acedeuce802

iTrader: (1)

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.

Occam’s Racer

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.

acedeuce802

iTrader: (1)

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.

mx5-kiwi

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?

endura

yeah, windows up not feasible at every track. looking forward to the info.

Madjak

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.

Morello

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

Occam’s Racer

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.

jspeed.713

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

jspeed.713

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Occam’s Racer

PM sent

Occam’s Racer

We tested four different tops, two wings, no wing, splitter, vortex generators, and combinations of them. It's a lot of information that would be hard to sum up in a forum post, and so it's all on my website. https://occamsracers.com/

None of this would have been possible without jspeed.713, thanks buddy.

Midtenn

iTrader: (15)

Great read. We run a Champ Car with a roof panel, but we've been debating building a fastback. This is what we needed to reignite that fire.

endura

Wow. Very impressive work here.

mx5-kiwi

Really enjoyed reading that. Thank you for all that effort!

It seems like a 9LR wing might be on my shopping list....you have me worried about drag with APR GTC200.

jspeed.713

Something to consider is the Lift/Drag efficiency of the wing in question. Drag is not a bad thing, as long as you are generating downforce at a much higher rate it can often pay off. Assuming the drag and lift impacts on pace and fuel consumption are attractive or acceptable, then the primary concern might lie in the balance of the vehicle. Does the ruleset and car classification allow for the front of the vehicle to make sufficient downforce and establish a driveable car in regards to balance? I have seen on a few occasions people compensate for the imbalance experienced in higher speed aero influence conditions by using mechanical adjustments to sway bars, springs, suspension kinematics etc. Relying on heavy amounts of mechanical balance correction often made the car a handful to drive at lower speed corners.

IanIsInTheGarage

Looks like I missed that in my reading. What was your concern with the APR GTC200 wing?

mx5-kiwi

Apparently very draggy. My lap times haven't improved as I expected with extra power so I wonder if the higher drag is giving me front lift as mentioned in that article.

I got to thinking and it seems like I "may" be subconsciousley compensating for a bit of understeer. just a wild *** theory, could be far from the truth BUT worth playing with.....

So further to my comment about buying a 9LR wing, I realised a few things I can test first.

- Will try running it lower (without the lifters).
- Will try without Gurney flap.
- Run it flat or even negative to see what that does
- Go back to the original endplates.

If it is related to the wing, the new fast back is likely to make it worse so just trying to plan ahead....

IanIsInTheGarage

I've always recommended the APR GTC-200 on S2000, owned a track S2000 with wing on Circuit of the America's and Harris Hill Raceway. I never got a back to back test at COTA, but Harris Hill I was able to pick up 5mph on a offcamber right hand turn (85-90mph). Same day removed APR wing and could not maintain grip on same turn. Being their both pretty similar platforms I always liked the concept of mitigating drag on center of wing due to aero affects of the chassis. To note, I never tested, but always was suspicious that risers did harm.

apexanimal

i really don't recommend running it negative... if, for whatever reason, you get turned around at speed bad things can happen