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Real-world testing

 
Old 01-24-2019, 09:26 AM
  #21  
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Originally Posted by Occam’s Racer View Post
I've looked up linear travel sensors, and there are quite a few available. Has anyone here put one on their car? I suppose I need one each front and back, and some kind of gauge with a max level readout. Is there a plug-and-play solution for this?
This would need to be logged, if you have any spare analog inputs in Megasquirt that would work. A max level readout won't work because a single bump in the road will skew the results. You'd want to drive at multiple steady state speeds, and average wheel travel during those speeds. Using suspension travel, spring rate, and motion ratio, you can calculate downforce. Then you can generate a downforce versus speed curve.
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Old 01-24-2019, 09:38 AM
  #22  
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Originally Posted by acedeuce802 View Post
This would need to be logged, if you have any spare analog inputs in Megasquirt that would work. A max level readout won't work because a single bump in the road will skew the results. You'd want to drive at multiple steady state speeds, and average wheel travel during those speeds. Using suspension travel, spring rate, and motion ratio, you can calculate downforce. Then you can generate a downforce versus speed curve.
I found this sensor and gauge, which pair together, but you're saying it would need to log the data.
Shock Absorber Suspension Travel Sensor with Rod End Joints
Mini Digital Display Gauge for Shock Absorber Suspension Travel Sensor

I wonder if there's a DIY solution that is more like putting a zip-tie on a fork tube kind of thing? Although Turn 7 may be where the most compression occurs, and that wouldn't be measuring what I want to measure, which is the downforce due to speed alone. If I can get a downforce measurement on the back straight at WGI, that would suffice.
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Old 01-24-2019, 10:09 AM
  #23  
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In response to the digital gauge, you'll find that the data is all over the place. It's not like you'll be able to look over as soon as you hit 100 mph and be able to say, "my suspension is compressed 28.5mm". Instead you'll be able to kind of read the gauge and say "I think it's bouncing between 26 and 30mm, so maybe it's like 28mm?". This is why it needs logged. Every pebble and bump on the road will skew the results, so being able to filter and average the data is necessary. Also it should be noted that for what you're trying to do, and the level of DAQ/analysis you have available, this testing would not be done on track, or at least at a typical track pace. It needs to be steady state, so hold at 60 mph for 5 seconds while recording data, hold at 70 mph, etc, etc. If you are driving for lap times, you are always on the gas or brake, both of which skew suspension travel measurements (unless you also have a way to cancel those forces out of the measurement).

In response to zip-tie on the shock shaft. This wouldn't work because you could never measure the travel you want. Imagine you have this zip-tie on the front shock, you're going 100 mph on an infinitely long glass bed and your suspension compresses 28mm due to downforce. If you ever go less than 100 mph after this, you've now decelerated and the body pitches forward, you've now moved the zip-tie more than 28mm. Then you're not on a glass bed and you hit a pebble, again the measurement is skewed.

The linear travel sensor at steady state will get you approximate downforce numbers. This is useful if you are trying to correlate CFD or input into a lap time simulator. If you have a good amount of test cases, you don't care about downforce numbers, you care about the result of the whole vehicle. Therefore, if you can get a consistent driver and record max speed at the end of the straight, average steady state cornering acceleration over the most consistent corner, and lap time, you'd get a real good idea of how the different setups are performing in relation to each other. Does the AIM solo allow you to plot a map with speed along the track map? That would be useful to overlay all the setups.
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Old 01-24-2019, 07:42 PM
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My Aim dash records G's for corners. presumably other race dashes do the same thing.

I wonder if averaging laps with one mod versus another to get a gain or decrease in the g reading is enough?

Aero is mostly aimed at achieving higher downforce and thus higher grip in corner, thus higher g. Isn't that closer to the end result and outcome required anyway?
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Old 01-24-2019, 11:53 PM
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Originally Posted by Occam’s Racer View Post
I'll have a RumbleStrip predictive lap timer in the car, and this will help keep a consistent speed at the exit of Turn 1, which determines the speed on the back straight (in a Miata, anyway), and can easily be over 110 mph (even in a Miata). The Aim Solo can get top speed on the bake straight, as well as G-forces in corners, lap times, etc. By overlaying data of multiple runs, large changes should be obvious, small changes probably not so much. I don't think I can test hardtop windows up at the race track, I'll probably be black flagged if I do that. But I'll coast-down test that one for sure. Hardtop window delete and hardtop vs fastback will be tested, as well as the other tops.
yeah, windows up not feasible at every track. looking forward to the info.
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Old 01-25-2019, 12:29 AM
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I made my own ride height sensors with a $5 potentiometer and a 3D printer. They work well if you have something that can log 2 x 5v analog channels. The resolution is probably enough to gauge downforce alterations if you are running softish suspension. In theory you can figure out drag by how much the front end lifts vs the rear, but that really only works if you are adding downforce at each end and not the middle of the car.

