43 replies to this topic

[dw1]

I had an incling that the Jimsan diffuser would have some effect on the car. Whilst it looks great, I secretly hoped for this as I don't like buying things for the VX that are just cosmetic, I like them to have a bit of function in there as well given the nature of the car. Whilst this is all in my opinion and tested using the sensations travelling through the seat of my pants I'm absolutely convinced that the rear lift has been reduced when travelling at high speeds. Having just come back from the Ring where there are 120mph+ sweeping bends with camber and gradient the car felt much more planted than last year. I didn't feel the end want to lift or go light like it used to at those same corners. It didn't give downforce but did cut out lift. I'd have thought Tats gorgeous looking long diffuser might also have some effect on the same basis.

[subseamac]

that's really interesting as there is precious little hard evidence of the effectiveness of all rear diffusers apart from thorney's early experiments with his CF one. Having done some reading on the subject it looked like there was v little to be achieved with diffusers as long as the cars were setup higher than 8cm, and even the bravest on here are running 10/11cm minimum height. The one thing that got me thinking was that the elise S2/exige produce downforce at the rear and very little lift but they also do not run anywhere near 8cm. so there's got to be something there. I am not sure if it's the vertical vanes (a bit dubious) but my money's on the undertray extension, effectively clearing the rear body by at least 5cm maybe even 10cm to minimise turbulence and lift. If that does work, then TAT's extended diffuser ought to be reasonably effective. The VX-s and some of the race VX's adopted rear diffusers that stick out a fair amount. The other contributor to rear lift must be the abrupt transition from the roll bar cover to the area above the engine cover. This has got to be a low pressure area (good for sucking hot air out of the engine bay) which is also contributing to lift. It also explains why there is hardly any airflow over the engine lid and boot even at speed. very interesting, maybe there's something to be said for a long TAT then!

[siztenboots]

Having done some reading on the subject it looked like there was v little to be achieved with diffusers as long as the cars were setup higher than 8cm, and even the bravest on here are running 10/11cm minimum height.

On track running 70mm rear , albeit with undertray and diffuser modifications

[subseamac]

7cm rear!! front? How do you clear speed bumps - for that matter how do you clear leaves, empty packets of crisps, etc

[Mr_M]

DW1 - I have to admit that, for what it's worth, my buttocks agree with yours I've had the Jimsan on the car for quite a while now (~9 mths) and must admit that, subjectively of course, I do feel that the back end is more stable - but it's only noticeable at high speeds - 80/90 plus. Not so long ago now I had the opportunity to do a max speed run (no - not on the public road) for only the second or third time since I've had the car - and for the first time with the Jimsan on. I have to admit that at 140+ I felt the back end of the car was a lot more stable than before.

Subseamac - I must admit that it's a very long time since I studied fluid dynamics, but I think you're (almost) on the right lines here. The well documented rear end lift that our cars suffer with has (I think) two sources - (i) the low pressure area over the rear deck (roll-bar to bootlid transition) and (ii) the upswept rear underside. There are two ways of tackling the lift issue which people often confuse - firstly reducing lift - and secondly, adding downforce.

The <8cm 'rule' I think relates to adding downforce from underbody airflow. I've not seen any underbody treatments on an elise/vx (outside of the Factory GT race cars) which I think would add dowforce - road cars just run too high. That doesn't mean however that an add-on diffuser can't reduce lift - personally I think you're spot on - the longer aftermarket diffusers (Jimsan, tat long, Thorney) are going to have some effect by extending beyond the rear of the clam and giving a clean separation of the air exiting the back of the car. They're definitely not going to add any downforce - but that doesn't mean they're having no effect.

If you want actual +ve downforce there's only one way to achieve it on our cars - stick a nice big wing high up in the rear aiflow




Spooky - whilst I was writing this, another forum member phoned me to discuss this very exact same thing (he hadn't seen this thread either) - cue twilight zone music.....

[subseamac]

something like this perhaps

http://www.lotuscars...6/rbf/race2.php

effective but not exactly discreet .. there's got to be a middle ground

[NickB787]

Lovely

[VIX]

Stunning! Not seen this before.

[Baron Von Scubadaddy]

I don't want to say I told you so but
oh how you mocked



I'm a visionary



do agree the jisman dose feel better!

