Lift or Down Force ?

[Cajun Kid]

By looking at a few still pictures, then finish line pics and videos can anyone tell if car is Lifting ? Has Down force ? or neutral?

The reason I ask is I remember the A2 commentary and the pics DW took of the Superbird. It was easy to see the lift and then the the corrective down force.

On my car, from both the drivers seat and the pics, I can not feel or see any lift.  Looking at the pics, I use the moon disc realtionship to the fender openings as a referance point as well as the rocker panel and front spolier height.

What are your thoughts or observations?

Car at rest




At 164 mph



180 mph finish line video (click to launch then go full screen)

[John Burk]

Clever method . A piece of masking tape on the body above each wheel would add accuracy .

[jl222]

  Charles the picture angles are different so its hard to tell, doesn't look like lift, you have a better view of car sitting in shop. It should be squatting some from spoiler downforce in rear depending on spring rate.
 Looks good though

                 JL222

[hotrod]

Yes if the car has major lift, or down force changes in ride height you can see them on good telephoto shots of the car.
As you are doing that, the best way is to measure from fixed reference points, such as the top of the wheel (moon caps in your case) and the wheel arch.
A couple pieces of light colored masking tape (on a dark car) or dark tape on a light car, will greatly help you make the measurements.

There are a couple issues to consider however. During the heat of the day mirage can significantly distort the appearance of the bottom of the tires and the rocker panels.
This distortion makes measuring the air gap from rocker panel to the apparent racing surface highly suspect in some cases. By measuring from the top of the wheel (not tire) to the wheel arch you get above the worst of this distortion.

If you want to make more precision measurements take a chalk line and with tape mark reference points that are even with the top of the wheel.
On a good high resolution camera and long telephoto image you can resolve details down to about 1/2 inch wide, so use tape at least 1 inch wide to be sure you can pick it up in the image. The most important issue is visual contrast, ie white against dark or dark against white. The higher the contrast of the reference point the easier it will be to pick it up on the image. To scale it use the known diameter of your wheel as your ruler (measured horizonatally in the image if the image is shot from near 90 degrees to the path of the car. If the image is at a significant angle use the vertical measure from the center of the wheel to the top of the wheel to minimize distortion of your reference point.

Or you can put a tape ruler on the side of the car near the wheel well.

I will post a couple images in a bit to show what you can see with a good long telephoto and then blowing up the digital image on the computer.

Below is your image resized several times. As you can see if you had a hard white reference point above the wheel center it would be easy to measure the exact body lift. The blocky (pixilized) character of the image is due to the low resolution of your original. If you did the same with your original image the quality would be much better.

Larry

[Jonny Hotnuts]

Charles are you data logging?

You can buy a linear potentiometer and data log if you are getting lift or down force, how much and at what speeds.
This one if from the Edelbrock Quickdata stuff (*expensive) but there are many out there and will work great.



~JH

[krusty]

Looking at the enlarged pic, I can't say if you are lifting at the front, but you sure are letting a lot of air under the car...   What is the measurement from the bottom of the air dam to the floor at ride height in the shop?  What is the height of your tire sidewall? (the height of the dam at speed looks about the same height as the edge of the moon disc. As I think about it, it sure looks like the front is up.     vic

[Peter Jack]

The budget potentiometer is a piece of welding rod with two o-rings on it. Mount it to either the frame or an A-frame and an eye mounted on the other with an o-ring on each side of the eye. You'll soon know what your suspension travel is and you can figure your body lift or squat from that.

Pete

[Stan Back]

Ron Joliffe told me years ago that he quit racing the original Rocket Science '50 Olds after seeing pictures of it in the lights.  As I recall, the top of the rear fender opening was about even with the top of the Moon disc while standing still.  In the lights there were inches of clearance above the top of the tire.  He had no sensation of lift, either, and is an experienced and sharp piloto.

[jl222]

Looking at the enlarged pic, I can't say if you are lifting at the front, but you sure are letting a lot of air under the car...   What is the measurement from the bottom of the air dam to the floor at ride height in the shop?  What is the height of your tire sidewall? (the height of the dam at speed looks about the same height as the edge of the moon disc. As I think about it, it sure looks like the front is up.     vic

  Yea Charles, if you take a card and square it with bottom of Hotrods foto the the air dam and rim look the same height, now all
you have to do is go measure
  And if Hodrod could blow up the rear tire 

           JL222

[SaltRat]

Thanks for that!
I used the "paperclip on the throttle linkage" to gain 15 mph in my mile speed (from totally shameful to merely embarrassing).  Big help, and thank you again!

The budget potentiometer is a piece of welding rod with two o-rings on it. Mount it to either the frame or an A-frame and an eye mounted on the other with an o-ring on each side of the eye. You'll soon know what your suspension travel is and you can figure your body lift or squat from that.

Pete

[Interested Observer]

Deducing aero forces from suspension travel is a dodgy endeavor unless the suspension rate is relatively low.  Many LSR cars have very stiff suspension and/or travel limits for the express purpose of restricting body movement and attitude changes.

The problem with the o-ring or ty-wrap technique is what happens when driving over a bump.

For one-off testing and instead of an LVDT* and data system, one could inexpensively rig up a common rotary potentiometer and read or record the variation with an ohm meter.

*LVDT = Linear Variable Differential Transformer.
It is highly likely that the “linear potentiometer” shown earlier is, in fact, an LVDT.

[Peter Jack]

On the Dallara Indy cars we ran we had load sensors on each corner that measured the load on each corner. Looking at that data after a run gave a pretty good indication of downforce. Those sensors are probably fairly available on the used market. The problem may be getting a data collection system that can read the results.

Pete

[MAZDA1807]

Charles are you data logging?

You can buy a linear potentiometer and data log if you are getting lift or down force, how much and at what speeds.
This one if from the Edelbrock Quickdata stuff (*expensive) but there are many out there and will work great.



~JH

If you are logging a cheaper way to moniter lift, up or down, is to use throttle position sensors.

[Cajun Kid]

I have no data logging at all.  

Looks like I may have to look into a logger.  

For Maxton I raised the cars ride height as to make certain nothing hit the track or pit/return road.

Since nothing hit (in 4 passes) I am going to lower the car both front and rear another 1/2 inch to start. Keeping the same rake as the handling is great, the bumps seem to have no effect, car goes straight and the suspension takes up the bumps without even a wiggle from the car  (the in car camera run shows my hands and sterring had no inputs other than to make the dogleg in shutdown).

One of the reasons I was trying to determine any front or rear lift is deciding on adding a Step Pan,,, and I can adjust the rear spolier lip angle.   I have  0 , 15, 30 and 45 degree lips,,in 1 inch lip and 2 inch lip lengths from flat,  all those runs used the 30 degree 2" lip.  If I was not getting rear lift,, I could reduce rear angle from 30 degrees to 15 and that would also bring the nose down (if it was lifting some) 

Charles

[MAZDA1807]

If you look at alot of the doorslammer cars that run in LSR they have a pretty hard rake. Just look at a pic of Mike Cooks Alpha. I have been told to rake my car because it's quite a handful.