4x4 lead/lag stress on drivetrain...what's the dif

[Anonymous Poster]

4x4 lead/lag stress on drivetrain...what\'s the dif

Hi,

Gordon referenced the following Firestone information page in another thread:

Firestone General Farm Tire Information

It is an interesting read. Here is a quote that I don't understand the reasoning behind.

<font color="blue"> But no amount of lag is acceptable. That’s because a lag condition puts excessive strain on the tractor’s transfer case, and it causes the front tires to wear out more quickly than they would under normal operating conditions.
</font>

Why would a lag situation put more strain on the front drivetrain than a lead situation, if the percent lead/lag being compared were equal?

Seems like the gears and other drive train components would feel the same stress/strain in eithe case, just in the opposite direction.

I can understand that overspeeding the front tires would help in control of the tractor...but am having trouble seeing how the gear train would like lead over lag, or lag over lead, as far as excessive strain goes... /forums/images/graemlins/confused.gif

In my mind, in a lead condition the front tires would try to drag the rear wheels over the ground...and in a lag condtion the rear wheels would try to force the front tires over the ground...

Wouldn't the drive train be stressed equally in both cases, but just in an opposite manner?

What's the fine [or corse] point I'm missing here? Might it have something to do with the tire tread slipping easier in one direction than the other? Doesn't seem like that would be it... /forums/images/graemlins/confused.gif

 

[gordon]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

Er um cuz thats the way it works.

On the seirous side. The reason that you want a lead ( front tires actually spinning quicker than the rear tires) is to keep control of the tractor. If it was the oppsite wheel hop would occur. Not pretty. Reason being the rear axle would be pushing the front. Think about that last sentence and what happens next. BINDING.

That I guess why once on the other thread Lamaur (sp) said that all they run is roller grip chains on the front that made some good sense. Not only because you have steering all the time but it's on your lead axle as well. Then if your in 4wd on the road your less likely to bind up.

We really need some of those real smart people that post here to give the dynamics of it. I just know what will happen if you do have a lag condition, been there done that.

I remember a long time back I did a real detailed post on this, let me do a search and see. I do know that my L4310 has a very slight lead condition and I mean very slight. I can under the correct conditions I can cause a slight power hop.

Gordon

 

[Anonymous Poster]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

Here's a post/thread I found that relates to my question....

Still trying to sort this out in my mind. Experience proves that a tire mis-match will cause jumping and so on...

AT the risk of illustrating excessive overthink, has anyone ever seen a study or some kind of tabulation showing a comparison of between what would be expected to happen if front tires lead VS lag?

For example, if the front tires lag, it would be expected that steering control would suffer. Sort of like having trailer brakes weaker and setting slower than those of the pulling vehicle, resulting in a jack knife situation...

I am sort of hung up on the statement that tires that cause a lagging situation on the front stress the drivetrain more than tires that cause a leading situation. I don't dispute the reality of people's experience, just want to understand why this should be the case...

If anyone can point me at an explanation of why this should be I would appreciate it. Intellectual curiosity only. I know there is no practical need to know the answer... /forums/images/graemlins/blush.gif

 

[thcri]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

Just what i was told a long time ago,

The first problem that you have is gearing to make a complete match. Now on tractors because of the slower speeds it would be easier. But pickups really had this problem due to, 10 miles per hour you may have lag, but at 30 you may end up with lead?? That is why it is so hard to match them up. Most manufactures liked the lead on the front because you do have better control over the vehicle. Rather than your front tires trying to fight against your backs, it was best to help pull. You steering was better also. It is kind of like if your speedometer is off. It may only be 5% off at 25 mph but at 60 mph you would be 10% off. I remember some of the older 4 wheel drive pickups there was major warnings about driving in 4 wheel on hard pavement. Speed wasn't a major factor with them. But on hard pavement there was no slipage and this is where you messed up the drive system. Your drive train comes back to one place and they have to match.

Hydro-Static will eliminate this to some extent

I hope I didn't ramble too much and I also know that things have changed and technology has bettered this situation. But even today in my brand new Ford Pickem-up truck with 4 wheel on hard pavement you can feel it buck.


Murph

 

[RickB]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

I think the main reason is to maintain control; lead keeps the tractor going straight, lag would cause the rear to shove the front wherever it could to a small degree. I can tell you that New Holland's TNF tractors with SuperSteer and PowerShuttle ARE engineered with a small lag factor. It has been explained to me that the traction management system used on these tractors controls the FWD function better with a lag ratio than with lead. Several years use on units sold here show no ill effects, including tire wear. Of course these tractors are designed to run with the FWD in auto mode, which means the computer shifts the front axle in and out based on rear wheel slip, steering angle, fore/aft tilt, and brake pressure.

 

[Woodbeef]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

You guys might not notice it with the CUTs due to the smaller tire sizes but on larger ag tractors equipped with mfd,even having more bars on one front tire than the other can cause lag. This can then cause binding of the front axle. That is why they tell you to replace both front tires at the same time,with the exact same tire brand,size,and model.

Now lead can be used very creatively,as can be witnessed in many Asian tractors. A slight amount of lead can become an agressive power steering setup. A large amount of lead can mean sharper turns,ala Kubota's bi-speed turning.

 

[gordon]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

Is the lag ok becuse the system doesn't kick in (front wheel assist) until the tractor see's (feels) 10 percent wheel slippage on the rear axle. On hard pavement it would be a lag condition but in the field with 5 percent of wheel slippage it's still a lead condition. Or becuase of slippage in the transfer case that is built in?

I'm guessing on the above. But I can say at work the New Holland that we have is a hoss for pushing snow. With both front and rear diff locks and a dregleman blade in the front she will really push a big pile.


One thing tbn always gets me thinking
Gordon

 

[RickB]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

The number of lugs is irrelevant, as is the brand of tire. A larger front tire increases lead or decreases lag, not the opposite. The informed buyer will match the type (radial, bias) of tire and pick a brand and size with a loaded radius very close to that of the OEM tire. I have seen identical new tractors equipped with different branded tires, either from one tractor to another, or from one axle to another. As long as the loaded radius of the front/rear combination creates an acceptible drive ratio, there will be no problem.

 

[thcri]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

RickB,

You mentioned the TN and supersteer. I have a TC29D with supersteer, do you know if they have lead or lag?

Murph

 

[Woodbeef]

Re: 4x4 lead/lag stress on drivetrain...what\'s the dif

And a mis-match of front tires will then do what ? One bigger and one smaller will make for trouble!! If you don't believe me go and ask a good reputable mechanic that works on mfd tractors bigger than 60 hp of many different colors. Some brands seem more problems with this then others. Not sure if it is the front end design or overall design of the mfd system.

Have you ever seen different branded tires on the same front drive axle? Me thinks not,because from manufacturer to manufacturer there are difference in tire heights even with in the same size.

As for the different amount of bars,stop and think a minute about your reply,especially with a limited slip front diff and what traction differences it will encounter. Why do you think in full time 4 wheel drive vehicles tire harmony is so critical,eh?

But then again as I stated earlier in compact tractors it might not be as much of a problem to begin with. Take a front tire of say........13.6x24 or larger then it becomes more of a problem very quickly,especially a mfd loader tractor!