Aerodynamic Formula

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Reverend Hedgash
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stability

Post by Reverend Hedgash »

A Mira?

There is a ute version called the Norkis, I dare you to race that!

The following have been our thoughts on track width for the tank. (Which we think we are calling the "Spirit of Sunshine" after its suburb of construction). Some of it may be relevent to you.

Final track width for us was primarily defined by our ford diff as we didn't wan't to spend the time or money cutting it down, so we made the front track width match that.

Apparently having the rear wheels of a narrower track improve straight line stability which we could do later if required.

Being a Lakester and having the wheels exposed to the air lead us to the following thoughts (correct me if I am wrong anyone):

Although having a narrower track means there is a smaller cross sectional area it is really only of the amount of the axtra axle exposed, ie a few square inches.

The effect of having the wheels closer to the body means that the turbulence from them will have a greater affect on that beautiful aero design of the tank (breaking laminar flow) which destroys the reason we want to use them in the first place.

Therefore it seems keeping the wheels out wide could improve the net aero shape, at the minor loss of of some cross sectional aero.

This has the advantage of being a more stable car in the event of a spin than a long sausage with wheels on the side. (The Rice Vigeant crash comes to mind although I believe its primary failure was axle breakage).

Keeping the wheels in line means a potential reduction in cross sectional area (compared to out of line) depending on the distance behind the front pair (and cross wind conditions).

For further stability we have kept our car's centre of gravity as low to the ground as possible, and in front of the centre of pressure (to be confirmed).

Our castor is 30degrees for steering stability as well.

Bless.

Rev. H

ps Thanks Dirty Dave for the info. I shall chase them up although we are having some success with our Gemini... It wouldn't do to have a new imported item in our car made of second hand aussie pieces!
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momec
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Post by momec »

Thanks Rev actually my Mira is the Vice Regal Norgis upgrade racing version fully tested in Brisbane traffic.

Decisions, wether I build a streamliner or a Comp coupe?
They will both be narrow and obviously plenty of narrow cars are built in the states on a narrow track and have been succesful

It was never to be a bellytank anyway. I tend to steer away from what others have already done so am leaning towards the Norgis Upgrade.
It will be different enough to keep my interest over what you guys know is a long build period.

I think a very heavy low c.o.g perimiter chassis (lead filled perhaps) as a basis. 30 degrees caster thats a truck load :o

I always go by if it looks right it is right and I can visualise a certain length Mira that would be stable.
I will chop the top straight up so it will only stand belly button height at the roof top anyway.

This thinkin is hurten my head :roll:
Seeya Chris
Acme Racing #251
Dr Goggles
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Different strokes

Post by Dr Goggles »

C Hanlon wrote:It was never to be a bellytank anyway. I tend to steer away from what others have already done so am leaning towards the Norgis Upgrade.
It will be different enough to keep my interest over what you guys know is a long build period.
I will chop the top straight up so it will only stand belly button height at the roof top anyway.
This thinkin is hurten my head :roll:
Seeya Chris



.......Mr Hanlon, this is no place for original thought , toe the line! :lol: :lol: ........personally I think you'll be pretty safe with the Norgis....style-wise I mean....as for stability? 'kknows til you try it .Are you gonna stretch the front like Burklands 120Y....4cam toyota 8.....is this the end of the ute?
...few understand what I'm trying to do , but they vastly outnumber those who understand why..
momec
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Post by momec »

Ohh don't you worry bout that no

I intend to push the crank out through the sump of the ute which I hope will take 3 years by which stage the Norgis will be ready.

Kathy intends to drive next year and once shes got the bug she'll just wanna keep going till she beats me but little does she realise but the tuneup can alter run to run and I control the tune up so we'll be racing till we're 70 at least :wink:

Get this I mentioned the hypethetical project to Kathy and she said well we'd have to put a trailer on the bus so we could take both, now from my side of the fence thats like saying go ahead and build the Norgis so I don't need much encoragement :lol: :lol:
Chris
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XW #901 180mph
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Post by XW #901 180mph »

Q1: Does anybody know much about wheelbase vs stability, ie would a Fairlane handle better than an equivalent year falcon at high speed as it is about 2" longer in the wheelbase and the XW is not very happy in the 170 - 180 mph range.

Q2: Also thinking about some Lead ballast on the rear floors next year but after my 175mph spin in 2005 I wouldn't like to have too much more weight leaning on the tyre (LH rear in this case) that had the majority of the damage done to it in the slide/spin.

Any Idea's

Regards Mark.
momec
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Lead

Post by momec »

G'Day Mark

Regarding the ballast, in my ute I ran four 3inch x 2 inch lead ballasts about 16 inches long bolted under the floor both sides just under and behind the seat area.
I can see no reason to remove them as ute was quite stable up to 175mph. As you know the salt can change morning to midday etc but the worst handling I've had is it wanting to use the whole track to stay in a straight line if you know what I mean.

