Ride Height

[Magpie]

Well I have finally got around to doing the install of the ride height sensors (Electro Optical ones, measure between 40mm and 300mm with a response of 39msec) and hooking them up to the data logger. The front sensor is on the centreline and below the power steering pipes (mounted on the splitter), the rear is just behind the diff on the centreline as well. Purchasing 4 sensors whilst may have given better data it was just not worth it.

Thepaln is to now record the ride height at various road speeds as well as do some cost down tests to see if my current spring rates will be suitable for 200 kmh and the down force of the wing/splitter. I will not be doing 200 on public roads, but I will calculate what the ride height will be at fast than road speeds based on the current/future data I have. Hopefully I should be able to balance the front rear downforce.

I have done a base line, and when I hop in the rear goes down 5 mm and the front 4mm (need to measue if the seat is central between the sensors).

[Magpie]

Well a little more data today, did not go out with the intent to collect, just do a club run.

Coast down run
Rear Front
Standing 120 108
70kmh 120 102
115kmh 113 100

Acceleration (11 sec)
69 kmh 115 100
117 kmh 114 113
however when I took the foot off the gas (2 secs later), the weight has moved to the front of the car.
119 kmh 116 109

If I have time in the next few days I will do some proper testing and change the angle of the wing to see if there is any detectable changes. The valves appear to be quite small, however using 8/6 springs it would appear that on the rear 7mm of travel equates to 84 kg (42 kg each side) of downforce at 115 kmh and on the front 128 kg. Easy enough to check I can get somebody to sit on the rear of a know rate and just check it out.

[Magpie]

This is more a blog

Did some checking today taking a break from a uni assignment.

When I put 97 kg above the rear sensor there is a decrease of 5.7mm (the front rises by 7.7mm) so the decrease on the rear is 17kg per mm.

Also doing coast downs from 120 to 70 (wing at 5 deg) the Drag coefficient = 0.363372279 and the Coefficient of rolling resistance = 0.006564279. Now to test the other wing angles (0, 10 and 15 deg).

[Magpie]

Coast downs with wing at 10 deg (6 runs in total 3 each direction):

Drag coefficient = 0.3405 (-0.02287 from 5 deg), interesting that the wing at 10 deg is less drag???? I did have my other half in the car for the 5 deg test but have taken her weight into consideration. Sounds like I have to redo the 5 deg tests, lesson is to keep things the same as much as possible.

However doing a pull in 3rd gear on a certain stretch of road (just me in the car) the results are:

starting with data from 50 kmh after 10 secs with 10 deg on rear wing speed is 109.8 and at 5 deg 112.3.

Next test is with the wing at 0 deg and removed. Plus I need to do a few runs holding 110 to see what effect the wing has at 10 deg and other angles.

This testing is not helping do the uni assignment but helping me understand the changes in the car with respect to aero and weight.

[manga_blue]

Rear Front
Standing 120 108

Interesting experiments, magpie. Any particular reason why you've started with 12mm rake? Are you compensating for rear downforce? Emilio posted 6mm as the ideal figure years ago and everyone has pretty much gone with that since.

The Miata seems to work best with about .25" positive rake (rear higher) measured at the pinch welds without driver in car and about 1/4 tank. It seems the the roll center axis doesn't like to be too far out of sync with the roll centers. In general, you can lower the rake to increase rear grip and improve transitional stability up to the point that the rear suspension begins to bottom. Lower the front to increase front grip and turn in response, again, until it begins to bottom the suspension excessively.

[Magpie]

There is no rake, it is just where the sensors are installed, it just so happens that the rear sensor is 12mm higher than the front one. The measurements that I am doing are from a baseline so are relative to when he car is stationary on a flat surface.

I picked the best spots to protect the sensors. For the front sensor I cut a hole in the splitter as it would have been too close to the ground and I would have just ripped it off. I will get some photos of where they are next time I jack the car up.

[Magpie]

Well did some 3rd gear pulls with the wing at 0, 5 and 10 deg

Starting from 45.7kmh (2,900 rpm) after 9.09 sec
00 Deg 108.2 kmh (6,730 rpm)
05 Deg 107.9 kmh (6,600 rpm) - 108.2 kmh was acheived at 9.26 secs
10 Deg 104.8 kmh (6,490 rpm) - 108.2 kmh was acheived at 9.83 secs

Therefore it could be argued that the wing at 10 deg is about 1 sec slower going from 45 to 110. It is not possible to use ride geights from these tests as the car is acelerating, next is to go on the highway and hold some constant speeds for a while with the wing at different angles to see what the downforce difference is.

