How to

Performance Tuning – part 4 Power Transmission

The term ‘power transmission’ makes quite obvious what we are looking at. Transmitting all that power to the rear wheel. After all what is a bullet that leaves the gun but does not hit the target?

Now the power we have made so far is at the crankshaft. It now has to go through the clutch and the gearbox. Our aim here is not to make any more power, since we cannot. It’s already made.

Our goal here is to

Minimise the transmission losses as much as possible to convert the power in form best useable on the road

Clutch: The clutch isolates the gearbox from the engine. The clutch has limitations to how much power it can transmit. So consequently when you make power, your clutch better not slips and wastes it all.

Basic things that you can do:

  • A simple method is to install harder springs. This will increase the pressure on the plates, but also increase the effort that your left hand needs to make to engage the clutch. Sometimes just adding washers below the springs will do the job, but in this case the travel of your clutch will reduce, which sometimes is not affordable.
  • Add a plate. Simple yet effective, but not always possible. Con: Reduce in travel.
  • Increase coefficient of friction in your pressure plate by sandblasting it, making dot punches, whatever. Con: Reduces clutch life.
  • Very sensible a thing to do is to make sure the available area on your plates is totally used. Misalignment of clutch plates causes only a partial area to grip. Hold the plates under a glass table and you will know what the situation is here.
  • If lifespan of the clutch is not needed beyond a race, then you can consider running a dry clutch. Oil bath serves only to increase life of the plates at the cost of some grip.

Gearbox: There is nothing subtle that you can do here. If your motorcycle has another model with an extra gear, then you can get lucky and get a conversion kit. The Yamaha RX series can be converted to 5-speed units thanks to the 5-speed versions that came in the late nineties.

One more thing that you can do is to install a different gearbox to suit your desired ratios of speed v/s rpm but that is a very drastic and very technical a modification.

One thing that you can do very effectively and should do on any motorcycle, performer or not, is to run good oil. Changing from mineral based to fully synthetic oil greatly enables the components of the motorcycle to withstand the increased stresses.

Sprockets: Functionally similar to a gearbox, you can alter the rpm v/s speed characteristics without touching the engine. The ratio of teeth between the front sprocket and rear sprocket determines the speed conversion between the tyre and the final drive on the gearbox.

If the rear sprocket is changed to one with more teeth, then the pick up increases through all rpm in the engine and top speed decreases for each gear. Similarly, if the teeth on the rear sprocket are decreased, the acceleration goes poorer and top speed goes up.
It is a matter of suiting the application.

Chassis: At first glance it does not seem like it’s a transmission element but in some ways it is. We are looking at performance enhancement not just power increase. Not really something to always tinker with, the things you could consider:

  • Depending on your application, you might make modifications. A drag motorcycle will need a long and low chassis with a shorter swing arm to prevent the front wheel from popping. If wheelies are your thing, leave it or shorten it.
  • Some extreme modifications require chassis strengthening to absorb the resulting vibrations. You might also consider rubber mounting your engine.

Now your bike is ready to put those horses down to work when you want it, how you want it, as scary as you want it. Get a good helmet and have the time of your life.

Performance Tuning Part 1
Performance Tuning Part 2
Performance Tuning Part 3

7 Comments

  1. hi………..

    I want to know whether there is any availability of power tuning box which can b installed to YZF R15

  2. Hey,
    Sorry for the late reply

    Haven’t got a chance to tinker with one yet, will let you know. Watch this space.

  3. Martin Alva says:

    Aftermarket ECU’s are available for the R15 in Indonesia. This aftermarket ECU will simply over-ride the factory rev limiter and will increase the power band slightly. there are sellers in Bangalore who have imported the ECU for testing road feasibility in India. Ill let you know the sellers name and more details after its finialised.

    cheers.

  4. pls,help me how we can increase sprocket,from were we can get increase sprocket for bullet electra,can we get frm bullet showroam,from were we get hard spring for cluth.
    i want to perfrom wheeli on bullet pls mail me deatail for modification for my bullet electra 2003 and pls tell frm were we can get all part,pls
    in deatail,if posiible send me deatail information with pic(images)
    maiul me pls i am waiting

  5. sir,
    do u have any idea to measure compresion ratio cos bore is like cylinder in shape so we can measure the volume but in head no proper shape in yamaha rxg how to measure please reply me in mail,tks

  6. Very nice article.

    You got it all.

    It will serve as a guide to build my bike.

    Thanks.

    Ron

  7. Sorry guys I did not realize that the comments section is still active. I will try and cover the questions and hopefully you will actually be checking after all this time..

    Sunny:
    Sprocket ratios need to be higher for wheelies. Simple math – Count the number of teeth on both sprockets. Divide the number of rear sprocket teeth by the number of front sprpocket teeth. The higher this number, the higher acceleration and lower top speed. Preferably increase the rear. Also you might need to partially reduce rear tyre pressure especially when learning

    Ananth:
    Compression ratio is a very important measure so you are probably on the right path. Since it’s an RXG (which is a simple two stroke), best thing to do is to remove the head, put the spark plug in and fill up the cavity (while the head is upside down) with a burette and measure volume. Try and also calculate the volume of piston that protrudes out from the cylinder when at TDC (but this is minimal). Some people also use plastecene to fill up the cavity to measure.
    For most purposes, it’s most important to know the compression pressure so think about whether you actually need the volume or really the compression pressure. If it’s the latter then simply get a good compression gauge!

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