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      06-10-2020, 12:37 AM   #45
rac
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Quote:
Originally Posted by fe1rx View Post
Behind the centre vent and at the base of the windshield, there is a recirculation bubble. I suspect this is due to the flow from the vent and is not present in the baseline. Flow reattaches over the windshield and remains well attached over the roof.
do you have the stock engine bay cowl fitted under the windshield? I see in one of the first posts that it is.

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      06-10-2020, 10:04 AM   #46
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Originally Posted by spxxx View Post
Have you considered trimming the radiator shroud behind the grills to allow for more direct airflow to the radiator?

I've also been working on sealing airflow around my intercooler, oil coolers and radiator but it seems there's an opportunity to increase airflow to the radiator with larger openings behind the kidney grills.


Btw... Love your car, such attention to detail. The flat bottom is great!
I watched a recent video of Tyspeed and his M2 where he briefly discussed that removal of the kidney grills all together allowed a discernible drop in water temps, something like 10-15 degrees.
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      06-10-2020, 11:16 AM   #47
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Quote:
Originally Posted by rac View Post
do you have the stock engine bay cowl fitted under the windshield? I see in one of the first posts that it is.
Yes. I think this is important to prevent high pressue air at the base of the windshield from flowing into the enging bay, and doing little or nothing to improve cooling.
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      06-10-2020, 11:19 AM   #48
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Quote:
Originally Posted by mKilgore View Post
I watched a recent video of Tyspeed and his M2 where he briefly discussed that removal of the kidney grills all together allowed a discernible drop in water temps, something like 10-15 degrees.
That is a lot and seems unlikely, but might be worth a try.
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      06-10-2020, 05:23 PM   #49
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Quote:
Originally Posted by fe1rx View Post
Quote:
Originally Posted by mKilgore View Post
I watched a recent video of Tyspeed and his M2 where he briefly discussed that removal of the kidney grills all together allowed a discernible drop in water temps, something like 10-15 degrees.
That is a lot and seems unlikely, but might be worth a try.
Yeah 15* seems a bit high but the shroud behind the grills look like they would disrupt airflow to the radiator.

I was considering ducting air from above the FMIC up to the radiator as I rarely have IAT issues and water/oil cooling is a bigger concern for me.

Do you plan to do any radiator -> hood ducting? There seems to be some room if the front turbo inlet is moved to the passenger side.
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      06-10-2020, 06:25 PM   #50
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Quote:
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Yes. I think this is important to prevent high pressue air at the base of the windshield from flowing into the enging bay, and doing little or nothing to improve cooling.
Do you think its plausible to raise the rear of the bonnet up with a guided spoiler enough to improve flow onto the windshield and also cause a low pressure at the base of the windshield (now underneath a spolier), enough to pull air out from the engine bay?
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      06-18-2020, 11:36 AM   #51
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Originally Posted by rac View Post
Do you think its plausible to raise the rear of the bonnet up with a guided spoiler enough to improve flow onto the windshield and also cause a low pressure at the base of the windshield (now underneath a spolier), enough to pull air out from the engine bay?
Given that the flow reattaches to the windshield, I don't think there is a need to improve the flow.

A spoiler at the back of the hood may work, but it will do so by creating an area of recirculation, which will delay reattachment further up the windshield.
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      06-29-2020, 09:03 PM   #52
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are you running data at the track? curious what each of these upgrades is giving you, i'm just starting down the HPDE / TT path with my 135i
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      06-30-2020, 10:52 PM   #53
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are you running data at the track? curious what each of these upgrades is giving you, i'm just starting down the HPDE / TT path with my 135i
Yes, an AiM Solo DL connected to the CAN bus and to a SmartyCam.
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      06-30-2020, 11:05 PM   #54
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Track Test

I have now completed two days at the Mosport GP track and can report how the modification has fared.

Heat – I kept an eye on both oil and coolant temperatures (recording data from the CAN bus o my AiM Solo DL). Both got close to the point where power is reduced (117°C water, 151°C oil), and on occasion reached these levels. Prior to Aero V3 I have never been oil temperature limited (only water temperature), although my current tune (Cobb AccessPort Stg 1+FMIC 93 OCT) was only installed late last year. The oil temperature issue is likely due to the tune rather than Aero V3.

