Fan operation.

[atomsplitter]

There should be a level when cold, right?

Not necessarily. You can run the bottle empty and you should be fine. The expansion of the coolant is contained by the bottle. If you have a level in it at startup there maybe insufficient volume available to hold all the excess volume from the radiator. There's a siphon tube at the bottom of the bottle to draw the coolant back ino the radiator during cooldown (when a vacuum occurs due to volume displacement).

The specific volume of a fluid is the inverse of it's density. If volume goes up density goes down and vice versa. The change in volume is directly related to a fluids temperature. Increasing the temperature increases the volume. If the system is water solid (like your cooling system) the increased volume shows up as a pressure rise since the coolant can't expand initially. As it expands against the rad cap the cap acts like a relief valve at a set pressure to relieve the excess volume to the tank. There is no check valve in the system (valve to prevent reverse flow) so when the bike cools down and the coolant density increases it causes a negative pressure in the system, drawing the coolant back out of the tank. If some air gets drawn in the air will get expelled the next time the coolant flows into the tank since any air circulating in the system will go to the high point (radiator cap).

To know whether or not you have too much coolant in the bottle when cold, simply start and idle the bike until the fan starts. The coolant will rise in the bottle as the system heats up to 200+ degrees. When the fan is running with the bike idling the temperature should be fairly constant (you can check that with a pyrometer). The engine cannot (theoretically) heat past that point since engine heat output to heat sink (atmosphere) are in equilibriium so the coolant volume (level) in the tank will stabilize. No more heat added, no more expansion. Viola.

 

[barbagris]

The Reynolds number for flow resistance in the radiator is ignored here because it gets way too complicated. (Trust Me)

Spoilsport!

 

[DCRocket]

Not necessarily. You can run the bottle empty and you should be fine. The expansion of the coolant is contained by the bottle. If you have a level in it at startup there maybe insufficient volume available to hold all the excess volume from the radiator. There's a siphon tube at the bottom of the bottle to draw the coolant back ino the radiator during cooldown (when a vacuum occurs due to volume displacement).

The specific volume of a fluid is the inverse of it's density. If volume goes up density goes down and vice versa. The change in volume is directly related to a fluids temperature. Increasing the temperature increases the volume. If the system is water solid (like your cooling system) the increased volume shows up as a pressure rise since the coolant can't expand initially. As it expands against the rad cap the cap acts like a relief valve at a set pressure to relieve the excess volume to the tank. There is no check valve in the system (valve to prevent reverse flow) so when the bike cools down and the coolant density increases it causes a negative pressure in the system, drawing the coolant back out of the tank. If some air gets drawn in the air will get expelled the next time the coolant flows into the tank since any air circulating in the system will go to the high point (radiator cap).

To know whether or not you have too much coolant in the bottle when cold, simply start and idle the bike until the fan starts. The coolant will rise in the bottle as the system heats up to 200+ degrees. When the fan is running with the bike idling the temperature should be fairly constant (you can check that with a pyrometer). The engine cannot (theoretically) heat past that point since engine heat output to heat sink (atmosphere) are in equilibriium so the coolant volume (level) in the tank will stabilize. No more heat added, no more expansion. Viola.

A little over my head! but . . . then why is there a minimum and high level marked on the "bottle" - I thought that was for when it's cold. When it's hot, meaning right after I shut down, the level is perfectly within the two lines. Are you also saying I do not need to replace the radiator cap? (Btw, thanks for the input Atom and Ruzzle)

 

[ruzzle]

Consider the radiator should be chockers ALL the time.

As the coolant heats up and expands, it can no longer fit in the radiator and the radiator cap seal lifts against the spring pressure, and allows it to flow out and into the reservoir.

Then as the coolant cools down, it shrinks, creates a vacuum in the radiator, and a little valve in the cap allows coolant to be sucked back into the radiator to keep it full.

If the system is not airtight, the whole idea breaks down. You will still get coolant pushed OUT of the radiator when hot, but there will be no vacuum and it can't suck it back in as it cools down.

So there can be coolant in the reservoir (or overflowing from it onto the ground) and yet the radiator level will be low.

The reservoir bottle Hi/Lo marks indicate near enough where the levels OUGHT to be in normal operation if the system was initially filled correctly. In other words, the difference in the reservoir levels is the amount the coolant expands/contracts in use.

So when hot the reservoir will be around the Hi mark, and when cold, the Lo mark.

Here endeth the sermon.........................

 

[ruzzle]

To continue the sermon (sorry!)...

If you overfill the reservoir, then when the coolant gets hot and is pushed out of the radiator, there will be no room left in the reservoir, the reservoir will overflow, and it will dump onto the ground.

NOW the sermon endeth..............

 

[atomsplitter]

A little over my head! but . . . then why is there a minimum and high level marked on the "bottle" - I thought that was for when it's cold. When it's hot, meaning right after I shut down, the level is perfectly within the two lines. Are you also saying I do not need to replace the radiator cap? (Btw, thanks for the input Atom and Ruzzle)

The Min/Max marks are for looks, not operation. If you ride hard and the bottle is only half full is the bike operating properly? Probably, don't let OEM equipment cloud the physics of the system.

Replacing the cap is a good start (may cure the problem in fact). When you pull the cap check if your radiator is full to the top, if so your cap may have a weak spring so replacement is essential. With a weak spring your system cannot hold pressure. This condition will put coolant into the reservoir sooner than normal causing heat-up rate to increase and reducing cooling efficiency. Loss of efficiency results in higher operating temps that cause more coolant to flow into the tank and reduces the available coolant mass reducing heat transfer. Recall that heat transfer rate is dependent on mass flow rate and differential temperature (heat source to heat sink). If you have an air leak into the system you will also have an air leak out of the system (tell-tale steam leak). So the cap should fix it. Then put a minor amount (like almost none) in the reservoir and see if you overflow after a hundred miles.

 

[DCRocket]

Great - thanks, guys!

 

[DCRocket]

And just to be clear: when hot the coolant is at the high mark, but when cold it IS to the top of the radiator though I cant see any in the reservoir --

 

[DCRocket]

Here's the potential culprit. Haven't had a chance to run more than 20 miles, but so far so good

 

[atomsplitter]

Let us know if the new cap solved the problem.