Belly Pans on Semi Trucks

[Diesel_Dave]

It occured to me the other day that I've never heard of belly pans being used on a semi truck and I'm a little puzzled as to why not. That industry is probably the most efficiency-conscience segment on the road. I've seen various other "aero-aids" (side skirts, air dams, trailer tails etc.) but never a belly pan. To the best of my knowledge, even Shepherd777's AirFlow truck doesn't have one.

Anybody know why?

[Shepherd777]

We did have one, albeit a mini.

The State Inspectors hated us enough already for the low ground clearance.
Although we were legal as 4" is the minimum. The next truck will have more on aero thought & devices on the bottom.

Here is a build pic of it. Aluminum sheet over corrugated plastic, shown with the protective blue plastic still covering the white aluminum. Light & tough. The cut-outs were for radiator brackets that were flat on the bottom. So we basically had a belly-pan from the leading-edge to the front of the steering tires.

[Piwoslaw]

Originally Posted by Diesel_Dave

It occured to me the other day that I've never heard of belly pans being used on a semi truck and I'm a little puzzled as to why not.

My guess is that since semi's have air dams in the front, often quite low, then adding a belly pan would see only minimal gains.

[Shepherd777]

Originally Posted by Piwoslaw

My guess is that since semi's have air dams in the front, often quite low, then adding a belly pan would see only minimal gains.

We did the one in the BulletTruck mostly to duct air to the cooling system package.

But I'm pretty sure it help on the aero side as well.

We have run SolidWorks CFD on simulated full belly pans on both the tractor, and the trailer believe it or not, and it helped the aerodynamics tremendously.

[aerohead]

Both NASA and Renault published findings from wind tunnel research which included belly pans on Semis.They help.
Renault's V.I.R.A.G.E. attained Cd 0.29 in part,by the use of a full-length belly pan.Pretty sweet for 1987.

[Diesel_Dave]

Originally Posted by aerohead

Both NASA and Renault published findings from wind tunnel research which included belly pans on Semis.They help.
Renault's V.I.R.A.G.E. attained Cd 0.29 in part,by the use of a full-length belly pan.Pretty sweet for 1987.

It would seem to me that a belly pan would be more beneficial than an air dam because it doesn't increase the frontal area the way an air dam does. Do you know if they used an air dam and a belly pan or just the pan?

[ConnClark]

I would think the main reason against belly pans on heavy haul would be it would hamper inspections.

[aerohead]

Originally Posted by Diesel_Dave

It would seem to me that a belly pan would be more beneficial than an air dam because it doesn't increase the frontal area the way an air dam does. Do you know if they used an air dam and a belly pan or just the pan?

I'm certain that Renault used both.
I'll have to revisit the NASA paper.Memory tells me yes,but I've long lost any confidence in remembering stuff.

[Coroner]

The DOT would hassle every truck driver or O/O to remove them for inspections. Any oil deposits would rack up violations pretty quickly. Even if it was just a drop, it counts as a leak, depending on the inspector. Belly pans then turn into liabilities, not gains.

[mechman600]

The same reason cars don't get belly pans: serviceability.

Considering 2007+ trucks are quite unreliable, sometimes needing monthly service work to keep them running, not to mention the regular bi-annual inspections and random roadside inspections, the the added cost of belly pan RE & RE would be high. Not to mention impossible in most shops - HD truck shops generally do not have lifts for underneath access. I can tell you that a truck with a belly pan would not be allowed in my shop, and it's a dealership.

Also, heat rejection. EGR HD diesels (generally 2002+) require a ridiculous amount of airflow for heat rejection, and it all goes through the grill, rad and downwards - around the engine, maybe some under the cab, but mostly under the truck. A belly pan would not allow this without drastically changing something else.