I removed two of my wheel well baffles to let some hot air out into the wheel wells, and my impression was that oil and water ran slightly cooler as a result. I will probably leave those baffles off, as I am nervous about the engine mounts getting cooked with them on. To try and avoid cooking things when parked, I cooled the car to 120°C oil temperature or below before parking it, and I parked with the hood open. There were no nasty hot smells at any point, but removing the covers later revealed that the transmission exceeded 270° but not 280°F (according to the temperature label) and the differential temperature exceeded them label maximum of 320°F and the diff exuded that characteristic gear oil smell.

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Also, the PVC cover on one of my rear brake lines (that allows for movement of the rear subframe) showed heat damage,

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as did the parking brake cable (both close the axle-back pipe).

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Also, the LH inboard CV joint boot was shrunken and distorted – not, as I first feared from cooking the rubber, but from pressurizing and then venting due to heating the air inside. Letting some air back into the boot restored it to normal appearance.

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Conclusion – I have some heat issues to deal with.

Mechanical – a P2A82 inlet VANOS code thrown at the end of last year prompted me to change the inlet VANOS solenoid over the winter. That happily fixed the problem, although I did get a 30FF boost under target code a couple of times, but with no discernible power reduction. All the aero bits stayed attached.

Exhaust – I heard no rattles from the exhaust. I was using OE exhaust hanger rubbers, which are very soft around the no-load position.

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I had ordered, but not received, some PowerFlex urethane exhaust hangers and had planned to use them to (hopefully) reduce the sway space occupied by the exhaust. In fact, with OE hangers I had no trouble with the exhaust contacting any other part of the vehicle under track conditions, but as I have now received the PowerFlex hangers, I have installed them. Before installing them, I tested their stiffness against the OE hangers, and they are about 10% stiffer than OE while also having no no-load soft spot, and are also slightly shorter at no load. This, of course, is at room temperature, and urethane softens with heat, so how they actually work under track conditions remains to be tested. The PowerFlex hangers are narrower than OE so I fabricated some Delrin clips to remove any lateral play.

I attribute the heat damage to a combination of radiant heating from the axle back pipe, and hot air coming through the tunnel. To reduce radiant heating, I fabricated some stainless steel shields, that provide a 1/8” air gap to insulate the pipe and reduce radiated heat.

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The shields are secured to the pipe with stainless steel tie wraps.

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The shields have been located to shield the brake hoses, parking brake cable, exhaust hanger, and the differential housing from radiated heat.

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Drag – if V3 has increased drag, the increase was negligible. My maximum speed exceeded 220 km/h, which I am completely happy with at this track.

Downforce – one interesting observation on my early runs was that above 200 km/h I started hearing some grinding noises from the rear strakes. After a few laps the noises stopped as the strakes wore into their minimum ground clearance state. The design ground clearance of the strakes would require the car to be hard on its bump stops to contact level ground, ignoring flexibility in the structure. I am certainly not achieving enough downforce to cause this. While the diffuser and strakes are pretty well pinned to the chassis, there is some local flexibility in the installation near the rear axle, and I think this is the primary culprit so I will beef up the attachment in this area before my next lapping day.

Unfortunately, the track photographer planted himself at the slowest corner on the track, not the fastest, so I have no pictures at high speed, to assess how ground clearance and attitude may have been affected by downforce.

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Feel – the car felt immediately very planted with the wing as settled on at V2. I experimented with reducing wing incidence to see how the feel changed. I got a bit of high-speed oversteer in off-camber turn 2 that got my attention, and no discernible drag reduction benefit so I set the wing back close to its original setting. I think this means that while the underbody changes have increased total downforce from V2, that increase is well balanced fore and aft as it did not require any compensatory change in wing angle.

Track – the track has had several areas completely repaved, possibly making direct year-to-year comparisons suspect. Ambient temperatures were nicely cool, at 20 – 22°C, making for good power and good grip. I started with 30 psi cold all around, and didn’t adjust them. Checked hot they were all about 38 psi.

Lap Time – with the driver green after the winter, I managed to get within a couple of tenths of my previous best (V2 configuration) on several laps, but that PB was set on a really cool (8°C) day that both the engine and tires loved. With RE-71Rs, the first hot lap is the one to set a time. After that the RE-71Rs get a bit too hot if pushed hard. I didn’t overdrive them to the point of making them greasy. With a cooler day and some good driving, I suspect that V3 will be capable of reducing lap times by 2 seconds. That claim needs to be proven though. If we accept that for a moment though, we can ask “is the added complication of V3 worth 2 seconds?”

