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Aerodynamics of hard top vs. soft top

 
Old 07-03-2019, 01:14 PM
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Default Aerodynamics of hard top vs. soft top

The purpose of this new thread is to try to quantify the aero advantages of a hardtop vs. soft top as they pertain to road course lap times. Some posts made by myself and others in THIS THREAD were the inspiration for this. There are many other threads looking at wings and splitters with the assumption that a hard top is already in place, but to my knowledge there is nothing that looks at the hard top specifically.

I recently got a hard top and experienced an improvement in lap times of roughly 2.5 seconds at the same track (Road Atlanta) at HPDE sessions one month apart. Details on the car:

- NA 1.6L base model. No A/C, power steering, ABS
- MKTurbo setup
- Torsen 4.1 rear end
- TSE 11.75" Big Brake Kit and Sport rear rotor conversion; PFC 97 compound all around. Wildwood prop valve.
- Xida GS suspension
- new 225 RS4's on 15x8 wheels
- GV style front lip.

There were a number of variables between these two sessions that confound the data as it pertains to the hardtop.

- Driver. I'm an intermediate HPDE driver with three years of experience, still improving and not very consistent compared to many of you. On the average I'm a couple seconds slower than typical SM lap times at this track. A very experienced driver who is already very consistent may not experience improvements of this magnitude.

- Individual Laps. I'm looking only at my best laps for these two days, rather than across all laps. Individual laps are affected by traffic, other drivers, etc. However, it does appear that on the average, my laps with the hard top were at least two seconds faster.

- Track Conditions. RA is a high speed track that probably rewards good aerodynamics more that typical. On both days, the weather was sunny and in the 90's with minimal wind.

- Setup. For the hardtop laps, I was running 1psi less tire pressure at all corners, and higher damping on my Xida GL suspension.

- Power. For the soft top (slower) laps, I was running 14 psi and 200whp dyno'd on Mustang. For the hard top (faster) laps, I was running 10psi and probably 20-30 hp less.

- Data Logging. I was using RaceChrono Pro on my Iphone, with onboard GPS. Not the most accurate system, although looking at the actual GPS source data does not indicate any inaccuracies or discrepancies.


So, on to the results:

Session 1 Road Atlanta Chin Track Days 5/26/19 Best Lap 1:54.02 Soft top up, rear window zipped down
Session 2 Road Atlanta SCCA TNIA 6/27/19 Best Lap 1:51.48 Hard Top, with soft top still underneath (not removed)

I don't know how to export the entire data overlay between these two fastest laps, but here is a sample looking at turns 10-12:



Things that I've observed in the data:

- HIGH SPEED CORNERS. Consistently 2-4mph faster with the hard top.

- BRAKING. Probably the biggest subjective difference. The traction threshold was much higher (presumably due to less lift), and I was able to brake quite a bit deeper into the corners.

- LOW SPEED CORNERS. Because the brakes were more effective with the hard top than I was accustomed to, I was actually over-braking into low speed corners (Turn 5, 7) and going more slowly. I paid this penalty twice because my exit acceleration was lower with less power. As I adjust, there is more time on the table to be gained.

- MAX SPEED. Generally the same, but remember I was running 10-15% less power with the hardtop. I would expect that at the same power levels, max speed would be higher with the hard top.

Conclusion: for my purposes, a hardtop has been a very worthwhile upgrade, possibly the most important one so far other than tires and suspension setup. If I had this information before, I would have gotten one a lot sooner. The biggest variable is still my driving skills, but the improvement that came with the hardtop is pretty hard to deny.

Last edited by Schroedinger; 07-08-2019 at 06:17 PM.
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Old 07-03-2019, 04:04 PM
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Ohhh neat!

Some values I'd be interested in seeing. What is your entry speed into the following corners for soft-top and hard-top?
T1
T5
T6
T10A
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Old 07-03-2019, 08:14 PM
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I’m learning a lot by taking this data apart- mostly about how inconsistent my driving is. The one consistent theme is that during the hard top laps I was braking later and harder, entering turns slower, getting on the power sooner and exiting faster. Basically driving better.

