Motorcycle CFD Reduction Tips

[donpearsall]

I apologize if this has already been posted. Here is a link to an interesting article on the results from trying different things to reduce motorcycle drag.
http://www.union.edu/academic_depts/mechanical_eng/students/projects/09/Barsukov_Kostyantyn_FinalPoster.pdf
Note that the best thing that you can do is cover up the radiator for a 7% reduction in drag (in wind tunnel tests they taped over it.) Also, contrary to what we have always been told, making improvements to the REAR of the motorcycle did little to reduce drag.
I am not vouching for the veracity of all this, but all testing was performed by an engineering student at Union College in NY.

Don

[Seldom Seen Slim]

Don, I am in the process of reading the article -- but my question remains the same until you or the article explains. 

covering the radiator -- just taping it, so no (or much reduced) air goes through?  That is -- the tape will just "replace" the front of the radiator fins with the front of the tape -- the same amount of surface area will be presented to the wind.

So -- big difference by simplyreducing (to near zero) the amount of air flowing through the radiator?  I think we could work to make the engine live with little or no air through the rad. for five miles.

Is that what he means?

[donpearsall]

I think the reason for this is that the radiator is more drag than  just the flat plate surface area of it. The air slows down quite a bit going through the fins, and then gets stagnant right behind the radiator. This builds up more frontal pressure than just a smooth surface that takes up the same area. And if you can round that surface so that the air does not have to slow down as much, then so much the better.

Don

[Seldom Seen Slim]

Okay -- I can see something there.  Thanks for your help (I think it's a help).

Back to Nancy's pasties...

[Stan Back]

I'm confused with the time zone changes -- is it lunch time there?

Stan Back

[Anvil*]

donpearsall:
"I think the reason for this is that the radiator is more drag than just the flat plate surface area of it. The air slows down quite a bit going through the fins, and then gets stagnant right behind the radiator."
---
Agreed. At high speed you would want a smaller inlet and use a shroud contoured to slow the air and recover air pressure. After the radiator you would want to split the air and contour an after-shroud to use pressure and heat to speed the air back up before dumping it out and back. Search through NACA engine cowlings and items like the P-51 oil radiator.

Once the air is fully turbulent the structure following is just added wetted area. The trick is delaying and reducing turbulence to keep the tail still somewhat usefull at reducing overall drag.

[PorkPie]

A simple test explained the effect of a radiator against airflow.

To show this we tested a crate of beer and a block of bricks with the same outside dimension as the crate in a windtunnel .

The cross section of the crate was smaller - due to space between the bottles - but the drag was much higher than the drag of the brick block.

The reason for, is, that the air has to go around the bottles - the airflow has to change his direction around every bottle - technically nothing other than creating turbulences.

On a naked bike to tape the radiator will reduce the drag - if the bike got a "streamline" partical bodyshell it's better to keep the "air intake" where the air goes inside the bodywork so small as possible, just enough that you get enough air on the radiator.

So less air has to go through the inside of the bodyshell, so less is the drag.

To the rearend of a bike - aerodynamically it's a desaster - but depends how much the rules allowed you got a lot of solutions to clean the airflow on the rearend to get a much better drag.

[A2WindTunnel]

almost any time you have internal cooling or ducting you will see an increase in drag.  The air is getting chewed up as it goes through the radiator and engine compartment compared to going around it.  Any time you can eliminate this you should see a decrease in drag.

However, this is not the only thing that you should be concerned with when sealing up any cooling or ducting.  An example: a race car with rear spoiler producing downforce will typically gain front and rear downforce as you close up the cooling while reducing the drag. A car that produces lift on the rear could gain more lift making it more unstable at high speeds.

[Commuta_Busa]

I was hoping it would also talk about the vents on each side of the fairing and the bottom.  I've been curious to know if opening them up more may help evacuate the air that's trapped inside the fairings.

I noticed his references pertain the Sport Rider article back in 2001. (I've read that probaby 20 times)

I guess the Hayabusa is a pretty established bench mark. 

[Blue]

Note that the best thing that you can do is cover up the radiator for a 7% reduction in drag (in wind tunnel tests they taped over it.) Also, contrary to what we have always been told, making improvements to the REAR of the motorcycle did little to reduce drag.
I am not vouching for the veracity of all this, but all testing was performed by an engineering student at Union College in NY.

OoooooooooKaaaaayyyyyyy, we need to take this with a BIG slab of salt.

A partially faired motorcycle is about the most complex aerodynamic object on the planet.  The flow is virtually all separated (not "turbulent") aft of the engine and rider.  Forks, radiators and fairings create as much stagnation as they do streamlining.

Tails make the largest difference to separation drag and stability, visit www.ihpva.org and explore the links.  Recumbants with no front and elaborate tail fairings out-perform bikes with front fairings and no tails.  I didn't see anything in the research about stabilizing the aft separation into stable vortex flow, let alone designing a tail that would prevent the separation like Bub-7.  Making a slightly larger passenger seat pad buried in the separation shadow of the rider would have little effect, and he did find that.  It shouldn't have taken CFD.

What the researcher did here was a simplified motorcycle model, probably of himself or a friend.  To get the mesh of this model accurate enough to see all of the separation and other effects would take upwards of a billion nodes.  To accurately mesh just the fork/brake/rotating-wheel interaction would take millions of model nodes, he didn't go that far, so the drag numbers are nothing more than WAG's.  The conclusions are even more so: they contradict the well-researched 50-year database of HPV science.  His work may still have value, and like any CFD researcher, he needs to resolve some of these blanket statements with the proven knowledge and data from real-world vehicles that have run for decades contradicting his conclusions.

In other words, get off the computer and tuft it.

[Harold Bettes]

Howdy All,

In a test at the Air Force Academy a scale model motorcycle (a very detailed one from a process of stereolithography) was placed in one of the many wind tunnels and studied with and without fairing and with and without mods. Lo and behold, it was found that the better package aerodynamically was without the fairing. Wish I had the report at fingertip, but I don't.

Way too many folks try to work on the entry losses and do not address the exit losses which are nasty.

Blue's comments about how complex the mc package is right on!

Regards to All,
HB2

[donpearsall]

The SCTA and AMA rules for sit-on bikes make it almost impossible to have a tail section that will re-attach the separated and turbulent flow from the bike and rider. For instance AMA says you cannot have any streamlining past the rear tire. And both SCTA and AMA require 1/2 the wheel to be visible, meaning very little streamlining of the rear lower portion of the bike. I agree that a fish shape like the BUB and other streamliners is best, but we have reality to deal with. So considering that the rules restrict rear streamlining so much, bikers need as much frontal streamlining advantage as we can get.

[1212FBGS]

Don
You might want to read page 117 of the little black book..... pay attention to the bold print 7.G.11..... then you can thank me for that one.....
Kent

[isiahstites]

Don
You might want to read page 117 of the little black book..... pay attention to the bold print 7.G.11..... then you can thank me for that one.....
Kent

Yep - It is better.....thanks!

[wobblywalrus]

Poorly designed front streamlining creates a large turbulent wake and rear streamlining will not help the situation.  Wedge shaped front streamlining is an example.  The turbulence behind the wedge cannot be reorganized into laminar flow by the rear section.

Well designed front streamlining has a more blunt nosed bullet shape.   The trailing surfaces are parallel to the bike and air is parallel to the bike when it flows over the trailing edge of the fairing.  Rear streamlining can be designed to maintain this laminar flow over the entire bike and it will significantly reduce wind drag.

Well designed streamlining can significantly reduce air drag, and rear sections and front fenders can help.