Rank: Driver's Permit
#1 Posted : Thursday, October 22, 2020 8:16:50 PM(UTC)
Hey everyone, long-time lurker! Pleasure to meet you all!

I've been having a problem tuning for grip using RWD vehicles like I'm sure most people have. I've seen it come up again and again. Anyway, I normally set my front ARBs softer and my rear ARBs stiffer because I was under the impression that in addition to the oversteer my cars usually need, a benefit of this is that the rear of my vehicle would be locked down a little tighter during cornering to prevent body roll at the back and hence, my vehicle losing traction mid-turn. However...

I recently watched a video showcasing a particular tune for the same RWD car I was using that had the driver crank up the stiffness in the front of their vehicle and soften the rear- exactly the opposite that I had been doing this whole time. I copied their method and voila! Suddenly my vehicle was gaining far better traction than it had ever had. And while this is great news and all, I still don't understand the 'why' of it. My only guess is that I simply had the stiffness set to high. High enough in-fact that the anti-roll bars were actually preventing the middle and outside of my tires from making full contact with the roadway during corning. Hence, my losing traction mid-turn but I'm still to inexperienced to know for sure.

Can somebody with experience here please explain to me what was that I was experiencing? Your help would be much appreciated.
Rank: C-Class Racing License
#2 Posted : Friday, November 6, 2020 3:58:50 AM(UTC)
I'd meant to reply to this earlier but got distracted and forgot.

How roll stiffness interacts with tire grip is a complex system. As a car enters a corner, weight (and therefore grip) is transferred from the inside tires to the outside tires. The catch is the inside loses more grip than the outside gains (don't ask me to explain why; it's been years since I read up on this stuff), so you end up with a net loss of grip. Increasing roll stiffness is one means of limiting weight transfer to the outside tires and therefore limiting the grip loss, but only to a certain point. If you increase roll stiffness too much, weight transfer can occur too quickly/harshly (resulting in a sudden change in grip) or the whole system becomes so stiff the tires start losing contact with the road surface.

The trick is to find a sweet spot between "stiff enough to limit roll" and "soft enough to still maintain good tire contact". You'll need to be mindful of potential tire grip, spring rate and ARB stiffness, body weight, the car's roll center, and the track conditions to get the most out of it.

Hope that's what you were looking for.
Rank: A-Class Racing License
#3 Posted : Friday, November 6, 2020 4:16:13 AM(UTC)
That's interesting. I thought too that stiffer rear means bigger force for rear tires against the road. I don't have time to test it but even here few people wrote ARB settings is opposite/switched. I am not big tuner so I rather wait for some other explanations.
Rank: X-Class Racing License
#4 Posted : Wednesday, November 18, 2020 8:19:39 PM(UTC)
Over 95% of cars, Rear ARBs < Front ARBs, then, depending on track and/or chicanes go lower/higher on overall stiffness.

DJ Saoco
Rank: C-Class Racing License
#5 Posted : Saturday, November 21, 2020 4:40:31 AM(UTC)
If you want to get into the physics of a car in motion but don't want to take postgrad-level engineering courses to figure it out, I found this website a while back that does a pretty good job of explaining it in a relatively understandable manner:

Autocross to Win

Just keep in mind he's talking about real-world math, and Forza doesn't give us real-world numbers to work with. The ideas are generally applicable, though.
Rank: C-Class Racing License
#6 Posted : Saturday, November 21, 2020 7:13:53 AM(UTC)
Set your arbs lower than 15.00 in the front/ 12.00. this will give you a good starting point. Do you have your springs set correctly, I mean it helps to have a good foundation.