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Discussion Starter #81 (Edited)
I got info from 3 other Slovenian 4C owners, that they have curved underbodies too, they are upward of 2015 built date.

I also jsut spoke to my Alfa Romeo dealership fellow tech and got the info after being restless, that there actually was a "silent" change of the alu undertray in all 4C cars, regardless of the market trim. First change of undertray (replaced with part of weight of 1.500g) was on 03.05.2013 (replaced with part of weight of 3.830g) and then was another silent (we shouldn't know about that) change, since early 2015. Perhaps some gearbox overheating issues or something, although I don't exactly see how upward curves on the undertray should help with that as this is a problem of hydraulic shift unit and too hot exhaust, not the gearbox itself, or perhaps there is some other reason behind that, and it is not downforce as any openings upward or downward don't help, more hurt the under car air flow. Ideally, the car should be closed completely up to the rear diffusor, but let's not get to deep with that.

Anyway, mine 4C is built March 2015 and has opening instead of the upward curved parts and weights 2.930g, the other 4C was November 2014. According to the FCA parts catalogue there should be only one part available and that is part nr.: 50530393 (the upward curved, with weight of 3.830g). I really didn't even for a second think, that there would be a difference in underbody tray, but I also never really understood why the stock rear arm has reinforcement (cross tube) in the wrong position, so we moved it and achieved significantly improved rigidity, we chose an uprated material and the result was super rigid and lightweight rear control arm. Unfortunately, the undertray issue is now causing me headaches.

Now, we have 2 options:

1.) Reposition GMS performance rear arms reinforcement tube. The only way is to position it just as the stock rear arms are. We will loose rigidity in the longest tube of the triangle, where the most of flex comes, but due to of use of better material it should still be better than stock arms, but it will be a compromise though.

stock rear arm study stress.jpg

2.) Modifying newer version of undertray by cutting away the upward curves.

undertray new.jpg

Of course, all the costs, regardless of your choice I will bare, as this is something it shouldn't have happened, but I really never though there was a change in underbody and with the luck I have, I had my 4C and other 4C with the same type of undertrays, both early builds.

Let me know your decisions, I respect them all.
 

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Lunch break from car work.... I received Gale's uniball pushers as I couldn't get the large washer symmetric between left and right. This was caused by one of the uniballs being slightly, and I mean slightly different press depth than the other. A quick squeeze with the pushers, and I was able to get the bushings close enough to allow symmetric large washer placement. Originally I had the large washers on each side of the bushings (OEM A-Arms), but had to go to two bushings on one side on the front, and symmetric on the rear bushing.

Rear Bushing...
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Front Bushing
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As Gale noted, the deflector and humps on the lower pan will interfere with its installment. Sorry to everyone that I missed it in my original post. Anyways, the cooling duct from the side skirt won't interfere with the A-Arms. It is just the lower pan changes...
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On a funny side note, the drill hole is an excellent spot to tie off the brake sensor cable for those of us that don't use the OEM pads.

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Discussion Starter #83 (Edited)
OK, after frying my brains out wha to do I got the conclusion and went out for a few drinks.

The conclusion is. There are racers and classic car lovers, but both would like the car to handle the car well. With single product we won't be able to cover both.

So we will offer 2 types of GMS performance rear arms. OEM style rear arms that will plug and play without any mods to newer version of undertray needed. In this case the reinforcement tube will be positioned exactly as in stock arm is, the material will be uprated 25CrMo4 / 4140, GMS ball joints and uniballs all around, but the design itself is a bit of compromise due to reinforcement tube not being positioned exactly optimal.

The second type. GMS performance rear arms as the way they are now. Optimal rigidity layout, with significant improved triangle strength, GMS ball joints and uniballs all around, but with the downside of requiring rear undertray bends to be trimmed on newer build date 4C's.

I will post an email to each customer tomorrow. In case of desire to swap for OEM style GMS performance rear arms, I will send OEM style performance rear arms to your address. Of course I would ask kindly to send the sent one back.

I would kindly ask for about 30 days of time frame to weld and assemble the new OEM style GMS performance rear arms. The CroMo material is on stock and the welder is ready, but we need to set up the CNC machine for new reinforcement tube position.

GMS's quest is to be nr. 1 4C tuner, known for the best parts and customer support. No shortcuts.

Thank you guys for your understanding. This time the 4C build differences got us. ❤
 

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First... Gale I think that is a very good solution. Over the years, we have all learned that the 4C is a bit of a hand built car and not everything is the "same" between the same model year cars. Your truly had to have Sector111 modify their harness bar, as my car wasn't the same dimension as their test fit car... Oh well... But we DO appreciate the customer service!!!

For those of us that will KEEP the "Performance" version, it looks as if the modification isn't too major.

