170 replies to this topic

[Dave Crevie]

Actually, we have. Cutting the backs out of our bodies did knock a tenth or two off the lap times. Although infinitesimal, there is some suction created under the body. Adding a lip spoiler at the rear of the body, like that allowed by IRRA retro rules, increases that effect. Before vac formed bodies, this wasn't possible. Wing cars take the aerodynamic downforce to the extreme. 

[Mark Onofri]

Ok, I used two (2) different types of cars to, hopefully,illustrate the difference.
I have to go on a tangent for a second. Mr.G.Bomd and I were discussing the theoretical possibly that benches do exist under slot cars and parts. Although my findings are inconclusive at the moment I discovered something more significant. If you play a original, not remastered version of Abby Road while you work on slot cars from the same era, it is the equivalent of working at-257°c. In layman's terms, the laws of boundary layer physics no longer exist. And, even more significant,it is a hell of a lot of fun.
Ok,in this picture,we have two semi real cars. The distance from the centre of the guide to the r.wheel is ~=.
In the second picture, I'll call them silhouettes, notice the centre of the rear wheel on these two? By the time I translate "Race Car Vehicle Dynamics"* to something everyone could understand,we will all be at the great track in the sky. Would you like to, please, explain it.
* Martin, I thought you might like the dynamic part of the title.

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[Dave Crevie]

Since absolute zero is -273⁰ C, and, at least theoretically, you can't get colder than that, (enthalpy and entropy cease below that point), you would be unlikely to be able to work on slot cars at -257⁰ C regardless of the background music.    (Nemst theorized that entropy gradually slows until total stop at absolute zero Kelvin. And if the water in your body doesn't freeze.) 

 

Anyway, to try and answer your question, several factors come into play here. First, the cars in the top photo have side dams (wings). Lets ignore those for now, since they are variables not related to the shape of the body. Also, let's assume that the cars all have the same type chassis. The bottom car in each photo has a canopy, which will interrupt, or at least redirect, airflow coming over the car. That could, and most likely does, reduce the downforce in that area. That changes the area where the greatest downforce is realized. In general, it moves that point farther back on the body. I usually compensate for that, at least partially, by moving the body farther forward on the chassis. The position of the rear axle can come into play here as well. A shorter distance from the guide flag to the rear axle can in effect move the center of the downforce farther back. (assuming that none of the cars have front wheels contacting the track surface and the guide flag alone is supporting the front of the car.) That seems to contradict the theory that a longer chassis is inherently more stable, and will handle better. Here's where you have to decide which is more important, downforce or stability. Adjusting the placement of the rear axle is not always easy, but you can change to a longer body, with more overhang behind the rear axle. This places the rear lip spoiler farther back, (remember our discussion earlier in this thread?)

 

One last little detail. I can't tell in the photo of the upper car in the lower photo (let's call this car #3) if it has spill dykes. But the bottom car does have them at the rear of the body. These short air dams normally run the length of the sides of the body, and are sometimes called "fences" on real cars. They have the same effect as STOL tips on real airplane wings. There, they keep airflow from spilling off the ends of the wings. On race cars, they keep airflow from spilling over the sides of the body, keeping it on top where it can do the most good. On slot cars, I'm not sure how much they effect aerodynamics, but if they work for you, go with it.   

[Mark Onofri]

Ok, I could have been wrong about the temperature. Totally agree with the rest. The part that, to steal a Who verse," I know what it means but.....I can't explain.
My friend Gordy, was fanatical about the centre of the rear axle being perfectly inline with the apex of the body. (Wing cars photo) I figured that the two (2) different types of cars would make it easier to illastrate.
Would you please take over from here?

[Mark Onofri]

PS: I do know that
1) the rearward placement of the wing can be used to generate/accelerate negative pressure under the body.
2) I'll get back to you on #2

[Mark Onofri]

Better? The left car is a Parma b&w chassis with a G-27 mura. The car on the right,is a racer products b&w chassis with a mura 12x. The bodies are, ironically, the same. The G-27 is a evilbay purchased so it's origin is unknown. The x-12 is by me or Gordy.

