how does the drivetrain work on a bike

[John]

I’ve been trying to figure this out for a few days now and can’t quite figure out how the engine, transmission, and rear tire all connect on a motorcycle I understand how the engine and transmission work but how do they connect together?

Topic

[alan]

It’s mostly by chains.

[Rob]

For any specific bike? It kinda varies per category/type of block (and type of final drive!)

For getting the power from the crankshaft to the clutch/gearbox assembly sometimes belts are used, sometimes chains, or in more compact blocks a extra shaft with gear is used to create an offset between the block and the gearbox. Gearbox is the same idea as in a manual car, exept it has a drum with several grooves to select the gears, instead of a H shaped grid in your manual car. The H grid allows you to control the gears via two cables/rods. The two grooves in the drum do the exact same thing, but in a fixed order. It has a ratchet like mechanism in it, always one gear/click up or down..
Clutch, is multiple layers of friction material, some held in the driven (outside) of the clutch, some splined to the drive shaft in the center. Not one big (dry) friction plate like in a manual car.. I think this is mainly for size. borrowed via google (pic)

Not all to sure how gearboxes typically are laid out for a bikes with a driveshaft (cardan?) and/or a engine thats mounted lenghtwise. Like those big bmw’s etc have.

I haven’t exactly taken apart 100’s of different bikes, pretty much exclusively japanese tour/sport bikes.. so maybe my ideal of what is common isn’t really that common at all. 😉

[Jeff Savoie]

In general, most modern motorcycles work the following way;

On one end of the crankshaft is a gear. This gear, through reduction (the gear on the crankshaft is around ten times smaller than the gear on the outer basket), connects to the clutch outer (*known as the basket). This reduction means that an engine can “trade” RPM’s for torque.

A stacked set of multiple plates, some with a fiber coating (*not unlike an automotive clutch) and some that are steel, stacked in alternation, these transfer the engines rotation to the tranmission.

The fiber plates have dogs around the outside edges (*sort of like a huge square toothed gear) and these dogs correspond with gaps in the basket known as fingers. The steel plates have a set of gear teeth on the inside edge that mesh with a geared inner basket which in turn is connected to the transmissions main shaft.
A series of four to eight springs “squish” this stack of plates to engage the clutch (*spinning both parts of the basket together)…. a throwout or thrust bearing is connected to the clutch lever, via a cable or hydraulic cylinder, which pushes the inner basket away from the outer, allowing the plates to spin freely of one another.
Again, in most modern motorcycle engines, these plates are “wet” meaning they are lubricated with oil to keep them from overheating.
*That’s just part of why the correct type of oil used in a motorcycle engine is so important… way back when Mobil introduced graphite into oils, lots of us ruined the clutch plates in our bikes.

So… you pull the clutch lever in, the outer basket continues to spin with the engine, and the inner does not (*if the motorcycle is sitting, stopped).
When you let the lever out, the inner slips until fully engaged, which sets the transmission mainshaft in motion. The abilty to slip is what keeps you from stalling the engine, or launching violently forward.

I suppose now is a good time to stop and explain the mainshaft and countershaft relationship;
As I stated the mainshaft is connected to the inner basket (via a set of splines)… lying alongside it is a countershaft. Each of these shafts is splined to allow some of a stack of gears to slide sideways on them, fixed in rotation with the shaft. Other gears are allowed to spin freely around the shafts.

Here’s the key;
If a gear on one shaft spins freely, its mate is fixed to the other shaft. Also, if a gear spins freely the gear on either side of it (*on the same shaft) is fixed.
On the sides of some of the gears are dogs, and on the gear next to them the gear has slots in it, that correspond to the dogs… these dogs and slots are what make a specific set of gears, let’s say first gear, to both become fixed (*on the mainshaft and on the countershaft).

A series of “forks” ride in slots on the various gears, and a “drum” with a series of odd shaped slots in it move these forks back and forth sideways, engaging the gears in order…. first through fourth, fifth or sixth, depending on the bike. The shift lever is connected to the shift drum.

On one end of the countershaft, is a sprocket (*the countershaft sprocket) and again through reduction this sprocket turns the drive chain, connecting it to the sprocket on the rear wheel (*the drive sprocket) which turns the wheel.

*Conversely the countershaft can drive a belt pulley, or a drive shaft that is connected to the rear wheel…. but that’s another thread, in and of itself.
**As mentioned, in some (*generally older) bikes, the engine and tranmission are housed in separate cases… this is known as “pre-unit” construction, and then, rather than by direct geared contact the crankshaft is connected to the clutch via a belt of more usually a chain. In the case of pre-unit construction the transmission functions in the same manner as above, and connects to the rear wheel via chain, belt, or shaft.

I hope this helps more than it confuses y’all,

Krusty

[Thomas J Ahern]

Krusty,
I’ve been to two motorcycle factory schools & I couldn’t have explained it any better! Of course if you’re not mechanically inclined , it’s all hard for most to picture! I’ll put a star on your report card anyway. Irish

[Author]

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