Thread: Why??

Originally Posted by isuzurover

Dougal, a colleague of mine used to be on the CFD team for Honda Formula 1 and also for several production cars.

I discussed the issue with him before my initial post. His words "flow fields around cars are extremely complex".

Indeed they are.

But the empirical measurement of the drag is quite straightforward and the only dominant atmospheric in those results is air density.

Hi Shane,

There are two issues when working on CD Carbies.

1) Is balancing between carbies - as shown ony your reply post whereby you adjust the idling on each carbie so they are sucking in air at the same rate.
There is a special balancing tool that you can buy to connect between carbies and has a guage that has "0" in the centre ...when you connect both carbies the one sucking more will pull needle to left or right of "0" and you adjust via the idle screw.
Most people as per video use a hose and their ear to adjust them, although I havent tried that as yet.

2) Mixture is done either by (I'm a bit fuzzy on earlier CD175 as I don't have them) by adjusting jets via an screw adjustment underneath float bowls I think?? where as later models like mine (81) are adjusted via piston dampners (using special tool) where you put in oil (refer my earlier post)

In order for the carbies to work efficiently you need to do both.

Maybe I give that a go this weekend.


Cheers

Baggy

Originally Posted by Dougal

Indeed they are.

But the empirical measurement of the drag is quite straightforward and the only dominant atmospheric in those results is air density.

If all my students were as pig headed I think I would shoot myself.



http://www.springerimages.com/img/Im...Fig24_HTML.jpg

Yes, clearly you are right and drag force has no relationship to viscosity (and therefore Re) and only changes with density. Furthermore the same BS genaralisations you find in 2nd year texts or cummins websites hold for space shuttles, trucks and cars...

I give up. Please continue to post though, all the fluid mechanists here are finding your posts hilarious.

I have no idea why you guys are going on about drag co-efficient... it's completely irrelevant IMO

This is how much you tell tell by how you can tell by the "look" of something.

Range Rover classic CD = .45
Jaguar E Type ... CD = .43
Citroen 2cv .... CD = 0.51

It really does seriously bugger all to do with how the old classic is running in hot weather ( FFS: a head wind will have far more effect).

Given the thing is running a ****load better around town at town speeds ....The drag co-efficient due to temperature will make as much difference to the way the cars as me taking a dump before I go driving it

seeya
Shane L.

Originally Posted by DoubleChevron

I have no idea why you guys are going on about drag co-efficient... it's completely irrelevant IMO

This is how much you tell tell by how you can tell by the "look" of something.

Range Rover classic CD = .45
Jaguar E Type ... CD = .43
Citroen 2cv .... CD = 0.51

...

You are right that drag coefficient or drag force is not the issue in your case.

I was just trying to point out to Dougal that his statement at the start is a foolishly oversimplified view of the influence of ambient temperature on how easy it is to move a car through the air. The graphs in my post above show (as you have posted), that the drag force vs reynolds number curves for different cars are completely different, and they are not the flat lines with respect to Reynolds number which would denote viscosity independant cases.

(Reynolds number is the ratio of inertial and viscous forces in fluid flow).

Higher temperature air decreases reynolds number slightly (due to the change in kinematic viscosity), which moves you slightly left on the above curves. In most cases CD increases, but it could decrease or stay the same depending on what speed you are travelling at, the type of car, mods to the car (bars/racks/lift/tyres/mudflaps), etc, etc, etc...

If you were travelling on the autobahn at top speed or freefalling through the atmosphere then the kind of relationships Dougal is suggesting would start to apply.

Originally Posted by isuzurover

You are right that drag coefficient or drag force is not the issue in your case.

I was just trying to point out to Dougal that his statement at the start is a foolishly oversimplified view of the influence of ambient temperature on how easy it is to move a car through the air. The graphs in my post above show (as you have posted), that the drag force curves for different cars are completely different, and they are not the flat lines with respect to Reynolds number which would denote viscosity independant cases.

Yep, and guess what, it all falls over as soon as you load a car and it squats in it's arse end ( not relevant for a Citroen or Classic with a working load leveler). But he impact the the drag co-efficient is enormous if a car is dragging it's @rse up the road

The work the went into getting something like the Citroen CX (named after the term for drag co-efficent Cd), in 1974 is immense. You just need to look *** under *** not at one to see the work that went into getting it down to 0.36 (yeah crap right). Throw some wider tires on it like the later models and it's all out the window anyway. No tailshaft, not diff, exhaust up into the floor cavity, air guide and undertrays front and back...

The talk of something simple like temperature of air friction versus "why is a car running a ****load better in the heat" ... versus all these other impacts on how well a cars travels through air is a bit like spitting into the wind with your eyes crossed hoping it will make a difference to how far you'll spit, and if it'll blow back into your face. Wind direction will have far more effect than temperature. Try towing a bloody caravan into the wind and away from it ... or on a hot or cold day and tell me which one you can pick the difference with

seeya,
Shane L.

Succinctly put......

Originally Posted by isuzurover

If all my students were as pig headed I think I would shoot myself.



http://www.springerimages.com/img/Im...Fig24_HTML.jpg

Yes, clearly you are right and drag force has no relationship to viscosity (and therefore Re) and only changes with density. Furthermore the same BS genaralisations you find in 2nd year texts or cummins websites hold for space shuttles, trucks and cars...

I give up. Please continue to post though, all the fluid mechanists here are finding your posts hilarious.

Pig-headed for pointing out a 10% change in air density?
Wow.

Your paraphrasing of my argument is completely incorrect and the link you provided is not accessible.

When your name appears in the text books re-writing the air drag formulae to include viscosity. Then I will happily accept you are right.
But right now, you're just digging down in an outward spiral trying to justify the strongly stated but completely wrong information you put in the first few posts. All the literature is against you.