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Looking for knowledge about Spicer auxiliary overdrive.

Posted By Bearmtnmartin Saturday, November 14, 2020 5:00 PM
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Bearmtnmartin
 Posted Saturday, November 14, 2020 5:00 PM
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I am repowering an old motorhome with a newer Cummins M11. It has 425 hp and 1550 torque which is quite a jump from the old NA 210 hp Cat. So I am well into this project now and have not finalized my gearing plans yet. The coach has an R155 rear gear which is the rear rear of an sshd. I am having trouble regearing it because it is currently 5.29 and I need 3.5 or 3.73 which were not common back then. And if I get the gears I need to also replace the carrier and other parts due to the much larger size of the ring gear. I could replace the axle with something newer but that's not easy because the axle is built into a four bag air suspension and the new axle would not fit it. So I am looking at a brownie which would be the easy button. I have lots of framerail to bolt one up (coach is a puller) and I could leave the axle alone. So my questions are first whether I could leave the box in overdrive or would it heat up. I have no need for more gears as it would be behind a 13 over so I would not bother with a shifter. And also whether the 7000 series would be big enough, given that I will gross about 35000 and I am pretty easy on equipment. I cannot find an 8000 series with the 0.75 ratio I need but there are a few 7000 series around to choose from. As I understand it the 7000 series would be rated for 700 pounds engine torque and the box would withstand that torque multiplied through the lowest gears in the main transmission. In my application I would likely only ever develop maximum torque in direct while pulling a hill so maximum input and through torque would be 1550 pounds. Sorry for the long post and thanks for any thoughts or advice.
Geoff Weeks
 Posted Sunday, November 15, 2020 7:53 AM
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Your best case argument is you are only going to put double the rated torque into it? No! 1st despite what you think, you are going to go way over that in the lower gears. Simple, best solution all around is to change carriers and rear ratio. Money spent upfront, is less then picking up pieces on the side of the road waiting for a tow.
While it is possible to exceed the rating of a trans a little and see no adverse effects, you are putting more than twice the rateing in with the main on O/D, every gear below that multiples the input. It can only go up, not down.
Geoff Weeks
 Posted Sunday, November 15, 2020 8:35 AM
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I think you'll find 3.73 is as fast as you can go with the SSHD, The good news is any carrier built for 4.63 or faster can be re-geared up to 3.73.
I reality I suspect 4.10 would be enough depending on which 13 you are putting ahead of it. If it is one of the newer RTLO series of 13 4.10 or 3.90 would be plenty.
By a "core" with a fast ratio carrier and re gear to your choice, not cheap, but done right and will not leave you on the side of the road.
Geoff Weeks
 Posted Sunday, November 15, 2020 8:46 AM
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Also, the pinion input splines are different on older units, would need to change to the newer fine spline yoke if your is the old 10 spline type. Not a big deal.
Geoff Weeks
 Posted Sunday, November 15, 2020 9:01 AM
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Just a quick look on the net, $500 plus core charge or exchange yours for a good used 4.63, add may $2000 for a re ratio with new gearing and your set to go.
Don't know where your located, but call around to gear shops, and get prices. They may be able to supply a core cheaper than you can.
Bearmtnmartin
 Posted Sunday, November 15, 2020 5:03 PM
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Thank you for the reply Geoff. Perhaps I should have just come here and asked about regearing instead. Can you clarify just what I would need to do to swap the gears out? Is it just the carrier and ring and pinion? How many different spline counts on axles over the years? Will any R155 carrier fit my 1981 SSHD case? I have asked all these questions at axles shops and I get blank stares or they tell me to bring my carrier and wallet and they will try and figure it out. No one has given me enough information to proceed, but certainly if I knew what I was getting into it is the better way to go. I can do the work no trouble. I just don't want expensive surprises.
Bearmtnmartin
 Posted Sunday, November 15, 2020 5:20 PM
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This diff is available locally:

https://www.99truckparts.com/parts/differentials/reconditioned-rockwell-differential-sshd-r-ratio-4-11/

And here is the ring gear I need on ebay

https://www.ebay.com/itm/WORLD-AMERICAN-R-P-SHR-R155-3-73-RATIO-Repl-Rockwell-Meritor-Diff-A40504-1R/303647168305?hash=item46b2c81731:g:aNgAAOSwtzZejzPK

Is this all I would need, or is there more to it? I keep getting told they need to tear mine apart to see what might fit. Bring my wallet.
From what I can see, the R155 is the rear rear of a SSHD? Do I have that right?