The Passenger side looks to only need a small notch or slight removal of the top most of the dome.
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The Drivers side is a bit more of a challenge, as the deflector is there and one of the three rivet locations holding it in place is right at the top of the dome. The "simple" way is to completely remove the deflector and cut the top dome just like the passenger side. Not sure if this is the best option though...
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The job isn't bad... What I did was fit the pan into the lip at the front and screw in just 2 bolts, then hold the pan up as high as you can (till it hits the arms). Then take a Sharpie or some other marker, and mark the pan on both sides of the A-Arm brace AND the cross bar it connects to. It will look something like this....

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I placed some painters tape along the marked lines and then increase the cut by roughly 1/2" to 3/4" and proceeded to cut with a dremel or rotary cut saw. I included measurements so people can get the proper scale. You need to mark the pan because the angle is a little funny in reference to the curvature of the pan.
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Installed, the cutout looks like this, and you shouldn't have any clearance issues....
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The Drivers side is about the same process, but we have to contend with the air deflector....

Mark the pan the same way as the passenger side, and increase the cut width...
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The first notch in the deflector wasn't enough, so I had to add an additional notch... The photo below is STILL NOT ENOUGH, for a fully unloaded suspension, we will likely have to cut all the way to the bend in the aluminum. I'll try this and test it in the next few days. The deflector isn't AS sturdy as it was, since you have to remove one of the rivets in the cutout. I may add a minor brace once I figure out how much more I have to notch the deflector.
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Discussion Starter #87 (Edited)
I will make an exact CAD render of the new, oem style arm design to check the difference in rigidity and stress zones.

Any more users with old style undertrays? So far we are only 3 with old, flat style undertray.

@DrPyro2k I don't have access to my car for a few days. What if we could completly cover the rear opening at the rear arms, with new undertray. Is it doable? If it is, then we add @cipsony side carbon covers and we get the 4C full flat bottomed with large rear difussor. That would be great...
 

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I'm going to look under my car today to see what sort of undertray I have. I have taken it off before, but never noticed the detail, but I have a feeling I know the answer as I have a vague recollection. I think I'm in denial! (I expect it to be the version causing the issues, as my car is early 2016 UK version.)
 

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Discussion Starter #89 (Edited)
The older version of undertray is not available through dealership as it was replaced with new part nr. and design.

Explanation of different types of rear arms:

STOCK REAR ARMS:
  • The least rigid design (reinforcement tube/brace not at optional location, lesser grade material – mild steel of unknown type)
  • Tube diameter 25mm
  • Lots of flex (starts occurring at 1.5G of lateral forces, and a bit lower for USA cars, which are roughly 150kg heavier). See CAD analyze below with 500 kg / wheel force applied
  • Recommended for street only driven cars, no semislick or slick tires (deformation of rubber bushings and deformation of rear arms once the rubber bushings get replaced by uniballs)
  • stock.jpg
GMS PERFORMANCE REAR ARMS (SPORT)
  • Improved rigidity design (reinforcement tube/brace at OEM location, superior grade 25CrMo4 / 4130 material)
  • Tube diameter 28mm
  • Zero flex up to 2.1G (The longest side of triangle still carries the most of stress, but due to better material and additional reinforcement gussets, this type of arms are 40% more rigid than stock rear arms.) See CAD analyze below with 500 kg / wheel force applied)
  • Recommended for high grip street, medium and hard compound semislick tires (UTQG > 140)
  • Lighter weight (unsprung weight reduction)
  • No modification of rear undertray required
  • gms street.jpg
GMS PERFORMANCE REAR ARMS (RACE)
  • The most rigid design (reinforcement tube/brace at optional location, superior grade 25CrMo4 / 4130 material)
  • Tube diameter 28mm
  • Zero flex up to 3.1G (The longest side of triangle is now exposed to much less stress due to reinforcement tube splitting the effective length of it and having additional reinforcement gussets and better material this type of arms are 107% more rigit than stock rear arms. See CAD analyze below with 500 kg / wheel force applied)
  • Recommended for soft compound semislick (UTQG < 140) and slick tires
  • Lighter weight (unsprung weight reduction)
  • Requires minor modification of rear undertray on cars with newer, curved undertray design
  • gms race.jpg
Conclusion:
Stock arms are OK for low grip tires. At 1.5G (semislicks) of cornering force and the car and driver weight of 1000kg (USA about 150kg more), the OEM rear arms start to flex. Most of the stress is taking the longest tube as you can see from the CAD analyze. The most significant improvement that can be done is the realocation of the reinforcement tube. The second important factor is reinforcement gussets, third is diameter of the tube, and fourth is the material itself.

If you are occasional tracker and aggressive street driver, GMS PERFORMANCE REAR ARMS (SPORT) are the one to go for. Still noticeably more rigid than OEM rear arms and no undertray mod required.

