Common 4X4 myths

[BusaDave9]

Myth #1: I often hear it said that an open differential will, at times, send all the torque to the one tire with the worst traction. And that a locking differential (or spool) is needed to send torque equally to both tires on an axle.

The above paragraph is almost completely opposite from the truth. Actually an open differential will always send torque equally to both tires on an axle. And a locking differential can, at times, send all the torque to the one tire with the best traction.

As an example lets say you have a 2 wheel drive car with 2 tires on dry pavement and the right tires are in snow and ice. If the car has an open differential and the driver were to hit the gas he would just spin his right tire and he wouldn't go anywhere. Some would say the right tire is getting all the torque but actually it is getting very little torque. It doesn't take much torque to spin a tire on ice. The tire on dry pavement is getting exactly the same amount of torque. An open differential will always send torque equally to both tires.
Now if the car had a locking differential he would be able to accelerate quickly. The left tire would account for all the acceleration because it would be getting all the torque.

I hear this myth stated incorrectly on a regular basis in books and magazines.

For an explanation of this myth click here: http://www.jeep-cj.com/forums/f49/common-4x4-myths-11138/index9.html#post134587

 

[BusaDave9]

In the above myth, people get torque confused with speed.
A tire spinning at high speed may not be getting high torque.
A tire that is not moving may still be delivering high torque to the ground.

 

[BusaDave9]

Myth #2: People often say anti-sway bars will help keep all 4 tires planted firmly on the ground. In an ad for their Anti-Rock Sway bars Currie says the kit will "Increase off-road traction by balancing and distributing the weight over all four tires".

Once again this is almost completely opposite from the truth. If you don't have anti-sway bars you are much more likely to keep all tires firmly on the ground. Completly disconnecting the anti-sway bars will increase off-road traction by balancing and distributing the weight over all 4 tires.

In this first example imagine you are driving your jeep accross flat ground and your right front tire rolls over a big rock. If you have very soft suspension and don't have any anti-sway bars you would be much more likely to keep all 4 tires on the ground. If instead you have anti-sway bars then as the front right suspension is compressed the anti-sway bar would try to push the right tire down while pulling up on the front left tire. The anti-sway bar would be more likely to pull the left front tire off the ground.

Now if you are going along a side hill it is true you will tilt more if you don't have anti-sway bars. Now lets say the front tire on the uphill side goes over a big rock. You better hope you don't have anti-sway bars on. Because if not the tire will raise over the rock easily. If you have an anti-sway bar up front it will cause your Jeep to tilt even more as you go over the rock. It is now possible the back tire could get pulled off the ground and risk a roll-over.

Don't get me wrong: Anti-sway bars are not all bad. The purpose of anti-sway bars is to hold the vehicle level when going around a turn and the momentum of the vehicle wants to tilt it to the outside of the curve. For that reason anti-sway bars are an excellent way to control sway of a vehicle as it goes around curves NOT for the tilting and swaying of a 4X4 as it goes over rough terrain.

 

[mylittlecj5]

references?

 

[BusaDave9]

references?

Let's start off with myth #1. My best reference is the book Differentials: identification, restoration & Repair by Jim Allen & Randy Lyman. (the same Randy from Randy's Ring and Pinion). This is an excellent book I highly recommend to anyone interested in 4x4 mechanics. Randy really knows his stuff. In the book he says the open differential is Limited by the tire with the worst traction but he also says the tire with good traction is getting the same amount of torque. Here is a direct quote from that book:

The rule with open diffs is that the available traction torque is two times the amount of traction torque available to the wheel with the least traction.

In all honesty both of these myths are so common you will see lots of sources that say the opposite of what I am saying. For example I was just reading Four-Wheeler's Bible by Jim Allen. He has it wrong saying that an open differential will send all the torque to the one wheel with the worst traction and that you only have true four wheel drive on vehicles that have lockers front and rear.

I just ask that you carefully read my posts and try to understand what I am saying. I will be posting references for my comments about anti-sway bars.

 

[mylittlecj5]

I just ask that you carefully read my posts and try to understand what I am saying. I will be posting references for my comments about anti-sway bars.

I'm not saying your right or wrong...but anytime someone posts on the internet and says everyone else is wrong about "xxx", and doesn't post references, he is just perpetuating whatever myth he is in support of. Thereby acting just as bad as the guy that posts the wrong info in the first place.

If your going to argue that everyone else is wrong, its not unreasonable to ask for sources and references to support your position.

 

[BusaDave9]

If your going to argue that everyone else is wrong, its not unreasonable to ask for sources and references to support your position.

I agree 100%. A healthy debate is good. Question me and everyone else on the Internet.
I can find all kinds of sources that support my position but I don't want to post anything unless it's from an expert that you have good reason to believe is credible. That's why I picked Randy Lyman's book.
Give me a little more time to prove my point.

 

[Petescj]

 

[derf]

I agree with the Myth 1.

