Suggestions for making a rear diffuser from scratch?

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#1

I searched and I did find a few threads about rear diffusers for cars, but I still have some more questions.

There currently are no diffusers made for my car, so I am thinking about making one completely from scratch. I would like to end up with something like this:



What would be a good method to go about getting started with this? I was thinking making one of some really rigid paper (poster board-type stuff) or something, then doing what I can to stiffen that up. Bolt it back up to the car to make sure it still fits after all the stiffening, then producing a mold from that.

Are there any products that come to mind for doing something like this? Surely paper can’t be the best thing.

#2

Bump. Anyone?

#3

Paper is not suitable. I would use a CAD system. (Cardboard Aided Design)

#4

Ok good, so I’m not crazy trying to use cardboard/posterboard? How would I ensure that it’s flat, smooth, and rigid?

#5

Cardboard Aided Design. :stuck_out_tongue:

Anthony - would this be your first composites project? There are lots of different methods to making a plug. A couple of options depend on whether you want to have a plug with just the outside surface accurate (the thickness of the part doesn’t matter), or you might want to make a plug that is the exact shape and thickness of the final part (I’d say this definitely involves more work).

Given that it’s a mostly flat shape I would probably start by making some large fibreglass sheets to cut up and use as the base of the plug (just CSM and polyester resin - if that’s suitable for your process. I.E. will it be cured in an oven, etc.?) You might like to use card to get your shapes right, then use the card as a template to cut out your fibreglass. Once you have the base pieces cut to size you can hold them in place and join them with more fibreglass or just mix a batch of filler and put it down over the joins. Also use the filler to create any small radius/curved areas. Note that using a lot of filler will quickly increase the weight of your plug.

Once you have the shape finalised you can either paint in 2k (you just want a paint that is chemically non-reactive - always a good idea to do a small test piece first!) or use something like the Duratec plug surfacing primer. (Duratec is much less work.) Then you have a plug that is smooth (the Duratec and paint can be polished to a mirror finish), flat (as long as your plug is correctly supported), and rigid (this can be controlled through the thickness of your fibreglass base).

That is how I would approach it. Hope that helps.

#6

Thanks a lot! I haven’t worked with composites yet, but I’m not sure this will be the first thing I try to make. It’ll probably be among the first 3 though. Maybe first.

#7

I used corrugated plastic board to get the correct angles/curve and then built my frame for layup.

#8

Thanks! That looks really similiar to what I want. I wanted to make a smooth curving one as opposed to hard angles. You have any idea if yours is effective or not?

#9

I havent hooked it up to any pressure sensors yet (I was going to make a simple solution using Arduino and Bosch pressure sensor but that got hung up). Some preliminary math tells me that the area under the curve should yield approx 600lbs of down force at my mean max. track speeds.

I have done some before/after testing with my SOloDL data aquisition and have gone from max 1.31 lateral g to 1.44 lateral g. So approx 10% increase. This is with the exhaust blown thru the diffuser. I’ll re-route the exhaust to the side next for an added increase.

When I get around to making it a little sexier this autumn, I’ll make it available as a kit for the E46 and E92 M3.

#10

Wow, 600lbs…

Do you know if curved ones are superior to angled ones? Just thinking about it, it seems like curved would be better. But I’ve seen so many angled ones. But also I think a lot of the ones available for any car are just for looks anyway.

#11

You can design for functionality or looks. The curve acts like an airfoil. In this case an upside down wing providing downforce (instead of lift). Technically, as long as you increase the volume of air passing under the car, you will create downforce. The airfoil design provides more.

http://www.formula1-dictionary.net/downforce.html

#12

You have it backwards mate…maybe just a typo. If you increase the volume of air passing under the car you will generate lift.

I’m curious how you calculated your figures for downforce. I’m also curious to see how you routed your exhaust for the exhaust powered diffuser. Can you share some pics?

#13

I have made a few diffuser’s
the magic number is at 17 degrees and must be fed from a flat floor from the front axle.
does the Lotus have a flat floor? this is the diff I make for the Elan

it is fed by this floor

#14

What car is it? I have a diffuser mould here

PS That diffuser is not very effective. The curve leading out to the back of the car needs to be as smooth as possible to release the air back to ambient pressure, otherwise it wont have a downforce effect. That diffuser has an angle in the middle of it.

#15

I have a question… I thought diffusers generally would not do much of anything if the bottom on the car was not flat. Also… In my case I have a corvette… The fwd edges aft side of the front wheel wells are rounded so air can escape the well. Brake ducts feed from the front bumper. On the rear quarter there is ducting from the rear well out the back through the diffuser if I remember right… I thought about getting rid of all that crap aside from the brake ducts.

Note on the front of my car I have a asthetic front splitter and then an actual ugly spoiler flap that hangs Down. Besides scraping everything I think the flap actually has a purpose but would love to make something that would get rid of the street sweeper spoiler flap thingy. Is the basic idea to Chanel the air from under the car straight out the back or is there more to it. Bournolli is suppose … Right or no.

#16

Quick answer: True

#17

My car is surprisingly flat on the bottom from the factory. This photo isn’t actually my car, but it’s just like my car. I’ve also been considering making it actually flat on the bottom in addition to the diffuser.

Toyota claims the muffler actually serves as a diffuser. I think what they mean is that the muffler keeps the air down and from slamming into the bumper cover. I’m getting a single exit exhaust, so I won’t have a muffler there anymore, thus creating a big plastic parachute at the back of my car. So even if I don’t create the most well engineered diffuser ever, it’ll at least be a flat surface to prevent the parachute effect.

#18

Anthony,

Check out the Borla diffuser, it’s a decent piece but requires the Borla exhaust to work. Now if you made a universal one…

#19

Yeah, that would definitely be a lot better than nothing. Do you have an FR-S/BRZ?

#20

No, not personally. 2 of my coworkers have and we also have a shop FR-S. We have the full Borla header back exhaust plus the diffuser. It’s a nice piece, but really expensive.