TURN 1
Soft top- VMax 106.0, turn in 75, mid corner 73
Hard top- VMax 111.1, turn in 69, mid corner 75

Turn 1 is the one spot where I think it’s actually slower to use the whole track. You can enter fast and track out all the way to the edge, but then you’re much later to get on the power and you have a long way to go to get back to the right for the entry into Turn 2. I’m faster when I carry less speed in, get on the power as soon as I’m on the hill, and track out a little more than mid-track.

TURN 5
Soft top- VMax 83.0, turn in 72.2, mid corner 67.2
Hard top- VMax 86.9, turn in 64.5, mid corner 66.5

I overcooked this one on the soft top lap, and undercooked it on the hard top lap. This is by far my worst turn on the track, and I learn it a little better every time I go. I know I can be going 70+ mid corner here in good conditions.

TURN 6
Soft top- VMax 100.0, turn in 64.2, mid corner 61.6
Hard top- VMax 103.2, turn in 62.8, mid corner 63.8

TURN 10a
Soft top- VMax 119.0, turn in 48.9, mid corner 48.3
Hard top- VMax 119.8, turn in 48.4, mid corner 52.5

Last edited by Schroedinger; 07-05-2019 at 08:14 AM.
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Old 07-04-2019, 07:19 PM
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Originally Posted by flier129 View Post
Ohhh neat!

Some values I'd be interested in seeing. What is your entry speed into Vmax prior to the following corners for soft-top and hard-top?
T1
T5
T6
T10A
Fixed that for you - you may have intended this interpretation, the other is tip-in speed. Vmax will be the best indicator of aero differences in this comparaison. We are not talking about wings, splitters and such like.

For comparative purposes, we need to compare laps (straights) which are preceded by corner speeds that are the same. My hypothesis would be that softtop/hardtop will not alter mid-corner or exit speeds, so where these are the same (near enough) for both cases, Vmax on the following straight will be a pretty definitive indicator of the relative aero efficiency. The only other variable that needs accounting for is wind speed and direction, assuming other considerations are more or less unchanged (boost, fuel load, ...).

Great work, I look forward to seeing the results.
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Old 07-05-2019, 08:19 AM
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Corrected my previous post to show VMax, I agree that is more clear terminology.

Important to note that I was running less power with the hardtop, and VMax’s were greater at all turns. The biggest subjective difference with the hardtop was more grip during braking from high speeds.
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Old 07-05-2019, 06:56 PM
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This is relevant to my interests since I too do track days with a soft top and I'd much rather invest in a trailer than a hard top. Given the data you have, your best bet is probably to take a couple data points around the course and see how long it took to accelerate from one speed to another near vmax where you're wide open throttle the whole time (for example, 100 to 115mph). Given your data was taken on different days and at different power levels, and 1Hz iphone data isn't very accurate, it'd be hard to draw any conclusions here.

For reference, I'm at about 210whp on wastegate pressure and was running 1:43-1:45 at the JZilla Road Atlanta event on 6/14 with 225 Rival S tires, Xidas (800/500), and a soft top (rear window up). I'm hitting about 126-128mph going into 10a according to my racecapture GPS.
I have access to a hard top through a friend of mine - I may try to do some coast down testing here in the near future if that would be valuable to the community.
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Old 07-06-2019, 08:24 AM
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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? I’m 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.
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Old 07-06-2019, 11:37 AM
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Would it be too hard to test on a soft top car and remove a hard top between sessions to do A B A B testing on one day? That would remove a lot of variables.

What about drag strip testing? Take a non turbo car and test trap speeds between the soft top up and the hard top.
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Old 07-06-2019, 09:23 PM
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The best (ie only) way of doing this is A/B comparisons on the same car, same day. Pick the longest straight. Look at the data, if the prior corner speeds for BOTH A AND B are the same, the Vmax at the end of the straight is the comparator.. If you aren't data logging, you don't have data. If you can't get the prerequisite corner speeds near enough identical, the data is at best incomplete or suspect.

The alternative is to pick something like Conrod at Bathurst, where just about everyone reaches an aero-limited terminal velocity, so corner exit speeds are somewhat irrelevant, and the only relevant variable is Vmax (again, making an assumption about car condition and wind speed and direction being unchanged which applies in all cases).

