Why Are Lightweight Vehicles Better Off-Road Than Heavier Vehicles, Or Are They?

[Humvee4us]

Lately, I've been reading up on off-roading. There seems to be two things that directly affect off-road performance and those are traction and flotation. As a heavier load pushes the tire down more and a wider tire disperses the lbs/inch² these two, traction and flotation, seem to be opposites of each other. You cannot increase one without decreasing the other. However, I keep reading article after article that for an off-road vehicle lighter is always better, but never explain why until I came across one article that read that it's because a Jeep or other light-weight vehicle has better traction and flotation than a heavier counterpart. How can it have better traction And flotation if you can't increase one without sacrificing the other? So my question is if, and if so why, a lighter vehicle is better off-road and how can a light vehicle have both better traction and flotation than a heavier vehicle, if anything the heavier vehicle would have better traction due to the added pressure on the tires, no?

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[Arterius2]

I think the science behind this is far more complicated than that. And I can’t even begin to explain it properly. But generally you’ll have find the good balance between wider surface area vs friction caused by gravity. The friction coefficient of the tire material is also another level of factor at play here.

The only time flotation matters is if you are traversing through some soft/liquid-like substance like sand, snow, or mud. Each has its own properties. Both shallow sand, mud and snow has lower friction coefficient on top but as you go deeper they become more solid and compact and thus higher coefficient on the bottom. So you want your tires to dig and reach the bottom. While deep sand, mud and snow has consistently low friction coefficient all the way through so you want to float on top and paddle your way out.

If you are just talking about hard stuff, then lighter is generally better (not always). Basically more weight your vehicle has, you need a higher coefficient in the tires to overcome the friction force caused by gravity. Again, it also depends on the surface you are traversing — is there a coat of the softer stuff on top of the more grippy harder stuff on the bottom? Is it wet or dry? I think there are just too many variables to have a one rule vs all sort of thing.

But just remember Newton’s first law of motion — An object at rest stays at rest until acted on by an unbalanced force. So lighter object = less force to get it moving = less traction is needed / less friction is acting upon it in opposite.

 

[Humvee4us]

Right, so striking a balance between flotation and traction. But this proves my point that you can't have both increased flotation AND traction, right? Also, why do many people keep saying that lighter is always better off-road? You could just increase tire size and width until you strike the same balance that a light off-road vehicle has but with the added advantage of bigger tires.

 

[RubiSc0tt]

So lighter object = less force to get it moving = less traction is needed / less friction is acting upon it in opposite.

It is essentially this. Keep in mind though, generally speaking- a lighter vehicle will also have less rolling mass (I think that's the term I'm looking for); Basically, it's going to be easier to maneuver and also have less forces acting on it in off camber situations- there are other factors to consider (like Center of gravity, weight distribution, etc), but generally speaking it's all related to Arterius2's explanation. Remember- you're not just dealing with a flat surface all the time. There are rocks, hills, side hills, etc.

You could just increase tire size and width until you strike the same balance that a light off-road vehicle has but with the added advantage of bigger tires.

At some point though, you're out of that balance, or your Center of Gravity/ weight distribution is so out of whack that you're actually limited. Sure, you can stuff 42" sticky tires on a Jeep and go nuts, but you're either going to have to do a lot of suspension engineering, to keep it capable, or start thinking about turning it into a buggy- because having 12" of lift just isn't practical anymore, despite what some people will tell you. Also: There are different tire compounds based on what you're doing: That's why you see full on buggies w/ 40+" tires able to climb walls.

Also: Contact patch: Terrain, tire, wheel, and tire pressure will all factor into this: low PSI will give you a greater contact patch, which will allow you traction on surfaces like rock. A greater contact patch will also allow you to float over terrain, like sand or snow. I've run 12.5" wide tires on a 10" wheel at >10psi on my old TJ in the rocks, and it went everywhere. I ran the same pressure in on 12.5" tires on an 8" wheel and had to winch on the same stuff- less of a contact patch. On the flip side: When I was running on the beach in Cape Cod, I had 31X10" tires @ 5PSI and they dug into the sand. They kept digging and i was getting nowhere. I had to drop them to 2-3PSI in order to float over the sand. The following year I was running 33x12.5" Tires again, at 2-3PSI and the Jeep effortlessly cruised over the sand.

If any of this is unclear or incorrect terminology I apologize, it's the end of a long night shift.

 

[Humvee4us]

I think the science behind this is far more complicated than that. And I can’t even begin to explain it properly. But generally you’ll have find the good balance between wider surface area vs friction caused by gravity. The friction coefficient of the tire material is also another level of factor at play here.

The only time flotation matters is if you are traversing through some soft/liquid-like substance like sand, snow, or mud. Each has its own properties. Both sand and snow has lower friction coefficient on top but as you go deeper they become more solid and compact and thus higher coefficient on the bottom. So you want your tires to dig and reach the bottom. While mud has consistently low friction coefficient all the way through so you want to float on top and paddle your way out.

If you are just talking about hard stuff, then lighter is generally better (not always). Basically more weight your vehicle has, you need a higher coefficient in the tires to overcome the friction force caused by gravity. Again, it also depends on the surface you are traversing — is there a coat of the softer stuff on top of the more grippy harder stuff on the bottom? Is it wet or dry? I think there are just too many variables to have a one rule vs all sort of thing.

