Weight reductions and impact on balance

[4Y_stack_acceleration]

Not doing these modifications anytime soon but just curious about what the outcome might feel like / general curiosity.

Referencing this thread for weight reductions: https://f80.bimmerpost.com/forums/sh....php?t=1077612

Referencing this thread for weight distribution (52.3/47.7 between the axles): https://f80.bimmerpost.com/forums/sh...d.php?t=966978

Starting point - 3,640lb F82 DCT non-ZCP with extended leather, carbon roof, 19" rims. 1,904lbs in the front half of the car, 1,736lbs in rear half of the car.

Delete rear seats: -100lbs (2x rear seat + middle seat)
Replace front seats with carbon recaros: -115lbs + 20lbs
Removing carpeting/insulation in back half of car: -100lbs? (according to Misha Charoudin Apex M4 video)
Chromoly studio RSR half cage: +60lbs

So that's about 235lbs of net weight reductions (3,405lbs). You're losing about 95lbs in the middle of the car (front seats) and 140lbs towards the back... shifting the weight distribution to something like 53-55% in the front and 44-46% in the back.

Does this make the balance more oversteery in a good way or bad way?

If you're not taking much load off the front tires (-4% vs -9% in the rear), do these weight reductions even help with reducing understeer? You lose some traction on throttle, car still understeers where it would otherwise, and now oversteers more easily when you overcook throttle mid-corner or on off-camber.

Anyone who has done this notice any change in rear wheel traction on throttle? Taking even more weight off the rear tires...

Let me know if I'm wrong on any of these weight reductions.

Can also throw on a team schirmer carbon hood (-40lbs net reductions), carbon doors (-80lbs net reductions), carbon trunk lid (-25lbs net reductions), and lighter wheels (maybe -40lbs reductions if you do a 1:5 conversion from unsprung to sprung weight reduction), but that's a lot more money and lower ROI than the items listed above.

[m5_mbb7]

Carbon hood I don’t think is 40lbs I believe they’re 20-27lbs of reduction. Carbon doors you must not do unless you have a real full cage (not a half cage or BS full cage like studiorsr sells) to support the door areas in the event of an accident else you’re asking for trouble.

I’ve done a lot of weight reduction stuff on my m2 which granted is a different chassis and slightly more front biased weight distribution; all driving characteristic type changes like becoming more oversteery with less weight in the rear etc can be mitigated by changing alignment, adjusting coilovers, spring rates, etc so it’s not a big deal. Not to mention corner balance which helps a lot too. I wouldn’t make all these weight changes on a stock suspension which isn’t very adjustable.

[robbo mcs]

You are overthinking it.

Weight reduction in general is good for the handling of the car, especially if it lowers centre of gravity. Have you ever heard of people adding ballast to make the cars balance better?

It will change the F/R distribution and cornerweights. However, in a track car you can address those issues by ride heights, rake, spring/shock settings, alignment, tyres etc.

[bimmerboyE92]

Weight reduction over anything in these cars. You simply cannot remove enough to upset the weight balance to the point of making the dynamics “bad”. A slight damper adjustment or more likely a desirable nimbleness will be felt.

[TractionControlDelete]

Originally Posted by 4Y_stack_acceleration

Not doing these modifications anytime soon but just curious about what the outcome might feel like / general curiosity.

Referencing this thread for weight reductions: https://f80.bimmerpost.com/forums/sh....php?t=1077612

Referencing this thread for weight distribution (52.3/47.7 between the axles): https://f80.bimmerpost.com/forums/sh...d.php?t=966978

Starting point - 3,640lb F82 DCT non-ZCP with extended leather, carbon roof, 19" rims. 1,904lbs in the front half of the car, 1,736lbs in rear half of the car.

Delete rear seats: -100lbs (2x rear seat + middle seat)
Replace front seats with carbon recaros: -115lbs + 20lbs
Removing carpeting/insulation in back half of car: -100lbs? (according to Misha Charoudin Apex M4 video)
Chromoly studio RSR half cage: +60lbs

So that's about 235lbs of net weight reductions (3,405lbs). You're losing about 95lbs in the middle of the car (front seats) and 140lbs towards the back... shifting the weight distribution to something like 53-55% in the front and 44-46% in the back.

Does this make the balance more oversteery in a good way or bad way?

If you're not taking much load off the front tires (-4% vs -9% in the rear), do these weight reductions even help with reducing understeer? You lose some traction on throttle, car still understeers where it would otherwise, and now oversteers more easily when you overcook throttle mid-corner or on off-camber.

