35 replies to this topic

[Tail slide]

Yes, but I've learned before that balanced aero is highest priority... and I can't run a rear Wing in the production car regs but for next Saturday, have made a more effective rear diffuser so now also fitting the little Tubby front airdams.

[JohnTurbo]

Also sneakily running the tubby spoiler and hoping they don't notice??

[Tail slide]

Also sneakily running the tubby spoiler and hoping they don't notice??

Allowed as an optional extra that was available ... and apparently you could also order a diffuser like on an Exige

[Winstar]

He's wanting to get some aero sorted out so was looking at my short diffuser etc.... an area I've learned a bit about from an aerodynamicist friend and will apply the underbody part to my own VX now. Unlike his class in roadgoing I can't have anything 'that changes the silhouette profile'. No big gay wing here then

When you say 'that changes the silhouette profile' is that the side profile only or is it the end on as well?

[Tail slide]

When you say 'that changes the silhouette profile' is that the side profile only or is it the end on as well?

Perhaps an area open to question!

The reg just says 'the silhouette of the vehicle', though as this is in 'Series Production' the other competitors and officials would take it up & ban you from the results if you appeared with something really race derived like dive planes.

[Winstar]

Perhaps an area open to question!

The reg just says 'the silhouette of the vehicle', though as this is in 'Series Production' the other competitors and officials would take it up & ban you from the results if you appeared with something really race derived like dive planes.

LOL typical motorsport regs, one my engineers is ex-Rudbull and he said there are almost purposfully gaps in the regs you can play around with.

One probelm with fitting a diffuser is that:
a) with out it extending past the body it'll do very little
the VX sin't really low enough to stop leakage from the sides
c) more down force will be created at the front of the car then the back

Generating down force at the back of the car is pretty difficult without a wing. If the siloette is only side then the aerial is in the perfect position to hide a row of Vortex Generators to turn more flow down towards on the the VTX spoiler.

One idea would be to fit an aerofoil into the diffuser and run tension links upto the subframe and long as it was positioned high enough it would be hidden within the silloette no matter what the viewing angle.

Other that that it's down to reducing the lift caused by the wheel arces by either trying the vent them to the rear of the car or seal them off but that is ride hight and droop dependent

[JamesC]

Are there classes that compete with minimal/no modification from a standard road/going car? Or do they all require harnesses/fule cut off switch etc??

[Tail slide]

Are there classes that compete with minimal/no modification from a standard road/going car? Or do they all require harnesses/fule cut off switch etc??

Yes you'd be eligible for 'Roadgoing Series Production Cars over 2000cc', and the tyres allowed are either normal road or trackday type as long as approved for road use (listed in book from MSA when you apply for your licence), but not slicks.

LoafingWafu did an excellent entry list summary here;
http://www.vx220.org...-and-hilclimbs/

[Tail slide]

Perhaps an area open to question!

The reg just says 'the silhouette of the vehicle', though as this is in 'Series Production' the other competitors and officials would take it up & ban you from the results if you appeared with something really race derived like dive planes.

LOL typical motorsport regs, one my engineers is ex-Rudbull and he said there are almost purposfully gaps in the regs you can play around with.

One probelm with fitting a diffuser is that:
a) with out it extending past the body it'll do very little
the VX sin't really low enough to stop leakage from the sides
c) more down force will be created at the front of the car then the back

Generating down force at the back of the car is pretty difficult without a wing. If the siloette is only side then the aerial is in the perfect position to hide a row of Vortex Generators to turn more flow down towards on the the VTX spoiler.

One idea would be to fit an aerofoil into the diffuser and run tension links upto the subframe and long as it was positioned high enough it would be hidden within the silloette no matter what the viewing angle.

Other that that it's down to reducing the lift caused by the wheel arces by either trying the vent them to the rear of the car or seal them off but that is ride hight and droop dependent

Interesting , thanks for that! Will consider those ideas along with trying to adapt the project info I already have for my TVR which was supplied by MIRA aerodynamicist Simon McBeath. Not entirely convinced about the diffuser needing much projection beyond the rear. The diffuser sides are now only 50mm from the ground, but I know that 10mm would be very much better, but impossible without ultra stiff springs!

Quite appreciate it's tricky to avoid the main effect of any faster flow being focussed on the front... but possibly achievable by arranging a venturi, via a slightly narrowing throat in plan section toward the centre of the car, which then opens up on a steady radius from there to the diffuser thereby reducing the pressure in plan as well as in section. Less lift-reduction overall but worth it if better balance perhaps. I'm used to experimenting with ally sheet so I'll give it a try. Might even take it to MIRA with a load of quick-fit options, never got around to doing it with the TVR though I had an open invitation.

