Topic: dual case ?

[twentyfootdaredevil]

Is it okay to drive around on the trail with your front case in low?


truck specs...
22re
40'' TSL
5.29
2.28 front case 4.7 rear

[BigMike]

Of course. Just like it is ok to drive around the trail with your rear case in low.

[RRMike]

I've heard of people breaking 21 spline inputs with the front case in low and the rear in high.  According to the guy he wasn't even doing much when it broke.  Maybe it was already twisted and getting ready to go from prior hard use but I try to leave the front case in high until I put the rear case in low.

Just an opinion and I do run my front in low from time to time with the rear in high but not when I'm getting on it.

When I get my gears I'll probably order a 23 spline coupler at the same time and do 23 spline gears so it's one less thing to worry about.

[BigMike]

We have not heard of 21-spline rear input shafts breaking when used behind a factory 2.28:1 gear set. This is unnatural and extremely uncommon. His input shaft must have had prior damage or fatigue for sure.

We encourage dual case customers to use 2WD low range, which consits of 2WD-HI in the rear case and Low range in the Crawl Box. In fact, I even recommend people to load and unload boats in docks in 2WD Low range if they wish.

The output shaft is far more likely to break than a factory 21-spline input shaft in a dualcase setup.

In twentyfootdaredevil's case, he certainly does not need to fear as he is using a 4.70:1 gear set in his rear case. Even if it is not a genuine Marlin Crawler Total Spline™ gear set, it will still be stronger than the factory 2.28:1 input gear.

With that said, twentyfootdaredevil should have no reason to fear his 4.70:1 rear input shaft when driving, towing, tractor pulling, mud bogging, terra grappling, remote vehicle cave spelunking, tar pit clutch dumps, or freelance ice bouldering with his Crawl Box in Low Range. There are other weaklinks (aka fuses) that are far more susceptible to failure than a 4.70:1 rear t/case input gear.

[Hottrod81]

Are you sure he shouldn't fear tractor pulling?

[RRMike]

http://www.pirate4x4.com/forum/showthread.php?t=487955&highlight=input+shaft+spline

Posts 7 and 8

[BigMike]

The only good thing about that guys situation is that he has an automatic, which is more gently on drivetrain components.

But come awn, his cases have been in and out of three different rigs, he has a freak'n Chevy 406 V8 with one ton axles and "big swampers". Are you going to try to tell me that his input shaft has not had a hard life?

That is crazy and very uncommon. It is much much more common to break a output shaft behind a 5.19:1 (double low range) ratio than it is to break a 21-spline input behind 2.28:1 (Crawl Box in LOW) ratio. I am not even talking about twentyfootdaredevil's 4.70:1 input, which is much stronger than a stock 1981+ 21-spline input.

Its a good story but everything will eventually fail, right?

twentyfootdaredevil and everyone else should feel perfectly confident with using their vehicles with the Crawl Box in Low and the T/case in High.

BigMike

[toyman2]

id have to disagree with big mike respecfully
i have broke a 21 spline input on the back shaft and seen a couple others.
that said ,it is ok to run but keep it in mind the increase of torq and gearing that the shaft wasnt designed for
if it is a stock input shaft.
id say you need to be aware of wheel speed and HP when in low range with the front case and in 4X4 HI in the back

[RRMike]

I wasn't trying to argue with you Mike.  I was just saying that it's something I've heard of more than once and is not almost unheard of like you make it sound.  Maybe the outputs should fail first but the simple fact is that they don't always and people do break inputs with the front case in low and the rear in hi.  Why do people run 23 spline inputs?  Why does Marlin even make them?  For the 1 in 10,000 who own turbo trucks?  Because they are stronger than 21 spline inputs.

That was the first thread I could find over on pirate so I linked it but it wasn't the only one I've heard of.

[BigMike]

Hey RRM, please don't ever worry about throwing up a debate Thats what makes public forums so great!

But seriously, this can NOT be a reoccurring problem and I will tell you why:

If Rear 21-spline stock T/case input shafts are failing, then I think we need to discontinue our ENTIRE 21-spline Dualcase line up.

