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Question for the experts


phoenix2013

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I had an off line discussion where the question came up.

 

"What are the pertinent differences that make the factory single axle carry significantly higher load rating than the tandem rear axle moved forward and singled"?

 

Tires? Bearings? Airbags? Differential? Shafts? All of these?

 

Perhaps folks like Scrap or those who purchased factory single rigs recently could throw some "enlightenment" on the subject.

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uneducated SWAG - single point loading? - why can a single tire carry more than a dually?

 

analogy - one tire on a rock carrying the weight of both tires --- in the iron spring days, one axle over a ditch (small dip) and the other axle carrying the first?? With the air suspentions of today, they would equalize but..... inertia on the point of ratings?

 

Just a guess bud.

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It could very well be regulatory. IIRC, a tandem axle set is only allowed a max weight of 40K, where as a single axle straight truck can carry 26K on the rear axle legally. Just a guess, but I'll bet the same exact axle is rated 20K in tandem, 26K as a single--just because of the laws.

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Mark and noteven are correct. For a given engine/tranny, the single axle will be significantly larger, both size and rating.

 

In the old days, you could walk past a truck and look at the axle ends and see the difference in bolts holding the axle in the hub. A 23k is much larger and has more bolts than a 19k. While I don't spend as much time gazing at axles as I did in my misspent youth, I would imagine the same applies today. Scrap can certainly tell us.

 

Those of us using a single 19k axle could be in for a rude awakening some day if they are close to the weight rating for said axle, and/or are running relatively high HP.

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Mark and noteven are correct. For a given engine/tranny, the single axle will be significantly larger, both size and rating.

 

In the old days, you could walk past a truck and look at the axle ends and see the difference in bolts holding the axle in the hub. A 23k is much larger and has more bolts than a 19k. While I don't spend as much time gazing at axles as I did in my misspent youth, I would imagine the same applies today. Scrap can certainly tell us.

 

Those of us using a single 19k axle could be in for a rude awakening some day if they are close to the weight rating for said axle, and/or are running relatively high HP.

 

I detect "the silence" from Scrap as a "research phase" into the question.

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If I looked it up I couldn't post it! Long ago I sold my soul and signed a NDA that takes all the fun away. :( Jack would wet his pants if he saw what I was working on today..... <_<:P

 

Anyways, they use the same R outer ends as each other so wheel bearings and hubs are the same. Iron hubs and U rotors are encouraged, and tapered axle shaft dowels are very highly encouraged. So that's the difference you'd see by looking at axle ends. [and y'all better put your chrome covers on before Rick gets to the rally or there's liable to be trouble.... :D ]

 

Yes the "class 8" singles do use the bigger axle. A 1/2" housing is required, but that is required for any trailing arm suspension so it isn't a big deal to y'all. Except that in your world you want to keep your unsprung weight down as much as possible so it kid of adds to the difficulties. They aren't required, but I've always figured that if you have over 1650 tq and the smaller "regular" axle then a temp gauge is a really, really good idea, don't make a nasty shift on a hill, and better stick to the pavement. Shock loads and long torque is going to catch up with ya. Back in the day you used to have to get a different leveling valve (or add an edge kit to the one you have) as the CR valves couldn't keep up with frame walking. But nowadays any of todays valves work just the same on both. Before RSD 4x2's got 16.5x5 front brakes and they'd get longer rear slacks depending on your ordered GVWR. The 121 park brake test requires what gets what. I have no idea about today's RSD trucks, it's complicated.....

 

All the singles use the 10" air bags and frame brackets. They use different front axle springs & spacers to control the rake and they get a 4deg slant engine mounts to make the drivelines work. The short WB single driveline trucks would get a high entry carrier that is the same as your interaxle carrier and uses the same dropdown gears but does not have the diff in it. They are heavy, add a bunch of unsprung weight, and suffer the same MPG loss as a tandem so are avoided if at all possible. But sometimes it just doesn't work any other way. You also get limited on transmissions as some work, some make the rear howl, some have too deep of a reduction, and on and on.

 

So that's about all I can remember for now...

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