Chimaera Suspension

Chimaera Suspension

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Discussion

shnozz

27,423 posts

270 months

Wednesday 6th October 2004
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markh said:
Shnozz
just athought but I've been speaking to this guy to see if this cerbera splitter could be converted to fit a chimaera?

www.carboncerbie.com/underconstruction.htm

Mark




VERY interested. Can you let me know what he says?

shpub

8,507 posts

271 months

Wednesday 6th October 2004
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Front splitters.... not quite as simple as you all think. Rake is generally a good idea as it can create some downforce by removing lift. However you don't get anything for free and the penalty is drag so the car will be slower post 100 mph. Too much and the car will be significant slower. The problem with most Chimaeras is that the body position changes quite a bit with weight transfer. As the car accelerates, negative rake will be introduced as the weight transfers to the back. The amount of rake needed to overcome this is big so when the car is cruising or braking or lifting off, the amount of rake is big and the car will slow down very quickly.

There are two types of splitters that are used on the front. Air dams which try and prevent air going under the car and building up pressure and lift and the horizontal type (a real splitter) that trap the air at the front of the car and creates a high pressure zone in front of the car which gives downforce.

The splitter works best when it is low down because air can't get underneath and cause lift which counteracts against the downforce gain. Not practical on a road car. Splitters etc also change how the air flows through the radiator and a high pressure zone can dissapear through the rad grill and again reduce the down force.

Been playing with this stuff for years and I am now using a combined version that has a front splitter for downforce and an air dam underneath it that directs the air that goes underneath up directly through the rad and out through the huge bonnet vent at the front. This prevents the lift build up under the splitter but there is about 1/16th of inch ground clearance in race trim on the air dam. The air dam is made of rubber and is flexible.

The front splitter is plywood as it is cheap and easy to replace. Pete H fitted mine after a long talk with me about my experiences. Used to use GRP but they are very vulnerable to damage. BTW the 520 has broken splitters at around 140 mph when the plywood structurally gave way which gives some indication of how good the design can be and it is noticeable when the car doesn't have one. I am now using 6mm ply and special mounts and had no problem. The TVR Le Mans cars used 12 mm plywood spoilers.

The key to all this stuff is preventing the chassis from moving so that the car's position is stable. This means that on a road car, the advantages are not so big. Remember the insurance companies need to now of any splitters etc as it is a mod. Only exception is the factory ally strip sorry lip spoiler.

You also get real problems with the rear of the car as that will also generate a lot of lift anyway but that is another story...

Steve
www.tvrbooks.co.uk

shnozz

27,423 posts

270 months

Wednesday 6th October 2004
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cheers steve, some things to think about there. I was planning on a "trial by error" route similar to what you have already followed so that post gives me a head start.

Thinking of sitting the TVR splitter about 6 inches back from the front of the car. Of course, clearance dictates that it wont be very low to the ground and my simple understanding of physics was simply that being angled in teh shape it is - ie an L shape pointing forwards, the air would hit the splitter and press against it, forcing the front of the car downwards. My simple physics also suggested that this would lose a few mphs from the top end (but at least give the driver an opportunity of finding the top end )

corner splitters were also the same principle I guess, more of a continuation of the TVR splitter more than anything else - just thinking that although the splitter in the middle will help, the air will be forced sideways and potentially towards the front wheels, thus leading to air forcing those in an upwards motion.

I know there is no simple answer, but am I completely on the wrong lines?

shpub

8,507 posts

271 months

Wednesday 6th October 2004
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Yes ish... It is not the force of air hitting the spoiler that generates the downforce. The small TVR lip thing disrupts "spoils" the air flow under the car and the turbulent air in front of the spoiler reduces the further flow. This reduces the air pressure and the lift. The reduction in lift gives you the downforce. It doesn't generate downforce... it simply reduces lift. Basically inverted wings are the main method of creating downforce. Everything else like splitters, spoilers etc are simply reducing lift.


On the TVR it also helps direct air through the rad as there is a hole in the under floor. However the air also has to go somewhere and it will build up under the bonnet and this will create lift. I had a CF bonnet on the 520 and at 70 mph it bulged with the air pressure in the engine bay and flew off leaving small areas of CF around four mounting posts so another thing to consider is how to duct air out to prevent lift building elsewhere. That is why the Tuscan has the fans where they are and the second engine cover. The Elise and GT40 use a big duct which is where I got the idea from. I realised this was the problem when the bonnet disintegrated. BTW CF is not as strong as you might think...

