For those with HRG .... a few questions

[RiotfunK]

Because you’re pre loading the spring. Put more risk at topping or bottoming out the shock since you’ve changed height of the assembly. And your bump stops aren’t the correct height anymore so now your stop is the shock travel limit. The shock will also rebound faster since you’re adding pre load. Hit a pothole get that pogo, bong sound.
Down travel will be ok but more abrupt. Up travel again limited to the shock travel. You gain an inch of height not an inch of travel. Need longer springs/struts for that, plus other bits.
For the majority of people, you won’t notice the difference.
Def need the height increase on these since the front end clearance is a few inches lower than the rear and hits everything.

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[Mark S.]

You'd still need to correct your circumference so your odometer is accurate(ish). Or are you thinking of some other measurement I'm not considering?

No, you're right, of course. Both the computer's computation and hand measurements are based on the ODO reading, so it has to be accurate to get accurate fuel economy. Took a mental TDY there. No need to be concerned; I'm back now.

Underbody turbulence doesn't add to the frontal area, it adds to the drag coefficient. The turbulence DOES disrupt the flow of air, so it certainly does affect fuel economy.

Oh, you want to get out into the weeds! Heh. Seriously, I was trying to keep it simple. I'm not an aero dude, but I understand some basics, and I agree with you on the technicalities. My point is turbulent air is not flowing air, and the air flowing around the car has to go somewhere. Yes some of it gets into the turbulent flow under the car (the compressible area you mentioned), but a lot of it hits that wall of turbulent air and simply gets pushed around it, thus the turbulent region becomes a wall that "essentially" serves to increase the vehicle's frontal area. Again, I realize this isn't technically accurate; I'm just thinking about real-world effects. For the more technically minded, the gentleman in this video discusses the difference between drag coefficient and frontal area. The upshot is total drag is a multiple of both drag coefficient and frontal area, which is what I'm trying to say, albeit very clumsily. NOTE: He makes a observation pertinent to this discussion at 4:25.

This online lesson from Colorado State about fluid dynamics discusses computing total drag on a moving vehicle. The formula takes into account both drag coefficient and rolling resistance. In it, the author notes that a typical value for the coefficient of rolling resistance is 0.015, and a typical number for drag coefficient is 0.3. This tells me aero drag has a far greater impact on total drag than tire rolling resistance.

I enjoy these types of thought experiments! Now someone needs to record real world before and afters. We'd need several tanks of gas under consistent driving conditions.

Agreed. We need someone to take some real-world measurements. Unfortunately, I don't plan to install a lift on my car until someone can quantify the effects of changing suspension geometry on driveline components. But I'm happy to accept data from some intrepid individual!

 

[tburner]

My point is turbulent air is not flowing air...

But that's not true. Turbulent air IS flowing air. Turbulence just means the pressure, velocity, and direction are rapidly changing. On a very small scale, you'll have pockets of air that may reverse direction as they tumble through the overall turbulent volume, but that overall turbulent volume continues to flow in one general direction. That's even illustrated in the video you linked at the 1:13 mark, where an image of a Mercedes SLS in a wind tunnel shows a smoke trail passing right under the car. Granted it's much closer to the ground, so here's a GLC and another SLS but on stilts. I don't know why, but Google seems to like Mercs in wind tunnels.

The upshot is total drag is a multiple of both drag coefficient and frontal area, which is what I'm trying to say...

Agreed. The lift does slightly increase frontal area (2 * tire width * 1.25) and slightly increases Cd (due to the larger volume of turbulent air under the vehicle), but I suspect those increases have minimal impact on fuel economy. Here's why...