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[Dave Crevie]

Looks like I have to do another drawing, but not tonight. I'll get it tomorrow morning. 

[old & gray]

Remember when you are doing your analysis of aero forces on chassis to include the placement of the pin mounts on the chassis. Simply assuming the center of pressure on the body translates directly to the chassis is a flawed assumption. This can (and was proven by experiment) change the handling of a wing car which is highly aero dependent. Moving the rear mount backward increases traction, moving it forward reduces traction but makes the car easier to drive as the aero downforce on the front of the car increases. Note this test was over a limited range of perhaps 3/4 of an inch. Another interesting experiment is to install a pin tube in the holes of the sides of a Parma hot wing on a Flexy One. then compare the handling of the car by moving the front mounting pin from the side pan to the front wing and back. 

 

When the rear spoiler is added there is a possibility the air piling up in front of it will "stack up" along the length of the body increasing the front downforce as well as the rear. This is probably very dependent on the stiffness of the spoiler used. I came to this conclusion when I did the testing for my senior thesis in collage. I tested a 1/24 model in a wind tunnel with separate force gages for the front and rear. The spoiler was rigid but removable allowing back-to-back tests with repetition for verification of observations.

[Dave Crevie]

OK. To start with, adding air dams and spoilers completely changes the way the air flows over the body. These figures might help show that;

 

High pressure closer to rear wheels.

 

 

High pressure concentrated farther back

 

 

Adding Lexan side dams does three things. It captures the airflow over the body, preventing it from spilling over the sides. They turn the body into a sort of flour scoup. They also tend to spread apart, increasing the frontal area that the air has to flow over. That increases downforce. In addition, they also effectively lengthen the body, adding an inch or so to the tail of the body. That increases stability. 

 

Now. I think your question more relates to mounting the rear spoiler. As figure 2 shows, at straightaway speeds, the spoiler tends to flatten out. At slower speeds, at the corners, it's natural springiness brings it back closer to it's natural shape. What this does is to keep the amount of downforce at the rear of the body more or less constant. At slow speeds, it has a larger angle of attack, at higher speeds it has a lower angle of attack. This is what Jim Hall was trying to do with his moveable rear wing. The driver could tilt it down coming into corners, for more downforce. Then he could flatten it out for less drag (wind resistance) on the straights.

 

So, for slot racing, the trick is to find the perfect balance of downforce. Your friend was trying to give you a good starting point, and if he only races at his local track, and if he sticks with the same body and Lexan add-ons, not much will change. It's when you start travelling to other tracks that you need to make adjustments. It's why the serious wing car racers bring boxes of bodies, each with different attributes. They use their knowledge and experience to choose the right body. 

 

I don't think I've answered your question, and that is because I can't. I don't know where you are racing and what kind of track you are on. The location of the body on the chassis, the side dams, the rear spoiler, will all change some depending on that. I spent a short time racing boxstock, and had a great teacher for that. But I only raced one track. When retro started, I decided to concentrate on that instead. So my experience in wing cars is very limited.   

[Mark Onofri]

they also effectively lengthen the body, adding an inch or so to the tail of the body. . .
😃 You nailed it, keeping with the long tail theme.
Tonight, I switched from the Beatles to the Yard Birds (of course with Jeff Beck) as much as I like the Yard Birds,I was more productive listening to the Beatles. Tomorrow I'll try the Beach Boys(good, good, good, good vibrations)
2) I remember what the 2ond thing I might know is. In "Race Car Vehicle Dynamics" Jack Brabham's Cooper F-1 was observed taking the turns at a amazing pace. The only thing they could find was a thin strip of wood running from left to right. I'll have to check but, I'm pretty sure Jim Hall realized that it was changing the pressure zone(s). I've been digging up my 20+ year old flexies for a series at Speedies Fast Lane. It almost all came back to me when I found a 1st generation Mossetti that I started to,add a basic form of diffusers based on the Cooper. I'm going to proceed with it now that I have a some what better idea of what I want it to do. 🧐

[Mark Onofri]

Mr.OG, you are absolutely correct. Remember when the latest, greatest thing was square tubing mounted at the extreme front with a shaker pin tube inside? And just a wire in the rear with a tube and the pin tube inside. Effectively turning the body into a plumber pan.

