Wheel offset and Aerodynamics

I searched this forum but didn't see posts related to this topic. I just recently purchased a set of wheels that fill the fenders very well, but also move out just a bit more than I was expecting them to. As a result I have been analyzing them with every familiar roads and turns at different speeds. Today I was traveling well over the posted speed limit on a highway and noticed more effects from the wind than I had noticed before fitting the car with these wheels. I have been driving with stock 7.5x18 on the front and 10x18 on the rear for several years. The car is a 99 C4 with an M030 ROW suspension. Has anyone ever noticed if their aerodynamics were effected after pushing the front wheels out this far? This may all be my imagination as the wind was a bit high today. Any racers want to chime-in with "before and after" studies they might have conducted? The photo of the driver's side is a 'before image' and the passenger side is an 'after'. Thanks a bunch. Jeff C.

You didn't notice aero differences, the tires have a bigger contact patch, so you don't fit in the 'grooves' of the road as easily and the car will be pulled in and out of them. There is a little negative effect of aero from wheels sticking out that much, but I doubt you will notice it at all unless you're doing high speed runs (note bonneville salt flat cars with moon hubcaps for aero). Here are turbo wheels on my c2: I like the stance of the wheels. Do you have a side profile picture?

Also, if the offset moved, it can effect the suspension geometry and THAT can make the car handle different. The wider tires will make more noise too.

You're feeling the effects of different tires/wheel size/looking for a difference, your aero was not affected.

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Interesting to note though that cars like BMW's go the extra mile to address the aero in and around the tires on most modern models. If you look underneath, most of them have a flap to deal with these kind of issues, hence the "ultimate driving machine", haha.

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You sure you're not looking at brake ducts? My Boxster has these small air dams ahead of the front wheel wells but they are intended to direct air to the front brakes. Also, for BMW and similar cars the aero around the tires is important and tires should not stick out as this increases the car's frontal area, a large part of it already made up from what portion of the front tires are exposed, but just as or more important is air flow under the car. This can affect the car's stability, fuel economy and how efficient the cooling system works and how hot the engine compartment runs, already a bigger concern for mid and rear-engine cars.

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Nope, these are specifically right in front of the front tires under the front bumper. The brake ducts are integrated into the bumper cover themselves. Another thing I like about BMW's is they have a pretty sizable underbelly pan cover which also aids in the aero dept. I like these kinds of details that BMW has thought about and incorporated on their models. Even detailed items like having the ECU getting rain data from the rain sensors for the wipers and ever so slightly activating the calipers just enough to shear off water so that you can get that instant braking in the wet, those kinds of things seems pretty slick to me.

Without a doubt, aero is unaffected at any level that you would ever notice.

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Ok. I note BMW is not the only automaker to think about underbody aerodynamics. The first time I was under my new Boxster (now some years ago) I was very impressed with the attention to aerodynamics in evidence from the number of panels. Pretty much the entire underbelly is paneled with gaps at just a few places. Quite unlike the nakedness of the underbellies of my previous cars. My Turbo has even more paneling, if that's possible, than the Boxster. I too like this attention to detail. On one car the tech pointed out how the water drains from some part of the car were positioned with the drain tube ends cut so as to create a low pressure area at the end of the tube to help expedite water out of the drain.

For any "sports" car brand or manufacturer with cars costing over 100k, I fully expect some kind of aerodynamic enhancements and R&D on their part. I was just speaking to the fact that most non sports cars have been generally open underneath with the exception of a few, like BMWs that have paid attention to these kinds of details.

I don't have a side photo. Just the one in the original post. I wasn't thinking the wheel width would cause what I was feeling since I'm running the same tire width. The car was a little bit more work to keep it positioned than before. In town at 60mph and lower, no ill effects. Just a solid, sticky car. But at 90+ things change. Maybe the car needs a different alignment since the wheels are wider and have moved out? I know the front track width increased with the 8.5 wheels but I'm not exactly sure how much. I'll get the numbers a compare later today. Jeff C.

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When's the last time you got an alignment or your alignment checked? Things tend to get amplified at higher speeds, especially coupled with wider tire/wheel combo due to the increased contact patch. If you have worn bushings, that can also be felt at higher speeds as well.

Whoa. Last alignment was about 15k miles ago.

I would like to agree with the recoendation of alignment. I changed over from a stock 17 inch, 45 offset to a 18 inch 55 offset in the rear and there is a lot of difference, some good, some bad. I have not tried driving any faster than 110 because i want to get a good alignment done. As for now, the tires are not wearing funny, so its been a non-issue.

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Till they stop working and you find yourself bent over the service writers table.

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I have new 18" rims also with 55 offset front and rear as well as larger tires front and rear and and yes you can feel car handle different none "bad" that I recall. It grips more so the only negative I can come up with is now I could use an extra 100hp. That said, makes no difference on Aero all the way up to 155 "ask me how I know" Agree on alignment. I did one with new rims and tires and one wen added x74 suspension.