Fictional Car - floor and diffuser help

[garyjpaterson]

Okay, please be aware I'm not in anyway an engineer, nor is this an engineering project - but felt this would be the most suitable forum. I do however have a great interest in vehicle dynamics, but am a total amateur and it doesn't take anything too complex before it goes over my head!

This is a purely fictional car I've designed, and am more than aware it would likely be difficult to manufacture, and probably not be an idea package. However I'm having fun, and that's all that matters!

Its meant to be a road legal vehicle, but definitely track focused (think KTM X-bow), and is definitely not meant to produce crazy downforce or anything like that - just to be reasonably low drag, with a floor producing a low amount of downforce with a front wing (/bumper) keeping it balanced. No rear wing (though there is an airbrake).

What I'm looking for is some hints and tips on creating a floor/diffuser that at least has the appearance of being beneficial in the real world. Right now its pretty much a flat (raked) floor, with a gradual increase in angle to the rear. I've left a bit of space under the rear wishbone to allow for wheel travel. And also it has 'skirts' or walls, with metallic skids on the bottom to reduce wear should it contact the ground.

PS. I'll try and get more focused (orthographic) images of the floor this evening, right now I just have general images.

[andylaurence]

Renault Twizy?

[wesley123]

Looks cool!

In terms of asthetics I'd look more into the height of the training edge rather than width.

Functionally I'm not seeing anything that would be a big no-no. The skirts over the top of the floor would be the main thing. In a diffuser you generally want the skirts on the underside to enhance the low pressure that is generated.

[garyjpaterson]

Renault Twizy?

Similar concept - thought this one is designed to race on the track, not be a commuter. Engine is a 1000cc 4 cylinder from a bike, making just under 200bhp. Includes an electric motor which helps a lot with low end torque which the engine lacks.

Looks cool!

In terms of asthetics I'd look more into the height of the training edge rather than width.

Functionally I'm not seeing anything that would be a big no-no. The skirts over the top of the floor would be the main thing. In a diffuser you generally want the skirts on the underside to enhance the low pressure that is generated.

Thanks! Not sure what you mean by this though - "height of the training edge rather than width." And yeah, there are skirts on the underside.

Few more pics, these first two are orthographic:




I've made the rest one colour as it helps distinguish between surfaces:

[wesley123]

Thanks! Not sure what you mean by this though - "height of the training edge rather than width." And yeah, there are skirts on the underside.

The car itself has a very tall appearance, while the diffuser exit itself is fairly low but full width. This gives the appearance that the diffuser is wider than it really is. A higher diffuser exit would aesthetically fit the car more imo.

[turbof1]

I'd add fences inside the diffuser to generate vortices. That'll speed up the flow.

Perhaps also add a small rear wing to help the diffuser extract flow.

Perhaps you could also make round corners at the diffuser trailing edge. that would look aesthetically better.

[garyjpaterson]

The car itself has a very tall appearance, while the diffuser exit itself is fairly low but full width. This gives the appearance that the diffuser is wider than it really is. A higher diffuser exit would aesthetically fit the car more imo.

Good point, I'll extend the sides of the diffuser down a bit to add some hieght - as I can't really increase the top of it much more as it would limit the suspension travel.

I'd add fences inside the diffuser to generate vortices. That'll speed up the flow.

Perhaps also add a small rear wing to help the diffuser extract flow.

Perhaps you could also make round corners at the diffuser trailing edge. that would look aesthetically better.

Definitely want to add some fences, just wasn't sure the best way to implement them; should how far forwards (towards the front of the car) should they extend? Also should they remain paraller or spread outward toward the rear?

I want to go without a rear wing as possible just to keep a clean aesthetic, but might consider some options just above the diffuser, sort of like a monkey seat or beam wing. And yeah, I'll probably round the corners a bit too

Cheers guys!

[turbof1]

Definitely want to add some fences, just wasn't sure the best way to implement them; should how far forwards (towards the front of the car) should they extend? Also should they remain paraller or spread outward toward the rear?

I want to go without a rear wing as possible just to keep a clean aesthetic, but might consider some options just above the diffuser, sort of like a monkey seat or beam wing. And yeah, I'll probably round the corners a bit too

Cheers guys!

Up to the diffuser "mouth", which in this case is the lowest point of the floor.

Given your diffuser exit is quite straight, the fences should be as well. Bended fences are more ideal when you sacrifice diffuser volume for a footplate and taper the diffuser sides at the rear to the the maximum width of the floor, creating outwash. Like this:


However, that requires you to completely redesign the diffuser, and it generally makes the whole deal too complicated for too little gain.

If you are looking further into aesthetics and a small gain, you might want to have some fun with the middle section of the diffuser:






But do add a beam wing. You are trying to work with ground effect, meaning the flow gets worked hard and might under circumstances detach. A beam wing can help out in this regard.

