Engine bay undertray aerodynamics

Some time ago I started to do some CFD analysis to see if I could do any simple changes that improved the aerodynamic properties of the car, without altering its appearance too much. It turned out that I had too much faith (or more probably - too little knowledge) of what the CFD software could provide me, and after putting in a great deal of work I finally gave up.

It is said that from an aerodynamic point of view, one of the most important features of a car's body design is how it looks underneath, and even if my previous CFD adventures failed it pointed me in the same direction. To fit an engine bay undertray is not very complicated and doesn't affect appearance much, but before I started I wanted to confirm if it would improve things or just be a waste. This time I made a much simpler 3d-model of the car, and had much lower expectations of what the CFD software could help me with.

Standard under body

The results should be taken with lots and lots of salt!

Standard
Drag: 879N
Downforce: 427N

Undertray only
Drag: 793N
Downforce: 455N

Undertray and skirts
Drag: 809N
Downforce: 590N

Undertray, skirt and splitter:
Drag: 858N
Downforce: 604N

All calculations are in an air speed of 160 km/h. The standard design does generate a bit of downforce, because the car body is at a small angle compared to the direction of the air flow. The ground clearance is 15 cm, minus the engine oil sump. The design measurements are just guesses as I didn't take time to go out to the garage and measure. (it does look a bit short). No rounded edges. The body is solid on the upper side.

An underdray reduces drag but doesn't give much improvement in downforce. On the other hand, extending the undertray with a 5cm skirt improves downforce quite a lot. I also tried to add a small splitter in the front and see if that made a difference (see top banner image) but the small gain in downforce is not worth the effort.

One apparent issue with fitting an undertray is of course heat. I have a plan for lowering intake temperatures (more about that later), but if the heat from the exhaust primaries will be just too much and start to melt things I don't know.

Caterham has a ready made undertray on their website, but I've already bought a sheet of 1 mm aluminium and started to cut out the under tray. I'll post more when (and if) I finish it.

MOT, autumn and nothing much

The yearly MOT was a bit different. First attempt they refused to inspect the car as the guy said he'd not manage to get into the car as he was too big. So he sent me home with my money back. I'll never use them again.

When I should do the second attempt I realized the car was just running on three cylinders! With just minutes to spare I found the fault was a loose wire to one of the coils. I managed to fix it temporarily with a piece of wire and insulation tape and got there just when my registration number was showing on the display.

He did the inspection with me waiting in the car, only to get out to pay the bill. No troubles, no complaints.

This year I had the cage still on the car. No-one mentioned it. I won't bother to take it off any more.

I haven't driven the car at all for some time. Today is a cool but dry autumn day so for the first time in a very long time I took a drive out on the country side.

This car is really fantastic to drive!

Just wonderful.

My wheel fell off on a trackday

At 137 km/h coming into a long right turn, my inner rear wheel fell off. A loud clonk and I could see how the inner rear wheel was bypassing me! I slowed down slowly standing on three wheels and could keep the car on track the whole time. A guy ran out on the track to help me, and we could easily push the car out from the track by lifting at one corner while pushing.

The loose wheel had hit the rear wing rivets loose. Other than that, no damages at all! No scrape marks, nothing. I put the other set of wheels on and continued driving the rest of the day as nothing happened.

But how could this happen?

The wheels have been used three seasons. Magnesium centres are known for the need to be inspected every season - but this is aluminium and they're supposed to hold together. Then I noticed that the rear centres had larger bolt holes than the fronts!

I've emailed the manufacturer and got the following replies:

Cracks forming on the other rear

I have spoken to two other owners  with the very same design billets,  I think I only made 5-6 sets like your before changing to the later profile as  the power and weight of customer cars was  going up so I thought to beef them up a little, The two I have spoken to  have no problems and have used them now for three seasons hill climbing here in the uk. I do not have an answer for you why  there has been a failure, but I can Only assume fatigue, The later billets you have are now used on lots of similar cars to your including some saloon circuit  cars around the 850kg mark with no problems reported 
I am of course more than happy to send out replacement centres .

And later:

I see the  old wheel bolt holes are larger, this is because when I originally worked with Meteor  we were going tom use floating taper washer that needed a bigger hole, using these washer didn't work out so I changed the hole to smaller dia, the set you have were simply a set form stock that were used up.

