Splitter

[Soul Reaver]

Hi Guys

I want to build a splitter this weekend for the Audi but have a couple fo questions.

1. Can I use 8mm marine plywood and then cover it with carbon vinyl or is there a better material to use to make it from?

2. How far does it need to extend out the frot of the car from the end of the bumper?

3. Does it need a rake on it? Pointing up or poitning down?

4. Do I take the centre section back to the subframe to give it extra rigidity?

5. If I use ply then the top I guess should be covered in thin ally to protect it from heat?

[andylaurence]

8mm is going to bend at speed. I've got a 6mm ply splitter with a laminate of fibreglass woven cloth on each side, a 20mm aluminium tube for stiffness and bolted to the body. It's about as stiff as the 12mm plain plywood one it replaced and it scrapes at speed from the leading edge bending down. The great thing about plywood is how cheap it is to buy and to shape though. If you knock it off on a kerb, just cut out a new one.

9 times out of 10, longer is better with diminishing returns. 6 inches wouldn't be too long. The air heading towards the car is going in a downwards direction at that height, so a slight upwards stance is complementary. Don't overdo it though. The splitter should ideally be the start of a full flat floor, so take the flat floor back as far as you can. All the way to a diffuser at the rear is ideal. I'm not sure what heat you'd need to protect it from - what's nearby?

Here's an example of a splitter that has a front diffuser element integrated into it leading to a flat floor and a rear diffuser. Cheap and simple...

[Soul Reaver]

Hiya

Thanks for the response, great. Ill go for 8mm marine ply then and reinforce both sides with cloth and resin and then wrap it in 3m carbon vinyl for effect on the top.

I just thought if it's near the downpipe it might get too hot and burn? If I bolt to the ally subframe on the Audi S3 and then again on the bottom of the bumper it will have a gap of about 8 inches to the downpipe and about 2 inches to the sump.

I was going to use ally sheet for the flat floor? What thickness though? 1mm or 1.5mm? The I was going to buy a universal carbon diffuser and make it fit.

Cheers
Mark.

[wadsapple]

You might find this thread I started a while ago a good read


http://www.pistonheads.com/gassing/topic.asp?h=0&a...

[Soul Reaver]

This is a very complicated area so my next question is

Am I going to notice anything on a 200bhp/ton at the wheels Audi to make it worth the effort other than it all looking rather cool

Cheers

[andylaurence]

Power at the wheels has little to do with whether aero is important or not. Lower drag means you're faster down the straights and more downforce makes you faster in the corners, irrespective of how much power you have. You can always improve one of them and sometimes both! Of course, your all-up weight is triple mine, so the impact will be around a third. 100kg of downforce will (rather simplistically) add 20% more grip to my car, but will add less than 7% more grip for you. What difference it makes to you depends on the events you typically compete at. If your average apex speed is 25-35mph, then it's unlikely to be worth the investment as you'll see very little downforce, but if you're cornering at 80-100mph, then you'll likely see a return. What's the car used for and what's the current spec?

[Soul Reaver]

Hi Andy

That all makes sense. The car will be used for team Trophy 60 minute races.

[Count Johnny]

Soul Reaver said:

Am I going to notice anything on a 200bhp/ton at the wheels Audi...

If your splitter is working, you might find yourself the driver of a very oversteery car in high speed corners (which is less than desirable) unless you do something serious about balancing it at the rear.

As I am wont to say - when these subjects come up - balance, balance, balance.

Any twonk can stick a big wing, or a mahoosive splitter, on a car and get some downforce. The trick is to balance the downforce (front to rear) and to create downforce with a minimised drag penalty.

[Soul Reaver]

Count Johnny said:

If your splitter is working, you might find yourself the driver of a very oversteery car in high speed corners (which is less than desirable) unless you do something serious about balancing it at the rear.

As I am wont to say - when these subjects come up - balance, balance, balance.

Any twonk can stick a big wing, or a mahoosive splitter, on a car and get some downforce. The trick is to balance the downforce (front to rear) and to create downforce with a minimised drag penalty.

