Inertia vs aero drag vs rolling resistance in urban and highway driving

[Logic]

Originally Posted by Ecky

How I meant it was, the typical new vehicle weighs near 2 tons. 20% from that is 800lbs, or ~400 kilos. That's a lot of weight to remove, and, having seen some cars go on extreme weight loss diets, I'm of the opinion it is wildly out of reach. A percent of weight loss may be more useful in most driving scenarios than a percent of aero, but a percent of aero is a lot easier to achieve.

As an example, one forum member here did an extreme weight loss diet in a 2004 Saturn Ion. It started at ~26xx and ended (if I recall correctly) at 23xx. To get there, he removed the interior, removed all seats but the driver seat, lightweight battery, cut the support webbing out of the hood and trunk, cut the crash beams out of the doors, and took a hole saw to and made swiss cheese of every bit of sheet metal that didn't face the outside. He cut the floorpan out of the car behind the rear seats and replaced it with coroplast. Door latches deleted. Window hardware deleted. Speakers and "unnecessary" lights deleted. Exhaust deleted. Power steering, engine balance shafts, A/C deleted. You name it, he cut it out. I believe he didn't quite reach 15% weight savings.

I'm not advocating everyone build boat tails, but MetroMPG found a boat tail (alone) on his Insight (an already very slippery car) to improve ABA fuel economy by 10% on the highway. That suggests a near 20% improvement in Cd from that piece alone. I guess you can get at least half that from a small rear kamm or well placed spoiler, front grille block, smooth underbody, smooth wheel covers and rear spats - most of which I could put together in an afternoon with $50 in materials, and which would not alter the function of the car in any way.

Simply put, weight savings are great (I do them too), but they're much harder and more expensive.

Ah ok. thx

Yes the discussed Saturn is just crazy! (a great experiment though!)
But if you are looking for something to do on a Sunday, removing the odd bolt and cutting off the excess length is an easy DIY mod that can done bit by bit.
(I have a lathe so can cut neatly and even drill out the bolt's center)

But here's the challenge:
If around 98% of all driving is urban.
So the question becomes; what did MetroMPG get in city, stop-go driving?
(Besides the inconvenience of parking etc)

Car manufacturer's could easily put boat tails etc on cars, but don't because 98% of the time they would be increasing fuel consumption on average by doing so?

So the question becomes:
What aero mods can be done light so they are worthwhile on your/my car if it spends most (98%??) of it's time driving around town, with only occasional highway trips?

Wing mirror deletes obviously.
But at some point the weight of a replacement camera system may outweigh the aero and weight advantage around town..?

I think Moon disks are one, if they replace std hubcaps of a similar weight and because the top of the wheel is moving forward at 2X the car speed.

Faired in rear wheels? They seem popular with car makes, but not the general public.

Front spoilers also seem to be a win as they are on most cars to some degree already.
But that may be a 'stability/safety at speed' thing..?

Fairings (of a sort) or lips in front of the wheels too. (What are they called again?)

Flat bottoms to some extent..?

This depends greatly on the materials and design etc too.
A carbon or glass fiber flat bottom with fillets in all the right places may be a win overall, whereas a steel plate with angle iron stiffeners and mounting means is not.

(EV's are 'all in' as the batteries are in the floor already to keep the center of weight down.
A 'Kill 3 birds with 1 stone' thing...)

ie: Let's list aero mods that don't add weight, or if they do; can be made light and still be a win for the world average driving cycle.


I think Dielectric Barrier Discharge (DBD) may be an easy DIY one that's overlooked.
It's basically some strips of aluminum foil and Kapton Tape in the right places, and a cheap and light High Voltage but Low Amperage (= low Power) PSU.
Science studies say yes..?
https://ecomodder.com/forum/showthre...tml#post696530
Thing is; on a practical DIY level, no one really knows..!

[freebeard]

Fairings (of a sort) or lips in front of the wheels too. (What are they called again?)

Spats?

[Ecky]

It's worth putting in a caveat for EVs and hybrids. Because they can recapture a large percent of energy when decelerating, the penalty for weight is much smaller. Perhaps less than half, in the case of my own vehicle? Where I live now, hybrids and EVs also make up almost 50% marketshare of new vehicles, and they are able to capture a larger portion of energy than older models. I do weight savings mostly because it makes the car more nimble and accelerate better.

With regards to weight, your most impactful savings will be in the wheels. I've seen various figures thrown around, but quarter mile testing with Miatas with various wheel weights suggests a factor of around 3:1 in acceleration. It would be 1:1 for rolling resistance, of course. I pulled 14lbs from my wheels, which should have been worth around 40lbs equivalent in acceleration and braking. I scored a deal on these wheels (around 1/10 new price), but if they were new, they would have been $3,000US. They decrease total vehicle weight by 2.3% and ought to improve inertia by ~7%.

A lightweight battery (4lbs?) for $200 saves around 26lbs. In my car this saves 1.4% total vehicle weight, though it would be closer to 0.8% in a more typical sedan or hatchback. This is one of the better value weight savings.

Based on the typical weight of coroplast, my Insight's belly pan weighs around 3.5lbs, without the fasteners. I'd estimate the finished weight to be under 6lbs.

I have Verus Engineering's fully underbody on my Miata. Their testing suggests the two central panels, which cover around 15% of the car's underside, are worth a ~3% drag reduction on their own. I would ballpark their complete kit to be in the range of 9-14% drag reduction, with the rear suspension covers and diffuser.

Regarding cameras, they're definitely much lighter than my already lightweight wing mirrors. The wing mirrors make up almost exactly 2% of my vehicle's frontal area. Since they also add drag, let's call it 3% of the aero of the car.

So, a belly pan + wing mirror removal could be 12-17% aero improvement. Even with the addition of cameras, it might actually be a net weight savings.


Grille blocks - between 2010 and 2011, Ford advertised a 6% aero improvement on their Mustang, and most of that was reducing the front grille size, and re-routing where the air went. I can't say if this can be extrapolated to other cars, but I run an 80% block on my Insight year-round, which ought to be better than a reduction and re-routing. The plastic for this has negligible weight.

[aerohead]

I believe we've already covered this material at the aerodynamics forum, because we had to.