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The coolant overflow tank should be the highest point in the system. I’ve seen some people tap and thread metal thermos type bottles to use as overflow tanks and mount them in the original position. I think some of them look pretty good if done right. The bikes are coming together nicely!
 

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Discussion Starter · #42 ·
The coolant overflow tank should be the highest point in the system.
Interesting!

I personally believe that cooling systems work by magic, so I would really appreciate some help/explanation.

When you said "should" do you mean you think it's the best way, or do you mean there's a scientific principle at
work where the overflow tank either will not work, or works poorly, if mounted lower than the rest of the system?

Problem with metal thermos type bottle is I can't see the coolant level without taking off the cap.
Problem with original position I will admit is personal-I just don't want it there, and that's one of
the aspects that's driving all these changes: coil on plugs, moving cooling components, etc.

Thanks!
 

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perhaps an explanation of the principal of flow and counter flow may help. perhaps an engineer could elaborate some.
but as I understand the system all elements increase in volume when they are heated some like liquids at a certain point lose the ability to hold all the elements bonded together and gas off or split AKA boil.

to prevent the liquid from boiling you have two options 1. contain it under pressure to increase the temperature it splits at (boils) or 2. cool it down to a point it doesn't split. (gas off/boil). the cooling system uses both first by pressurising the system then cools it using the radiator/fan.
now as the coolant expands as it gets hot the cap holds the pressure to a point but the expanding liquid being uncompressible has to be allowed to escape through the pressure cap and is caught in the expansion bottle as it escapes.
it stays there while ever the engine is hot, but as the engine cools the liquid contracts and is sucked back into the radiator by capillary action. (provided there is no air in the system) until enough as gone back in to refill the radiator.

now as I see the issue will the system refill by capillary(syphon) action alone or does it need a helping hand through plain old gravity. afraid I don't know that answer. but if the expansion bottle is below the rad cap it has to depend on self returning of the liquid. whereas if it is above the rad cap gravity will help. the other point to consider is should any air enter the system there is no way the liquid would return. so in my view it would need to be above. all the systems I have seen that are effective are all above.
 

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Discussion Starter · #44 ·
Thanks, glad to have @yorkie in this discussion.

2 things to consider:

1. Your description on expansion and contraction sounds right on. The part I don't understand is how the capillary (suction) action can push coolant back into the radiator; isn't it blocked by the pressure plate of the radiator cap that's spring loaded? You're right, it does indeed suck coolant back into the radiator, I just don't yet understand how that happens, and understanding that might help decide whether the relative vertical position is important.

2. " all the systems I have seen" Check it out on Google; search "motorcycle coolant overflow tank" and switch to images. They're all over the place, and the standard position for the CX500 was back behind the bottom of the motor. As you said, there may be an advantage to setting the recovery/expansion tank higher to get aid from gravity, but I'm thinking the science of expansion, contraction and capillary or suction action will not change depending on vertical position. I think that's why all these folks can locate their overflow tanks practically anywhere.

Cheers!
 

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I’m no engineer but I believe the answer is it should be the highest point not that it has to be the highest point.

It’s all about pressure and coolant expansion/contraction. The radiator cap serves multiple purposes. First it holds pressure on the cooling system which allows the coolant to reach a higher temperature before boiling. This lets the system operate more effectively. Once the pressure rating of the cap is reached, the larger outer spring will compress allowing coolant to flow out of the system preventing it from over pressurizing and damaging components or blowing off hoses and draining all the fluid. The fluid the cap allows to flow out of the system flows into the overflow tank where it remains until the coolant begins to cool down. As the coolant cools, the pressure in the cooling system drops which creates a vacuum. The cap experiences the vacuum and the smaller inner spring inside the cap compresses which allows the coolant in the overflow tank to be sucked back into the cooling system. Once the pressure equalizes again, the cap will completely seal off and prevent coolant flow in both directions.

If the overflow tank is not at the highest point it can cause some potential issues although the issues can be mitigated. A radiator cap should be at the highest point in the pressurized cooling system since boiled coolant gas has a lower density than liquid coolant so it rises. Remember how the radiator cap opens and allows stuff to flow across it? Well if the boiled gas rises to a point that it can’t escape then it will remain in the system as a gas. Gas in the cooling system drastically reduces the cooling efficiency of the system and will lead to localized areas of extremely high temperatures. You need to maintain fluid at all times in the entire system. The radiator cap allows the gas to escape the system along with some of the liquid coolant. Now is where pressure comes into play again. The pressured cooling system raised the temperature at which the coolant boils. As the hot coolant leaves the pressurized system through the radiator cap, it enters the tubing that connects to the overflow tank which is at atmospheric pressure. This lower atmospheric pressure means the coolant will boil at a lower temperature. What happens is the liquid coolant flows through the radiator cap then boils creating gas. This gas then rises to the highest point. The highest point is generally the air pocket at the top of the overflow tank. If the highest point is not the air pocket in the overflow tank, then that gas can be sucked back into the pressurized system. The gas will usually collapse back into a liquid when it reenters the pressurized system, but it is also possible it prevents flow back into the system or remains a gas long enough to create localized hot spots. The other issue when the overflow tank is not the highest point is making sure the tubing from the radiator cap to the overflow tank is completely primed with fluid and that no air is present. This is the bigger issue I believe. If you fill the pressurized system, put the radiator cap on, then fill the overflow tank which is at a lower location, then you will trap air in the lines. This air can then get sucked back into the pressurized system reducing its cooling efficiency creating localized hot spots that may damage components.

