Effect of Front Unsprung Weight Reduction?

[Jianlun]

Sorry for another "weight reduction thread"... hear me out..

I would like to ask the Suspension Gurus in this forum to advice on what the effect a 20% unsprung weight reduction (ie 16lb or 7.5kg) in each front corner will have on a stock GR86 front suspension behavior. @CSG Mike @Racecomp Engineering !

I plan on swapping to 2022 BRZ front knuckles, AP Sprint calipers/299mm rotors, RPF1 17x7 wheels. and thats a 20% reduction in unsprung mass (see my calculations below), which seems a lot.

I am thinking the stock bump/rebound damper rates and spring rates were tuned by the almighty Toyota (Ahem... Moziro) to deal with a certain unsprung weight and i am effectively giving the system a lot less weight to deal with. what happens to the grip, wheel control etc? Eg when the wheel hits a bump and the unsprung mass bumps up, wouldnt the stock bump damping be too "stiff" for the unsprung mass since its 20% lighter (and thus doesnt take much to settle it down)? And during rebound, would the lower inertial of the unsprung mass now mean it "flies" towards the ground faster (for the same spring rate) but then it is held back by a (now) overly stiff rebound damping? (And at exactly this moment my mind exploded)

Before
Stock 18" Wheels + Tyres: 43.1lb (Stock 18" wheel + tyre)
Stock Calipers: 7.8lb
Stock Rotors: 17lb
Steel knuckles: 12.4lbs
Total: 80.3lb

After
RPF1 17x7: 15lb
215/45R17 tyres: 22lb
AP Sprint calipers (CP8350): 5.4lb
AP Sprint rotor (CP3862 ie 299mm): 12.2lb
Aluminum Knuckles: 9lbs
Total: 63.6lb

63.6/80.3 = 80%. ie 20% reduction in unsprung mass.

Sorry i didnt cite the sources but they are generally googled from within this forum and i am deeply grateful to the community for all the info floating around.

Obligatory photos of my GR86 RC imported from japan heh heh. (Ignore my steelies, they are not even in the calculation above, else maybe the %reduction will be 100%...)

[CSG Mike]

You'll have better compliance in high speed suspension movement.

Have you driven the car stock to see what you like and dislike first?

[Jianlun]

Not on track yet. But theoretically less unsprung weight should be all good and no drawbacks? Even with such a drastic reduction % wise?

[Racecomp Engineering]

It will be a good thing.

I can give you numbers on how it actually changes things later...I'm on vacation.

- Andrew

[drift86]

Check out this simulation on different unsprung weights and it's effects.

Granted it's a mountain bike and it's for off road use, some of the same principles will still translate to cars.

[chipmunk]

Keyword: unsprung rotating mass.

As opposed to stationary

[Tcoat]

Originally Posted by chipmunk

Keyword: unsprung rotating mass.

As opposed to stationary

Rotation has nothing to do with it. ANY weight below the suspension (and part of the weight of the suspension itself actually) is unsprung weight.

[Racecomp Engineering]

Busy day, but 2 other things to think about:

1. The hollow front swaybar on the BRZ. Apparently in terms of stiffness it's approx 2% stiffer than the standard 18mm solid. Not much...but it should be lighter. Surely someone has weighed it yet compared to the old 18mm one but we have not.

2. The front BRZ control arms for 22+ models are a different part number than the old arms for steel uprights. Not sure what the difference is...bushings are the same. It might be a fitment/geometry thing with the new aluminum uprights. Perhaps @Opie knows since he had a set. Might be nothing important.

I've been thinking about doing something like this with my 1st gen but we've been busy!

- Andrew

[chipmunk]

Originally Posted by Tcoat

Rotation has nothing to do with it. ANY weight below the suspension (and part of the weight of the suspension itself actually) is unsprung weight.

he's talking about wheels and rims.

Edit: To further clarify, lighter rotating components also have lower inertia plus lower unsprung mass, whereas lighter suspension arms only affect the mass.

[Jianlun]

Originally Posted by Racecomp Engineering

Busy day, but 2 other things to think about:

1. The hollow front swaybar on the BRZ. Apparently in terms of stiffness it's approx 2% stiffer than the standard 18mm solid. Not much...but it should be lighter. Surely someone has weighed it yet compared to the old 18mm one but we have not.

2. The front BRZ control arms for 22+ models are a different part number than the old arms for steel uprights. Not sure what the difference is...bushings are the same. It might be a fitment/geometry thing with the new aluminum uprights. Perhaps @Opie knows since he had a set. Might be nothing important.

I've been thinking about doing something like this with my 1st gen but we've been busy!

- Andrew

This is a very informative page. Shows the ins and outs of a 2022 BRZ knuckle swap into a last gen (and i assume applies to the 2022 GR86 wholesale also).

https://minkara.carview.co.jp/smart/...4954/note.aspx

You would need to run Google Translate etc. Says some bolts need to change due to longer lengths needed for the alu knuckle. But also an interesting problem about the outboard ball joint (not possible to buy separately) of the FLCA being not long enough. Maybe thats the reason for the diff part number rather than any geometry changes?

Wonder if an (even lighter....) aftermarket FLCA (cusco, SPL, racerX) can fit the alu knuckles instead. Since the stock FLCA cannot plug and play per se. What the writer did was get a toyota castle nut which is thinner where it counts to allow the pin to successfully engage still.

[Racecomp Engineering]

Originally Posted by Jianlun

This is a very informative page. Shows the ins and outs of a 2022 BRZ knuckle swap into a last gen (and i assume applies to the 2022 GR86 wholesale also).

https://minkara.carview.co.jp/smart/...4954/note.aspx

You would need to run Google Translate etc. Says some bolts need to change due to longer lengths needed for the alu knuckle. But also an interesting problem about the outboard ball joint (not possible to buy separately) of the FLCA being not long enough. Maybe thats the reason for the diff part number rather than any geometry changes?

Wonder if an (even lighter....) aftermarket FLCA (cusco, SPL, racerX) can fit the alu knuckles instead. Since the stock FLCA cannot plug and play per se. What the writer did was get a toyota castle nut which is thinner where it counts to allow the pin to successfully engage still.

Ah yes I knew I'd seen a link like that a while back. Not a simple plug and play it seems but certainly doable.

- Andrew

[ML]

in the name of weight loss has anyone else also tried to swap a 22 86 roof panel to a previous gen? they look to be about the same shape but the 22 is aluminum, this would lower the center of gravity essentially.

[Tcoat]

Originally Posted by ML

in the name of weight loss has anyone else also tried to swap a 22 86 roof panel to a previous gen? they look to be about the same shape but the 22 is aluminum, this would lower the center of gravity essentially.

Forget about where you would even get one that wouldn't cost way more than it is worth what would you save? Maybe 4 or 5 pounds.

If you really want a reduction you get a whole 10 pounds by going with an existing carbon fiber replacement.

https://www.ft86club.com/forums/showthread.php?t=129182

Not sure that the ROI is worth the cost and work but hey it is weight reduction.

[ML]

Originally Posted by Tcoat

Forget about where you would even get one that wouldn't cost way more than it is worth what would you save? Maybe 4 or 5 pounds.

If you really want a reduction you get a whole 10 pounds by going with an existing carbon fiber replacement.

https://www.ft86club.com/forums/showthread.php?t=129182

Not sure that the ROI is worth the cost and work but hey it is weight reduction.

they are about 400$ from subaru. Marsh is backed up over a year on carbon ones, and with my roll cage about to go in and having a dented roof I'm thinking about doing it.