my question of the day - downshifting

Discussion in 'Questions From New Drivers' started by LostBoy, Jun 7, 2007.

  1. PTX

    PTX "Electronically Involved"

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    Jan 29, 2008
    Dallas, TX
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    OK, drivers... don't take this the wrong way, but here comes yet another 4-wheeler telling you how to drive.

    I'm No Expert(tm), but I've been driving manual transmissions for about 26 years now. You look at my profile and say "that's not right, it says he's 34." Yup. Since I was 8. I try not to "know-it-all" and I certainly don't know everything... but one thing I do know is the dynamics of a manual transmission, and various types of such.

    One thing I notice in this thread is a lot of drivers talking about cold hard numbers, either RPMs or road speeds.

    Nothing wrong with that... in your world, the numbers are real close together, and it makes the differences minor, in most cases. In the world of a gas-power vehicle, let's say a 4-cylinder pickup, it may make torque at 3800 rpm and hp at 5200 rpm. And it's transmission is set up to use that sort of powerband in the way that's going to give it peak efficiency in at least 80% of it's use. If I were hauling anything near the max GVW, a 30% gear span for 2-3 and 3-4 would be unacceptable on anything but the flattest of terrain.

    And that's precisely what we're dealing with here. A gearbox is a mish-mash of steel brass and aluminum that does one thing... divides and multiplies. That's right. It is a fraction. A percentage difference. A ratio. Everyone knows that if I move the shift lever from 1 to 2, I have changed the ratio that the engine is turning the driveshaft.

    But few other than professional drivers and racers really understand more than that.

    And therein lies the rub for the driver who needs to learn to downshift.

    Let me try to explain...

    First of all, if you don't know where your engine's powerband is, you cannot make educated decisions as to when to downshift or upshift. Secondly, if you don't know how the percentage span between gears is going to affect the RPM, you cannot make educated decisions as to when to downshift or upshift. What I mean by this is that if you upshift too soon, you will undershoot your torque peak and may lug the engine. If you downshift too soon, you will overshoot your governer and damage your engine.

    So- let's say you're equipped with a CAT Acert C15 rated at 550 HP # 2100 rpm and 1850 ft-lbs # 1200 rpm. Let's say you have an 18.5% span between gears I'll call "4th" and "5th", adapt it to whatever transmission you enjoy running. If you shift out of 4th for 5th at 2100 rpm, you will drop by about 390 rpm to 1710 rpm. On the other hand, if you don't claw for the sky and shoft out of 4th for 5th at 1900 rpm, you'll drop in at 1550 rpm. If you pull out 4th at 1750 rpm you'll slot in 5th at 1425 rpm. That last one is only a 324 rpm drop compared to the 390 from 2100 rpm.

    It's not so important you know the actual percentage span between each of your gears. It is important to know that it's not always cold hard numbers- even something as simple as two identical trucks, but one has all virgin drives and the other has all wear-bar-showing drives... will make them shift out at different road speeds and even show different speeds on the speedo... minor, but it's still different than a cold hard number, though a cold hard number will help you find a feel for it.

    So what the heck does any of that have to do with this driver's plea for downshift help? EVERYTHING... because once you have a feel for the span between gears, you will get a feel for how much you need to raise the engine speed to match the transmission speed in a downshift.

    For example, in that previous example, lets say you're in 5th at 1400 rpm. Having watched on your upshifts and after guaging the span, you'll know that you must raise your engine speed to about 1750 rpm to slot in 4th.

    So there you go. Whatever it's worth to ya. :biggrin_2559:
     
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  3. Wiseguywireless

    Wiseguywireless Road Train Member

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    Dec 21, 2007
    Petoskey, MI
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    I was driving a peterbilt with a 8 speed man. and Auto 9th
    Why they do that is beyond me... But I couldn't find my slot for that at all when I started my first 60 miles. I shut down for the weekend. asked on here about it. Got some good advice, But what worked the best was when I got back into the truck, I watched the tack when the truck autoshifted in 9th. It told me everything. The truck wanted to shift down when it dropped to 900 rpm, and it jumped back in at 1250... LOW rpms I thought..... But it worked. I had no problems going the last 400 miles with that truck. Now I am picking up a international to take to Las Vegas. Loaded too. I am glad for that. I hate emptys.
     
  4. Cybergal

    Cybergal Road Train Member

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    Oct 20, 2008
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    Iam used to the eaton fuller 10 speed!:yes2557:
     
  5. Wiseguywireless

    Wiseguywireless Road Train Member

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    Dec 21, 2007
    Petoskey, MI
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    you got that right.... I learned on one and I am used to that.
     
  6. lilillill

    lilillill Sarcasm... it's not just for breakfast

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    Nov 7, 2007
    Possum Booger, Alabama
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    ...or actually, use the governor.

