Gear Ratios: Misinformation at its worst.

Discussion in 'Experienced Truckers' Advice' started by Hammer166, Apr 28, 2021.

  1. Hammer166

    Hammer166 Crusty Information Officer

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    Differential Gear Ratios

    We see a lot of posts asking about gearing and transmissions, and most are a battle to correct misinformation. So it really doesn't help when a source considered reputable by a vast segment of trucking, puts up a blog post so full of BS. (it's the same as the column in the Movin' Out paper, where I saw it.) I mean I'm ready to start calling him Back Row Bruce, because his info is about as valid as what came out of party row on the CB, back in the day.

    Two points I want to make, to correct the most blatant errors. The first is the double over crap. Double over has a very specific meaning, it means the power path travels through 2 sets of overdrive gears. How many gear ratios are OD at the yoke has nothing to do with it, nor does the high gear ratio. (10 speed double over, AYFKM!?)

    The second relates to his comments about power lost in overdrive. He's correct that a .73 OD only puts out 73% of the torque that a direct gear does, at the yoke. However, there isn't some huge HP loss because of the OD, it's only a couple percentage points. And when comparing a direct drive/low ratio setup to an OD/standard ratio rig, he fails to point out that the differing rear end ratios result in both setups basically putting nearly equivalent torque to the ground. The extra torque of a direct transmission is lost in the lower ratio rear end. Basically, it doesn't matter what the gearing combinations are, if two trucks are going down the road at the same rpm and speed, they're putting down nearly identical torque to the ground. The advantage the direct has isn't in a huge power gain, but rather a small efficiency gain of the power path not traveling through as many gearsets.

    Rant over, we now return you to your regularly scheduled programming.
     
    Last edited: Apr 29, 2021
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  3. Bean Jr.

    Bean Jr. Road Train Member

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    I knew that this would be a good thread when I saw your avatar next to this post.
     
  4. Accidental Trucker

    Accidental Trucker Road Train Member

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    Bruce does live in the '70's and this century, all at the same time. "you'll have to call the manufacturer to find out your engine's peak torque".

    Good grief, every engine web site has the torque curves all over them; most of the modern ones are flat from 1050 to1500.

    I ended up with a "direct drive" set up in my old cornbinder. I'd rather lose the 1 or 2% in HP and fuel economy again to have those two gears back when climbing. A 13 geared to run in 11 is like having a 10 speed with bad gear spacing.
     
  5. Linte_Loco

    Linte_Loco Road Train Member

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    TY for the info

    Above my station I admit :oops:
     
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  6. Hammer166

    Hammer166 Crusty Information Officer

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    Somebody on here has a glider spec'd that way, except with an 18 instead of a 13. I would imagine that low and reverse are way too fast with a 13.

    Edit: it's Cowpie. Took a bit for the name to come to me!
     
    Last edited: Apr 29, 2021
  7. abyliks

    abyliks Road Train Member

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    I couldn’t even read the whole thing it makes my brain hurt.
     
  8. abyliks

    abyliks Road Train Member

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    Depending on if it’s an a b or c pattern, some 13s are geared with low and reverse as low as an 8ll, some are closer spaced to first, I don’t remember which is which, but I know it’s backwards for 13-18 (ie low low is a on a 13 and b on an 18)

    the rtlo is also the “double over” 13 he is referring to, the rto was the single od and the rtoo was the true double over
     
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  9. abyliks

    abyliks Road Train Member

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    I’m sure there’s a reason paccar won’t build anything below a 3.08, it’s tripping over dollars to pick up pennies, any small savings will go out the window when you snap an axle at a job site or have to replace a clutch after a few hundred thousand miles, most guys spec 13 or 18s for a reason, and know trucks, where as bean counters THINK they know trucks, but have no clue about anything in life

    The 10/15 speeds are also single over drives, some have the .73, the older rto are .79, all those little numbers on the side of the trans do actually mean something.

    running 3.36-3.55 in 8 direct around 60-65 is about perfect then use the big hole for high rolling/empty, IMO, also I’ll take 11rs over low pro any day,
     
    Last edited: Apr 29, 2021
  10. Hammer166

    Hammer166 Crusty Information Officer

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    It's related to an argument that more drivers are familiar with, which is the old "my truck with 3.90s will out pull my buddies truck with 3.55s!"

    Well, yes and no. If you're talking about in the same gear, of course it will. It's got better torque multiplication. But if you drop a gear or two in the truck with 3.55, so then both trucks are at the same RPM, they will both pull just as hard.
     
  11. Linte_Loco

    Linte_Loco Road Train Member

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    Thanks. That I get. I’m getting my own truck soon. I’m definitely gonna study that
     
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