The thing is, the torque supplied to each wheel is very small. When you have an energy sink like a free spinning tire, the engine is under very light load. And when that happens, it doesn't produce very much torque to begin with.

All of the dynos you run a car engine on test the engine under load. On an engine dyno, there is some kind of piece inside that resists spinning. On a chassis dyno, the tires have to spin huge heavy drums. That load causes the engine to work harder than if it was free spinning.

It's the same principle as watching a vacuum secondary carburetor while you're sitting in the driveway. You can rev the throttle all you want but you'll never see the secondaries open because the engine is not under load. Adding the weight of the vehicle causes the engine to work harder. This extra work builds the extra vacuum needed to open the secondaries.

So when you're just free revving the engine (either in the driveway or spinning a tire on ice) the engine itself is not delivering very much torque at all.



So it is technically correct to say that the same amount of torque is sent to both wheels because the amount of torque being made by the engine is significantly lower. There just isn't enough torque being produced to overcome the the inertia of the vehicle to get it moving.

The proper thing to say is that all (most) of the energy is going to the spinning wheel. A little is consumed by the friction in the differential but the energy is mostly being spent spinning that tire. Torque is just a force. Add movement and you get energy and that's what is going to the spinning tire.

 

[BusaDave9]

Can I add Derf as a reference? I think he's a credible expert.
I still have some persuading to do regarding myth #2 but I have to be heading out.
I'll be back.

 

[derf]

Myth 2 is not entirely correct.

It is correct that if you have no swaybars, you'll get your tires touching the ground more than you would with the swaybars attached. But that doesn't mean you're getting any real weight applied to the drooping tires when you're at or near full droop. The compressed springs hold most of the weight of the vehicle on those tires. The drooped out springs put no force on that side of the axle. The tires will lightly touch the ground but will not carry much more than the weight of that half of the axle. My old XJ had an open front diff and when I disconnected the swaybar it would flex great but would still get no traction on the drooping tire at full droop because there was no real weight on it.


I'm not sure what the advertising people say about anti-rock swaybars. But the whole point of having them is to tame a super flexy suspension. There is such thing as too much flex. When you're on an obstacle, your body can roll significantly and cause the weight to shift so much you end up with tires off the ground or even flopping over too easily. The anti-rock swaybars are really supposed to keep your really flexy vehicle from flopping around side to side and becoming terribly unstable.

A friend of mine had this ridiculously flexy buggy that wouldn't sit straight on flat pavement. He could flex it so that the axles were each 60-70 degrees off flat at the same time. He could drive right over any RTI ramp without lifting a single tire off the ground. That is, if he had someone with a strap keeping him from flopping over. That thing was always leaning to one side or the other. It would tip over on even moderate side camber. He added some Antirock swaybars and it leveled itself out while still allowing enough flex to be useful on the trails. The thing is, with those Antirock swaybars, he was less likely to tip on side camber hills. The swaybar kept his body from rolling so far and it kept his center of gravity closer to the center.

Now, with swaybars of any kind on modern factory vehicles, the suspension is not flexy enough to cause this kind of problem. There is no question a stock-ish suspension will benefit from not having any swaybars at all when going off road. You will get more articulation and you'll be less likely to tip over. But that's because factory style suspensions provide their own limitations to flex. Even with an off the shelf lift kit, you get some binding in the suspension limiting articulation. But when you design out 100% of that binding, you need something to limit flex or you're just going to flop around.

The thing is, admitting that fact won't let them sell more Antirock swaybars to TJ and JK owners so that's probably a motivation for the marketing people to color the truth a bunch.

 

[derf]

Can I add Derf as a reference? I think he's a credible expert.

Well, I did stay at a Holiday Inn Express last night.

I still have some persuading to do regarding myth #2 but I have to be heading out.
I'll be back.

I thought your myth 2 needed a bit of clarification. But it was mostly correct.

 

[BusaDave9]

Okay let me clarify myth #2.
Let's say you drive one front tire of your Jeep up on top of a huge rock. It makes no difference what kind of suspension you have or whether or not you have anti-sway bars the spring on the tire that is on the rock is compressed hard. The tire is rarely up that high into the wheel-well because it takes a lot of force to push it up there.

Let's say the Jeep in this picture has his anti-sway bar disconnected. Just after the picture is taken he backs up and connects up his anti-sway bar. He drives over the same rock but now his front left tire comes off the ground. Why? Because the anti-sway bar tries to keep the front tires level. How hard it tries depends on the torsion qualities of the anti-sway bar. Some anti-sway bars are adjustable. In the above picture the anti-sway bar would pull the left tire up into the air by pushing down on the tire on the rock.
With an anti-sway bar you are always more likely to have a tire come off the ground.
If you don't have an anti-sway bar your weight is more evenly distributed across all 4 tires. As I said before, it always takes a lot of force to compress a tire up high into the wheel-well but an anti-sway bar makes this uneven weight distribution more pronounced.
This is where I think Currie is completely wrong saying their kit will "Increase off-road traction by balancing and distributing the weight over all four tires".
If you want to increase off-road traction by balancing and distributing the weight over all four tires then you should remove your anti-sway bars.
Derf, you talked about a buggy with extremely flexible suspension. He may love it off-road but that sounds very dangerous on the curves. It doesn't sound like a street legal buggy but he should be careful even going around a curve off road. An anti-sway bar will help there. As I said before the purpose of anti-sway bars is to hold the vehicle level when going around a turn and the momentum of the vehicle wants to tilt it to the outside of the curve. For that reason anti-sway bars are an excellent way to control sway of a vehicle as it goes around curves NOT for the tilting and swaying of a 4X4 as it goes over rough terrain.

