Transmission shaft 1 is fixed to the crank is called the main shaft.
Counter shaft is chained to the rear wheel.
The main shaft can be called a fixed gear, being it's fixed to keep spinning.
The main and counter shafts have what is known as free-wheelers and they are not fixed to the shafts.
The sliding gears are fixed and they slide via the forks moving when the foot shift moves up or down, you get side to side so the forks engage.
At this point, you have the 3 Stooges action happening. Moe is the main shaft that is constantly slapping the free-wheelers. The 'slide to side' is a fixed gear 'dog' about to enter a slot. That dog end looks more like an erect nipple. Usually 3 or more are placed around the fixed gear. These are spinning with the crank. The free-wheelers are going to be locked together when the slider dogs enter the slots.
The free-wheeling gears on the main shaft are connected to the fixed gear on the counter shaft. So when you are in N, all the free-wheelers are not engaged so the only shaft moving is the main off the crank. So when you stab the trans into 1st gear, you are stooge slapping the slots with the dogs.
Why is this happening? Well, if you didn't have the clutch plates so close to each other, they keep spinning with the oil being separated from it is one. You have 'lube holes' down the shaft so it cools the plates with oil. The plates sort of drags for awhile when you pull the clutch lever.
So there are a few tricks you can do to either stop the noise, stop the wear at the dogs and slots, is to start out in gear with engine off. Coast to a stop at a light with the engine off and now shift down to 1st. Using a clutch lever with the engine off finding gear~ 'Bleedsquid.' So, you have some rider in a car looking over at you finding N with a dead engine and clutch lever going in and out like that will help... We be laughing from here.
And if that does not cure the problem that is 'normal' as most said, is because of the design and all that. Hold the clutch to the grip and let it sit. This lets the plates stop eventually, maybe. And now after 4 minutes of waiting, maybe it will not clunk. So to recap the 3 options...
1. Engine off = Engage 1st gear says no noise present.
2. Engine on = Engage down to 1st gear before you stop, then hold the clutch in till the light turns green or it is safe to move.
3. Engine running meaning = Hang time before you engage 1st gear is more a crapshoot 50% of the time. Engine off is no noise 100% of the time.
My MO is always option #1. I want zero material floating in the oil pump gears is if it goes thru there it goes upstairs too.
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