While I drove a complete season last year with a single set of splitter wear pucks without wearing them out, two days were enough to fully wear out a set of wear pucks this year. V3 runs the pucks 12 mm lower than V2 so this is not entirely a surprise.

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That, and the strake wear, tells me that I can’t run the underbody any lower without stiffening the suspension substantially. So for now I am happy with my V3 geometry, subject to adding some more beef to the diffuser mounting just forward of the kick-up.

Where to from here?

I would like to keep the transmission and differential below 250°F. To that end, I have installed some additional baffling to direct downpipe-heated air away from the transmission.

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And I have added an additional baffle at the transmission crossbrace to direct transmission cooling air over the crossbrace and to some extent over the exhaust tunnel heat shield.

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To understand the scope of the cooling issues, I have installed thermocouples on both the differential and transmission and at 6 other points along the air path.

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Being attached to the cases, these thermocouples will under read the fluid temperatures, but still give a reasonable idea of what is going on. I would like to keep the fluid temperatures below 250°F. As this is likely to be a particular challenge with the diff, I have changed its fluid from 75W90 synthetic to 75W140 synthetic.

Other points I am measuring air temperatures at are:

- the O2 sensor connectors
- below the transmission thermocouple
- the fuel/brake line cavity
- the air passing over the exhaust tunnel heat shield adjacent to the giubo
- the air exiting above the exhaust tunnel heat shield at the diff
- the air exiting below the exhaust tunnel heat shield at the diff

The thermocouple wires pass through a grommet in the floor into the cabin.

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I have done some road testing of temperatures. That shows that under normal driving conditions all the measured locations show “reasonable” values. In order to collect meaningful data under track conditions l have ordered a 4-channel data logger so that I can record the temperatures of interest for later review. I need data before I start thinking too hard about more baffling, ducting or venting (or heaven forbid, a diff cooler). An upgraded oil cooler(s) is almost certainly in the cards though.

Before I reinforce the diffuser, I would like to get some video of exactly what deformation is occurring at speed. Rather than try to run a GoPro in extreme proximity to the ground, I am planning to install an endoscope camera connected to my phone. These are available very cheaply from Amazon these days. I will try the same approach to look at suspension deflection under aero load.

I have not done any intentional flow visualization over the underbody surfaces, but some dirt deposits on the diffuser show that the flow is nicely attached and the differential NACA duct is flowing properly (i.e. completely swallowing the vortices generated by the duct edges). I want to do more underbody flow vis testing.

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Of course, the final goal is improved lap times. I am quite certain they will come with familiarity and optimization. With the higher load now applied to my tires, I am fairly certain that lower starting air pressures will be advantageous, as will really making that first hot lap count.
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      07-01-2020, 10:29 AM   #55
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Definitely keep us posted on the heat/temps. When discussing a custom fully sealed underfloor and aero with my workshop, their main concern was heat retention and the negative effects on cooling, so I'm keen to see real-world data, especially on the 1-series platform.
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      07-01-2020, 04:44 PM   #56
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Sweet update!!

Any thoughts or plans to maybe use a finned differential cover from BMW? Although if it's any bigger than what you have now it might cause an clash with the underbody aero.
Also, it might not be too hard to make a little diff plug/sensor adapter if you have access to a lathe.
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      07-24-2020, 02:36 PM   #57
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Downforce Measurements

I have received and installed my 4-channel temperature data logger onto a bracket attached to the removable cup holder. The cup holder was sacrificed in the process. Along with a single-channel Fluke meter, I can now watch 5 channels and log 4.

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While all of the thermocouples are providing valid signals, I have discovered that the two bolted on thermocouples (transmission and differential) are of the grounded-junction type, and the logger is not happy to have two grounded-junction thermocouples connected simultaneously. Whatever mV difference there is between these two ground points shows up as common mode error. Moving one of those thermocouples to the Fluke meter solves that problem, but I cannot log both the transmission and differential temperatures simultaneously. Some extended highway driving (110 km/h at 25°C ambient) resulted in stabilized transmission and differential temperatures of 67°C and 70°C respectively – so reasonable. Upon parking the car at these temperatures, both crept up over the course of 60 seconds – the transmission to 77°C and the differential to 75°C. I suspect that the core temperatures basically migrated to the entire housings in the absence of cooling air flow. Assuming that, I conclude that the transmission temperature under reads by about 10°C while in motion, and the differential temperature under reads by about 5°C while in motion – again reasonable, as long as I keep that in mind.