​

 

[CJim7]

the purpose of anti-sway bars is to hold the vehicle level when going around a turn and the momentum of the vehicle wants to tilt it to the outside of the curve. For that reason anti-sway bars are an excellent way to control sway of a vehicle as it goes around curves NOT for the tilting and swaying of a 4X4 as it goes over rough terrain.

​

This pretty much explains #2 as clearly as can be.

 

[mylittlecj5]

To Busa,
OK so lets say I agree with you...

Since a anti-sway bar was originally designed for a street-driven car, and it does exactly what you explain in the original and subsequent posts on said cars, would you not agree that they are not designed for off-road vehicles? The fact that people have them on a vehicle used off-road is irrelevant, if they are disconnected.
So at the most, all I see to your argument is the apparent fact that you have a problem with the words chosen by Currie's advertising agency...

I personally don't have a anti-sway bar on my Jeep, and it is very "tippy" on the street (a combination of a very soft suspension, worn shocks, and no anti-sway bar), so much that I will be installing one as I work on the CJ this winter...But it will have disconnects installed as well



BTW, I'm just playing Devil's Advocate here, I really am not trying to start a fight....lol
And everyone here knows I don't always phrase my thoughts on a keyboard very well...

 

[BusaDave9]

To Busa,

would you not agree that they are not designed for off-road vehicles?

all I see to your argument is the apparent fact that you have a problem with the words chosen by Currie's advertising agency...

Yes, I would agree that anti-sway bars are not designed for low speed off-road vehicles such as rock crawlers. Sure some off-roaders like to blaze across the desert. There anti-sway bars will help stability as they go around curves.

It's not just Currie's advertising that gets this wrong. I also see magazines print that an off-roader would keep the tires planted on the ground better if it had anti-sway bars and be less likely to have a tire come off the ground.

BTW, I'm just playing Devil's Advocate here, I really am not trying to start a fight....lol

I know your not trying to start a fight MyLittleCJ. Everyone should play devil's advocate if I don't present my facts in a clear concise manner. As I said before a healthy debate is good. There is a lot of information on the Internet and much of it is BS.

 

[derf]

Yes, I would agree that anti-sway bars are not designed for low speed off-road vehicles such as rock crawlers

I'll agree with you here 99% of the time. However, I have seen an extreme example of a buggy that was completely unstable because of its suspension design. In order to make it perform reasonably well off road, they had to add a swaybar. But that is an extraordinarily rare counter example. For just about everyone else, no swaybar (or disconnected swaybar) off road is the way to go.

 

[Kane]

I All of the dynos you run a car engine on test the engine under load. On an engine dyno, there is some kind of piece inside that resists spinning. On a chassis dyno, the tires have to spin huge heavy drums. That load causes the engine to work harder than if it was free spinning.

It's the same principle as watching a vacuum secondary carburetor while you're sitting in the driveway. You can rev the throttle all you want but you'll never see the secondaries open because the engine is not under load. Adding the weight of the vehicle causes the engine to work harder. This extra work builds the extra vacuum needed to open the secondaries.

So when you're just free revving the engine (either in the driveway or spinning a tire on ice) the engine itself is not delivering very much torque at all.

This is backwards. Vacuum holds the secondaries shut. Spring tension opens them but only when the vacuum is reduced enough for the spring to take over. An engine under no load has its most vacuum and under load has very little.

 

[derf]

This is backwards. Vacuum holds the secondaries shut. Spring tension opens them but only when the vacuum is reduced enough for the spring to take over. An engine under no load has its most vacuum and under load has very little.

Typically, at least in a Holley, the spring holds the secondaries closed. The vacuum used to open them is ported vacuum, not manifold vacuum. As the airflow is increased in the primary venturi (i.e. as the load on the engine increases), the ported vacuum increases putting more pressure against the spring, opening the primaries more.

 

[Kane]

Typically, at least in a Holley, the spring holds the secondaries closed. The vacuum used to open them is ported vacuum, not manifold vacuum. As the airflow is increased in the primary venturi (i.e. as the load on the engine increases), the ported vacuum increases putting more pressure against the spring, opening the primaries more.

Even I had it partially wrong. You are right. It is not using manifold vacuum. sometimes I type without fully thinking.