Ive bought a rear wing too. How do you go about finding the optimum balance between the two ends?

[andylaurence]

I guess team trophy means big circuits, so generally reasonably fast corners. There is more benefit to downforce there, of course.

Finding a balance can be done subjectively or objectively. Subjectively, if you're getting understeer at low speed and oversteer at high speed, then you've got more downforce at the front than the rear. Oversteer at low speed and understeer at high speed would mean more downforce at the rear. What's important is that the front-rear balance of the downforce matches the front-rear weight distribution of the car.

Objectively, you can measure sprung downforce with a data logger and suspension pots. If your S3 is available with HID lights from the factory, then there will be sensors on the suspension already to run the auto-levelling headlights and if you're really lucky, they'll be analogue. If not, get some MINI units, which are about £50 each and produce a lovely 0-5v output for your logger. Correlate average damper displacement with speed and you will see how much downforce you're getting as speed increases. You can use bags of sand to see what this equates to in kgs if you like!

As a half-way measure, you could objectively measure oversteer and understeer with a data logger, either directly with a yaw sensor or through derivation of yaw.

[Web13]

Soul Reaver said:

Ive bought a rear wing too. How do you go about finding the optimum balance between the two ends?

Wind tunnel or more realistically just by driving with different set ups and finding a good balance in the high speed stuff.

In simple terms, high speed oversteer you need more on the back or less on the front, high speed understeer and you need less on the back or more on the front.

[wadsapple]

Don't forget if you are adding down force you will need to go up on the springs to compensate

[Web13]

wadsapple said:

Don't forget if you are adding down force you will need to go up on the springs to compensate

Honestly, I would put it on and drive it first. You wouldn't change the springs for a passenger or a high fuel load? I would be surprised if your whole areo package is giving you more than a 100kg flat out, never mind mid corner and that's before you take lift into account. If you make too many changes at once your going to risk getting lost in your set up and find yourself confused about what has improved or upset the handling of the car.

Edited by Web13 on Wednesday 4th December 12:29

[Count Johnny]

andylaurence said:

Objectively, you can measure sprung downforce with a data logger and suspension pots. If your S3 is available with HID lights from the factory, then there will be sensors on the suspension already to run the auto-levelling headlights and if you're really lucky, they'll be analogue. If not, get some MINI units, which are about £50 each and produce a lovely 0-5v output for your logger. Correlate average damper displacement with speed and you will see how much downforce you're getting as speed increases. You can use bags of sand to see what this equates to in kgs if you like!

This

[Greensleeves]

http://www.aprperformance.com/index.php?option=com...

Hardly measurable results below 80 mph. If you can get your Audi to corner at 120 then all the hard work is worth it.

At entry level / club race level, your time and money would be better spent on tuition or circuit learning, unless you want to do the mods for cosmetic reasons, then knock yourself out. Don't know enough about you to know if you're close to winning and trying to shave a tenth off or running a couple of seconds off the back and trying to keep up?

[Soul Reaver]

Greensleeves said:

http://www.aprperformance.com/index.php?option=com...

Hardly measurable results below 80 mph. If you can get your Audi to corner at 120 then all the hard work is worth it.

At entry level / club race level, your time and money would be better spent on tuition or circuit learning, unless you want to do the mods for cosmetic reasons, then knock yourself out. Don't know enough about you to know if you're close to winning and trying to shave a tenth off or running a couple of seconds off the back and trying to keep up?

I did win twice in TDT and had a second so I guess im not too bad but I watch the videos and just think I'm crap and make loads of mistakes all over the place LOL. TT is more competitive with some good cars at the front so I want all the advantages I can get for next year.

Started making it this weekend and also fitting the rear wing. Both look good although not finished yet. Quite a bit of work involved. need some custom brackets making now to support the splitter from the front subframe arms.

[Count Johnny]

By the way...a bit more on balance.

This is a complex area and of you can also find that your rear wing is producing much more downforce than your splitter - cue high speed corner understeer and instability under braking.