[MAXR]

I have fitted the jimsan Diffuser and must confess to have been very sceptical at first. However, I also agree with the comments about the max <80cm height for the ground effects to work properly. But I my rear is much more planted especially through high speed bends. The trouble is I have adjusted my rear spoiler at the same time as well as having a new geo, so I'm not sure exactly why. My car is currently about 90mm of the tarmac. In a month's time or so, it will be about at about 75mm of the deck (I trailer my car to the track) as I am having some custom ohlins dampers & springs made up & my steering rack will be machined to allow for better camber settings & the lower ride height. Obviously a complete custom geometry setup will be done by plans. Apparently, fitting full length veins to the underside would increase the ground effects, has anyone tried this? Oh, & I will be shortly booked in for my stage 3 S/C conversion at Courtenay's, 4.17 ratio gearbox swap (after much thought) and I am finally having a full roll cage fitted with about 30kg's being removed to compensate for the 22kg increase. Lastly, my brakes will probably be upgraded as they will have to work much harder to remove excess speeds entering corners.

[Baron Von Scubadaddy]

Having done some reading on the subject it looked like there was v little to be achieved with diffusers as long as the cars were setup higher than 8cm, and even the bravest on here are running 10/11cm minimum height.

On track running 70mm rear , albeit with undertray and diffuser modifications

Mr boots

what is going on with your car in this picture ?

haven't you read the weight limit on the boot liner ?

you really should take your tool kit out of the boot

[Mr_M]

Oh, & I will be shortly booked in for my stage 3 S/C conversion at Courtenay's, 4.17 ratio gearbox swap (after much thought) and I am finally having a full roll cage fitted with about 30kg's being removed to compensate for the 22kg increase. Lastly, my brakes will probably be upgraded as they will have to work much harder to remove excess speeds entering corners.

- Give us poor tubby drivers a chance Max

[subseamac]

Max re ground effect you'll be able to experiment at 7cm off the deck. I would suggest a race front splitter as per thorney's green race car with a fore-aft adjustable plywood lip, a couple of side skirts from badster, you're right you need little v shaped vortex generators infront of each front wheel arch to reduce drag. The rear undertray would then need to sweep up at approx 12 degrees, have the sides extend as far down as poss (in line with the front and mid undertrays and skirts) and stick out by at least 10-15cm to clear anything off the body including the exhaust. That should work perfectly with your wing. Could you get on with the work and post some pictures on here post-haste! Mr M you could tune your tubby to 400bhp a la klasen, lighten it to 850Kg like Chris and Rapidos on here and job done!

Edited by subseamac, 16 May 2008 - 12:28 PM.

[Winstar]

Actually this remindes me I did these a while ago when I was bored (yes I am a sad engineer)

The car model is only roughly VX shaped as I the creaky old code I used is limited the 1 million cells, I'd really want more like 5 million for a full car, but the shape of the diffuser in both cases is accurate and has a dense mesh.

Stream lines of flow - No diffuser



Stream lines of flow - With diffuser



What adding a diffuser that extends out the back of the car does is to stop the flow recirculating up the back of the car and creating lift.

[MAXR]

I have a friend who is an aerodynamics engineer who works for Noble (I didn't realise they had them....he..he..he! I just thought they hired clay sculpture artists) Anyway, he has promised to pull a few strings to get my car tested at a facility in November. He also promised to help design a front ply splitter, under-tray and tweak the rear diffuser. Mind you, come to think of it, he's promised me many things in the past but has never delivered...Bugger! He has told me that they have used coated ribbed polystyrene sheets stuck to the undersides of several test cars in order to develop some of their later cars? Tell me subseamac, did you buy the CF engine lid on fleebay? I had my Sniper ready, but alas my setup bid was too low.

[Adam_B]

i think i need one of these in my life. how long does it take for the Jimsan to come from ordering?

[Mr_M]

Winstar - my point exactly - though it has to be said, a picture's worth a thousand words Subseamac - LOL - had occurred to me - though with 400bhp I'm not sure I'd worry about shedding those last 40kg's or so! Give me time...

[pj_sibley]

I have to admit that I agree with dw1 on this one. I was out at the 'Ring a few weeks ago and through Schwedenkreuz at 125mph, the car felt really planted. This was very different to how another VX I went out in last October felt in the same place. Probably no down-force being created, but certainly helping to eliminate some of the lift and twitchiness at high speeds. Plus they look great, and that counts for a lot in my book

[Winstar]

Winstar - my point exactly - though it has to be said, a picture's worth a thousand words

I've got some more plots that show the effect better somwhere I'll try and dig them out tonight.

I was going to do a better car model but just never got round to finding the time.

[NickA]

What adding a diffuser that extends out the back of the car does is to stop the flow recirculating up the back of the car and creating lift.

So if I'm reading this correctly it also means I wouldn't have to spend so much on cleaning products because the back of the car wouldn't get as much muck thrown up it?  Result! - I wonder if I can convince my other half it would make it cost effective..