center of gravity ?

[beanie]

Is there a simple rule to find what is a safe vertical center of gravity for a build, in other words when does it become unsafe and unsafe at what angles?

Anything is unsafe at 70 degree + on it's side.

However I have seen some road side mowers on a bank so steep sides-ways I would have already bailed from and the guy driving it was half asleep, like ho-hum.

I know lower is better, my question is how to tell what is unsafe other than the simple rule : If it rolls over on flat land sitting still it is unsafe

Thanks again for a great site and you input to my endless questions
Dan

 

[Pito]

hi i dont know that there is any rule for centre of gravity but surely common sense would prevail you will need to decide where you intend to use it & decide your ground clearance then it would all take place from there What motor & speeds do you think you will be running?

 

[kibble]

Don't know that there's any single particular "rule" but like pito said; common sense is a big one. Don't go putting all the heavy stuff on one side without balancing it out on the other. if it's a really low vehicle then this usually isn't a big problem. If it's a bigger vehicle maybe you could abide by: "Wider is better". Having the seat right in the middle for a single person kart is a good idea as the majority of the weight will be in the center.

If it flips over on solid, level ground, then you got some problems!

 

[beanie]

Thanks I agree wider is better and common sense is always a good start.

My concern comes from my recent build. When I compare it to my commerical ATV which is almost always rock solid and the build being a wee bit too top heavy. I would never consider using it on anything except almost flat land as opposed to I will chicken out before the Yammaha gives up.

I guess suspension plays a large part in that as well as low and wide. I am looking for a design rule such as:

What is a good starting point for width vs height of CG.

 

[ryf]

assuming 100% equal weight distribution vertically and horizontally(which isn't likely or really possible), if your height is greater than the width, your triangle of weight will be higher... you can off set that by mounting the motor lower. the better you can center weight, motor/you/everything possible my ATGC is 4x8 footprint and about 4 ft tall, and I would run it anywhere without fear, it is FAR MORE stable than a 4 wheeler, my engine, trans, fuel tank, and me are centered... if you get the right length to width ratio, and get the weight low, reasonable height doesn't matter nearly as much as where the weight is. another thing that matters ALOT is steering, the right steering ratio will make it far easier to feel safe slow and fast and feel in control.

no offense intended by this, is your fear based in its real performance or because you question your building skills? when I first go up to 45 mph, I thought, what if I screwed something up??? you get comfy with time if it IS right.. if its not you must identify the problem.

 

[beanie]

"no offense intended by this, is your fear based in its real performance or because you question your building skills? when I first go up to 45 mph, I thought, what if I screwed something up??? you get comfy with time if it IS right.. if its not you must identify the problem."

It isn't fear at all. It is just that when I build something else in the future I was looking for some guidelines to use during the design phase.

I do however respect the way the thing I built reacts to a side grade, but then it was intended to be run fairly slow on flat ground and with that in mind it fits it's role with more than adequate stability at any speed it will run. I am thinking that the addition of front shock will help quite a bit by keeping the front axel in a horizontal plane instead of allowing a free pivot around the center point of the chassis.

When designing a new "something", i.e. a off road kart, I want a lot of stability at much higher speeds. I do trust my building abilities but not being a vehicle engineer I am learning as I go and having fun while I do it!

I can see that with the centered weight and low slung weight and correct steering geometry that a 4 x 8 platform would be very stable.

My ATV is 50" wide and a low CG, very stable.

Thanks!

 

[Pito]

think about all of this logically from your own point of view, this is only meant to be an example but if you are 10 inches off the ground & your kart is 20 inches wide its not going to feel very safe [you thinking yet ?] that tells me to start at ,at least 2 or 3 times the height for your width / length & try to do things in proportion maybe sit on a board on the floor & mark out where things feel right for you.Take your time with this as a mistake at this stage could be quite expensive [time wise at least] if it is planed properly at this stage i believe you will have a fair bit of trust in it from the start & hey we are all learning here anyway i dont know that there can be any hard & fast rules in this stuff because there are too many variables as we all build things to different specs & strengths anyway just trust yourself in knowing what feels right for you if it dont work just change it hope this is helpful i know it all seems a bit daunting at the start

 

[ryf]

quads are stable because AT LEAST 1/5 of your total riding weight (you and machine wet) can be leaned into a turn a lot farther as well as standing up to get more weight shift. they are very very different. if you want something to scale, look at a vw dune buggy. make it smaller, it will still work.

if your using a front beam like a rider mower, thats most of your problem... mower racers weld those because they are unsafe and do weird things at speed. check out Heymow.com and search for "weld front beam" or replace beam with "end"
my advice go cut that beam in half and do a single a-arm front end like my first front end (which worked really well, we changed it to get a slightly shorter wheelbase and make more room for other things) this is a pic at my first front end, stable at 45 mph



when I say as tall as is wide, I mean including you if you have no roll bar, to the ground. this is an oversimplification to make it easier, you still want to get everything as close to center and as low as feasible. my motor is higher than I want, but it was to help when I cross water, if my butts wet my motors still dry. I off set that by keeping the bulk of my framing low, as well as my seat. if you know how much everything weighs you can kinda get it to where it works out

even if you are 90% flat ground, use bigger tires, soaks up the defects in the ground and makes the ride more fun, squirrelly can also be a terrain issue, I have 23" balloon rears, and 21" fronts, rolls over everything without a hitch.

 

[beanie]

Thanks Guys, good info!

Most things can be fixed but time/money saved by doing it right the first time can be spent on some other hot project.

 

[Pito]

centre of gravity

hi beanie found a couple of sites that might help with your problems hope they are helpful
about.com:metal
spidercarts.com
diygokarts.com
again it all depends what & where you are going to do with it

 

[beanie]

Thanks!