel than there is below it which means you could measure the angle and then work out exactly where the center of gravity is.

You would have to make sure the tyres wont slip either as you would typically have quite high angles.

example:

If the car tips at 60 degree's then you would take the center line of the car and draw a line away from 60degree's so it connects to the outside edge of the tyre.

http://www.mole2k.co.uk/501x/_MG_5438-cog.jpg

note: this is only my interpretation of that suggestion, i might be totally wrong :eh?:

Why exactly do you want to know all this???

Usually you have all sorts of other things to be concerned with in offroad with bumps in the track surface and weight transfer that you induce by running the car a little higher to get a particular cornering characteristic you are after. You can't set the car up for lowest CofG anyway and just see that you place heavier items like the speedo on the lower chassis plate.

Thanks for the diagram mole - greatly appreciated. That's what I thought was meant, and it's basically a flawed method. Although it will work, the car has to be basically vertical.

http://www.glypo.com/general/cog/bad-idea.jpg

I used one of jimmys photos to illustrate my point. No tyre will have this grip. On 80's style monster trucks, sure method will be fine. But any racing car, C.O.G. is too low.

I think I have a better idea. Basically similar principle, different method. Tilt the car so until you find the point it balances on two wheels. then using something like the chassis plate, measure the angle. Here I put a camber gauge up to show how it would be done, my camber gauge only goes to around 5 degrees though, and I think this car balances around 10. Plenty of other devices would measure this though.

http://www.glypo.com/general/cog/good-idea.jpg

The maths is very easy. Firstly measure the width of the car, then divide by two. This is the adjacent side. The angle you measure will be something like 75 degrees, the tilting angle will be around 15 degrees (i.e you just take away the angle you measure from 90).

These are the two bits you need to know. You will need to use the tan rule. tan(angle) x adjacent = z height of C.O.G.

A working example.

My Kyosho Lazer ZX-5 balances at around 75 degrees, so that's 15 degrees from vertical. It is 240mm wide, so half of that is 120mm which is my adjacent length. tan(15) is 0.2679. If you don't have scientific calc, see here for table of tan(angle). Excel can easily handle tan.

so:

tan(angle) x adjacent = z height of C.O.G.
0.2679 x 120mm = 32.15mm

Which sounds totally reasonable.

Yeah I was thinking grip would be the hardest thing given how low the CoG is going to be on the cars. Something like some new minipins on something like a carpet tile could probably give enough grip, either that or else just have it propped up on the ground and lift it up. I guess as long as the exact method used is the same between measurements it would be pretty accurate. Just tried mine and its 34.85mm

As to how usefull this information is to an off-road rc car, i've no idea.

I think even '80s monster trucks would struggle with grip if they were part way up the board. See the magazine pic below, the idea is they rest against the ground.

http://www.oople.com/forums/picture....&pictureid=513

Board or no board you'd probably need to lock the suspension somehow at the desired ride height if you wanted to accurately work back to C of G height. Even without this at least it is repeatable and if done like in the photo probably a pretty good way of comparing two vehicles.

I dunno, what is it you want to do haulin?