I intend to add more ballast for next time just behind the existing ballast and about the same amount again. I'm also adding trac rods to control rear spring wind up which was giving me pinion angle shake/vibration

Bobs XP coupe is quite stable on the same wheelbase as me (which is a bit shorter than yours) and he is running over 200 mph

I don't think its a wheelbase issue causing yours to spin IMHO its more to do with body shape and aero lift at the rear as speed increases. More ballast should help this and/or reducing air flow under the car.

You can legally run a big airdam under the front to help reduce air flow under and this is something I'll be experimeting with as I go faster.

Keep at it
Seeya Chris
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DirtyDave
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Post by DirtyDave »

do you guys have a preferable weight distribution, from from to back, ether 50/50 of 60/40 ??
this would give you guys some idea of centre of gravity..
I assume the front naturally is much heavier and you would like to weight the rear of the vehicle for a more even weight distribution. :roll: :roll:
Don't Worry, It only seems Kinky the first time..
momec
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Post by momec »

Yeah I have no weight behind the rear axle and the motor has only two plugs foward of the front axle.
I have'nt done corner weights to confirm what mine is but your right. Its a battle with production bodies to get sufficient weight on the rear axle.
Centre of gravity is also formost in my mind as this helps everything even if the balance front to rear is out a bit.
Good question also side to side balance? Something that must be looked at, all adds to stability.
Chris
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Reverend Hedgash
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Weight distribution

Post by Reverend Hedgash »

Hey Mark et al,

The following is how I understand it:

Well placed ballast should reduce the liklihood of a spin in helping it become more stabile than it was.

For stability you really have to have your centre of gravity in front of your centre of pressure, regardless of car length.

Think of it like a dart with a weight up front and fins at the back; you can throw it back to front and it will end up straight.

For a rear wheel driven car having all the weight up the front like a dart this is less than ideal though as the weight is not efficiently utilised over the driving wheels.

Although adding weight is an aero drag free way to add downforce, it does increase rolling resistance and will slow your acceleration rate with the potential of not having enough track to get up to full speed therefore you want to use it in the right places overall.

Therefore it seems to make sense to position it in a place where the centre of gravity is ahead of the centre of pressure but far enough back to maximise its efficiency at getting the load to the driving wheels.

Centre of pressure can be moved rearword by the addition of fins at the rear, or other similar aero tricks but they may affect class.

As the good Dr.Goggles says, trying to get a car of your shape to go 200mph is like "trying to shove a chest of drawers up a frog's bum".

The pressure on the front is so high that the back just wants to step out.

With all that said 2" more would be an improvement if comparing cars of equal shape and weight distribution.

Please correct me if anyone sees some fuzzy logic.

Rev. Hedgash.
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momec
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Post by momec »

Goggles, what size frog should I practice on?
Can we use cane toads instead as we would be favouring the enviroment up here and have less hassle from the RSPCA.
Sounds like a much cheaper way to go testing, thanks for the tip Rev. :lol: :lol:
Chris
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Dr Goggles
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Kermit

Post by Dr Goggles »

......search for Sumner Patterson's site under "Purple Sage Trading Post"........he's got a lot of info from Tom Burkland's site as well and TB's right on the money when it comes to aero stuff.....he has the car streamliner record and reckons he has run with ( 4 wheek drive ) his wheels going 130 mph faster than his ground speed....and the car went straight...not many people could claim or survive that.. :shock:

....can't see any reason that cane toads shouldn't be pressed into service for our more than noble cause........

cleaning up the underneath of the engine bay and front floor can't hurt in moving the centre of pressure rearwards ........putting a front dam on might prevent air that will give lift getting underneath but it also may increase the apparent frontal area and move the centre of pressure forward........ ---k knows :? what the best solution is !!!!!!, Get out the rule book and get the XW on a hoist would be a start .....did anyone see the grill on the HK this year..... :wink: ...there's a tip......
...few understand what I'm trying to do , but they vastly outnumber those who understand why..
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Lynchy
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Post by Lynchy »

I've tended to use Dettol on the cane toads, being the sensitive man that I am, with the occasional use of 9 iron (it's important to get loft into the equation when clearing the ugly little fellas from the back yard). I have been known to eradicate them with the lawn mower when they stray into it's path, but this is the first I've heard of using a chest of drawers.

I have a couple of side tables and a chest of drawers that have been taking up valuable garage space that I had intended to donate to Vinnies but may try the Goggles theorem out prior to their passing. I guess I can always clean them up with the Dettol before they get picked up????
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