Drive Train Basics

Discussion in 'Modifications & DIY how-to' started by jt money, Mar 9, 2006.

  1. jt money

    jt money 350hp mmm mmm Good! Supporting Member

    Power/Drive Train

    Clutch, Flywheel

    [​IMG] [​IMG] [​IMG]
    The clutch, flywheel and transmission make up the Power Train or Drive Train.

    Fly wheel
    The fly wheel is what is connected to the engine. The engine is cranking the fly wheel around. Basically, you can look at it like it is a large metal disk that is connected to the driveshaft. Every time a cylinder fires, it helps spin the fly wheel.

    Once a heavy fly wheel is in motion, the momentum (mass * velocity…the heavier the fly wheel, the more momentum) smoothes out the engine/cylinder “pulses”.

    It is this momentum and the ability of the fly wheel to maintain a constant speed that allows you to launch a car smoothly. If a fly wheel has little mass, it has a harder time keeping the engine at the same speed and can stall when the gear is engaged.

    It is for driving/launch ease that most manufacturers use a somewhat heavy fly wheel. A lighter flywheel does not have as much ability to maintain the engine speed when a load is applied to it. While that makes launching the car a little more difficult, it makes the engine much more “rev” happy! No longer is an engine slowed down in its acceleration/deceleration by the momentum of a heavy fly wheel.

    A lighter flywheel makes a huge performance increase!

    The clutch is the “connector” between the transmission and the engine.

    When you work the clutch pedal, you engage (pedal out) or disengage (pedal in) the clutch. In other words, when the clutch is engaged (pedal out) the transmission is connected to the engine. When you press the clutch pedal in you disconnect the engine from the transmission (disengage).

    The clutch engages the engine through use of a pressure plate. Think of it like when you are with a bunch of your friends and a “hotty” walks by. As the “hotty” walks past you, your friends shove you in the back, right in to the hotty. You now have no option but to interact with the object of desire (you can apologize or strike up a conversation…or whatever). You are now “engaged” with the hotty. The pressure plate pushes the high friction, clutch surface in contact with the fly wheel…until they are “engaged”.

    When things are too powerful…
    Back to our example…

    If the “hotty” is too “hot” for you (or too powerful in the case of an engine), you may initially “slip” or fumble in your connection. This is called “slippage”. Harmful in dating and to the clutch in your car!

    When you add a lot of power, or do a lot of high RPM launches, the stock clutch does not have the ability to adhere or clamp to the fly wheel. It is a sure way to go through a clutch quickly.

    High performance clutch have higher clamping power. That means that they are less likely to wear out quickly, but most importantly, can handle higher engine power output.

    The downside to a high performance clutch is similar to its advantage…no or little slippage. Some slippage reduces the immediate shock to the transmission and also makes for a “smoother” launch (if that is what you are going for…I prefer a rocket launch :^).

    For more information, view this articles source.( )
    Last edited by a moderator: May 24, 2006
  2. jt money

    jt money 350hp mmm mmm Good! Supporting Member


    Shouldn't the question really be why not? Subaru believes every driver should have the benefit of all-wheel drive. And Subaru is the only manufacturer to build it into every car they make. It's all part of the Subaru commitment to active safety. It's simple, really -- a safe vehicle is one that helps you avoid an accident, not just survive one. And the Subaru All-Wheel Driving System actively works to help you do just that, every time you take the wheel.

    The Subaru All-Wheel Driving System is a powerful combination of full-time, All-Wheel Drive, a horizontally opposed boxer engine and an advanced, long travel four-wheel independent system. All of which work together for superior traction, handling and driving confidence. Otherwise known as The Beauty of All-Wheel Drive.®


    Consider this. If all-wheel drive is designed to help you avoid accidents, why should you have to stop to think about it? Not to mention the time it takes to engage four-wheel drive in most cars. By the time it's all said and done, you've most likely gotten yourself into a messy situation.


    All Subaru models sold in the U.S. feature the Subaru All-Wheel Driving System. This system consists of several subsystems working in harmony to ensure maximum traction availability without any driver input. Unlike many four-wheel drive or all-wheel drive systems, the Subaru all-wheel drive system consists of power transfer mechanisms that are small enough to fit inside the transmission case. These small components not only hold down weight and power loss, but also ownership costs of Subaru vehicles, as no extra maintenance is required for the system.

    Subaru all-wheel drive works differently in manual and automatic transmissions. Read on for more technical information.

    All-Wheel Drive (Automatic Transmission)

    Active all-wheel drive is a term coined by Subaru to differentiate the all-wheel drive system in the automatic transmission from other "reactive" all-wheel drive systems on the market today. What makes this all-wheel drive system so special is its ability to anticipate traction needs and act before a wheel slips.

    The mechanism that transfers torque fore and aft is contained within the transmission’s tailshaft. To the casual observer it looks just like a typical hydraulic clutch found in any automatic. The key difference in this clutch pack is its operation. It’s designed to slip according to how much all-wheel drive is needed. When an automatic’s clutch slips, it is due to a malfunction and will eventually burn up. But the multi-plate transfer (MPT) clutch uses a special friction material that easily withstands the friction loads generated during torque transfer.

