Weird GT 86 factory spring rates in GT5?
 

So, Gran Turismo 5 came out with an update yesterday that includes the new GT 86. Awesome, I think, I'd love to try it out in the game.

So in the setup and settings menu, I noticed that the stock spring rates were 6.0 kg/mm front and 2.0 kg/mm rear. This strikes me as being strange for a few reasons. 6/2 seems like a much wider differential in rates in a car as well balanced front to rear as the 86. Also 6.0 kg/mm is a pretty damn stiff spring to put on a stock car which weighs as little as the GT 86.

For those unfamiliar with GT5, I know it's just a video game, but they are usually anal retentive when it comes to modelling and accuracy. Just for fun I went and checked the rates for the GR STI and they were 4.2 front, 4.0 rear and that sounded pretty accurate to me (or at least close, I'm only familiar with the USDM models).

Any thoughts out there?

Just for fun checked another car...

1991 Acura NSX

'Official' from NSXPrime
F: 170 lb/in, R: 220 lb/in

GT5
F: 179, R: 213

Sounds funky to me. How does it drive?

- Andrew

Exactly how you'd expect it to based on the info out there so far. Nice and lithe, not a ton of power but enough to get it to move with the light weight. Jacked up the power a bit, and you could feel that the stock tires did not offer enough grip for that power level, and the understeer at the limit becomes more apparent. But really fun, and nothing surprising.

Also, RCE I love your stuff.

In stock trim, to me the car did feel stiff with very minimal front end compression, little body roll (compared to the real Toyota 86 reviews which seemed to have a lot more roll) and the steering was very responsive. I was impressed for it being an unmodified car and if this thing is as accurate as it is in the game as it in real life, this car is going to be something special.

Good points. Agreed on there being less body roll than one would expect to see in real life.

Thanks for that. We've got big plans for this car and hopefully we'll have some awesome stuff coming for it as soon as we can. :)

I'll have to test the car out soon on GT5....gonna convince Myles to bring the PS3 in!

- Andrew

It does have to do with the leverages an mounting angle on the suspension, too. (I'm just learning about this, but hopefully Old Greg or some other knowledgeable person can correct any errors I make here...)

In the front it is a MacPherson strut design, which are pretty close to 1:1 leverage on the spring. So the wheel only sees the spring rate of the mounting angle.

In the back it is a multi-link design (3 lower links and an upper wishbone). So depending on where the coil-over mounts to the lower link, relative to the outer most link, there will a change in leverage. So it won't be the same 1:1 as the front struts.

So even though the spring rates are very different, the effective wheel rates are probably much closer to being the same.

^ You're correct that the motion ratio must be taken into account to get actual wheel rates and I do not know how or if GT5 deals with this.

BUT the rear motion ratio is less than 1....probably somewhere between 0.7 to 0.9. This means that the rear 2kg/mm spring (according to the game) will actually be softer "at the wheels." It has less leverage than the front spring.

So really it's like 6 kg/mm front and 1.3 kg/mm rear if it's a .8 rear motion ratio. And I would be shocked if those are the actual spring rates. It's not impossible, but that would be....interesting.

- Andrew

Hmmm... I couldn't remember which way it went, so I weaseled and just said that it was different. :) This makes it even more odd.

With the low CoG it won't need crazy rates to resist pitch and roll. But what does the difference mean? They made a big deal about 'dynamic balance' with this car.

With more front it will resist dive better, and softer rear means it will squat and plant the back end easier, right? Roll can be balanced with bars.

Fronts being stiff will also reduce camber change, maybe?

Seems that it may be setup to rotate easily, but the soft rear makes it safer when you get on the throttle? So it turns nice and when you get on the gas it just straightens out?

Something like that?

yeah i was thinking about how to get that to make sense. it could just be something as simple as using a sway to make that up and keeping the bounce freq more streetable. or maybe the multil link allows for much wider range of useable camber. i mean, i think you would want the softest suspension possible while staying in a desirable suspension geometry. more likely that any of that, its just a stupid game. the stock suspension on the ae86 wasnt close iirc

Almost all of that was right, but the huge disconnect from reality is the ludicrously stiff front spring rate.

6kg/mm would mean ~2.1 Hz ride frequency in the front, which is stiffer than most race cars. While the rear would be ~1.2 Hz which is somewhat stiff for a passenger car. I would be willing to bet at least two quarters that the front spring rate is actually 1.6 kg/mm and somebody at either Toyota or Polyphony Digital just screwed up. 1.6 would be a much more realistic number; It would put the front ride frequency @ 1.1 Hz which, aside from being a sane number; is just a touch less than the rear ride frequency and that is entirely right and proper from an NVH standpoint.

Glad to see that you and Myles are on board. I know of two local customers (old nasioc guys) who will be picking up cars, one an FRS and the other a BRZ. Both are going to have planned down time for some component development. Perhaps we can work on getting some components to you for measurement and then you'll have to chassis' for guinea pig testing too.

-Micah

P.S. the guys are in Falls Church and Hagerstown and the cars will be on the lift in Hag...

6kg/mm is ~335 lbs/in and that doesn't sound like a huge number. 111 lbs/in on the rears sounds soft.