How prototype undertrays for your car

This covers how to make undertrays that work. It might not be the longest lasting (or the prettiest), but with a couple of hours and about $10 in materials (depending on scrounging ability) you can see if it's worth building properly.

Front undertrays are aerodynamic aids that serve to reduce drag and lift. The rear one does much the same. The theory is that the easier the underside of the car cuts through the air, the less positive pressure is present there, and less drag and lift is developed. That's a bit simplistic, but it will do.

The finished product will look like this... Yes the front one has a downward angle, but read step 1 to see why.

Step 1: Assess the situation

Look under the front and rear of your car. I was confronted with this view. It's not the best aerodynamically, but that thing jutting down does actually have a purpose (first pic). In the second pic you will see that behind it is the oversized gap between the front of the engine and the radiator (that's what you get when you have a V6 where a straight 6 can fit), and in the third pic you can see the radiator and air-con condenser.

The plate currently does three things.

1. Provide a negative pressure behind it to extract air from the engine bay
2. Provide a slightly higher pressure in front of the radiator (to encourage airflow through it
3. Act as a stone guard for the cast alloy sump

I want to keep all 3 of those aspects while improving aero. Which means I have to run a sheet from the bottom rear of the front bumper to that plate. I also wanted to block air from going into the part in front of the wheels.

The rear section is just untidy. The other side is more fuel tank in place of the exhaust. I wanted to cover this, but unfortunately the exhaust meant that I could only block in the middle and right hand side of the car.

Comments

[bpfh]

My Jetta (diesel) has a factory undertray. A PITA when you want to service, but it gives you better fuel economy and noise protection.... but yes, any wildlife your car catches, your car keeps...........

[TinKicker]

Well, did you do your mileage testing yet? Curious...

[BLSTIC]

Yeah I did. Best steady state (110km/h) went from 7.5 to 6.8l/100km. Average highway went from 8l/100km to about 7.6l/100km. Since sold the car (less the undertrays) so I have no repeat testing to go on.

However there was no loss of stability or cooling performance.

I estimate my $5 undertray saved me about $25 on fuel that trip...

The only reason I can see it not being a factory offering was that nothing can fall out of that gap I covered any more. Seriously, I caught a bird in a car that didn't have that gap, it's there for a reason.

I'll do it again, with better quality pictures next time, and maybe even an entire undertray...

[Scott_Tx]

As soon as you run into your first concrete parking stop you'll know why the factorys dont do this.

[BLSTIC]

There isn't actually much less ground clearance than factory, 10mm at most. But whichever way I make the front undertray, it is more likely to catch while reversing than the factory plate. The rear, however, I can put a small lip on in front of the fuel tank, and it won't catch on anything that wouldn't ordinarily rip the fuel tank out... And it won't hurt the aerodynamics the slightest.

[Scott_Tx]

I used to have an rx-7 and you really had to be careful with that thing it was so low

[MrK]

the factories dont do this only because you don't see the undercar when you go buying cars... they won't make a car more expensive if it is something you won't notice only new electric and hybrids car have undertrays because they are more concerned about mile range you can get in electric mode