What's the reason for shifting at a given rpm?

Discussion in 'Questions From New Drivers' started by Trukker_1736, Mar 24, 2021.

  1. Trukker_1736

    Trukker_1736 Bobtail Member

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    Trying to understand why you shift at a given rpm. As a general rule when upshifting, you shift out at a higher rpm and then shift back in at a lower rpm. I'd like to try and explain briefly why I think this is the case and determine if I'm on the right track or completely off.

    So the lower gears are physically larger than the higher gears. The radius of a lower gear is larger than that of a higher gear. This is desirable as a gear with a larger radius provides more torque. But because the lower gears have a larger radius they also complete less revolutions in a given period of time than the higher gears. I'm making the simplified and probably inaccurate assumption that the gears driving the higher and lower gears (on the countershaft connected to the engine input shaft) are the same size. But this makes it easier to ask my question.

    So if you're engine is running at a given rpm the larger gear would have to be completing less revolutions than the smaller one. This means that when connected to the wheels (output shaft) the larger gear would complete fewer revolutions of the wheels than the smaller gear would.

    So when you upshift you want to try and sync up the rotational speed of the larger gear with the smaller one. This is why you shift out of the lower gear at a higher rpm (say 1600 rpm) and then shift into the higher gear at a lower rpm (say 1200). 1600 rpm of the lower gear roughly equals 1200 rpm of the higher gear. Or put another way, your road speed (the rate at which your wheels are spinning) is roughly the same with the lower gear spinning at 1600 rpm and the higher gear spinning at 1200 rpm.

    If you tried to immediately shift into the higher gear at the same rpm that you shifted out of the lower gear, that higher gear would be spinning too fast to match the rate at which your wheels are turning and you would have difficult shifting into the higher gear.

    I realize I've oversimplified a lot of things but I'm wondering if this is the basic idea? I feel like this reasoning could extend to essentially any gear system (say that of a bicycle) as well.
     
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  3. uncleal13

    uncleal13 Road Train Member

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    The bicycle is a good analogy.
    You can start off in top gear, but you would have to stand on the pedals pretty hard, so you select a slower gear to make it easier on yourself to get going. Then if you tried to go 20 mph but still in a lower gear you would be pedalling so fast you couldn’t keep up.
    An engine has a certain operating rpm range. If you go too far under it the engine will buck, if you go too far over it, the engine will fly apart. So you select appropriate gears for a given speed to keep the engine within it’s preferred operating range.
     
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  4. Ol'Shusquatch

    Ol'Shusquatch Light Load Member

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    Sorry have been busy on an oil thread in a different forum.

    Now the question. For me personally, I have always done it because the truck gives me a big thank you and a sigh of relief.
     
  5. JolliRoger

    JolliRoger Road Train Member

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    A lot of these shift patterns are Greek to me. In 53, the old F8 Fords did not have tach's, shifted by sound and feel...(If that makes sense.) Later the IHV195 had a tach, and 8 speed RR, but drove it the same way. Just kept the engine sound in go noise..
    H63 Mack had the stack centered behind the cab, just noticed the RPM to not go near lugging on a long pull to blow a head gasket. Mostly run it by sound and feel. Same tractor all the time probably accounted for it. Olden days, lot different.
     
  6. Brettj3876

    Brettj3876 Road Train Member

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    You mean over complicated
     
  7. kemosabi49

    kemosabi49 Trucker Forum STAFF Staff Member

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    It didn't take me long after I started driving to be able to hear when to shift. I rarely look at the tach. When on a good pull I might glance at it but the pyrometer had more influence on when I would downshift.
     
  8. RockinChair

    RockinChair Road Train Member

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    You've got the right idea about torque multiplication and gear sizing, but the technically correct answer is this: you want to match the rotational speed of the gear(s) on the input side of the transmission to the rotational speed of the gear(s) on the output side of the transmission. I'm certainly not a mechanic but I seriously doubt that a modern transmission has just one gear on the input shaft.

    Don't worry about the actual gears themselves, just remember that the gear ratios indicate the number of times the transmission input shaft turns relative to one revolution of the output shaft. In all of the underdrive gears (x:1, where x>1), the input shaft will turn faster than the output shaft. In direct drive (1:1 ratio) both shafts will turn at the same speed. And when in overdrive the input shaft will turn slower than the output shaft; the typical overdrive ratios are 0.86:1 and 0.73:1.
     
  9. seagreg

    seagreg Light Load Member

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    It is the gear selector fork that needs to mesh and not the gears themselves. Synchronized gear boxes have synchros that match the speed but trucks are not synchronized. But the gear teeth themselves are always engaged in modern transmissions. They just are not locked to the shaft they are on.

    [​IMG]
     
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