80 replies to this topic

[subseamac]

Well ive just got back from speaking to Liam, ended up there for hours and is a nice bloke.

He's taken a few measurements for the rear diffuser and is going away to have a look and create some designs. Ive pointed him to the two diffuser threads on here and also to JawZ's site for some ideas.
Then im going back on thr 23rd where he'l have the car for a week or so. This will give him some time to work with the CAD engineers and fabricators. Hopefully then there will be a prototype and we'l do some test fits and adjustments. Then there should shortly after be a finished product.

After this there will hopefully be the option of a front splitter and other aero aids. He's been very busy currently designing stuff for the S1 and so make take some time.

I think it would be fair to say to wait for a prototype, where we can get pictures fitted and then get some feedback as to any possible improvements. Once a completed product is made then we could then start a typical vx220.org style group buy thread...

That's excellent news A13x what were the odds that you'd live nearby AND could leave the car with Liam for a week for him to experiment!!

I'm really hopeful we will end up with a cracking aero diffuser design, and more aero goodies too in the future.

[Dave E]

Great work guys

[A13X]

The funny thing is he only lives 1mile from me, same town!! We offered up the diffuser that he makes for the S1(the 1 that had the ebay link) and thats no where near to fitting. The biggest problem is coming up with the design around the exhaust area. In using 1 flat undertray you would be left with something that would just clear the clam below the exhaust but the ends would have a much larger gap to the clam. The simplest option and most cost effective would be to use 1 flat panel like the one he already sells. Personally i think this would look pretty rubbish and would want something that atleast looks like it was suited to the car. As im no aero expert i dont know what would be the best design and thats why i hope subseamac can fill Liam in on what is required. Personally i think a design has to be made that both works better than anything currently being made, but also has some appeal in aesthetics. Something like the jimsan but a little longer? For me the gulf VX diffuser is pretty much what im after(and if i could actually use this site properly instead of it freezing all the time then id link a picture, ffs) but its on page 4 on the diffuser thread in tuning and mods. Maybe we could get a set of designs using current and new ideas and set up a poll of what people want?

Edited by A13X, 13 June 2008 - 05:00 PM.

[Crabash]

I have been reading up on aero in general for 6 months now (Competition car aerodynamics), from what I understand ( I may be wrong but the book I used seems to do a good job of explaining it) you will not create downforce at the rear with "better diffuser" but simply more DF across the flat under tray that is available, this is done by "accelerating" (read allowing the full velocity potential) the flow under the car by "diffusing" the air more efficiently at the back. Some race car's use the low pressure area created by big wings to "drive" the diffuser. However if other things are wrong that may be creating probs, I susspect without having any data that our cars dont really have very good flow attachment in the diffuser due to it not being a smooth flat pannel therefore lowering velocity under the car. From what I've read a 7 deg angle is regarded as a good place to aim for, however that is only for flow attachment reasons and a general rule is that the steeper and hence shorter you can retain attachment then the longer the flat floor will be and so the area the DF can be gained from, as you can see this will not create any more DF at the rear than the front, in fact it may be the other way round. I think the best you can do is to make a new undertray for engine and a new diffuser focusing on smoothing it all out not having a join directly after the bend (should really be before the bend by a reasonable amount) and obviously lengthening it overall, oh and keeping the flat bits flat as mentioned elsewhere. I know alot of you may know all this stuff, but I constantly get the impression that most people seem to think that the diffuser creates some sort of vacuum to suck the car down which is understanderable but wrong.

[carl1977]

I was more under the impression that i dissuser will help combat the uplift created at higher speeds, that occurs due to the way the back of the car was designed, rather than using it to create artificial downforce.

[Winstar]

The funny thing is he only lives 1mile from me, same town!!

We offered up the diffuser that he makes for the S1(the 1 that had the ebay link) and thats no where near to fitting. The biggest problem is coming up with the design around the exhaust area. In using 1 flat undertray you would be left with something that would just clear the clam below the exhaust but the ends would have a much larger gap to the clam.

