The Fastback bloc

[freebeard]

Persisting in capitalizing aerohead's handle is passive-agressive.

[JulianEdgar]

Originally Posted by freebeard

Persisting in capitalizing aerohead's handle is passive-agressive.

Um, where have I capitalised anyone's name? (Well, I guess I do. But that's because a name is a proper noun - and I am a teacher of writing.)

I have used inverted commas, because that's not his name - just as I assume you're not known as 'Freebeard' on your official documentation.

[JulianEdgar]

My Insight with rear spoiler and fins that measurably reduce lift and drag (fins also provide much better lateral stability):

[freebeard]

Um, where have I capitalised anyone's name? (Well, I guess I do. But that's because a name is a proper noun - and I am a teacher of writing.)

e.e.cummings mocks your rubbish education[al bent].

2—a word or combination of words by which a person or thing is regularly known
likes to go by the handle "Champ"
Synonyms for handle

appellation, appellative, cognomen, compellation, denomination, denotation, designation, moniker (also monicker), name, nomenclature, title

https://www.merriam-webster.com/thesaurus/handle

You made the conscious choice to conflate your handle and your given name. Not all so choose. Show a little respect, m'kay?
________________

Ontopic: Aero or no, I'll aways have a soft spot for the 1949-52 Chevy Fleetline. ....in Powder Blue, with a slight rake.

[Fat Charlie]

Originally Posted by freebeard

We appreciate your insight.

The Porsche was pretty nice, too.

Originally Posted by JulianEdgar

just as I assume you're not known as 'Freebeard' on your official documentation.

Actually, he goes by "freebeard" here. If he doesn't want a capital F, I try not to burden him with it.

[JulianEdgar]

I am not quite sure what point is trying to be made - clearly a rather trivial one, I would have thought - but I will continue to capitalise proper nouns, including names.

(In fact, Freebeard's example - Champ - did so, so I am not sure that was a very good reference to draw upon.)

[freebeard]

merriam-webster.com/thesaurus? [Throws up hands]

The point, sir, would be the distinction between the Queen's English and Netiquete.

en.wikipedia.org/wiki/Etiquette_in_technology#Netiquette

I don't know why we're stuck on this....just Be Kind.

I haven't given up hope, ferinstance now aerohead puts a space after a sentence, like everyone else since the 1600s. So I think there is hope for everyone.

[JulianEdgar]

So all fastback shapes have a lot of lift - but does that actually matter?

After all, even in the case of the relatively high rear lift Porsche Taycan described above (well, high lift before the rear spoiler deploys), the lift is still only a small proportion of the car's mass - especially at the speed limits in most countries.

But my experience with the Insight, and also based on some of the SAE paper research I did for my aero book, is that yes, lift does matter. In both cases, there seems to be evidence that the aero lift is disportionately felt.

I quote some papers in the book where lift - especially rear lift - appears to be oscillatory in nature. (This would not be shown in a wind tunnel that uses averaged Cl figures.) Oscillatory forces (ie they fluctuate rapidly) have the real potential to excite resonances in car suspension. Pretty well all car suspension have natural frequencies in the range of 1-2Hz (it's easy to measure suspension natural frequency with a smartphone and an app, but off topic here) and it appears that the oscillatory aerodynamic forces are within this range. (Perhaps they are caused by Karman vortex shedding?) Thus this appears to be a mechanism by which the car may become disproportionately upset by aero lift - especially at the back.

Another (anecdotal) example is a guy who follows my YouTube channel - Nate - who put a really good undertray on his Passat wagon and immediately commented that the car felt "just planted to the road and incredibly smooth" - see https://youtu.be/jKRrhhk5zj4.

That's also my experience with the Insight (that now develops measurable downforce) - the car is just so different from 80 km/h (50 mph) and upwards. In my car, the ride quality is literally smoother (as the effective weight of the car increases) and stability is much better.

But what if you're chasing just low drag, or you drive only at low speeds (sub 80 km/h - 50 mph)? In those cases I don't think that lift matters much. I would have once said that because lift causes induced drag, you don't want any lift at all for lowest drag, but I note the lowest drag production cars in the world (eg the Taycan) achieve that with lots of lift (although the solar race cars, still on another planet in terms of drag, chase zero lift).

So, TL;DR:

• Fastback cars develop lots of lift
• Lift - and especially rear lift - really matters
• But if you drive really slowly all the time, it doesn't matter much

[JulianEdgar]

And if you want to see how much lift your fastback shape is developing, it's pretty easy - and accurate too. What you do is to measure ride height - and then see how much it increases at speed.

I first did this back in 2008 using the ride height potentiometers fitted as standard to my car (a Lexus LS400). They are used in that car to auto-level the HID headlights.



To dial-out the variation in ride height caused by suspension deflections over bumps, I used a Fluke Scopemeter to monitor the potentiometer output, doing firstly the front and then the back. The Scopemeter has an averaging function, so you could set it to zero, then drive (say) a few kilometres on a freeway and see the average ride height. Note that value, then do the same, but at a faster or slower speed, and then see the difference.



These days, lots of multimeters have an averaging function that works with quite high speed sampling - eg my Fluke 287 can do this. It's also possible to build a very simple circuit (a resistor and capacitor) and smooth the voltage output of the pot, allowing height to be monitored with any multimeter.

On my Insight I use the suspension height potentiometers (ex Range Rover) that I have fitted as a major input into the air suspension control system (eg I automatically lower the car at speed). I also take the output of these sensors (two front, one back) and feed them into my MoTeC dash. I then apply a smoothing (averaging) function to the display so that I can see ride height 'live', averaged over about 5 seconds on my dash.



Another way to measure ride height is to use an ultrasonic sensor looking down at the road. I have experimented with such a system, using an Arduino and XOD code that I wrote to display measured height. This worked - kind of - and showed that my wife's W212 Mercedes has a fair amount of rear lift. (That matches my driving experience of the car.) However, the ultrasonic system is accurate only to about 5mm and so I am not totally happy with it. I have bought a LIDAR sensor but I am yet to do anything with it.



Once you can measure how much the suspension height changes with speed, it's easy to turn it into actual kg of lift or downforce. For example, if the front ride height increases by 5mm average at 100 km/h, all you do is replicate that height increase with the car stationary, eg by pulling the front upwards via spring balance (crane scale, etc). The amount showing on the scale is the lift generated.

On the Insight, I measured downforce in this way by loading the front axle line (and then the rear) with bags of sand until the deflection downwards matched what I experienced on the road. (When doing this also bounce the suspension to take out damper stiction.) This approach also takes into account varying motion ratios in the suspension. You can measure at two different speeds and ensure that the measured force has gone up as the square of the speed.

[Taylor95]

Originally Posted by JulianEdgar

My Insight with rear spoiler and fins that measurably reduce lift and drag (fins also provide much better lateral stability):

That is a pretty good looking spoiler. Do the lateral fins make a noticeable difference?

Is that tape on the back or a light strip?

This is giving me ideas for a future Jeep project.