If performance is the only factor, than GMS PERFORMANCE REAR ARMS (RACE) are the one to go. With zero flex up to 3.3G this is the most rigid option available, providing the most precise handling with zero dynamic alignment changes. It requires minor modification of rear undertray on cars with newer, curved undertray design.

All the arms are "trackable", including stock, but as we have experienced with the rubber bushings that were replaced later by uniballs, dynamic alignment changes start to appear when G-forces increase. If you are planing to run semislick tires and track the car, then I would suggest "sport" version of GMS performance rear arms. If you are planing to seriously track the car, use soft compound semislicks or slick tires then I would strongly suggest "race" version of GMS performance rear arms, to have absolute control of the rear end of the car, regardless of the G-forces you are yielding.

Note:
Those of you who will go for “SPORT” version instead of “RACE”, will receive new GMS PERFORMANCE REAR ARMS (SPORT), the ones you currently have (RACE) cannot be modified. I will ask for received “RACE” arms to be returned though.

It will take up to 30 days for us to dispatch new GMS PERFORMANCE REAR ARMS (SPORT) for those of you who will go for this option. Meanwhile, you can already send the received GMS PERFORMANCE REAR ARMS (RACE) arms back.

Options:
  1. Keep GMS PERFORMANCE REAR ARMS (RACE) and modify the rear undertray (picture above)
  2. Send GMS PERFORMANCE REAR ARMS (RACE) back and receive GMS PERFORMANCE REAR ARMS (STREET)
Thoughts:

Perhaps I over reacted, but when i got photos from a customer showing the undertray not fitting I was shocked, I didn't expect that. I do have a few parts from other vendors on my car and few of them (intercooler, front arm uniballs, harness bar) required more involving modifications than this, but I'm a fabricator and wrench my own car, so I don't have problem with that. However, I do understand that some of you are not exactly a mechanics and prefer to have everything plug and play. I understand and respect that. So once again. I will take care for each and all of you, according to your needs and decisions, but having ultra rigid rear arms will require undertray mod, there is no way around. For all the future orders, as mentioned, two types of GMS PERFORMANCE REAR ARMS will be available SPORT and RACE.

I sincerely wait your reply and decision to: [email protected]

thank you for your understanding.

Best regards,

Rudi
 

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@GMS - I'm not sure it is possible to completely cover the back end and A-Arms and have small notches for the arms (like a "real" race car). There are two issues that I see, first securing the extra material (you run out of attachment points on the frame). Second, I would be concerned about cooling. We can install additional NACA ducts to add the cooling, but we need to be careful. Once the modifications were made by Alfa for gearbox cooling, some of us have hammered the cr*p out of the car in brutal heat and have had zero issues. Yes, we loose a little power due to heat soak, but no real issues.... I would not want to mess that up.....

Rudi, obviously I'm keeping mine... lol
 

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Discussion Starter #91
I will look in to rear downforce matter, until then some tech info...

Most of the overheating issues for the gearbox overheating was solved by flashing TCT unit (TSB info release), which modified operating parameters of the hydraulic unit (shorter time between clutches engagement and revised pressure - on lower gears the pressure was lowered and on higher gears altered. Because of that less heat is generated, problem solved). The quickest way to overheat hydraulic unit is to shift as much as possible and it doesn't depend of the engine power. Also heat shield was added after early 2015 build date cars to protect hydraulic unit from excessive heat generated by exhaust. Since then, I never had gearbox overheat issue again but engine bay heat soak remained problematic, especially on remapped ECU. 3" exhaust helped a lot regarding the IAT (intake air temp) and consequently the ignition timing (retarding when IAT gets too hot = power down). Even more, currently we have in works rear engine bay cooling hatch for 4C coupe and custom ECU and TCT tune to further optimize the track performance. We'll see...
 

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I'm going to look under my car today to see what sort of undertray I have. I have taken it off before, but never noticed the detail, but I have a feeling I know the answer as I have a vague recollection. I think I'm in denial! (I expect it to be the version causing the issues, as my car is early 2016 UK version.)
My tray is the later type, so would need modification. My overriding thought (but not final decision) is that I would prefer to retain the "Race" arms for fitment, but to know of a way of modifying the tray, while being sure I am not compromising cooling in any way. In discussing this with my brother in law, we were wondering if, once the necessary cut out from the scoop / channel has been made, there could perhaps be added a side wall on each of the long sides of the cut out, facing upwards, so as to maintain a air channeling effect which may otherwise have been lost by the removal of material. I assume the scoop (the bit which needs to have a part cut out) is to purge air rather than force it in?
 