[Mark Onofri]

-273°C ,-459°F,15 Milli kelvins I was close

[Allan Feldman]

Mark; the body you have is indeed the 365P2 long tail, and probably the Lancer version shown in the link in Mike's post. The NART version had a long tail designed by the Drogo brothers.

 

365P in it's original configuration 

365p.JPG

 

365 P2 long tail. Usually called the 365 NART. Notice the louvers in the tail.

365p2lt.JPG

 

Different configurations could be done simply by changing body sections. Here is David Piper's car as a Spyder, with the roof section removed. He also had a slightly longer tail section built.

365p2dp.JPG

 

Mark; The car you show in your post #7 is a 512S long tail. 

512slong1.JPG

512slt2.JPG

 

Some ear candy;

 

https://youtu.be/USB...RbIDp57GLLMn9fW

 

 

Just for info the Drogo bodied NART 365P Longtail was originally first seen at the 1966 Le Mans with longtailed body with no fins; number 18. The number were White on a dark Blue Roundel

It returned in 1967 with fins added to the body. The car was painted white like the previous year. The tailfins where painted Dark Blue with White motifs of an Elephant on them. number 26.

The colour scheme was USA since the team was the"North American Racing Team" The car was known as "The White Elephant\"

[Dave Crevie]

Bob is correct about boundary layer "stack up". I didn't want to get into how the difference in thickness of the spoiler effects the aerodynamics, but a thicker spoiler, which has less flex, pushes the center of thrust forward because the air compresses and forms a "false wall" ahead of the spoiler. That false wall has the same effect as moving the spoiler forward. This can be good, or bad. It requires that certain variables remain constant. Those being room temperature (air temp.) and barometric pressure. On the good side, as the car speeds up, there is more compression ahead of the spoiler, and the center of thrust moves forward, helping to keep the car in the slot on the straights. As the car slows down, there is less compression, and the center of thrust moves rearward. The downside here is, that as the car slows down, you have less downforce in general. As for the variables, a hotter room will have less downforce because the air is thinner. And conversely, a colder room will have more downforce because the air is denser. Barometric likely won't change much during a single race, but could if it is a multi-day event. A storm or cold front could move in, and effect that.    

 

-273°C ,-459°F,15 Milli kelvins I was close

 

If we are going to knit-pick, Max Plank determined absolute zero to be -273.15^o C, (-459.67^oF), the point at which carbon atoms (theoretically) cease to vibrate. 

 

Oh. And The Yardbirds over the Beatles, any day. At least until Peter Grant took over, when they started go go on a tangent I wasn't interested in following. "For Your Love" was one of the first songs I learned on guitar. Three chords. Can't get simpler than that. 

[old & gray]

Mr.OG, you are absolutely correct. Remember when the latest, greatest thing was square tubing mounted at the extreme front with a shaker pin tube inside? And just a wire in the rear with a tube and the pin tube inside. Effectively turning the body into a plumber pan.

 

I remember when tech inspectors would question if the "original" chassis came with the front hinge tubes, and I remember a discussion of whether a steel pin tube in back was a significant change from a brass tube. 

