Those are good examples of convertibles vs fastbacks, and it surprises me that the fastback makes about no difference in top speed. I wonder if there are cars that go faster with the fastback, like maybe a Corvette or something? Anyway, it's great info to support the status quo and beat down the new guy, which is always a fun game.
For me, there were a a few good reasons to build a fastback. For one, I could sell my old hardtop and build a fastback for half the price. It was also less than half the weight. And I think it looks cool, which was maybe most of it. Since we race with windows down, I also wanted to optimize airflow around the open windows. The stock hardtop is wider at the rear than the windows, and I'd imagine this acts as an air scoop, feeding more air into the cabin. My hardtop is very different in this regard, and I have some vents to extract cabin air as well. Does it work? I don' t know, the only evidence I have is my last race at WGI where the engine was running terribly. However, an e30 was clearly out accelerating me and walking away on corner exits, but every lap I'd catch him on the back straight and pass him going into the inner loop. I don't know if it was that his aero was terrible, but mine was clearly better. How much of that was the fastback is debatable, maybe a lot, maybe none. I live 25 miles from WGI, and I have a factory hardtop on my street car, so I could do an A/B test and see what the difference is. There's a foot of snow on the ground, so not tomorrow, but maybe this summer I'll get around to it. It would be interesting to find out. I won't be happy to be proven wrong and start over, but the point of innovating is failing and improving. To change the topic slightly, what are people doing to optimize aerodynamics when running with the windows open? |
Originally Posted by Blackbird
(Post 1511315)
I have a car w/ a CCP fastback here, it's actually not half bad for a race top, but it's not as light as you think.
IIRC we scaled it at 24 Lbs for the top and trunk lid that goes with it.
Originally Posted by Blackbird
(Post 1511319)
Yes, skin with a couple strips for reinforcement in key areas and gel coat.
Fit and finish was not OEM, but definitely better than the garbage skin tops that are commonly sold at least around here in socal.
Originally Posted by Savington
(Post 1511321)
I have a couple of the CCP tops in the shop as well, all OEM shape. They are nice race tops. Fitment is 9/10, if OEM is 10/10. I have seen far, far worse. I have only seen one nicer race-style OEM top, and it was 3x the price, and you cannot buy one today.
#justbenthings Guess i should consider the fastback if my lack of quality issue is an anomaly. |
Originally Posted by Occam’s Racer
(Post 1511357)
it surprises me that the fastback makes about no difference in top speed
Originally Posted by Occam’s Racer
(Post 1511357)
if there are cars that go faster with the fastback
Originally Posted by Occam’s Racer
(Post 1511357)
beat down the new guy
Originally Posted by Occam’s Racer
(Post 1511357)
For me, there were a a few good reasons to build a fastback. For one, I could sell my old hardtop and build a fastback for half the price. It was also less than half the weight. And I think it looks cool, which was maybe most of it.
Originally Posted by Occam’s Racer
(Post 1511357)
The stock hardtop is wider at the rear than the windows, and I'd imagine this acts as an air scoop, feeding more air into the cabin. My hardtop is very different in this regard, and I have some vents to extract cabin air as well. Does it work?
Originally Posted by Occam’s Racer
(Post 1511357)
I have a factory hardtop on my street car, so I could do an A/B test and see what the difference is. There's a foot of snow on the ground, so not tomorrow, but maybe this summer I'll get around to it. It would be interesting to find out. I won't be happy to be proven wrong and start over, but the point of innovating is failing and improving
|
-re. open windows racing, I've eliminated rear window completely. haven't done a apples to apples comparison yet. It was much faster in cold weather but I don't have a baseline from temps. that low.
-disagree on 1.6 miata under 100mph top down/128mph top up. (the top up part only) -I agree fastback should have less drag, I just think it's not as much as everyone thinks/assumes. -OP DIY fastback is fantastic, wish I had his skills/resources. -all the speed figures are either from car and driver.or manufacturer. before, when the magazine was much thicker, they would do actual top speed runs on their own. I'm assuming their budget cuts killed that in the past 7-8 years. -everything is drag limited. no governor/redline limits. found the corvette. -1998 C5 coupe Cd .29 171mph, conv. Cd .32, 167mph. (160mph top down, back when C&D budget permitted top speed testing.) -2009 coupe 190mph. convertible 185mph. https://cimg2.ibsrv.net/gimg/www.mia...76934ec462.jpg https://cimg4.ibsrv.net/gimg/www.mia...7986453283.jpg |
Originally Posted by endura
(Post 1511372)
everything is drag limited. no governor/redline limits.
