MB 917 vs 1120/1124?

[Geo.Lander]

Front and rear axle ratios are always the same. What is printed on the labels on front and rear axles? What does the Mercedes Benz parts catalog show when you check your VIN? What are the SA codes starting with A..?

According to the VIN my axles are mixed, fast on the back, medium on the front.. with my limited understanding on the low level workings of differentials I can imagine a mixed axle ratio can either be compensated for in the hubs and/or plays a part of rear wheel drive bias (unless there is a slip in the rear wheel then they pickup more torque from the center diff). I have not also asked this question on LN2 forum.

AL 3/8 D-4,7 - FRONT AXLE, RATIO 39:8
HL 4/29 D-9,2 - REAR AXLE, RATIO 43:10

 

[VerMonsterRV]

According to the VIN my axles are mixed, fast on the back, medium on the front.. with my limited understanding on the low level workings of differentials I can imagine a mixed axle ratio can either be compensated for in the hubs and/or plays a part of rear wheel drive bias (unless there is a slip in the rear wheel then they pickup more torque from the center diff). I have not also asked this question on LN2 forum.

AL 3/8 D-4,7 - FRONT AXLE, RATIO 39:8
HL 4/29 D-9,2 - REAR AXLE, RATIO 43:10

I had just assumed after looking at the rear axle ratio for our truck in the EPC that they would be the same front/rear. Well your post prompted me to check, and it just so happens we are setup with "mixed" axles also with the same ratios you have.

 

[charlieaarons]

That’s way too much difference: 4.875 front, 4.30 rear. 13.4% difference; unless tires are different sizes. Either the gears inside are different or something is at high risk of breaking.
And the rear is faster than front. Get on ice in 4wd and the truck will be undrivable. Something is wrong. Planetary ratios should be included in the ratio posted on the placard. And planetary ratios by themselves are usually in the vicinity of 2:1, not 1.13:1. If they were, the actual planet gears would be tiny. Too tiny.

 

[Geo.Lander]

That’s way too much difference: 4.875 front, 4.30 rear. 13.4% difference; unless tires are different sizes. Either the gears inside are different or something is at high risk of breaking.
And the rear is faster than front. Get on ice in 4wd and the truck will be undrivable.

well, it has been operating fine for 29 years in one of the snowiest districts of Norway so im assuming it drives fine in winter condition..

 

[Geo.Lander]

I had just assumed after looking at the rear axle ratio for our truck in the EPC that they would be the same front/rear. Well your post prompted me to check, and it just so happens we are setup with "mixed" axles also with the same ratios you have.

it makes sense if we go back to the torque bias of the rear. With a higher rear ratio the center diff will send more torque to the rear, in my mind anyhow..

Or will it be the opposite way around and send to the front?

 

[charlieaarons]

well, it has been operating fine for 29 years in one of the snowiest districts of Norway so im assuming it drives fine in winter condition..

I will bet you just about anything that one of the labels is in error. If you take the diff cover off or pull the diff center if it’s a banjo type and count teeth, and take the cover off the (rear) planetary and count teeth aand apply the proper formula, and calculate ratios (assuming equal sized tires), you will find the front ratio equal to the front or at the most 1% faster.
If you didn’t purchase the vehicle new, you don’t know if for instance someone purchased a used axle housing with a different ratio and transferred original gears to it.

 

[Geo.Lander]

I will bet you just about anything that one of the labels is in error. If you take the diff cover off or pull the diff center if it’s a banjo type and count teeth, and take the cover off the (rear) planetary and count teeth aand apply the proper formula, and calculate ratios (assuming equal sized tires), you will find the front ratio equal to the front or at the most 1% faster.
If you didn’t purchase the vehicle new, you don’t know if for instance someone purchased a used axle housing with a different ratio and transferred original gears to it.

No argument from me charlieaarons, I just looked up the VIN number I haven not even check the labels on the axles.. I would not know where to look even..

 

[Geo.Lander]

charlieaarons, according to LN2 guys you are mostly likely right. They told me to check on the Axles directly and most likely they are both 39:8, i have asked to clarify how they know its 39:8 but i have presumed because of the AL prefix..

apologies for highjacking the thread..

 

[palebluewanders]

apologies for highjacking the thread..

I love that this discussion is happening, it's the sort of thing I'd never think of otherwise and it helps to follow every detail of your process since I'll hopefully be starting the same one in a few months. ?

 

[Sitec]

I'm not saying I'm right, but I'm tipping I'm close... The HL7 is a Hub Reduction axle (pictured) It can have a slim close ratio diff (higher speed and less torque) then allowing for final reduction in the hubs where the torque is needed. This means that everything in front of the drive wheels can be made lighter and spun/geared faster because its running at speed, and then all the work is done in the hub right at the wheel. Our 1222a has a normal diff in the front and a Hub Reduction axle in the rear. The plate has the overall ratios very similar, but the diff ratio in the rear axle is different...

Our rear is HL7/10S-10 with a ratio plate of 26:24x4.000-4.333
Our front is AL3/9DS-6,8 with a ratio plate of 43:10 1=4.300

 

[VerMonsterRV]

So here is a pic from the EPC of the gears in the transfer case, #20 is the front gear and #41 is the rear gear. From the picture they appear different. Other than taking things apart and trying to put them back together I know very little about this, so this may be off base. But, is there any chance the EPC is correct and front/rear ratios are different and the difference is balanced out inside the transfer case with different gearing front/rear?

 

[Sitec]

One gear will be low range, and the other will be high range. They are both being driven off '56' which is the intermediate gear. 20 and 41 sit on the same shaft, but are running on bearings. When low or high is selected, the dog (no 11) either engages on 20 or 41. . Also worth noting that there is no center diff in that transfer case so it would point to it being a selectable 4x4 transfer box... (assuming that the output flange (80) connects to the rear prop and the front prop engagement system is not shown in this drawing...).

 

[VerMonsterRV]

One gear will be low range, and the other will be high range. They are both being driven off '56' which is the intermediate gear. 20 and 41 sit on the same shaft, but are running on bearings. When low or high is selected, the dog (no 11) either engages on 20 or 41. . Also worth noting that there is no center diff in that transfer case so it would point to it being a selectable 4x4 transfer box... (assuming that the output flange (80) connects to the rear prop and the front prop engagement system is not shown in this drawing...).

Ok, I am trying to follow along. So I think the parts diagram I attached was the input shaft from the transmission (80). This is why there is no output flange. The diagram below is the shaft for the front/rear with the expected output flanges.

 

[Kurben]

The data sheet of my 1124 in the Mercedes Benz EPC shows the same differential ratios as you your trucks (front: 39:8 / rear: 43:10). But the SA code list for my truck in the same data sheet has also „A60“ listed. „A60“ translates to i=4.75 which is an axle ratio of 38:8. I checked the labels on the axles of my truck and both have 38:8 stamped on it. It looks like the SA codes overwrite the BM standard ratios.

 

[Sitec]

Ok, I am trying to follow along. So I think the parts diagram I attached was the input shaft from the transmission (80). This is why there is no output flange. The diagram below is the shaft for the front/rear with the expected output flanges.
View attachment 627291

Ok, this paints a different story.. What I said earlier still stands in that it's high and low gears. This next pic shows both output shafts and an epicyclic gear set between the two output shafts, which then does make it full time 4x4. I think 122 is the sliding dog to lock the center diff. Clever use of an epicyclic unit.