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Off-throttle understeer ?

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152
120
Exp. Type
Time Attack
Exp. Level
Under 3 Years
Greenbrier, TN
As far as I understand it, @blacksheep-1 is exactly correct.

There's a good video on YouTube by a guy named Andre Marziali called Physics of Racing. It's about 90 minutes long but there's some very good information in there. Explanations for the 'why' to the things you hear.

The SN95 is like a 56/44 weight split in general. Yours is likely better, but let's say more than 50% is on the front wheels, static. In a 3000# car, that's ~840# on each front tire and 660# on each rear. [You can find coefficient of friction tables for tires I've heard.] Under braking or deceleration, how much weight transfers to the front tires? Say it's 8%.[?] Now you've got 960# on the front and 540 on the rears. And, how much more on the outside front tire now that you're attempting to change direction? Probably well over 1000#. You can quickly see how a good portion of the weight transfer ends up on your LF tire and is overwhelming it's traction abilities.

My first time at GingerMan, my instructor thought I was turning in late because I was overdriving the car so bad that I missed every apex. I raced dirt bikes when I was a kid, so I understood braking into a corner. But, I was attempting to trail brake an already nose heavy car with not enough tire under it to support the speeds and weight transfer. He tried to tell me that getting off the brakes and back on the gas would help and I thought he was nuts. But, turns out, as I later discovered, he was correct. When I decided to listen to him understeer almost went entirely away (as long as I didn't overcook the entry) and my corner speeds were much faster, because by getting back on the gas I balanced out the car. Thus, unloading the outside front tire.

What's your alignment like? What springs? Sway bars? Have you corner balanced the car? If it's happening in turning one way and not the other I would assume it's inconsistency's in alignment settings R-L or a balance issue.

And, if you think about it, it happens on right handers, and you're on the left side of the car. So it's likely heavier and more likely to overload the LF tire in a right hander, especially when you let off and cause weight to shift forward, because that's the tire attempting to do the brunt of the work.

Sorry for the long winded post...hope it helps.
Absolutely! This is all great info that even I can understand. Thanks.
 
308
279
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
You listed 2 situations. The first was cold tires and the second was when slowed by traffic. In the first scenario, it's not usual to have a big cold tire (turn the wheel and no one's home) type push for a lap or 3 as the tires get to temp and pressures come up. Rear tires tend to come in first.

Also, both on cold tires and in traffic, it's difficult to load the front end of the car correctly early in the corner because you're limited on how hard you can brake. Even if you lift the throttle, you haven't approached the corner with the normal amount of pitch in the car and it won't want to respond. In both scenarios, you're best to over-slow the middle of the corner just a bit so you can get the car turned early and then get a good drive off the corner.

You mention turn 7 at Road Atl. That's a flat to off-camber corner which invites *everyone* to overcharge, especially since you've just hauled ass through 6. Get the car slowed just a couple more mph at entry so you can ask more of the steering early in the corner and I bet you'll not only have a better front end, but you'll open your hands earlier which allows for a better exit. Remember, these big dumb cars don't make time in the center of a corner (like a Miata or whatever), they make it in getting a good drive off.
 
6,345
8,135
As far as I understand it, @blacksheep-1 is exactly correct.

There's a good video on YouTube by a guy named Andre Marziali called Physics of Racing. It's about 90 minutes long but there's some very good information in there. Explanations for the 'why' to the things you hear.

The SN95 is like a 56/44 weight split in general. Yours is likely better, but let's say more than 50% is on the front wheels, static. In a 3000# car, that's ~840# on each front tire and 660# on each rear. [You can find coefficient of friction tables for tires I've heard.] Under braking or deceleration, how much weight transfers to the front tires? Say it's 8%.[?] Now you've got 960# on the front and 540 on the rears. And, how much more on the outside front tire now that you're attempting to change direction? Probably well over 1000#. You can quickly see how a good portion of the weight transfer ends up on your LF tire and is overwhelming it's traction abilities.

My first time at GingerMan, my instructor thought I was turning in late because I was overdriving the car so bad that I missed every apex. I raced dirt bikes when I was a kid, so I understood braking into a corner. But, I was attempting to trail brake an already nose heavy car with not enough tire under it to support the speeds and weight transfer. He tried to tell me that getting off the brakes and back on the gas would help and I thought he was nuts. But, turns out, as I later discovered, he was correct. When I decided to listen to him understeer almost went entirely away (as long as I didn't overcook the entry) and my corner speeds were much faster, because by getting back on the gas I balanced out the car. Thus, unloading the outside front tire.

What's your alignment like? What springs? Sway bars? Have you corner balanced the car? If it's happening in turning one way and not the other I would assume it's inconsistency's in alignment settings R-L or a balance issue.

And, if you think about it, it happens on right handers, and you're on the left side of the car. So it's likely heavier and more likely to overload the LF tire in a right hander, especially when you let off and cause weight to shift forward, because that's the tire attempting to do the brunt of the work.

Sorry for the long winded post, I've had a lot of coffee this morning...hope it helps.
I completely understand your mindset, I raced karts for 30 years and I'm ruined for life.
 

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