Anyone who has done this notice any change in rear wheel traction on throttle? Taking even more weight off the rear tires...

Let me know if I'm wrong [...]

Hey OP, just wanted to offer some more accurate and specific information because there is some guesswork here that over-estimates some reduction efforts. For context, I’ve gone through a whole slew of weight reduction efforts with my first M4 and got the weight down to 3398 with a full tank of fuel.

Stock Aluminum Hood - 22lbs
Carbon Fiber GTS/CS Hood - 16.5lbs
True weight savings: 5-6lbs

The rear seat removal only nets 42lbs

With that out the way here are some of your options with specific data and true weighs:

1) The way I reduced weight by 139lbs quickly was like this: Replace front seats (115 for both) with 15lb/each race seats, harness bar (adds 16 lbs), remove rear seats (42), carpeting (4), and went to the 513M wheel package (24)

2) For a more intense weight cut of just under 153lbs I did the following:

Engine M Power cover = 1.23 lbs
Interior User manual binder = 2.2 lbs
Interior Front floor mats = 2.76 llbs (for the pair)
Interior Rear floor mats = 1.64 lbs (for the pair)
Interior front leather seat = 57.6 lbs (each, Source: PSI) 85lbs saving with lightweight
Interior F82 rear seat = 41 lbs
Interior oh-shit-handles w/ plugs = 0.21 lbs (each, based on driver's handle)
Trunk cargo net = 0.58 lbs
Trunk F80 floor panel = 7.00 lbs
Trunk M Mobility kit = 3.74 lbs
Trunk F80 tray (left side) = 0.35 lbs
Trunk lid warning triangle mounts w/ screws = 0.11 lbs
Trunk battery access cover w/ tool kit = 1.82 lbs
Trunk batter access cover w/o tool kit = 0.53 lbs
Trunk tool kit = 1.29 lbs (cover, tow hook, screwdriver)
Trunk tool kit screw driver = 0.10 lbs
Trunk tool kit towing hook = 1.05 lbs
Trunk tool kit cover = 0.14 lbs
Trunk spare tire well tray = 2.66 lbs
Trunk lid arm covers = 0.48 lbs

Total loss of 152.68lbs


3) With the above weight reductions in mind I’ll offer you the following for context:

The power to weight ratio is approximately…

7lbs of weight per HP (6.92)
9lbs of weight per LB-FT (9.2)

Which essentially translates to 1 horsepower for each 7 or so pounds lost. In other words the 139-153lb reduction will be like having 20-22 more horsepower which is basically the equivalent to the power boost going from the Base F8X to the Competition Pack.

4) And finally, to answer your question regarding the subjective feel and impact on the car:

Personally, the way that I found the car changed was during trailbraking - specifically, the car rotated quicker as I came off the brakes and I liked how the car reacted when coming off the brakes while feeding more throttle.

A second big benefit was the addition of the race seat but not due to weight… I liked the seat because I wasn’t moving around as much so I could focus on my inputs as opposed to using my legs and arms to hold myself in place while simultaneously controlling the car.

The final change that was discernable was the use of lighter wheels. The car subjectively and objectively accelerated faster which isn’t too surprising. I was also using 200TW tires so the change stood out even more.

5) All in all, when I got my current M4 I focused on only the areas with easily perceptible changes and on my driving in general. The driver modifications had the biggest and most rewarding impact on the car (no shocker there)**

I did choose to swap the springs to Swift Spec R, I use lightweight Apex 18x10 wheels with 200TW tires, and I still loose the rear seats/carpets for track days but that’s about it. So my suggestion is to just enjoy the car and learn to become one with it. The weight reduction somewhat helps but on an already light car the impact isn’t as large as you might expect. A proper track alignment, great tires, and good brake pads/fluid are all the car needs.

Hope that helps. Do whatever makes you happy, just sharing my experiences.

**Driver Mod = I am now 1.89 seconds faster at Sonoma in a stock M4 ZCP 6MT, than I was in a M4 6MT Base with the weight reduction and mods I described above. And 1.1 second faster at Laguna Seca.

[4Y_stack_acceleration]

Originally Posted by TractionControlDelete

Hey OP, just wanted to offer some more accurate and specific information because there is some guesswork here that over-estimates some reduction efforts. For context, I’ve gone through a whole slew of weight reduction efforts with my first M4 and got the weight down to 3398 with a full tank of fuel.

