Models that don't work on real cars

[JulianEdgar]

Originally Posted by kach22i

I found the first few sections of the link below informative.


https://www.sciencedirect.com/topics...rodynamic-lift

Looks good, but just be careful with the 'upper flow has to travel further' part. This theory of lift has been largely discredited. I can recommend some good aircraft aero books on wing lift if you're interested.

[bentring]

Although I've long been fascinated with automobile aerodynamics, I don't have enough knowledge and interest to speak of the science, so I usually don't bother parroting what's accepted as common knowledge here.

I know there's been prior debate of the subject of the template I haven't read, so if this information has been discussed to death feel free to disregard this post; but I feel like information, pictures, and videos on this page would be relevant to the discussion.

[freebeard]

Thanks. Bubblemania is an interesting site. They cover things like Luigi Colani and Ed Roth and domes.

The page you linked is more than just the Schlorwagen, which is iconic; then Persu et al. I've stood next to the Lewis Airomobile in a museum. I remember the Riley heads in old Ford roadsters, but I'd never seen what must be the Ur-Lakester:


http://bubblemania.fr/wp-content/uploads/CI-600x426.jpg

In case you haven't seen it, I'll repost my motor home design.



I keep reposting it in case it turns up in a Google Bing DuckDuckGo search for Elon Musk so he sees it. Not touching Twitter.

[JulianEdgar]

Originally Posted by bentring

Although I've long been fascinated with automobile aerodynamics, I don't have enough knowledge and interest to speak of the science, so I usually don't bother parroting what's accepted as common knowledge here.

I know there's been prior debate of the subject of the template I haven't read, so if this information has been discussed to death feel free to disregard this post; but I feel like information, pictures, and videos on this page would be relevant to the discussion.

With respect, this is likely to just send us back to the BS from which the original garbage descended.

Note: I am not saying that these cars are of no interest, or they they were backward at the time. In fact I am a great fan of them.

But....

The Cd values recorded at the time for these cars are highly unreliable, as has been demonstrated when full-size cars of the past have been tested in modern wind tunnels.

...and...

The designers of the time cared nothing about lift, and so basically all the shapes shown are of such high lift they would never be released by any responsible car company today.

...and...

In general, the designers were not aware of the drag resulting from trailing vortices, and so attached flow was often seen as an end in itself.

More than anything else, these factors explain why no current cars look anything like these cars. For exactly the same reasons, it also explains why using some sort of standardised approach (a template) based on these ideas is usually a pretty bad idea, and using such a template as some sort of model for changing an existing car is an even worse idea.

[Taylor95]

So it is generally better to not use any kind of template then?

Makes sense given the big differences that exist between different vehicles.

[JulianEdgar]

Originally Posted by Taylor95

So it is generally better to not use any kind of template then?

Makes sense given the big differences that exist between different vehicles.

Yes I think it makes vastly more sense to trial aerodynamic changes and then test them. Nearly all aero changes can be tested with low cost (often free) changes - about the only one hard to mock up out of cheap materials is a true wing. Otherwise, you use cardboard, tape, stiff sheet plastic - or my favourite, thin marine plywood.

Then we have lot of tools at our disposal to measure nearly every aspect of car aero on the road at low or zero cost. Here's a few:

• locations of separated and attached flow
• panel pressures anywhere on the car
• relative changes in drag
• lift and downforce, both front and rear

That way, you find out what works for your specific car in terms of exactly what you want to achieve.

For car aero, guesswork and blindly following what others have done on different cars in the past is exactly the same as for any other area of car modification. And what's that? It's likely to result in much poorer results than could have been achieved through some simple test and development.

[RedDevil]

I'm not worried about lift much, at least not at reasonable speed. If anything it slightly lowers rolling resistance, but I don't expect much of that. It will also slightly reduce grip of course, but again not by much.
If lift is the price to pay for less drag I'll take it with both hands.

[JulianEdgar]

Originally Posted by RedDevil

I'm not worried about lift much, at least not at reasonable speed. If anything it slightly lowers rolling resistance, but I don't expect much of that. It will also slightly reduce grip of course, but again not by much.
If lift is the price to pay for less drag I'll take it with both hands.

Ah well, it depends on what is 'reasonable speed', doesn't it?

I am biased - I spend 90 per cent of my driving on country (rural) roads at 100 and 110 km/h (~60 - 70 mph) and the change in lift is obvious on my car (I have measurable downforce at those speeds). I also overtake at considerably higher speeds (often on narrow and poor roads). And of course, any headwind simply adds to the airspeed.

But that's completely your decision - if you're happy with lift (and maybe lots of lift) and lower drag, then that's a decision you can take.

It's an interesting area. Despite driving a lot of cars, I didn't realise the significance of lift until I changed it on my Insight. Then I realised the 'it feels better, the faster you go' that I'd associated more with good suspension damping is probably in large part due to a lack of aero lift.

[RedDevil]

Overtaking? What is that?
I'm happy if a slowpoke ahead gives me an excuse to slow down, and though I keep a respectable distance sometimes the wake can be noticed in extra reduced fuel consumption.

Then I have other reasons. I have tinnitus (whistling ears) and it gets worse with more external sound. And I like my car, am in no hurry to arrive at my destination to flee it.

I never have the feeling that I suffer from excess lift.
I do have instability in side wind though, which I think may be related to the rounded corners of the rear bumpers (where a Prius and many other cars now have a sharp fold) but it about vanishes with a mildly raised tire pressure at the back, especially when on 16" rims (versus the 15" narrow steels for my winter tires). It is an useful indicator now to the point where any instability makes me check tire pressure at the next stop, with a couple of hits (slow leak due to a screw in the tire!)

[Taylor95]

Originally Posted by JulianEdgar

Yes I think it makes vastly more sense to trial aerodynamic changes and then test them. Nearly all aero changes can be tested with low cost (often free) changes - about the only one hard to mock up out of cheap materials is a true wing. Otherwise, you use cardboard, tape, stiff sheet plastic - or my favourite, thin marine plywood.

Then we have lot of tools at our disposal to measure nearly every aspect of car aero on the road at low or zero cost. Here's a few:

• locations of separated and attached flow
• panel pressures anywhere on the car
• relative changes in drag
• lift and downforce, both front and rear

That way, you find out what works for your specific car in terms of exactly what you want to achieve.

For car aero, guesswork and blindly following what others have done on different cars in the past is exactly the same as for any other area of car modification. And what's that? It's likely to result in much poorer results than could have been achieved through some simple test and development.

Thanks. I have an ambitious project that I will definitely do some of these tests on rather than using the template. With the amount of time and money I will be spending, I really don't want to mess up the aerodynamic modifications. My project will start with a Jeep Grand Cherokee, so the aerodynamic modifications are likely one of the most important aspects of the build.

I like the idea of using cardboard to test modifications. Obviously it is much cheaper than bolting/welding sheet metal to the body.