cutting holes in your rear bumper

[Christ]

Not sure where you got that from, honestly.

Cutting ANY hole in the bumper represents a collusion to the coanda effect. It WILL increase air turbulence more than a smooth transition from one point to another, and create vortices at the edges of the hole. That's not a point that can be argued.

Also not to be argued is that cutting holes in the bumper will not produce any effect at all. It will lessen another effect, which is where you're apparently confusing terminology.

I agree with you that cutting the entirety of the parachute portion of the bumper cover is the best option, short of cutting and smoothing the transition. I never said differently, that I recall.

Those points made (again), I still don't see the relevance of your tirade.

[winkosmosis]

Originally Posted by Christ

Not sure where you got that from, honestly.

Cutting ANY hole in the bumper represents a collusion to the coanda effect. It WILL increase air turbulence more than a smooth transition from one point to another, and create vortices at the edges of the hole. That's not a point that can be argued.

Also not to be argued is that cutting holes in the bumper will not produce any effect at all. It will lessen another effect, which is where you're apparently confusing terminology.

I agree with you that cutting the entirety of the parachute portion of the bumper cover is the best option, short of cutting and smoothing the transition. I never said differently, that I recall.

Those points made (again), I still don't see the relevance of your tirade.

What tirade? I was just complaining.

If the holes are big enough, the effects you're talking about won't matter much.

I don't know what you're talking about with the producing vs lessening effects. What effect did I say the holes would produce?

[aerohead]

They're wrong,it doesn't work that way,and they'd have lower ETs if they sealed 'em back up.
There will certainly be found a different explanation for the lower times.

[Christ]

winkosmosis -

The paper you linked suggested that you were expecting the holes to act as an underbody diffuser, to increase rear downforce, when it would in fact not happen that way.

IF the holes did anything positive, and I have my doubts, it would be nothing more than lessening the effect created by a parachute of the rear bumper area. I still have ill feelings when I think that even that could be achieved without doing more harm than good. Airflow doesn't like to be redirected, let alone through jets that would increase the pressure of the flow. If you're forcing air through a smaller area than it's already in, you're increasing its pressure. The energy required to force the air through the smaller area has to come from somewhere. Your car's inertia provides the energy.

I *once again* agree with you about cutting the bumper to remove the overhanging area and combine it as a flat upward slope from a lower section of the body. That would be the definition of a rear belly pan, though, only modified to remove the same section of the bumper that we would normally just cover up.

[aerohead]

The original boat tail for my CRX had an open duct,patterned after the Mustang GT.
At Bonneville,I learned that by completely taping it over I realized a higher top speed and significantly better mpg on the return trip home.
The opening encouraged air to go through a torture chamber,and allowed air that would have otherwise flowed lower and unobstructed,to also collide with the undercarriage.
You''l notice that most modern autos incorporate a very deep valance below the rear bumper,which extends down to intersect the imaginary 2.5-4-degree diffuser angle.
The valance allows air to "pool" against the underside of the car,"drowning" all the under-hanging components in a mass of stagnant air which is dragged along with the car.
All the air passing beneath,essentially skips off this stagnant pool,as if it were a bellypan,passing below all the obstructions,with a straight shot to the rear.
By cutting the bumper open,you destroy what cost millions of dollars to create in the wind tunnel.
SAE journals and transactions and Hucho's book are good places to look for "science" rather than anecdotal heresay.

[Frank Lee]

Those holes are the rice equivalent of a net gate and we know all about net gates right?

[aerohead]

Originally Posted by aerohead

The original boat tail for my CRX had an open duct,patterned after the Mustang GT.
At Bonneville,I learned that by completely taping it over I realized a higher top speed and significantly better mpg on the return trip home.
The opening encouraged air to go through a torture chamber,and allowed air that would have otherwise flowed lower and unobstructed,to also collide with the undercarriage.
You''l notice that most modern autos incorporate a very deep valance below the rear bumper,which extends down to intersect the imaginary 2.5-4-degree diffuser angle.
The valance allows air to "pool" against the underside of the car,"drowning" all the under-hanging components in a mass of stagnant air which is dragged along with the car.
All the air passing beneath,essentially skips off this stagnant pool,as if it were a bellypan,passing below all the obstructions,with a straight shot to the rear.
By cutting the bumper open,you destroy what cost millions of dollars to create in the wind tunnel.
SAE journals and transactions and Hucho's book are good places to look for "science" rather than anecdotal heresay.

I found my timing slips from Bonneville.
The boat tail with Mustang-style open duct added 1.21 mph to the top speed.
When I blocked the opening with cardboard and duct-tape I saw a 5.51 mph increase with the tail.
If you stand far to the rear of an automobile and can see any of the undercarriage components,it's valence is NOT LOW ENOUGH.
Drop it down even with the floor of the trunk at least.It will NOT act like a parachute.The air in front of it is stagnant.It has very little kinetic energy.

[vtec-e]

So low hanging bumpers actually create a virtual rear bellypan then? The conventional wisdom here indicates that it acts as a parachute. Which makes sense. But so does the virtual bellypan.
Where have i gone wrong in my thinking? What am i missing?

ollie

[aerohead]

Originally Posted by vtec-e

So low hanging bumpers actually create a virtual rear bellypan then? The conventional wisdom here indicates that it acts as a parachute. Which makes sense. But so does the virtual bellypan.
Where have i gone wrong in my thinking? What am i missing?

ollie

Ollie,imagine that the air captured within the boundary between both bumpers and the rocker panels just travels with the car.Like a pool of water upside down,with no influence from gravity.
If air WERE moving there,it would be like a literal torture-chamber for the air,with nothing but separated flow and mass quantities of turbulence.
By "capturing" air underneath there,all the air flowing below,will pass by undisturbed,with an un-obstructed path to the rear where it can inject it's kinetic energy into the wake to dilute it's drogue effect,That's where the drag is,behind the car.
If you cut a hole,now you've let the air out of the dam and re-opened the torture-chamber.And then there is the chance that ALL the air under the car will be turbulence,adding to the wake resulting in higher drag.

[Christ]

Is this application specific?

I mean, vehicles with higher ground clearance aren't going to exhibit the same behavior, are they?