Thanks!
Geoff Weeks
 Posted Monday, November 16, 2020 2:24 PM
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As the ratios get faster and faster, the pinion gets larger in diameter, this means the ring gear has to move sideways to accommodate it. Generally, there are a range of ratios that can be fitted to each carrier, once you move beyond the ratios that carrier is designed for, another carrier is required to put the ring gear in the proper place.
With your model there are two carriers, Standard and Fast. the break point is 4.63, that ratio and anything faster takes the "fast carrier" anything lower takes the "standard" carrier.
Since you want to jump from lower than the cut-off to higher, you need to change carriers.
Sometimes things like pinion bearing cages have to be changed also, I didn't look at all the related parts but I don't think they change on the SSHD diff's
I had to change them on a 2 spd Eaton I was re ratioing but it was available and not too expensive.
Whoever build the diff needs to match the side gears to the axle spline you have now, you notice the one for sale near you says they can change the side gears for a different application.
I am going to try and attach the parts manual in .pdf form for your axle. The manual covers many older Rockwell axles.
Find it Parts, the ebay seller of the gear set is high priced in my book. I can't remember where I bought my Eaton gearsets from, it was somewhere in Oregon IIRC. Much cheaper then other places and very knowledgeable.
Before you go through all the trouble to change gearset, make sure the choice will work well with your trans and tire size.
I am have a 15 O/D (.73 top ratio) trans on 11x 22.5 rubber and found that a 4.11 will give me a top end of 80 MPH and much better cruising than the truck had with 3.90's Just how fast do you intend to push that old thing? With an older 13 spd with a .86 top ratio, 3.73 would be about right., with the newer RTLO 13's with a .75 (IIRC) top ratio I'd be looking at that 4.11. IF you are running 11x 24.5s you'd better chart out the engine speeds at different road speeds to see what ratio will be best.
Re ratioing isn't hard, but it can be expensive for the gearsets, so is not something you want to get wrong and have to change all over again.
When I dropped from 3.90's to 4.11's I gained almost a full mpg and had a much more useful truck but I did the math and plotted out every change on paper before I did anything.
If a shop doesn't know what gears can be installed or what is required to install the ratio you want without seeing, they don't know what they are doing! All the info I posted is from the manual I attached.
The SSHD uses a common center section (Diff) that is used the 44k axles and heavier, the differance between the axles is the thickness of the tubes and sometimes the bearings, The axle shafts get bigger the higher the capacity requiring different side gears but the basic center section is the same.
For that reason, you need to know the diameter and number of splines on the axle shafts you have now, and get side gears to match. IIRC side gears for my Eaton cost around $35-$45. so not too bad.
Monday, November 16, 2020 2:26 PM by Geoff Weeks
Bearmtnmartin
 Posted Monday, November 16, 2020 5:47 PM
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Thanks for the info Geoff. Very informative. I think the best thing is to pop my diff out and take it down to compare.

I have calculated the numbers a hundred times. I don't want to go fast, but I do want to be toward the bottom of the Cummins recommended RPM range at 70 for best economy low noise in a pretty light vehicle. I would actually like 3.55 because I would be idling at highway speed, and always have the option so drop a half gear if need be. But 3.73 would be fine as well. Cummins tells me 1200 to 1600 RPM for best economy.
Bearmtnmartin
 Posted Friday, November 20, 2020 5:17 PM
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I found a shop that will replace the gears in my own differential for $1000.00 so I went that route. Thanks for the help Geoff.

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