Fiddling about with my kit car- some ideas.

[jondoeclsm]

Ok- so I own a kit car. Its called an Onyx Firecat. For years I couldny find a hard top, so in November I ended up having to buy an entire car... I will now sell my original car as sadly they are built slightly differently and the new car has a bar around the windscreen to make it stronger (good news with or without a hard top). The paint finish on the new car is less good, so chopping it around is less of a concern.

Less of an 'eco' car but it has lots of potential for that with its low weight but aerodynamically it could be better! It has a Fiat uno turbo engine and weighs 650Kgs.

I did a quick and very basic test the other day, one set of 'runs' from 4500 to 6000 revs with the hard top off and then with the hardtop on. I did the runs several times and chopped the results up on my video editing equipment, thus getting averaged times (although only accurate to the frame rate). This showed a half second difference between the two in favour of having a hard top on. Roughly speaking the time taken to get from one point to the other is 5 seconds, so to knock half a second off is great news.

Ok, so that's the introduction.

In this thread I hope to throw my crazy ideas out there and welcome the results. One idea, I dont think has been done before, but maybe with good reason!

I will add some pictures, but for reference I will add a video so you can see my original car (it had a soft top which is shown in the vid which has nothing to do with this). ***

Thanks in advance for your thoughts!!

I am too new to add a video- youtube search Onyx firecat rare

[jondoeclsm]

(I say too new, the forum wont let me add a link, not that I am too new to be bothered!).

My ideas are not in order of priority. This first idea I want to get off my chest!

(i). At the rear of most cars, the sudden 'gap' of the rear facing panel with the lights creates drag of a low pressure. With this in mind, I was thinking of routing air from somewhere else to exit in between the rear lights.
There is an 'EVO' which does this from ducting from behind the front seats taking air from the floorpan and it is ducted through to the rear. That is not possible with my car.

How about a tube going from the side of the boot space and cutting to the rear? Repeated on the other side as well. I am sure this wont be totally ideal but my gut feeling is that helping that vacuum at the back should be beneficial.

At 7 seconds on the video on youtube, I am talking about going from behind the 'dot' which is rear of the door, through to between the lights.

(ii) Modern cars have a flap in front of the front and sometimes rear wheels. If this flap becomes too big or too close to the wheel, it creates a high pressure in front of the tyre and can be bad for handling. I would like to create a v shape on the horizontal plane in front of the tyre (like a bow of a ship) to reduce the air to the tyre but also to create a low pressure which I hope may have the opposite effect of the 'big flap' problem. With this idea I hope to be able to go with a bigger 'flap' than if it was forward facing, and hopefully decide where I am trying to push the air.. to the inside of the car or to the outside.

(iii) Bonnet vents. Around half way down the bonnet, the slope is actually heading back downwards towards the windscreen. So I hope I can get better cooling by venting but before the high pressure at the front of a windscreen.

(iv) To reduce the 'hit' of the windscreen, I am thinking of somehow building up the bonnet so that it goes further up the windscreen.

(v) Ok, so where the firewall is at the bottom as the engine bay ends, there is some high pressure build up. Is it even worth ducting between the passenger's legs to jump the gap at a 45 degree angle from the vertical ot the engine bay to the horizontal of the floor of the car.

Enough madness- hope this hasnt angered you all too much or caused injury by facepalm!

[freebeard]

Enough madness- hope this hasnt angered you all too much or caused injury by facepalm!

Actually, it's quite entertaining. As of reading this, you have 4 posts. Once you have 5 you can post pictures. The little landscape with a yellow sky will get you an input box.

I took a look at the video. Some of those panel gaps could be tightened up. If the front of the door jamb isn't an exhaust vent, it should be.

Ducting air around internally fails for the skin friction that reduces the volume of air, whereas doing anything effective requires a massive amount of air. The duct needs to be short and open.

[jondoeclsm]

(i) By cutting across, I was hoping it would be moving towards what some cars have as a cone tail. I was less sure how this affects the air after the intake of the vent further to the rear of the car.

[jondoeclsm]

Wow, 5 posts already!

So, would it work but 'not very much'? I am looking to do what I can, to give a combined effect that will be better. The ducting on the EVO I mentioned was pretty thick stuff.

