Crawler theory - Weight.

[Pygmy]

Hi,
I'm trying to piece together all info I can find on crawler setup theory. For offroad buggies there's lots of tuning / setup theory documentation, some of it is valid for crawlers as well, most of it is not.
So for crawlers, let's discuss weight, and weight distribution. I'm just going to write down my thoughts, and I'd like you to pick up from there - correct me where I'm wrong, and add any information you think is relevant.
And please explain it like I'm 5 years old since everything I know from the high-speed offroad scene is probably wrong

Weight.

On a crawler, I want to have the lowest center of gravity possible, to make sure the car doesn't fall over when climbing steep hills. Right ?
Then again, a crawler needs as much bottom clearance as possible to avoid getting stuck.
So here's the first balance-point to consider - Low COG vs. High Clearance.

One way to help improve this balance would be to move as much weight as possible down towards the ground, and the chassis as much as possible up so it's harder to get the chassis stuck on something.

So - we move the weight off the chassis, onto the axles, hubs or wheels.
If we add extra weight to the wheels, we're adding rotating mass which influences acceleration / deceleration.
If the weight is applied on dynamically on the tires (f.e. I've seen people mentioning filling tires with water, or putting ball bearings in them) it will shift the COG when the vehicle is on an incline, and influence acceleration / deceleration. (If you stop a fast-moving car with water in the tires, the water will move to the front of the tires dragging the car forward)
If we add extra weight to the steering knuckles / axles, we're moving weight down but we're not adding rotating mass... which sounds like a better idea, but the weight is only down as far as we can get it - probably not as far down as the bb's in the tire would have been if we were on horizontal ground.

This is just the stuff I can think up as a noob so I'm probably
a) wrong about some stuff
b) forgetting about more important stuff

So what is the general consensus on weight distribution / placement on crawlers ?
Does a "crawler theory bible" exist with info I can read up on ?

[JokersWild]

That's about it..... Low cog, high clearance...... Keep out of wheels due to rotating mass ruins bearings. Comp crawlers went lighter over the years from being heavy pigs. The low cog helps roll overs and side hills. Weight distribution around 60/40 for most comp crawlers front to rear and some scalers like that or go to 50/50.

If going to add weight knuckles is the best bet vs wheels.

Also changes in rigs weight changes speed and amount of tq used to steer wheels.

This is my theory and part of no bible.... But this is perfect world...... Some top heavy rigs do very well.

[crawl-o-matic]

I'm very much in the minority, but I don't add any additional weight to my comp rig.

[Crawllife]

I'll jump in just to be confusing. I typically build with light weight parts so I wind up adding up to 8 oz of weight to my knuckles. It's alot if weight but I still fall in the 4.4 lb range.

[Hardline]

All upper weight is bad. Low weight is less bad on a shafty than on a moa because the shafty has unavoidable upper weight in the chassis, so saving weight from the most obvious place, wheels & tires, hurts the CG worse than on a moa. Best low weight would be D-shaped, lower half knuckle weights. An ultra-lite shafty has trade-offs. Proper ballasting will improve performance on angles due to lower CG. But minimum weight pays off on nimbleness, jumps up/over ledges etc. Many like a final weight distribution of 60-62% front. Higher front percentage = better climbing, worse descending. And vice-versa. No one configuration is best for everyone and everywhere.

[AddictedtoRC1]

Whether your rig weighs 5.5lbs. or 3.5lbs., the key is distribution. A forward bias is what you're going to want no matter what your rig weighs. Everything as low as possible obviously. A rig with a clearance of 2.5" and a 60/40 bias in theory will do just as well as a rig with 3" clearance and 50/50 bias on breakovers.

[jeepgeek2002]

My Gmade is ass heavy with the battery pack.. in my case ..i should relocated the battery to the front axle. i put some wheel weights on the servo and on the diff skid to see if it will drop the front down some. I noticed on the table the swampers are starting to mush a little. so what seems little to me, in scale might be more than i think. I like the gmade with FunJac high clearance arms for the LGG.. ($250 for a rolling chassis and portal axles)
it also depends where you live matters and what kinda terrain you ll throw at it.

