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Old 12-16-2010, 05:49 PM   #311 (permalink)
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COP and handling

Quote:
Originally Posted by euromodder View Post
Why would Jim consider adding area to the rear, while the centre of pressure is obviously still forward enough not to cause handling issues in sidewind conditions ?
A fin wouldn't cure an aft shift in CoP, it'd make it worse.

If not perfectly symmetrical to eliminate "lift", a vertical tail surface would add a permanent resultant sideways force working on the vehicle.

A fin also adds to the frontal area, and thus to the aerodynamic drag, the reduction of which is why the tail extension was built in the first place.
Been looking at this and have ended up on same side of fence as lunarhighway.
Low speed understeer is preferred by carmakers selling to drivers that might freak out in an oversteer condition. And rather than require opposite-lock steering and throttle modulation to control the drift,the driver simply 'steers' harder in the direction they want to go.
In Jim's situation,if the car IS breaking loose first at the front,then as the car yaws,it grabs more air,increasing the yawing moment,and conceivably,the car could go to an out of control situation as lunar has mentioned.
Kamm was obsessed with this concern and put fins on all his personal cars.His last project,a DKW station wagon was festooned with roof-mounted slotted fins.
Ditto, Jaray's fast cars.
Ditto ,Walter Korff.
Dr.Alberto Morelli put fins behind the rear wheels underneath the CNR car to guarantee a rearward COP bias behind the CG.That car generates zero lift,so un-weighting the front axle was not an issue.
With Jim's tail,I'm pretty sure there's separation under there and wake pressure ( vacuum ) is no doubt communicating forward under the tail,but I suspect that there is so much pressure bleeding between the flanks and underside,that a pressure moment arm,as in race cars, cannot take form.Tufting in that crucial area would tell us a lot.
Thinking out loud,I would be tempted to construct as deep a rear wheel fairings as approach/ramp/and break-over would permit,which in addition to streamlining the wheels,would help with the COP as well.
P.S. GM's Epcot car movable rear valance would cancel the separation and move the wake completely behind the car canceling any lift should it exist.

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Old 12-16-2010, 07:58 PM   #312 (permalink)
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Quote:
Originally Posted by lunarhighway View Post
....have you considdered experimenting with a dorsal fin? if i interpret your view of the situation correctly you assume the cars handling has remained stock even with the tail and therefore not an issue to be adressed...
Hi Lunar,

Yes, I am very glad that the tail has not moved the center of pressure too far rearward, and in fact the handling of the car in sidewinds seems to be the same as before.

Why is this important?

My biggest concern with the tail is making sure it continues to stay on the car. There were several posts by other EcoModders that had items coming their cars at speed, only to alarm the drivers behind them.

The tail has proved strong enough to withstand 60 mph sidewinds at this point which is a very good thing.

As far as making a fin on the back, this does give me pause, as I'm not sure that I would want to bother tuning this to get the right balance fore/aft.

Secondly, if the fin were large enough in area, what is the possibility that it would also force the car off the road if the sidewinds were high enough and I was still running over slick ice?

On the other hand, if someone else would like to go through the effort and build a tail with a large fin I would certainly follow their progress with an open mind.

Jim.

Last edited by 3-Wheeler; 12-16-2010 at 08:18 PM.. Reason: add the word "fin"
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Old 12-16-2010, 08:01 PM   #313 (permalink)
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Quote:
Originally Posted by Varn View Post
3 wheeler be careful or you may end up in the ditch during your under-steer moments.
Hi Varn,

Well if I were to end up in the ditch, at least I know that it would not be due to the addition of the tail, as I have not detected any change in handling so far.

If the ditch it is, then the winds would be so forceful as to wish for a safe parking place for the car until they pass.

Jim.
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Old 12-16-2010, 08:14 PM   #314 (permalink)
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Quote:
Originally Posted by aerohead View Post
...Low speed understeer is preferred by carmakers selling to drivers that might freak out in an oversteer condition.
Personally, I would prefer neutral steering in sidewinds.

Back in the days of rear-wheel drive cars, I used to really enjoy mildly "power sliding" the rear end of the car with a light touch on the gas pedal in the turns with snow on the ground. So in the this case, oversteer was preferred.

