Roof or no roof...weight v Aero.
 

I have been trying to buy a damaged race roof cheap from a supplier but he is out of town...was hoping to have it fitted and ready to race on Saturday....

Between the race weight roof and the new f/glass race weight boot lid I was hoping to drop a decent amount of weight and have a crack and my PB lap time.

Turns out neither the roof or boot will be ready and I suddenly thought had the brain wave... save even more weight and just take the whole bloody roof off.

Now I can't decide whether the Aero loss will be offset by the weight loss....

Anyone care to elaborate....?

track is approx 2.8km long with only one main straight that will have a top speed around `90 - 200 KPH.

aero > weight in this case. I've found the hard top to be worth 1-2 seconds depending on the track.

Do you intended to sawsall the windshield off?

not at this point... :)

Without a doubt run the top! Weight is ~ linear penalty, Aero penalty is squared.

Hard top is worth the weight. There's a local track here with a huge straightaway, even on a 120 to 140HP Miata it's worth about 6 to 10MPH at the end of the straight. Lap times are much better at all tracks with it on.

Theres a picture somewhere where 949 (i think, may have been someone else) ran a piece of coroplast on top of the cage, and recieved some aero benefit.

Roof is always better.

Even with autocross?

Maybe not for something like an ES car. CSP might be a wash. Competitive SSM cars have them.

Thanks!

I know AJ Hartman Racing did this on his chumpcar miata to avoid OEM hardtop penalty.

Edit - Looking at it he obviously used either carbon fiber or glass that he had laying around. Not coroplast. Similar idea.

(sorry for potato quality, only shot I could find)
https://www.miataturbo.net/attachmen...ine=1455135907

This is where virtual dyno is really cool. Make a 4th gear pull with the top and without the top, don't correct for the weight difference, and it'll give you an answer which is faster for any given speed.

I logged a couple pulls a long time ago in a NA8 with headlights up vs down and the areo difference showed up pretty big above 40mph.

I feel like you could pretty near pinpoint the speed where your areo advantage overtook the weight advantage and it'd give you a clearcut answer to your question.

https://www.miataturbo.net/attachmen...ine=1455138370

Only a couple do. It's not worth the weight, like cool though. WE don't go fast enough for drag to matter and if you can't drastically over power your front aero with your rear then your wing sucks

Speaking of, do you template for a stock NA8? I downloaded virtual dyno the other day, but all the settings it needs looks like Greek and I just wanted to mess around with it for fun.

How much does the "race roof" weigh? They are available here in the states at about 11lbs before you add the poly window.

What about hard top vs soft top? Any lap time time difference there???

I've never been to a track that allows the soft top to be up.

I imagine the same, its just a fibreglass outer skin so should be in the ball park...we have to have a 5mm Poly windows (and ali bracing) where some countries can use 4mm.

Two guys who race in the same Classic Japanese class I do have done it already and say the weight saving is significant. they told me the figures at the time but i forget now...I just know its the next stage of car delveopment :)

Mine weighs 11 lbs with poly window. I think it is worth 11 for autocross but not if it is 45 lbs like a stock one. On the track you want the top regardless.

They look similar... but the top one is 25 lbs and the bottom one is 4 lbs. I later added poly window and some gaskets, I believe it's still under 10 lbs. Dry carbon so thin it would flex at speed, had to add some reinforcing ribs to the underside so it held shape at speed.

https://www.miataturbo.net/attachmen...ine=1455295672

As it was before the reinforcements, it was fine up to 100mph. I think the 4 lbs was worth it in autox in that form:

http://i175.photobucket.com/albums/w...psnesyawfy.jpg

On the track, even 40 lbs for a full top is unquestionably worth the weight.

The only 2 SSM Miatas in the trophies at Nationals were the only 2 SSM Miatas running hardtops.

Jusy saying. Pretty positive it's worth it.

You guys are allowed to run aftermarket hardtops? Pretty positive you're not.

Edit: 2015 16.1N says you can. Never read that far. Was thinking 16.1I which says hatch with glass can't be replaced plus SP rules that carry over but 16.1N supersedes either way.

Yeah, but a non-OE top doesnt count as a roll bar so if you are also running a roll bar with a non-OE top so that you can have a harness legally it makes more sense to just run the OE top and no roll bar.

The way I see it is the weight that high could be bad/negligible when you are on a slalom/quick transition intensive course. In a long sweeper type course it would be worth the weight of even a factory type top.

Yea. Guess Bob and I just did it wrong #shrug

I was agreeing with you. A lightweight top should always be better than no top even at autox speeds. But 42 lbs all above the drivers shoulders in a constant switchback type course may not be, IMO. It would still represent an aerodynamic improvement of course but I would argue that much weight up high would counter the effect on handling that the roof provides. On a sweeper intensive course, or road course, not the same effect.

Plus I *think* there may have been other factors contributing to your cars performance other than just the roof.

Yea the stock top is not faster at autocross speeds its too heavy and high. cut its weight in half or down to 1/4 the weight of stock and its worth it.

what about the consideration of the roof stiffing up the chassis?

Most cars (here in race prep anyway) will have a roll cage so it wouldn't be a factor.