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Aerodynamics of hard top vs. soft top

 
Old 07-09-2019, 08:21 AM
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Originally Posted by Gee Emm View Post
Looking forward to the CFD results too! Marcus, will you be able to separately quantify the major components - perhaps lift/drag from bonnet, roof/turret, rear deck, rear vertical face, and underbody?

Schroedinger I hope you can remember the source of your lift memory too, and that also reveals where the lift is being generated. My guess is that it is mostly the rear deck and the front end, but would be very interested in the component generated from the roof.

And yes, I agree it is 'conventional wisdom' that the hardtop is 'faster' than the softtop, I had not heard of the Spec Miata experience, but I am surprised that the top down and hardtop speeds were so close - what sort of speeds are we talking about there shuiend ?

Returning to the use of lap (sector?) times as the basis for analysis, we need to remember that while hardtop/softtop comparisons can be made, the hardtop case involves an addition of significant weight, mounted high in the car ( or are we talking about removing the softtop, not just putting the hardtop on over the softtop?). How much weight will depend on the particular top, but it could be expected to adversely affect braking, cornering and acceleration, and therefore times and also Vmax for the same reasons.
I’m sure one of the Megasquirt geniuses here can figure out how to use the accessory channels to log a potentiometer on the sway bar, or a shock travel sensor. I’m also thinking that building such a device with an Arduino wouldn’t be that difficult. Godless Comments always get the creative juices flowing...

I haven’t found my source yet for the lift figures, but I do recall that they didn’t break it down into components. As noted, the lift comes from low pressure areas over the hood, the trunk lid, and the roof itself. I think it’s common to underestimate the effect of the smooth, uninterrupted hardtop surface vs. the textured, rippled surface of the soft top. It has been a long time since fluid dynamics classes, but I recall that relatively minor changes in surface texture can make relatively large changes to flow adherence.
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Old 07-09-2019, 04:52 PM
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If I remember correctly the bottom picture was photoshopped
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Old 07-09-2019, 05:03 PM
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It definitely looks photoshopped; and not a very good one to be honest.

The shock travel sensor idea is doable, but ideally you would want just a straight flat section of road for such shenanigans. Drive at certain speeds, log suspension travel, filter the data for noise and average across multiple runs in the same speed, repeat every 5mph or so. At the end of the run coast down in neutral, so you can also estimate drag differences as well. We have done this 6 years ago, when developing our 1st ever FSAE aero pack and had to validate CFD results to see if we were even close to our estimates on a budget.
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Old 07-10-2019, 02:43 PM
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Originally Posted by Gee Emm View Post
Also shock travel sensors. You would need to massage the data to remove noise, but that should be doable
Based on the sources we've talked to this is actually pretty difficult to achieve. It ends up needing LOTS of consistent tests to get a proper base-line and that still is pending a relatively smooth surface.

But hey, it's awesome to see the effort into projects like this for science!
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Old 07-10-2019, 07:38 PM
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Bummer! Another idea sunk by reality!

I guess this also applies to the mid-point swaybar mounting too, certainly insofar as undulation type effects are concerned (two wheels moving together). However the magnitude of the effects of other movements would be reduced, so there might be more scope for useable results here?

Re the mounting, I put some effort into a string pot to measure rear DF on my racecar, using the rear swaybar mid-point movement and feeding the data to my ECU datalogging. I mocked up a lever clamped to the bar, but found complications (details of which I have forgotten) with the string path. The project got overtaken by the part-out of the racecar, so resolution of those issues never happened. I never looked at the front bar, but I am guessing that mounting there (perhaps using a split pulley) would be more straightforward.

Fine travel resolution will be required, as the sensor will have to tolerate full-travel wheel movement, while giving high resolution data to identify the much smaller (tiny?!) movement generated by hardtop/softtop aero effects. In my exercise that was less of an issue, as I was looking at DF of over 100kgs (said the maths) so the travel would be much more significant that this experiment.

Laser ride height sensor? Mounted on centreline on the car ...
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Old 07-10-2019, 07:49 PM
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We have some pretty smooth freeways here, and I can do whatever speed I dare.

