So is your plan to use an APR splitter or are you going to make that out of alum as well? If so, how far back can you take the piece? If you take it a long way back (a good thing) can you use it a bit like a Croxx plate to stiffen the chassis?

Also, you may wish to consider having a hole to drain oil and get at the filter or a trap door of sorts. I would also wonder about the effect of a few vents to help move air in the right places?

Just pondering but I would LOVE to see a flat bottomed Evo with a real and effective rear diffuser as well.

 

I've thought about the potentiometers, but my suspension is pretty stiff and I'm afraid I won't get the resolution I want. I could always put the stock suspension on I guess.

I'm actually thinking about force sensors placed in spacers between the top hats of the shocks and the body. If I put a relatively low preload on the bolts which hold the top hats down, I can calibrate the sensors to remove that offset and then any additional force will be downforce.

I have considered building my own splitter for a long time, but at this point the APR was the easiest and most cost effective. It's not as stiff as I'd like and I could use it sticking out a little more but whatever. I am going to stretch it by adding aluminum. I'm going to just work around the oil filter and drain and leave that area open; it doesn't loose much potential area as it's pretty close to the control arm. I guess with the correct bracing the extended tray could replace the lower chassis braces. I was just planning on attaching the tray to the existing bracing.

As for a flat bottom, I am really thinking about it. It's a pretty big area under there and making it easy to remove is the real trick. I might make separate full length side skirts out out polypropylene and then the pan out of thin gauge aluminum. I'll build a rear diffuser as well at some point. I just need to find the time...

 

Just run a more sensitive potentiometer then, but I doubt your rates are anywhere near stiff enough to not get a decent measurement from your off-the-shelf PDT, which can be had for pocket change now in decent resolutions.

You can do it with a load cell, but they are extremely sensitive to off-axis forces (bending moments) and are more expensive. Damaging them is easy if mounted improperly not to mention your data would be off anyway. For suspension mounting of any sensor, I'd recommend using a position measuring sensor just for ease and longevity. Take a look into string potentiometers as well.




Devin

 

I have been looking at load cells, and it's true they are more expensive, not to mention I would have to build adapter plates to mount them under the top hats. I do think it would work though.

I really haven't put too much thought into it yet, as I'm still building the parts. But the more I do think about it I may be leaning towards linear pots like you are talking about. I'd need to have good resolution with around .020"-.050" displacement. Then the numbers are only as accurate as the tolerance of the pot and the spring rate. Seems like the tolerance on the spring rate could be pretty large. I'm running on KW V3's with KW springs, I'd need to go and see if I can get a tolerance on their stated spring rates.

 

If you have ever had your car corner weighted or have access to scales, you could establish a reasonable idea of what the actual spring rate is based on the weight at the corner and measuring the compressed length and uncompressed length of the spring, assuming they are not preloaded.

Either way, hopefully the tolerance of the spring is held decently tight and you wouldn't have to go that route. Just an idea, though, in case you have to.


Devin

 

Solo Evo,

Thanks for the input today! I can do a string pot setup pretty easily now that I've done a little research. I'll pull out the shocks and calculate the spring rate using the pots with a known weight, that way everything is calibrated with the same hardware.

I can log all the data in logworks and tie it to vehicle speed, that way I can relate downforce to speed and use coast down do figure out drag (thanks Indy Evo for that idea).

Now I just need to sell more seat brackets to fund the effort,and my stroker motor build isn't helping either.

 

load sensors on the strut top would be silly, you would get tons and tons of noise.


string pots or a long stroke linear pot is the way to go. i've measured downforce on UTA's formula SAE car a number of times for aero testing. the string pots worked, and so did the linear pots. the testing i did to measure the downforce included al lthe following test procedures:

1. stationary calibration. car facing one direction of the path the test will be done. measure voltages for all 4 sensors.

2. stationary calibration. car facing return direction. measure all 4 voltages.

3. 30mph steady state velocity in one direction, logging all 4 voltages so they could be averaged (you'll need to log RPM and or forward velocity to get really accurate results when averaging because you'll need to scale with slight variances of speed).

4. same thing in the return direction (the return direction is done so that wind speed is taken out of the equation.

