Crank Pulley - How do they work?

[Ray_32]

I recently talked to a fellow FRS owner and he mentioned that he had his crank pulley swapped to a lighter one. I was looking through the net and most people vaguely mention that it improves throttle response, less rotational mass etc. Sounds great but I'm still not sure how they work exactly. Forgive me as I'm new to this.

I'm very interested in getting one since they're rather cheap (and not too difficult to install) but I'd like to know in more detail how a good crank pulley will affect the FRS in general. Will appreciate any input especially from those who have swapped theirs and some recommendations regarding which one to get wouldn't hurt

Many thanks in advance!

[Captain Snooze]

I have lightweight crank pulley installed. Although I know many people on this forum reckon they are great I could not be sure if there was any difference after installation.

[Admiral Ballsy]

There's two potential effects. If it's lighter, the engine will rev more quickly as there's less inertia fighting it. Downside is poorer vibration damping.

If the diameter is made smaller, then the accessories are driven more slowly and consume less power - downside is potential issues with cooling (if the water pump is belt-driven) and charging/air conditioning.

[CaptainSlow]

Any engine has a specific peak output. That is, a certain amount of horsepower/torque that it is capable of producing. Everything attached to or run by the engine take a certain amount of that power away.

Think of it this way...you are riding an exercise bicycle with no resistance on it. It's pretty easy to pedal...takes almost no effort. However, once you start adding resistance, it becomes progressively more difficult (takes more effort/power) to keep pedaling at the same speed. It works the same way with an engine.

Every pulley on the engine is another source of resistance. In the same way it is easier to spin a pencil in your fingers than it is a steel rod, the more mass something has, the more difficult it is to spin.

The resistance of any physical object to a change in motion is referred to as "inertia" (p). The formula for inertia is p = (mass) * (velocity). As you can see, at a constant velocity (speed), something that has more mass (weight) will have more inertia than something that has less mass. Therefore, a heavier object is going to be more resistant to changing speed than a lighter object. This is the reason why sport bikes can accelerate almost as fast as supercars with just a fraction of the power.

This means that a heavier pulley will have more "parasitic drag" on the engine than a lighter pulley. Lighter pulleys reduce the inertia of the pulley system, which allows the engine to rev faster (better throttle response) and frees up a little of that HP lost due to parasitic drag.

Fun fact to round this all off - A Top Fuel engine's supercharger has a parasitic loss equivalent to around 600HP! That means you'd need 3+ FA20 engines just to turn the supercharger at speed!

[troek]

there is so much debate about crank pulleys, but the way i see it the crank pulley is lighter there for less mass for the engine to turn and less parasitic power loss. pretty much cut and dry. i look at it as dropping weight, no one part you remove from your car is going to be noticeably faster, but things add up, and every little bit helps.

edit: captain slow types faster than me i guess, lol.

[Captain Snooze]

Originally Posted by CaptainSlow

The resistance of any physical object to a change in motion is referred to as "inertia" (p). The formula for inertia is p = (mass) * (velocity).

This is only true for linear motion. Angular momentum is expressed as moment of inertia x angular velocity.

[Bg8780]

I've heard these pulleys causing issues and possible damage. Anyone care to elaborate?

[Ray_32]

Originally Posted by CaptainSlow

Any engine has a specific peak output. That is, a certain amount of horsepower/torque that it is capable of producing. Everything attached to or run by the engine take a certain amount of that power away.

Think of it this way...you are riding an exercise bicycle with no resistance on it. It's pretty easy to pedal...takes almost no effort. However, once you start adding resistance, it becomes progressively more difficult (takes more effort/power) to keep pedaling at the same speed. It works the same way with an engine.

Every pulley on the engine is another source of resistance. In the same way it is easier to spin a pencil in your fingers than it is a steel rod, the more mass something has, the more difficult it is to spin.

