Question about chromoly birfields and longfields (1 Viewer)

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djsherif

djsherif

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Jun 13, 2008
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Location
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If the chromoly birfields and longfield super axles are so tough, why are they only for off road. What do they do differently on road that makes them deteriorate faster than stock parts.

I would think the opposite would be true that since they're so tough they'd last forever on road.

Thanks. Hopefully not a dumb question.
 
They are tough because the metal is softer and can take more "twisting" without snapping (tensil strength?) but not as well suited for 80's due to being full time 4x4 vehicles. As a softer metal, they wear more rapidly. That's my understanding anyway.
 
Chromemoly (chromoly) shafts, as I understand it, are of a harder material which is less prone to twisting, which would be the benefit of a higher tensile strength; but with one downside: they wear faster than the softer stock units.

I would like to hear some better technical data on this, which I am sure is on 'Mud somewhere.

From Sleeoffroad's web site:

"Chromemoly units are made to withstand higher shocks loads and twisting forces. This is achieved by making the joint from chryogenically stabilized aviation grade steel. This design makes them a good choice for vehicles with larger tires and are used in more hardcore application, but they will wear faster."

Also from Slee:

"Light duty replacement units are intended for vehicles with smaller tire sizes and/or driven more road miles. Due to the hardness factor, they wear better, but do not provide the same protection against shock loads as the Chromemoly heavy duty units. Supplied with ABS ring."
 
Seems I was mistaken on the information that I heard. Instead, it would be a good idea to read some threads where this subject has been discussed from different angles, and by those with direct/indirect experience.

Read the following for some "verses" debates with good points:

https://forum.ih8mud.com/80-series-tech/253555-oem-birffield-verses-chromoly-longfield.html

https://forum.ih8mud.com/80-series-tech/348543-chromoly-inners-need-advice-2.html

And another:

https://forum.ih8mud.com/80-series-...ended-birf-type-larger-tires-air-lockers.html
 
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See I always though chromoly was tougher, but in this case it seems it has more "give" so as to flex and not crack.
 
Because there is a lot of debate in many of the threads on the subject of chromoly (chrome moly?) versus stock axles and birfields, I think pasting quotations from a multitude of posts would keep this thread clean.

In other words, try and extract from these opinions a good overall idea of what our forum members have to say.

"chromoly is softer than regular forged steel, and will wear slightly faster"

"Definitely run OEM or equiv. for street use."

"It is near common knowledge that chromoly birfields are not for a daily driver rig"

Regarding chrome moly: "...don't expect to get more than 15K miles out of these if you drive on the street."

"A 20 mile round trip to work 3-4 times weekly is a daily driver, IMHO. Add trips to the store and whatever and you're putting way more miles on your truck in town then off-road. That combined with your stated intention of running intermediate-level trails with 33" tires suggests that you would be best served with stock birfields and axles. They wear better than Longfields. The Longfields are best used on a mostly trail truck running 37"+ tires on difficult trails with lockers."

Regarding Newfield's vs. OEM: "As to how many miles, that is up in the air. The lower cost light duty ones are just that. It is for the soccer mobile that doesn't see much off-road, but needs to replace clicking birfields."

"There is a direct relationship between hardness and how brittle steel is. Hard steel has better wear characteristics than soft metal. Hard metal lasts longer under normal loads. Soft metal wears faster but also flexes rather than breaks under shock loads. So given the same metal type the soft one will work better for wheeling, or shock loading. That same metal will wear very quickly under a constant dd load. So for dd you want a harder birf, OEM's. For wheeling you want a softer birf, Longfields.

With all that stated, you are running tires and gears way out of the "normal" range. You run a higher risk of shock loading your birfs than the rest of us "normal" mudders. Consider the risk if you are going to wheel. Your choice is to carry spares on the trail and be ready to do a trail repair or do more regular pm on your softer birfs."

Hope this helps. :cheers:
 
Thanks for all the input guys. Makes more sense
now.

Found this definition on the web if it interests anyone:

"The 4130 is a code of the American Iron & Steel Institute and defines the approximate chemical composition of the steel.

The "41" denotes a low alloy steel containing nominally 1 percent chromium and 0.2 percent molybdenum (hence the nickname "chromoly"). The "30" denotes a carbon content of 0.30 percent. In addition, as normal constituents of plain carbon and low alloy steels, there will be around 0.2-0.5 percent silicon, 0.5-1.0 percent manganese, and well under 0.1 percent of each of a dozen or so other elements whose presence is unavoidable, in a few cases deliberate, and generally not harmful. The remaining 97-98 percent is iron (Fe)."

so with 35" tires, mostly DD and occasional trail runs I should be ok with stock birfs.

Are there replacement normal use birfs that are better than others?
 
This is the one inherent problem with an full time system and the reason why sooner or later i'm going to have to part time the rig.
 
The problem is that 4130 cannot be surface (case) hardened and it is the surface hardening that prevents wear. 8620 is just about as tough, but it can be surface hardened. It is typically used for gears and it might be a better choice for 80 series birfields than 4130.
 
Pin_Head said:
The problem is that 4130 cannot be surface (case) hardened and it is the surface hardening that prevents wear. 8620 is just about as tough, but it can be surface hardened. It is typically used for gears and it might be a better choice for 80 series birfields than 4130.
Click to expand...

Is 8620 a more expensive alloy?? So much so that it would price itself out of the market??
 
I doubt that it is much more expensive. The machining is a much larger component of the overall cost than the materials. 8620 is used for gears like ring and pinion sets, so it can't be that expensive.
 
Pin_Head said:
I doubt that it is much more expensive. The machining is a much larger component of the overall cost than the materials. 8620 is used for gears like ring and pinion sets, so it can't be that expensive.
Click to expand...

Ok, so has anyone brought this up with bobby, or poly performance before? It would seem a simple solution to just change the material.
 
I would image that Bobby considered it. 8620 may not have as high a tensile strength as 4130 in the hardened condition. The specs I can find are in the normalized (not hardened) condition and 8620 and 4130 are comparable. There are other alloys that have a higher strenght and toughness than 8620 and are surface hardenable. En36A and En39B sound good.


Case Hardening steels
8620 General-purpose case hardening steel suitable for comparatively lightly stressed components. Used for automatic components such as transmission gears, crown wheels, ring gears, hypoid gears and racers, king pins and pinions.
Atlas 6587 The high chromium content produces high case hardness with excellent core strength. Used for gears, bearings, sleeves, pins, bushes, shafts, plastic moulds and any highly stressed or high wear component.
En36A Used extensively for highly stressed gears in trucks, agriculture and mining machinery, pins and brushes, heavily loaded shafts and other applications requiring a hard surface with a tough shock-resisting core. Core tensile 55tsi minimum after hardening and tempering.
En39B An exceptionally high hardenability steel for large or highly stressed gears where core strength and toughness is required. May be used as an alloy engineering steel.Core tensile strength 85tsi minimum after hardening and strengthening.
 
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Maybe we should get in touch with him and ask him to chime in on this thread. I know he has a Mud account.
 
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