We are really looking at which of A/B has the highest drag coefficient, and Vmax is our surrogate. Because drag rises at the square of speed, the faster the better. Hence the longest straight. If big hp turbo, the longer the better, need to get to a point where the car has stopped accelerating, or the rate of acceleration has slowed - ie approaching car's absolute terminal velocity, or max aero drag. The more drag, the easier see a difference in the A/B Vmax numbers.

ETA: Assumes car has scope to go faster, ie not reaching redline in top; A drag strip would not be my choice of test environment, for reasons evident in my last para above.

Last edited by Gee Emm; 07-06-2019 at 09:36 PM. Reason: Clarify
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Old 07-07-2019, 07:20 PM
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This strategy would be good for measuring coefficient of drag, but not for grip resulting from less lift. I think that both are important when we’re talking about lap times.
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Old 07-08-2019, 06:08 AM
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Yes and no. Lap time is made up of speed through the twisty bits, and speed on the straights. You didn't mention a rear wing/aero, so I assumed you have no rear wing/aero. Please clarify if I have that wrong. Please also clarify what 'lift' you are speaking of - lift generated by the hardtop/softtop ithemselves, or lift generated over the boot lid?

I'm going to get radical and suggest that rear deck lift will be affected to such a small degree that neither you nor I, nor our dataloggers, will be able to tell, and a comparison of lap times/sector times will be more affected by minor variations in lines, braking and braking points, and general variations in our techniques, than that particular aero effect. Ironically, improving airflow over a bare boot will increase lift, so the better of the two may slow lap times/corner speeds. That may change if you have a spoiler or wing on the boot, the better airflow should improve its effectiveness, to the extent that it is effective in the first place, but again it would (I suspect) require a really skilled and consistent driver to be able to produce back-to-back sessions that had sufficient consistency to be able to confidently draw conclusions.

OTOH I think that there may be some measurable difference in drag/Vmax from the slightly different shapes of the hard/softtops. I think the hardtop is slightly lower, and I think its peak is a bit further forward compared to the peak of the softtop. The rear of the hardtop sits further back compared to the softtop, and these differences (I think) create a subtly different profile, more curved vs more angular. This might improve the airflow aft of the rear window, and it is this turbulent airflow that a) affects the effectiveness of any rear aero, and b) creates drag which reduces acceleration and limits Vmax.

TL;DR It will be easier to get Vmax deltas than any other measurement, and conclusions/inferences can be drawn from that data about airflow over the rear deck.
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Old 07-08-2019, 11:42 AM
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I would expect air coming off the soft top to be a little more turbulent, but I have no idea if the flow stays attached better at the back of either the hard or soft top.

My gut feeling is that the hard top has a bit less drag than a raised soft top but that neither is close to ideal. The front windshield and the rear glass are both at very steep angles. This car wasn't designed to be driven at triple digit speeds. It was meant for driveability at highway speeds with the top down. It's not excessively buffety and you even stay dry in light rain at nearly any speed above 40mph. The soft top is designed to fit between the rear seats and the trunk, not support good airflow for a wing.
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Old 07-08-2019, 11:58 AM
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This thread is conveniently timed, we'll(9LR) will have CFD on soft-top vs hard-top vs fast-back on a NA soon. Obviously there's many other variables, but the CFD gives base numbers at least.


Not to over-simplify this, but I have no doubt hard-top is better than soft-top.
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Old 07-08-2019, 02:34 PM
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Originally Posted by flier129 View Post
Not to over-simplify this, but I have no doubt hard-top is better than soft-top.
I thought that was known for years. Way back in the mid 2000's I learned that at Summit Point. There was a 4-5mph speed difference at the end of the back straight in spec miata with a hard top over top down. I think the spread was a little less with soft top up.
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Old 07-08-2019, 05:45 PM
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Originally Posted by Gee Emm View Post
Yes and no. Lap time is made up of speed through the twisty bits, and speed on the straights. You didn't mention a rear wing/aero, so I assumed you have no rear wing/aero. Please clarify if I have that wrong. Please also clarify what 'lift' you are speaking of - lift generated by the hardtop/softtop ithemselves, or lift generated over the boot lid?

