Geoff Weeks

If your going to run a lot of highway miles a steep O/D gearing is going to put a lot of engine power into heat in the transmission and rear axle. In an ideal world, you would have top gear direct and be able to get all the road speed you want. The world is not ideal, and for many reasons that is not easy to achieve and be usable. Most efficient is straight through drive, where all the gears are just "along for the ride" in top gear. Gearing up in front and down in back has lots of losses in the form of heat. It does allow smaller driveline to a point. That is why it was used of old. @Mark T is spot on about driveshaft speeds and critical rpm. It is easy to drive a driveshaft at speed that it will come apart. Shorter shafts and carrier bearing are required to keep the shaft from "whipping" at higher speeds. Spicer has a good online critical speed calculator.

It may be as simple as the 18 and 13 use different low (range) and OD ratios and if you got it to function, it would leave you with ratios that aren't really usable. A 15% splitter ratio for the 13 and 18% for the 18. So the 18 has a lower "low range" ratio so when in OD low it is close to low in the 13. If you take a normal 13 and add an O/D without changing the low range ratio you don't get the ratio spread you need to be usable.

always thought if adding an underdrive rear splitter box to an overdrive front, to make what Eaton finely did in the RTLO series. Gives you two O/D ratios .86 and .74, direct is one stick position down splitter in direct.

Let be start by saying I have zero experience with the newer RTLO series. All the roadrangers I worked on were the older RT-RTO series. I haven't been inside one in almost 10 years now 😮. I remember there being a reason the old series can't take a 13 and make it an 18 with just a knob change. So I am going to disappoint just about everyone including myself and say I don't remember why. There is no reason not to use highside low on a 9 spd or a 13 (or 18) where it becomes more risky is when you use the splitter gear. You are multiplying the input torque through the the front box, then passing though the splitter gearing, and it isn't designed for that. In direct the power flows straight through the back box. The reason it is not generally used in a 9 spd or others is it is a ratio in between 4th and 5th (or 3rd and 4th if you use the "low" terminology) and the step between that isn't a even step. If you're climbing a grade it may be tempting to use "funny gear" as it used to be called (highside low) but the jump down is a bigger one then it is to low side high, and it means any further downshift will also be another un-even drop. The low range gears are larger and better able to take the torque input for long periods than high side low. Empty I also have used funny gear many times, 1-2-3 (or if you must, lo-1-2) flip to high and go back to 1st, it is a bigger jump than continuing on in low and the jump from there to the next gear up is bigger jump than shifting as you "should". I only did it empty, not saying it is better or great but no harm will come. sorry not to remember better, a mind is a terrible thing to loose!

If your worried about spitting a driveshaft, you have the math backwards. Lets say you have 600 ft.lbs at the flywheel with the engine at idle, clutch engagement. If you have 12.57 low ratio, you have 7542 ft lbs into the driveshaft (assuming no losses in the transmission) if you have 20,08 low, you have 12048 ft/lbs into the drive shaft. In most cases you'll loose traction before you stall, front wheels sink in and the drive tires don't have enough traction to move you forward. Lower gear will not help with that. once the torque exceeds grip of the tires they will spin. Lower gears come in handy when moving heavy loads slowly, but if your tires can't grip enough, it doesn't matter. Soft ground it is all about what your tires can transmit to the soft ground. I did heavy haul and started loads of 170K with 12.56 low gear. backing that load in a tight space, a lower gear would have been handy, but risks the driveshaft if the operator isn't careful. Only way to increase reduction but not stress on the driveshaft is a lower rear ratio, either single speed, two speed or planetary hub. Superload prime movers use planetary hubs to lessen the strain on the driveline all the way to the hub.

Larry, you can take the 4 motor bolts out of the air starter, flipping the motor down low and re clock the drive. I don't know if that will give you enough wiggle room to get the motor out with the filter on, it will however give you a little more room on top to attach the large air hose. Not sure where your steering gear come to in relation to the engine, so that may be a problem.

A 14069B has a low of 12.57 Where as a 15618 has a low of 14.71 so that is not true. However what the low ratio is doesn't tell the whole story, it is the steps between gears and what the power band of the engine is that will make or break the set up. An Eaton 18 has a low of 14.71 and a top of .85 with 17 steps between The Mack 2080 B has a low of 20.08 and a top .67 with 7 steps between. Each step has to be huge to make that spread, and if the engine isn't built to allow that RPM drop between gears it will lug and may not even make enough torque to get to its powerband to be able to use that spread.

The issue @terry and I are trying and failing to get across, is that an engine built to run in front of a close ratio 9 sped is fundamentally a different engine than one built to run in front of a wide ratio transmission. The 350 and 9 spd has higher torque and hp but in a narrower band than a 300 designed to work with a wide ratio box. Maxidyne vs Econodyne. I hope showing the Charts for the two types would get the point across. While placing a broad torque band engine in front of a close ratio trans will do no harm but not use the engine to its best, going the other way (placing a narrow torque band engine in front of a wide ratio box) can do some serious damage as the engine will lug at every gear change and may never have enough torque to make it into the power band of the gear it is in.

I think what he needs help with is the torque rise of the engine in question, and why some need close ratio steps and others in the Mack family can work with wide ratio steps. The engine torque rise has to work with the steps between gears. You know Mack engines better than I and can advise.

I was hoping we'd have heard back by now!

Yeah, and the Gatewaypundit always gets the story right. Dream on! https://en.wikipedia.org/wiki/The_Gateway_Pundit

Wider ratio gap between gears, means the engine speed may be too low for the load in the next higher gear but too high in the present gear. Torque rise of the engine has to be matched to the gears available. An engine set up for a 9 speed with closer steps will not be matched to a transmission with wider steps.

The cab looks like a match to the picture, or at least the door looks the same.

No KW sold to Unit Rig.

Really not enough to ID on. Cab may be a KW or Chicago Mfg cab. Unit Rig, KW, Dart, Pacific or even IHC (Payloader). IF forced at gun point, I'd go with KW on the cab shape alone.