Think about it. If you have your Crawler in Double Low Range, isn't not the front Crawl Box in LOW Range? The same amount of gear reduction is entering the 2nd input in double low range as it is with just the Crawl Box in low range.

Therefore we should be warranting massive amounts of failed Marlin Crawler 21-spline non-ultimate dualcases, but yet I am willing to wager that in our 10 years of selling them that we have yet to warranty a single broken rear 21-spline input gear.

If you can not run the Crawl Box in Low range and the T/case in high range, then you can not run both in Low range either.

Now it comes down to the argument about theoretical torque vs. applied (or "seen") torque, whereas higher final drives see a higher load on parts than compared to lower final drives, which is probably true for drive ranges in excess of ~200:1, but if you are comparing 47:1 vs 108:1 then I am almost certain that the rear t/case input gear is seeing more torque with both t/cases in double low range than with just the front Crawl Box in low range and the t/case in high range.

Regardless of the fact, if these are breaking in such a fashion that would warrent a caution to use Crawl Box Low Range only, then I think we should stop selling 21-spline non-Ultimate Dualcases and offer a disclaimer that says never to use the Crawl Box in low range and never use both cases in low range because the rear input is likely to fail in both situations.

If you think about it that way, if you think about the fact that in double low range that input is seeing the same amount of torque if not a higher load, then you can start to see where I am coming from by firmly stating that I have absolutely no reason why not to run just the Crawl Box in Low range and rear t/case in high.

Regards,
BigMike

[BigMike]

Remember though that the opening question of this thread is not related to the post on pirates or anyone else who has broke a stock 21-spline input gear.

twentyfootdaredevil has a 4.70:1 rear input gear which is stronger than a 21-spline stock input gear, so he above all has the least to worry about here and that is why he certainly is not at risk in his proposed situation. Furthermore he only has a 4cyl, which is what the 21-spline input was designed for.

And if there are others who are running Chevy V8s on a 21-spline drivetrain, which is something that Marlin Crawler does not support nor endorse, then simply upgrade to a 23-spline drivetrain. That is why the 23-spline is available, for any application with 170+ Engine Torque.

But if a 4cyl is being used, then by all means feel free to run both cases in double low range or just the Crawl Box in single low range; it won't make any difference.

Regards,
BigMike

[RRMike]

Now it comes down to the argument about theoretical torque vs. applied (or "seen") torque, whereas higher final drives see a higher load on parts than compared to lower final drives, which is probably true for drive ranges in excess of ~200:1, but if you are comparing 47:1 vs 108:1 then I am almost certain that the rear t/case input gear is seeing more torque with both t/cases in double low range than with just the front Crawl Box in low range and the t/case in high range.

If you think about it that way, if you think about the fact that in double low range that input is seeing the same amount of torque if not a higher load, then you can start to see where I am coming from by firmly stating that I have absolutely no reason why not to run just the Crawl Box in Low range and rear t/case in high.

Regards,
BigMike

I guess that's where we disagree.  I believe all of these contentions to be false.  If you give more mechanical advantage after the given drivetrain part (lower gears, smaller tires) the part sees less load.  Why do locker makers have warranties that limit tire size?  Same reason.  The more gearing you have (taller) the more strain you're putting on the parts upstream in the drivetrain by adding resistance.

Having the rear case in low cuts the strain on everything in front of that (including the input IMO) in about half.  That's my contention.  I've seen it hashed on Pirate but the general consensus seems to be that this is the case.  People talk about upping diff gearing (lower numerical) for stronger R&P's and what this will do to add strain to TC's.  I agree with them.

I would say think about it this way:  A 7.5 inch ring gear can hold up to clutch dumps in a 300HP Z-28 because the tires are tiny.  Put that same R&P in front of some 39.5 inch swampers with that motor and see how long they last.  They won't.

I'll get back on topic though.

Twentyfoot:

Feel free to do whatever you want with your truck.  I'm not a MC or Transfercase expert by any means.  I'm just relating anecdotal stuff and discussing general theory.  Mine's never broken.

[badassfullsize]

Good post i was going to ask this same ?.  Getting ready to do dual cases on my yota and just trying to figure out the do and dont's with this setup.