The simplest option and most cost effective would be to use 1 flat panel like the one he already sells. Personally i think this would look pretty rubbish and would want something that atleast looks like it was suited to the car.
As im no aero expert i dont know what would be the best design and thats why i hope subseamac can fill Liam in on what is required.

Personally i think a design has to be made that both works better than anything currently being made, but also has some appeal in aesthetics. Something like the jimsan but a little longer?
For me the gulf VX diffuser is pretty much what im after(and if i could actually use this site properly instead of it freezing all the time then id link a picture, ffs) but its on page 4 on the diffuser thread in tuning and mods.

Maybe we could get a set of designs using current and new ideas and set up a poll of what people want?

Sorry missed this thread, I don't come in this room very oftern.

I'm sort of on the same page as A13X that just a flat sheet diffuser wouldn't look great, but it wouldn't take much of a difference in angle of the center channel to create the visual effect required, say 2 degrees then use this hight to set the angle of the others. The other thing that would possibly help IIRC is that adding extra vanes will act like reducing the angle of the diffuser. The only problem is that both of these things will add cost to the finished product.

[subseamac]

The funny thing is he only lives 1mile from me, same town!!

We offered up the diffuser that he makes for the S1(the 1 that had the ebay link) and thats no where near to fitting. The biggest problem is coming up with the design around the exhaust area. In using 1 flat undertray you would be left with something that would just clear the clam below the exhaust but the ends would have a much larger gap to the clam.

The simplest option and most cost effective would be to use 1 flat panel like the one he already sells. Personally i think this would look pretty rubbish and would want something that atleast looks like it was suited to the car.
As im no aero expert i dont know what would be the best design and thats why i hope subseamac can fill Liam in on what is required.

Personally i think a design has to be made that both works better than anything currently being made, but also has some appeal in aesthetics. Something like the jimsan but a little longer?
For me the gulf VX diffuser is pretty much what im after(and if i could actually use this site properly instead of it freezing all the time then id link a picture, ffs) but its on page 4 on the diffuser thread in tuning and mods.

Maybe we could get a set of designs using current and new ideas and set up a poll of what people want?

Sorry missed this thread, I don't come in this room very oftern.

I'm sort of on the same page as A13X that just a flat sheet diffuser wouldn't look great, but it wouldn't take much of a difference in angle of the center channel to create the visual effect required, say 2 degrees then use this hight to set the angle of the others. The other thing that would possibly help IIRC is that adding extra vanes will act like reducing the angle of the diffuser. The only problem is that both of these things will add cost to the finished product.

so are you thinking a Jimsan-like diffuser but one that sticks out more to clear the body/exhaust tips and also has taller vanes?

[Winstar]

I have been reading up on aero in general for 6 months now (Competition car aerodynamics), from what I understand ( I may be wrong but the book I used seems to do a good job of explaining it) you will not create downforce at the rear with "better diffuser" but simply more DF across the flat under tray that is available, this is done by "accelerating" (read allowing the full velocity potential) the flow under the car by "diffusing" the air more efficiently at the back. Some race car's use the low pressure area created by big wings to "drive" the diffuser.
However if other things are wrong that may be creating probs, I susspect without having any data that our cars dont really have very good flow attachment in the diffuser due to it not being a smooth flat pannel therefore lowering velocity under the car.
From what I've read a 7 deg angle is regarded as a good place to aim for, however that is only for flow attachment reasons and a general rule is that the steeper and hence shorter you can retain attachment then the longer the flat floor will be and so the area the DF can be gained from, as you can see this will not create any more DF at the rear than the front, in fact it may be the other way round.
I think the best you can do is to make a new undertray for engine and a new diffuser focusing on smoothing it all out not having a join directly after the bend (should really be before the bend by a reasonable amount) and obviously lengthening it overall, oh and keeping the flat bits flat as mentioned elsewhere.

