Weight placement and Balance and COG

[soundcolor]

I was recently asked this question over a PM, and thought that it was an excellent question. I decided to answer it in a regular thread, because the answer would be a little long for my FAQ post, and because I thought it would be benificial for some of us who dont know to learn about.

Here is the PM I recieved

"I dont know who to ask this but i have yet to search this question as i do not yet have a MRC yet but hope to have one by this end of this month for my b day. But i think this question may come up in some peoples minds. And i think it would warrent being on your MRC FAQ thread. Whats the difference from putting weight in the tires compared to weight on the axle housings, or other places that are not in the tires? And the advantages of such. Sorry if this has been asked already or not or has been added already. I jus looked over the thread again and didnt see it. If i am wrong about this jus let me know and dont worry about. But i have a lot of free time on my hands due to my job and have yet to seee this question answered..."



Weight placement directly affects both weight balance and Center of Gravity. Lets take a minute to examine what weight balance is. Imagine if you will placing one scale under each tire of your MRC. The entire weight of the rig is resting on those four tires, which is being show in the scale as pounds or ounces. Weight balance could then be described as the percentage of weight that each wheel carries. We can then take those numbers for each wheel, combine them in the appropriate manner and find out what our front/rear weight balance is, or our left to right weight balance. (this is just another way of saying weight distribution, as you will see in most automobile magazines).
Now that we understand what it is, lets set some guidlines that will help you in properly balancing your rig for optimal perfomance. Getting the perfect balance can be a very difficult thing to do. It requires alot of advance planning, and consideration. However once you get it right, it can really change every aspect of how your crawler works.
One of the first things that people often do with a new MRC is to move the electronics to the 3 links, and mount the battery to the top of the servo. But, really the only explination they ever get for doing this is, it puts more weight over the front axle. Thats true, it does. But why should we do that. Whats wrong with that battery being in the center of the chassis mounted on the battery tray. Now, please bear with me, because we are going to overlook, and skip right over the COG (center of gravity) issue right now.
Having more weight up front helps us because it puts more downward pressure on the front tires, causing them to deform, which makes the tires contact patch larger. It also reduces the effects of weight transfer.
What happens when you are sitting at a redlight, the light turns green and you mash the throttle to the floor of your car? The rear of the car squats down, the front lifts and you go. The reason for this is weight transfer. It isnt something you can see, but basically it is physics in motion. Now, the same thing (only in a much, much smaller scale) happens with our rock crawlers. When we go from a standstill to moving, all the weight gets transfered to the rear of the rig. We dont want that. The basic idea is you want your front tires to pull you up a climb, rather than the rears push you up.
Here is a neat little experiment to try, and it really explains it perfectly. Set your MRC on the ground, reach down with one hand and hold onto either of the rear tires, now give it throttle and watch. Repeat with the front.
Did you notice how when you held the rear, the rig wanted to flip over backwards... But when you held the from it didnt. That is why you want alot of the weight toward the front of the rig.
Out on the rocks, weight often acts like your hand did in that experiment. If you have alot on the rear, the rears want to dig in and bite hard, causing your rig to want to flip. Whereas putting that weight up front will make the front tires dig in and bite hard, pulling you up the climb.
Now that we understand weight balance we can get into some of finer points, and answer some tougher questions.
So, which is better, having the weight on the axle or the wheel? Well, ideally you want the weight as low to the ground as you can get it. Obviously that would make the most obvious choice to mount the weight to the bottom of the wheels, as thats as low as you can get it. However, weight wont stay there because the wheels rotate. There isn't alot of room on the MRC's axles to mount much weight, so that is why most people go with adding weight to the tires.
Now, if you imagine the MRC as a lever then you can understand something critical in dealing with weight balance. If you are sidehilling and the slope is running downward to the left, then the obvious place you would want the weight is on the far right side of the rig to stop that side of the rig from lifting off the ground. Add enough weight to that side and it will crawl a vertical wall sideways. But we can't do that, because we arent always going to be crawling on that particular slope. And that is the other reason we put the weight in the wheels instead of on the axle. Getting the weight as far out to the corners as possible helps keep the outside tire planted to the rockface reducing its potential to lift.
How much weight should you run in your wheels, and how do you add wieght to them. Well, if you have the stock servo (please if you do, do not ever remove the servo saver) you should place no more than about 2 to 3 oz's. Yes, it may be able to turn more, but understand this, what it will turn on a flat, smooth surface it wont do when you get to the rocks. All you will end up doing is turning the servo, while the tires keep pointing straight. If you have an upgraded servo, and depending on what you have, You shouldn't need any more than say 8 oz. Most people run in the 6 to 7 oz range. To add the weight to stock wheels you need to perform the stock beadlock mod. Once you have that done, or if you already have beadlocks then here are some options.
1st, run a couple of rows of BB's, held in place by some tape around the inner channel of the wheel
2nd, get some lead stick on wheel weights for a 1:1 car (or for an RC airplane)
3rd, use lead rope, flattened out and wrapped around
4th. lead fishing weights
Some wheels have a built in weight system, if they do you should use that.

