measuring rear downforce on a lsr car

[wayne petty]

i have been thinking about how to measure downforce / loading of the rear tires on the back of race cars for a while.(without a windtunnel). i read and saw the videos of the spins up on the salt and remember seeing all the nextel cup cars loosing the rear and swapping ends.

my ideas. optional and open for debate or whatever.

using bimbo double acting cylinders in a master / slave set up piggybacked to the rear shock mounts. left and right slave cylinders in cockpit agenst a scaled backplate. so when the car goes down under load the master is compressed at the rear end and the slave is extended or retracted depending on which way it is plumbed. video cameras record both the run and the movement of the slave cylinder motions.    

i have followed a trench plate shoring co. flatbed trailer out here in los angeles and they have a gauge mounted in a tail lamp hole.  yes i was that close. the gauge bumps around but shows how much weight is on the trailer.  i guess it was cheeper to install that then getting nailed for being overweight.  it looked like it was attached to the air suspension on the airbag side of the control valve.

idea #2  uses the gauge on the dash. and a custom short stroke hydraulic cylinder to mount the rear springs to. or the coil overs.
this would not be hard to build.  one could almost use a disc brake caliper cut in half.  cut in half means removing the stationary side of the caliper. or maybe even leaving it whole and inserting a block ( i am thinking of a gm style front brake caliper) to stop the piston from coming totally out if the car goes airborne.

idea #3 uses electronics and sensors from lincoln mark 7 air suspension. i have not researched what voltage in and out these level sensors use.   someone will know...  and i thought i knew everything.

these could be hooked to a data logging system that may be installed on the cars.

i just think a bit more down force might be better. i've never been to the salt. el mirage a few times decades ago.  both my autoshop teachers 78/79 were members of the 200 mph club. roger gates and monte wolfe so i kind of got lead that way.  i used to run a renault daulphene powered sand rail up there as a kid. OCIR was a favorite haunt and i worked for big willie robinson at terminal island raceway back in 83 84.

at ocir in the late 70's i ask a wheel manufacturers  rep if they could cast directional blades in the wheels. he said it was impossible.  i guess he was wrong. hmm  and i ask my brother in law a professional plumber back in the early 80's if lead is so poisioness whey are we using it to soldier pipes?

wayne

also got me thinking some of you remember my last boat trip.  i navigated with a handheld gps.  has anyone installed a gps unit on the dashboard  with a rooftop antenna to accurately  measure groundspeed. and map it out after the run.

[kb426]

Several years ago when Ric Galles had the Valvoline deal, he had a small wind tunnel set up in there shop. The thing that interested me the most was a wing dyno. One of my friends who lived in Albuq. took our wings over and put them on the dyno. They didn't have as much down force as we thought or was current theory in drag racing at the time. We checked them at 100, 150, and 200 mph. This deal had a load cell on the mount and that would be my suggestion to your question. A load cell under each mount hooked to a recording device and you'll have real time info.

[1FATGMC]

Hey Wayne interesting ideas.  On the salt we try to achieve most of our down-force with weight vs. aero devices.  Since almost all of our HP is going into overcoming aero drag anything that contributes to it is avoided.  Any wing or spoiler that adds down-force also has some aero drag associated with it.  

Some of the classes allow them and some don't. You will see some lakesters have a wing on them for down-force.  I'm not sure if this is the way they intentionally wanted to go of if they were forced into it with no place to add weight.

It is unbelievable how much HP is required to overcome the aero drag on high speed cars.  It takes 8 times the HP to go twice as fast.  I figured for us to go from the low 240's to the low 250's would require about 150 more HP than we had in 2006.  With the added boost we had it, but the track wouldn't let us get it to the ground.  Hooley was able to run at 100% throttle for on short periods of time before wheel spin set in.  Hard to believe you can spin the tires at 240 mph, but you do trying to put 850 HP to the ground through 5 inch wide tires on a service that only has about .4 to .5 the traction as pavement with a wall of air in front of you that might just as well be the side of a building

We added about 150 lbs of weight to the front of the car this year and about 200 to the middle of the car.  More weight is in store for next year for both positions.  It doesn't hurt acceleration as Hooley was only able to use 25% to 29% throttle in 1st and 2nd and our 1st is a 1.90 ratio with 2nd being about 1.3, really tall gears.  3rd was 1 to 1 and 4th was .93 overdrive.

We would like to add some data acquisition to see if the car is going down or raising while it is at speed.  We ran out of time this year for that. It could be as simple as some markers on some rods that showed the limits of travel up and down.

c ya,

Sum

[kb426]

My two cents worth on wings at Bonneville would be for stabilization. One of the first things you learn when you go to Bonneville is weight is important. When you see belly pans out of 1/4" plate, that should tell you something. The lack of traction is so different than most forms of racing that you can't take what you're used to and think it will be fine. Bonneville is probably the most interesting place I've been to because of the diversity and the fact that as a pavement racer, most of my thoughts don't work there.

[1FATGMC]

   

My two cents worth on wings at Bonneville would be for stabilization. .................

That is why we run a spoiler that we hope is neutral as far as down-force goes.  The spoiler allows us to run the side Spill Plates (vertical fins on the side of the spoiler) that help to keep the car running straight like the tail on a plane.  We have them as large as the rules allow.