On a related note, I drove in 12 countries so far (including the US and Canada), and the smoothest, velvetiest, the most pool table top perfect roads were in, get this... Albania.
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Old 07-10-2019, 09:48 PM
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I was messing around with an Arduino kit a few months ago, and was impressed with the proximity sensor. One could just put one of those under the car near the center facing down, and measure distance to the ground. It would probably have to be accurate to a few mm’s though, and I’m not sure that it is.
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Old 07-11-2019, 02:47 AM
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It doesn't need to measure distance as such, it just needs to output a (measurable, recordable, repeatable) signal that varies according to distance; a calibrated distance scale would be ideal of course, and if that is required it may be able to be empirically determined. We are looking for a delta between the two cases. It's actually a bit more complicated than that, but that will suffice for now.

I don't think accuracy to a few mm will be enough. Do the math on spring rates and movement ratios, and how much force is required to move the whole car say 5mm. Put another way, what is your estimate of the force, and the delta force, of the lift you are trying to measure? On a hypothetical example, say 6kg/mm springs, and ignoring motion ratios for simplicity, that requires 24kgs to depress all four springs 1mm. I think, unless you can get measurements at 200kmh (where the aero effects are four times those at 100kmh), or better, you would be looking for resolution at around 0.1mm at say +/- 0.025mm accuracy. I should add that I have chosen these values for the example because my wet finger in the air is telling me that the delta lift over the top is of the order of single digit kgs, if that, up to (I don't know, say) 150kmh. 150-200+kmh the aero forces will be increasingly greater, and the lift correspondingly greater - maybe 10s of kgs? That still translates into very small movements in ride height. If your 100mph/160kph figure is correct, that is about 80kg, 20kg/corner, so a bit more than 3mm delta ride height; I would relax the resolution a little for those numbers, say 0.5mm?

The other factor not being discussed is rear end lift, this is different to roof lift because, presumably, the roof being more or less in the centre of the car, lift induced by the roof will be broadly spread around all four corners. There is likely to be a difference in airflow coming off the two tops, if so it is likely that there will be a difference in lift produced over the rear deck, and for this reason two sensors would be required, one around the front axle, the second at the rear. If this effect exists, it will be impossible to isolate values for the roof and the rear deck, only a single value being obtainable (two counting a front axle sensor, but it will be little help in isolating roof and rear deck numbers}.

If lift is your interest, that doesn't matter. All that matters are the numbers at each axle - they will determine aero-derived handling variations, whether at different speeds or because the top has been changed. Maybe CFD can quantify the various contributors to those numbers, and maybe that will enable options to be identified to deal with them individually. Or maybe we say stuff it, and just throw wings, spoilers and splitters at the lift until it goes away.

Making lift go away means, generally, more drag. Which is where I came in, and since I am one of those 'throw wings and splitters at it' people, I will I will now go and do that, and let you all get on with the task at hand without further distractions from me. I wish you luck with it, and will follow your progress with great interest,
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Old 07-11-2019, 04:26 AM
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if anyone really wants to do this, they're welcome to contact me. I can use arduino schematics and the appropriate program code to transfer all 4 wheel positions to the megasquirt, where we would save everything. I would then evaluate the data records. The more data the better. There are no flat roads or repeatable driving necessary only a lot of data to be able to filter everything.
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Old 07-11-2019, 10:59 PM
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Others have said it, but it's impossible to use shock transducers to measure lift or downforce on a moving car. Say you're looking at a 50lbs difference between the hard top and soft top (I doubt it's even close to that much), acting on the center of the car. I have 800lbs/in springs on the front of my car - so that's 50/4 = 12.5lbs per corner or .015 inches of displacement on the front springs. When your noise is two orders of magnitude larger than your signal, you have no chance of "filtering" it out.

If we're interested in the air flow, tufts are the way to go. I'd be interested to know how close to laminar the flow over the decklid is with the soft top up, because that will help me decide if the blackbird spoiler is helpful or if I need to go with the big **** (hoping not since I don't want to look like a tool when I drive this thing to work)

Originally Posted by Schroedinger View Post
It would be good to get some data with a more experienced driver. Are you coming down for the JZilla event at RA in September? Iím doing both days. Since I still have my soft top under my hard top, we could try the hardtop on my car and your car in alternating sessions to see if we can get more rigorous data.
Most likely, and it looks like miatas before the gap as well if I can get the day off work. Could do some hot lapping with/without the hard top!
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