Making a splitter that really works on a production car is really difficult (little or no diffusion, little or no interaction with the ground, etc) and - even on a sports racing car - it can be still be tricky to achieve. If you subscribe to Racecar Engineering, you can go through your back issues and find Simon McBeath's article concerning wind tunnel testing an ADR, wherein you'll learn that they actually had to remove the rear wing, in its entirety, to achieve anything like a workable balance.

Obviously, changing your wing's angle of attack is a means of 'tuning' your rear downforce - and a second element, if you have one, is another tuning tool but all of this takes lots of testing (albeit, cheap testing on a nice flat airfield).

Basically, what I'm saying is that it's very easy to end up with a car that is slower down the straights and a pig under braking and in the corners.

Edited by Count Johnny on Sunday 8th December 10:49

[wadsapple]

Web13 said:

wadsapple said:

Don't forget if you are adding down force you will need to go up on the springs to compensate

Honestly, I would put it on and drive it first. You wouldn't change the springs for a passenger or a high fuel load? I would be surprised if your whole areo package is giving you more than a 100kg flat out, never mind mid corner and that's before you take lift into account. If you make too many changes at once your going to risk getting lost in your set up and find yourself confused about what has improved or upset the handling of the car.

Edited by Web13 on Wednesday 4th December 12:29

Not saying your wrong not to get confused with too many changes at once. I went from 1:12.5 round Castle combe at the start of 2013 at the last race of GT's I went round in 1:09.93 I'd have to say that aero even if it doesn't do much physically certainly did mentally for me and my racing, ended the season as class D champion and missed the overall championship by one point loosing out to a sports prototype cars. My front spring rate went from 750 up to 1200 over the year because of grinding the front out under braking. What I would say is what works for one car may not work for another. The pictures show what I started with and what I ended with at the close of this years racing.

[andylaurence]

Count Johnny said:

Making a splitter that really works on a production car is really difficult (little or no diffusion, little or no interaction with the ground, etc) and - even on a sports racing car - it can be still be tricky to achieve. If you subscribe to Racecar Engineering, you can go through your back issues and find Simon McBeath's article concerning wind tunnel testing an ADR, wherein you'll learn that they actually had to remove the rear wing, in its entirety, to achieve anything like a workable balance.

I think that says a lot about the front end on the ADR at that point in time. The rear diffuser and (dual plane) rear wing were an effective combination, but the front diffuser was not good at all and was severely compromised by the sidepod design. That re-inforces your point about generating front downforce on a saloon, of course! I've added lots of front downforce to mine and reduced the drag at the rear by lowering the wing. Hopefully, when I get to the track in the new year, I'll have a reasonable balance...

On the subject of negligible difference below 80mph, citing the figures on the APR performance website, that's about 35kg of downforce at 60mph from a single plane rear wing. If that was matched at the front on a 1500kg car, that's a 2.3% increase in normal force, which is around the same increase in grip. To put it another way, that's about 0.7mph for the same car or more than 30cm gained every second. For a lighter car, the gains increase massively. The same absolute improvement gets me 2mph, which is a gain of 90cm every second - that's a car length in a single corner. Worthwhile, methinks.

[Web13]

wadsapple said:

Not saying your wrong not to get confused with too many changes at once. I went from 1:12.5 round Castle combe at the start of 2013 at the last race of GT's I went round in 1:09.93 I'd have to say that aero even if it doesn't do much physically certainly did mentally for me and my racing, ended the season as class D champion and missed the overall championship by one point loosing out to a sports prototype cars. My front spring rate went from 750 up to 1200 over the year because of grinding the front out under braking. What I would say is what works for one car may not work for another. The pictures show what I started with and what I ended with at the close of this years racing.

I do wonder looking at your photos if the drop in ride high is on of the main causes of it grounding out? The nose dropping under braking just causes it to touch the floor. Rather than any massive increase in areo increase on the front axle?

I found it to be a strange sensation when the splitter is on the floor on the brakes and turn in, I seem to get more areo grip from the ground effect until I started to bleed off the brakes and the splitter came away from floor and the nose would wash out. Did you get a similar sensation? just curious really.