Stock Aluminum Hood - 22lbs
Carbon Fiber GTS/CS Hood - 16.5lbs
True weight savings: 5-6lbs

The rear seat removal only nets 42lbs

With that out the way here are some of your options with specific data and true weighs:

1) The way I reduced weight by 139lbs quickly was like this: Replace front seats (115 for both) with 15lb/each race seats, harness bar (adds 16 lbs), remove rear seats (42), carpeting (4), and went to the 513M wheel package (24)

2) For a more intense weight cut of just under 153lbs I did the following:

Engine M Power cover = 1.23 lbs
Interior User manual binder = 2.2 lbs
Interior Front floor mats = 2.76 llbs (for the pair)
Interior Rear floor mats = 1.64 lbs (for the pair)
Interior front leather seat = 57.6 lbs (each, Source: PSI) 85lbs saving with lightweight
Interior F82 rear seat = 41 lbs
Interior oh-shit-handles w/ plugs = 0.21 lbs (each, based on driver's handle)
Trunk cargo net = 0.58 lbs
Trunk F80 floor panel = 7.00 lbs
Trunk M Mobility kit = 3.74 lbs
Trunk F80 tray (left side) = 0.35 lbs
Trunk lid warning triangle mounts w/ screws = 0.11 lbs
Trunk battery access cover w/ tool kit = 1.82 lbs
Trunk batter access cover w/o tool kit = 0.53 lbs
Trunk tool kit = 1.29 lbs (cover, tow hook, screwdriver)
Trunk tool kit screw driver = 0.10 lbs
Trunk tool kit towing hook = 1.05 lbs
Trunk tool kit cover = 0.14 lbs
Trunk spare tire well tray = 2.66 lbs
Trunk lid arm covers = 0.48 lbs

Total loss of 152.68lbs


3) With the above weight reductions in mind I’ll offer you the following for context:

The power to weight ratio is approximately…

7lbs of weight per HP (6.92)
9lbs of weight per LB-FT (9.2)

Which essentially translates to 1 horsepower for each 7 or so pounds lost. In other words the 139-153lb reduction will be like having 20-22 more horsepower which is basically the equivalent to the power boost going from the Base F8X to the Competition Pack.

4) And finally, to answer your question regarding the subjective feel and impact on the car:

Personally, the way that I found the car changed was during trailbraking - specifically, the car rotated quicker as I came off the brakes and I liked how the car reacted when coming off the brakes while feeding more throttle.

A second big benefit was the addition of the race seat but not due to weight… I liked the seat because I wasn’t moving around as much so I could focus on my inputs as opposed to using my legs and arms to hold myself in place while simultaneously controlling the car.

The final change that was discernable was the use of lighter wheels. The car subjectively and objectively accelerated faster which isn’t too surprising. I was also using 200TW tires so the change stood out even more.

5) All in all, when I got my current M4 I focused on only the areas with easily perceptible changes and on my driving in general. The driver modifications had the biggest and most rewarding impact on the car (no shocker there)**

I did choose to swap the springs to Swift Spec R, I use lightweight Apex 18x10 wheels with 200TW tires, and I still loose the rear seats/carpets for track days but that’s about it. So my suggestion is to just enjoy the car and learn to become one with it. The weight reduction somewhat helps but on an already light car the impact isn’t as large as you might expect. A proper track alignment, great tires, and good brake pads/fluid are all the car needs.

Hope that helps. Do whatever makes you happy, just sharing my experiences.

**Driver Mod = I am now 1.89 seconds faster at Sonoma in a stock M4 ZCP 6MT, than I was in a M4 6MT Base with the weight reduction and mods I described above. And 1.1 second faster at Laguna Seca.

This is a trove of good information, thank you.

I don't know if this is correct, but Misha Charoudin said the Apex M4 rental saved probably 200kg (440lbs) of weight from gutting the back half of the car (video link with timestamp):

The Apex M4 page says it weighs 1,491kg (3,280lbs) (160kg less than stock):

https://www.apex-nuerburg.com/bmw-m4-coupe/

No carbon bodywork for their car, just carbon Recaros, completely gutted back half, and a half cage. Curious whether they didn't actually measure the car or if the carpeting/insulation really weighs that much.

[TractionControlDelete]

Originally Posted by 4Y_stack_acceleration

This is a trove of good information, thank you.

