Diffuser Design help - Andylaurence

[andylaurence]

I'm looking to build a new diffuser on my car for next season and would appreciate any assistance from the clever people here! I've sketched the space I have available in Google Sketchup, which is bounded by the chassis, tyres and wishbones. I can extend beyond the rear wheels for 1500mm, according to the rules, but my trailer isn't big enough, so I've limited the length to 1050mm.

The static ride height is 60mm, but I don't have the tools to measure ride height on the track. Apex speeds vary from ~25mph at the slowest corners on the slowest tracks to ~80mph. Average apex speeds are ~60mph. Top speed is under 130mph. There's also the potential to reduce the height of the rear wing to interact with the diffuser more effectively.

Here's a side view of the car as it stands today.


The area shows the space around the chassis/wheels that's available. The floor ahead is flat (aside from rivets) and 1680mm wide. The small plate above the plan area shows the maximum height at this point, thanks to the wishbones. Both views are from the left rear of the car.



I've cribbed a basic diffuser setup within those boundaries and I'd be interested in your thoughts. The first section of the outer diffuser is ~4.2 degrees ramp angle, but widens at the same time, increasing expansion ratio. The second section of the outer diffuser and the central section are ~10 degrees ramp angle.

[Smokes]

What are your target goals? Diffuser can produce low drag high down forcebut, what improvement are you expecting to happen if you fit a diffuser?

[DRCorsa]

I have designed and built a carbon fiber diffuser for a customer in Australia. His car though is lot less aerodynamically performant than yours, it is a 7-type replica kitcar. I have studied it in CFD but i will be waiting for his thoughts as soon as he test it on the track.
The dimensions of the diffuser is 1050 wide, 940mm long (lengthwise).
For more details, or if you may need any CFD help, please send a pm. Thanks!

[hardingfv32]

DRCorsa

Generally, how is the diffuser fabricated? How do you get the stakes in place?

What does the unit weigh?

Brian

[DRCorsa]

Brian,
The diffuser is made by resin infusion and the strakes are bonded on the shell with aerospace epoxy adhesive.
The weight of the diffuser with all the brackets/bolts/nuts weighs 4.3kg.
The diffuser alone without the hardware weighs about 2.5-3kg maximum, i don't remember the exact figure right now.

[wesley123]

@andylaurence; I think it is quite important you give more information about the car you're running. I mean sure the diffuser has an height limit and other things to take in account.

I think you can rather give too much info than too little, apart from the dimensions of the 'box' you can fit it in I think balance, weight distribution and even driving style can be quite useful. Although on such car I think an understeering car seems quite unlikely, you never know.

[hardingfv32]

DRCorsa

I am familiar with aerospace epoxies, but that is still impressive that the bond is strong enough. These strakes could be expected to vibrate a little in use. Are the perimeter flanges also bonded on?

I am a DIY fabricator just starting to branch out into laminates. Aluminum has always been faster to use for me.

Thanks for the info
Brian

[DRCorsa]

Brian, the adhesive is really strong. I have made several tests on test pieces prior bonding the strakes, and it's the carbon that breaks at the end while the adhesive remains intact!
The adhesive has nothing to do with "plain" epoxy resin which is very hard and brittle. This formulation is a bit "soft" and is designed to "absorb" vibrations.

edit: If you mean those 25mm vertical lips at the back of the diffuser, yes they are bonded on. And they are supported by small triangular plates at the opposite face.
I added this lip to give additional stifness to the structure without adding more layers of carbon which would add weight.

[andylaurence]

Thanks for all the helpful replies so far.

@Smokes
My target is to increase downforce with minimal increase in drag. Given that the existing setup is as per my basic design up to the point of the first angle change, that means the central section is simply a flat floor and the outer sections are just 49mm high. I would my target is achievable.

