We have an 85 Ford L9000 that has a rust jacked frame that has split. Its a straight truck set up for hauling grain and forage. Long wheel base around 250" with a pusher set 9' in front of rear group. 20k front, 20k pusher, 40 k rears. My grandpa built the truck from a salvaged rig, state police came out, inspected and gave it a titled GVWR of 70,000. As we are looking for a replacement, I'm also learning about vegicle configuration and load limits and wondering why so many heavy spec trucks have 40 or 46 k rear tandem groupings if you can't put more than 34k on them together? What's the benefit? I understand using all of the max allowable front axle rating of 18k when properly equipped in Michigan, but I'm missing the point of putting a heavier set of rears in that you can't legally load heavier. I'm not concerned about hauling off road. We haul grain to market and so staying legal on road is all I'm concerned about. Are the heavier rear groupings just for working off highway or am I missing something else here?
Strength of components and being able to pick up lift axles loaded for traction.
HeavyEquipmentForums This is the truck I referenced above.
Biggest reason you see 40 and 46 rears are component strength and oil capacity. If you were running a truck over the road and needed to buy weight for a permit you can get 40,000 on a tandem just about everywhere and 46k a lot of places. You'll see a lot of vocational trucks running 46's because the don't cost much more, weight much more but live a lot longer. My haul trucks were all 52k rears. More oil capacity, bigger bearings, gears, axles etc. Most of the trucks you look at were a tractor of some sort I'd bet and to achieve desired GCWR the engineers call for that size rear end. Now they're a dump truck, grain truck, flatbed truck etc I'd have 46's in a heartbeat if I had the chance. Lay the axles next to each other (40 vs 46) and you'll see one obvious difference. Usually the thru-shaft is bigger and the power divider is a bit heavier as well which is nice when you're cutting ruts in a field and you're loaded. When allowing a truck to be spec'/built the factory engineers look at GCWR, slip torque, engine power, trans, application (tractor, bobtail etc). All that tells them what rears to use, gear ratio, suspension style, driveline size etc etc It gets fun when you build them for anything over 200k GCWR. They will tell you certain things flat WON'T work and also proceed to tell you things that WILL. Often they're bass ackwards because they've only seen trucks through a drawing or on an assembly line. They've never worked one. Clear as mud?
You can get a overweight permit in il that allow 48000 on a tandem in a tractor trailer combination.
Axle housing wall thickness has allot to do with just how much weight can be had on a axle. For instance Eaton 404 vs 461 housing. There are many trucks with 404's and RS480 suspension from factory-those usually start a crack along the center of housing or at beam hangers. The 404 housing should never have made it off the drafting table when spec'ed with 48,000 lb suspension. The 404 housing is just under 3/8 thick the 461 housing is just under 1/2" thick. The old 402 P housings were just over 3/8" thick just right for RS460 suspension. IMO Truck Shop
My etc was covering housings I got lazy and forgetful!
I appreciate the insight here. Truck shop gave me something else to inspect when looking over prospects. The knowledge is appreciated. I haven't looked into permitting, didn't know that buying weight was an option. I do know in Michigan, with 11 axles properly spaced, you can haul up to 164,000 legally without a permit. We see guys at the ethanol plant with these giant 11 axle, dump rigs hauling in 2000 bushel loads. The straight truck pictured above can legally haul 725 bushel. The box is huge for hauling forage though, not grain. The hoist isn't strong enough to lift a full box of grain.