I see the damaged inner rim. No problem for me to send a replacement under the circumstances.

After investigating the new design I'm confident they're much stronger than the one that broke, and I feel assured they'll hold. If you have similar wheels, I suggest you check them and see if you have the larger or smaller holes, and replace them if you do. If you have the new style centres I wouldn't worry.

I must say I got an immediate and professional response from the manufacturer, and they did everything they could have done in this situation. However, I don't think I should have gotten the weaker style centres in the first place.

New style centre from the inside.
 Note that this picture doesn't show that the centres are also thicker than before.

Old centre, with big bolt holes.

New rims with stickers on

New 13" rims from Force Racing, 7" and 9" wide with brand new Dunlop slicks - "stickers".

I bought the rims directly from Force Racing this time and they were very easy to deal with and I can really recommend them.

The rims are very lightweight - only 3792g/3528g according to my kitchen scale* and look stunning.  If you want to order yourself, refer to this post and they know what to do.

Note that you need to modify your rear calipers to fit 13" rims on a CSR!

The slicks are the same dimensions I usually run - used Radical PR6 slicks 230/570/13 and 190/535/13 - but this time in soft compound instead of medium which the Radicals run. We'll see how that goes. I bought them from HP Tyres. The rims are a bit narrow for these tyres, but I think they work ok.

I plan to practice on worn used slicks and put these on when I really want to go for it. I've already started building a simple rack for my trailer.


* - my kitchen scale is known for messing up recipes 

Gearbox disaster

My gearbox failed with a bang on on the King of Mantorp TA-event. Totally unexpected on the day's last cooldown lap. Looking at the log file I didn't even have 50% throttle when it happened.

The good news was it was easy to see where the problem was. The bad news was I it looked rather serious. When opening the top lid everything looked normal which at least gave me some hope.

We don't have a company named Road'n'Race Transmissions here, so it took some effort to find somebody that was even interested to look at the gearbox. As it turned out, the man I eventually found knew a lot about type-9 gearboxes and I must say gave me very good confidence it could be fixed, and with an helpful attitude.

Bertil Carlsson in Vejbystrand, described what have happened as a locking nut had shaken loose, causing the big end nut unscrew itself through the casing. Something that he'd never seen before.

The case was welded, the nut was replaced, and while he was at it he replaced the sync-ring between gear 5 and 6 which was very worn. Everything prompt and for a very reasonable price. (I'm not sure what could have caused the worn sync-ring. The only thing I can think of is that I had a cracked clutch plate a long time ago that made gear changes difficult at high rpm.)

That said - someone (not me and not Bertil) did really mess up the order of the bolt from Caterham, and therefore this little disaster took a big part of the summer. I'm a bit disappointed about this but I guess I should be happy it turned out as good as it did, and that I didn't have to buy a brand new Sadev sequential gearbox with paddles.... ...because I couldn't afford it.

All right - time to put the engine back in and see what's left of the summer!

TimeAttack - "King of Mantorp"

Valtonen Motorsport RX-8

Normally I don't like to compete but somehow I participated a Time Attack competition this weekend. Seven type cars are not allowed in their series, but this time they had a "no limits" class. Some of the competitors car's was completely insane, as the tube frame, all carbon RX8 to the right, accompanied by a complete race team.

Photo: Felix Eriksson

I managed to get the 4:th place. I'm really satisfied with that. Not so much for the final time (1.23,4), but that I kept my head together and managed to get a good run in the super final which was one lap only!

In fact, the qualifying lap and final lap was one second faster than all my practice laps during the whole day!


The official noise check reported 97.5 dbA drive-by which they said they allow today but not tomorrow. I'm getting there...

Everything else worked very good, except....

Gearbox problems!

Just when the day ended something happen with my gearbox on the cool-down lap. I works but makes some chirp noises and the gears are hard to engage. .... .... ghh

Update: I got a couple of seconds of fame in this movie:

Intake cone filter and air pressure

The Pipercross airbox I bough was supplied with a built in air-filter rated up to 200 hp. As don't want to strangle my engine I now have fitted a cone filter with larger filter area.

102mm to 80mm silicone reducing elbow from do88, a plastic tube and three hose clamps from the local hardware store. The filter is left overs from the CSR200 engine.

