38 replies to this topic

[markiii]

Having spent a good few ours reading and searching (obviously not in that order) I still can't find an answer. So I'll buy one even if it's just for looks, in which case the Tats one imho looks the best. However as form should follow function is their any imperical evidence that any diffuser is better than another? Obviously in theory longer is better, but any facts? thanks in advance Mark.

[p4cks]

I think they are purely aesthetic. Unless someone can show me a VX in a windtunnel with and without a diffuser on I'll stick with the same opinion.

[ChazUwe]

I guess the longer and deeper the veins the better. I have the Jimsan one which is probably somewhere between the out and out TMS ones and the more cosmetic TAT ones. It maybe in my head but I would definately say it's more stable at high speed. Was solid as a rock at 140mph on the Autobahn back in sept, was hunkering down nicely! Obviously it probably helps if you have the car lowered also as there is less air under the car then too!

[2kracing]

Are the Tat ones still available? Where from?

[mandarinvx]

Are the Tat ones still available?

Where from?

Send a PM to Tatonspeedy

[i need nos]

of course theres going to be a diffrence in downforce with what diffuser you are using, when a car is designed and tested in wind tunnesls they select a design thats suitable for a car, if u put a very large diffuser on the car you will notice the diffrence, small ones like tat u will harly notice a thing, tms ones on the other hand u will feel more at high speeds and corners..

[Crabash]

I have seen a lot of "theories" plastered around about diffusers on road cars, and while I am no expert, I am an engineer and therefore tried to find out more about them as a lot of theories seemed to me to be conflicting at best.

One place I found some good info was http://www.ten-tenth...hlight=locost47
There were many other sites with good info on the actual physics behind how and why diffusers work and the other components that can aid or be aided by them or the opposite. (google "rear diffuser design")

After a few hours research I came to the conclusion that most designs for aftermarket road car diffusers are based on misunderstandings of how they and other aero dynamics work and their main advantages, or simple designed to look good with no regard for function.

If I were looking for an effective diffuser I wouldn't be looking at any of the premade items available for the vx, on the other hand if you just like diffusers then it simple, you buy whatever YOU think looks best.

Edited by Crabash, 08 December 2007 - 03:10 PM.

[Stu-7]

I guess the longer and deeper the veins the better. I have the Jimsan one which is probably somewhere between the out and out TMS ones and the more cosmetic TAT ones. It maybe in my head but I would definately say it's more stable at high speed. Was solid as a rock at 140mph on the Autobahn back in sept, was hunkering down nicely! Obviously it probably helps if you have the car lowered also as there is less air under the car then too!

Actually - exactly the opposite happens....

With a flat undertray and no diffuser fitted, the the lower the car is, the greater the effect of rear-end lift.
This is because the greater the effect is, of low pressure air flowing underneath the car (flowing faster than over the top), compared to high-pressure air over the top of the car.

The VX suffers badly from rear-end lift. Lowering the car without a diffuser will amplify the effect as you are increasing the effectiveness of the front splitter, as well as generating a higher flow rate of low-pressure air under the car. The VX is no different to the S1 and S2 Elise in that respect: Flat undertray leads to air passing more quickly under the car than over it. But, the air clings to the upswept clam, creating more turbulence at the rear, which results in lift.

The theory of fitting a diffuser is that you separate the lower pressure air from the rear of the car. If you do this, as speed increases you are actually creating small levels (less than 10KG's) of downforce, as opposed to lift. When you take this to it's extremes (as in F1 cars or sports cars eg LMP1's) then you can create significant levels of "free" downforce ie: that doesn't cost you straightline speed (vs wing-produced downforce, which obviously affects straight line speed)

My understanding from various comments, is that the S1/S2 and VX suffer from 30-50 KG's of rear-end lift at 120mph.
This is fine if you are driving along in a straight line with no compressions or crests but - take this onto the track - a place like the Nordschliefe, and there is potential for handling issues.

This was rather frighteningly demonstrated by a friend of mine on one of the 'ring trips we had this year. For those of you who know the 'ring, you will be familiar with "Lauda Kink" - it's a fast, very fast kink, which appears just after the half-way point (Breidscheid/Ex-Muhle) and is approached at about 120-130mph in my car. It takes big cahoona's to keep it ABSOLUTELY FLAT on turn-in and apex here. Most of the time I have a confidence lift on turn in then mash the throttle at the apex. I have taken it flat once or twice before but it was rather too close to the limit for my liking.

My friend (in a Honda Elise) lost it before the apex ie: the car went light on turn-in. Given the camber of the track at that point, it's a rather strange thing to happen. The car *should* be completely stable however, with an Exige from splitter it is estimated the front end produces crica 40 KG's of downforce. Now, with 40 KG's of lift at the rear, any aggressive change in the dynamic weight distribution of the car will accentuate this. The track was moist, in places but 95% dry. He did 3/4 360' spins and rather fortunately barely kissed the barrier, before coming to rest on the entry to Bergwerk, with all 4 tyres badly flat-spotted and a soiled pair of pants. He's now fitted a big feck-off wing to the car as well as a diffuser.

