Front Splitter advise

[wadsapple]

I am modifying my front splitter to make it 15mm thick with a blunt front edge,it was just made from a sheet of 4mm with a rounded edge before.Questions as follows:

Why does it have to have a blunt front edge.
Why does it work best being 15mm thick,
Does the 15mm thickness have to be all over the splitter.(I am just modifying mine to have the blunt front leading edge and i am glueing on a strip of plastic that is about 100mm wide if you understand me creating the front edge and giving the first 100mm of the splitter the 15mm thickness )

I gleened the thickness and the blunt edge info from the Vickers project mazda car in "racecar engineering"

[tapkaJohnD]

Being blunt. How would you feel if it was sharp and struck a paddock person in the ankle?

15mm. Can't think why it MUST be this thickness - airflow on either side is independent. Could it be that an extra 11mm nearer the ground bought some ground effect? It becomes critical (very influuenced by small changes) as a surface approaches the ground. Glueing thickness on top of the splitter may not help, unless you are too near the ground clearance regs of your class.

JOhn

[indigorallye]

I recently emailed Simon McBeath and he was very helpful

[kimducati]

If you sign up online for Trackdriver magazine (its free) I think you can access back issues online.
Simon Mcbeath has been doing a series of articles on aerodynamics, and goes into splitter design in some depth.

Kim

[stacy]

Well, as its one of my cars we are talking about... =:-o

As has been mentioned do look at Simon's articles or even his book on aero. He's far more knowledgeable than me so I don't claim to understand, but yes the advice was to have something thicker than I was expecting. Specifically the advice was 15-18mm, for it to protrude as far as I could get it, and crucially to take it as far back as I could too.

I will dig out the specifics but some of the thickness was about rigidity and stopping it vibrating and flapping about. If it was to be carbon then presumably it could be thinner. Other specific advice was not to sharpen it to a point, but just to round the edges. Again - I don't know why, air can be confusing - for example a flat front at the sides can generate twice the downforce of a shallow ramp which is counter intuitive.

Will see if I can get a link to the relevant articles somewhere. TrackDriver is about to have a web revamp with extras so its a discussion going somewhere.

S.

[wadsapple]

Thank you Stacy,more than some help,post up what you can it's a mind field this air flow thing

[Count Johnny]

stacy said:

...be thinner. Other specific advice was not to sharpen it to a point, but just to round the edges. Again - I don't know why, air can be confusing...S.

I suspect - without knowing too much - that it may be because sub-sonic air doesn't like sharp edges.

Here's our own, home-grown, effort:



The splitter's probably 25mm or more thick and the forward edge is radiussed on the underside. The splitter probably extends up to 150mm (or more) from the junction with the front clamshell. As you can see, we have BIG diffusers which do a lot of the work.

Without getting too complex about things, we get over 130kg of front downforce at 137mph (nicely balanced by a quarter of a tonne at the rear).

[Count Johnny]

PS

Splitters (proper ones) need to be STRONG. I'm talking a big heavy bloke jumping up and down on it STRONG.

When we built splitter MK1 and the associated support structure, it simply wasn't strong enough and would allow the splitter to touch down, instantly killing the downforce, thus allowing it to pop up, instantly creating downforce, which then allowed the splitter to touch down (again) instantly killing the downforce (and so on) turning the car into a very scary high speed pogo stick.

For underbody aero systems, like those on our car, dynamic ground clearance and pitch are absolutely critical and we put a lot of effort into managing these when the car is in motion.

[X SP]

Count Johnny said:

PS

Splitters (proper ones) need to be STRONG. I'm talking a big heavy bloke jumping up and down on it STRONG.

Funny you should say that, I once did some "high speed testing" in a TVR Tuscan S and had the front splitter rip off at 1.7 Leptons. The fear was so great I don't think I even had time for a brown trouser moment but thankfully the car stayed on the road. Shocking piece of under-engineering!

[wadsapple]

This is where i'm at so far.I have increased the leading edge thickness to 16mm extending 100mm back from front edge.What do we think ?

This shows the splitter underside..



This shows the splitter from ontop so to speak with the air dam joining the bonnet...



This shows the plastic edge i have used to thicken the splitter which is made from strong 3mm Aluminium...



This is the car in question for reference.......

[tapkaJohnD]

I think you need to maintina the smoothness of the underside for good effect. The step behind your thickening will induce turbulnce and increase pressure just where you wnat to keep it down.

