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I have been reading a lot about aero for cars lately and the tradeoffs for drag and downforce. Given that I'm an engineer and like to geek out on things, here is a link I found really interesting: https://occamsracers.com/miata-aero-test-at-wgi/
One thing that I found really interesting is that a rear diffuser can both reduce drag and increase downforce. There are various wings and splitters available for MINIs but very little real data on how well they work, Sneed seems to be the only vendor that has any measured data published. So I thought I'd start a thread to see what other people are doing and hopefully share real data.
Last weekend I finally completed my long desired upgrade to the GP2 diffuser - buying the diffuser kit, buying the JCW rear bumper cover and getting it painted, figuring out and buying all the bits and pieces for the lights and trim. I haven't added up what it all cost because it was a LOT, and I don't want to know. Why do it? Honestly because it looks so cool, and it should be functional too - less drag and more downforce, right? I don't have a way to measure downforce (yet) but coincidentally I was doing some hyper-miling experiments so I have a really good understanding of my long term average MPG and also what is possible by changing my driving behavior. (About a 25% increase in MPG over my daily commute, and a 1000% increase in boredom!) Any reduction in drag should show up as improved MPG. Stay tuned.
Did you also install the under body panels that go from the front of the car to the rear diffuser? that is, the panels that connect to the rear diffuser.
Reason i am asking is because those are very important.
I have had the front panel under the engine for a long time. I haven't bought the panels that go between the front and the diffuser yet. Stuff I have read says the diffuser by itself is a big improvement, and a flat floor makes it work better.
I have some ideas about adding some DIY panels from the forward end of the diffuser channels to the body, I think that will get me about 80% of what the full length panels would do.
I have had the front panel under the engine for a long time. I haven't bought the panels that go between the front and the diffuser yet. Stuff I have read says the diffuser by itself is a big improvement, and a flat floor makes it work better.
I have some ideas about adding some DIY panels from the forward end of the diffuser channels to the body, I think that will get me about 80% of what the full length panels would do.
Exactly, those under panels make the air go smoother. I ran into this issue a while ago when i bought just the diffuser and what happened is that at high speeds the air accumulated between the diffuser and the chassis causing damage to the diffuser. Clips were broken and the screws were somehow released from the high vibration that caused the air.
Yah, I looked at it and had the same concern about that space between the front of the diffuser tunnels and the body being an air scoop. My car has a plastic tab that is maybe 6" wide by 1" tall that sticks down from the body on either side ahead of the rear suspension, it appears to be an air deflector to maybe help prevent the whole area in front of the rear bumper from being a big parachute. My idea is to use the studs those tabs mount to as a front mounting point for some plates that will connect to the front edge of the diffuser tunnels, the way the full length panels do. I need to get the car back up on the ramps and crawl under it to do some CAD then cut the panels. I'll either get some thin aluminum sheet or maybe a sheet of alumalite to build them.
It does seem kind of oxymoronic to mention MINIs and aero in the same sentence. But I'm sure there are things that can be done to improve the aero performance even for a brick. I'm looking for ways to measure and quantify the impact, good or bad, of changes, and not just sticking something on the car because it looks cool. (Though I have to admit that the GP2 diffuser does look really cool and that's part of why I installed it.)
So far the difference in MPG after installing the diffuser is so big that I don't believe it, I'm going to wait for a few hundred miles more before making any claims. Meanwhile since I don't have a wind tunnel I'm thinking about what I could do with some bits of yarn and the GoPro. And there's some inexpensive digital manometers available on Amazon that could be fun to play with.
Yah, I looked at it and had the same concern about that space between the front of the diffuser tunnels and the body being an air scoop. My car has a plastic tab that is maybe 6" wide by 1" tall that sticks down from the body on either side ahead of the rear suspension, it appears to be an air deflector to maybe help prevent the whole area in front of the rear bumper from being a big parachute. My idea is to use the studs those tabs mount to as a front mounting point for some plates that will connect to the front edge of the diffuser tunnels, the way the full length panels do. I need to get the car back up on the ramps and crawl under it to do some CAD then cut the panels. I'll either get some thin aluminum sheet or maybe a sheet of alumalite to build them.
Just try not to go too fast in the mean time because when that happened to me I was going no more than 100 MPH, actually i would say 80 to 90 MPH.
