Poor Man's Ride Height Sensors: some (more) data

[Jack Olsen]

Some of you may remember that I was trying to find ways to measure the effects of different aero setups on my 911, and was considering different options for ride height sensors. Since the laser sensors start at around $500 each, I went a different route and located some infrared sensors that are used in photocopiers (and cost $12).



They claim to have a .3mm resolution, and output analog voltage for my data logger to read in the following, not-quite-linear way:



I tested them out, and realized the amount of suspension travel I would be trying to measure would probably be too small, given the amount of noise from bumps and the relative stiffness of my suspension. I had an idea for amplifying the suspension movement as a way of artificially increasing my system's resolution, and I threw it together, even drilling a hole in my chassis. (Hey, it can be patched back up.)



I did a setup in the back as well. When I did a quick test, I wasn't getting a good reading from the rear sensor. And since I ended up not having time to test the wing on a straight road before I tested it on the track, I basically forgot about the sensors.

Well, when going through last weekend's data of the wing and splitter versus a ducktail alone, I realized the ride height sensors were still attached. The rear one is still not working. I think it's wired wrong, but I haven't looked at it. But the front one was giving readings, and -- surprisingly -- the readings weren't just a lot of random noise.

Here are full laps graphed next to each other of a session with the ducktail and a session with the wing and splitter.



As you can see, when I group similar laps together, there are repeatable patterns to the front ride height. You can even see the front coming down sharply for each braking zone.

But here's where the surprise comes. When I overlay a lap of the wing/duck/splitter aero kit against a lap where I'm just running the duck and no splitter, my front ride height is higher with the aero stuff in place than it is with just the ducktail.



If I also had rear readings to compare, I'd be more comfortable saying this, but it looks like the big wing in back might actually be lifting up my front end at high speeds.

It could also be that I'm simply working with bad data. I never really finished the lever system for either the front or rear, and there's still more play in the mechanism than I'd like. But I'm surprised at how readable the ride height changes appear to be, and how clear the differences are between two different aero packages on the same track on the same day.

I'll definitely do some more testing.

[Chuck Moreland]

Very cool and interesting.

Looks like the big wing is creating enough downforce to cause a "wheelie" effect! It is sitting a couple feet behind the rear axle, so you have a teeter-totter.

You could prove this out by comparing low speed sections with high speed sections. We would expect smaller height differences in the low speed sections.

Have you been able to calibrate the graphed data to actual height change? Unit-less data may be exaggerating the differences.

[Jack Olsen]

At this point, I'm only half convinced the data is meaningful. But it was an interesting thing to find.

The next step is to get the rear sensor working, and then do some straight-line tests over the same piece of flat road, repeating and repeating with different aero combinations.

[DW SD]

why not try to calibrate the data using a jack to raise the front end a bit and see if you can see how much the front end may have been lifting from the large rear aero? It might make the data more meaningful.

Interesting information.

Doug

[Jack Olsen]

Yes. Static tests will be part of the next round of testing.

[DohertyCM]

Jack,
You have got to be my hero...

If you have enough time and money to sit around and play with this stuff....at what seems to be full time.....you have got to be my hero......

But I remember my neighbor complaining about his car
at high speeds because the front end was pulled up by the down force of the rear end so much that the front would float.

If your "duckwing" setup is raising your front end it would appear that you achieved your goal. But now you have to address a new area in keeping your front end down.

The idea setup would be to push the whole car down towards the pavement...dont you think....

Im not a pro at this, just an amature who is thinking out loud..

Chris

[Jon Merck]

Amazing Jack...you need to be working for a race team or something.

I wonder if graphing the torque/hp cycles from your engine and overlaying it would reveal anything?

[Zeke]

Indeed, some race cars (mainly formulas, me thinks) are designed to do this exact same thing, float at high speeds. When the front is up, the front (wing, in the case of the formula) is at a lesser attack angle gving less resistance to the drag. You don't need downforce in a straight line unless you drive in the NHRA and have 1000's of HP. Hit the brakes, car comes down, downforce balance is achieved until you get up on the air again.

At least that's what I was told by the formula guys. Of course, the NASCAR boys have a different take on this. Springs that are so weak that he car rides on the stops most of the time and on the ground to prevent air from creating a pressure zone under the car. The formula car has little underside and the air is managed well.

I wonder where the 911 fits in with its relatively small footprint. Probably closer to the stock car. Still, it would be interesting to see if additional front downforce actually resulted in faster lap times rather than having the effect of holding the car back.

I think Jack will know the answer to the question before long.