Now, lets talk about side to side weight balance. You obviously dont want your rig heavier on one side than it is on the other. The narrower your chassis is, the less chassis weight matters. Idealy the best place for all of the chassis weight would be perfectly center and forward. Since there is no way to accomplish this with an MRC (due to suspension constraints, and packaging) we have to do the best we can.
I cant really give you any tips or tricks, or mounting locations for anything in particular, but, if you hold the rig by the center of the skid (fingers on the front and back of the skid, not the sides) and the rig wants to tip either way more than a few degrees, or flops over to one side, then you need to work on your left to right balance.


I hope this helped, I will post up about Center of Gravity in my next post

 

[soundcolor]

COG, or center of gravity

Center of gravity is very easy to understand,
Basically, how high off the ground is the weight.
The higher the weight, the more your truck will feel tippy, and want to roll over.
Have you ever noticed how go-carts just dont want to flip over. Sure they will, but it takes an act of congress to get them to. Its because all of their weight is super low to the ground. Which is the same reason we see ultra high performance cars and race cars that are only mere inches from the ground. The lower the weight, the harder it is to pick up.
Now with a rock crawler we have to make some compromises. We want the weight to be as low to the ground as possible, but we still need some ground clearance to be able to get over obstacles.

With our MRC's being so small, its hard sometimes to find a good place to mount things down low. There just isnt enough room, or wheelbase to do what we need to do.
When we speak of wheel weights, obviously, we are always going to have an equal amount of weight on top of the tires as we do below, because the tires rotate, however this is not the case with the axles, suspension and chassis.
Alot of people assume that COG is a stationary thing, but it is not, Gravity is.
Gravity is the invisibly force that pushes/pulls everything down. However, when you are out on the rocks, and are climbing or sidehilling then gravity is pushing/pulling you downward, but the distrobution of gravity is diffrent. Its no diffrent then if you are standing up on your feet, or laying down in your bed. Yes, gravity is still applying the same amount of force to you either way, however, if you are on your feet then all of your weight is being applied to those small contact patches at the bottom of your legs (your feet) whereas if you are laying on flat on the ground, then all of your weight is being stretched out over a much larger surface (your whole body). Sure you weigh the same, but the load is not the same. The same thing can be said for your crawler.
Now, consider this, when your crawler is sitting on a flat level surface, all of its weight is being sent straight downward and into its tires. However, drive your crawler up a hill, and all of the sudden we see all of that weight that was at all four tires being transfered to the rear tires. Weight is always applied and transfered in the thing closest to the ground. Its a fact of physics. You can have a 200 million ton bridge, but all the weight of the whole bridge is being transfered to the supports.

Understanding that gravity acts in one motion, but Center of Gravity is relative to location is the key here.

To get your COG where you want it, consider moving anything you can move down as low as you possibly can. We are starting to see a great trend in the new chassis that are coming out. They are cutting a recess into the floor of the skid so that the tranny/motor sits as low as possible.
Every ounce counts in this game. You may not notice 1 extra ounce left up high on your rig, but you will notice 5 or 10.

Getting the COG down low is why you see so many of the best rigs with a drooped suspension.
Here are some steps to take to get your COG as low as possible.
1st, move that batery. Its big, and heavy
2nd. move those electronics, they are small, and dont weigh much, but they are easy to move and every ounce counts
3rd, rework your suspension to get your rig lower to the ground
4th ADD weight. add wheel weights, add weights to your skid if you have it takes roughly double the weight down low to counter act the weight up high (another reason for wheel weights)


Now, alot of times you will see and hear people tell you, "only add weight to the front tires"
This is very good advice, and almost always works. When you are descending a hill, chances are you aren't going to be applying the throttle (however if your rig starts to flip over fowards, then apply just a little, and the rear end will settle back down).
Having an equal amount of wheel weight as the front is only going to hurt you. Most of the comp rigs out there will only have between 1 to 3 ounces IF any in the rear. Its just not productive. The amount of times you are climbing a hill in reverse are rare. And when if you were, then the tires would be rotating in a diffrent direction than if you were descending head first down the same hill.
When you are climbing up a hill, you dont want the fronts to have to pull any more weight then neccessary, adding rear weight makes them have to pull more, and as discussed earlier, makes the rear bite. We dont want that.

So, keep the weight down low. Be obsessive about it.

 

[gottorque]

Props Soundcolor. That was very well writtin and it served it purpose and i learned some

 

[DareDiver]

Thats the perfect answer, thank you soundcolor. Im lookin forward to your freemod thread, also. Thanks keep up the good work....