A lot of people are cutting holes in their rear bumper to "diffuse" the high pressure and drag. A far better solution is to weld a sheet metal piece to the bottom of the car (at the back of the rear footwell). Then rivit it to the bottom of the rear bumper cap. Cut out the area for the rear suspension and tires.
I would also weld in some braces to hold it up off the ground. If done correctly this setup will produce some serious downforce at the rear. Ideally the sheetmetal will angle up at about 7 degrees to the rear bumper.
I did a setup like this a while ago. It only cost about $40, and 4 hours. Saved over 3 tenths in the 1/4.
<FONT COLOR="red">[B]Speed[/B]</FONT>: [B][I]" It's really about judgement and intelligence, so long as your brain can go faster than your going at any time, you're all right, and it isn't fast."[/I] [/B]
Im not done yet.... [img]http://images.honda-tech.com/set1/smile/emwink.gif[/img]
Re: solving the rear bumper drag problem (tgh99si)
Theres really not that many cars done correctly. A couple of pro fwd cars have it done nice like Levon eisles , Lisa Kubo , Team GM , Shaun Carlson and so on. I'm not even sure that those are correct for down force . but they look real nice as far as smooth air flow. Hotrod cars that have been half backed have just a flat piece from the b post back, but they don't appear to have any angle to them.
As far as that guy who picked up three tenths , hes full of ****. I haven't seen a fwd car going fast enough to benefit from that type of areodynamics be consistant enough to make a statement like that. A fwd car with that kind of power can fluctuate a hell of alot more than 3 tenths every run. Even team gm or bothwell wouldn't make statement like that.
Re: solving the rear bumper drag problem (2.0Coupe)
Great idea. You dont' really know what angle to place it at, because if its pushing down on the back, the front will come up, and vice versa. Rememer that if you create lift on one aspect, it also creates drag as a biproduct. I think if you're going to do this, do it to the front here, as seen on this skyline, taken from autospeed.com ...But it's underneath the car where the real action is. At the front is an undertray that extends the full width of the car prior to the wheels, then back under the engine. This smoothing of the airflow helps the front spoiler/splitter work, in addition to making the airflow passing under the rear undertray less turbulent. What rear undertray? you ask.
A full rear diffuser is fitted. This comprises a flat panel that is angled upwards towards the rear of the car. It is sub-divided into three areas of flow, with one path partially blocked by the protruding muffler and tailpipes. The idea of a rear diffuser is to accelerate the air around the throat - the part of the device that has the initial change from being flat to being tilted upwards. This higher speed flow creates a low pressure, pulling the car downwards.
I did see the post about the mold being made for the defuser on the Celica (http://honda-tech.com/zerothread?id=769081), though, I was somewhat supprised there are no fences (vertical stabilizers) running the length of the diffuser-- it seems as if the air pressure wouldn't be high enough as it is now for it to be as effective as it could be, but I know that Dejan is a pretty smart guy when it comes to aerodynamics.
Re: solving the rear bumper drag problem (Quik Chris)
There needs to be vertical straights in it, but we haven't done it yet. We've been to busy getting the car ready for the track. Good eye! One a good note, in testing yesterday it went a 1.34 60' right out of the box, first pass! Things are looking good so far.
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