BT Tech spoiler

[Bob Allen]

I noticed the BT Tech spoiler ad on the side bar of this page. I checked out the photo and have a few observations that are more like questions.

I'm a flight instructor, and somewhat versed in aerodynamics. The blurb that shows up in the ad mentions the lift effect which is created by air moving faster (and thus at lower pressure) over the top of the car than the air moving under the car. That's true, and probably would have a negative effect on the handling of the Prius at highway speeds. The BT spoiler looks like it would disrupt only the flow aft of the car, not the airflow moving over the top, which would be unimpeded by the BT spoiler. What would happen, then, is that the BT spoiler would create a lot of drag by introducing turbulence where you least want it, at the airflow's exit point.

This brings me to wondering if the BT spoiler would interfere with the OEM spoiler mounted on the car. The ad makes no mention of the purpose of this spoiler, which, I believe, is to reduce the turbulence created behind the vehicle, a major source of drag. To illustrate: If you had a solid object shaped like a teardrop and wanted to move it though a fluid with the least possible drag, you would aim the big rounded end into the fluid flow rather than the pointy end, because the greatest amount of drag is the "suction" created by vorticies behind the object. The flatter the object's stern end, the greater the vorticies. If the sharp end were facing aft, away from the airflow, it would allow the air to move smoothly back into the flow because air has a tendency to cling to the surface over which it passes. This is the purpose of the OEM spoiler: to reduce the vortices created by the relatively flat end of the car (you couldn't have the Prius end in a point), and thus reduce the overall drag.

The BT spoiler will create more turbulence, not reduce it. I honestly don't know if this would improve the handling or not. But to lessen the lift effect, the airflow needs to be disrupted further forward on the "wing". Spoilers used on jet aircraft, are mounted near the top of the airfoil curve, not at the rear of the wing.

Nitpicking, I know, but since they are asking 200 bucks for this thing, I'd be really curious to know how and if it works.

Bob

[efusco]

It would be fun to see one of those wind chamber smoke stream tests done before and after the little spoiler.

I'm still a bit of a skeptic on this thing too Bob. Did you see the thread on teh mpg test..by Tumbleweed I think? He noted no adverse affect on mpg on a pretty well concieved test.

But even so, that doesn't answer the question as to whether it really improves handling.

[BT Tech]

Hello Bob and thank you for your comments below. The purpose of our spoiler is to disrupt or "spoil" the laminar flow going over the back of the car in order to reduce the amount of lift. Yes, there is drag but as you know this is the purpose of a "spoiler". The disruption of the air flow going over the car is what is reducing the amount of lift and creating down force which aids in the cars handling and stability at highway speeds.

I can tell you that there is a HUGE difference between driving the car with and without the spoiler. Unfortunately, Prius Chat suffered a server crash where there were a lot of posts regarding the handling differences our part made.

I am sure that those that have installed it will chime in here and give you their unbiased impressions.



Thanks!!

Brian
BT Tech
305-652-3115

Originally posted by Bob Allen@Oct 28 2005, 07:34 PM
I noticed the BT Tech spoiler ad on the side bar of this page.  I checked out the photo and have a few observations that are more like questions.

I'm a flight instructor, and somewhat versed in aerodynamics.  The blurb that shows up in the ad mentions the lift effect which is created by air moving faster (and thus at lower pressure) over the top of the car than the air moving under the car.  That's true, and probably would have a negative effect on the handling of the Prius at highway speeds.  The BT spoiler looks like it would disrupt only the flow aft of the car, not the airflow moving over the top, which would be unimpeded by the BT spoiler.  What would happen, then, is that the BT spoiler would create a lot of drag by introducing turbulence where you least want it, at the airflow's exit point.

This brings me to wondering if the BT spoiler would interfere with the OEM spoiler mounted on the car.  The ad makes no mention of the purpose of this spoiler, which, I believe, is to reduce the turbulence created behind the vehicle, a major source of drag.  To illustrate:  If you had a solid object shaped like a teardrop and wanted to move it though a fluid with the least possible drag, you would aim the big rounded end into the fluid flow rather than the pointy end, because the greatest amount of drag is the "suction" created by vorticies behind the object.  The flatter the object's stern end, the greater the vorticies.  If the sharp end were facing aft, away from the airflow, it would allow the air to move smoothly back into the flow because air has a tendency to cling to the surface over which it passes.  This is the purpose of the OEM spoiler: to reduce the vortices created by the relatively flat end of the car (you couldn't have the Prius end in a point), and thus reduce the overall drag.