    The MPT’s operation is controlled by the Transmission Control Unit (or TCU) and constantly changes dependent on how the vehicle is being driven. To get more all-wheel drive, the TCU increases the hydraulic pressure to the clutch for less slippage. Less all-wheel drive calls for more slip and the TCU reduces the hydraulic pressure to the clutch.

    Under normal, dry pavement operation torque split is about 90% front and 10% rear. This distribution helps to compensate for the car’s weight distribution and resultant smaller effective rolling diameter of the front tires. As weight transfers to the rear of the vehicle, (i.e., under acceleration), the TCU shifts the torque split more toward the rear wheels. Under hard braking, torque is directed forward. Torque distribution is changed based upon how the vehicle is being driven. Throttle position, gearshift lever position, current gear and other factors combine to influence the TCU and it, in turn, selects a software map that determines how aggressively torque split will be adjusted.

    Two speed sensors are used by the TCU to detect wheel slippage. One sensor monitors the front axle set, the other the rear axle set. Pre-programmed variables help the TCU differentiate between slipping wheels and normal wheel speed differentials as what occurs when cornering. A speed differential (front-to-rear) of up to 20% signals the TCU that the vehicle is cornering and torque is distributed to the front wheels to help increase traction during the turn. Anything above 20%, however, indicates to the TCU that wheel slippage is occurring and torque is then distributed to the rear wheels.

    Another feature of the all-wheel drive system is its interaction with the anti-lock brake system. When ABS is engaged, the transmission selects third gear, reducing the unpredictability of engine braking and, thus, reducing the possibility of wheel lock-up. But all four wheels are still connected to the engine through the AWD system and are brought back up to overall vehicle speed quicker and can, therefore, be controlled again sooner. In a two-wheel drive system if the locking wheel isn’t a drive wheel, it can only be brought back up to overall wheel speed by whatever traction exists between it and the road. The quicker a wheel is controlled the better the stopping performance

    All-Wheel Drive (Manual Transmission)

    The 5-speed manual transmission’s all-wheel drive is referred to as a continuous all-wheel drive system. It uses a center differential located inside the transmission case that is controlled by a viscous coupling device. In effect, the center differential is a limited-slip differential.


    This is a differential with no gears. The input shaft from the transaxle (at the front wheels) and the output shaft to the rear wheels each carry a series of plates that are alternately intertwined and closely spaced. All the plates swim in a special fluid that transfers power from the input to the output plates when needed. If the front-drive wheels begin to slip, their shaft and plates spin more quickly than the others. This speed differential within the housing churns and heats the fluid, which thickens it and more tightly bonds the alternating plates. Some torque is now sent to the grippier wheels until spinning ones regain traction.

    In normal operation, power is distributed equally to the front and rear wheels. Plates are alternately attached to the front and rear output shafts inside the viscous coupling. When a rotational difference occurs between the front and back wheels, the plates inside the viscous housing shear inside the contained fluid (a type of silicone) heating it and causing the fluid to thicken. The thickened fluid causes the plates to transfer torque from those that rotate faster (the slipping wheels) to the plates that rotate slower (the wheels with the best traction).

    This no-maintenance system is simple, compact and virtually invisible in its operation. The system can distribute torque from a 50:50 torque split for maximum traction to mostly front or rear wheel drive.

    For more information, view this articles source.( )
    Last edited by a moderator: May 24, 2006
  3. Alex

    Alex Community Founder Staff Member

    Awesome! Thanks for this series of knowledge installments.
  4. bluetwo

    bluetwo Active Member

    Good info for sure.

    Where can you find more about the WRX's torque split? I've noticed my front wheels seem to slip more than the back ones, but surely my car doesn't have a front wheel bias. I've always thought it was more of a 50;50.
  5. mamock116

    mamock116 New Member

    great read, learned alot
  6. moose

    moose Infina Mooooooose!

    Think of where the weight transfers when you accelerate, and you will see why the fronts have less grip. :)
  7. CKings21

    CKings21 Member

    thanks, appreciate it alot i been looking for more stuff like this to read and its just seems like a maze finding it.. thanks again
  8. nsvwrx

    nsvwrx Active Member

    your missing the 4 eat awd system in the wrx.. its 45/55 along with a few other add-ons
  9. kingwrex

    kingwrex Supporting Member

    nice write up, that should explain alot of questions people have
  10. ScoobyMike

    ScoobyMike OG Mod

    Thanks JT!!
  11. MarkM2016GTI

    MarkM2016GTI Supporting Member

    That's a great write-up JT...Thanks...

  12. 03BluWRX

    03BluWRX New Member

    Gear Matching Tranny Problems

    I have heard many different conversations and input on gear matching ( or releasing clutch slowly to slow down) when downshifting. " Because of all wheel drive it can mess up your tranny". Could you elaborate if there is any negative effects of this driving behavior.
  13. slowwrx

    slowwrx Supporting Member

    There is no negative effect of slowing down in this fashion other than faster clutch wear. If you are rev matching (heel toeing) then you will experience virtually no clutch wear if done properly.

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