6.25 lbs. of drag = 1 hp

[JohnPaul]

I've been talking to Gary Hickman about fabricating a new set of corners from aerodynamic tubing. His calculations say we will be saving almost 45 pounds of drag by replacing the round tubing on the car. I did a little research to see what that equals in actual HP and I came across this post that I thought was pretty interesting, I'm wondering if it is correct and maybe it can help others:

6.25 Pounds Drag @ 60 mph = 1 Road-Horsepower

By definition,
1 HP = 33,000 foot-pounds work/minute
One mile = 5,280 feet
1 hour = 60 minutes
60 mph = 1 mile/minute
So divide 33,000 foot-pounds by 5,280 feet to get
6.25 mile-pounds/minute = 1 horsepower

In other words, if a side view mirror is resisted by exactly 6.25 pounds of wind resistance at 60 mph, it is consuming a steady 1 horsepower. Big truck mirrors consume considerably more than 1 horsepower at 60 mph.

Would you like to test how much horsepower specific attached devices like mirrors and exterior spotlights consume at various road speeds? Here's a test procedure that can give sufficiently accurate comparison results to guide decisions.

Attach a common drawer slide to a vehicle's free-air-flowing attachment surface. Lumberyards sell low-priced, sufficiently-strong drawer slides which have low friction. Their end facing the air flow will add little wind drag. If you'd like, you can even zero your spring scale at 60 mph with no test device attached. Your test slide may be attached and detached with removable Velcro adhesive strips. Attach objects you'd like to wind-drag test to the slide. Attach a thin cable or strong fishing line to the slide, pulling it forward about half way between its front and rear stops. Attach a spring scale positioned so you can read it from inside your vehicle while driving. We don't want the scale to chip paint, so tape on some bubble wrap to create a soft bumper. Secure your spring scale's front to a forward attachment point on your vehicle with more cable or strong fishing line.

Wind drag will push your test objects rearward. Your spring scale & cables will resist that rearward push with tension. You will read that tension from inside your vehicle by reading your spring scale. By testing at 60 mph, we can divide observed tension pounds by 6.25 pounds to reveal road horsepower consumed at 60 mph.

Wind resistance pounds increase by speed squared. Horsepower required balance against speed-squared wind resistance increases by speed cubed. Start with 60 mph horsepower values, then adjust for speed accordingly.

If you run some of these tests, please post your results telling exactly what device you tested, wind resistance you observed at 60 mph and your calculated horsepower estimate.

If you're not sure where actual 60 mph appears on your speedometer, adjust your cruise control until Interstate mile markers come up at exactly 60 second intervals. Survey teams set those mile markers pretty accurately.


Does this look calculations look correct? so by these calc's if I reduce drag by 45lbs I'd be picking up 7.2 hp at 65mph, what happens at 120mph?? I can now see why the switch to 6-8's have been so succesful

Also: I found this racing battery that will cut 10lbs off my weight. Maybe it can be helpful for someone trying to cut weight:
http://carbon-speed.com/prod.htm

[Speed Sport Engineering]

6.25 lbsf of drag is actually a lot of drag. By my figures, a 6" x 6" object with some depth to it with an estimated Cd of 1.2 at 60mph in normal air is only about 2.76 lbsf of drag. Without digging up my old data, I did some wind tunnel tests with mirrors to figure out the HP loss at 80, 100, 120 mph. Think in terms of sub 1 hp.

To reference your hope of saving 45 lbs of drag by switching to aero tubing arms, I can't say for sure. But a 2ft long round tube .625" diameter versus aero tubing .625" cross:

15 in2 area. Assume round tube has a Cd of .6, aero tubing .2, then at 100 mph the drag on each would be 1.6lbsf and .35 lbsf. You'll need a lot of exposed tubing to make up 45 lbsf of drag. Better estimates can probably be found on actuall Cd values, but I think those are safe numbers.

It's not to hard to make a good spreadsheet to figure everything out. I made one that goes as far as to take engine / gear ratio data and use aero changes to estimate changes in top speeds. Started working on figuring out ways to convert that data to actual time differences at various tracks, but never finished it.