The resistance of any physical object to a change in motion is referred to as "inertia" (p). The formula for inertia is p = (mass) * (velocity). As you can see, at a constant velocity (speed), something that has more mass (weight) will have more inertia than something that has less mass. Therefore, a heavier object is going to be more resistant to changing speed than a lighter object. This is the reason why sport bikes can accelerate almost as fast as supercars with just a fraction of the power.

This means that a heavier pulley will have more "parasitic drag" on the engine than a lighter pulley. Lighter pulleys reduce the inertia of the pulley system, which allows the engine to rev faster (better throttle response) and frees up a little of that HP lost due to parasitic drag.

Fun fact to round this all off - A Top Fuel engine's supercharger has a parasitic loss equivalent to around 600HP! That means you'd need 3+ FA20 engines just to turn the supercharger at speed!

That's a fantastic explanation, Cap'n. THANKS! Really appreciate it.

Anyone who's installed a lightweight pulley felt any differences so far? Or any issues which pop up? Curious to know.

[CaptainSlow]

Originally Posted by Captain Snooze

This is only true for linear motion. Angular momentum is expressed as moment of inertia x angular velocity.

Just wanted to make it as simple as I could for this case

[CBR600RR]

Originally Posted by CaptainSlow

. This is the reason why sport bikes can accelerate almost as fast as supercars with just a fraction of the power.

I agree with everything you said but this.

I have had my Honda CBR600RR cross the 1/4 mile at 10.9 seconds at 132mph. With a pro rider and my bike stock, it could do 0-60 in 2.7 seconds and the 1/4 mile in 10.6 seconds.

This is faster than the majority of super cars.

with that said; my bike tops out at 170 mph, where as super cars usually top out aroung 200 mph


BUUUUUT this is way off toipic of the thread

[frs10]

so which crank pulley are you guys going with that seem to actually have a better performance after the swap??

[AdrianG]

Originally Posted by Bg8780

I've heard these pulleys causing issues and possible damage. Anyone care to elaborate?

Some people have had damage due to installation issues, as there's a crankshaft spacer that sits between the pulley and the crankshaft which can easily fall out and is incredibly difficult to put back in if it does:

http://www.ft86club.com/forums/showthread.php?t=26386

However, if you're careful and hold the spacer in when you remove it, there's not much to worry about.

The loss of mass on the pulley means better throttle response and a tiny bit more power to the ground. It also means the engine has a little less intertia, which in some applications means a rough idle and the engine is easier to stall with a MT and street driving.

I think on the FA20 if you installed all the lightened pulleys and and a lightened flywheel, it might make daily driving a bit difficult and you might stall and look like a chump in front of the cutest girl in your high school.

[Ray_32]

Originally Posted by AdrianG

Some people have had damage due to installation issues, as there's a crankshaft spacer that sits between the pulley and the crankshaft which can easily fall out and is incredibly difficult to put back in if it does:

http://www.ft86club.com/forums/showthread.php?t=26386

However, if you're careful and hold the spacer in when you remove it, there's not much to worry about.

The loss of mass on the pulley means better throttle response and a tiny bit more power to the ground. It also means the engine has a little less intertia, which in some applications means a rough idle and the engine is easier to stall with a MT and street driving.

I think on the FA20 if you installed all the lightened pulleys and and a lightened flywheel, it might make daily driving a bit difficult and you might stall and look like a chump in front of the cutest girl in your high school.

Thanks for the info! But high school? LOL I wanna meet the high school student who has a FRS.

[Calum]

Originally Posted by Captain Snooze

I have lightweight crank pulley installed. Although I know many people on this forum reckon they are great I could not be sure if there was any difference after installation.

After I took mine off I noticed the quicker throttle response missing, until then I didn't think it was making any difference.

@Ray_32 I've used bot the Perrin and Kartboy crank pulley. The Perrin pulley comes with directions that will lead you to over tighten the bolt if you follow them. If you get that one, DO NOT follow the directions, just torque it to the specified amount. The Kartboy pulley is good, but a few of us have had issues with a whining noise from it. Apparently a different belt will take care of it. I hasn't bugged me enough to bother fixing it.