Uni assignment is not getting done, shame it is on electronic law and not car testing.

[Magpie]

Drag coefficients (Wing Angle):

00 Deg: 0.25293
05 Deg: 0.36337
10 Deg: 0.34050

Wing 0 Deg (F/R ride height)
000 Kmh 118mm 98mm
110 kmh 117mm 106mm
130 kmh 115mm 103mm

Wing 5 Deg (F/R ride height)
000 Kmh 118mm 98mm
110 kmh 114mm 100mm
130 kmh 113 mm 105mm

[Magpie]

Ooooops I just noticed that the front and rear are around the wrong way


Wing 0 Deg (F/R ride height)
000 Kmh 98mm 118mm
110 kmh 106mm 117mm
130 kmh 103mm 115mm

Wing 5 Deg (F/R ride height)
000 Kmh 98mm 118mm
110 kmh 100mm 114mm
130 kmh 105mm 113mm

[rascal]

This is more a blog

..using 8/6 springs it would appear that on the rear 7mm of travel equates to 84 kg (42 kg each side) of downforce at 115 kmh and on the front 128 kg. Easy enough to check I can get somebody to sit on the rear of a know rate and just check it out.
....
When I put 97 kg above the rear sensor there is a decrease of 5.7mm (the front rises by 7.7mm) so the decrease on the rear is 17kg per mm.

just so this isnt just a blog...

Did you add the 97kg directly above the rear axle centreline? (length-ways and width-ways) ie on the small body panel behind the rear window. If not then you may have a lever effect happening which may account for the diff between the expected 12kg per mm (for a pair of 6kg/mm springs) and the measured 17kg. Though over 40% diff seems a lot still..

Also it seems the ride height decreases are not consistent with speed/angle,
eg at 110kmh ride height dropped by 1mm at 0deg and 3mm at 5 deg - this sounds feasible.
however, at 130kmh ride height dropped 2mm for 0 deg, but only 1mm for 5 deg??

Would love to see more (consistent) figures so can then make more accurate guesses about downforce produced.

ie above figures show wings downforce at 130kmh is likely somewhere between 36kg and 51kg at 0 deg and b/w 60kg and 85kg at 5 deg. (50% increase) and we could extrapolate that out to higher speeds (assuming downforce increases with speed in a linear fashion (?) then that would suggest downforce figures well in excess of 150kg the closer to 200kmh you get. And the stronger the mounting your aero bits need to have to not break under the load.

Lastly, kudos for the testing/analysis work and for sharing your results. Impressive stuff. and very interesting what your results show..

[Magpie]

Rascal thanks for the comments.

No I did not add the weight to the car, I worked it out from the spring rates. I have minimised the lever effect by the placement of the sensors. The front is on the centreline of the car and between the front wheels. The rear is also on the centreline and just behind the rear of the diff.

I will be repeating these tests again soon as I noticed the inconsistencies that you pointed out. The plan will be to do them again but on a different road surface, the section I used was also concrete pavement and this can cause problems. Another reason could be that after a certain point the wing can stall, that is despite an increased AOA no more downforce is generated however drag is just increased.

Plan is to first build another splitter to test out and measure air pressure on the bottom of the splitter as well as on other places. The Innovate gear I have in the car means I can measure 2 instances of pressure (vacuum and boost) with a range -14.7 to 29.4 PSIg. All I need to do is run tubes to where I want to measure the pressure and hopefully this will give me some good data. However I’ll be restricted to about 110 kmh and the data will have to be interpolated until I can get some track time.

The problem with the track is it is very rare that the car will be cruising as it will either be accelerating or braking so getting data of downforce is difficult. I have collected about 100 mins of data from my recent track day and will see what sense I can make of it. However I’m not collecting brake/TPS so it is a bit of a guess as to when brake/accelerate is, I could use the g-force at a pinch.

Also I’m looking at using a Arduino to collect some data, this is my December/January project. The Arduino can take sensors, like infrared temperatures, yaw and log these to a SD card.

