Motorcycle Repair: Comparing dirt bikes, 4 stroke engines, dirt bikes

Question
Hello eXpert Chris,

My name is Nagy and I am a fellow expert in Electronics, home theatre and satellite Pay-TV.
I live in Australia and I have an import business.
Due to the local demand for Dirt Bikes, I have been communicating with Chinese manufacturers as to source from them.
In return I was confused with all the jargon they used which I doubt that all end-users would know but since they affect the price greatly I would appreciate all help in deciphering them.
So basically, number of engine strokes, number of valves, alloy vs steel parts, normal or upside down absorber, water vs air cooling.
I understand it could be too much for you to discuss and write but it will be equally helpful if you attach some links for me to read, I'm good at it.

THANKS HEAPS

NAGY

Answer
Hi Nagy.
 Actually, the phrases that you are asking about are very common amongst all serious riders.

1) Number of engine strokes.
I am assuming you asking about the difference between 2-stroke and 4-stroke engines.  There are not a lot of differences between the two, mechanically speaking, but there are differences and they are very big and important.

 First of all, you must understand what a "stroke" is.  It is the up and down cycle of the piston in the engine.  A stroke is one up and down cycle of the piston.

 Basically, a 2-stroke engine runs on a mixture of petrol and oil (usually 50:1 ratio).  It also has a 2 stroke firing cycle.  That means that it takes 2 strokes to go through the cycle of intake, compression, firing, exhaust.

A 4-stroke engine runs on petrol only (no oil mixed in the fuel) and has a 4 stroke firing cycle.

Most of that can be found in any encyclopedia that has a section on recyprocal (either automotive, motorcycle or aircraft) engines.  all recyprocal engines run in the same basic manner, but the real difference is whether it is 2-stroke or 4-stroke.

2) Number of valves.
 This should include valve shape.  The number of valves determines how efficiently the fuel is transferred into the cylinder and the exhaust is transferred out.  The valve shape has an effect as well (don't ask me why, cause I haven't a clue) :).

3) Alloy vs Steel parts.
 This has mostly to do with weight.  Alloy parts, while nearly as reliable as steel, are lighter than steel and therefore reduce weight.  This is an issue which is critical for racers more than anyone else, since racers tend to want to shave every ounce off of their bike's weight to help make them faster.  Alloys can also sometimes be less susceptable to heat damage than steel, which is another reason that they are a favorite amongst racers.

4) Upside down (inverted) forks.
 It is exactly what the name implies.  The meaty end of the fork, instead of being at the bottom of the fork is now at the top.  These cannot be retrofit to a bike that is not designed for them unless the triple-tree (the part that attaches the forks to the handlebars) is replaced as well.  The inverted fork is a relatively recent design change, mostly for street bikes and almost exclusively for the race-style bikes.  I am not sure why this is done, but I suspect that it is an effort to reduce aerodynamic drag.

5) Water cooled vs air cooled.
 This one is exactly what it says.  Cars and trucks are water (liquid) cooled, while most motorcycles have been, traditionally, air cooled.  That trend is quickly changing.  Liquid cooling requires an engine that is heavier than an air-cooled one, because of the addition of a radiator, coolant lines, water pump, thermostat, cooling fan, thermosensor and internal engine ducting for the coolant to remove engine heat.  Liquid cooled engines do not have cooling fins on the cylinders since the do not need them.

Air cooling requires only that the engine be able to shed heat as air moves past it, thus the fins on the cylinders of air-cooled bikes.

I hope that's helped you.  Sorry I don't have any links to send you, but I don't normally need that kind of information to reference.

Good luck.
FALCON