The jury is really out on the lip spoiler but it is cheap and worth playing with and if it helps then fine but don't be surprised when it wont.

Steve

>> Edited by shpub on Wednesday 6th October 14:46

JonRB

74,402 posts

271 months

Wednesday 6th October 2004
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Would the bonnet vents on the Chimaera not be sufficient to allow the air out?

shnozz

27,423 posts

270 months

Wednesday 6th October 2004
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JonRB said:
Would the bonnet vents on the Chimaera not be sufficient to allow the air out?


from the lift I already get around 120mph at the windscreen edge of the bonnet I would have thought "extra" air in there is only going to worsen the situation.

chris547

Original Poster:

87 posts

236 months

Wednesday 6th October 2004
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I'll aim this at Rude girl for being so tetchey. What have i done to upset you?? What is your agenda??

Thanks Jools for being so sensible and ever helpful to people needing support. See if you can pass on some of your skills to the aptly named dame.

Obviously I am going back to the garage but now I understand the issues better.

Not certain but I suspect the 2 front dampers are not working at all. The springs are indeed oscillating. I have adjusted them from 3 to 30 and its just the same. What has confused me is the rear is working so it was all rather odd when you make adustments and the results don't match expectations.

This stuff on splitters is fascinating and rude girl surely you too have learned something from so many peoples experiences.

I have decided for now not to fit one.

Could someone not do a wind tunnel test with wheels on a load cell to see if any of the designs do in fact provide benefit?

Finally something on Quality Control. You used to get guys who wore brown coats that took customers cars and checked them before the customer got his car back. I think they were called foremen. The old ways were the best but of course you did not have computers and forums in these days so there's another reason Rude Girl for not just going back to the garage. They don't always get it right

JonRB

74,402 posts

271 months

Wednesday 6th October 2004
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chris547 said:
Could someone not do a wind tunnel test with wheels on a load cell to see if any of the designs do in fact provide benefit?
It would indeed be interesting, but who is going to pay for it? Not TVR, that's for sure.
In an EVO article a few months back they featured the boys at Caterham fettling the new revisions to the Seven (which include inboard front dampers. Mmmmm) and they mentioned a figure of £8/minute for a wind tunnel. And that was for one without a moving floor.

shpub

8,507 posts

271 months

Wednesday 6th October 2004
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I have looked into wind tunnelling the 520 and it works out all in at around several thousand a day... There are cheaper ways including informed trial and error as the technology is pretty well defined and armed with some good reference books and some perseverance, a lot of improvements can be made. Can be very worth while though but it is out of my budget. I have data logging on board so can immediately see if something is good or bad so that is a big help. I can see from the playback what the car is doing and its behaviour.

However like suspension mods, it is important to remember that you need to know what the problems you are trying to solve in the first place, work out some solutions and then consider what the drawbacks and knock on effects will be. Then it is a question of what compromises you will live with and thus you go for it.



19560

12,722 posts

257 months

Wednesday 6th October 2004
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shpub said:
Yes ish... It is not the force of air hitting the spoiler that generates the downforce. The small TVR lip thing disrupts "spoils" the air flow under the car and the turbulent air in front of the spoiler reduces the further flow. This reduces the air pressure and the lift. The reduction in lift gives you the downforce. It doesn't generate downforce... it simply reduces lift. Basically inverted wings are the main method of creating downforce. Everything else like splitters, spoilers etc are simply reducing lift.

Splitters and spoilers do generate down force which can be well in excess of g so that the car could drive upside down.

ATG

20,485 posts

271 months

Wednesday 6th October 2004
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19560 said:
Splitters and spoilers do generate down force which can be well in excess of g so that the car could drive upside down.
Big downforce is not produced by splitters and spoilers. It is provided by wings and venturi tunnels.

GreenV8S

30,150 posts

283 months

Wednesday 6th October 2004
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I think wind tunneling is a waste of time unless you are looking for detailed 3-d flow visualisation.

