Difference Between Sprung & Unsprung Weight

[Wildman]

As I was working on the Jeep I started thinking about this and how the two affect your rigs traction.
I know a lot of people switch to aluminum armor for their Jeep. The reason being that it's been proven to help with traction. That is your sprung weight but how does unsprung weight affect your traction? I've heard about people putting water in their tires to help with traction.

I'm not knowledgeable enough to understand how this works. So I'd like to hear others thoughts about this.

 

[Rick Flair]

In on this, always heard this but I’m not 100% understanding the practice.

 

[Bill M.]

All our tractors have water and 30% calcium chloride in the tires for traction.

 

[WallyWest]

It's weight that stays at the bottom I guess? Doesn't really seem like there would be enough increase in weight to make any real difference.

 

[Brianj5600]

Sprung weight is supported by the springs. Unsprung includes wheels, tires, axles, tie rod, steering stabilizer, steering ram and half the weight of the following, drag link, coil springs, shocks, control arms, track bar(s) and driveshafts. That will get you pretty close.

 

[Bill M.]

It's weight that stays at the bottom I guess? Doesn't really seem like there would be enough increase in weight to make any real difference.

We fill the tires nearly full. You do get a low CG though.

 

[JEEPCJTJ]

I remember growing up in the 1970s and the neighbor's two 8N tractors had calcium chloride in the rear tires. For the several times that I was sure that they were going to roll over sideways, the center of gravity was so low it never happened. Rolling was almost impossible, either due to the calcium chloride or the whole tractor being unsprung weight.

 

[Apparition]

Unsprung weight is especially bad for acceleration, braking and mileage, in the form of rotational inertia.

 

[Wildman]

I'm not just talking about filling the tires. I'd like to discuss how each affect traction and performance off-road.
We try to save weight on our rig but I don't see people talking about the weight of rims or tires.

How does a rig that weights 500lbs more perform compared to the lighter rig? And what if half that weight is unsprung weight how does it affect each rig?

 

[toximus]

A tractor is an entirely different beast than what (some of us) do with our Jeeps. Unsprung weight is difficult to control from a suspension perspective and leads to decreased performance. It's also very hard on components.

Years back my understanding is that guys ran water in their tires for SRRS and the KOH Backdoor Shootout. But once they understood that shock tuning performed better than weighted tires that practice stopped. Somebody can correct me if my history isn't correct.

 

[Vinman]

I'm not just talking about filling the tires. I'd like to discuss how each affect traction and performance off-road.
We try to save weight on our rig but I don't see people talking about the weight of rims or tires.

How does a rig that weights 500lbs more perform compared to the lighter rig? And what if half that weight is unsprung weight how does it affect each rig?

I’ve been harping about tire/wheel weight for 2 decades but most folk I talk to just shrug it off.
An extra 10 pounds per wheel/tire combo has way more negative effect than throwing 10 pounds in the back of the heep.
I think the formula was either 10-1 or 4-0, ie: 10 lbs unsprung rotating weight is like 40 or 100 lbs of sprung weight.
Any time I shop for wheels or tires the very first spec I look for is what they weigh.

 

[Wildman]

I’ve been harping about tire/wheel weight for 2 decades but most folk I talk to just shrug it off.
An extra 10 pounds per wheel/tire combo has way more negative effect than throwing 10 pounds in the back of the heep.
I think the formula was either 10-1 or 4-0, ie: 10 lbs unsprung rotating weight is like 40 or 100 lbs of sprung weight.
Any time I shop for wheels or tires the very first spec I look for is what they weigh.

If I have a tire & wheel combo that weigh 100lbs and another that weighs 140lbs how does this effect the rig off-road?
And then how does having 200lbs of gear in your rig effect it compared to not having that weight?

 

[Vinman]

If I have a tire & wheel combo that weigh 100lbs and another that weighs 140lbs how does this effect the rig off-road?
And then how does having 200lbs of gear in your rig effect it compared to not having that weight?

Generally more weight has a negative impact in most situations.
I’ve seen a bare bones YJ on 35’s with stock open diffs put a much heavier Rubicon on 37’s to shame climbing a steep, slippery incline more than once. There was probably a thousand pound difference between the two rigs.

I get the adding liquid weight to the tire to increase weight down low but how is the weight dispersed once the tire starts spinning more than a few mph? And what is the impact on the drivetrain if one adds 40 or 50 pounds to each tire?

 

[mrblaine]

I’ve been harping about tire/wheel weight for 2 decades but most folk I talk to just shrug it off.
An extra 10 pounds per wheel/tire combo has way more negative effect than throwing 10 pounds in the back of the heep.
I think the formula was either 10-1 or 4-0, ie: 10 lbs unsprung rotating weight is like 40 or 100 lbs of sprung weight.
Any time I shop for wheels or tires the very first spec I look for is what they weigh.