I'm going to get radical and suggest that rear deck lift will be affected to such a small degree that neither you nor I, nor our dataloggers, will be able to tell, and a comparison of lap times/sector times will be more affected by minor variations in lines, braking and braking points, and general variations in our techniques, than that particular aero effect. Ironically, improving airflow over a bare boot will increase lift, so the better of the two may slow lap times/corner speeds. That may change if you have a spoiler or wing on the boot, the better airflow should improve its effectiveness, to the extent that it is effective in the first place, but again it would (I suspect) require a really skilled and consistent driver to be able to produce back-to-back sessions that had sufficient consistency to be able to confidently draw conclusions.

OTOH I think that there may be some measurable difference in drag/Vmax from the slightly different shapes of the hard/softtops. I think the hardtop is slightly lower, and I think its peak is a bit further forward compared to the peak of the softtop. The rear of the hardtop sits further back compared to the softtop, and these differences (I think) create a subtly different profile, more curved vs more angular. This might improve the airflow aft of the rear window, and it is this turbulent airflow that a) affects the effectiveness of any rear aero, and b) creates drag which reduces acceleration and limits Vmax.

TL;DR It will be easier to get Vmax deltas than any other measurement, and conclusions/inferences can be drawn from that data about airflow over the rear deck.
I have read that a stock Miata with soft top generates 180 lbs. of lift (not downforce) when traveling at 100mph. I wish I could remember the source, I’m pretty sure it was in one of the links in Emilio’s Aero History sticky. I need to see if I can find it. Regardless, this lift is due to turbulent flow over the top of the car and would greatly reduce grip during turning and braking. If the hard top make flow over the top of the car less turbulent and more laminar, it would also reduce lift and increase grip. I don’t think your assumption that this is a nominal effect is valid without evidence. My (admittedly limited) data shows substantially shorter braking distances, for instance. I do agree that this effect may be harder to measure directly than drag.

To get information about the total effect of the hard top, we need to quantify it’s effect on both drag and lift/downforce. As noted I have no other aero aside from a GV lip spoiler. I think for the purposes of this discussion, we need to isolate variables and look at the hardtop alone.

And yes, it is known and assumed that the hardtop is a good thing from an aerodynamic standpoint. We just don’t know how good. Marcus’s CFD data should be very interesting. My preliminary data suggests that it’s worth at least 20-30hp in the straights, and if true that is no small effect.

Last edited by Schroedinger; 07-08-2019 at 06:13 PM.
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Old 07-08-2019, 06:09 PM
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I always assume that everyone has seen these photos, but they’re worth posting here anyway.

Soft top:



Hard top:


That turbulence behind the roof has two main effects: it creates lift which reduces rear grip, and it increases the suction behind the car which creates drag. I think in many cases the suction behind the car has a greater effect on speed than the high-pressure area in front of the car.
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Old 07-08-2019, 06:30 PM
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Would it be possible to hook up a TPS (or some other suitable potentiometer) to the front and rear sway bars and log their output together with VSS data?

That way any amount of lift or downforce could be seen without doubt at identical speeds using a soft top and a hard top, and the exact amount of lift or downforce (in lbs or kilograms) could be calculated by adding or removing ballast to a stationary car. The removing ballast part would require a jack that can measure load - like a jack sitting on a robust scale.

Install potentiometers at sway bars, take a reading at rest with driver, etc in the car.
Then drive at various speeds with both tops, compare readings. Lift would raise, and downforce would lower the car.
Add/remove ballast to match potentiometer voltages in a stationary car, rinse and repeat.

What do you think?
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Old 07-08-2019, 06:31 PM
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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.
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Old 07-08-2019, 06:52 PM
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Originally Posted by Godless Commie View Post
Would it be possible to hook up a TPS (or some other suitable potentiometer) to the front and rear sway bars and log their output together with VSS data?
What do you think?
That is doable. Also shock travel sensors. You would need to massage the data to remove noise, but that should be doable, and anyone who runs these has data already (maybe not comparative data though, and maybe only data contaminated by wings and splitters). Does Spec Miata allow aero mods, and do they use loggers? Any other classes that use non-aero cars and allow logging?

This would also be a great calibration/validation tool for the CFD results.
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Old 07-09-2019, 08:02 AM
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In for some godless commie pictures with doodles of how an ARB pot sensor would attach.
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