The GP2 aero parts are beneficial, even if the car is a brick. I added the GP2 front skid tray to my Clubman, and noticed a few point is additional mpgs. I wish there was a suitable option for rear diffuser that would work on the Clubman.
Here's a pic of the tab I mentioned in post #5 - it is the greenish looking thing (funny effect of the flash) just ahead of the gas tank. There's another one to the right that is further back, just ahead of the rear wheel.
A little bit of CAD (Cardboard Aided Design) and I have a template for a panel that will use the mounting studs for the tab and connect to the front end of the diffuser tunnel. Today I picked up a big piece of leftover 3mm ACP from a local sign shop for $20 to cut the panels out of - pics when it's done.
Thanks to PG&E shutting off the power to a good chunk of Northern California, I didn't get any garage time in last weekend. Finally got power back on after 3 days of flashlights and takeout. At least we still had hot water. I hope to cut and install a first try this weekend.
After about 500 miles with the aero bumper and diffuser, my long term average MPG has gone from 31.1 to 33.6 - an 8% increase! I find that astounding - why didn't every MINI come this way? If I change my driving habits slightly I can get to 34 easily, if I hyper-mile I can get to 40+.
Left side is done, here's a couple of quick pictures. It is hard to see in the first picture, there is a S bend formed into the panel on the left side so it flows nicely over the gas tank to the attachment point on the underside of the car. The second picture shows the panel mounted on the car, the step is visible at the far end. The ACP will bend but only over a large radius otherwise the aluminum on the outside of the panel will tear. It took some experimenting with scrap pieces to figure out a jig that would create a smooth S bend across the width of the panel without tearing the outer skin. The material is really easy to cut, I used my radial arm saw for the long straight stuff, the jigsaw for the smaller stuff, and the dremel to round off the corners.
I came up with an idea for a better way to quantify any change in drag. One way is to do a "coast down test" where you measure the time to coast from say 80 mph down to 50. But doing that on public roads would have a good chance of getting noticed. Another way is to monitor the MAF reading while holding a constant speed using cruise control and average the readings over as long a period as possible and driving both directions. Measuring the air going into the engine should directly correlate to the engine load required to hold a constant speed. There's a road near me that's perfect for this - long, flat, straight, and a 50mph speed limit. (A couple of times over the years, late at night I have come across drag racers lining up on that stretch.) So I'll try that, comparing data from before and after installing the panels.
I finally got some garage time today and finished up the panel for the other side. Here's a pic of both of them in place (need to install the screws to connect to the diffuser tunnels.) Now I need to find some drive time to do the data logging to see if I can measure a difference between panels / no panels. This won't be as good as the much longer underbody panels but my total cash outlay is $20 so it is good cheap fun experimenting.
Also interesting is that my mileage has dropped recently, it is still better than the long term average before I installed the diffuser but not the big gains I saw earlier. There are two possibilities - one is CA switches to a different gas blend in winter which will affect mileage, the other is I have the stock suspension swapped in while I have the Ohlins out for rebuild so the car is sitting about an inch higher, therefore more air under the car. Maybe it is some of both. I'm planning to do direct comparisons with/without the panels, and before/after swapping the Ohlins back in - same day, same tank of gas.
In general it's hard to change the overall front aero shape of the MINI - front bumper, bonnet, windshield, hoodline.
Front lower lip/ aero splitter, rear diffuser and rear wing are possibles. Aero side mirrors and lower suspension with possible mild rake could help. Canards are not that useful
With the coronavirus restrictions I'm supposed to be staying home, I have been working from home the last two weeks and hardly going out at all. Yesterday I broke quarantine and finally got out to do a back to back comparison without the panels and then with to see if I could measure any difference. I used Torque to record the MAF sensor output (g/sec) with the cruise control set at 60mph on a long straight stretch of 2 lane road. I logged both directions since there are some slight elevation changes, total distance for each run was about 2.5 miles. Exported the logs to a spreadsheet and averaged the MAF readings for the time at 60mph... drumroll... adding the panels reduced the average MAF reading by 1.5%.
It's fair to ask - is that meaningful, or experimental error? Was I going fast enough to see a difference? Is one 2.5 mile trial for each case enough to get good data? I expect the differences to be small, but measurable with careful procedure, so I am optimistic that there really is some improvement. I'm going to see if my long term average mileage improves with the panels on. Unfortunately it is going to take a while to get any meaningful data since all of CA is staying home.