As I already have a MAP-sensor connected to ECU, I decided to do a road test and see if I could see any pressure drop at the intake. Ideally I would compare the three different filters against each other, both regarding pressure drop and maybe 100-200 km/h acceleration, but I rather not on public roads.

RPM vs air pressure (click to enlarge)

The lowest pressure seen was 0.9918 bar, which equals a drop of 8.2 millibars or 0.82%. In theory that would decrease power with the same amount, or in my case 2.2 hp*. The pressure increase seen on this chart is interesting. I have an hunch of what's happening but I don't have enough knowledge to either explain it or fully understand what the result is. Could it have something to do with air moving but the engine's volumetric efficiency is declining and that causes pressure build up?

I was worried the intake temps would suffer with the filter close to the exhaust manifold, but it appears that they are lower than they used to be with the big sausage type filter and never went above 28°C, with the ambient temperature being around 18°C.

* Source: Four-Stroke Performance Tuning page 26, A. Graham Bell.

Attempt to redirect exhaust noise

As I previously wrote my noise levels are now down to 94-96 dbA on the intake side and 101 dbA on the exhaust side.

In an attempt to reduce the latter I did a quick test to fit a 90° steel tube and point the exhaust towards the ground and see if it made any difference.

The result was a bit unexpected. The exhaust noise went down an half dbA, but the intake noise raised to 98 dbA.

I've been complaining about this before, but noise metering appears to be quite random.

Intake airbox

As I wrote in the previous post the track noise regulations in Sweden has become much harder lately. The current limit is 95 dBA drive-by measured from a 10 m distance.

I've fitted a Raceco silencer and now a Pipercross PX600 airbox. Now the car is much more quiet - huge difference! I can't even hear the revs because of the wind. A quick drive-by test with a cheap china noise meter gave 96-97 dBA but after glueing on some insulation material on the airbox the readings dropped to between 94 and 96 dbA on the most important intake side of the car.

On the exhaust side the noise level is 101 dbA and still too high. I'll try to mount a bend or something to point the exhaust to a different direction and see if that helps.

The airbox have an integrated filter that I suspect rob a bit of power. Next step will be an external filter with larger filtering area, and maybe some cold air ducts on the nose cone side or sides.

The good thing with this airbox is that it can be mounted within minutes if the occasion requires. Direct fit, but I had to remove the steel bonnet/nose cone support tube.

Raceco silencer

The track noise regulations in Sweden has become much harder lately. The current limit is 95 dBA drive by measured from a 10 m distance. This limit is set by the SBF (Swedish motorsport federation) for all swedish circuits.

I know my car sounds a lot more than that, around 105 dBA. So far I've never been blacked flagged but I know it is a question of time. Also I wouldn't mind if the car was quieter as I'm not a big fan of loud noises.

The first step is to quieten the exhaust and that is now done with a Raceco 2.5" titan silencer built by Mike Riley. The high price made it a painful decision but it is very nice built and also a lot more quiet than the previous silencer. It take the edge of the bad part of the noise and what is left is a nice round sound. In fact the exhaust noise is now masked entirely by the (very nice!) induction noise.

It was a straight fit and no brackets needed to be fabricated.

It is also a bit lighter than my previous silencer - about 1.5 kg.

The car is much more quiet now but I think it still have a bit to go before I reach 95 dBA. So trying to reduce induction noise is next step.

Rear diffusor and front splitter inspiration

I more or less gave up on my CFD simulations. At least for a while. I could not get consistent results and the obstacles were just too many to keep me carry on. But before I gave up I did some simulations with an engine bay under tray and a rear diffusor, and according to the simulations it did quite a big difference on both drag and downforce.

Today I received some inspirational pictures from Paul Cardy on his previous CSR. I think I'll try to do something in the same lines.

Paul writes:

"The diffuser was attached to the chassis using these brackets with threaded inserts as per picture 1, the rear end of the diffuser was held using these hanger brackets in picture 2, and the main body was held in position using P type clips with steel surrounded by a rubber sleeve and clamped round the chassis tubes. 

The diffuser was made by Freestyle in fiberglass and I had him glass in some carbon on the last section for the look. I made up the front mounts and cut up the diffusers to fit as this was the first ever to be fitted to a CSR so its was a make it up as you went along. 