Half the trick to figuring out aero data is figuring out the right Cd to use. But with good data gathering these days, it's possible to work backwards from wind tunnel / on track information to estimate Cd's.

[Rick Ross]

JohnPaul,

I did a quick check, and I think your "6.25 lbf per 1 hp" at 60 mph is about right. I came up with 744 watts, which converts to just under 1 hp.

Drag force is proportional to the square of the airspeed, and power required is proportional to the airspeed cubed. So if you double your speed from 60 to 120 mph, the drag force will increase by 4 and the power required will increase by a factor of 8.

If it were in fact possible for you to reduce your aero drag by 45 pounds at 60 mph, then at 120 mph your drag would be reduced by 180 pounds. At 120 mph the power required would be reduced by about 57 hp. These numbers are obviously unrealistic.

Perhaps Gary was targeting a savings of 45 pounds drag at a much higher speed? At 140 mph, 45 pounds of drag consumes 16.8 hp. If obtainable, such a drag reduction would obviously have a significant effect on performance.

The real question here is what amount of actual drag reduction can you expect from switching to aero tubing? What assumptions were made when calculating the 45 lb estimate?

[JohnPaul]

Maybe I got it wrong and I don't want to put Gary on the spot as this thread was not to question his calculations. Plus I may have misunderstood him (I'm totally retarded when it comes to this stuff)and got the figures wrong. And to be fair to him he said it was very hard to calculate and may only equal 2-3 mph's at high speeds (120-130mph) But what I think I understood what that an average round tube creates something like 5-9lbs of drag while streamline tubing like 1-2 lbs. So I added all the round tubes hanging out in the wind and came up with 45 lbs. savings. Ok so am i way off and look like a dope?
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[Brands]

Just to throw a spanner in the works the best aero performance I ever saw during a suspension wishbone aero section test was with round tube! To be fair we had small diameter round tube wishbones that were used as a carrier for the rapid prototype aero section cladding and we ran that just for fun. It was by far the best for downforce and drag! F1 being F1 the optimised wishbones would often have a nose up L/E and a nose down T/E. That was before we got into varying the incidence along the length of each leg.

[Modo]

I have retractable mirrors for sale for $550 a piece, push a button and WHOOoooosh!!! Presses you right back in the seat!!

PS "movable airfoils", NOT ALLOWED!! movable safety equipment, HMMmmmm!! don't get any ideas Radon!!

[Modo]

Only kidding, couldn't resist, I like the serious discussion, please continue!! remember it only took 15 HP to maintain a typical American car (3500lbs+) at 55 mph on a level surface (Florida) back in 1974 (gas shortage, new dismal speed limit for our new Interstate highways, tuff to swallow but might be low carbon today and trendy as well), sumting like dat as I remember

[Rick Ross]

Depending upon wing settings, a Swift 014a Atlantic produces around 550 lbs of aero drag at 150 mph. The power required to overcome this drag is around 220 hp.

[Wright D]

Was the car still accelerating at the end of the straight when you gathered those numbers? If so, the HP to over come the drag would have been less.

[brownslane]

John Paul, thanks for the lead on the battery; I know of a few guys in DSR who are using another source for lightweight batteries...a supplier in FLa?

Has anybody tried any of these super-lightweight batteries?

After all, if we are using a jumper battery to start the car and have a charging system, we really don't need thtat much of a battery reserve.

Thanks, Tom

[Wren]

I don't know if it is still available, but there used to be a plastic aero cladding that you could put on round tube that would probably be a 90% solution for a lot less money.

[Rennie Clayton]

Was the car still accelerating at the end of the straight when you gathered those numbers? If so, the HP to over come the drag would have been less.

Dustin - those are wind tunnel numbers.


Cheers,
Rennie

[Wright D]

I have made some bits for brake line covers that would work well too.

Basically it is just a really thin piece of aluminum sheet metal (.010 thick flashing) that I bend into a long V shaped section and tape onto the round tube.

some thing like this... o>


Thanks Rennie..just wanted to know how/where the numbers came from.

