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Clutch control and torque

  1. Aug 16, 2011 #1
    Does anyone understand this? How do you get more torque by slipping the clutch? I slip the clutch to reduce torque to the wheels until I get going while maintaining power by keeping the rpms high.

     
  2. jcsd
  3. Aug 16, 2011 #2

    Drakkith

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    As the clutch slips, you can increase the engine speed to any RPM while the tire RPM stays the same. The increased RPM of the engine can put MORE torque on the clutch while it is slipping than when it is locked. For example, lets say your engine puts out (totally made up numbers) 100 ft lbs of torque at 2,000 RPM and 200 ft lbs at 4,000 rpm. If you are going down the road at 2,000 RPM and you want to accelerate, the maximum torque available at 2,000 RPM is 100 ft lbs. However, if you let the clutch out a bit so that it starts slipping and rev up to 4,000 RPM it is possible you will have more than 100 ft lbs of torque available from the engine. MOST of that extra torque is lost due to the slippage, but not all.

    USUALLY you don't use the clutch in this manner. A clutch really isn't designed for that specific purpose, it is instead used because without a clutch you wouldn't be able to accelerate from a stop as the engine cannot operate at 0 RPM. Once you are up to speed it is 100% possible to shift gears without using the clutch at all.
     
  4. Aug 16, 2011 #3
    Ah that is a better explanation. I wish my engine had 100ft lbs of torque at 2,000 rpm!! Sign me up!

    My gears don't shift unless I disengage the clutch.

    People often say I am not very usual.
     
  5. Aug 16, 2011 #4

    Drakkith

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    You should be able to shift without disengaging the clutch, but you have to match the transmission and engine speeds. For example, start in 1st and accelerate. At about 3-4k RPM there should be a point that you can simply pull the shifter down out of 1st and into neutral. Immediately after that let the engine rev down to 2k or under I believe and there should be a spot in that RPM range that you can push the shifter into 2nd without using the clutch. Note that you cannot still be putting pressure on the transmission and engine, you have to *match* the speeds, not simply hit that RPM with the engine still trying to accelerate. I don't know the actual RPM ranges, but the general idea is the same.
     
  6. Aug 16, 2011 #5
    The comment "So slipping the clutch actually gives more torque to the wheels even though the fraction of power wasted in the clutch increases much faster."

    The clutch doesn't give torque. The engine can increase torque output by using more input, gasoline.

    1.)Torque is measured at the wheels, cause that's what matters.
    2.) Engine speed has little correlation to torque output.

    The torque measurement requires a force input, 4k rpm is not a force, and rotation speed is not a measure of torque.
     
    Last edited: Aug 16, 2011
  7. Aug 16, 2011 #6

    Drakkith

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    Nitsuj, I think they mean that the MAX torque at a given RPM is set, and you would need to increase engine speed to put out more torque. The torque from the engine is output to the wheels via the clutch and rest of the drivetrain. Slipping the clutch allows you to get to that higher RPM to get that higher max torque.
     
  8. Aug 16, 2011 #7
    Yea, so if the clutch is releasing the resistance to the engine and as a result the engine speeds up, doesn't mean more torque is at the wheels. Giving the engine more gas & engaging the clutch puts more torque to the wheels.

    at wide open throttle trying to go up hill in 5th gear with the engine speed slowing will not be cured by slightly disengaging the clutch. What would cure it is greatly improving torque to the wheels by shifting into first gear and easily climb up the hill with the new found torque via gear ratios, not specifically increasing engine speed by disengaging the clutch.

    To ensure you are transferring maximum available engine torque to the wheels keep the clutch engaged.
     
  9. Aug 16, 2011 #8

    Drakkith

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    You are mentioning two different effects here. At any given RPM you WILL have more torque applied to the wheels if you have the clutch engaged fully. But that is not what the discussion is about. Nor it is about what the BEST way to get more torque to the wheels. It is about whether this method CAN give more torque to the wheels. I believe it can as long as the clutch is engaged enough and the engine RPM is high enough.
     