I know alot of you may know all this stuff, but I constantly get the impression that most people seem to think that the diffuser creates some sort of vacuum to suck the car down which is understanderable but wrong.

yes that's how a proper difuser work, but thats not actually the issue with the VX the issue is the design of the rear of the VX is such that the flow recirculates up the back for the car and causes lift as a function of vicous force not due to a pressure difference between the under side and above the car, shown below

Flow Velocity Vectors without Diffuser


Flow Velocity Vectors with Diffuser

[subseamac]

ok so anything that sticks out enough should eliminate that effect - what's "enough" 5cm/10cm/15cm ??

[Crabash]

ok so anything that sticks out enough should eliminate that effect - what's "enough" 5cm/10cm/15cm ??

I would take a guess that even a quite small protrusion would virtually eliminate this effect.

Also if the start of diffuser can be moved back slightly this might also improve the aero balance of the car, obviously if better flow attachment can be retained with the use of a smoother joint or better placement of the joint and the extention at the rear that would also cover where the clam joins the back of the original diffuser you should be able to achieve better attachment for the same angle of any given design.

I originally started looking at this after visiting the Nurburgring, the car felt great for most of the time, but at high speed the rear end lift became unnerving, particularly at Flugplatz (very fast double apex right hander after 1st big crest) where the back felt like it was very unhappy.
It's not so much wanting more DF or less uplift as wanting the balance at speed to be more like the low speed balance. Naturally if we can have a little DF as a bonus I wouldn't say no.

[subseamac]

ok I will take all this into account. Liam emailed me at the WE saying he'll have some preliminary designs in a couple of weeks. In the mean time I'll continue to gather feedback and hopefully we can have a fully functional diffuser that also doesn't cost the earth. Some science behind the dimensions (how far it needs to sticks out, how deep the vanes are and how many of them, and the ideal angle of the diffuser vs engine undertray) would be most useful at this point.

[barrybethel]

Does anybody agree that some proper, scientific testing needs to be done on any aero devices developed for the vx? Wind tunnel would be ideal probably but expensive. Maybe people could club together? The book Competition Car aerodynamics that Crabash refers to is, iirc, written by Simon McBeath. He made and supplied the rear wing I had on my vx (now RichyRoffs) and he was very helpful. I am sure he would be happy to help, he seemed like a really nice bloke. He does the aero column in the magazine Race Car Engineering which often details wind tunnel tests of not especially sophisticated aero components fitted to road-car based racers. If wind tunnel testing is too expensive then what about proper, timed track testing? This is what I intended to do with my car to assess the performance (or not!) of my car with big wing and big air dam but unfortunately I had to sell it before I got chance. Before and after stuff is what's required, ideally with a very good, consistent driver. Mallory on a wednesday is cheap and has fast and slow corners. I would willingly come along to help.

[Winstar]

I was thinking something along these lines



with the center channel tight up against the bump therfore giving an air gap for the cooling holes above the other channels.

It's only roughly the right size as I've not got the measurments with me, as for how far it sticks out the further the better, so maybe we do 2 lengths one thats only 20mm past the back of the car and one thats 50 or more mm the difference will only be a couple of quids worth of material.

Wind tunnel testing is too expensive, it could be worth doing a ribbon test of the car in motion with a chase car taking a vid of the ribbons movment in the flow streams.

[danger7]

Hi all, Rather than wind tunnel work, what about CFD? Winstar are you familar with ANSYS? Cheers Ian

[Retset]

I've been following all the diffuser threads with great interest. I intend to black out a few bits of my silver car in the fullness of time and would definitely be up for a reasonable priced black one if it is genuinely effective. Easyish removal would be a bonus.

[Winstar]

Hi all,

Rather than wind tunnel work, what about CFD? Winstar are you familar with ANSYS?

