mx5cartalk.com

BARMY will continue to be a heavy old thing, with air con, power steering, sound system, cruise control, and all the other stuff a real racer would remove. But like most of us, it's also my daily driver, and I like my creature comforts. I will also never be removing the (also heavy) soft top as it's one of the main reason I have an MX5 (my top-down threshold is 8 degrees, unless it's raining hard).

So I'd appreciate people's experiences on the track of what happens to lap times with:

Roof up (and, judging by photos, air being trapped in the cabin and expanding the roof fabric. Mine does not have the unzippable rear window).

Roof down (which is waaaay more fun, and you hear more feedback from tyres and engine and other cars, but probs doesn't improve the aerodynamics much).

Factory hardtop, leaving the soft top on.

Lightyear fastback, leaving the soft top on.

I seem to recall a while back something about this.

The basics were:

Hardtop made for faster top speed and laptimes on faster tracks due to aerodynamic advantage, but was slightly slower in corners due to the extra weight. (from memory this was with the softop still attached but folded)

Softop up (closed) was almost as good on the straights, but better in corners (lighter)

I think the general concensus was that removing the softop and instead using a light hardtop was the way to go (which makes sense since that's what you see most often on MX5 racecars)



If you want to keep the softtop you're best to close it, since the extra weight of a hardtop is likely to be slowing you more than the slight aero advantage can give back.




As for the softop parachuting, look into some window netting, it helps reduce the air coming in (i'm assuming the car gets too hot with the window closed)

The parachuting is with the windows closed. No more overheating inside the cabin now the gearbox tunnel is insulated underneath and inside the cabin.

davekmoore wrote:my top-down threshold is 8 degrees

Such a Pom.

davekmoore wrote:Lightyear fastback, leaving the soft top on.

this is on many peoples xmas list


anyway, i think you should stick with soft top. its a road car, you use it daily, you like the roof down sometimes. do what makes you smile.
When/if you have a dedicated track car where you are chasing every 10th, then the situation changes.

davekmoore wrote:The parachuting is with the windows closed. No more overheating inside the cabin now the gearbox tunnel is insulated underneath and inside the cabin.

How?

I honestly cant see any reason the softop would baloon out with the windows shut, except if you've got the interior fan on full and on fresh air setting (in which case it's not adding to any wind resistance at all since it's just pressurising the cabin with the fan)

Have a look at the cars on the track...I've seen a couple of pics of mine going down the straight - the soft top does balloon out. Air pressure?

There's an easy 7+ tenths at Lakeside between roof up and roof down. I had been doing 65.7s there one day for 3 sessions with the roof down. Next session I put the roof up, and Racechrono showed that I was half a second ahead by the time I hit Hungry corner on my first flying lap. I then speared off in circles at 102kmh and didn't get to complete the lap, so I can't say with certainty how many more tenths were there to be found in the final high speed stretch on the straight, but I'm confident there was at bare minimum another 2 tenths without any trouble. Considering it's a 2.4km track, a long highspeed track like Philip Island would have to be 2 seconds I'd reckon.

Best choice would surely have to be removing the soft top and fitting a lightweight hardtop, for aero slickitudinocity with minimal weight gain, if any at all. NFI if a decent fastback would increase or decrease drag measurably compared to a normal hardtop.

hks_kansei:
Roof does balloon. Also loves trying to open itself:

Trackphotos:
Car is DD. Soft top will be staying.

Anyone else able to share experiences of roof up v roof down?

The roof balloons due to the low pressure behind the widescreen from the air travelling up and over it. Has nothing to do with inside air pressure. Soft top up and down IS worth 2 seconds at P.I. Your really stuck with using the softop. Only option would be to use a lightweight roof for track days. But quicker times would be gained from doing more track days and improving lap times by practice, and confidence.
Maybe you should remove the softop, get a bikini top, and use a lightweight roof for track days.

lightyear wrote:The roof balloons due to the low pressure behind the widescreen from the air travelling up and over it. Has nothing to do with inside air pressure.

That makes sense.

Dave beat me to it.

Red is high, blue is low pressure. This shows why the radiator works, why cowl intakes and cowl oil coolers work and why bonnet vents work(you can just see the blue on the surface of the bonnet and gurney flaps increase this substantially).


Dann

Have a look at this thread http://mx5cartalk.com/forum/viewtopic.php?p=723899#p723899

orx626 wrote:The results are in!

Mazda factory hard tops do increase the beaming and torsional stiffness of a standard MX-5 chassis!

Danny

Blue zones is where the air is traveling faster (low pressure) and if you have slower air (higher pressure) under the car you have lift.

lightyear wrote:Has nothing to do with inside air pressure.

Actually it does have something to do with inside air pressure.
The ballooning is due difference in air pressure between the inside and outside.
if you had an unusually low air pressure inside the car, the roof wouldn't balloon.

beavis wrote:

lightyear wrote:Has nothing to do with inside air pressure.

Actually it does have something to do with inside air pressure.
The ballooning is due difference in air pressure between the inside and outside.
if you had an unusually low air pressure inside the car, the roof wouldn't balloon.

........ and the driver would ultimately be unable to breathe?

Might bonnet vents create more airflow over the top of the car causing more roof ballooning?

Ultimately lightyear's advice to get more seat time is no doubt the best way to get more speed. More talent and bravery would also help.

low air pressure and low oxygen content aren't the same thing Dave...

Surely the ballooning is ultimately constrained by the physical limits of the soft top - i.e there's a point at which no further pressure differential (outside the absurd) will cause ballooning because the fabric just won't stretch further?

maybe it's not all bad. ballooning = smoother lines = better airflow?