AC GT3 @ Zandvoort - Sunday 4th September 2022

[RasmusP]

Do you have a video of that spin?

Yep, just uploaded it:

The spin didn't surprise me. I just thought "Yeah I touched the kerb, it was a bit rougher, rear tyres were hopping, I kept the throttle, wheelspeeds peaked, rear started to slide, end of story".
Then I looked into Motec and was like "Errrrr no that didn't happen like I thought it did!"...

Now watching the replay again to record the video I did touch the grass at turn in and the tyres were already squealing.

So I went back into Motec to check the "Dirt Levels", that AC outputs.
And well, when I touched the kerb, the dirt levels were all zero...
Here's the entry, kerb, touching the grass at the end of the spin:

These are the wheelspeeds, marker when the Slip Angles start to increase into the slide:

I checked every single tab in Motec and there's absolutely no hint about the slide happening apart from "Guess you touched a slippery point of the track".
Basically everything stayed completely normal apart from the one thing that's not visible in Motec:
Surface grip multiplier!

So it's either that the tyre was simply at its limit and the kerb surface grip multiplier OR the bouncing was too much (although the MX-5 doesn't tend to slide away on you in that case) OR the 1-point-model decided that the tyre had 0% grip and that was it.



I don't want to complain much about it though! As I said I fully expected the spin while driving. I touched the grass on the outside of the entry, went too far over the kerb, kept the throttle floored and spun.
It's only when I looked into the data that it really didn't make any sense


This is the suspension trace from the little Ligier race last Thursday. I spun in T1 due to hitting the inside kerb while at the limit.
Clearly visible though:

Here's the suspension trace of the MX-5 spin:

To set this "since wave" into perspective:

1= straight
2= turning in!
3= slide/spin
4= going off to the inside+hitting the armco

 

[demetri]

Yep, just uploaded it:

Ah, so it was the previous corner then. Well, I don't know about the MX-5, but in a mid-engine GT3 car I wouldn't go full throttle until I'm off that curb for good. I don't think I can explain why, but I just don't feel like I can stomp on it until the exit, so I go with half-throttle there:

I think you might've tried to carry too much speed there (you're hitting the apex at 120 km/h, while I'm at 132 in an aero car on fresh soft tires, so tons more grip, and it was about 125-127 in the race on mediums ), maybe because you clipped the grass on entry and didn't brake enough. And if your setup was on the oversteery side, it could've resulted in a spin if you tried to force the car to still make the corner

 

[RasmusP]

Ah, so it was the previous corner then. Well, I don't know about the MX-5, but in a mid-engine GT3 car I wouldn't go full throttle until I'm off that curb for good. I don't think I can explain why, but I just don't feel like I can stomp on it until the exit, so I go with half-throttle there:

I think you might've tried to carry too much speed there (you're hitting the apex at 120 km/h, while I'm at 132 in an aero car on fresh soft tires, so tons more grip, and it was about 125-127 in the race on mediums ), maybe because you clipped the grass on entry and didn't brake enough. And if your setup was on the oversteery side, it could've resulted in a spin if you tried to force the car to still make the corner

Yep, I agree with everything!
What I find confusing is that I took the corner faster, more grass at the entry, more kerb at the apex and everything in between throughout the rest of the race.
After the first spin in lap 3 I expected to lose the car again about 5x but it never happened again...

Setup was completely default, which has low camber on the rear so a bit "slidey"

 

[Kek700]

Ironically, I was going to say the same @RasmusP , I must have gone through the same corner a 100 times in practice and the race. Never even a hint of it happening there, I even had a fresh set of softs on, three laps old.

 

[RasmusP]

Thought I'd check every lap just for fun.
And I recorded all the laps too
Watching this straight one after another, I can see why the car slid away. The weight is more settled on the other laps when I take a similar line across that kerb.

Apparently the lateral load combined with the weight not being settled plus giving it some longitudinal load from the throttle was just too much for the tyres. No wheelspin though!
The acceleration simply took away some amount of the maximum lateral grip and the kerb was too much then.

I still think that it's a bit "weird", but at least I can think of a possible explanation now.