This is only true because you have removed the things manufacturers and tuners use to stop bad things happening. Specifications are there for a reason. With a particular RPM limit and gearing you can only do a certain speed regardless of the output of the engine. You just get there quicker. Once you get obnoxious power levels top speed will be limited by RPM and gearing only. This is likely why the R8 is exactly the same but the Mustang is not. Emilio's Vegas in standard trim rather than aero would rip to the limiter without difficulty but a more subtle tune would (with the same gearing) just get there slower. Neither will be drag limited. That is not to say reducing drag on a car that isn't drag limited is not a worthy thing to do. It is. Very worthwhile really as the air can be used more efficiently with other devices.
Originally Posted by endura
(Post 1511372)
disagree on 1.6 miata under 100mph top down/128mph top up. (the top up part only)
The top down speed is a hilarious moment I my driving history having only done this back to back over the course of an abandon airfield "run what you brung" day.
Originally Posted by endura
(Post 1511372)
corvette. -2009 coupe 190mph. convertible 185mph
It doesn't demonstrate the coupe is drag limited though. I'd love to see more information on the 2005 Mustang. Assuming it has the same specifications the fact the softtop has higher top speed means it has less drag. What amuses me is that means for some reason the coupe with all the smooth panel design is worse that the badly shaped canvas top for drag which is impressively bad given the funding supplied to manufacturers. |
Originally Posted by rrjwilson
(Post 1511360)
Very impressed with your construction. Did you ever ponder the connection between your fastback and the softop edges being smoothed?
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Originally Posted by Occam’s Racer
(Post 1511395)
I'm not sure what you mean by soft top edges.
https://cimg8.ibsrv.net/gimg/www.mia...be829e75d5.jpg Soft top edges |
Something to keep in mind, drag differences is only one aspect to consider. Buffeting and inconsistent flow structures have great impacts on performance. Buffeting generates oscillating downforce which in turn increases tire contact patch load variation, which is typically a negative thing for performance.
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Confirmed best i could to compare cars with no governors and theyre not hitting redline in top gear/top speed. That, to me, leaves only aero drag. If (otherwise identucal) corvette coupe with ideal fastback and convertible with miataesque rear window slant/long rear deck have a 5mph difference at 190mph, what am i gaining on my 250 whp miata at a fast track where I'm hitting 130mph? 2-3 mph tops.? I agree that it will gain, just disagree that it's significant. The gains are probably much more significant with a proper wing /fastback combo. If the wing on a fastback can get clean air/lower, drag should go down significantly compared to hardtop with wing at roofline for clean air. Regardless, it's all somewhat educated guesswork without a wind tunnel or actual testing. Also, I'm going to build a fastback this winter as a-even if it's 1-2 mph gain, I'll take it, b-I love how a fastback miata looks.
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Originally Posted by rrjwilson
(Post 1511398)
The edges of where the soft top used to be not covered by your main fastback area. I've highlight in green the locations i mean.
Soft top edges |
Originally Posted by endura
(Post 1511408)
Also, I'm going to build a fastback this winter as a-even if it's 1-2 mph gain, I'll take it, b-I love how a fastback miata looks.
|
Originally Posted by Occam’s Racer
(Post 1511417)
I think that area you highlighted might be a very important part of the drag reduction.
Maintenance of the corner between window and panel as you have done should allow air to feed along better than standard but I would be it will cause more separation than desired. Smoothing of the corner into the fastback shape would likely cause less separation and reduce drag for those areas. Downstream effects may be worse off but as far as the hardtop goes that would likely be better. |
Originally Posted by rrjwilson
(Post 1511431)
Agreed but I think the reduction is due to reducing the separation of flow via smooth transition.