The front splitter is a sheet of carbon cut to shape its fixed to the chassis with the P clips again. The front upright section is aluminium folded and cut to follow the underside of the nose cone and fixed to the splitter its wrapped in black vinyl."

I also would like to cover the engine bay. But before I do that I must take care of the already high intake air temperatures. But that is a different subject.

Aerodynamics - CFD simulations

After some vacation reading of the book "Race car aerodynamics" I have had lots of thoughts of how to improve the car's aerodynamics, especially the lack of down force.

The common approach on this subject is that the seven is a hopeless case and don't even bother. That might be true, or could it be the other way around? Since it does have the aerodynamic properties of a brick (or worse) - even small mods can make a great difference?

One problem is that you don't want to change the classic look of the seven. That is also true for a majority of seven owners. But I believed something like a flat underside could make a big difference. You can also add removable elements that you only have on the car when you're really going for it.

The question was, how much can be gained, and is it worth the effort?

Autocar did an article a long time ago (?) about wind tunnel testing of a Caterham. You can find it here:
#1 #2 #3

Some time ago I did some simple simulations of how much some downforce would affect lap time. The result was that 160N more downforce @100km/h would shorten the lap time with over a second! As a reference, F1 cars generate more than ten times of that, at the same speed. (source: the book mentioned.)

Wind tunnels are not accessible for most of us. At first I thought I would do road testing with string potentiometers measuring the suspension compression and connect it to the data logger. Straight roads are not that easy to find where I live, and going 200+ km/h on public roads are not that great either. The number of variables are endless and if something works or don't work I probably won't know why.

The book mentioned above briefly talks a bit about CFD - Computational fluid dynamics - and concludes that it is very expensive, complicated and not something for others than high end race teams. Well, time has passed since the writing of that book, and now the CFD software is not that hard to use and 30-days trials can be downloaded over the internet from many different software companies.

It turned out that the book was quite right. After experiments I now understand why top race teams still use wind tunnels. CFD simulation is very complicated, and it takes lots of effort to get accurate results. But for rough estimates it could still be very useful!

Baseline

So I made a rough simplified CAD model of my car. It may sound easy, but for a complete 3D modelling newbie it took quite a lot of time learning.

I started with high ambitions, but after a while I lowered them just to get some results at all. In these simulations the wheels are not rotating and the ground is not moving.

@160 km/h
257N front downforce

1935N drag

Note, these are rough numbers. 

I use to complain about front end lift when I drive, but this is pretty much the opposite. 

Anyway I plan to do some changes to the model and see how it affects the output. I also plan to investigate some individual parts in detail with finer meshes and higher accuracy.

Fuel system with Swirlpot

It was not long ago I uprated my fuel pump and since then I only used my car once. But I never felt satisfied with the new system. I feared the fuel surge while cornering problem would be even worse than before, so I decided to do it properly with a swirl pot and a low pressure pump.

Pumps

The fuel demands for a 280 hp NA engine is according to the book Four-stroke performance tuning: power * k = 280 * 6 = 1680 cc/min = 100.8 liters/hr including 30% oversupply.

The pumps and filter is located under the trunk.
A pity to hide those expensive AN-fittings?

According to Cosworth their 280 hp crate engine should have fuel in excess of 1.4 l/min = 84 liters/hr.

I want to be able to run E85 in the future if I decide. That is another 30% = 131.0 l/hr. And as before, I don't want the pistons to melt if the alternator fails and the supply voltage drops a volt or two.

The low pressure pump doesn't need quite as much flow as the high pressure pump as it always have the swirl pot as a buffer. The Facet Posi-flow pump can deliver 32 gal/hr = 121 l/hr which will be enough. It turned out that the Facet fuel pump is extremely noisy! I don't think I can stand it and I probably need to find a more quiet one.

Generally it is not a good idea to just fit the biggest pump you can find because all that fuel flowing back and forth will increase the fuel temperature. High temperature could mean detonation problems or less power. As the stock CSR fuel system is a non-return type with the regulator in the back, that might not be a big problem as the fuel never goes that far. Also, the fuel tank is cooled by the wind. But anyway.

A one litre swirl pot in the trunk compartment. 

The classic Bosch 044 pump is too much with >250 l/hr @4 bar and 13.8V. It also uses a lot of power and is very expensive, at least the originals.