5. up the speed to 45 mph and repeat test in both directions

6. 60 mph and repeat

7. 75 mph and repeat (we didn't go faster then this because our racing never really went above 75 for autocrossing.



for reference, we thought we had stiff enough springs, but we made so much more downforce then previous years that we bottomed the shocks out and had to go almost 100lb heavier spring (and for a small 450 pound car, thats a big adjustment).

here are the wings that made crazy downforce:



and one other thing..

we repeated the test with only the undertray on the car (no wings)
and repeated the test with front wing and under tray
and repeated the test with rear wing only and undertray.

we did that to find any effects between elements, and to be able to distinguish how each element added to the total balance of the car.

(we also had extensive wind tunnel data for the 4 element wings, and scale model cars in the wind tunnel, oh and tons and tons of fluent analysis for everything )

 

I had a splitter fabricated which attaches to the stock 8 bumper using nut-serts placed between the mounting holes for the stock front lip.

It's an alumilite panel riveted to a box-aluminum "backbone". The back backbone mounts to tabs welded to the chassis in front and in the back it bolts up using the stock cross-brace bolts.

I would love to wind tunnel test it but I don't have access to one here in NorCal. At a NASA TTA event at Infineon in April it was effective. I didn't really have a chance to push the car even 95% but overall it's helping.

Point is, with 500lb springs in front, 2.5" of clearance wasn't enough. I was scraping at several points on the track (which as a lot of elevation changes).

I believe 3" will be enough clearance. It won't be quite as effective but there are practical considerations (I don't trailer the car).

Can anyone tell me if vented front fenders would help keep air under the car?

 

I've got a set of string pots on order requesting delivery tomorrow so I can work on this over the weekend. I should have a descent test setup on my car to be able to record down force and drag.

Currently my car has the stock wing, vortex gen, and an APR splitter. I don't plan on taking any of those parts off. So that configuration will be my baseline. I'll then test each part individually as well as in combination with other parts.

 

Oh the UTA FSAE car.....so much cooler then the VT car........

Keep up the good work! Looking forward to the results.


- drew

 

I went to BYU and our FSAE car (at the time) was electric and they focused on the electric drivetrain, so it was only available to the Electrical Engineers : They just used an off the shelf chassis and never even played with the setup -lame-o. They also had an electric drag car.





The mechanical engineers had Mini-Baja, but that was stupid in my opinion I could care less about 5hp Briggs powered white-trash mobiles.

Now BYU finally cowboyed up to the real deal.



Evo's are way cooler though!

 

From what I understand vent the wheel wells serves to reduce the high pressure area inside the wells. You don't want a high pressure zone in close proximity to a zone you are trying to keep the pressure low (underneath the car).

Not venting the fenders will add air underneath the car increasing the pressure, you need to reduce the amount of air underneath the car.

 

Got the string pots in this morning, now I have a project for the long weekend. I'm afraid my suspension is too stiff to make the resolution valid. I'll test it out, but I think I'm going to have to put the stock stuff back in.

I'm still trying to decided on the calibration procedure here are my options.

1. Go off the stated spring rates
2. Pull shock out, load them directly with a known weight and find the true spring rate.
3. Place a known weight in the trunk and front of the car, separately, to simulate the downforce as seen through the body of the car.

To me #3 seems like a good way to go, for example, if the wing on the trunk creates 100 lbs of down force it is carried through the body in the same manner (or pretty close) as placing 100 lbs of weight in the truck. There will be some losses in the chassis for sure, so I think it's more representative of real loading. I can always compare it to what #1 calculates out to be as well.

 

There won't be loses in the chassis because 100lbs of force = 100lbs of force, you can't lose that. Unless you are talking about transient loads likea gusty of wind changing the downforce or hitting a pump in the road. But what can happen is that the sum of forces from the aerodynemic piece can cause downforce on one axle and lift on another. You probably know this though since you know of how strain guages used.

 

Well I spent the weekend installing the sensors. It was more of a project than I expected. My car was sooo dirty from 2 years of daily driving 100 miles and never washing the undercarriage or wheel wells. I had 1/8" of crud caked on my suspension, brakes etc. I had to spend half a day cleaning just to be able to work.

I ended up putting my stock suspension on. That way I could weld on brackets for the sensors and not feel bad about it. It took me three tries to find a location for the front sensor without it hitting the body or tires!

Anyway, most of my time was spent getting the sensors installed. I did get out and make a few quick logs and the results are not great.

From what I can gather the car is creating no downforce or lift at 60-70mph. When I calibrated the sensors I used myself as the weight (165lbs) and I was able to make a noticeable input on the sensors. Even 50 lbs is within the resolution. So far it looks as if the speeds required to create a noticeable change are higher than 60-70 mph.

I'll find some road to bump it up to 100mph and see what happens...