Smoothing of the corner into the fastback shape would likely cause less separation and reduce drag for those areas. The reason I chose a 12 degree taper was to avoid separation. I’m sure there’s turbulent flow along the side, but avoiding separation was paramount. |
Lots of people spend a ton of resources and collaboration on big engine builds but ignore aero. Using the relationship between drag and hp required to over come it, Hp required=(Frontal Area^Coeff of Drag*.00256*MPH^3)/375, using this we can see that power needed is proportional to mph^3, so to increase top speed by 10% i.e. to 1.1 times its original top speed we would need 1.1^3 amount of hp to overcome the drag, I.e 1.33 original hp. To summarize it takes 33% more power to achieve 10% more top speed. This is a slippery slope and puts quite a bit of strain on powertrain reliability and increased vehicle consumables. The coeff of drag is not cubed, it is proportional to Hp required, reducing drag is effectively dynamic horsepower, power that doesn't show up on the dyno when classing your car in race series that use adjusted pwr/weight ratios. If you do the calcs for a car with frontal area of 20f^2, at 85mph (assumed avg track speed), and reduce coeff of drag by .05 it is worth ~4hp which is important in some race series, Assuming the engine on avg uses .6 lbs fuel/hp/hour we can relate this reduction in drag to an increased fuel economy of 2mpg, In a longer race this is critical, the fuel consumption relationship to vehicle weight distribution can have noticeable impacts on vehicle balance at grip thresholds, (over/understeer at the limit as weight distribution shifts foward assuming fuel cell is near rear axle) All these benefits with very little impact on powertrain reliability and rate of vehicle consumables.
Make Sense? Did I miss something? |
Originally Posted by Blackbird
(Post 1511315)
I have a car w/ a CCP fastback here, it's actually not half bad for a race top, but it's not as light as you think.
IIRC we scaled it at 24 Lbs for the top and trunk lid that goes with it. https://fuuz.com/p/6Kmlqw1vsfTP34j.jpg https://fuuz.com/p/zL0V6c95Df8R15T.jpg |
Let me share a couple online tools I use. I don't know how accurate this stuff is, but it's fun to mess around with.
First, this HP Wizard calculator allows you to play with different body shapes and estimate the total drag. I chose some values to represent a Miata and got Cd of .376, which is close enough to published figures of .38. I messed around with the shape of the top and got these values: .376 Standard top, closed windows .395 Fastback, open windows .424 Standard top, open windows .462 Convertible top down So then I go to this other top speed calculator (scroll to the bottom and use 18 sq ft for the frontal area and 2400 lbs). Using the open-window drag figures, here's some interesting results. To go 120 mph, the convertible needs 123 hp, the standard top needs 114 hp, and the fastback need 107 hp. You can reverse the whole process and find top speed. So let's say the same Miata with 123 hp, the calculated top speed of the convertible is 120 mph, the standard top is 123 mph, and the fastback is 126 mph. The standard top with closed windows is 128 mph. These are big numbers for a Miata, so let's take a look at what happens at 90 mph. The convertible needs 55 hp to get there, while the standard top uses 51 hp, so it has another 4 hp available. And the fastback uses only 48 hp to go 90 mph, and so it has another 3 hp available over the standard top, and 7 over the convertible. If you have a 300 hp Miata, then you may not care about single-digit hp. But if you're endurance racing a stock-ish Miata, you take all the free HP you can get. In addition, a 10% reduction in drag is about a 4% increase in fuel economy (at 70 mph), which can be a big deal if you're racing on a stock gas tank. |
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What do you all think about the difference between the OEM-style hardtop and the treasure coast chop top? With windows open, I'd imagine it wouldn't be all that different - lose some smoothness from the over the top air, but reduce the drag of the open window-cabin air interface.
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I bought two of those half tops because it was the same price to ship one or two. If nobody else has done this test, I can try open top vs hardtop vs half top next spring.
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https://s3.amazonaws.com/amv-prod-ca...pless-toys.pdf
From Aug. 2001 Car&Driver. Corvette and Boxster S are ungoverned. Top speed, top up/down. Corvette 162/158 Boxster S 155/144 No thoughts, just more info for the pile. |
Originally Posted by Occam’s Racer
(Post 1511265)
When I was building my fastback I did some research and found that the ideal rear window angle was around 12 degrees. I used more like 15 degrees for better visibility, but it's still good. The Miata hardtop rear window is really close to 30 degrees, which is apparently the worst angle you can have, worse even than a vertical square back.
https://encrypted-tbn0.gstatic.com/i...VswWH9Qm_Kln5u This random paper shows that the notchback (45d) was only 10% lower Cd than the fastback (21d), .28 vs .31, and the squareback (no window, flat to the rear of the car) was 35% more draggy @ .38. In reality, the curved edges of the OEM glass would further reduce the difference. One of the big fastback advantages might be lower drag in corners. As rrjwilson pointed out, the other benefits of the fastback are there, but i doubt the shape alone is as big a reduction as most of us think. |
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