I probably need to replace the pipes on the coolant tank on my bike in due course, which should be pretty easy to do, however, much to the dread of regular posters it's got me thinking again...

Now, for some reason unknown, I've always assumed the the top pipe on the tank was fed from the top radiator and that the bottom pipe went back to the engine. I've got no idea why I thought this considering that this would mean the tank would be pressurised to the same pressure as the system and blow the filler flap open as soon as the bike got warm. However, this does leave me a little confused. The bottom pipe leads to the top rad, and excess pressure will cause coolant to be vented into the tank. Nice and easy to understand that. I make the uneducated guess that the top pipe is an overflow, and drains somewhere under the bike if the tank fills to the brim.

This arrangement will mean that once vented, coolant cannot return to the system, and is effectively lost unless you were to remove it from the tank and put it back. Unless you've got air in the system causing it to boil or suffer chronic overheating, nothing should come out in normal operation, therefore nothing need replacing (unless you have a leak). So why does the tank have a minimum fill level marker on it if all it does is act as an overflow to stop the stuff pissing out all over you and the bike? Or have I got something dramatically and embarrassingly wrong as usual?

You pose a very interesting question here.....

My thoughts are based on experience of bleeding my 30 system.

I would say that there is a minimum fill level due to what happens to the coolant level when you rev the bike. I remember when bleeding the system that revving the bike pulled the coolant into the system as such and the level in the top rad dropped.

Now if there was no coolant to suck in from the overflow bottle it would pull in air, obviously no good for the system.

But then I'm probably chatting shit here as the HRC overflow bottle is literally just that, an overflow bottle to catch excess coolant, and not feed it back into the system.

Coolant CAN return to the system.

You will notice if you take the cap off that in the middle there is a small valve that you can open by pulling it down.



The cap keeps the coolant under pressure, as the temperature goes up the pressure will also increase. The cap is designed to open and vent the coolant once a certain pressure is reached (1.1 bar is common pressure)

Now, lets pretend some coolant got vented out... perhaps a long idling / overheat in traffic. You park the bike up and switch it off.

As the temperature drops this also causes a drop in pressure inside the radiator (which is still sealed with your cap) This drop in pressure is like a suck and pulls open the small valve in the middle of the cap, causing the coolant to be sucked back in from the bottle as the pressure equaliseses.

This is why there is a minimum fill, otherwise it would suck air in.

Hope that makes sense.

That makes perfect sense. I didn't know that the cap had the second valve, but it would explain why the overflow pipe goes to the bottom rather than the top. I figured that this and the minimum level should indicate that there was a two-way process involved, but I didn't have the foggiest idea how!

What I'd still like to know is why the HRC overflow only looks as though it works one way?

er, 'cos it's a racebike thing and it's assumed you aren't going to be bothered about having to refill the rads once you've finished the race?