[freebeard]

Pictures? What does the hardtop look like? Where do you plan on routing these ducts?

Shooting in the dark here, but generally the rear is where you can make the most difference. Miata are similar, take a look at these fastback fiberglass tops:


https://www.google.com/search?q=miata+hardtop+fastback

For the lower rear, a diffuser. Here's an example on a Lotus:
[IMG][/IMG]

For the airflow in the cowl area:


Air vents as at B would fit well with the generous panel gap at the front of your doors.

[jondoeclsm]

Yes, glad you mentioned the fastback, I almost have a solution planned for that (should, if it works be useful for other fastbacks).

There is one of these cars up for sale, with one of the only 3 hardtops I know of in existance:
onyx firecat with hard top | eBay

Look at me posting links!

My fastback would stretch back to the end of the bootlid. Hopefully with a sharper tip/ flat horizontal plane as I have read this is better.

Ok, so regarding front air flow, I need to let go of a lot of air out of the top of the bonnet. I do need plenty of flow.

The rear lower section is going to be the most awkward to sort out but pretty vital. Is there any data (I will google but as yet have not) for adding a rear diffuser and drag reduction achieved? My car is front heavy and light, the addition of a little weight, especially at the rear is no issue at all.

It will gain a front air dam, splitter and rear wing for track handling. The rear beam has been stiffened up and polybushed to give a lot more anti-roll, so the wing is needed to stick it back down at the rear.

[jcp123]

Cool little car. I'd add a good turbodiesel

Seriously though, I'm a bit out of my league with your questions. I do like the car!

[aerohead]

Originally Posted by jondoeclsm

Yes, glad you mentioned the fastback, I almost have a solution planned for that (should, if it works be useful for other fastbacks).

There is one of these cars up for sale, with one of the only 3 hardtops I know of in existance:
onyx firecat with hard top | eBay

Look at me posting links!

My fastback would stretch back to the end of the bootlid. Hopefully with a sharper tip/ flat horizontal plane as I have read this is better.

Ok, so regarding front air flow, I need to let go of a lot of air out of the top of the bonnet. I do need plenty of flow.

The rear lower section is going to be the most awkward to sort out but pretty vital. Is there any data (I will google but as yet have not) for adding a rear diffuser and drag reduction achieved? My car is front heavy and light, the addition of a little weight, especially at the rear is no issue at all.

It will gain a front air dam, splitter and rear wing for track handling. The rear beam has been stiffened up and polybushed to give a lot more anti-roll, so the wing is needed to stick it back down at the rear.

*For the bonnet you might check out modern cars like Corvette/Camaro/Mustang etc. to see how they handled under bonnet airflow.All these designs were targeted foe high-speed stability.
*For the diffuser to work you need a smooth belly pan which has shown a drag reduction of 0.045 on an Audi 100 III.With a diffuser they got - 0.07.
*Also,check out the BMW Z1 with aerofoil-shaped muffler and venting slots into the rear bulkhead.This setup was mostly for high-speed stability at the track.Here's a link
http://www.bmwz1.co.uk/aerodynamics1.JPG
*For the boot,the fastback would be what Kamm would recommend.He did this for one of Briggs Cunningham's LeMans race cars in the 1950s.
*If you need more rear downforce,you could incorporate a spoiler,or add one.A smaller wing,mounted up into the clean air would provide more downforce with less drag.And it could be directly over the rear axle so as not to aggravate front lift.

[freebeard]

Here's what you ask for, but I wouldn't put too much stock in it:



Mainly because it shows an angle and flat plate, whereas I think the shape should be the top half of an exponential horn, IOW curved side walls and a curved roof. The Elipse has a flat plate and fanned fences. Here's one I like:


It has curved sides and no fences. Notice that it starts immediately inside the tire and curves out approximately the tread width. That right there will block turbulence from the wheelwell from getting under the car and throw it out into the ambient flow.

Then the expanding section slows and pressurizes the air (I think... aerohead?) to raise the base pressure.

As for the fastback, I think the prettiest car ever is the Shelby Daytona coupe. Here's the electric version by Renovo:

http://jalopnik.com/the-renovo-coupe-is-an-electric-shelby-with-1-000-lb-ft-1621969757