Transmission and auxiliary transmission

OK, if you happen to decide on no Aux. trans then with the 11R/24.5" tires I would look at 4.30 or 4.56 ratio. Depends how often you are grossing 200K+ and how fast you want to run when empty. Have dealer give you mph calculator to determine top speed or google search for one if you know 18spd. ratio. The tire will be 44" tall.
Another thing I do want to ask is what speed will you be running loaded? Remember a 2spd. aux you low side ratio is an 8.28 ratio with 4.10s. Do you think you will be using that ratio often? If not then the other option is 4spd. Aux. of course. With the overdrive you may consider a 4.30 or 4.56 ratio, so you can run in direct or low when loaded and use overdrive when empty.
Im just not a fan of an Aux. as you can tell, but of course this is your truck and not mine.

 

If you do get a 4 speed auxiliary, the main thing is to stay out of OD when loaded heavy. If you've ever looked inside one of these boxes, you will see how small the OD gear is compared to the others. Also, there is a small needle bearing between the main shaft and the output shaft that gets a workout when in OD. I know a guy that worked for a small HH outfit, his truck was a heavy spec Ford 9000, with a Spicer 1241c auxiliary. He drove that truck for several years without problems, pulling some monster loads. The outfit he was working for sold out and another larger HH outfit bought the truck. The new owner of the truck Let multiple drivers in the truck. I ran into my friend the other day and asked if he had heard how his old truck was doing. He told me that since the new owner had the truck, they've had the auxiliary out 2 times to work on it.

The truck I'm setting up is a Mack RD822sx. E9 Mack 500hp engine, Mack 12 speed main trans, Mack 65K rears with 5.87 ratio. This setup will be for short haul superloads, grossing up to 300K with my current trailer. I'm currently installing a Fuller AT1202 auxiliary. It's a 2-speed, that isn't designed to be shifted on the go.

 

You can get a no slip in a two speed.

 

Turner runs a lot of two speed rears. 4.88 on high and 6.50 something on low. They claim to have less problems than with an aux

 

I can see the 2-speed rears being more reliable than a auxiliary. When you put in an auxiliary, you add more driveline components, and even the strongest boxes can grenade if you abuse them. The main disadvantage of 2-speed rears is they usually don't provide the ratio drop an auxiliary does.

Another interesting concept is the DT air-shift planetary hubs. I think they have been bought out by Capital Gear but are still available. The best part about planetary hubs is it does the final reduction at the hub, eliminating strain on the rest of the driveline. The good part about the DT hubs is it gives you the advantage of planetaries when you need it, and the ability to disengage them for hwy use.

 

I don’t know much about planetary hubs. I have seen them is use on some coal trucks up in ky and wv. Truth is I don’t know much about any of them. I personally wanted to go with two speed rears a while back. The problem is, I have rockwell rears. They no longer offer a two speed tandem, so I would have had to change housings too.

 

I asked about an aux box, they told me a short wheelbase truck it’s not recommend because of driveline geometry. They said it would be almost impossible to eliminate vibration.