[wings_of_fire]

Originally Posted by Pygmy

Hi,
I'm trying to piece together all info I can find on crawler setup theory. For offroad buggies there's lots of tuning / setup theory documentation, some of it is valid for crawlers as well, most of it is not.
So for crawlers, let's discuss weight, and weight distribution. I'm just going to write down my thoughts, and I'd like you to pick up from there - correct me where I'm wrong, and add any information you think is relevant.
And please explain it like I'm 5 years old since everything I know from the high-speed offroad scene is probably wrong

Weight.

On a crawler, I want to have the lowest center of gravity possible, to make sure the car doesn't fall over when climbing steep hills. Right ?
Then again, a crawler needs as much bottom clearance as possible to avoid getting stuck.
So here's the first balance-point to consider - Low COG vs. High Clearance.

One way to help improve this balance would be to move as much weight as possible down towards the ground, and the chassis as much as possible up so it's harder to get the chassis stuck on something.

So - we move the weight off the chassis, onto the axles, hubs or wheels.
If we add extra weight to the wheels, we're adding rotating mass which influences acceleration / deceleration.
If the weight is applied on dynamically on the tires (f.e. I've seen people mentioning filling tires with water, or putting ball bearings in them) it will shift the COG when the vehicle is on an incline, and influence acceleration / deceleration. (If you stop a fast-moving car with water in the tires, the water will move to the front of the tires dragging the car forward)
If we add extra weight to the steering knuckles / axles, we're moving weight down but we're not adding rotating mass... which sounds like a better idea, but the weight is only down as far as we can get it - probably not as far down as the bb's in the tire would have been if we were on horizontal ground.

This is just the stuff I can think up as a noob so I'm probably
a) wrong about some stuff
b) forgetting about more important stuff

So what is the general consensus on weight distribution / placement on crawlers ?
Does a "crawler theory bible" exist with info I can read up on ?

Very well written. I like the way you thought about it in a systematic order.
You probably are, or you could be a good engineer

[Pygmy]

Originally Posted by wings_of_fire

Very well written. I like the way you thought about it in a systematic order.
You probably are, or you could be a good engineer

gameprogrammer ;)

[jado6006]

While your being givem good advice, i want to remind you that its all relative. Just an example-These days everyone is advocating weight at the knuckles & not in the wheels. Well its a definite trend, like the ever lighter rigs being built. If your not going to compete, you put it wherever it's easiest. Dudes have been running wheel weights for years with excellent results, until lately. Its out of favor now. Reminds me of doctors telling ya to not drink wine after advocating drinking a glass a day, cuz its good for your heart. Bottom line, you can go broke buying the latest hottest gear. Just don't think you have to.

[Hardline]

I think he's just trying to get a handle on the subject. And doing a good job. I just want to point out there are no absolute correct answers. Major events across the country have been won by both heavy and light rigs. I can tell you for a fact that when I take a properly tuned good performing heavy shafty and knock all the wheel weight down to minimum, it no longer climbs the slabs it used to climb easily. But it handles "ugly" rock better. It's all a matter of where you want to set your compromise. But that's weight...as to weight distribution I think nearly every hot rig I've ever read about balanced at/around the lower link rod ends.
I've heard people talk about 50/50 rigs, but if they mean it balances at the center of the wheelbase, I've never seen one. And know I'll never own one.

[catzuk]

Its very similar to 1:1 rigs, the trends are changing. If one is just slow crawling, then wheel weights are cheap easy to fit and tune in your wheels. But as things move to the want to go fast as well as the crawling, weight on the axle is key for the COG, but keeps the rotational mass down so your wheel speed/acceleration is better. Then the weight bias as mentinoned, 60/40ish seems to favor great climbing without killing downhill drops. But as the speeds go up, a more 50/50 tune can help in the handling/stability. Ultimately, it all ends up in what the user wants out of their rig.

[Rodeo]

You also have to work with your budget and it depends how much you are willing to spend. I run nuckle weights on my rigs, but the ones I use cost about a hundred dollars a set and you can buy some lead weight at Walmart for about five dollars and rap it around your wheels.