[nulrich]

I don't know if it is still available, but there used to be a plastic aero cladding that you could put on round tube that would probably be a 90% solution for a lot less money.

Currently this is not legal, at least in FC. My original design for the Radon wishbones was round tubing with extruded plastic aero covers. I asked the CRB for clarification, and after some discussion they decided to not allow this approach. I believe they were concerned about the covers becoming much more than just fairings.

Wishbones made from round tubing would cost a fraction of those made from streamline tubing, and the plastic covers cost about $.10/foot.

I have made some bits for brake line covers that would work well too.

Basically it is just a really thin piece of aluminum sheet metal (.010 thick flashing) that I bend into a long V shaped section and tape onto the round tube.

some thing like this... o>

This is illegal, at least in FC, although probably one of the most ignored rules out there. We made our fairings out of steel sheet to strictly comply with B.5:

B.5. Suspension
All parts shall be of steel or ferrous material, with the exception of hubs,
hub adapters, hub carriers, bell cranks, pivot blocks, bearings and bushes,
spring caps, abutment nuts, anti‑roll bar links, shock absorber caps, and
nuts. Titanium is prohibited.

Nathan

[nulrich]

Depending upon wing settings, a Swift 014a Atlantic produces around 550 lbs of aero drag at 150 mph. The power required to overcome this drag is around 220 hp.

And a late model Van Diemen produces between 95 and 115 lbs of drag at 85 mph in the wind tunnel. That's at a range of "medium" downforce settings with Radon and Cooper series style wings. That translates to 300 to 360 lbs of drag at 150 mph...if you could get there.

Nathan

[Wright D]

well heck, then tape and zip-ties are out too.....

[nulrich]

I thought duct tape and tywraps were exempt from all rules?

Those carbon fiber brake line fairings that some guys run are illegal as well. It would be silly to protest someone for that, though!

Nathan

[Rick Ross]

Dustin - those are wind tunnel numbers.


Cheers,
Rennie

Sorry, perhaps I should have provided a source for those numbers. Rennie is correct.....those are drag estimates taken directly from the Swift 014a aero manual. As far as I know, this data was based primarily upon wind tunnel testing, and not CFD. Swift had their very own rolling-road tunnel that they used for the 014 aero development.

I would assume that with 550 lbf of drag at 150 mph, the car would have very little excess power remaining for acceleration.

[Wren]

Currently this is not legal, at least in FC. My original design for the Radon wishbones was round tubing with extruded plastic aero covers. I asked the CRB for clarification, and after some discussion they decided to not allow this approach. I believe they were concerned about the covers becoming much more than just fairings.

Thanks, that is good information to have.

I would never have considered the streamlining or brake line clips to be a part of the suspension since the GCR gives this definition of suspension:

Suspension is defined as the system of springs, shock absorbers, control
arms, links, etc., supporting the vehicle on its axles.

I'm curious, did you get a paid for ruling or just talk to a CRB member? I think we have a few CRB members making up rules as they go and acting like their word is the final authority.

BTW- did you find an actual source for the plastic, or were you going to have it extruded for you?

Wishbones made from round tubing would cost a fraction of those made from streamline tubing, and the plastic covers cost about $.10/foot.

I agree completely. It would be kind of nice to be able to make everything fit up with a pair of scissors instead of lots of hand fitting.

[nulrich]

I submitted a letter to the CRB via their web page asking for a rules clarification allowing plastic fairings. The result would have been some additional language in the FC rules. I was told that the FSRAC reviewed it and eventually decided that such a clarification was not a good idea.

I designed a custom plastic extrusion and had it quoted, but stopped the process when I learned of their decision.

I did not submit a compliance review, pay my $300 and get a ruling from the Court of Appeals, so it is possible you would survive a protest if you ran plastic fairings. I would consider fairings to be part of the control arms, but I don't know how the Court of Appeals would rule. There are many people running brake line fairings of various flavors, ranging from duct tape to carbon fiber, maybe someone has tried protesting those?

The Piper has anti-intrusion bars on their front wishbones made from aluminum, and those are clearly illegal, but who's going to protest them? Every FC car I've seen is illegal in some way or another, but not necessarily in a way that provides a competitive advantage.