Wayne I forgot to mention that some people do run gps at the salt and record their run that way.  We data log rpm, so I know our speed at any moment, but I don't know where we are on the course exactly at that moment.

c ya,

Sum

[Crosley.In.AZ]

Would a GPS provide a more accurate speed ?  

The tires possibly have some wheel slippage against the salt ?


Comment on the down force ..... I seem to recall the first year for 974 the car was dropping down on the bump stops at speed??  Suspension was bottoming out?

[1FATGMC]

Would a GPS provide a more accurate speed ?  

The tires possibly have some wheel slippage against the salt ?


Comment on the down force ..... I seem to recall the first year for 974 the car was dropping down on the bump stops at speed??  Suspension was bottoming out?

Some guys data log front wheel speed, rear wheel speed, rpm, and drive-shaft speed.  Then you can tell what is slipping.  Of course the only real slip that counts is the SCTA one they give you at the end of the run.  We don't have the ability to data log more than 6 inputs at this time and we have the most important ones data logged now.

We Hooley tore the motor down this winter the clutch was pretty severely burned, so that was slipping last year.  I can see rpm bumps on the data logs and thought it was wheel slippage, but now it might be that and clutch slippage.

Tony I don't think we knew where the suspension was sitting the 1st year, but when we added weight in 05 you are right the car was bottomed out front and rear.  Not good at all since that was the year the course was so rough.  We got the front working and were working on the rear when the blower seat blew and we were done.  That is probably the year you were thinking about.

For 06 and 07 Hooley has different shocks and springs that give us more control, but there aren't enough runs to really try much and analysis it.

c ya,

Sum

[Bruce Dorsi]

... but there aren't enough runs to really try much and analysis it.

Sum, is that because there are:
(a) too many vehicles being run?
(b)because of weather and track conditions?
 or (c) because of mechanical problems?

I've never made it to Bonneville, yet, so I have no idea of how the actual event unfolds.

[1FATGMC]

... but there aren't enough runs to really try much and analysis it.

Sum, is that because there are:
(a) too many vehicles being run?
(b)because of weather and track conditions?
 or (c) because of mechanical problems?

I've never made it to Bonneville, yet, so I have no idea of how the actual event unfolds.

Yep a, b, and c.  Over 500 entrants this year.  2 courses, but delays for crashes, blown engines/parts lost, and the wind.  Also remember that running on a 1/4 mile track or even Maxton's mile that the cars/bikes clear the track very quickly.  A run takes much longer and ties the track up much longer with the cars running to the 3 or 5 mile and then taking a mile or more to slow enough to clear the track.

We made 4 runs in 04 (all were license runs) and broke, 4 runs in 05 and broke,  about 9 runs in 06, got a record and didn't break.  2 of those runs were licensing passes for Patrick.  This year we made a run, broke, worked for 4 days and then got in 3 more runs.  You can average about 1 run per day for the first 5 days and maybe 2-3 for the last two days.  Also when you start running over 200 mph you just can't make laps.  You need to check the car out more thoroughly between runs.  

The other big variable is the track.  We aren't running on a consistent surface.  It might have a coefficient of traction of .6 in the morning and .4 in the afternoon.  Besides that the track can go away as it gets tore up.  We ran into that on Friday and to some extent on Thursday.

Density altitude can be in the high 4000's in the morning and the high 7000's in the afternoon.  We now need to try and time our runs for the morning if we want to maximize our efforts.

So in a good week you are probably lucky to get in 5-6 good runs with a fast car.

c ya,

Sum

[wayne petty]

   

My two cents worth on wings at Bonneville would be for stabilization. .................

That is why we run a spoiler that we hope is neutral as far as down-force goes.  The spoiler allows us to run the side Spill Plates (vertical fins on the side of the spoiler) that help to keep the car running straight like the tail on a plane.  We have them as large as the rules allow.


hmmm... that statment brought up an old idea.   a hollow spoiler with an open lip... like a siphon. the upper half is a conventional shape.. the lower half follows the shape of the rear of the deck lid or body down.. inside it is hollow and there are holes drilled into the rear panel so as the car gets to speed the low pressure across the tip of the spoiler sucks the air out of the car body. or if it is ducted properly out from between the belly pan and the bottom of the car...

i thought of this a long time ago when i spent a weekend working the pits on a aa/fc that mike dunn was driving.. my though was to get the air out from under the back of the body where there were only a few holes..  the car owner was not impressed with the extra idea that all of the flames from the engine fires that they had been having would become a flaming peacock type of display routed through the spoiler..  that was the season mike dunn lost the engine, burned the chutes off, burned the rear tires off, burned the body off, and hit the catch nets at 175. then jumpped out of what remained like something big happend..


i would have loved to have spend the rest of the season with them.

the guy next to us with a s/s XX looked like he had a lot of pressure in his slicks...  he did ... like 32 psi.. he said it stopped the back of the  car from swinging on the top end...

wayne...  thinker and entertainer....


ps  did anybody see the artical on the Hindenberg.. seems they painted it with aluminum powder and iron oxide in a nitrocellulose base. guess they never welded railroad track. talk about explosive...