I did a lot of background reading around the subject, and also spent some time mapping out the major flows round the car and measuring the actual lift, before I added the splitter. It is by no means as simple as screwing a plank under the front of the car and voila more grip. To start with, if the splitter is effective there is a very real risk that it will provide lift at the back as well as downforce at the front. Rear end lift can be a serious problem because it reduces high speed stability. Most of the effects from adding a splitter or air dam come from the changes in pressure over the rest of the car rather than from loads on the splitter/dam itself. However, as Steve points out, the aerodynamic loads on a splitter can be immense. I could stand in the middle of the MK1 version, but at 150 mph it deflected so far that it scraped the ground. I thought it must have broken when it started making graunching noises, but as soon as I slowed down the noises stopped and when I got back to the paddock it looked fine and was still solidly mounted, just with a massive flat worn on the front edge!

The other thing to consider is that splitters and air dams do essentially the same job, which is to reduce the amount of air flowing under the car by lowering the stagnation point. Broadly speaking, moving the stagnation point forward by an inch has the same effect as moving it downward by an inch in terms of the effect on air distribution.The major difference is that and air dam that is well back from the stagnation point produces a high pressure zone in front of it which cause a small amount of lift; a splitter produces a high pressure zone on top which produces some downforce. (But as I said earlier, most of the downforce comes from the change in pressure over the rest of the body.)

My lift measurements turned up one result that I found very interesting. On the V8S, within the range I measured it (50 - 100 mph), the weight on the back wheels did not vary with speed. Not at all, in the slightest, even a teensy bit. (I could measure weight transfer from acceleration/braking, bumps and dips in the road, wide sweeping bends that were all but invisible to the eye, side loads from minute changes in road camber, I could even see the effect of the wake from vehicles around me on my - ahem - private test track. However, the weight on the rear wheels did NOT vary with speed.) This doesn't mean that there is no lift at the rear, just that the weight transfer caused by vehicle drag just happens to exactly match the lift, in this car, in the conditions I measured it. Just as a sanity check I can confirm that I got a nice V-squared lift profile at the front, although the amount of lift was small compared to all the other factors.

What this shows is that the amount of grip lost due to aerodynamic lift is actually surprisingly small. The reason it is so important is that the center of lift is beind the center of gravity so this lift reduces the lateral stability of the car.

The biggest effect of the splitter is the improved cooling due to the reduced underbonnet pressure. People do say it affects the high speed stability on Griffiths and Chimaeras, but if the results from the V8S are anything to go by I think the effect is largely psychological. However, I expect it would have an even bigger effect on the cooling on these cars because unlike the V8S with the big front air intake, the main air intake on these cars is almost parallel to the air flow rather than square on to it. You would only need to change the direction of air flow by a few degrees either way to make a big difference to the air through the radiator, and an air dam just behind the air intake is just what you want to achieve this. On the other hand adding a splitter in front of the air intake is likely to have the opposite effect.

>> Edited by GreenV8S on Wednesday 6th October 23:04

ATG

20,485 posts

271 months

Wednesday 6th October 2004
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Peter, how do you measure the weight when on the move? Sensors on the shocks, strain guage on the springs, ...?

GreenV8S

30,150 posts

283 months

Wednesday 6th October 2004
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I used string pots, A-to-D conversion, and a data logger that also recorded speed, GPS position and dual axis acceleration. These other inputs enabled me to quantify the factors other than lift, that affected weight on wheels. It was quite fiddly to set up and took a lot of work to compensate for all the other factors (video cam to show where I was in 'clean' air, GPS to show actual speed etc as well as a lot of work to smooth and filter the data to eliminate transient effects from various sources) but is a nice low tech solution. It is also very cheap, which suits me fine!

JonRB

74,402 posts

271 months

Wednesday 6th October 2004
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Pete -

shpub

8,507 posts

271 months

Thursday 7th October 2004
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Just to echo what Pete was saying about rear lift. I was banned by class regs from having any rear spoiler and the car was very noticably twitchy at high speeds and especially under braking at around 130+ The amount of lift is immense. I forgot to push the bootlid down at Goodwood and halfway down the back straight, the boot lid simply flipped up and came off, braking the hinges. You would think that the forward motion and air flow would keep the boot lid in place. It doesn't as the lift is very big. The boot lid weighs 9 kG and imagine how much force is needed to brake/rip out hinges.

This got me thinking about rear spoilers and after consulting my various I-Spy books of Spoiler Design, I fitted a simple Gurney Flap design out of ally. The high speed stability improvement was unbelievable but it started to show up some problems with the front splitter design.