@DRCorsa
Your diffuser looks fantastic! It's a little wider than I could accomodate and longer, although I note it doesn't start to sweep up until some way down its length. I assume this is to avoid the differential. Yours looks like the ramp angle at the end is somewhat greater than my suggestion. Is that the case?

@wesley123
You're right - I've missed out lots of information. Aft of the existing flat floor (the missing area between the outer sections) and the wishbones, there are no restrictions on the height of the diffuser and not far behind the existing flat floor, my width limit is 1680mm. My driving style is probably best described as erratic, although you can get an idea from my onboard videos. Weight distribution is about 45%/55% front/rear, but I've been unable to weigh the car with me sat in it. I don't know the current aero balance but it seems more stable at speed; I've only spun it at <50mph and on the slower tracks (Silverstone Stowe and Crystal Palace) the back end feels very lively!

[wesley123]

I love the sound of the car!

Based on what I see in the vids I would say the car is very well planted, but as I see it I am in the opinion the increase in rear df should also be coupled by a increase of front df.

I have to say, I have no degree in anything relevant to aerodynamics. but what I noticed was a really calm entry and quicker exit where there seems to be a bit of oversteer and wheelspin. Therefore I think there should be added a bit of front df too, because judged by the vids I would say the increase in rear df will cause understeer.

I think that DRCosta can give an better explaination/better opinion but this is how I see it based on what I know.

Also there is a lot of info in This topic showing lots of CFD with for example DRCosta's diffuser.

[andylaurence]

The sound is wonderful, but makes my heart skip a beat at the noise test

The oversteer on exit is often me chasing the throttle too much. It's only about 185bhp, but I still use it too aggressively. I agree that I should try and balance out the front too. I have plans for that, but the diffuser is first on the list.

I've read every single post in the thread you referenced and found it a very interesting read. I can't get my head around the step after the transition and I doubt I could effectively reproduce it without CFD or a wind tunnel. The variation in the width of the outer tunnels in my basic design does vary the rate of expansion in a similar manner to the concave to convex design that is common in F1. The widening of the tunnels during the first ramp angle ceases before the transition to the steeper angle. Whether that's good or bad, I'm not sure but it fits the space available and I like to think it is!

[PNSD]

The car does look well planted, so if you were to add a diffuser, I would suggest possibly adding dive planes at the front to help you adjust the balance?

[DRCorsa]

@DRCorsa
Your diffuser looks fantastic! It's a little wider than I could accomodate and longer, although I note it doesn't start to sweep up until some way down its length. I assume this is to avoid the differential. Yours looks like the ramp angle at the end is somewhat greater than my suggestion. Is that the case?

Your car looks (and sounds) better than my diffuser..

I know that your car has only a few things in common with a "7 brick" ( )
but the idea was to extend the flat floor of the car and actually add something to its small length.
I attach a drawing to see the shape. Yes, the differential clearance was a factor of course, but anyway with the current setup of the car (it has a Cortina axle at the back) the diff case is never going to go lower than the floor plane.

[andylaurence]

I plan to vent the front wheel arches into the sidepods to reduce lift at the front, which should help balance out the bigger diffuser. The outer sections start before the rear axle, which means the CoP will be further forward than the one DRCorsa has posted, for example. Dive planes are a possibility, but I'd block up the front air intakes before going down that road, which should also reduce lift.

I'm interested in the CFD results of your design, DRCorsa. By eye (yes, I know!), it looks around 20 degrees at the rear of the diffuser. I'd have expected separation by that point. Is it the combination of a taller ride height and the strakes creating vortices that keep the air attached? I didn't want to risk stalling, so I've stuck to ~10 degrees, especially as I run a fair bit of rake, as can be seen in the photo, which is taken pretty much at the apex. Of course, a fair bit is still only a degree or so!

[wesley123]

For front df you could also just add a gurney tab a cm in front of the front wheel arch, creates low pressure and helps front wheel air extraction. Or extending the front splitter. First and a set of diveplanes should already be enough I guess.