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Lowering intelligently my GT86

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Hi Racecrafters!

I have a bunch of different questions I would like to ask: I'll start by this one.

I have a 2018 GT86 racing edition (Brembo brakes) bone stock.

I'm planning to lower my car, install coilovers, changing wheels and tyres and add adjustability to the setup.

I watched the entire Wheel Alignment Course (amazing) and now I want to do it more than ever.

I have to tell you that this is my first real car and I am trying to learn as much as I can. I love the technically aspect as I am an engineering student.

This should be essentially a road car which is trackable and offer adjustability. I use the car 2/3 times/month so I don't need comfort but unfortunately I can't build now a track only car (so I thought to avoid uniball and use stiffer suspension bushes to increase the steering precision and response).

I am trying to list all the components that are needed or recommended in order to do it.

Starting from the obvious:

• Coilovers

What else do I need?

Having watched the course I would say

• Roll centre correction kit

• Stiffer suspension and steering bushes

• Front and Rear lower control arms ?

Speaking about lower control arms, I went to the Whiteline website and looked for them: while they seem to have rear ones, they don't have the front ones.

I emailed them and they told me that aren't selling from lower control arms because the adjustability at the front is reached by using their "Front essential vehicle kit" that you find attached below.

I saw that on the Racecraft GT86 they're using SPL lower control arms and they are dramatically different from the solution proposed by Whiteline.

What do you think of this solution?

I am so sorry for the long theme but I have, as you can understand, a lot of questions to answer before starting my project.

Any help is appreciated!

Drive safe :) and fast

Attached Files

Hi Mattia, all good questions!

I would also add upgraded anti-roll bars and drop links to your list. Also, spend the money on good quality tyres that are suited for running on track, this will be some of the best money you spend anywhere on the car!

I would say your choice on front lower control arms depends on if you want the car to be closer to road car or race car. The Whiteline kit will likely last longer and also be quieter and less harsh to drive. A lower control arm with solid bearings like the SPL is more suited for pure track use.

I also see that the Whiteline kit includes steering rack bushings which is a nice upgrade to help the steering feel more direct.

Hi Tim, thanks for helping me out!

I will definitely add to my list

• Steering rack bushings

Given that the streets in my country are really bad i would tend to avoid solid bearings... But I could also choose front lower control arms with stiffer bushings!

Maybe the best solution would be:

Lower control arms with solid bearings for a track only build, lower control arms with stiffer bushings for a road/track build (more road oriented tho) and the Whiteline solution for a road car you want to make more enjoyable.

Do you agree? :)

Speaking about the tyres... Yes, I am planning to run on semi-slick. I am evaluating every semi-slick model which is homologated for road and is produced in my size.

I would like to ask for an advice regarding tyres size.

My car, as I said, is bone stock, and my tyres are 215/45/R17 mounted on the stock 17x7.5 et48 wheels.

In Italy we can't change the tyres size or wheels size without problems and we must homologate every change. Not every measure can be homologated. On the GT86 platform, I can only homologate 225/45/R17 or 225/40/R18. (Definitely not interested in R19 on this car).

My goal is to drive fast on track and I think that a slightly bigger tyre would be a nice upgrade.

225/45/R17 is a very economical tyres size with plenty of models and I could buy a new sets of wheels with performance tyres and keep the stock ones with a more sacrificable one. Unfortunately, the augmented side wall worries me in a performance application.

225/40/R18 is still economical (but less than 225/45/R17) and offers less side wall compared to the stock 215 tyres.

Not very important in a performance application but R18 looks cooler.

Your help would be really appreciated.

For a trackday/play/light road use car it will always be a question of what compromises you can put up with.

I'd suggest paying a bit extra for adjustable dampers - or at a minimum single ajustment on the compression, as you can back that off to help compliance on broken road surfaces and firm it up as required for the track.

17/18" wheels may come down to visual preference. For the road I'd probably go for the 17" for compliance and, MAYBE, 18" for a dedicated track set of wheels/tyres. Depending on the specific wheels used, the smaller ones 'should' be lighter and that can make more difference than having a lower profile tyre - if buying the 18"set, look around for lightweight forged wheels - they will be more expensive but are stronger and lighter.

Don't forget to give some consideration into brake pads as often even 'sporting' pads are compromised for cold road response - you will need to try your OEM ones in anger before deciding - but at least change the brake fluid for a high boiling point type.

Good suggestions Gord!

Mattia, it sounds to me like keeping the car as a comfortable road car with a little bit of track use is your priority. I would avoid solid spherical bearings for this case. It's true the ultimate performance will be better, but the harshness is not good for road cars.

You are probably better to go with the more commonly available tyre size so you have a wider range of tyres to choose from.

Hi guys, it's always great to see a notification from you!

Yes, the car should be trackable and adjustable but driveable on public road.

My car has actually the Brembo brake systems from factory (in Italy is called the 'racing edition' GT86).