A very thin panel of alloy on top (or rather beneath) your thickening would smooth flow and give enormous stiffness to the whole assembly. Simon McBeath himself writes of filling such a thing with expanding foam, but being careful to allow it to vent, else you will end up like this



Uploaded with ImageShack.us

[stacy]

WHS

S.
My kind of car..

[wadsapple]

tapkaJohnD said:

I think you need to maintina the smoothness of the underside for good effect. The step behind your thickening will induce turbulnce and increase pressure just where you wnat to keep it down.

So what about thickening the top of splitter from leading edge to the air dam and taking off my first attempt from underneath if you get my drift.Saving the weight of another skin of ali and the cost as well.The splitter was flat all over on the underside before i stuck my thickening plastic on it so taking it off and putting it ontop instead should do the same thing do you think

[Count Johnny]

wadsapple said:

So what about thickening the top of splitter from leading edge to the air dam and taking off my first attempt from underneath if you get my drift.Saving the weight of another skin of ali and the cost as well.The splitter was flat all over on the underside before i stuck my thickening plastic on it so taking it off and putting it ontop instead should do the same thing do you think

Obviously, that will alter your ground clearance, but I doubt that this is critical on your car, as I think you will be seeing very little (if any) ground effect that would translate into downforce.

[andylaurence]

Putting the plastic on top will mean more air going under the car, which translates to more air going through the diffuser, if you have one. Of course, it'll also change the expansion ratio of the diffuser. If you have any underbody aero, I expect you're better off moving the plastic to the top of the splitter. If not, then you need some underbody aero as that'll make much more difference than the thickness of the splitter! For reference, my splitter is 6mm ply with a laminate of one layer of fibreglass cloth each side, a 15mm tube laminated into it to stiffen laterally and 1.5mm aluminium sheet folded to angle to add bracing. It's pulling out the rivets holding the aluminium sheet in place, despite the body also being bolted to the leading and trailing edges to help with stiffening. Make it strong!

[wadsapple]

Count Johnny said:

Obviously, that will alter your ground clearance, but I doubt that this is critical on your car, as I think you will be seeing very little (if any) ground effect that would translate into downforce.

I take it that you think splitter needs to be lower to the ground,other wise i'm not with you why you think i will not get any downforse from my splitter.

[wadsapple]

andylaurence said:

Putting the plastic on top will mean more air going under the car, which translates to more air going through the diffuser, if you have one. Of course, it'll also change the expansion ratio of the diffuser. If you have any underbody aero, I expect you're better off moving the plastic to the top of the splitter. If not, then you need some underbody aero as that'll make much more difference than the thickness of the splitter! For reference, my splitter is 6mm ply with a laminate of one layer of fibreglass cloth each side, a 15mm tube laminated into it to stiffen laterally and 1.5mm aluminium sheet folded to angle to add bracing. It's pulling out the rivets holding the aluminium sheet in place, despite the body also being bolted to the leading and trailing edges to help with stiffening. Make it strong!

Hi Andy,

Thanx for that i do have a rear diffuser of sorts going on at the back and will be adding some tweeks to that before next time out which is only 4 weeks away.I do intend to put side skirts on at some point but don't want to get too lost yet as it is a new build car with only 3 outings so far so i am still trying to get a handle on it.


Here is the rear of the car....

[tapkaJohnD]

Did say I didn't think that there was much GF to be got there, but there is so much air going through, and limiting it will tend to accelerate what's left. Bernouille should do the rest, right under the car, if wadsapple's smoothing will kept it laminarish.

But come on, admit that apart from Adrian Neway who can SEE the air, we are all guessing a bit.
John

[Count Johnny]

wadsapple said:

I take it that you think splitter needs to be lower to the ground,other wise i'm not with you why you think i will not get any downforse from my splitter.

Apologies. I wasn't saying was that you wouldn't get any downforce from your splitter. I was just saying that any appreciable downforce was unlikely to be as a result of any ground effect and was more likely to arise from stacking air on the upper surface - meaning that, in your application, ride height was less critical.

[Count Johnny]

tapkaJohnD said:

But come on, admit that apart from Adrian Neway who can SEE the air, we are all guessing a bit.John

Absolutely. Aero systems are complex, have to work in harmony to be effective and we were (50%) fortunate that we achieved BALANCED downforce with our car.

The other 50% came from listening to lots of advice, thinking about things A LOT, spending a good few grand on consultancy, even more on getting or making the right bits and lots of aero testing with potentiometers on our suspension playing with ride height, rake and flap angles. And, still, we may be missing out on available dowwnforce because we don't actually know what the air is doing - some modelling and CFD work is planned for the end of the 2013 season.

My point with the downforce from ground effect thing was not that groung effect would not exist - it would of course - but merely that at a ride height of what looks like 100mm, it would be negligible and would not be aided by an impossibe to tidy underfloor.