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Old 07-15-2012, 02:27 PM   #1 (permalink)
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Tailpipe placement affects rear wake?

I was reading a motorcycle article a while ago talking about the placement of the exhaust exit affecting the wake behind the bike and actually slowing the bike at speed.

2 reasons it stated that incorrect placment hurt the bike.

1: the exhaust stream can actually disrupt the wake behind the bike causing more drag

2: the exhaust being blown in the air stream makes it harder for the exhaust to exit thus reducing power.

It was found that if the pipe was pointed so that the exhaust exited toward the center of the teardrop there would be less drag and it would help draw out the exhaust increasing power.


I wonder if it would have any effect on a car in the same way?

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Old 07-15-2012, 03:41 PM   #2 (permalink)
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It stands to reason that incorrect exhaust placement would hurt a car's aerodynamics in the same way that it would hurt a motorcycle's aero.

That being said, cars tend to have a lot more frontal area than motorcycles. Thus, side disturbances, like incorrect exhaust placement, would not be as noticeable as for motorcycles.

However, it may be beneficial to move the exhaust on a car such that the exhaust could take advantage of the low-pressure wake. Could be why so many cars nowadays have exhaust pipes arranged as they are.
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Old 07-16-2012, 06:54 PM   #3 (permalink)
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teardrop?

Was the bike an enclosed streamliner? If they mentioned 'teardrop' in the article,it suggests that there was a body on the bike.
At Bonneville,the MC streamliners have drag coefficients on the order of Cd 0.11.
The flow is completely attached with the exception of the skids,minor wheel openings,ducts,and tail area,where the tailpipe and parachute tubes are located.
If the tailpipe was forward on the 'fuselage' the exhaust would be dumping into a lower pressure area compared to the wake,but would upset the boundary layer,potentially triggering flow separation which would cause a drag increase.
Could you dig up that article? It would be nice to scrutinize it.
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Old 07-16-2012, 08:03 PM   #4 (permalink)
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This was dealing with race bikes, MotoGP type stuff and I will see if I can find the information again.

I added "teardrop" to help you visualize what I was describing
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Old 07-17-2012, 02:35 AM   #5 (permalink)
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Was it this one? I found this while looking around a while back.




Ducati Desmosedici RR | Exhaust - aerodynamics of underseat, side, upward configuration | robotpig.net
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Old 07-17-2012, 03:09 AM   #6 (permalink)
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There is nothing I love more than made-up drawings of flow, complete with arrows. I once mocked such a drawing by drawing a cube with those beautiful, flowing arrows effortlessly slipping around it.

Volume of exhaust vs volume of surrounding air flow???

I'm more than a little skeptical. Seems to me the placement of the canister re how it is situated in the flow would be the biggest factor. But I'll leave the door open on this one that there could be a little something to it.

I think the main purpose of nonsense like this is to create some hype for this year's "next big thing" thus obsoleting last year's crap and inducing the boy racer types to open their wallets while they dream about that gaining that extra fraction of a second advantage over... who exactly?
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Old 07-17-2012, 03:46 AM   #7 (permalink)
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No it was not that article, but I enjoyed reading it.

I would test this theory since I am doing a custom exhaust on my car and could easily change the outlet position, but this install is including a nice header and performance oriented exhaust so my numbers would not really help much.
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Old 07-17-2012, 06:06 AM   #8 (permalink)
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The Ducati article noted that it mattered more for stability than for actual fuel economy... and yeah, I'd be dubious about how much exhaust volume there would be compared to total flow around the bike at anything other than medium speed high rpm riding...
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Old 07-17-2012, 09:20 PM   #9 (permalink)
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If memory serves me correctly, the point of it was not to discuss the "next big thing" frank but simply to point out why certain teams were winning vs others. All other things being equal this was the only difference between the winning and losing teams bikes even from the same manufacturer.

I figured if it could have some merit for something as aerodynamically dirty as a motorcycle, it could have some merit for a car so I posted it for discussion sake. The article that i got it from was not geared toward fuel economy at all, but making more power from subtle tricks that wont show up on a dyno chart.
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Old 07-17-2012, 11:02 PM   #10 (permalink)
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Thanks for posting it up. It reminded me of back when I used to get all the motorcycling magazines though, where they breathlessly hype up the new model with it's 1mm smaller base circle hollow cam like someone's gonna lap the field with that.

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