Over and Under or around the sides

[marcush.]

I really really like this thread.
I thought about the flow under the car,which has to pass between the tyres first front then rear tyres.
With a flat underbody you obviously get some choking in the wheel area ,don´t you? I assume creating somespace for the air in this area could would help reduce drag? (raising the floor in that area a bit like a wave).

[tok-tokkie]

The technical challenge you are faced with probably looks trivial to the uninformed but is fascinating when we are party to informed opinion. I had not understood what the real purpose of vortex generators was until I read Breezy Racers post. I really look forward to your report on what benefits you find they deliver.

Breezy Racers link gave me this picture. I had made the suggestion that minimal frontal area and wheels in spats seemed the best way to go. Here is exactly that approach and the wheels & spats are spectacularly narrow. An entirely different problem so not directly transferable to yours. Your streamlined shape follows from informed advice given here whereas my suggestion came from a much less informed person (me).

[greenpower dude reloaded]

There is a balance to be had. In Greenpower, for years we have had this Large FA teardrop Vs. Small FA Wheels out argument. I personally have always been more on the side of the large Frontal Area side, but that's not to say I haven't been tempted otherwise.

Here are the a sample of some of the thoughts that I went through designing this car and as a comparison to the Lotus.

Wheels out is draggy, there are no two ways about it. You can indeed spat them as the picture of the Lotus 119b I posted (incorrectly) and breezy reposted shows however in 1.5hr races or occasionally the 4hour races we compete in spats are tricky. They will suffer a lot of vibration etc, and a lot of being blown about at 40mph. Which as they can only really be supported from one side has always left me thinking that it sounds like it could cause a reliability issue.

This left me thinking ok, I need to go for wheels in. I have now squeezed the overall width of the car down to 680mm at its widest and the driver is sat as low which I think gives a total frontal area of 0.33m2 roughly. I personally would wager that that Lotus isn't far off. The width must be 450-500mm to accomodate the shoulders of the driver then a bike hub (assuming the spats are no wider) is around 85mm which brings the width up to 670 you then have the axles, and the housing for the kingpins disc brakes etc etc. and the driver is sat higher. Our car is 450mm high at it's very highest those wheels are either 24 or 26" (i can't remember) ours are 16" which actually provides lower rolling resistance as well.

ANYWAY you can see where I'm going with this, I'm sorry if it sounds like a rant. It really isn't It's just a case of expressing the trials and tribulations that face an electric racing car constructor.

Marcush, Very interesting idea, but unfortunately too late to intergrate in to this car .... Maybe next year ..and I'm glad to hear your enjoying this thread.

[BreezyRacer]

You're actually getting lower rolling resistance from a 16" wheel/tire than the larger ones? Have you tried some coast down experiments on that? I'm quite surprised that equal for equal you could do that. What type of bearings do you use? Fairly exotic, I'm sure.

[greenpower dude reloaded]

HAHA! Research into it by two seperate parties (a tyre manufacturer and an eco marathon team that achieved 12,666 us mpg) actually shows that a smaller wheel has less rolling resistance bearing drag counts for very little when it comes to wheels. Bearing drag is something rediculous like 0.0001% of overall wheel drag, Its more about the contact patch. The load tends to flatten the bottom of the tyre so there is more to topple over every time it turns so the smaller the wheel the higher the efficiency.. well within limits anyway. Plus I have a few tricks with bearings. You can make standard cup and cone ones spin for about 15mins if you set them up right. I would love to try at least semi ceramic but can't afford them.

[BreezyRacer]

While I'm thinking about it, what drops in speed do you see while cornering? Is grip ever a problem? I really don't know anything about the track you run on. Is it a single location track or do you race on a variety of surfaces? Any hills, or is it always flat?

[greenpower dude reloaded]

Lots of different tracks, all pretty flat. We raced at Rockingham motor speedway, flat out the whole way, Goodwood, again flat out. Dunsfold park bit tighter and therefore a touch slower but all open and very little drops in speed the whole way around. Wind is probably the biggest factor.

[marcush.]

yes I am aware that this is definitely not feasible during the season ,but I guess it is worth considering during layout of the next iteration..

[Carlos]

I like Breezyracer's idea of a neutral chord wing with rounded tips on a strut to mount a few (4>6?) CG's. I had a similar idea of using a neutral wing at the back of the car to smooth drag, guess I got it backwards. The March 711 front wing is sort of related to the idea; but maybe 30 cm/12 inches wide, perhaps with drooping or rising ends, maybe not but with 4>6 small CG's? Or maybe a very narrow boxwing about half that width, how about using them both fore and aft? Maybe full length strakes under the floor would also help reduce drag? Hmmm
The March 711 front wing design is closely related to the idea.
http://autospeed.com.au/cms/title_Blowi ... rticle.htm
An article and pictures of small CG's. Could be an easy DIY to make them.


March 711

A CG a few inches wide. Could it be enlarged and used as that 30cm/12 inch neutral chord wing?

Or a Boxwing for the Boxfish?

[BreezyRacer]

Lots of different tracks, all pretty flat. We raced at Rockingham motor speedway, flat out the whole way, Goodwood, again flat out. Dunsfold park bit tighter and therefore a touch slower but all open and very little drops in speed the whole way around. Wind is probably the biggest factor.