I also feel a similar effect over the crest just before Schwedenkreuz: approach at 140+ mph - take the crest dead straight on the right hand side on a trailing throttle and in theory you should land nice and firm. In the VX it's all a bit wibbly-wobbly. The extra lift at the rear causes the car to rotate...only by about 1-2 degrees but enough for the car following to notice that, as I was landing, I had rotated a little so I was *landing* on the sidewall of the OSR tyre - it flexing noticeably as the car settled itself. I can feel this in the car. Following an S1 closely over here at same speed, I witnessed the same - it's quite scary actually. Another car who was with us had a big wng on the back and it was totally planted on landing.

Back to diffusers <gulp> - the Thorney one will be the best for a track user as it extends the furthest from the back of the car and will elimintae all rear-end lift and probably give you some (a few kilo's) of downforce - not that you would notice the downforce. Unless you are going very quickly or racing, you will look like you reversed into Halfords if you fit one (no offence meant John).
There are various diffusers which match the same dimensions of those fitted to an S2 Exige - which has obviously been windtunnel tested by Lotus - so I would recommend purchasing one of those as they are [erfectly effective at eliminating rear-end lift and look perfectly stealthy on the back of the car.
I myself am waiting for Jimsan's next production run.

Sorry for the long answer but, I hope that explains the situation

[cheeky_chops]

it is possible to get data, but it is expensive and the product producer has to be confident he will achieve cost/sale benfits

EG Reverie http://www.reverie.l...rearwing_06.php

Anything less is just halfords tat*

*ps i have the jimsan diffuser, looks great

[Arno]

My understanding from various comments, is that the S1/S2 and VX suffer from 30-50 KG's of rear-end lift at 120mph.

Not the case for the S2 shape.

The S1 has some downforce at the front, but (indeed) some lift at the back at speed. (this was after the re-design by the wintunnel guys as shown in the dicovery documentary, the original design was even worse.. Old shape audi-TT before they added the little spoiler on the back was also notorious for this issue..)

The S2 has a more 'equalized' setup where there is less downforce on the nose compared to the S1, but now also a small but measurable amount of donwforce at the back. This makes the S2 shape more predictable in it's response when speeds increase.

On the S1 a lot of the lift can be negated though by fitting the Sport160/111S 'spoiler' on the back. As the name implies is does not really generate any downforce, but it breaks up the airflow from the back of the clam to cut the generation of lift.

On the VX the body shape is quite different, but it's quite likely that the little 'winglet' on the VXT's serves the same purpose as the 111S spoiler and that is to reduce/negate the lift.

Diffusers only really generate any noticable downforce when you get the vertical panels very, very close to the road.. As in between 10 to 20mm or so.. Totally impractical for a road car.

In the same light as the small spoilers on the back of the car any iaftermarket/bigger diffuser may, however, help to reduce or cut the development of lift on the back of the car, making the car feel more stable at higher speeds.

Try getting hold of the book 'Competition car downforce' for an in-depth discussion on spoilers, wings (very different things!), diffusers, etc.

Bye, Arno.

[Winstar]

Actually - exactly the opposite happens....

With a flat undertray and no diffuser fitted, the the lower the car is, the greater the effect of rear-end lift.
This is because the greater the effect is, of low pressure air flowing underneath the car (flowing faster than over the top), compared to high-pressure air over the top of the car.

The VX suffers badly from rear-end lift. Lowering the car without a diffuser will amplify the effect as you are increasing the effectiveness of the front splitter, as well as generating a higher flow rate of low-pressure air under the car. The VX is no different to the S1 and S2 Elise in that respect: Flat undertray leads to air passing more quickly under the car than over it. But, the air clings to the upswept clam, creating more turbulence at the rear, which results in lift.

It the other way around the fast flowing air creates a high pressure region under the car and the curvature of the flow over the car created low pressure above it*, therefore generating lift as high pressure pushed towards low pressure. I'm not sure that lowering the car does actually increase the lift, but as you say it will significantly increase the front end downforce so upset the balance of the car at high speed.

* clicky for the proper explination of lift.

it is possible to get data, but it is expensive and the product producer has to be confident he will achieve cost/sale benfits

EG Reverie http://www.reverie.l...rearwing_06.php

Anything less is just halfords tat*

*ps i have the jimsan diffuser, looks great

Hmm that is what we would term Gaffa Dazzel, while the Analysis may be valid it has no experimental results to back it up.

It also only considers the wing in a free stream of air and not when the air has just passed over a car, which is what makes designing diffusers so difficult as the whole car and the wake needs to be modelled to capture the effects.

Edited by Winstar, 10 December 2007 - 08:55 AM.

[Thorney]

In simple terms the further they extand out from the body, the longer they extent down to the floor the more effective they are. Look at DTM cars.