[jeffgrant]

Jack... do you have any specifics on where to get those sensors? Very interesting!

[Jack Olsen]

Originally posted by DohertyCM
If your "duckwing" setup is raising your front end it would appear that you achieved your goal. But now you have to address a new area in keeping your front end down.

That's one of the things that makes this data unexpected (and possibly unlikely). If you add a ducktail to the back of a car with no front spoiler, you still increase downforce in the front of the car. Front and rear balance is a funny thing with these cars; it's not as much like a teeter-totter as it appears.

Originally posted by jeffgrant
Jack... do you have any specifics on where to get those sensors? Very interesting!

I got the sensor here. But I'd caution against ordering it based on this data. There's still a lot that could explain this as 100% wrong in its findings.

And yesterday I ordered a pair of potentiometer-type ride height sensors from an old Lincoln Continental model that might do the job better and simpler. They were $4 each, so I figure it's worth a shot.

[Tyson Schmidt]

Jack, the ducktail and the wing are not comparable.

You shouldn't take the data from one and use it for the other. The duck is a spoiler, and is reducing lift. The wing is providing net downforce, and is doing so high away from the body independent of the rest of the car.

It would be a mistake to assume that the wing will affect the front end the same way the duck will.

[Jack Olsen]

That's true.

But in this case, I didn't have any expections of the rear wing lowering the front end. I just didn't think it would raise it. After all, I was also running a 5-inch splitter up front -- which was removed for the ducktail only tests.

But the data might be misleading. The tail might not be lifting the front at all. I'm going to have to do some more tests before I believe those charts.

[jeffgrant]

Originally posted by Jack Olsen
I got the sensor here. But I'd caution against ordering it based on this data. There's still a lot that could explain this as 100% wrong in its findings.

And yesterday I ordered a pair of potentiometer-type ride height sensors from an old Lincoln Continental model that might do the job better and simpler. They were $4 each, so I figure it's worth a shot.

Thanks Jack... I thought I recognized that sensor. I remember seeing an article about using something similar in a Lego Mindstorm range sensor (Mindstorms ROCK), which was interesting, because it required a linearizing function, and it walked the end-user through the process of determining that function.

Turns out that the article is actually hosted on that site! How convenient.

I'm in the process of writing my own data analysis software that basically interprets data collected by other systems. I've been throwing around the idea of allowing just such a linearizing function within that software so that it could adjust the output plot of the raw sensor input based on either a formula or a series of data points.

Not really interested in picking up those sensors, was more interested in the specs on it.

Thanks for the link.


...jeff

[Vereeken]

Ok this is it he final straw Jack, I am selling all my garage stuff.

I am struggling with getting a bleeding coded lug nut from my wheels and then you post someting like that.

For sale: Mildly used garage equiment. Location: Belgium.
Will ship overseas.

Michel

[randywebb]

"you have a teeter-totter."
- Do you have a power spectrum analysis you can run the data through?

Also, we need to get you into a grad. program somewhere...

[JohnJL]

Shiznit Jack. Nice work. Skin would have gone on my wing this weekend but the car came back early and wants to be driven...

[Zeke]

Originally posted by randywebb
"you have a teeter-totter."
- Do you have a power spectrum analysis you can run the data through?

Also, we need to get you into a grad. program somewhere...

Hey, what about me? Can I carry his books?

[Quicksilver]

Originally posted by Jack Olsen
But here's where the surprise comes. When I overlay a lap of the wing/duck/splitter aero kit against a lap where I'm just running the duck and no splitter, my front ride height is higher with the aero stuff in place than it is with just the ducktail.



If I also had rear readings to compare, I'd be more comfortable saying this, but it looks like the big wing in back might actually be lifting up my front end at high speeds.

If I remember correctly Paul Van Valkenburg's vehicle dynamics book went into this. Either way, the cantalever effect is a well known property of a rear wing.

I wonder if you can use the nose lift data to corroborate the downforce data from the rear...

[911pcars]

"I wonder if you can use the nose lift data to corroborate the downforce data from the rear..."

Jack,
Maybe some well-placed 50-100lb sacks of Ready-Mix or rice on the car (at rest) can replicate the on-road data to arrive at downforce numbers.

Sherwood

[Chuck Moreland]

Originally posted by 911pcars

Maybe some well-placed 50-100lb sacks of..... rice on the car (at rest) can replicate the on-road data to arrive at downforce numbers.

Sherwood [/B]

Instead of putting all that rice on the car, he can just go with a double layer wing and maybe some Type R stickers