Weird GT 86 factory spring rates in GT5?

[Racecomp Engineering]

Originally Posted by Dimman

A quick cut & paste from something I was working on last night:


I’m going to have to question some of what’s been suggested here. Maybe double check some math or something. I’ve been going over more of my books and came up with this:

Using the following:
2680 lbs
170 lbs driver
53/47 split
75 lbs/side front un-sprung (150lbs total)
70 lbs/side rear un-sprung (140lbs total)
14” lower arm for the front suspension (not really important since a MacStrut is 1:1)
14” lower arm for the rear suspension with coil-over mounted 8” from chassis-side pivot
330 lbs front springs, 290 lbs fitted
110 lbs rear springs, 100 lbs fitted

Results for frequencies were 86.7 cycles/min front (1.44 Hz) and 94.9 cycles/min rear (1.58 Hz).

I get much different natural frequencies than that. The rear is not going to have a higher frequency than the front with rates that soft and the different motion ratio. Check your work.

- Andrew

[old greg]

Originally Posted by Dimman

So, thoughts? If you guys think I’m way off, let me know. Maybe I’m the one with the bad math, and it’s coincidence that the results look like they work…

As near as my forensic math skills can tell, you didn't square the front motion ratio, you applied the rear motion ratio inversely and didn't account for the fact that the spring rates are per corner, not per axle. But I could be wrong.

[Dimman]

Originally Posted by old greg

As near as my forensic math skills can tell, you didn't square the front motion ratio, you applied the rear motion ratio inversely and didn't account for the fact that the spring rates are per corner, not per axle. But I could be wrong.

D'oh!

I'm not using one that deals with motion ratios. It separates the fitted rate and leverage, so it's not 100% accurate as the suspension goes through its travel (fitted rate changing, although there is an appendix on how to calculate/graph this as well). And in that case it doesn't matter on the fronts as the MacStrut's leverage of 1 is still 1 when it's squared.

The rear suspension leverage confuses me, and from what I'm hearing here I wonder if it is a mis-print. They are given as AB/CB with A being the hub-side pivot, B being the chassis-side pivot and C being the spring mounting point. So MacStrut=1 and Wishbone-type link >1.

I'm using the calculations from Allan Staniforth's Race and Rally Car Sourcebook. (don't have it with me at the moment so I'm not 100% sure if I did the AB/CB thing right...)

Strong possibility that I goofed, though...

[Embarrassed]

I think we need to take a double take on the front suspension again. My not-recently-calibrated eyeball doesn't think the motion ratio looks like 1. The control arm mounting locations have been pushed outboard, the strut top is inboard compared and the strut is nearly vertical when compared to all the other strut suspensions I have looked at.

That would make the wheel motion to spring motion ratio closer to 1.5 than 1. Changes the calcs pretty dramatically.

[Dragonitti]

Dang...sure are a lot of GT5 threads.

[Dimman]

Originally Posted by old greg

As near as my forensic math skills can tell, you didn't square the front motion ratio, you applied the rear motion ratio inversely and didn't account for the fact that the spring rates are per corner, not per axle. But I could be wrong.

Double d'oh!

Going through the spreadsheet I made, I'm guessing that you guys and Mr Staniforth will tell me that I need to divide the fitted rate by the leverage^2...

Revision with curb weight halved and appropriate leverage:

Front: 2.04Hz
Rear: 0.73Hz (way less accurate than front guess because of leverage variable)

Could be Polyphony Digital just used a modified Mk3 Supra physics model with less weight and different power (wheelbase and f/r distribution are similar, double wishbones f/r but the rear shock mounting is very far outboard) as they didn't have any solid info on the FT86C at the time. And then they just re-did the body model and adjusted the weight and power again for the GT86/FR-S?

Or they got it backwards? 2/6 gives front 1.20Hz, rear 1.265Hz.

I started to lose faith in GT5 when I learned about how 'easy' it is to adjust camber on a solid axle rear suspension such as the AE86. (It involves an oxy-acetylene torch.) This isn't helping. But Forza let me change the driveshafts of FWD cars so

[Dragonitti]

Originally Posted by Dimman

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?

That's exactly how it drives in GT5

[trackmagic]

The spring rates on the game might be a little extreme (or maybe not?), but the spring rates are normally higher in the front because the engine is in the front and there is slightly more weight in the front AND the car can brake harder than it can accelerate.

This crowd sounds like they know what they are talking about though (seems like a lot of MEs here).

[Racecomp Engineering]

^ Again, there's more to it than just weight distribution. Gotta look at the motion ratios too.

The spring rates in GT5 are incorrect anyway.

- drew