    Downshifting 101 for newbies:

    1) Floor the throttle in neutral to see what the governed RPM is set at.

    2) Check the RPM drop on an upshift--usually 400-500 RPM's.

    3) If the RPM drop is, say, 500 RPM's and the governed RPM is 1900, you can downshift by flooring the throttle when the RPM reaches 1400. Like this...

    You hit an exit ramp and let off the throttle. As soon as the RPM slows down to 1400, put her in neutral, floor the throttle to the governed RPM and stick it in the next lowest gear... it'll go every time--even without the clutch. This allows you to downshift without trying to rev and look at the tach at the same time (a dangerous thing to do).

    Eventually, you'll be able to downshift at lower RPM's by blipping the throttle 400-500 RPM's, by ear. You'll know what the engine should sound like when the RPM is right.
     
  7. GasHauler

    GasHauler Master FMCSA Interpreter

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    Sorry but if you do that in my truck you'd be walking home.
     
  8. Wiseguywireless

    Wiseguywireless Road Train Member

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    Petoskey, MI
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    I am afraid that if I let out the clutch after only dropping 500 RPMs from max Gov. RPMs. I would fly thru the front window.
     
  9. lilillill

    lilillill Sarcasm... it's not just for breakfast

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    Why is that? It's a lot easier on the tranny when it goes in without grinding. And I didn't recommend to do it on a permanent basis... it is a method to learn downshifting without having to look at the tach too long.

    I'm failing to see the problem... enlighten me.
     
  10. GasHauler

    GasHauler Master FMCSA Interpreter

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    One of the main problems that these CDL schools fail to teach is taking care of the equipment. Running the engine up to the max governer speed will take it's toll long before the tranny goes. The object of shifting is to match your road speed to the rpm's and knowing what gear to be in at any given speed. Would your run your car engine up to the red line?

    I'd would much rather have a driver that takes it easy with the engine than running the piss out of it. I'm sorry that you can't understand that. One poster stated that he learned from the auto shift and it was shifting at a lower rpm than just pegging the motor. Don't you think that there's some reasoning behind that? Any good driver will tell you to stay below the governed rpm's and to drive RELAXED.

    When the fuel squeezer engines came out long before your time a new shifting pattern was recommended and that was progressive shifting. I've driven both types and you don't reach your destination any faster by shifting up agains't the peg. If fact your get more milage out of the engine when you shift at a lower rpm. We had a truck that had over 750,000 miles on it before anything was done to the engine. That was rare back in the 70's.

    On down shifting you don't have to hit every hole when slowing down and you're just over working yourself if you do. I slow the truck down while in high gear then when the engine rpm's are at it's lowest I'll skip gears and pick the gear that matches the road speed. For example, with a 10 speed I'll exit the highway let the engine drop to about 800 to 900 rpm and shift skipping two or three gears. You only have to rev the engine up a couple of hundred rpm's and the gear falls into place. Of course every truck is different but it doesn't take long to know what gear is good for what speed you're going. Believe me you'll get more milage out of that motor before any work has to be done if you stay away from the max rpm's.

    You need to let the truck work for you instead of you working the truck. What you have described about "flooring" the pedal is forcing the truck into gear and it's just not recommended by any truck engine manufacturer. Not only is it not good for the engine but it's not good for the running gear. Downshifting your way has a good chance to "Shock" the drivetrain and cause damage to the u-joints, driveline, and the differential. You need to treat your money maker with care so you can make as much as possible.
     
  11. lilillill

    lilillill Sarcasm... it's not just for breakfast

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    I'm afraid we'll have to "agree to disagree". I turned wrenches on trucks long before I started driving them. A diesel that has the governor set at the proper RPM will run up against the governor all day long without damage.

    Now there was a time or two when I'd get a Screamin' Deee-troit in the shop that somebody had "turned up". I had to put new blower seals on an 8V-71 once that had been turned up to 2500 RPM's. After reinstalling the blower and setting the rack, I set the governor back down to spec... 2100 RPM's. The guy came back a while later complaining about his truck wouldn't go as fast--he was used to running it all day at 2500. I turned it back up for him and he never came in with a blown motor. The old 2-strokers would run up to about 4500 RPM's before they blew apart (seen it happen when a rack was misadjusted).

    As for driveline shock... when the gear speeds are matched perfectly, there is no driveline shock. Perhaps some more clarification is in order--I'm not advocating downshifting all ten gears (or how ever many). One or two gears to go from highway speeds to city speeds--or to go around a corner. I shifted the first truck I ever drove this way for a good while, until I learned to downshift by ear--I never broke a u-joint or blew out a rear end. Most of the time, the stick would slip into gear so slick that you couldn't even feel it.

    At any rate, this is probably one of those religious battles that has no solution. So... no hard feelings... I just don't agree.
    :biggrin_25525:
     
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