But just remember Newton’s first law of motion — An object at rest stays at rest until acted on by an unbalanced force. So lighter object = less force to get it moving = less traction is needed / less friction is acting upon it in opposite.

Correct, you need a higher coefficient because you'll be applying more torque which all other things equal would cause the tire to slip sooner than with a lighter weight vehicle. However, doesn't the extra weight by pushing the tire down harder provide you with that higher coefficient?

 

[multicam]

My simple crocodile brain explanation is that a vehicle that meets all the safety standards and feature expectations of the 21st century is not going to be so light that it has to worry about losing traction because of lack of weight. Even my ~3300 lbs YJ did not have this problem.

 

[Humvee4us]

My simple crocodile brain explanation is that a vehicle that meets all the safety standards and feature expectations of the 21st century is not going to be so light that it has to worry about losing traction because of lack of weight. Even my ~3300 lbs YJ did not have this problem.

True, but I mean because moving more mass requires the application of more torque then wouldn't the extra pressure from the extra weight pushing down harder on the tires provide that extra grip in order to get traction?
Also, if you widen and enlarge the tires to a certain degree then you'll basically distribute the weight of the vehicle and as such shouldn't the extra torque applied to move the vehicle also be distributed? This would, in turn, be exactly the same as a much lighter vehicle, no?

 

[D60]

Light is might.

Less weight = less to haul up the hill. Rock climbers and polka dot jerseys know this (a little doping program never hurt)

Less weight = less load on joints, less breakage. Ever seen knee pain disappear when people lose weight?

Everyone else have fun with the enginerd discussion but the overall concepts are simple. Don't bother quoting, I won't be revisiting this thread.

 

[Humvee4us]

Light is might.

Less weight = less to haul up the hill. Rock climbers and polka dot jerseys know this (a little doping program never hurt)

Less weight = less load on joints, less breakage. Ever seen knee pain disappear when people lose weight?

Everyone else have fun with the enginerd discussion but the overall concepts are simple. Don't bother quoting, I won't be revisiting this thread.

But if you have significantly more torque from a larger engine then it really doesn't matter if you have more to haul up because you also have significantly more torque to do so.

 

[Headbarcode]

But if you have significantly more torque from a larger engine then it really doesn't matter if you have more to haul up because you also have significantly more torque to do so.

Yes in theory, but in practical use that usually ends in wheel spin when off road.

Adding down pressure to the drive wheels for more traction is good on level ground, assuming the vehicle has ample torque to overcome that added weight. But the higher the incline becomes, the less weight is pulling the vehicle straight down towards the ground and more weight is pulling the vehicle down the hill. That's where the wheel spin becomes increasingly inevitable.

 

[wolfdog]

I've driven sand and towing boats on sandy beaches with all Jeeps from CJ5's to heavy Grand Wagoneers. Oversize tires, little tread wide rims and low air they all "floated" great" However CJ5 and 7's best! Don't really know by the numbers the "science". "Contact patch" is the secret with loose stuff and also with a bit more air rock. I do not believe that airing down in snow is a good idea, there I think the right tread is the secret?

My 2 cents if that! As someone above said "don't quote", I'll probably not be back, just causing trouble and running! LOL

Go Jeep!!!

 

[Odyssey USA]

My thought goes like this. Lighter =less breakage. Lighter than another equal vehicle also means I can run a narrower tire yielding a lower rotational weight with equal flotation and traction... generally.

 

[Notorious]

There seems to be two things that directly affect off-road performance and those are traction and flotation.

There’s more than just two things and you’re forgetting the single most important element that influences off road performance.

What that element is, I’m not telling you just yet...

 

[Humvee4us]

Yes in theory, but in practical use that usually ends in wheel spin when off road.

Adding down pressure to the drive wheels for more traction is good on level ground, assuming the vehicle has ample torque to overcome that added weight. But the higher the incline becomes, the less weight is pulling the vehicle straight down towards the ground and more weight is pulling the vehicle down the hill. That's where the wheel spin becomes increasingly

This makes perfect sense actually. I've asked five mechanics and two engineers this past week for this answer and none of them could give it to me. Thanks Headbarcode!
So in theory to get over this problem I could increase tire size and width in order to spread out the lb/inch² of the torque that I applied to get the vehicle up the hill and this way the wheel won't slip?

 

[Chocolate Thunder]

This makes perfect sense actually. I've asked five mechanics and two engineers this past week for this answer and none of them could give it to me. Thanks Headbarcode!
So in theory to get over this problem I could increase tire size and width in order to spread out the lb/inch² of the torque that I applied to get the vehicle up the hill and this way the wheel won't slip?

Assuming you have adequate torque (which is evident because it’s enough to spin the wheels) the only variable in this scenario that you can affect is traction. Gravity is a constant pulling you backward down the hill. If you’ve got enough torque to break the wheels into a spin, the only thing to improve ability to climb is more traction to utilize all of that torque. Wider tire contact patch (airing down) and a higher friction compound and construction of your tires are the things that’ll help you get up that hill.

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