Cheers
Ian

yes, that what the mesh for the CFD I did above is, unfortunatly the problem with CFD is finding the time to take all the relavent measurements, create an accurate enough solid model of the back end of the car and invert it into a fluid zone. Then the only code i have access to is a in house code with a 1million cell limit, not that I have access to a cluster to run much more.

Even then using CFD to genetate anything other than flow paths can be hit and miss. So I can do a simplified representaion of the car as I did above but TBH it would take me literally days of time that I don't have at the minute, and as with all of these things people always want them yesterday.

Edit unless someone happens to have a solid model of a vx lying around.

Edited by Winstar, 17 June 2008 - 02:44 PM.

[p4cks]

I like how this thread is progressing. Kudos to you subsea!

[A13X]

Another thing to consider is the original undertray has cut outs for the lower wishbone. Now as my car is lowered on nitrons it doesnt look like the wishbone will ever catch on the undertray if these cut outs were filled in. Does anyone know if this would be the case? As would be both easier to fabricate and offer better earo. Alex

[subseamac]

Winstar that looks like a great design for our diffuser, only I assume the vanes' edges would be vertical vs what looks like an angle on your diagram? Also it would be good to ensure that the vanes do not intefere with fixing tow-hooks on the two subframe M8 bolts (mind you even that's not good enough as the tow rope would bend the undertay vanes when towing .... Hmmm will need to think about alternative rear towing locations. ideally the entire diffuser could serve as a tow structure with an integrated eye in the centre where it sticks out for towing, but that would imply a diffuser with enough strength to pull the VX which would add too much weight/bulk ?..... or maybe that centre panel under the exhaust hump could be reinforced and act as the tow "bar"?? drawing board on that one) I'll get Liam to look at that + feasibility etc. We may have to sacrifice towing for a good/cost effective design. So the consensus so far is that as little as 5cm out would do the trick, any more detail you could give on the extension vs effect would be extremely useful as a diffuser that sticks out enough but not too far will encourage the biggest number of people on here to buy it and will therefore keep cost down. I think that 2cm is too close to what the Jimsan etc gives albeit with shorter vanes etc, so min 5cm sounds like a good objective A13x re cutouts for suspension, Reverie specify that with their wider aero undertray cars need to have their suspension droop reduced. I don't know what that entails, but maybe add-on plates can later be developed for that unless somebody can contribute info now?

[Crabash]

Does anybody agree that some proper, scientific testing needs to be done on any aero devices developed for the vx?

Wind tunnel would be ideal probably but expensive. Maybe people could club together?

The book Competition Car aerodynamics that Crabash refers to is, iirc, written by Simon McBeath. He made and supplied the rear wing I had on my vx (now RichyRoffs) and he was very helpful. I am sure he would be happy to help, he seemed like a really nice bloke. He does the aero column in the magazine Race Car Engineering which often details wind tunnel tests of not especially sophisticated aero components fitted to road-car based racers.

If wind tunnel testing is too expensive then what about proper, timed track testing? This is what I intended to do with my car to assess the performance (or not!) of my car with big wing and big air dam but unfortunately I had to sell it before I got chance. Before and after stuff is what's required, ideally with a very good, consistent driver.

Mallory on a wednesday is cheap and has fast and slow corners. I would willingly come along to help.

I do agree although it may not be that easy to pull off, but if I had some spare cash at the moment (the ring comes 1st at this time of year and I need to do other jobs for that )I would be looking into making a rear tray and diffuser based on what I understand ( I am also learning about laminates from a book from the same guy which is also very good IMO) and doing some airfield testing. After reading the above mentioned book I realised most of the stuff I had read previously on various forums was based on complete missunderstanding of what a diffuser if for. Also with no data to back any claims up I'm affriad I will assume anything on the market is at best a cosmetic item and at worst someone just telling you what you want to hear so they can sell you something although I don't blame them for trying to make money but I am the type of person who needs to understand things and not just take peoples word for it.

Edited by Crabash, 18 June 2008 - 03:37 PM.