In order to understand my rear strakes unexpectedly wearing on the ground at 200+ km/h, I have installed a GoPro in the rear diffuser to see what is happening. The results were interesting. Here is the diffuser at low speed:

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And here at 210 km/h:

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I did not correlate speed with the GoPro video, but at some moderate speed the door began opening, and at about 200 km/h the diffuser skin began sagging significantly as shown in the above image. Lucky I installed bail on the door, or it would have been ripped off. Clearly the diffuser deflection is undesirable, so I designed and installed an additional bracket to support the diffuser:

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I repeated the 210 km/h test to confirm that the bracket functioned as intended:

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Of course I still need to deal with the doors. The open doors will upset the flow in the outer diffuser channels and reduce the diffuser’s effectiveness. I will do that, and also plan to extend the inboard strakes to further stiffen the forward part of the diffuser, but first I wanted to get a better idea of the downforce I am seeing.

To that end I have designed and built a system to measure the ride height by using wifi endoscopes to record the ride height as a function of speed. This will allow me to calculate the downforce resolved to the front and rear axles and to characterize the aero balance.

The rear ride height gauge utilizes a wifi endoscope and pointer attached to the chassis, and a printed scale attached to the rear damper. The scale reads in mm units of ride height, so each graduation is 0.792 mm apart to account for the motion ratio of the damper. Any dynamic deflection in the upper and lower damper mounts will result in an erroneous reading to some extent but these should be steady under smooth road conditions (which is the only time the readings are useful anyway).

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The image from this gauge is very legible under daytime ambient lighting.

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The front ride height gauge is significantly more complicated, with Delrin bracket secured to both the upper and lower spring perches.

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A stainless steel rule is secured to the top mount and slides through a slot in the lower mount with suspension travel. The scale graduations are on the inboard side of the gauge so are not visible in the above image. As with the rear, a sharpened pointer provides a precise reference. The endoscope is mounted vertically in the mount, viewing the scale through a 90° mirror. A loop is incorporated in the endoscope cable to allow for steering and suspension travel.

Images collected by the front gauge are mirror images and are poorly illuminated by the ambient light in the front wheel well. While the endoscope incorporates a variable illumination feature, it doesn’t work well in this application. I will install a separate light source in the future, but for now the images are dark but legible. The following image has been flipped horizontal to remove the mirror image distortion.

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My actual method is to record 10 seconds of video at a fixed road speed held by the cruise control, and then to read the average ride height during a period of relative smoothness. The above images are screen captures from these videos. I am using a local toll road to collect data as it is the smoothest road I can find and it provides good sightlines for extra-legal speeds. While downforce at extreme speeds is interesting, generally I can get all the info I need at 160 km/h or less. I am using GPS speed to avoid any speedometer error. Correcting for headwind is dubious without a pitot tube, so the best bet is to collect data during calm early morning conditions.

To be strictly authoritative, air density and wind need to be accounted for, in order to gather data that can be directly compared between different sessions/days. Density altitude is readily available from aviation weather reports, and this can be converted to density with a bit of math. As this is going to be my wind tunnel for development of the aero mods, eventually I will do that, but for now I am looking at relative as opposed to absolute front/rear values, so I don’t need that level of data reduction for today’s discussion.

Having installed my supplemental diffuser bracket (but not done anything with the doors) I set out to collect front and rear ride heights as a function of speed. One of the first things I discovered was that rear deflections seemed improbably high, and were slow to stabilize after hitting a bump. What I realized was that my dampers were set too hard, and the car was jacking down over bumps. This, and the previously noted diffuser deformation at high speed goes a long way to explain the strake wear that I observed in my first track outing. The new bracket likely increases rear downforce from what I saw on the track, and fixing the suspension doors will likely increase it further.

I set the dampers back to Ohlins’ recommended street setting for this data collection, while also realizing my track performance would likely benefit from some softening from my previous track values.

I have made myself a spreadsheet that accepts ride height and speed inputs, along with air density, to calculate front and rear lift coefficient and aero balance, I will save the details of that for another day.

In the current configuration, I can report front downforce of approximately 130 lbs and rear downforce of approximately 290 lbs at 160 km/h. This corresponds to negative lift coefficients of 0.22 front and 0.50 rear and a front aero balance of 31%.