The stock Brembo brake pads are about to end and since they were too expensive for my pockets (about 900€ for a set of 4) I bought a set of Ferodo Racing DS 2500 brake pads. I don't know it this brand is known in your country. From what I could read online, they seem reliable on track and are usable on the road after a little heat up.

Speaking about the brake fluid, as my car got 2 years I swapped all the fluids and put Motul RBF 600 DOT4 which has a typical dry boiling point of 312 °C and a typical wet boiling point of 205°C. Maybe I could go with the more extreme RBF660.

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Regarding the tyres and a few friends experience, I would like to try Toyo Tyres R888R, Bridgestone RE71R Yokohama AD08RS, Michelin Pilot Sport Cup 2 or Federal 595RS-R tyres. They're all semi slick tyres with good reliability and, expect for Toyo and Bridgestone, they all have a very good price point in my country! The Federal, for example, are about 90€/tyre and the Sport Cup 2 about 100€/tyre!

If you have experience with them, please let me know.

I will also surf on the internet and look for different wheels based on my budget and see which weight they have. I'll go with the lightest wheels I can afford!

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Speaking about the dampers...

My stock Sachs dampers are not adjustable and I want to buy a set of coilovers.

Unfortunately, sometimes they get very expensive.

I was thinking at something like Twin Flex A which is about 1200/1400€ in my country (they're twin tube). Or like Tein Monosport which are like 1600€ (they're mono tube).

The Ferodo is a well known brake pad for us too. Pagid is also another company that tends to make a wide range of compounds for different applications, so worth a look too!

The RBF 600 may well be fine for your application. It really depends on the specific heat dissipation for your car as well as the way it is driven. I recommend seeing how this fluid goes on track and upgrade if you find the fluid boiling causes you problems (the brake pedal will got spongy)

All these tyre options seem like good tyres, I don;t have any specific recommendation between these as car and driver preference is a big part of tyre choice. I would start with recommendations from people who are running the same model car as you. We only run a full slick tyre on out GT86.

The dampers are a place I would suggest buying the best you can afford. The likes of KW, Ohlins etc. I would stay away from anything cheap!

No cheap dampers, got it! It seems like my build (like all the other builds) is going to take some time to save all that money 💰💰

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I was thinking about a previous post in this thread about wheels weight and tyre profile.

I am going to ask a question whose answer is probably going to be "it depends" but I really want to know how I could understand more.

Wheel weight vs stiffer tyre sidewall

225/45/R17 Vs 225/40/R18

let's say that we pick the same wheel brand and model (so we aren't expecting a dramatic weight change) and tyre brand and model - basically in talking about two setups where the only difference is in size.

How could I have an analytical approach and quantify the difference between a slightly lighter wheel with more tyre movement and a slightly heavier wheel with a stiffer tyre?

I wish I concluded my engineering study (currently at 50% of my aerospace engineering course) to understand more by myself.

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In Motorsport, lighter is better. I think we all agree.

But sometimes more rigidity and more stability could end with better lap times. I also need to consider that fact that, in order to get my new set of wheels to be homologated, I have to choose between a 225/45/R17 on a 17*7.5 wheel or a 225/40/R18 on a 18*8 wheel.

I know that, generally speaking, a 225 tyre is okay on both 7.5 and 8 wheel. But which one to prefer?

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You'll hate me sooner or later.

From my perspective, the change in vertical stiffness from changing sidewall sizes is not something I would be concerned about. It's true that the vertical stiffness of the tyre is part of the suspension. But when you are talking about relatively low profile tyres (in this 40-45 range) this difference in vertical stiffness will be negligible. Also, there is nothing to say that having less vertical stiffness in the tyre is a good thing for your particular application anyway! For the same tyre, maybe you give away a small amount of transient response with a higher profile, but you may end up with a more compliant tyre that helps give you more grip on rough surfaces anyway!

From a pure performance perspective for your case, I would bias my decision to the lighter wheel. It's not just as simple as the advantage in unsprung weight (which may be negligible as you say). It's also the difference in rotational inertia, which as an engineering student, you know is quite sensitive to the radius! This extra inertia needs to be accelerated and braked all the time, so reducing it is freeing up some engine (and braking) power that you will then have available to go faster!

Another practical consideration is the size of your brakes. If you are taking your car to the track, the factory brakes will not likely give you the performance you need. Have a look around at brake upgrade kits and check that they will fit under your 17 inch wheels. If so, then great. If not, it's just something to keep in mind when making the wheel decision as I'm sure you will need to upgrade your brakes in the future.

I have found that the Braking system design and the available tyre sizes to suit your vehicle are the determinate of wheel size, there is no value in going to an 18" rim if you cannot find a suitable tyre for your vehicle that uses that rim size.

You may also find that some of the motorsport specific rims, due to the design considerations of the rim, can fit a larger brake package under a 17" rim, than you can fit under a 18" or 19" "road car" rim as they will tend to have a flatter profile on the inside of the wheel drum.

Thank you guys for the useful info!

I didn't know that motorsport specific rims could clear larger brakes than their road corrispective.

I can't wait to follow the data analysis course, buy and install a logging system in my car and starting this project!

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