A rooftop tent, an awning, an extra passenger, and several hundred pounds of camping gear dropped me about 7 or 8 MPGs. The tent and awning not only gave a hefty bump in frontal area, it wrecked what's supposed to be the most efficient path for airflow around the car. The Cd impact from the tent alone would likely be huge compared to the lift. But if we assume Cd was unchanged, and consider only frontal area (because we can actually quantify that), the increased frontal area from the tires (2 tires x 1.25" lift x 9.6" width of 245/65R17 tires) is 34x less than the tent (17" height x 48" width, per Yakima - ignoring the rack towers, cross bars, and tent attachments). Meaning drag from the lift is 34x less than drag from the tent. Further ignoring the weight penalty from the cargo, the tent (and awning, but we're ignoring that, too) caused an 8 mpg loss. If we believe some dude that is, or at least was at some point, GM's technical fellow of aero, the relationship between drag and fuel economy is reasonably linear. So this overly-simplified math problem would suggest a 1.25" lift would result in a 0.24 mpg penalty (actually much less because of everything we ignored).

Also interesting: a 2018 RAM 1500 Quad Cab has a Cd of .35. A 2018 Ford Focus RS, which sits much closer to the ground and has a more swept profile, also has a Cd of .35. Taking it a step further, the RAM probably has more frontal area, so it probably experiences more drag. But, surprisingly, the RAM gets better gas mileage than the Focus if you spring for the diesel.

 

[Mark S.]

But that's not true. Turbulent air IS flowing air.

It's not SMOOTHLY flowing air. I understand the air is still flowing, but it's not flowing as fast as the undisturbed air, and that slow down means some of the air trying to flow through can't, causing an increase in overall drag. I understand this simplified explanation isn't technically correct, but the net effect on overall drag is what I'm interested in. How much increase in overall drag can be attributed to that turbulent air under the car? That's where we disagree. Until someone is willing to do the before/after testing we won't know who's more correct (I say more correct because I believe we are both right to a degree).

Also interesting: a 2018 RAM 1500 Quad Cab has a Cd of .35. A 2018 Ford Focus RS, which sits much closer to the ground and has a more swept profile, also has a Cd of .35. Taking it a step further, the RAM probably has more frontal area, so it probably experiences more drag. But, surprisingly, the RAM gets better gas mileage than the Focus if you spring for the diesel.

The missing factors in this comparison are down force and cooling. The Focus RS has an advertised top speed of 165 mph. I would imagine the aero design of that thing has to include killing any lift to keep it planted at speed. Some manufacturers avoid the aero penalty of required down force with moveable spoilers, but I don't believe the Focus RS has one. Further, the RS is getting 350 hp out of a 2.3L engine, which means a lot of heat. There are two ways to reduce heat with small-displacement turbo engines: forcing a lot of air through heat exchangers (the RS has four I believe) and increasing the amount of fuel in the mixture. The more air you force through heat exchangers the more you increase overall drag which will affect fuel economy. The more fuel you add to the mixture will also affect fuel economy.

 

[tburner]

The missing factors in this comparison are down force and cooling.

That's not true, either. Drag from downforce and cooling aren't missing. The penalty from downforce is captured in Cd. That's why F1 cars, which have track-deforming amounts of downforce but are otherwise very aerodynamic, have Cds of .7 and up (which is in the realm of school buses). The aero penalty from redirecting air through heat exchangers is also factored into Cd. The lack of vents, grilles, and the like is part of the reason full electric cars tend to have better Cds than ICE vehicles.

Cd quantifies how efficiently air flows around the system as a whole. That includes active and passive aerodynamic elements like air dams and spoilers, ground effect, cooling vents, body shape, and even the wake behind the vehicle. The only thing really missing is skin friction drag due to surface texture, but for the love of all that is holy let's not pull on that thread.

But you were tracking in the right direction. The reason a truck and a hot hatch can have the same Cd is because Ford designed for track performance whereas RAM designed for highway efficiency. The RAM gets better gas mileage because it uses a more efficient power plant (diesel) tuned for highway duty (instead of track duty) and experiences less parasitic power loss in the drivetrain (2WD vs AWD).

 

[Mark S.]

That's not true, either. Drag from downforce and cooling aren't missing.

"Missing" as in you didn't mention it in your comparison. I was explaining why the Cd of the RS is the same as the truck.

 

[tburner]

"Missing" as in you didn't mention it in your comparison. I was explaining why the Cd of the RS is the same as the truck.

Ah gotcha, my bad! Post edited

 

[Winds of Change]

Just happen to come across this. While skimming through, Are you guys discussing the aerodynamics of a Brick?

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