I ended up not running them because I found my lap times were fairly consistent and I could evaluate changes easy enough. But then I haven't done much with aero yet.

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Old 01-26-2019, 02:23 PM
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I have some professional experience in vehicle (specifically tire) testing.
AIM solo is good enough for lap times.
Driver mod will be the hardest part for lap testing. You need a 5 lap standard deviation of < 0.25s (including tire evolution) to be meaningful.
A great way to test drag are coast down tests. Using the same stretch of pavement, coast from high speed to low speed in neutral (80mph -> 20mph) and measure the distance using the AIM Solo (we used a vbox). Shorter distance = higher drag. Need to have a weather station set up to make sure wind isn't too high (5m/s was max for us) but do it on a calm day and you'll be fine.

If you bring it down to VIR or Road Atlanta I could drive for you
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Old 01-26-2019, 05:20 PM
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Originally Posted by Morello View Post
I have some professional experience in vehicle (specifically tire) testing.
AIM solo is good enough for lap times.
Driver mod will be the hardest part for lap testing. You need a 5 lap standard deviation of < 0.25s (including tire evolution) to be meaningful.
A great way to test drag are coast down tests. Using the same stretch of pavement, coast from high speed to low speed in neutral (80mph -> 20mph) and measure the distance using the AIM Solo (we used a vbox). Shorter distance = higher drag. Need to have a weather station set up to make sure wind isn't too high (5m/s was max for us) but do it on a calm day and you'll be fine.

If you bring it down to VIR or Road Atlanta I could drive for you
Is distance more important to measure or time? I found a neat coast-down calculator on Grassroots Motorsports, and their spreadsheet uses start speed, end speed, and time. But not distance. It measures drag in pounds, and I'd also like to be able to convert that to Cd, if someone knows how.

I'd love to take you up on VIR, it's so on my bucket list. But the thing about WGI is that 1) it's in my back yard, and 2) I have 14 hours of open track time (no run groups).

The driver I'm looking for can easily do 5 laps within .25 seconds. I'm not a great driver as evidenced by these overlays, but these three consecutive laps are all within .1 second. (This was from a tire test my brother and I did at Thunderhill). My brother kind of pokes fun at me for not being consistent enough, and so I'm looking for someone who can do five laps and make this chart look like one line.


Last edited by Occam’s Racer; 01-26-2019 at 05:31 PM.
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Old 02-02-2019, 11:28 AM
  #29  
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Warning! Short story ahead!


I am in the process of developing a setup for aero testing on track. Uses a pitot tube (dynamic pressure with yaw sensing), sensors for relative humidity, ambient temperature, barometric pressure, strain gauges bonded to camber plates, strain gauge temperature, strain gauge amplifiers, and analog to can bus converter. I am calibrating the microstrain output of strain gauge in a load cell then correcting on the car with offset coefficients to correlate for corner weights using some freshly calibrated corner scales. There are many ways to skin the cat but this is my approach for what might be considered steady state aero tests. I correlate corner loads to dynamic pressure instead of solely GPS based or wheel speed to eliminate some uncertainty in testing, (head wind, tail wind, crosswinds). For total drag force, I have been using the primitive approach of taking average longitudinal g force during deacceleration while coasting down correlated to dynamic pressure and the taking into account the mass of the vehicle. I then subtract low speed rolling resistance force found in low-speed coast down test with some components of wheels, brakes, and rotating component inertia are taken into account. I import the frontal image of the vehicle into Solidworks with a meter stick in the image which allows me to find the frontal cross-section area of the vehicle to be used with aero loads and drag to generate Coefficient of lift and drag values. The plan is to plug in test results from aero setup changes into Lapsim or chassis sim, or a similar lap simulation tool for various tracks and configurations and evaluate which setup is the best compromise for the track day on hand.

(Right) Pitot tube mounted using Go Pro Camera Suction Cup (hasn't fallen off yet, key word being "yet")
(Left) PCB board with environmental sensors and CAN-Bus Converter
(2nd image) Omega strain gauges



Last edited by jspeed.713; 02-02-2019 at 11:32 AM. Reason: Image Order
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Old 02-02-2019, 11:29 AM
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Old 02-03-2019, 08:02 AM
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