Have you done any testing comparing straight line speed compared to flat out cornering car speed? Ackerman chassis settings should go a long way to minimizing tire drag in cornering.

[Ciro Pabón]

After reading the quite correct explanation about tyre Crr, I have a question: have you tried Michelin low drag tires? I read about them by chance a while ago. They were designed for Shell EcoMarathon. I think they are more or less affordable (better yet, that's what I've read). The latest I've read about (after googling for them right now) are the Michelin 45-75R16.

These are the figures, this is the source: The world's most fuel efficient vehicle: design and development of Pac Car II (it might give you a couple of ideas, if you haven't read it).



A regular bike tyre woudl give you a Crr around 0.006, a Michelin low resistance tyre gives you around 0.0008. That's significative.

I apologize in advance, I imagine everybody uses them at your series... this is just to post about them for our dear forum.

[BreezyRacer]

Here's an example of a wing tip vortex like I've been talking about ..
http://www.centennialofflight.gov/essay ... TH15G3.htm

I tried to find a better offering but in short time this works. Note the wing diagram at the bottom. The outer tip produce a spiraling airflow, aka a vortex. That lowers the air pressure in that region. Now imagine this wing mounted in the vertical position and the vortex it creates being lined up with the roll hoop/helmet area. That should reduce drag.

I hope this helps.

As for trying something like this ahead of the front nose section, again to reduce drag, you would have to allow some space for it. Depending on your length limitations that might be a problem. But wing tip vortexes are pretty strong and you might be able to make it work in an area as small 250mm in length with careful planning. Realistically I would figure more like 400mm or so.

If you modify your model to include a wing tip, be sure to mesh it pretty tightly behind the tip. Getting a vortex to show up in CFD requires a fairly high level of detail.

BTW, the windscreen idea should curve all the way back to the roll hoop to try to get rid of the turbulence that is created at the body/roll hoop intersection. You might find some drag reduction in careful designing of the profile of the roll hoop bodywork junction. You really don't want that turbulent ribbon if you can keep from it.

[flynfrog]

After reading the quite correct explanation about tyre Crr, I have a question: have you tried Michelin low drag tires? I read about them by chance a while ago. They were designed for Shell EcoMarathon. I think they are more or less affordable (better yet, that's what I've read). The latest I've read about (after googling for them right now) are the Michelin 45-75R16.

These are the figures, this is the source: The world's most fuel efficient vehicle: design and development of Pac Car II (it might give you a couple of ideas, if you haven't read it).



A regular bike tyre woudl give you a Crr around 0.006, a Michelin low resistance tyre gives you around 0.0008. That's significative.

I apologize in advance, I imagine everybody uses them at your series... this is just to post about them for our dear forum.

Michelin bridgestone and dunlop all make low draw solar car tires.

[greenpower dude reloaded]

Ah they are lovely tyres those, and yes I have read the book on Pac Car II really fascinating. Interestingly the Radial actually fits on a 16" motorbike rim

Our tyres arent as good as those but not that far off really. At 18mph the Michelins soak up 14.2watts each where as the tyres we run are at about 15.9watts each the tyres most people run on the other hand 27.8watts each

They make a big difference! The michelins are great but massive, about 50mm+ wider compared to ours which are about 30mm and roughly 18/19" in diameter. So they maybe be slightly more efficient but they are far less practical and push the frontal area up a fair, which at 40mph is a bigger factor. plus the michelins are very difficult to come by and farrr more expensive than our £80 for a full set.

[tok-tokkie]

Here's an example of a wing tip vortex like I've been talking about ..
http://www.centennialofflight.gov/essay ... TH15G3.htm

I tried to find a better offering but in short time this works. Note the wing diagram at the bottom. The outer tip produce a spiraling airflow, aka a vortex. That lowers the air pressure in that region. Now imagine this wing mounted in the vertical position and the vortex it creates being lined up with the roll hoop/helmet area. That should reduce drag.

I hope this helps.

As for trying something like this ahead of the front nose section, again to reduce drag, you would have to allow some space for it. Depending on your length limitations that might be a problem. But wing tip vortexes are pretty strong and you might be able to make it work in an area as small 250mm in length with careful planning. Realistically I would figure more like 400mm or so.

If you modify your model to include a wing tip, be sure to mesh it pretty tightly behind the tip. Getting a vortex to show up in CFD requires a fairly high level of detail.

BTW, the windscreen idea should curve all the way back to the roll hoop to try to get rid of the turbulence that is created at the body/roll hoop intersection. You might find some drag reduction in careful designing of the profile of the roll hoop bodywork junction. You really don't want that turbulent ribbon if you can keep from it.

When I was at University ('72 - '75) wingtip vorteces were said to be as a result of the different speed of the air over the top compared to underneath - resulting from the different camber & the angle of attack of the two sides. The diagram you link to was used. The analytical theory showed that mathematically there is circulation of the air about the wing which is shed at the tip. So what should be done here? An aerofoil with different curvature each side or a symmetrical one set slightly off straight ahead? Or do you get them from just about anything. I am aware that smokestacks shed vortexes but they come off one side then the other with opposite rotation & buffet the smokestack badly (hence the spiral rids on the outside to prevent vortex shedding).