Biltema 52-203 pump flows a bit less with about 156-174 l/hr @4 bar and 13.2V (source). It is also very cheap and quiet!

Hoses and fittings

Rubber hoses tend to leak a few fuel molecules now and then. Teflon hoses doesn't. Instead they have the problem with build up of static electricity and small sparks can jump through the hose to the mantel and cause small holes in the hose and even cause a fire. To solve that problem some manufacturers have added a bit of carbon to the hose, allowing the electricity to flow and avoid this problem. The cheaper hoses (including Torques' hoses) doesn't have any carbon in them. Some say that the fuel flow in a car's fuel line is too small to cause any problems. I don't have enough knowledge to trust that statement, so I went the safe path and bought more ordinary nylon braided rubber hoses. If someone would like to calculate on this, I found a good source of information including formulas and samples.

I bought all AN6 hoses and fittings from Torques.

One challenge was to connect the system to the already mounted hard fuel line that goes through the car all the way to the engine. The fuel line's thread is 16x1.5mm. I couldn't find an AN-6 adapter that fit, so I used the existing fuel filter adapter and a Saginaw AN-6 adapter from Earls part number 9894DBJ.

The end result.

Doing the plumbing was much harder than I thought it would be. It was like a puzzle to get the pieces to fit in a small space without occupying the whole trunk. The braided hoses are very rigid and did not make it easier. I also realized I should have ordered more 90-deg fittings and less straight ones, but too late for that now. Also, holding things in place with zip ties is probably not considered the best engineering but will do for now.


Allright, I haven't tested this properly yet, but at least there is fuel flowing to the engine :-)

Oil filler cap lock

It has happened more than once that the oil filler cap jump loose and spew oil on the exhaust primaries.

Hopefully this will fix it.

Oil temperature sender

I'm reading a book right now that among other things has a lot to say about oil. One of the things it says is:

"An engine should never be driven hard until the oil reaches 70°C". 

And later:

"In a race engine, bearing failure is a possibility any time the oil temperature goes past 130°C, and generally hp is lost over 115°C".

That is interesting. The first quote goes without saying but the second one makes me concerned of something I've neglected.

The CSR already has a 1-pol oil temperature sender on the oil return line from the dry sump pump, but it goes nowhere, at least on my car. Now when I have this fancy logger I can easily log the oil temperature and display it on my Dash.

As the sender is 1-pol I used a 2.2kΩ pullup resistor to the reference +5V. So far so good, now it only needs to be calibrated.

Right...

As I'd already done the wiring I thought I do it in the garage right next to the car. I put some engine oil in a stainless steel cup and heated it on my camping stove while watching the voltage on the dash and the oil temperature using a kitchen thermometer. The idea was that it would give me real world values despite all tolerances and the wire's resistance. The problems were that the oil temp sender did respond much slower on temperature changes than the thermometer. Also the temperature differed very much between different locations inside the cup. I ended up heating in steps of about 20°C and then waiting a few minutes until the temperature has settled before reading the voltage and temperature. (No, the smell of hot engine oil was not very pleasant). Maybe a better approach would be to measure the sensor's resistance in the oven instead.

°C	Volt
11,7	3,260
37,6	1,882
57,7	1,070
79,2	0,600
97,7	0,358
116,5	0,204
136,5	0,126

I entered the values in the Analysis software that came with the logger and it gave me the formula:

-12,2 * x^3 + 74,2 * x^2 - 155 * x + 149

It is quite cool that the logger can calculate this real time and display it on my dash! But is it accurate?

Well not quite. It will give me a hint within maybe 5-10 °C and maybe that is enough. Regular interpolation would probably give better results. But why stop there?

Steinhart–Hart equation

(aka I got too much time because it is holidays and the children are sleeping)

The Steinhart–Hart equation is a model of the resistance of a semiconductor at different temperatures. The equation is:

After testing some different values I found that A=1,47E-3, B=2,40E-4 and C=8,77E-8 gave me a curve that looked more like my measured values.

Logarithmic chart of temp vs voltage.

Temp in °C = 1/(A+B*LN(R)+C*(LN(R))^3)-272,15
Where R = V*2200/(V-5)

So the final equation I use for my DL1 logger is:
1/(0,00147+0,00024*LN(-x*2200/(x-5))+0,0000000877*(LN(-x*2200/(x-5)))^3)-272,15

I think it works.