I’m more than happy to share the results and if needed the raw data. Another nice would be doing the tests on a stock NA but that would mean moving all the data logging stuff I have to it and I’m not in the mood 

[greenMachine]

This is great stuff. I am interested in building a wing/splitter for what is basically a CAMS 3J car (Supersprints Type 3), which has highly restrictive rules on permitted aero, and am following this with great interest.

A couple of points/comments:

I would not assume linearity with aero effects, either AoA or airspeed - datalogging is the only way to go imho;

tuft testing would well worth trying, both to visualise the airflow in the vicinity of the wing, and to see the behaviour of the airflow on the surface of the wing. If you can film it from the side (and underneath the wing (which is the important bit)), so much the better. This might involve an amount of preparation, including camera setup and the frames to hold the wool tufts, but would make a world of difference to your ability to interpret the data from your sensors.

[Magpie]

Well I have had a chance to look at the data from the recent QR session. I played around with the wing settings a few times, however the main purpose of the day was to run the car in not play with the aero setup.

I filtered the data (logged at 0.18 second intervals) for when the car was at zero G (acceleration), however ignored the sideforce G's. The data shows how far above 'at rest' the car was. The data at 115k needs to be investigated more as it would appear that there may be some influence from sideforce G's. I'm going to look at a track map for spots where my speed was around 115k and see if it was in any of the turns. The sensors whilst on the centreline of the car will still be influence by the tilt of the car sideways... more work looking at the data.

This is the first time that I have used data from the car when it is in motion. The previous data collection was done in very controlled conditions, coast down tests.

Looking at the data separately the front (in general) above 120k the front splitter started to have no effect as its downforce started to decrease.

With the rear, the wing starts really working at 115k, but with the wing at 5 deg it starts to overpower the downforce of the splitter and starts to lift the front of the car. However, unlike the front splitter the rear wing does not produce enough downforce to get the rear end below its at rest height. There is almost 11mm difference between having the wing at 0 and 5 deg and using spring rates this equates to almost 130kg more downforce.

[rascal]

hmm, these results are really surprising/confusing...

so, If I have read this right, 5deg on your wing gives 130kg more downforce than 0deg but still doesnt meet resting height? (meaning significant kg of lift from the body alone, which the aero cant overcome)

Also very surprised that faster speeds show a reduction in downforce. I would have thought that more speed should at least show a linear increase and perhaps even an exponential increase in downforce.

Is it possible that the tyre's rolling circumference is expanding with centrifugal motion, thereby raising the height of the sensors and distorting the results? (plucking ideas of out a** to try and understand why downforce is decreasing relative to speed above 120..)

[Magpie]

Rascal, all good questions!

The old data I posted is now useless because the car has changed significantly since the last lot of testing. Hence the need to repeat it all again.

I would like to do a baseline regarding speed/height however going over 120k even in 110 zone is not worth getting caught or even the attention.

Yes the rear wing can be cranked up to almost 15 deg, however this will seriously over power the front downforce. However I do plan to repeat the coast down test (from 120) with a new designed splitter as well as the wing at various angles.

Now that I can get the logworks software to put the info into manageable tables I can do more analysis. The results were not that surprising to me, however I was surprised to see the splitter effect start to taper off as the speed increased.

You also need to consider that drag (generally) is proportional to the square of speed. Simply, doubling the speed of the car produces four times the aerodynamic drag and uses eight times more power. Therefore the graphs for the rear is showing this, however the data at 115k needs more analysis. I have data up to 160k, however I excluded it because of the sample size (under 50 points' roughly 5 secs of data)

I have some thoughts on a new splitter design and since I can make one in a day I can experiment over the few weeks I have off. Plan is to also measure the air pressure on both sides of the splitter whilst doing the coast down tests.

Thanks for the suggestion on tyre size increase, I will see if I can account for this effect, more research.

Have a look here http://ecomodder.com/forum/tool-aero-rolling-resistance.php to see the effects of drag. Based on my numbers 161 hp will get me to 225 whereas 33 hp gets me to 130k. An extra 128 hp for 95 extra k. Hence why keeping drag down is a good idea for low powered cars. Having a huge AOA for the rear wing will cause too much drag, lighten up the front end too much and upset the balance.

Once again thanks for the feedback.