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Model 20 Strength

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jpnjim View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Dec/01/2017 at 7:07pm
Originally posted by tufcj tufcj wrote:

Originally posted by DaemonForce DaemonForce wrote:

In the case of wheeling, would it make more sense to rely on a deep geared Model 20 or Dana 44? Would the axle still be the mechanical fuse in that setup @2.72 or 3.08 gearing? I've always had questions about the Model 20 axle and can never find it. I think it's my actual go to preference because it would be a drop-in swap but I'm also curious about the 44.


What size tires are you running? 2.72 or 3.08 on the trail would be terrible with anything but 26" street tires. I spun a couple of 2 piece AMC 20 axles with 35" tires on my CJ. I went to a Dana 44 rear before 1 piece axles were available for the 20. The 44 held up fine, the axle shafts in the Dana 30 front became the weak link. When I went to 37" tires, I upgraded to Dana 60 rear and 44 front, never broke another axle. The U-joints in the driveshafts became the weak link. I ran 3.73s in the AMC20, ran 4.27s in the 44. I went to 4.88s with the 37s.

Bob
tufcj

Hummers run Model 20's with 2.73's and 37's WinkWinkWinkLOL
71 P-code 4spd Javelin/AMX
some Jeeps and some Fords
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tufcj Quote  Post ReplyReply Direct Link To This Post Posted: Dec/01/2017 at 10:34pm
Originally posted by jpnjim jpnjim wrote:


]Hummers run Model 20's with 2.73's and 37's 


BUT.... a Hummer has a portal axle that gears down at the hub. It has a final drive somewhere somewhere around 4:1.

Bob
tufcj
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 10:13am
Originally posted by tufcj tufcj wrote:

Originally posted by jpnjim jpnjim wrote:


]Hummers run Model 20's with 2.73's and 37's 


BUT.... a Hummer has a portal axle that gears down at the hub. It has a final drive somewhere somewhere around 4:1.

Bob
tufcj

Smile

That's what all the winkie faces were for Wink


Back to your original post,
I'm surprised you went through all the driveshaft U-joints
In Cherokees and Comanches the little 1310 would survive no matter how stupid I got.

Eventually I went 1410 when the 5:1 T-case went in,
but I wonder if the greater joint angle in a CJ was what was killing your stock joints.
71 P-code 4spd Javelin/AMX
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Post Options Post Options   Thanks (0) Thanks(0)   Quote one bad rambler Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2017 at 4:00pm
This is almost like the 998 trans vas 727 question....if you take away all the weaknesses like you did you will be just fine..Gary with the 68 American wagon ran a model 20 for years with out an issue until he switched to a different type of slick and got violent tire shake causing damage to the ring and pinion...
68 AMX 390 4 Speed,68 American,64 American 2 Door Wagon Altered Wheelbase,78 Concord Build 360,727,8.8
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 79redspiritgt Quote  Post ReplyReply Direct Link To This Post Posted: Nov/04/2018 at 4:14am
are you spraying that 550hp?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PROSTOCKTOM Quote  Post ReplyReply Direct Link To This Post Posted: Nov/04/2018 at 8:54am
Comparing a Model 20 to a 9" Ford and saying, The Model 20 has a 8-7/8" ring gear compared to the 9", so it will be plenty strong is a bad comparison of overall strength. You only have to look at the physical thickness of the ring gear and of the pinion diameter to see which one is superior. Then look at the pinion bearings. The 9" has 3 bearings instead of 2 on the Model 20. Now the concerning part I read from the original posters thread is the use of a Detroit Locker with 550 HP on a dragstrip. A Detroit Locker is not a full spool and it will at times not distribute power equally to both tires. I have a friend that took out a Christmas tree at the starting line running a Detroit Locker and ended up parked in the side of the car in the other lane. If you really care about your car put a spool in it and leave the Detroit Locker for the rock crawling guys. The dragstrip is not the place to find out what unequal power to the tires is all about.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Nov/04/2018 at 10:07am
Originally posted by PROSTOCKTOM PROSTOCKTOM wrote:

Comparing a Model 20 to a 9" Ford and saying, The Model 20 has a 8-7/8" ring gear compared to the 9", so it will be plenty strong is a bad comparison of overall strength. You only have to look at the physical thickness of the ring gear and of the pinion diameter to see which one is superior. Then look at the pinion bearings. The 9" has 3 bearings instead of 2 on the Model 20. Now the concerning part I read from the original posters thread is the use of a Detroit Locker with 550 HP on a dragstrip. A Detroit Locker is not a full spool and it will at times not distribute power equally to both tires. I have a friend that took out a Christmas tree at the starting line running a Detroit Locker and ended up parked in the side of the car in the other lane. If you really care about your car put a spool in it and leave the Detroit Locker for the rock crawling guys. The dragstrip is not the place to find out what unequal power to the tires is all about.

Tom


I agree about the strength of a 9" vs our 8.875" axles,
there's no comparison,
 the 9" starts out stronger, and there's a much (much) higher ceiling when you figure in all the aftermarket support.

My only disagreement would be the 9" uses a smaller 1.313" pinion shaft than the M20
(^the gear head of a 9 is MUCH bigger tho and probably what you were referring to),
 either way, the 9's 3rd "pocket" bearing makes all the difference in the world,
 and adds a ton of strength and support (when you have a factory "N" case, or better anyway).

Another of the 9"s strengths is even the weaker style factory sheetmetal housings can be easily reinfoced, big bearing 9" truck housings are pretty strong right off the bat, and aftermarket plate housings can be found everywhere.

By comparison M20 tubes and center sections are pretty weak, and more difficult to reinforce.
Jeep Truck housings are supposed to be stronger to start with, but I've never gotten into one of them to know that for sure.

Even if the truck housings are thicker castings than the passenger car & CJ versions (not sure about that)
and if the axle tubes are beefier (pretty sure this is true),
 you still have the same 8 bolt ring gear where everything else in this strength range has 10 or 12 bolts holding the gear.

The 2.25" pinion offset of the 9" also makes it stronger by design.

I don't know where the crossover point is for dropping an M20 for a 9" (or better),
 depends on so many things like weight, traction, transmission, traction, driving style,
 but people will know when they've reached the point of more money not fixing the M20. Ouch

Good point on the Detroit Locker going peg-leg on you at the track Shocked

That would be bad.

I daily drove a Jeep MJ with a spool for a year or two,
 it's not as bad as it sounds,
 but in a Hot Rod I bet I would have gotten a few "Harsh Noise" tickets for squealing tires around some corners no mater how slow I went LOL
(concrete road surfaces were the worst Big smileLOL)


Edited by jpnjim - Nov/04/2018 at 10:21am
71 P-code 4spd Javelin/AMX
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