[garyjpaterson]

Definitely want to add some fences, just wasn't sure the best way to implement them; should how far forwards (towards the front of the car) should they extend? Also should they remain paraller or spread outward toward the rear?

I want to go without a rear wing as possible just to keep a clean aesthetic, but might consider some options just above the diffuser, sort of like a monkey seat or beam wing. And yeah, I'll probably round the corners a bit too

Cheers guys!

Up to the diffuser "mouth", which in this case is the lowest point of the floor.

Given your diffuser exit is quite straight, the fences should be as well. Bended fences are more ideal when you sacrifice diffuser volume for a footplate and taper the diffuser sides at the rear to the the maximum width of the floor, creating outwash. Like this:
https://scarbsf1.files.wordpress.com/20 ... bdflow.jpg

However, that requires you to completely redesign the diffuser, and it generally makes the whole deal too complicated for too little gain.

If you are looking further into aesthetics and a small gain, you might want to have some fun with the middle section of the diffuser:
https://www.auto123.com/ArtImages/12065 ... inline.jpg

http://cdn-2.motorsport.com/static/img/ ... 1-team.jpg

http://www.motorsport-total.com/bilder/ ... o/z011.jpg

But do add a beam wing. You are trying to work with ground effect, meaning the flow gets worked hard and might under circumstances detach. A beam wing can help out in this regard.

Awesome, great pointers - thanks very much!

[garyjpaterson]

Rear end just now:



Thinking of adding some shape to the profile of the driveshaft covers, to align a bit better to the beam wing.

[andylaurence]

Nice. That beam wing looks a bit bolt on and doesn't fit well with the suspension. It might well work better lower down and probably with end plates too, possibly integrated?

I see the renders are from Assetto Corsa. Have you done the physics too? I wouldn't mind giving it a few laps!

Have you thought about the electric implementation? I've not seen anyone do it to a bike engine as there's nowhere easy to plumb the engine in. I was looking at it a few years ago on my ADR. That ran a gsxr engine and I was going to mount a pulley on the end of the generator with a 6 rib belt and a ~50hp motor. It was to be used out of corners at higher revs and full throttle.

[garyjpaterson]

Nice. That beam wing looks a bit bolt on and doesn't fit well with the suspension. It might well work better lower down and probably with end plates too, possibly integrated?

I see the renders are from Assetto Corsa. Have you done the physics too? I wouldn't mind giving it a few laps!

Have you thought about the electric implementation? I've not seen anyone do it to a bike engine as there's nowhere easy to plumb the engine in. I was looking at it a few years ago on my ADR. That ran a gsxr engine and I was going to mount a pulley on the end of the generator with a 6 rib belt and a ~50hp motor. It was to be used out of corners at higher revs and full throttle.

Agree, will probably have some end plates that merge with the diffuser.

Yep, its been making progress physics wise alongside the model, not perfect but a lot of fun nevertheless.
As for the electric implementation, it works really well in Assetto Corsa. Would it work in real life taking into account packaging etc? No idea, probably not, but its fun to pretend

Right now there are a couple different 'modes'; one gives a boost of torque at lower revs to help pull the thing out of slow corners, and the other is almost full torque throughout - though is controlled to help 'fill' any gaps in the ICE torque curve.

The gearing is very tall (like 70-80mph in first gear) but very close, meaning getting going is tricky on a vehicle over twice the weight of the bike, but once you do you're always in the power band. The e-motor helps a lot with that.

[andylaurence]

I used to run a ~70mph first gear and getting going was hard work. I used to find that 8000rpm launches solved most problems! Later on, I swapped to shorter ratios and driving in the paddock or off the line became easier. I also found I didn't often run out of revs as it's not common to be over 120mph on track with a 470kg car and 180bhp. Interestingly, that showed the ADR either had lots more drag or the 180bhp wasn't quite as advertised, as the 170bhp car that replaced it managed 142mph at Castle Combe when it was remapped to 200bhp. I digress...

The electric assist I was looking at used brushless motors for which there isn't really a suitable controller for use from standstill. The current is a killer at low speed. I got around this by only engaging it at higher revs (over 7000rpm), which is actually lower than I ever went on track. It was planned to be just a button on the wheel that would give full beans on the motor if certain criteria were met; full throttle, revs over 7000, etc. The motor was only a few kilos and the controller a few hundred grams. It was cheap too. I was worried about the safety aspects of the batteries, so I canned it. It was about as heavy as a supercharger kit but delivered less power.

[garyjpaterson]

Just a heads up for anyone interested, this project is nearly done, and will be released (without unforeseen issues) in the next week or so.

Can't guarantee great numbers in a CFD package or in the wind tunnel, but its fun nevertheless.

[Phillyred]

Kudos to you sir! Very "neat" design.. I'm sure you're learning a ton in the process and that's what it's all about.. Are you gonna run it through any CFD demos?