I don't ever shrug it off. But there again I've helped folks fill tires with weight for their competition crawlers and once you try to roll a tire that weighs 300 lbs, you get it instantly.

 

[mrblaine]

Generally more weight has a negative impact in most situations.

I get to see the difference a bunch. Some folks show up in a rig that is just overweight. We call them lumbering pigs because they have zero grace and agility traversing the trails in JV. They struggle on every little thing that lighter rigs don't even know are there. Weight is not your friend.

 

[mrblaine]

You do get a low CG though.

That bears some very deliberate discussion and research. I could very well be fully wrong but the way I remember my conversations with folks smarter than me is the COG doesn't really change. To tip a TJ over or any vehicle for that matter, once the COG moves outside the tire box, it's going over no matter what it weighs. Change my mind.

 

[TJ Hunnicutt]

Unsprung weight is a double-edged sword...with a distinctly sharper edge on one side.
Imagine if you were to fill your tires with Mercury. They would be VERY heavy and would dramatically lower your center of gravity because a massive amount of weight would be below the usual center of gravity and this, by itself, would be a good thing...in that it would help you stay upright as long as possible in off-camber situations.
Now, the bad news. Those Mercury filled tired would also have massive...uhm...mass. Following good old Newton's first law of motion; the amount of energy required to move an object is [EDIT: directly] proportional to the mass of the object so, the greater the mass (weight), the more power it takes to make it move...and the greater the strain on all of the components of the drive train that's trying to make it move. Bad news two; Newton's second law of motion states that an object at rest tends to stay at rest and an object in motion tends to stay in motion and, in both cases, this tendency is directly proportional to the mass of the object. The more something weighs, the more it resists changing speed. Sooo, your Mercury-filled tires are now much happier to try to crash through the square-edge jaggy than to roll over it and, once they do get moving up, are far more resistant to stopping going up than a light, air-filled tire.
In short, unsprung weight should be kept as low as possible so that you drive line components don't have to fight the inertia of your heavy wheel, tires, etc. as well as the dealing with the bounces, bumps, and jolts of the terrain.

of course, if you only crawl at relative turtle speed,the lower center of gravity and the simple fact that heavy tires will tend to dig in and "hug" the terrain will far outway any inertial downside.

 

[Mike_H]

When I was back in my racing days, we used to roughly equate a pound of rotating mass to 5 horsepower. So, if you could reduce rotating mass by a pound, it was like adding 5 Hp to the engine.

I'm not a fan of extra weight, anywhere, especially rotating mass. We're in Jeep land though and that means bigger, heavier tires and wheels. So... You add gearing to make up for both the increase in overall gearing from bigger diameter tires and the extra weight your trying to sling around.

Sprung vs Unsprung... There is less effect of acceleration from sprung weight vs Unsprung. However, sprung weight will effect handling much more than Unsprung weight. It's all about inertia.

 

[TJ Hunnicutt]

That bears some very deliberate discussion and research. I could very well be fully wrong but the way I remember my conversations with folks smarter than me is the COG doesn't really change. To tip a TJ over or any vehicle for that matter, once the COG moves outside the tire box, it's going over no matter what it weighs. Change my mind.

Attempting to change your mind is, on its face, is a dangerous prospect but, in this case, I wouldn't dare.
It is true that, once you exceed the proverbial "tipping point", you will be going over. And, greater weight below the COG, will actually increase the roll because, once the vehicle is inverted, it becomes weight far above the (new) COG.
The advantage to the weight below the (right-side-up) COG is that it delays the tipping to very near the tipping point (again, keeping in mind the second law...so a bump near tipping point can actually be worse) and, in general, stabilizes the vehicle by putting a bunch of weight below COG.

With that, I'd like to amend my earlier statement that, at very low speeds, the heavy tires/wheels would be all good side with a mention that those massive tires/wheels would still add incredible strain on the drive components, again, due to the second law. Think of multiplying the herky-jerky feeling of being in granny gear and goosing the gass or going over a sudden bump.

 

[Mike_H]

That bears some very deliberate discussion and research. I could very well be fully wrong but the way I remember my conversations with folks smarter than me is the COG doesn't really change. To tip a TJ over or any vehicle for that matter, once the COG moves outside the tire box, it's going over no matter what it weighs. Change my mind.

I would like to further discuss as well. How much weight would it take to actually make a difference?

Loading the tires should lower the CoG. By how much? Who knows.

I can say that if we approximate a 35" tire on a 17 in wheel, there should be around 25 gallons of available volume to fill with something. If we choose water, we now have loaded our tires with an additional 207 lbs, each. That's 828 lbs total, all located lower than the unloaded CoG. Does it matter? How much does it "flatten" the box?

Oh... This rig with loaded tires is going to be an absolute pig to drive. But for the purpose of discussion, let's ignore that for now.