The next step is to modify the suspension doors to prevent them from opening at speed. I will then repeat the testing to see if that has a measurable effect. I will then play with wing angle to achieve what I think will be a desirable aero balance (I am guessing about 45%) and then take that to the track.

The rear wing’s position behind the rear axle means that wing downforce increases down force at the rear axle but decreases downforce at the front axle. With my wing position, 100 lbs of wing downforce appears as 124 lbs more downforce on the rear axle and 24 lbs less downforce on the front axle. With my aero balance, the opposite perspective is more interesting – 100 lbs less wing downforce will result in 124 lbs less downforce on the rear axle and 24 lbs more downforce on the front axle. Coincidentally, doing just that would give me the aero balance I think I am looking for.

Last edited by fe1rx; 07-24-2020 at 02:42 PM..
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      07-25-2020, 10:00 PM   #58
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"....measure the ride height by using wifi endoscopes to record the ride height as a function of speed."

Of course, what else would one do!!? I chuckled/shook my head when I read this. I mean this is probably "normal" in the world of suspension development / your background and industry, but for the average layman having the techniques brought to light is delightful.

Oh and I love the use of the mirror for the front!

Carry on fe1rx, these threads are like reality TV / crack addiction for the rest of us!
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      07-28-2020, 10:33 AM   #59
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Quote:
Originally Posted by fe1rx View Post
Yes, an AiM Solo DL connected to the CAN bus and to a SmartyCam.
are you tracking the improvements in grip or sector times as you make changes? I'm curious which modification gained the most on the track

My car understeered horribly on entry, apex, and exit until it power oversteered. -2.8 up front was not enough and it lost significant camber due to body roll. High speed sweepers were unstable, I did not trust the rear too much on these. I found a splitter and aerofoil wing with 0 AOA helped considerably but I've not taken it back to the track to measure the difference. I've not measured how much downforce I have though, your method looks pretty slick!

My MPG on the street sure did not appreciate the splitter
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      07-28-2020, 09:42 PM   #60
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Quote:
Originally Posted by imported_mega View Post
are you tracking the improvements in grip or sector times as you make changes? I'm curious which modification gained the most on the track

My car understeered horribly on entry, apex, and exit until it power oversteered. -2.8 up front was not enough and it lost significant camber due to body roll. High speed sweepers were unstable, I did not trust the rear too much on these. I found a splitter and aerofoil wing with 0 AOA helped considerably but I've not taken it back to the track to measure the difference. I've not measured how much downforce I have though, your method looks pretty slick!

My MPG on the street sure did not appreciate the splitter
Interesting. I have -3.2 in front, -2.2 rear and no rear areo (splitter up front) and don't really experience understeer issues with this setup.

Relevant mods:

Suspension: Custom Clubsports with Swift Springs and GC Camber plates: 450 front, 750 rear.

RE-71R tires: square 255/35/18 setup

M3 control arms

What are you running?
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      07-29-2020, 06:17 AM   #61
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Quote:
Originally Posted by AndyW View Post
Interesting. I have -3.2 in front, -2.2 rear and no rear areo (splitter up front) and don't really experience understeer issues with this setup.

Relevant mods:

Suspension: Custom Clubsports with Swift Springs and GC Camber plates: 450 front, 750 rear.

RE-71R tires: square 255/35/18 setup

M3 control arms

What are you running?
my car is now basically thee same as yours (camber, tires, spring rates, m3 arms, splitter) but also has a rear wing, I bet I dont even need the rear wing these cars are so unbalanced from the factory with so much understeer. By far the biggest improvements was the front splitter, it transformed the car, a little more camber helped but not as much as the splitter

i'm curious if fe1rx had similar results where one change net big rewards in handling or laptimes
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      07-29-2020, 10:03 AM   #62
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Quote:
Originally Posted by imported_mega View Post
my car is now basically thee same as yours (camber, tires, spring rates, m3 arms, splitter) but also has a rear wing, I bet I dont even need the rear wing these cars are so unbalanced from the factory with so much understeer. By far the biggest improvements was the front splitter, it transformed the car, a little more camber helped but not as much as the splitter

i'm curious if fe1rx had similar results where one change net big rewards in handling or laptimes
I managed to get good mechanical balance before I added the aero, so my splitter definitely needs the wing to balance it.

As for big changes:

1) nothing will ever be right without M3 or stiffer rear subframe bushings. When I say "stock", I subconsciously mean stock wih M3 RSFBs, because without them the car was a disappointment I would rather forget. So maybe this was the biggest change, moving the car from useless as a track car to ok.

