Transmission and auxiliary transmission

I would go manual and the four speed maintains the gears you want and you have four aux. Choices, for difference ratios

 

The only difference between the auto shift and the manual shift is who’s shifting the gears. I’m curious why the 4 speed would be better behind one and the two speed behind the other.

 

Interesting

 

Only automatic for heavy haul is an Allison

 

I know where there’s a decent size fleet of 18 speeds autoshifts pulling loads over 200,000.

 

I'm going though this myself, except I'm looking at grossing around 300K. The most common 4 speed auxiliary, the Spicer 1241c, isn't really rated for modern high torque engines. TTC does make a upgraded version of the 1241 that is stronger than the original Spicer. I know some people running them, but the they will even tell you that if you don't baby it, you will have problems.

I'm in the process of putting in a Fuller AT1202 2-speed auxiliary. I think the rating on it is 35,000 ft/lbs in low range. You've got to remember, the max torque to the auxiliary is the max engine torque times the first gear ratio of the main trans.

All that being said, I've got a Spicer 1241c that I've went though that I'd sell. It was what I was going to use until I did more research. I've done extensive research on auxiliary transmissions, if you need any other info, let me know.

 

Would like to hear more about what trailer you are going to pull and max gross weight, but initially I would give my opinion and say manual 18spd, with no Aux. trans and 2spd. rears instead. Aux. trans are nothing but extra weight and problems. Like m16ty said a 4spd. Aux will not handle the torque and he is correct as I have had some owner operators tear them out from under a truck running on 10 axles. If you insist on an Auxiliary make the practice of pulling it out every 100k and having it rebuilt regardless. Th at is the only way it won't leave you stranded in the middle of the road. A 2spd. will handle the torque, but you can not shift in motion as it is a 2.02:1 reduction. On the other hand a 2spd. rear end adds least amount of weight and you can shift in motion. A 4.10 final ratio will have a 5.61 ratio on the low side I think. You can still run 55mph on the low side if you feel is necessary. A 2spd. gives a 4.10 and 8.32 ratio on low side. Can't run over 35mpg just a guess and again you have to stop to be able to shift back on the high side.

 

I run a 20918b and a 4spd spicer with a 600hp 1850tq motor on 10/11 axles and have had.no issues to date. I love the set up and would order it the same way next time. I know the 3 spd fabco is rated for a bunch more torque but it was not available at the time and a custom order was cost prohibitive - like $25k more than my solution.

I do not run in overdrive except empty and then only on flat terrain.

As I understand, you can not get lockers with 2spd rears and my guess is a guy that runs in the backwoods of BC would really want lockers...

Just my opinion