Nathan

[Dick R.]

FYI one of the knowledgeable guys from way back told me recently that if the car is still supported with the part removed it isn't a suspension part. The context of the question was aluminum tie rods and toe links for pre 1986 FF's.

Dick

[Brands]

And a late model Van Diemen produces between 95 and 115 lbs of drag at 85 mph in the wind tunnel. That's at a range of "medium" downforce settings with Radon and Cooper series style wings. That translates to 300 to 360 lbs of drag at 150 mph...if you could get there.

Nathan

Just for interest about 10 years ago at Williams on a good day we had about 3000lbs of downforce at 150mph (at our datum ride height) with an L/d close to 3. Not too shabby but that's what you get if you tunnel test 24/7.

[JerryH]

As far as round tube drag vs aero tube drag.....remember that any published data is most likely from testing done in clean air. I've seen data that shows round tube in turbulant air can have less drag than aero tube......unless the aero tube us aligned with the local air fllow.....and we are always in turbulant air.....except for the forward most component.

Of course, as with all race car tweaks, monkey see monkey do......and chassis manufacturers have to supply what people will buy.

I wuold not spend money updateing an older car to aero tubing. Lots better things to spend the money on.....with more noticable results.

Jerry

[Rick Ross]

I wuold not spend money updateing an older car to aero tubing. Lots better things to spend the money on.....with more noticable results.

Jerry

I agree. There are probably some better things to spend money on.......like some new tires!

[Billy Wight]

I was running an analysis and had nothing to do until it finished so I created a spreadsheet that calculates aero drag forces on various suspension arms with inputs for length, velocity, etc. It looks like it would be reasonable to assume about a 6 hp equivalent gain at 100 mph by switching to aero tubes. You can download the spreadsheet in the resources section of my website if you're interested in taking a look.

[LJennings]

Based on some wind tunnel test data that I have seen, the Reynolds Number quoted seem to be low for a complete system model. Maybe Nathan can enlighten us on what Nr they saw in the tunnel.

[Frank C]

Mr. Jennings:
The Reynolds numbers for the streamline shapes in Billy's chart were thickness Reynolds numbers. One has to be careful because lots of airfoil information is presented as a chord Reynolds number (which would be larger) and with a planform area for the drag coefficient.

Jerry has a point, as turbulent approach flow could reduce the cylinder drag by tripping the cylinder boundary layer so it was turbulent with a later separation point and a lower drag coefficient as in the higher Reynolds number case Billy calculates. Laminar airfoils are sensitive to effective angle of attack and can stall, increasing drag.

- Frank C

[LJennings]

Frank, I will agree with you on your statement, but if you base your development program on the lower Nr alone you will probably be disappointed on the track.

I know this will probably bit me in the butt, but I had a friend working for Ralt back in the 80's, and one of their biggest customers came to Mr. Tauranac with a request to fit aero tube wishbones to the RT5 and RT4 instead of the flattened furniture tubing.

I was told that Ron being the business man that he was, did not want to change due to increased cost, so he ran a few windtunnel test and found that the turblent flow from the flattened front leg of the wishbone actually help move the air across the rear leg and in turn was the lowest drag.

[Steve Demeter]

Remind me to never stay near a RT-4 or RT-5 if one should show up for a club race. Flattened furniture tubing for wishbones. A bit scary

[Marty Nygard]

It's British humor. They sometimes refer to carbon steel tube shapes (round, square and rectangular) as OCM (office chair material) to differentiate it from chrome moly. Properly sized, carbon steel is an effective and less costly option to chrome.
Didn't Swift also use flattened round tube?
Marty

[JerryH]

What most forget is the local air stream direction varies wildly in the area where the wishbones live. Flow off the wings. flaps. tires etc differs vastly in it's direction from straight.....which can result in the aero tube presenting a larger frontal area to the wind than a round tube in the same location.....and at angles of attack exceeding the stall angle......thus more drag.

Jerry

[Frank C]

Exactly, Jerry. The A-arms also have some sweep which makes simple analysis with 2-D drag coefficieints a little problematic.
- Frank C