2) good tires (Direzza Star Specs initially, NT01 once, RE-71Rs for a few years now) are of course a given.

3) a decent mechanical package of Ohlins coilovers, big rear bar, camber plates, LSD and spherical bearings - but that was developed incrementally so the improvement did not arrive in one big hit.

4) the most dramatic improvement (at least specifically at the Mosport GP track) was the aero V1 and V2 configuration vs no aero (about 4 seconds/lap). V3 needs some tuning to show its full potential, but I expect a smaller incremental improvement than what the initial V1 and V2 configurations produced. The improvement on slower tracks was still there, but not as dramatic.

Regarding your earlier sector time question, yes. Once I get V3 a bit more refined I will post a comparison between stock / V1 / V2 / V3.
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      07-30-2020, 11:15 AM   #63
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Math

Time for some math to get a sense of what can be achieved with aero mods on our cars.

1) The dynamic air pressure “q” is what we have to work with. It is:

q = ½ rho V^2

SI units make the math easier. Speeds are, of course, air speeds.

Assuming sea level standard day air density rho = 1.226 kg/m^3

With V in units of m/s, q has N/m^2 units

Some sample values with conversions to useful units:

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2) The lift equation relates q to lift:

Lift = q x S x CL

Where S is the reference area (frontal area for a vehicle, planform area for a wing). Obviously, these areas are fixed for a given vehicle. Our cars have a frontal area of approximately 2.15 sq m (23.1 sq ft). So, at any speed the only unknown to calculate the lift/downforce is the value of the lift coefficient.

3) If we pluck a realistically attainable lift coefficient from the literature for our class of car, we can estimate a realistically attainable downforce.

4) An interesting thing about the lift coefficient for a wing is that it is constant for a single angle of attack, regardless of speed. Let’s assume that our vehicle lift coefficient is constant, regardless of speed and see where that leads.

5) For our thought experiment, let’s choose a (negative) lift coefficient of 0.55, with an aero balance of 45% (i.e. 45% of the total lift being reacted by the front axle and 55% reacted by the rear axle to assure high-speed stability). Let’s calculate at 160 km/h:

Downforce = q x S x (-)CL
= 1211 N/m^2 x 2.15 m^2 x 0.55 = 1432 N (332 lbs)

Front Downforce = Total Downforce x 45% = 644 N (145 lb)
Rear Downforce = Total Downforce x 55% = 788 N (177 lb)

6) Knowing our front and rear wheel rates is useful. The front axle rate is twice the front wheel rate. Similarly, the rear axle rate is twice the rear wheel rate. Knowing this we can calculate the ride height change, front and rear, at any speed due to downforce. Let’s use my wheel/axle rates:

Front Wheel Rate = 55 N/mm (315 lb/in)
Front Axle Rate = 110 N/mm (630 lb/in)
Rear Wheel Rate = 45 N/mm (257 lb/in)
Rear Axle Rate = 90 N/mm (514 lb/in)

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7) These calculations give a first order approximation of the kind of downforce values we expect as a function of airspeed, which is to say that headwinds and tail winds will have a significant influence on the actual downforce values. Also, these values are for sea level standard day conditions. They must be scaled by the density ratio of actual density to standard density. The simplest way I have found to determine the density ratio is by first getting the density altitude from an aviation weather report for the closest airport to the test area. Adjust this by the difference in height above sea level between the test area and the airport. Airport elevations are published, other locations can be found in Google Earth. Then apply the formula to calculate the density ratio. For example:

Closet airport elevation = 600 ft ASL
Test area elevation = 550 ft ASL
Airport density altitude at time of test = 2700 ft ASL
Test area density altitude = 2700 – 500 = 2200 ft ASL = DA
Density Ratio = (1 – DA/145539)^4.256 = 0.937

Forces and deflections would both be scaled by this ratio under these conditions. This correction will be necessary when calculating Lift Coefficient from measured deflections under known non-standard day conditions.

8) Ultimately, I want to calculate front and rear lift coefficients from test data. Why? Well, it is a good reality check as to how effective the aero is. The following excerpts from Joseph Katz’s “New Dimensions in Race Car Aerodynamics” provides some perspective. The 1991 Mazda GTO with rear wing example is most similar in terms of technology. A (-)CL of .53 is claimed, and Katz later indicates that a (-)CL of 1.0 is probably possible for this class of car. My earlier assumption of 0.55 should be attainable, with effort, and twice those values – probably not.