The long and the short of it is that the car is now extremely stable and planted. The Data logger shows that it is pulling about 1.2 to 1.5 G through corners on slicks which is getting close to what some more aerodymacally deigned race cars are acheiving. I think there is some improvement to be had as the limit now seems to be the amount of roll. Need to uprate the suspension design again... About to fit position load sensors to the data logging to confirm what I think is happening is happening before going to the next stage.

It does take a lot of development work as Pete says but that is half the fun...

deeen

6,079 posts

244 months

Thursday 7th October 2004
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Hi GreenV8S. Fantastic post. I didn't know anyone had put so much work into the aerodynamics. Your posts always make sense, but I had no idea you did your reaserch this thoroughly!

I have been considering aerodynamic changes to my Chim, and it would be nice to make sure they are improvements! In this spirit, I wonder if you could give your thoughts on a couple of pints? I mean points?

High speed stability. I understand it's best to have the centre of lift at the centre of gravity. Have you discovered how much taking the roof off moves it? Intuitively I would guess the airflow is so disrupted that there is less lift at the rear? I guess shpub your missing bootlid experience was roof on, if you were racing?

Have you tried a splitter or aerodynamic device under the rear end? I think the Chim is more upswept than the S at the back, I was wondering about reducing rear lift without compromising the looks too much.

I agree with you about wind tunnel and lift, it would be useful for checking drag though? I would love to know the benefit of simple mods like faired in headlights, or spats where the bodywork curves under the wheels. I guess it's a high drag body as standard, it's supposedly faster than the latest porsche 911 to 60 but slower to 100, mainly cos the porsche is slippery, I would guess. Mind you I have to change up at 90, must check the 0-90 times...

Your grinding splitter - as well as the wood flexing, were you compressing the front with downforce (or lifting the rear) and getting the splitter closer to the ground, maybe?

underbonnet pressure - I thought a splitter increased it, by directing more air through the front grille? I was thinking of reducing mine with vents at the back of the bonnet. nitpicking point - the Chim has a vertical rad, more like your car than a Griffith.

So for my Chim, my thoughts were front air dam extending to the outer edges of the wheels, vents at the back of the bonnet and leave the roof off! And maybe try something under the rear.

Weight transfer caused by vehicle drag - would this be enough to make the steering feel lighter? Maybe (as standard) the steering would feel more positive with the roof on then - less drag?

Dont know if you will have time to answer this, but thanks again for a most informative post.

shnozz

27,423 posts

270 months

Thursday 7th October 2004
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blimey its getting technical now! deeen, cant remember ever seeing such a serious post from you!

What are you thinking about for the rear? I would rather avoid an evo style spoiler! Are you thinking a sort of diffuser set up underneath the rear bumper so the air travelling beneath the car assists?

Sorry if I missed anything here - I am only experimenting in aerodynamics for the first time and using only common sense re teh air flow over the car and forcing it into pockets to assist downforce.

shpub

8,507 posts

271 months

Thursday 7th October 2004
quotequote all
On the 520, roof off screws up the aerodynamics big time. I have done quite a few acceleration tests with the roof on and off and roof off reduces the top speed by about 5-10 mph and adds significantly to the 0-60 time by about 0.5 s. This with the car otherwise standard form.

I had the roof fly off at goodwood again at about 130 mph when tyhe body/windscreen flexed and it popped out. Went out with the top down and the difference was very noticeable. The car was very twitchy and moving across the track with side wind. Retrieved the top, put it back with a tie down strap just in case and the handling came back to its normal planted self. Unless I'm poodling about, I never go out with the roof down.

shpub

8,507 posts

271 months

Thursday 7th October 2004
quotequote all
shnozz said:
blimey its getting technical now! deeen, cant remember ever seeing such a serious post from you!

What are you thinking about for the rear? I would rather avoid an evo style spoiler! Are you thinking a sort of diffuser set up underneath the rear bumper so the air travelling beneath the car assists?

Sorry if I missed anything here - I am only experimenting in aerodynamics for the first time and using only common sense re teh air flow over the car and forcing it into pockets to assist downforce.


For an Evo style wing to work, it has to be high enough to be in the air movement to get a good flow over it. Failing that a gurney flap on the back actually does as good or even better. Used on nascars in particular. As for channeling the air from under neath, quite difficult to do and get right. I have removed the lower rear spoiler on the 520 as it did nothing, added 20 kg to the weight and actually prevented air from getting out the back. Venturi /ground effect also can cause overheating as it can change the flow of air etc.

If this stuff was easy, it wouldn't be any fun...