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Discussion Starter #93 (Edited)
If you have side openings in the side skirt (limited edition), then this cooling side duct is meant to force air in to the gearbox. It is directed straight in to the gearbox, How exactly is the undertray scoop on the left meant to work is a bit of mystery to me. The airstream under the car is sucking the car towards the ground, so ideally it should be closed completely, but I'll leave that for someone more experienced in aero to explain. It looks like not much of the "dome" in the undertray must be removed, but the undertray scoop is a bit more in a way. Once I get the USA car next week, hopefully with newer undertray, I'll take a look at it and try to design replacement under tray scoop that could be rivet-ed to the undertray as it is now. Of course first I would prefer to understand the job of the undertray scoop.
 

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I think the purpose of that deflector is 2 fold. As you note, there is a fair bit of "suction" down there, and since it is very open open in that area, I would expect most of that air from the side duct to actually not "hit" the hydraulic assembly as it is not THAT close. That deflector shield will direct the air from that side skirt and some from below up along the side of the gear box/hydraulic assembly. Second, It also directs some air farther along the hydraulic assembly, presumable to keep it cooler.

@GMS - I would be interested in that modified scoop, I need add some extra support to hold the deflector
 
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Discussion Starter #96
We'll have first "sport" arm prototype fabricated by the end of this week, then it is off to stress test. If everything proves OK, which I suppose it should, as CAD analyze shows roughly 40% rigidity increase compared to stock arms, we go in to the fabrication of the first batch within 14 days. ETA 30 days until first dispatches. We'll try to sort it out ASAP, but I really don't want to send a chasis / suspension part, important as control arm, without assuring it has the required safety factor.
 

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Sorry to "hijack" the thread but it is related to the rear undertray and side diffusers. Below is a picture of a member that installed the side diffusers and center diffuser - It is visible how they "mix" with the undertray. With my design I choose not to cover the vents for the air because I believe that part of the air is meant to enter those vents and exit through the top of the boot lid and through the rear spoiler. Next I plan to close the holes where there are the elevator arms.

Cutting those protrusions (from the OEM undertray) to install the "race arms" I think it's ok.

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* Note: This is not my car because mine is never this clean. I provided the parts and I hope the owner doesn't mind.
 

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@cipsony - I had forgotten that you had developed something, till I started digging around the side skirts. I think its a very good solution hence my inquiry.
 

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Discussion Starter #99
Today we had in work another 4C. We have taken a look underneath and the mod to undertray won’t have noticeable affect on overheating or aero. More details why, below.

We also got the material for fabrication of new batch of GMS performance rear arms in "sport" version. Lots of questions are being asked and I'll try to make a quick FAQ list here:

How should I set my alignment?
Please use the supplied alignment specs.

My 4C has flat undertray, will "race" arms fit? What about "sport"?
Yes, both versions will fit without need for modification.

Will "street" version be rigid enough for my use (track)?
Yes, although 40% sounds not that much it is a lot compared to stock arms. Even if you run slick tires you can safely use "sport" GMS performance rear arms. Some flex will start to occur beyond 2.1G, but won't affect the handling nearly as much as rubber bushings did.

Will "race" version affect my transmission overheating? The riveted dome to the undertray has pretty much the function of directing the air that comes from the side intake on the left side skirt and hits the transmission, to go under the car. Without one, majority of this air won’t go under the car but straight out through rear grills at the exhaust level height. You can drive car safely without overheating issues due to this mod. We offer free replacement with "sport" version if problem ever arises.

I plan to use slick tires and semislick tires, which version of rear arms should I choose? Both will supersede the performance of stock rear arms, but race version is on another level in terms of rigidity. While the difference won't directly result in lower lap times, the "race" version will provide rigidity beyond the level other suspension components can withstand. In the world of racing everything counts. If extra rigidity, extra precise feedback and super stable dynamic alignment is the key than "race" is the version to go.

I have GMS uniballs, do I need to upgrade to GMS performance rear arms? With GMS uniballs the flex in rubber bushings is gone and translated in to the rear arm flex. While not critical and absolutely neccessary, it is still advisable to upgrade to GMS "sport" version for frequent track use with semislicks and "race" version for race use with slicks. This will result in more precise feedback and steering of the rear axle.

How involving is the modification on newer upcurved undertray to fit "race" version of rear arms?
0.5h -1h. It requires basic cutting tools (angle grinder).

When do I need to tell whether I want "sport" or "race" version? Due to easier organizing of the projects, we appreciate your decision within 20 days.
 

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"...Will "race" version affect my transmission overheating? The riveted dome to the undertray has pretty much the function of directing the air that comes from the side intake on the left side skirt and hits the transmission, to go under the car...."

My car doesn't have this side intake (if it's the one I assume you mean, on the outside of the car just behind the left side door, down low), so I wonder what is the function of my undertray dome. Maybe, it still enables air out from the engine comparment, perhaps by creating low pressure from outside air flow.
 
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