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Old 02-29-2004, 07:47 PM   #21
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onezman, mase, first off i owe you an apology, i truly misspoke my self about the excess power issue-- i was thinking of my own rig which is carrying 21,500 on the rear axle, yep i know thats more than i'm sposed to but i'm giong to try to not put a tag under it for 2000 lbs.. hope you agree with my thought process on this. i put 15,000 on it this past summer and crossed the divide 4 times, and did a loop around colorado with no p'bms. take care. mase
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Old 03-01-2004, 01:08 AM   #22
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Onezman said: A 10 speed will get the job done 99% of the time for all OTR applications. 18 and 15 speed transmissions are becomming a thing of the past in the market place.
Generally, 18 speeds are only used for very heavy haul applications. These transmissions were popular when HP ranged from 250 to 350 with torque ranges from 1050 to 1250.
With the advent of higher HP higher torque engines, that many gears are entirely unnecessary.
For RV porposes, even a 10 speed is too much. The lower gears will almost never be used. For towing, 4th gear is sufficient for starting off. I use 5th gear starting off bobtail. The only time an RVer will use lower gears is for "creeping."
**
Actually, there is a different basis. The 18s are pretty new, derived from the 13s in the early 90s, when the 3406B and Series 60 were changing the rules and did a decent job, with adequate torque and wider powerbands.

The 5 & 3/5 & 4 2 box transmissions are more like the situation you are describing - a response of the 60s to the lack of power, and at least as important, the narrow power bands, in the OTR trucks of the time. In the 70s, the folks who had the K series Cummins had to have such trannies because that was the only way to handle the torque of those large and heavy motors. Now Fuller and others make trannies that can handle a lot more, but a friend of mine saw a Fuller 10 blown up during a dyno testing and tuning session of a 6 cylinder Mack in the 70s.

For many purposes, a 10 is now adequate. However, for people hauling max loads (80K, not heavy haulers), the ability to split gears is of great value. The gap in a 10 speed is pretty big, and can make a hell of a mess on a grade. I ran out of the midwest, and a 10 is adequate there, with enough power. I would not have one for most purposes if I ran as far east as PA or West as Colorado; the ability to split the top gear on a pull is great. Depending on how much improvement there has been lately, I might reconsider that. A 10 was adequate for me running a V8 Mack in the late 80s, but it had far more torque than anything else around. I could run up grades at 65 that others could barely top at 30. (I had to jake going UP Jacob's ladder on the Mass. pike to keep down to the limit, and could get away with using the car lane on Sideling Hill in PA, running at 60+ when almost everyone else was at 20-25.) I see trucks having a hell of a time going up Vantage to Ryegrass (I90 W/B, from the Grant-Kittitas County line in Washington); often they are down to 40, which is inexecusable. I saw a heavy hauler from the Midwest, obviously underpowered, down to about 10 MPH.

Downshifting sucks fuel, and my time with a 9 speed and a 350 Mack (able to pull like a 400 Cummapart of the times) was not fun in some ways. The way to make speed and have decent fuel economy in a truck is to get a decent speed, like 62, set the cruise, and never slow down or downshift.

15s are not at all like 18s. 18s are good for heavy haulers due to the small spread on each shift, but it takes a lot of weight to make that important, and that's not our situation here. 18s, like 13s, are based on the 9 speed. In the 13, one simply can't split the bottom 5; in the 18, each gear can be split. A 15 is actually a 10, with a lower range below it, allowing for more multiplication at low speeds. (Besides, the max rating for a 15 is 1650 ft-lbs, which is a small motor in the OTR setting unless one never leaves the midwest.)

For an RV, it is almost certain that a 9 or 10 speed is ample, with one exception. The rule for picking a starting gear is that you use one that allows you to let out the clutch at idle (NO THROTTLE) - you have to get the clutch out and keep your foot off it, in fact have it flat on the floor next to the pedals! This should not be a problem with the relatively light weights involved. However, a nice low gear option (forward and reverse) would be great in a tight spot such as an RV park. The Mack trannies have it all over all the others in this issue; they have options for super low first and reverse gears. The Fuller 15 would be good on this issue, for that reason. Other than moving around in a tight spot, one need not use that creeper range, so it is really a 10 speed. Less shifting is a good thing for RV use, as it can be hard on the clutch if one is enough of a fumble bum to need it (although shifting is a LOT easier when one is closer to 80K than when light). This would be a good argument for an autoshift.