[Mark Onofri]

Dave, I wasn't nitpicking, I just thought it was funny.
Jeff Beck has a album/disk titled
" You had it coming"
Amazing!!!
Now, back to our regularly scheduled programing.
When the perimeter edm,etc. chassis became all the rage, the lotus 88's and it's variants were coming to fruition, I built a 1/32 wire version but, the owners of Miniature Grand Prix of Westvaill were adamant in prohibiting me from having ANYTHING except for the wheels and guide touching the track. Even my proposed felt side skirts were unaceptable. The playfit motor was years off so, the first iteration used a monstrous 16-D. I had purchased a mura s.wasp but, I got sidetracked when I bought a black 85 ElCamino.
Now that having the front literally touching the track is all the rage, I'm concidering trying it again. It's been (lonely) in the back of my mind, on and off since then.

[Dave Crevie]

Well, Jeff Beck won't be making any more records down here. He has a permanent gig in the sky. Any guy who loves guitars and can build hot rods is a god to me. 

 

My experience with having any part of the body touching the track has been that that area will pick up rubber and crud off the track and act more as a brake than an aerodynamic enhancement. After a practice run I always check any parts of the body that might be too close to the track surface, and if it shows any black, I trim it back a sliver. If that area is the front diaplane, I raise the front of the body a taste. I can tell you, Chicagoland sells a tech block with an adjustable insert where the guide flag sits. You can set that insert to match the braid height of the track to see exactly what your ground clearance is. 

[Mark Onofri]

Thanks, I'll tell that to Dan Wiggins. Weather or not it will make a difference, I've attached nickel Teflon tape to the front ears. All long with some other strategically placed tape sudo defusers.
Getting back to the significans of the centre of the axel in relation to the apex of the body, I tried it with a Parma intrepid. Granted, I had to cut a significant amount of the rear coach work off to get it to go. The results were impressive. Not quite legal but, impressive.

[Mark Onofri]

Should we stay on the
Tails
Or, start a new thread on fluid dynamics?

[Bill Breck]

Bob is correct about boundary layer "stack up". I didn't want to get into how the difference in thickness of the spoiler effects the aerodynamics, but a thicker spoiler, which has less flex, pushes the center of thrust forward because the air compresses and forms a "false wall" ahead of the spoiler. That false wall has the same effect as moving the spoiler forward. This can be good, or bad. It requires that certain variables remain constant. Those being room temperature (air temp.) and barometric pressure. On the good side, as the car speeds up, there is more compression ahead of the spoiler, and the center of thrust moves forward, helping to keep the car in the slot on the straights. As the car slows down, there is less compression, and the center of thrust moves rearward. The downside here is, that as the car slows down, you have less downforce in general. As for the variables, a hotter room will have less downforce because the air is thinner. And conversely, a colder room will have more downforce because the air is denser. Barometric likely won't change much during a single race, but could if it is a multi-day event. A storm or cold front could move in, and effect that.    

 

 

If we are going to knit-pick, Max Plank determined absolute zero to be -273.15^o C, (-459.67^oF), the point at which carbon atoms (theoretically) cease to vibrate. 

 

Oh. And The Yardbirds over the Beatles, any day. At least until Peter Grant took over, when they started go go on a tangent I wasn't interested in following. "For Your Love" was one of the first songs I learned on guitar. Three chords. Can't get simpler than that. 

 

Four chords: E minor - G - A - A minor. 

[Dave Crevie]

You are correct. Getting on to 60 years since I played that tune. So I'll call it a "senior omission". 

 

Who would have thought Jimmy Page started as a bass player?

[Mark Onofri]

He is not from the plant earth.

[Mark Onofri]

Les Claypool is a bassist. Presumably from the plant earth 🤔
Roger Waters is also a basist, who's origin is unknown 🧐.
I think I'll play
" Interstellar overdrive" and paint a body for Friday.
Speaking of body painting, they actually have re,re,reruns of Laugh In on TV again. Body painting as it was meant to be.

[Dave Crevie]

As long as we are going to drift;

 

Claypool ;

 

https://youtu.be/dZz...B0m0IpU2xaT6XHU

 

Waters;

 

https://youtu.be/QrW...CqcbnWzYe1NJS7d

 

Enjoy.

[Mark Onofri]

💥🤯💥
💥💨🧠
👍