As you know, front wheel drive cars simply go straight and loose all steering if the front wheels are going faster than the car in loose conditions. Dry pavement is a whole other matter.

Quote:
Originally Posted by aerohead View Post
...With Jim's tail,I'm pretty sure there's separation under there and wake pressure ( vacuum ) is no doubt communicating forward under the tail,but I suspect that there is so much pressure bleeding between the flanks and underside,that a pressure moment arm,as in race cars, cannot take form. Tufting in that crucial area would tell us a lot.
I tend to agree that the rear of the car has a lot of flow separation, since the underside is way past the ideal of 4° taper or so.

There should also be minimal lift/down force from wind approaching the car from the sides, as there are sharp creases on the top and bottom surfaces to break-up attached air in those locations. This is by design.

Quote:
Originally Posted by aerohead View Post
....Thinking out loud, I would be tempted to construct as deep a rear wheel fairings as approach/ramp/and break-over would permit,which in addition to streamlining the wheels,would help with the COP as well....
Thanks for the confirmation AeroHead, as when the spats are added next year, I will keep your comments in mind. This is certainly nice to know!

Jim.
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Old 12-17-2010, 04:10 AM   #315 (permalink)
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my initial reason for bringing it up was that your car could accomodate such a setup and it might benefit you, but of course, it's your car and if you're happy with the handling i won't loose any sleep over it either I just figured it might make this an even better car than it already is. The size of fin i envisioned would be something that starts on the rear window and than perhaps stay parallel to the road as the tail papers down or even smaller.

Quote:
....Thinking out loud, I would be tempted to construct as deep a rear wheel fairings as approach/ramp/and break-over would permit,which in addition to streamlining the wheels,would help with the COP as well....
this is indeed a smart comment combingin may functions in one shape so no drag is added.
i have very small finlets behind my rear tires to act as semi boat tails and while given their size their impact is likely minimal and my measuring is to inacurate to attribute any changes in fe or stability to them, neither have decreased either and crosswind stability is very good
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Old 12-17-2010, 07:45 AM   #316 (permalink)
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I agree that tuft testing will be key, and as Darin's brief experiment with wheel strakes showed, they will help a significant amount. And they should act to stabilize things by improving the flow underneath.

This car is the culmination of everything we have been learning here on Ecomodder. And the workmanship is wonderful. With a couple of good photos, this project deserves a place front and center on the Ecomodder homepage.
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Old 12-17-2010, 01:19 PM   #317 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
I agree that tuft testing will be key, and as Darin's brief experiment with wheel strakes showed, they will help a significant amount. And they should act to stabilize things by improving the flow underneath.

This car is the culmination of everything we have been learning here on Ecomodder. And the workmanship is wonderful. With a couple of good photos, this project deserves a place front and center on the Ecomodder homepage.
Neil,

Thanks for the kind comments, and I was thinking that Darin was waiting for the finishing sanding work to be done this summer before asking to post pictures on the home page.

Jim.
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Old 12-17-2010, 02:00 PM   #318 (permalink)
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Hi Jim,

I'm sure a main page post (or full article!) could be updated with "finished" photos?
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Old 12-17-2010, 03:00 PM   #319 (permalink)
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I get through the items on my to-do list at about the same rate Jim constructed the tail.

However, it'll definitely get there!
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Old 12-17-2010, 03:48 PM   #320 (permalink)
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Quote:
Originally Posted by aerohead View Post
Been looking at this and have ended up on same side of fence as lunarhighway.
If we start from the basic Insight, then adding a tail extension (i.e. more area at the rear) surely is going to shift the resulting pressure point for a sidewind further aft.
Fortunately, the design of the extension is such that the wind can't get too much hold on it.
The CoG is also slightly moving aft by adding more weight at the rear, so the effect of having more area at the rear is at least partly compensated for.

The extreme situation would be adding so much area to the rear, that the car starts to severely weathercock and would become undriveable.
This is a situation where one might end up by adding a vertical fin to the extension.
It'd add a lot of area for a sidewind to work on and cause additional stress on the boattail's structure.
Same goes for the additional weight of a vertical fin.

I wouldn't do it.

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