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9) Getting valid CL values takes really good data. Speed errors are squared, so underestimating air speed by 10% will result in a 20% optimistic CL calculation. BMW speedometers over-read by 5 km/h, ± the reading error due to non-standard tire sizes. GPS speeds get rid of the speedometer error, but headwinds will also result in optimistic CL calculations. A 10% error in air speed due to headwind is not at all unlikely if not controlled for. Absent a pitot probe, dead calm conditions are essential for calculating valid CL values. All this said, aero balance can be measured directly provided both front and rear ride heights are being recorded simultaneously, albeit with some uncertainty as to the airspeed at which the measurement is being taken. Without controlling for atmospheric conditions, including wind, direct comparision between different test sessions will be dubious with respect to CL and total dowforce, although aero balance comparisons will still be useful.

10) An important observation from my sample calculations is that both ride height and rake change with speed. Accordingly, both CL and aero balance also likely change with speed. As rake decreases with increasing speed, the effective throat of the underbody will move aft, moving the centre of pressure aft. This may be a good thing for stability, but a bad thing for overall grip. In any case, it suggests that increasing static rake would be a way to ensure that the desired rake is achieved at the design cornering speed. (I am using 160 km/h as a design speed.)

11) So how much benefit will downforce give in a corner? Well, one problem is that we don’t know what the baseline CL value is for the car. We didn’t measure it. Katz suggests that most road cars have a slightly positive lift coefficient. Let’s conservatively assume that the baseline CL for the 135i is 0. If the baseline was actually positive the benefit would be more. Downforce increases the load on the tires increasing their grip. Assuming that the baseline non-aero vehicle is capable of 1.0 g cornering a 160 km/h and weighs 3320 lbs, our calculated total aero load of 332 lbs represents an increase in tire load of 10%. Ignoring tire load sensitivity, grip will be increased by 10%, so 1.1g should be possible at 160 km/h and apex speeds in such a corner will increase by 5%. This is optimistic, because tire load sensitivity implies that grip increases don’t quite keep pace with tire load increases, and because a 45% aero balance means that only 90% of the added aero grip is available for performance while 10% adds stability. Added high speed stability means added driver confidence though, so maybe that confidence can be turned into some additional speed too.
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      08-12-2020, 09:15 PM   #64
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"My actual method is to record 10 seconds of video at a fixed road speed held by the cruise control, and then to read the average ride height during a period of relative smoothness. The above images are screen captures from these videos. I am using a local toll road to collect data as it is the smoothest road I can find and it provides good sightlines for extra-legal speeds"

why not use potentiometers? you could have data logged the information and easily made rolling averages of data plotted against speed etc. this method is a lot of manual work to assess a small amount of data.

some aero discussion came about on another forum and it was pointed out to me that the oem headlight height adjustment sensors could be used for this purpose after i had posted a link to this video:




component wise is a h8ll of a lot cheaper than buying aftermarket suspension pots. just need to plot voltage output versus height in the workshop to calibrate the geometry relationship and then log the data.
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      08-12-2020, 11:16 PM   #65
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I saw someone in another forum using boot struts from a mercedes for the same purpose.
Unfortunately that forum is down right now so I can't grab the details.
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      08-16-2020, 12:09 PM   #66
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Quote:
Originally Posted by rac View Post
[I]
why not use potentiometers? you could have data logged the information and easily made rolling averages of data plotted against speed etc. this method is a lot of manual work to assess a small amount of data.

component wise is a h8ll of a lot cheaper than buying aftermarket suspension pots. just need to plot voltage output versus height in the workshop to calibrate the geometry relationship and then log the data.
That assumes I have a method of logging voltage data. Solving that would not be too much a problem, but the endoscopes were a dirt cheap solution. To be really useful you would want to simultaneously log speed data.

My current data logger is an AiM Solo DL, which does not accept any additional data channels beside those that come from the CAN bus. Too bad the existing ride height (headlight leveling) sensors aren't accessible on the CAN bus.

Shock pots and a AiM Dash Logger are definitely on my wish list. Mostly because without being able to look at shock velocity histograms, shock tuning is a bit of a black art, but it would be cool to be able to plot average ride height against speed using the Race Studio software.

The rotary headlight leveling style ride height potentiometers are certainly an ecomomical way to go vs. linear potentiometers.
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