Interestingly, I panelled in the rear underside wheelarches on the TVR attaching light ally sheets to the wishbones with side vanes, which seemed to work to some extent judging by the 'attached flow' lines across them and into the diffuser after wet weather running. Helped me set a few TVR class records at the faster tracks like Goodwood.

[Winstar]

Interesting , thanks for that! Will consider those ideas along with trying to adapt the project info I already have for my TVR which was supplied by MIRA aerodynamicist Simon McBeath. Not entirely convinced about the diffuser needing much projection beyond the rear. The diffuser sides are now only 50mm from the ground, but I know that 10mm would be very much better, but impossible without ultra stiff springs!

Quite appreciate it's tricky to avoid the main effect of any faster flow being focussed on the front... but possibly achievable by arranging a venturi, via a slightly narrowing throat in plan section toward the centre of the car, which then opens up on a steady radius from there to the diffuser thereby reducing the pressure in plan as well as in section. Less lift-reduction overall but worth it if better balance perhaps. I'm used to experimenting with ally sheet so I'll give it a try. Might even take it to MIRA with a load of quick-fit options, never got around to doing it with the TVR though I had an open invitation.

Interestingly, I panelled in the rear underside wheelarches on the TVR attaching light ally sheets to the wishbones with side vanes, which seemed to work to some extent judging by the 'attached flow' lines across them and into the diffuser after wet weather running. Helped me set a few TVR class records at the faster tracks like Goodwood.

length means you get more of a diffusion effect low sides will help to stop leakage, but you need to be careful trying to apply somthing to a car with a diiferent layout as the flow routes will be different.

not sure how you'll manage to fit a venturi within the regs. you don't need to full scale test these days with the availability of cheap computing power, my phone is more powerful than my workstation was when i first started working in CFD 10 years ago. Here is some i did of a short diffuser

Edited by Winstar, 17 September 2012 - 11:44 PM.

[Tail slide]

Length - I guess up to 6" or so beyond the back of the car makes sure the flow stays fast right up to the edge of the diffuser, otherwise it'll slow down straight away. :: like the graphics! The watermarks on the roof of my diffuser stay a lot straighter than that, though still pointing inwards a bit, perhaps helped by the two intermediate vanes. If I add plates under the suspension, I'll add a number of vertical vanes there to induce vortices, and probably more to the diffuser like the Gt Astons/Ferraris.

[Winstar]

Length - I guess up to 6" or so beyond the back of the car makes sure the flow stays fast right up to the edge of the diffuser, otherwise it'll slow down straight away.

: : like the graphics! The watermarks on the roof of my diffuser stay a lot straighter than that, though still pointing inwards a bit, perhaps helped by the two intermediate vanes. If I add plates under the suspension, I'll add a number of vertical vanes there to induce vortices, and probably more to the diffuser like the Gt Astons/Ferraris.

Cheers there are a load more simulations in the premium members section of the site, this is the latest work I've done looking at diffusers, from which it looks like you'd get a big benifit from extending the outer vanes along the underside of the car.

From the OEM and short style diffuser results below you can see that there is only a small are of low pressure, if you look a the stream lines it can be seen that this is because the high depression in the center of the car causes the most of the flow to be pulled down the center of the diffuser. Additionally you can see that the air flow under the body upto the rear wheel archesis pretty uniform but after that there is flow leakage from the rear wheel arches.





So to stop this you need to have multiple vanes in the diffuser that extend as far to the ground as possible, these are 40mm from ground, and extend the full length of the undertray


However you can get almost the same downforce by having just the outer vanes extending the full length of the undertrayt to stop the leakage from the wheel arches


You could probably fine tune the diffuser a bit more to get a few more % increase but cutting off the leakage from the wheel arches is what make the difference.

[Tail slide]

That's what I'd hoped - the outer vanes being the main contributor. The only other area I'll think about is how far I can go on the regs with subtle but low skirts underneath to keep the flow going fairly undisturbed between the tyres. Thanks for sharing. Looks like I ought to apply for premium membership!

[Tail slide]

I'm easily tempted, now a Premium member and enjoying your full analysis. PS pleased to hear news about ears

Edited by Tail slide, 26 September 2012 - 08:40 PM.

[allsteel]

Anyone got the full spec of turpin's yellow vx. Sounds the business that.

[Tail slide]

Thnaks for reminder - I'll send him an e-mail