rear-mounted radiators and ballast tanks

[aspera]

Most of the aero stuff on this forum is the same stuff you'd see at Bonneville. You guys should lurk around landracing.com - Home

One of the tricks you see on the salt is rear-mounted radiators. That's a trick that's used in all sorts of motorsports from ice racers to rallycross cars to rock crawlers to swamp buggies. The new Lexus supercar uses the same trick, too.

The idea is to keep the airflow as clean as possible as long as possible. Wait to dirty it up right before you dump it into the already turbulent wake of the car. Readers of SportCompactCar might be thinking about this article.
P51 Mustang Net Thrust - Net Downforce - Sport Compact Car Magazine

Land speed racecars also use ballast tanks instead of radiators. Those tanks contain cold coolant. After the end of the run the tanks are hot and are allowed to cool down. The ballast tanks allow the cars to seal up the front grille entirely. Running without a radiator wouldn't work on a street car, but adding a ballast tank *IN ADDITION TO* a radiator would create a buffer so the car wouldn't overheat AS QUICKLY. That might help some of you guys that only have a postage stamp sized hole in your air dams.

One other thing I've seen on racecars and hotrodded cars is a hood vent behind the radiator. The Mitsubishi EVO has a good one. It might be worth your while to vent your stock front mounted radiators this way (with a small Gurney flap) and seal the underside of the nose with a splitter/skid plate/sump guard.

[aspera]

...plenty of room for one in a boattail...
{hint hint}

[NiHaoMike]

Why couldn't they thermally couple some coolant lines to the shell of the vehicle? Then the shell becomes a radiator. The delta-T should be sufficient for a significant amount of heat dissipation despite the relatively small surface area.

[MechEngVT]

Originally Posted by NiHaoMike

Why couldn't they thermally couple some coolant lines to the shell of the vehicle? Then the shell becomes a radiator.

The single-word answer? Lawyers.

You can't have any surface of the vehicle over 140 deg F (or 110 IIRC for a metallic surface) excluding solar loading. If by design surfaces get that hot without sunlight and someone were to lean on it they would be instantly burned and could sue for negligent design. This isn't so much an issue on racing vehicles since they generally aren't exposed to large numbers of the mind-numbed populace.

[Big Dave]

I agree. Fear of litigation. Howard Hughes' boys tried that using the skin of the car as a heat exchanger for a steam car. An errant Model A Ford showed them it was a bad idea.

Jet aircraft use hot bleed air to de-ice wings and provide air conditioning for the cabin. The tubing is welded to the leading edge sheet metal.

All the same, for very light-duty vehicles, a rear-mounted radiator is an idea with some merit.

[aspera]

Why do you say for very light duty vehicles? Almost all of the examples I've been able to find were on race cars.

Good info on the legal problems with production cars heating the surface. There have been some tube-frame drag cars that piped the coolant through the tubing, but a radiator still seem to be better answer. Most cars and trucks stick with the traditional solution of mounting ONE radiator front and center. That works but it isn't the only solution. In my opinion it isn't the best solution either.

Since you guys brought up heating the surface of a car, I've always wondered why some of that waste heat from the radiator wasn't put to good use keeping ice from building up in my front wheel wells in winter. A couple of coolant lines routed inside the front part of the rocker panels or inside the fenders would make winter just a little bit nicer. It only needs to warm up to just over freezing, so lawyers wouldn't have anything to fuss over. Besides, the underside of the car is a hot place anyway. You can put hot bits anywhere you want under there.

[Peter7307]

Originally Posted by aspera

Since you guys brought up heating the surface of a car, I've always wondered why some of that waste heat from the radiator wasn't put to good use keeping ice from building up in my front wheel wells in winter.

Many years ago I owned a locally built version of the Mini (before BMW built their variant) which had the radiator exhaust venting into the front wheel well.
Never had a problem with snow or ice build up on that car !

Pete.

[Sunwapta]

I've always wondered why they don't eliminate the radiator or cut its size by half by just running one or two cooling lines under the car to the rear then back to the engine bay. If necessary add a couple miniature rads midway back.

[beatr911]

I used to have a '72 911 that had a retrofit A/C fitted. The condenser was big and flat and mounted under the vehicle. Makes sense in that the 911 has no front grille opening. At first I thought that it was wierd but it did actually work. Worked until road debris wore a hole in it that is. Also I had to be moving for the condenser to give up heat from the freon (no fan) and the floors would be a little warmer than without the A/C unit.

[MechEngVT]

Originally Posted by Sunwapta

I've always wondered why they don't eliminate the radiator or cut its size by half by just running one or two cooling lines under the car to the rear then back to the engine bay. If necessary add a couple miniature rads midway back.

Another consideration that affects this suggestion (and the body-surface exchanger as well) is manufacturability. The assembly plant has to fill the cooling system with fluid after it has been assembled in the vehicle and is often one of the last things done before starting the engine and driving away. Filling fluids is time consuming and messy, both of which are abhorrent in a mass production environment.

Complicated fluid pathways are more difficult to fill. Air only comes out of the system at the top and if there is not a continuous path upward the system will not purge. Some factories use vacuum-fill where a considerable vacuum is drawn to purge the cooling system of air "dry" and then fill the void with coolant, but there will still be air. Inserting exchangers under the body will almost surely cause air traps and long horizontal runs of line tend to pool air bubbles without a clear upward path to an air bleed. Excessive air bleed screws make filling hard and messy, especially if tool access to the bleed screw is difficult. Even if many of these challenges can be overcome for initial production, servicing the cooling system can be dramatically more difficult as draining/filling partially wetted systems is even tougher and more likely to trap air than filling a dry system.

Folks here pay attention to cooling systems as a necessary evil that robs us of aerodynamic efficiency, but I'm pretty sure that cooling system engineers at all the auto companies are trying to please many different masters not the least of which is reliably cooling the engine (something air traps and off-putting maintenance costs don't help).