Rear defuser.

[Moleman]

Just fitted a rear defuser on the car. It looks ok but I'm not sure if I'm supposed to trim around the corner as they stick out a bit from the car.
What do you think please? 

[Lucky Ed]

Most that I have seen stick out like yours, so I would leave it.

..... alternatively I could tell you it looks rubbish, so you could take it off and give it to me

[Facial Hair Optional]

Yeah - what Ed said!!!! 

You know what this means don't you lads? He's gonna be even bloody quicker now 

[Gary RH7]

Dont know if it would make a difference to the handling but see this.

[Moleman]

Thanks guys. Sorry Ed I'm keeping it.

I don't think it will make that much difference though Steve.

I think it's supposed to have the side bits for a reason Gary. Don't know why though. 

[benchmark51]

I think I would leave it as it is. Doesn't stick out much and the chances of it causing any problem minimal.

[sanzomat]

I think I would leave it as it is. Doesn't stick out much and the chances of it causing any problem minimal.

What Dave said. I think it works best if you leave the air channelled for as long as possible. If you cut it down the air flow might break up too soon and reduce the effect? Not that I'm any kind of expert.

['The Gaffer']

I agree, leave as is.

[Moleman]

Thanks for the info guys.

[foggy@smr]

Got the same one I think, so now I have an idea how it looks, I like it 

[robp]

Nice job. 

Yep, leave the edges.  Something for everyone to kick as they walk around the back of your car!  It happens all the time with mine!

I don't know much about aerodynamics, but I think those edges are to keep the low pressure area under the car as far as possible.  You don't want air rushing in from the side.  To be fair, aerodynamics aren't really what Sevens are about.  Although.....

I have had two problems with a diffuser on my Westfield. 
(1)  First was having it low enough to clear the rear suspension wishbones during their full range of movement.  Fortunately, I have a lowered floor and this meant the whole diffuser was about 40mm lower.
(2)  Downforce at high speed.  Yes, they do actually work at high speed.  How do I know this?  Well, when I first fitted mine it was only attached at the front edge and the rear section.  At Donington, at 100+mph I heard this loud scrapping noise under the car.  When I slowed down it went away.  When I got back to the pits and had a look under the car, the middle section of the diffused was scored as it had been running along the tarmac.  The diffuser had bent  so much due to suction it had been scraping the track surface.  I took it off and tried to bent it myself (made of glassfibre) and it took quite an effort get it to bend as much as it had on the car.  I have added a pair of central supports to stop this happening again.

I think the main advantage of having a diffuser on a Westfield is it smooths the airflow under the rear of the car.  The rear panel of the Westfield is like a parachute so covering that improves the top speed of the car (on track of course).

[Moleman]

Thanks for that Rob. Mine is a lowered floor too & the front is fixed almost level with it, with a long thin strip of metal to support the whole front edge so hopefully it won't bend in the middle or rub on the suspension. I haven't tried it out yet as it has done nothing but rain since fitting it. 

[Lucky Ed]

I think Rob's diffuser does work, as he seems to pull away at 100+, with supposedly less power, and I think he's right about there being a lot of turbulent air around the back end of the car. I keep waiting for the lid of my boot box to blow off with the pressure that must be built up below it, maybe I should remove it for track days and see if it makes any difference?

I've been thinking about getting one, does anybody know where to start looking? I could ask MAC#1, but I probably wouldn't see it before Mr Corbin is elected if I ordered from them.

Anybody heard anything about this lot, it doesn't sound that cheap, and they look a bit heavy!

http://www.sa-motorsport.co.uk/rear-diffusers.html

[Gary RH7]

Obviously the fins do make a difference because they all keep them and have multiple ones

As regards boot box lids. Whose flew off last year at Combe and stopped every one in their tracks
But was that before the diffuser Rob?

[sanzomat]

As my car has been in bits in a dampish garage for a few weeks I think this is the kind of diffuser I'll be needing.

[Moleman]

Not sure Ed they do look good but also heavy & not cheap. 

Dave are you sure it isn't because you have been eating a lot of curry? 

[Iancider]

Anybody heard anything about this lot, it doesn't sound that cheap, and they look a bit heavy!

Ed,

That diffuser is not heavy it is only 1.5mm Ali.  It looks like 3Kg approx - see calculator at http://www.aalco.co.uk/online-tools/weight-calculator/

Ian

[Facial Hair Optional]

I think I would leave it as it is. Doesn't stick out much and the chances of it causing any problem minimal.

What Dave said. I think it works best if you leave the air channelled for as long as possible. If you cut it down the air flow might break up too soon and reduce the effect? Not that I'm any kind of expert.

I think you are correct as usual Dave.

[Facial Hair Optional]

I think Rob's diffuser does work, as he seems to pull away at 100+, with supposedly less power, and I think he's right about there being a lot of turbulent air around the back end of the car. I keep waiting for the lid of my boot box to blow off with the pressure that must be built up below it, maybe I should remove it for track days and see if it makes any difference?

I've been thinking about getting one, does anybody know where to start looking? I could ask MAC#1, but I probably wouldn't see it before Mr Corbin is elected if I ordered from them.

Anybody heard anything about this lot, it doesn't sound that cheap, and they look a bit heavy!

http://www.sa-motorsport.co.uk/rear-diffusers.html

Looking at that site and the diffusers, they all have fins. Did I imagine it or does your not have any Colin? I'm not even sure what they do?? Maybe less turbulence as the air stays in channels?

[benchmark51]

They vary a lot, so does the price.
I made mine using 0.7 galvanised steel sheet, just less than 1 sq metre. This I fixed with sealer and rivets. The 8 fins are 3mm aluminium bonded in pairs and riveted to the steel. All made from stuff I had lying around, probably about a tenner. However it is very effective in reducing drag from the boot area.

[Brandy Barrel]

The fins stabilise the airflow
Ideally the diffuser should actually stick out the back a bit and overhang.
The smaller channels created by the fins allow for a higher speed flow as it's not effected by turbulence from other areas.
Higher speed means greater suction.
B

[Brandy Barrel]

http://www.racecar-engineering.com/technology-explained/diffusers-engineering-basics-aerodynamics/

[Moleman]

I don't have any fins on mine & it has a lowered part to allow for the rear wishbones so it ie as far out as it can be. Well I guess it's better than nothing. 

[Facial Hair Optional]

It's bound to help Col, even if you don't get much downforce it will give you smoother airflow underneath, less drag and hopefully more speed.

[sanzomat]

Need to get one of these...


Sad part is I'm old enough to remember the BT46...

[Moleman]

I wouldn't mind the big fat slicks & rear wing as well Dave. 

[Vindicator]

I was just trying to add the exact same picture myself but you beat me to it Dave. Awesome handling from the car first time out if I remember 

[Facial Hair Optional]

That was the famous 'fan car' wasn't it? They soon banned it if I recall?

[Vindicator]

That was the famous 'fan car' wasn't it? They soon banned it if I recall?

The same.......Drivers complained it used to throw debris at them if they were behind it (as most were). The real reason is thought to be a major manufacturer who had invested heavily in turbos brought about the ban. For the fan to work, it needed ducting under and around the engine. With the turbos, there was too much pipework to allow them to design the ducting so they fought for a ban. Turbos became commonplace.