  10. Aug 16, 2011 #9

    rcgldr

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    A mechanical clutch doesn't change the amount of torque, the output torque is the same as the input torque. The input torque can be higher if a fast spinning engine is being slowed down by the clutch due to angular momentum, used to help with launches on pro strock drag cars.

    Most clutches on street cars have a relatively low kinetic (sliding) coefficient of friction, perhaps to reduce the shock on the drive train during a launch. Many automotive magazine testers find it's faster and more consistent to launch by dropping the clutch at some engine rpm and spinning the tires instead of slipping the clutch. In some cases the kinetic coefficient of friction decreases as the speed differential of the plates increases and also as heat increases.

    The top classes of drag cars use a single forward gear with a mechanically programmed clutch that slips allowing the engine to run a reasonbly fast rpm at high torque output with the goal of keeping the tires from spinning.
     
  11. Aug 16, 2011 #10
    Consider the clutch limiting torque to the wheels, it relieves that force by slipping.
     
  12. Aug 16, 2011 #11

    Drakkith

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    IF the torque from the engine stays the same, yes. But what happens if you let the clutch start to slip a little and then increase the RPM and torque from the engine?
     
  13. Aug 17, 2011 #12
    Man, I am NOT letting any of you drive my truck, I hate the smell of burning clutch and replacing them is a pain.


    Interesting discussion though. On one of those "build something out of the stuff in our junk yard shows" one group used a rear end differential with a brake on one wheel as a clutch to control the speed of the other wheel. Yeah, it got very hot, but it did what they wanted.

    And they were not too worried about the longevity of their cobbled device.
     
  14. Aug 17, 2011 #13
    The engine speed increased, the torque ouput is the same if you haven't applied the accelerator more, the engine sped up because there is no longer as much resistance on the engine.

    If an engine is reporting it max torque is at 4k rpm, it doesn't mean if the engine isn't under a load and you rev it to 4k then it's producing the max engine torque.
     
  15. Aug 17, 2011 #14
    This makes sense to me. My car will accelerate faster if I slip the clutch and rev first, rather than flooring it and waiting for it to rev up. I can chirp the tires going into 2nd. :biggrin:

    @Drak
    I can't shift without disengaging the clutch at least a little. Must be a Japanese thing.
     
  16. Aug 17, 2011 #15

    russ_watters

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    Try it sometime. Or try applying your logic to the extreme case: Can you apply more torque to the ground with a slipped or engaged clutch when stationary?

    Also, for people who drive stick, making smooth but fast shifts requires easing off the gas to lower the rpm. Failure to do that will make the car lurch forward due to the extra torque.
     
  17. Aug 17, 2011 #16

    Drakkith

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    Let the clutch start to slip, hit the gas, then ease off the clutch a little bit. Not enough to cause it to fully engage, but just enough so that when you hit the gas pedal fully down you can get to 4k RPM and keep it there. You will be applying more torque through the clutch to the wheels even though the clutch is slipping.

    WARNING: I don't recommend this, as I like my clutches NOT on fire.
     
  18. Aug 17, 2011 #17

    russ_watters

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    Not that it's a good idea, but there is this certain hill on my way home from work....

    It has a mild slope, then a sharp left curve, then a steep slope. I'll be going 40+ in 3rd gear at the beginning, then coast through the curve to 30mph. Now I'm in the wrong gear. Occasionally, I'll keep my gas pedal foot on the floor and push in the clutch a little to let the engine rev up, rather than shifting into 2nd.
     
  19. Aug 17, 2011 #18

    Drakkith

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    Actually I think I do something similar myself russ. I'll be in 2nd at about 20 or so, go around a curve and slow down to 10ish by pushing the clutch in and coasting. Instead of shifting I'll just let the clutch out and stay in 2nd.
     
  20. Aug 17, 2011 #19
    That is just to prevent the engine from stalling though.
     
  21. Aug 17, 2011 #20

    russ_watters

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    Same diff, isn't it? You're running the engine at a higher RPM to keep it at a better power point, so you can better deliver torque to the wheels.
     
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