Targets for Improvement

[Mighty Mira]

This is from the SAE paper and featured in this article, by Volvo’s Dr Simone Sebben (SAE paper 2004-01-1307). It shows the contribution to drag from different parts in the automobile.

Interestingly enough, the exterior is only 32%. This indicates that with some modification, the target should be a 68% drag reduction on a standard car (although that is certainly pushing things, it does give us something to shoot for). So perhaps anywhere up to a 40-50% drag reduction is possible if we fare the car correctly.

This also allows us to estimate the reductions various modifications would bring.

e.g.
Grille blocking: 30% (allowing some holes for ventilation)
Front wheel skirt and deflector: 13%
Rear wheel skirt and deflector: 7%
Undertray: 15% (all the other misc stuff added together)
Door shaving, caulking etc: ? This comes off exterior.

Remember, this is for one particular car. Each particular car will have its low hanging fruit.

Question: is this believable?

I think so. Consider that the average auto might have a Cd of 0.35. If all these can be applied and we get 33% of the initial drag, then that is roughly 0.12.

Obviously we aren't going to get down there that far as each modification cannot remove the associated drag completely, but seeing as the prius hasn't even attended to all of these issues and gets a 0.26 Cd, obviously there is some room for improvement.

[Mighty Mira]

This just goes to show that either the auto industry is either conspiring to give us crappy mpg or that styling is a very large factor in consumer preferences.

All these modifications will not add more than $1000 to the price of a new car. I have no clue why this hasn't been done before.

Also, considering that there are hundreds of models of motorcycle, why isn't there a single car equivalent of a motorcycle with low drag and appropriate engine?

WHY??????

[Matt Timion]

These are interesting numbers. I plan on attacking the front and rear wheel skirts sometime in the next year. I would also like to block the radiator, etc. and install an undertray. I see no reason why this cannot add an extra 10mpg on a regular car driving on the highway.

Most of these modifications will have neglible effects for city driving. For city driving LRR tires are your best friend.

[Mighty Mira]

Originally Posted by Matt Timion

Most of these modifications will have neglible effects for city driving. For city driving LRR tires are your best friend.

Yup. That and removing a back seat to reduce the weight.

[philmcneal]

wow very interesting data thank you! it seems grille blocking is the best drag reduction AS WELL AS providing quicker warm up times due to the radiator being blocked from the cooled air. As long as you don't rip it up, your comparment temps should not get too hot.

[JanGeo]

In the rain I see spray blowing out of the wheel wells noth front and rear - how do you suppose that happens? Pumping air out from under the car or from the tire itself spinning?

[The Toecutter]

Excellent article.

My 1969 Triumph GT6+ racecar has no undertray. Estimates from racers have this car's drag coefficient estimated at .32, from using the coast down method and estimating frontal area as width * height * .8. I expect to get about .25-.28 after aeromodding the hell out of it, grille block, rear wheel skirts, full underbelly, lowered ride height to 3 inches, wheel spoilers, rear spoiler for improved finess ratio, shaved door handles, removed rain gutters and chrome trim pieces, removed antenna, smooth wheel covers, basically everything that is feasible. I've been told I could get far lower than that doing those modifications, but I'm being on the conservative side of things.

I'm converting this thing to electric. I'm going to need all the improvements I can get. Getting Cd down to .25 would give me about 100 miles range at 65 mph with ~1,000 pounds of lead acid batteries, assuming other mods are taken as well like LRR tires, synthetic transmission oil, and such. If I could get it down .20, range would be much higher, perhaps 130 miles at 65 mph.

This would be without any advanced batteries, just off the shelf lead acid. I'm not expecting for theory to meet practice, and I'll be happy with 30 miles range, but if it does work, it will show just how viable EVs are.

[civicminded]

Very interesting data Mighty - and great bullseyes it paints!

Thanks for posting it up. Mucho Appricianado.

[Mighty Mira]

No problems, this data is hard to find but if we accumulate most of it here we will be well on the way to solving this problem.

[Bunger]

Great info! I think I'm going to get started on blocking off my grill also. I just gotta find where I put the damn radiator fan. With the huge 2 core integra radiator in there, I don't have any overheating issues, even with the car idling for 15 minutes.

[Mighty Mira]

Originally Posted by Bunger

Great info! I think I'm going to get started on blocking off my grill also. I just gotta find where I put the damn radiator fan. With the huge 2 core integra radiator in there, I don't have any overheating issues, even with the car idling for 15 minutes.

You have to remember that the above was calculated for that particular model of volvo, not every car. If your car has a small grille, you will not see that much benefit from blocking it.

For all we know, the volvo could have been this one:

[ZugyNA]

Originally Posted by Mighty Mira

[b]Grille blocking: 30% (allowing some holes for ventilation)

My take on the grill blocking thing:

1) you have to have some air flow thru the rad and around the engine to cool it...not very wise to block all flow

2) some of the mpg gain from this type mod probably has to do with increasing fuel and engine temps?

3) it's possible that extending the air dam down to the general ground clearance for the car would increase the vacuum pull thru the rad opening and help pull air thru the remaining openings under cruise conditions.

[SVOboy]

I honestly don't see why increasing fuel temperatures does anything good.

Anyway, I think it's mostly drag as the radiator's design creates a lot of drag, that's the point of it, I guess. But yeah, the engine temperatures won't do nearly as much as the drag, *shrug*

[DaX]

I think someone should dimple the entire exterior of their car and report back with results. Kind of hard to ABA that though.