[siztenboots]

Front to rear angle of rake is important too in several areas. As eluded to above, length, AoA, end fences and leading edge , there is a lot more to it than that.

[Stu-7]

I'm not sure that lowering the car does actually increase the lift, but as you say it will significantly increase the front end downforce so upset the balance of the car at high speed.

Slight misunderstanding - probably due to my wording
If you take the theory of "Ground-Effect" (pioneered by Colin Chapman)which we are discussing here, one of the main ways to achieve this is to design the flat panel undertry of a car so that incoming air from the front of the car, is accelerated through a narrow slot between the undertry and the ground. Ergo: the narrower the slot, the greater the acceleration of the air.

I was trying to say that, lowering the ride-height of a VX which does NOT have a diffuser fitted will greater accelerate air underneath the car, meaning it exits the rear of the car faster, thereby increasing the *effect* of rear-end lift (if you don't have a functioning diffuser fitted that is).

[Thorney]

Front to rear angle of rake is important too in several areas. As eluded to above, length, AoA, end fences and leading edge , there is a lot more to it than that.

I did say in simple terms.

I did a lot of research on them and worked with the guys who do the aero effects for the Bentley le mans team.

[EdButler]

Oh my god cant anyone write this concisely!? Let me have a go! The rear Diffuser merely accepts the air coming from the front-underside of the car, and diffuses it through the combination of a wider, longer and upward angled space to increase the volume of space the air needs to fill. Air displacing to a larger volume area (L x W x H) within the diffuser creates low pressure underneath the rear-end which causes suction to the ground. The Jimsan increases length and width and height (as it protrudes outward at the original angle) of the original. Therefore it should be theoretically better than the standard item.

[JawZ]

-Various Diffuser designs for the VX220-

[Tim F]

Ed wins this one! However it does appear that Thorney has spent more time developing his diffuser with a proper company rather than just bolting something good looking on.

[Thorney]

Sorry, I've typed this out so many times it gets tiring. Same applies with the wing and front splitters, all were done with either Lotus themselves or ex Lotus people along with some people in le mans racing (Bentley) or Opel (DTM). Firstly forget comparisons with the S2, the two bodyshpaes couldn't be more different, the fron the VX carries significant downforce already due to the bluff front shape, its the reason that even the race front splitters aren't exactly huge - they don't need to be. The problem with the VX is that the rear end was designed to carry this flat edge that that meant the rear undertray was 'swept up' from the flat floor a lot earlier in the chassis than was designed (have a look under an S2, the flat floor comes a lot further back towards the rear of the clam). This upswept angle (22 degrees iirc) means all the flatfloor generated airflow is essentially washed out the back of the car causing both lift and drag - nice combination really. its the reason your rear bumper gets dirtier than the rest of the car as well as the rear window (same effect occures off the roof). When we designed the diffusers the concept was to extent the flat floor effect as far was we couldand prevent this washing out of airflow. To do this you need: 1. leading edge of diffuser must be smooth to the flatfloor, ie no lips and no ability for airflow to above the diffuser and the floor - there is a different effect for tiered diffusers for this and even a 2-3mm gap (as we've seen on some designs) will actually make things worse. 2. Vertical planes need to be that - vertical, even a minor angle on the downplanes wastes the flow to render them near useless, the longer they are (ideally right down to the ground) the better 3. Rear extension, ideally at least 6 inches from the rear clam but any less than 3 " extension would be near useless (in terms of any real effect) 4. Heat. We've only really come accross this as a problem on some of the other designs in that a few people are removing them as they are melting clams. Even with extra cooling holes drilled in (which again will affect the role diffuser play) we've seen melted number plates and clam blistering, indeed we removed one from a customers car the other week. We've never had this with ours, probably because we use the separate side design but the main purpose of this was that if someone had a minor hit at an angle one side could be replaced whithout needing a whole new diffuser. Ours are designed for function and we have new race only ones that are even larger but Ihave the same opinion on the others as I do for cooling, anything is better than nothing but don't kid yourself that you're buying them for anything other than looks. We spent a whole day at Brunters on ours, same car running it round 120l over and over again with the diffusers off,on, wing off, wing on and every possible permutation and the net result was that for diffusers alone the corner entry was more planted and we gain mid and exit corner speed, that combined with the input from those who helped design them meant I'm happy with what we did.

[Crabash]

If I'm not mistaken void filling from the diffuser is also quite important as the void creates a lot of drag, too long a diffuser can make it worse, although this is not an issue with a road going car as you can't make them long enough within the law to be a problem I would think.
I believe also that having the 2 undertray's join where the floor kinks up into the diffuser (if it is not a clean join)reduces the effiency and can cause the air to "break away" from the surface earlier limiting length and angle. (Sorry Jon I see now you already mentioned this)

Bernoulli's Principle is what dictates what happens under your car, if you want to understand it try here:

http://home.earthlin...19/venturi.html

Edited by Crabash, 11 December 2007 - 11:33 PM.