Today's powerplants have a LOT more torque, get better mileage, and broader powerbands. The first truck I really drove OTR, when a driver got sick, was typical of the old style. It was a '77 Freightliner, with a 400 Magnum small cam II and a 13 speed. The damned thing had no powerband. It ran like a scalded cat if you kept in the power band, and with 4.33 gears would easily pull 80+. However, it had to be kept perking like a 2 stroke racing motorcycle. The powerband was 1800-2100+, and at 1798, it fell on its face. Those days are over. The last of the N series Cummins (the same block as this motor) had a lot more power, a power band well over twice as wide, and was more tolerant of the drop in RPM.

What we have here is a guy looking at a used truck for his starter. It won't be perfect, but he (like me) does not know enough for sure to go order one and get specs right (I'm still not sure what I would spec for an engine/tranny combination; it depends on the feasibility of a driving front axle and who will make a truck with one). An 18 auto will skip gears as needed; it has a clutch (critical - a full automatic is a BAD idea because of the torque and heat involved, although a much smaller risk in an RV); and it is already there. For my money, the idea that an 18 would be silly to order in a new truck used for an RV, which is clearly correct, is not relevant here.
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Old 03-01-2004, 03:11 PM   #23
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Doug...I had to read your post 4-5 times to understand it all....so much good info in so short of a post!....thank you geof
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Old 03-01-2004, 04:04 PM   #24
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doug, thanks-- i'm glad to see your post. i've got an 18 auto shift eaton. i love the "split-ability" of my tranny. one of the hardest pulls i made last summer was out of bishop up to mammoth lakes. lots of the hills in the rockies made me glad to keep the rpms in the torque band as opposed to having to simply have to grind my way to the top.--- mase
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Old 03-01-2004, 04:39 PM   #25
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I don't know about hauling 80K up a grade, but I know that hauling 20K up a grade does not require a single downshift if the driver has at least 400 HP and 1,450 torque. If the driver keeps the boost up, only traffic or curves will dictate a downshift.

I can't recall ever going below 8th gear using a 10 speed on any grade in any situation on an interstate highway.

http://onezman.tripod.com/yourrvhaulercom
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Old 03-01-2004, 07:50 PM   #26
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I would say that is correct. I note for example that the 380/410 Mack delivers 1560 ft-lbs. That is probably more than enough, even with a tandem and building your Class 8 RV to the max (as I probably would, using it for more or less full timing). I strongly suspect that even the 310/330 would have enough torque for such a unit.

The 9 speed Mack with the low gears has only one disadvantage. If one drives over the entire country, cruising speed and speed limits can vary from 55 to 75. Gearing for one end means one is either racing or lugging at the other, or at least has that risk. While most motors are more tolerant than they were 20 years ago, they have a sweet spot where they give their best fuel economy for a given speed.

I did the math playing with the Roadranger road speed calculator some time back, and I concluded that the 13 speed could address that range best due to the small split (and since the 18 is fundamentally the same as to this issue, one could do it with that tranny, such as when buying a used truck, although I would find it hard to justify getting one if ordering new for RV use). With the figures I used, the difference in cruising speed at the same RPM was about 10 MPH or a little more. Set it for 58-60 in 8L and you have about 68-71 at the same RPM in 8H. Not bad, eh? And, if you don't need the extra flexibility of the 13, you don't have to split it, which means it's a 9 speed with an extra button.

In case anyone is thinking I am advocating for Mack over others, the answer is no. It can be a PIA to get a Mack worked on sometimes, and Mack parts tend to be more expensive. I am, however, looking at either a Mack or a Western Star, due to the ease of special odd orders for these makers. I think I want a driving front axle, depending on what I learn from a dealer, and that could be a controlling factor. The Mack website no longer mentions them.

These are of course, options based on my preferences. I hate to fly, need room for the dogs, might be functionally full timing, etc. What YOU need for YOUR life is a different issue, and in spite of my pontificating, I am not living your life. I used to run OTR, and generally hate automatic transmissions. I doubt that applies to most here. Some of these issues are personal preference - I prefer certain breeds of dogs, with certain attributes of size, hair length etc. That doesn't make Rotts or certain obscure Mastiff breeds right for you.

The issue at hand was the suitability of this used truck for this member. While it might not be the truck any of us would order, if it is a good condition used truck and he feels confident in that, the transmission would not be the reason I would pass on buying this truck.
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