I’m writing this in bed on my phone so I apologize if I’m all over the place and make no sense.
 

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makes perfect sense to me and is along the same thinking lines as myself. air kills the function of the system it compresses unlike coolant lowing the system pressure. and stops any (lets call it a syphon) effect.
that is why when filling the system you need to get all the air out. it is also the reason a leak in the head gasket forces all the fluid into the expansion bottle but doesn't allow it to return to the point all the fluid runs into the bottle overflowing it but none returns back to the system.

I understand why @haybaler wants to do this. I had the same problem when building the cruiser. that is getting rid of all the sticky out bits that were hidden under the plastic but exposed now there are no fairings to hide them.

I worked round the issue by fitting the system off the ER which has the thermostat housing and filler cap attached to the top of the coolant tubes. but this will not work with the stock EX tank as it is flat bottomed.
you have to use a tank with a void underneath as the assembly sits far higher than just two rubber tubes as stock. the ER tank obviously has this void and it fits the frame (sort of) so was my choice.
the expansion bottle is off a Daytona it's very small (150ml) but seems to work just fine. it is marginally higher than the filler cap and attached to the front tank mount bracket.

one could experiment of course using a clear bottle with a tube at the btm fill the bottle 1/3 full mark the level run the engine up to temp through a heat cycle mark the bottle with the highest level reached then as the engine cools see how much liquid is returned to the system. do this with the bottle both above the cap and below it. see if the system works at both locations then you have a definitive answer.
 

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The lengthily explanation of the system above is incomplete. in the rad cap there are 2 one way valves out going set at 13 PSI. and incoming set at less that 1 PSI. if all system are intact gravity has no function. IE the expansion tank will work below the highest point until the weight on the coolant exceeds the strength of the contraction .

Engineer

FOG
 

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The lengthily explanation of the system above is incomplete. in the rad cap there are 2 one way valves out going set at 13 PSI. and incoming set at less that 1 PSI. if all system are intact gravity has no function. IE the expansion tank will work below the highest point until the weight on the coolant exceeds the strength of the contraction .

Engineer

FOG
incomplete yes of course it is I was writing a post not a book. although I fail to understand how gravity has no function otherwise how would the coolant have any "weight".
 

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What I think Fog was hinting at is that gravity will pull down on the coolant while the contraction will pull up on the coolant. When the pull from the contraction is stronger the coolant is sucked back into the system. If the pull from gravity is stronger then the coolant will not get sucked back into the system. In either case, the coolant system will equalize pressure although that pressure may be very slightly different depending on the location of the overflow tank. Since the system will equalize pressure in both cases, gravity doesn’t play much part (that is assuming the tank is not very far below the cap, which for a motorcycle is always the case).
 

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this is the whole point of the discussion. relocation of the expansion bottle and in finding place for it to go where it would still function as intended.
in order to know the maximum distance and level it could go below where it is sited in stock form, you would have to know 1. the force (in newtons) exerted on the coolant in the bottle by the reduction in volume as the coolant shrinks.
opposed to 2. the force (in newtons) of the opposite force of gravity trying to stop it. at some point 2. is greater than 1. then it will not work.
I have neither done the calculations or the experiment to find out positively so do not know. but in most cases the coolant overflow is above or a least almost level with the rad cap. so the force exerted by 1. must be very weak.
I would also point out the EX doesn't have a rad cap it's a sealed type the pressure cap is on the thermostat housing (the highest point on the cooling system) in order to remove all the air from the system.
 

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Discussion Starter · #51 ·
Excellent technical analysis, thanks to all. Now that I have a better idea of the forces at work, I'm ready to try a few different mounting locations.
I think I'll rule out the rearward location just on aesthetics alone. Will try small overflow bottle next to radiator, will post pics and results, thanks!
 

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All that really matters is the opening of the tube being at the bottom of the expansion tank. The coolant level in the tank must cover the opening by a few inches.

That way, when coolant expands and contracts it doesn’t suck air into the system. If the tube opening is placed up high so expanding coolant can exit but not return the system will not operate as it should.

You can mount the tank anywhere you like pretty much so long as you keep that relationship between the tube opening and the coolant level in the expansion tank.

That said, I wouldn’t place the tank way down low or anything like that. Similarly, I don’t think a tail mounted tank would be ideal. Mainly because the depth of the tank would limited by the available space.....sean
 

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Discussion Starter · #53 ·
Found a cute little coolant overflow bottle, works great so far:

53991


And it definitely supplies coolant to the system; after running for 10 minutes, shut off, then half the coolant tank just drained right back into the system, cool!

Another angle:

53992


Might try to cover bottle with stickers; also have larger Blue Bottle, aluminum unit, but it needs work to function properly.

Test drive Monday when I get a new clutch cable.

Cheers!
 
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