need to figure this out, chevy front, yota back?
 

So, like many of you probably have been doing, I've been paying attention to chevy and toyota, and how they have been allowing aerodynamics. Chevy seems to be focusing on the front end and panel gaps, and toyota seems to be focusing on a clean break of the laminar airflow on the sides of the bumper and a diffuser. Everyone seems to be using tire spats, more on that in a bit.

The car already has front and rear under trays, a relatively smooth bottom, filled panel gaps, filled door handle recesses.

The point of this thread is to discuss the next step for me, front deflector, tire spats, smooth wheels, and rear diffuser.

I studied the chevy line of cars, and the standard deflector height is roughly 6.25 inches across the board. I then walked over to the volt, and measured about half that height. My bumper sits over an inch higher than any of these. with the ride height compensated for, the volt (same car as the cruze, and very similar to mine) gives me a goal of a 2-4 inch deflector. At only about 3.5 inches from the ground, gm has omitted front tire spats from the volt. Thoughts?

Smooth wheel covers, sources?

Rear diffusers, there has to be some merit to this, since toyota is sticking them on everything not a large suv. Now, the number and size varies by vehicle size, but one thing I've noticed is they always tend to put them as far forward as possible with the camry having it's diffuser placed lower than the bumper, instead of at the same level like on the others. Thoughts on diffusers? Does the deflector limit the need for a diffuser?

Some car’s chin spoilers/air dam/front deflector/whatever you call it are higher in the center lower by the sides near the wheels, acting as a tire spat. You mentioned you already had a front under tray and relatively smooth car bottom, what is the height of that under tray relative to where you are wanting to place that deflector? Conventional thinking for low drag (without a wind tunnel for testing) is that a front air dam doesn’t need to go lower than the lowest point on a car, several examples on here of too aggressive air dams adding drag and needing trimmed. On the tire spats, look at oem spats and you will notice how far towards the center of the car they are, because the airflow in that area is angled pretty heavily, aka don’t put them right in front of the tire.

I would also note that the OEM’s are focusing on a lot of different things, and I am sure the engineers at both companies are focusing on the whole car, however they also have to balance in things like style and cost of manufacture. Use ideas from the best of both, it isn’t that one strategy is better than the other.

The front tray is just a filler piece between the bumper and radiator, so the same height as the bumper cover. A bigger front tray is out of the question on this car, I have nowhere to attach it and it would be so misshapen it would mess up airflow. I agree, but the cruze and volt are the same car, with the cruze only being an inch or so lower than the cruze, yet the deflector is so much lower. .. I found this interesting. More time at the chevy dealership is needed.

I don't buy the whole "an airdam only needs to go as low as the lowest component on the underbody," when, in every Bonneville class that allows an airdam, you'll see cars with that thing extended all the way to the ground--because the benefit comes not from "shielding" something sticking into the airflow, but from disallowing as much air to get under the car in the first place, instead sending it around the streamlined upper body (which is why even cars like the hydrogen Fusion tout a benefit from a smooth underbody and a very low airdam). So, I think if you're going to go with an airdam, make it as low as practically possible. The Volt's airdam used to be much lower, in fact, until enough owners complained of scraping that GM raised it.

Agreed with Vman. I have to wonder if the "only as low as the lowest component" advice has more to do with practicality (clearance) than maximum drag reduction.

Don't they want some downforce at Bonneville? Or at least, no or minimal downforce BUT no opportunities for airflow to get under the vehicle at speed potentially causing lift or instability? That isn't a problem on the street unless your name is Hermie.

We have seen that there is a sweet spot for the ground clearance re: lowest drag; too low and too high lead to more drag. I don't know why the air dam clearance would be any different. My guess is too low adds unwanted frontal area... FWIW.

Yes, racecars have front spoilers that go almost all the way to the ground; however race cars are not exactly focused on low drag, as they are more worried about stability.

Lower than the lowest existing component puts you interesting place, because you may continue to reduce CD, but you could be adding frontal area. Maybe the CD is reduced enough to offset the gain in area, maybe not. Probably depends on the specific vehicle.

I think as low as the lowest existing component is a good guess for a baseline starting point, any higher or lower can be tested. If a new ecomodder starts out making super low ground scrapers and it just so happens their drag increases and mileage decreases, good chance they will get discouraged with the whole idea.

Those sound like wise words.

A good lower bound for the airdam is the height of those bump strips they put in parking lots and residential streets. I'm at 2 3/8" to the tow hook and 3 1/2" overall. It clears about 90% of them , but still bottoms out crossing Main Street downtown and a railroad track in Harrisburg.

The young folk in their stanced Jettas run at around 1 to 1/ 1/2" clearance. They must have a smartphone app that tells them when a maintenance crew throws down a steel plate over a hole in the pavement.

One reason to put a relief in the center of the airdam is so it won't high-center on gravel driveways.

Carlos made one that flips up or down based on speed.