Underbody panel testing

[RobertSmalls]

Originally Posted by jimepting

It does look to me like there is adequate clearance on the torsion beam rear axel to use one large coroplast sheet to cover up everything remaining open, but JMO.

Jack the car up, off its rear wheels, to find out what the suspension looks like near maximum deflection. Options include a flexible panel spanning the axle, or an underbody panel mounted to the axle.

As to whether the performance improvement would be measurable, well, how good is your measuring technique? Also, a boat-tailed Insight is more sensitive to airflow at the aft underbody than most cars.

On the subject of diffusers, MetroMPG posted a promo video a while back that accompanied the release of the Insight. It was not written for a technical audience, but the voiceover guy mentioned that "the coefficient of drag was reduced by keeping frontal area to a minimum"*, and the (7° by my measurement) slope behind the rear axle "reduces turbulence underneath the car".


*Not as bunkum as it sounds. Keeping height to a minimum improves fineness ratio and Cd, not just CdA.

[jimepting]

Originally Posted by RobertSmalls

Jack the car up, off its rear wheels, to find out what the suspension looks like near maximum deflection. Options include a flexible panel spanning the axle, or an underbody panel mounted to the axle.

I ran this experiment slightly differently, but the bottom line is; there is plenty of clearance. I jacked up one side until the tire just cleared, the theory being that this represents the limit of weight transfer, and took the clearance measurement. There is 1.5-2 inches of clearance remaining, so a straight forward large panel covering the entire rear works.

[QUOTE]As to whether the performance improvement would be measurable, well, how good is your measuring technique? [/UNQUOTE]

I've been doing some "testing of testing," but I remain convienced that small incremental improvements will be below the measurement noise on a FE twichy car like the Insight. An approach which tests several "known" improvements at one time has its own shortcomings, but the combined improvements will probably be measurable.

[QUOTE]On the subject of diffusers, MetroMPG posted a promo video a while back that accompanied the release of the Insight. It was not written for a technical audience, but the voiceover guy mentioned that "the coefficient of drag was reduced by keeping frontal area to a minimum"*, and the (7° by my measurement) slope behind the rear axle "reduces turbulence underneath the car".[/UNQUOTE]

I pretty much agree that using a rear belly pan to smooth up the rear and extend the effective area is going to make the diffuser area more effective, particularly if you install a boattail. The lower spring perches and the lower shock mounts will protrude, but even these could be covered by small bulges.

[Nevyn]

Some of the effects of increased drag may also be due to Venturi effect, maybe?

[Otto]

Assuming the parachute effect with panel 1 in place is correct, i.e., panel 1 prevents air from properly escaping the engine compartment, how about:

Use aluminum window screen mesh, held taught on a frame, as panel 1.

That way, the mesh lets cooling air escape from engine compartment, preventing parachute effect, but the flat, taught surface would smooth airflow from front to back. The alu mesh would also withstand exhaust pipe heat, and being an excellent heat conductor would dissipate hot spots.

[MaxHedrm]

I was considering a piece with louvers cut in it for that reason. that would also actually create a bit of suction to help remove the air from the tunnel.

Looking again a the drawings, I'm not sure that a parachute effect is the issue. There should be plenty of room for air to escape from the gap left aft of #4 for the front suspension.