The BT spoiler will create more turbulence, not reduce it.  I honestly don't know if this would improve the handling or not.  But to lessen the lift effect, the airflow needs to be disrupted further forward on the "wing".  Spoilers used on jet aircraft, are mounted near the top of the airfoil curve, not at the rear of the wing.

Nitpicking, I know, but since they are asking 200 bucks for this thing, I'd be really curious to know how and if it works.

Bob

[snapback]146953[/snapback]

[NuShrike]

I still think the best way to reduce lift is to put it on the hood, the front spoiler, or right at the beginning of the rear window on the roof (instead of the current tail end) where most hatchback spoilers are...

[Areometer]

Do you mean something like this?

It mounts onto the hood.

[NuShrike]

Heeyyy.. if it works.. Maybe it'll create enough of a wash to keep stuff off the hood or windshield too.

I was thinking, can't we confirm some of these spoiler ideas with just two cars, 3 people, vidcam and a bunch of string and tape?

Find a mostly empty stretch of road, have a lot of strings taped in a way to follow air flow, one driver drives the Prius, the other drives a car, and the passenger tapes.

[200Volts]

My 2 cents.
If the BT Tech rear spoiler was further forward you would be disrupting more laminar flow and therefore you may add more drag (lower mileage). I think it is creating a new, higher pressure area in front of the spoiler (ala a Porche Boxster S rear spoiler) and turbulent flow over the stock spoiler(less lift).
Spoilers at the furthest point forward and aft on a vehicle have the most leverage/least drag effect (a smaller wing can be used for the same downforce if it is placed furnter from the wheels- this is why F1 and Indy cars have wings located furthest aft and forward).
The stock rear "spoiler" has very little extension past the back of the car so it's streamlining effect (straightening of turbulent flow behind the vehicle) is very little in relation to the rear width/height distances.

[Kiloran]

In the end analysis, every pound of downward force will cost you approximately a pound in drag force (assumption: spoiler at approximately 45 degrees to airflow).
Where you apply the downward force (front and/or rear) won't affect this but can make a difference in handling.
For example, if the rear of the car were subject to greater aerodynamic lifting, you'd want to concentrate your spoiler downward forces there.

[sotagear]

Here’s my take on the effectiveness of the BT spoiler. I ordered both the spoiler & the chassis stiffening plate from BT Tech, of which the spoiler arrived about 3 days before the plate. I installed the spoiler as soon as it arrived & luckily had a few hour+ freeway drives to test it on. Then 3 days later I installed the plate & drove 3000 miles across the country & then back again a few weeks later. So I have a lot of drive time with both, of which a few of those days were with just the spoiler. In my estimation, based on those 3 days, sans plate, I would say the spoiler made the largest recognizable difference in helping out the handling on the freeway of the 2 devices. I just can't quantify what changes I have felt with the addition of the plate like I could when I drove the car once the spoiler was installed. That little piece has definitely made a difference to my experience.

Now this is all very unscientific because it's about different days with different weather patterns & wind speed/directions, etc. But I did drive many of the same freeway stretches with the stock Prius & now with the spoiler & it's quite noticeable how much it helped to make the driving experience more controllable at speeds above 70mph on the freeway.

The way I look at it is I just spent $23k on a car that basically handles like crap above 70mph on the freeway. However, I still love the car, so a small $200 investment for a potential handling improvement wasn’t even worth debating. The car is still not perfect even with these 2 items & I anxiously await further upgrades that could make the freeway driving experience a bit more predictable at all times. But until then, in my experience, this was an improvement that helps right now.

[Godiva]

I'm doing the exact opposite.

I've already installed the stiffening plate and I've noticed an improvement. My car wasn't that bad to begin with, but it has improved the previous sway I used to get when taking the 270 degree on and off ramp turns in So. Cal.

I'll be installing the spoiler this weekend. I don't drive all that fast because I *like* high mpg. But I do get passed by the occasional truck, bus or whatever and I hope to feel a bit of difference there. I'll let you know.

I think for the small investment in a high quality product, any little improvement is worth it. Both the plate and the spoiler are crafted very, very well. And the support from Brian has been phenomenal. Fast replies to e-mail and even Wilma didn't slow him down too much. (That would be the hurricane....not his wife.)