I don't know if this is correct, but Misha Charoudin said the Apex M4 rental saved probably 200kg (440lbs) of weight from gutting the back half of the car (video link with timestamp): https://youtu.be/SxK0N_Vn_Gk?t=418

The Apex M4 page says it weighs 1,491kg (3,280lbs) (160kg less than stock):

https://www.apex-nuerburg.com/bmw-m4-coupe/

No carbon bodywork for their car, just carbon Recaros, completely gutted back half, and a half cage. Curious whether they didn't actually measure the car or if the carpeting/insulation really weighs that much.

It seems that the interior is stripped all the way to the aluminum. So the car is essentially front door cards, dash, and racing seats (potentially as light as 8 lbs each). In addition, lighter wheels and lighter BBK helps. One factor we don’t know is the starting weight - obviously, the higher the initial weight is will necessarily result in greater weight losses. Also consider that a full tank of fuel in the F82 is 100lbs so just by weighing it full then stripping it down before re-weighing it with 1/4 tank knocks off 75lbs that don’t even relate to the reduction… so unless we know the initial specs/parameters, we can’t really gauge the integrity of the weight cut.

Either way, the first 250lbs of weight loss (even accounting for the added weight of a cage) is very doable on this platform. But the second 200-250lbs requires some big sacrifices.

The F82 GT4 weighs 3,150 FWIW (with a starting weight of 3,803 *cough***bullshit***cough*). Source: my F82 is 3,580 with a full tank but I guess weight inflation to 3800+ is cool when trying to sell a more expensive GT4 but a mere 3,300 when the base car’s press release came out in 2014 :

https://youtu.be/9nCRPqIjOmY

https://www.cnet.com/roadshow/news/b...-drive-review/

[SYT_Shadow]

Originally Posted by TractionControlDelete

.

5) All in all, when I got my current M4 I focused on only the areas with easily perceptible changes and on my driving in general. The driver modifications had the biggest and most rewarding impact on the car (no shocker there)**

I did choose to swap the springs to Swift Spec R, I use lightweight Apex 18x10 wheels with 200TW tires, and I still loose the rear seats/carpets for track days but that’s about it. So my suggestion is to just enjoy the car and learn to become one with it. The weight reduction somewhat helps but on an already light car the impact isn’t as large as you might expect. A proper track alignment, great tires, and good brake pads/fluid are all the car needs.

Hope that helps. Do whatever makes you happy, just sharing my experiences.

**Driver Mod = I am now 1.89 seconds faster at Sonoma in a stock M4 ZCP 6MT, than I was in a M4 6MT Base with the weight reduction and mods I described above. And 1.1 second faster at Laguna Seca.

I could not agree more. The hardest part is modding the driver!

I'd go a step further and say that not modding it will make you a faster driver.
Learning how to deal with soft suspension and lots of weight transfer will aid you later on in your driving progression.
Working hard to beat people that run slicks when you have 200TW tires will teach you how to get more out of the tires.
Figuring out how to take a turn at the speed other cars with aero take it forces you to dig deep. The day you have aero, you will blow past those same cars.
It's hard to have patience, but it's a great way to get fast

[4Y_stack_acceleration]

Originally Posted by SYT_Shadow

I could not agree more. The hardest part is modding the driver!

I'd go a step further and say that not modding it will make you a faster driver.
Learning how to deal with soft suspension and lots of weight transfer will aid you later on in your driving progression.
Working hard to beat people that run slicks when you have 200TW tires will teach you how to get more out of the tires.
Figuring out how to take a turn at the speed other cars with aero take it forces you to dig deep. The day you have aero, you will blow past those same cars.
It's hard to have patience, but it's a great way to get fast

Agree with all this. But found your comments on aero interesting. Does it actually make a difference for a 3,600lb car? Is there any point to just running a rear wing without front splitters/downforce? And how big does the wing need to be - the GTS wing looks too low / below the roofline to make a difference...

[SYT_Shadow]

Originally Posted by 4Y_stack_acceleration

Agree with all this. But found your comments on aero interesting. Does it actually make a difference for a 3,600lb car? Is there any point to just running a rear wing without front splitters/downforce? And how big does the wing need to be - the GTS wing looks too low / below the roofline to make a difference...

aero makes a huge difference, but until you don't take the car to the limit without aero, what use does aero have? How would you even know the difference?

You really want to learn how to handle the car with stock stuff so it gets out of hand at lower speeds. As you add suspension, less weight, sticky tires, aero, every time the 'oops' happens at higher and higher speeds. It is not the same to correct a booboo at 72mph than 86!

The GTS aero makes a difference, and it can be felt, but it is not a massive aero. The GTMore aero makes a larger difference as you'd expect.