How much is too much?

ExSRT8Guy

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Did my second track day in the Mustang at Thunderhill last week. For the first time, probably 10 - 15 minutes into the run, the back end started to come around in a tight turn at probably 40 mph. I caught it, but was a bit unnerved after.

The guys in the paddock said my tire pressures were too high. Measured 'em when I came off the track, and they were 38+ in the front, 36+ in the back. I come from the time when you braked before the corner, since otherwise you'd overload the front tires and it'd push, big time (trail-braking was unheard of), and you double-clutched every downshift to keep from grenading your synchros. So, to me, these pressures aren't high at all.

Setup:

  • 255/45R18 Michelin Pilot A/S, ~50% tread
    18 x 8.5 Enkei Tuning, 35 mm offset (~1 1/4" wider track than stock)
    Tokico D-Specs, four turns out from full hard
    FRPP "P" springs
    GT500 upper mounts
    BMR front bar on middle setting
    FRPP rear bar
    BMR poly/poly LCAs and relo brackets

Can I get some guidance from those in the know?
 

Voltwings

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I'm just going to be "that guy," because it is always a possibility, but maybe you just weren't tracking out?

Granted, we have very different cars, tracks, driving styles, etc... but i target 36 psi hot and the car always felt, what i consider to be, neutral.

Granted, you are on all seasons it seems, and it's not very hard to get an all season greasy on the track. I'm sure there's AN issue here, but i'm not convinced it's a pressure issue.
 

46addict

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Was anything done to shorten the rear bumpstops? Those nubs on the axle housing.

And LCAs with poly bushings on both ends is not recommended for road racing as they tend to bind up and self destruct. The gurus say to use a spherical on the axle end and poly on the chassis end for a street driven car. If NVH is not an issue, spherical/rod ends on both sides is best.

Try the stock LCAs on the next outing and leave the relo brackets installed. The stock rubber bushings aren't the best for racing but at least you'll know if poly bushings are causing issues or not. And FWIW my Nitto NT555s used to start sliding when tire pressure got above 35psi on my 08 GT with FRPP K springs and Koni STR.T shocks.
 

Sky Render

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BMR's poly bushings articulate. I do not think the G-forces created by all-seasons are enough to make them bind.

I think you need better tires.
 
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stevbd

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Really need more info about what you were doing when the back came around. Corner entry, apex, exit, trail braking, trailing throttle, power on, how much steering input, etc.?

There are a whole bunch of ways to get the back end loose of course, even with the car set up well. And all of them will happen at lower levels with A/S tires.
 

Juice

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Get a infrared thermo. Check tire temps after a run, inside/middle/outside. You want the middle temp to split the outside/inside temp.

36 psi hot is "in the ball park". I see about a 5-8 psi build up in a 40 minute run session (outside front has the most buildup) , and tire temps are spread evenly across the tire.

You need better tires for HPDEs
 

ExSRT8Guy

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I'm just going to be "that guy," because it is always a possibility, but maybe you just weren't tracking out?

Granted, we have very different cars, tracks, driving styles, etc... but i target 36 psi hot and the car always felt, what i consider to be, neutral.

Granted, you are on all seasons it seems, and it's not very hard to get an all season greasy on the track. I'm sure there's AN issue here, but i'm not convinced it's a pressure issue.

"Tracking out?" Don't know that one. Clue me in, please.

Was anything done to shorten the rear bumpstops? Those nubs on the axle housing.

And LCAs with poly bushings on both ends is not recommended for road racing as they tend to bind up and self destruct. The gurus say to use a spherical on the axle end and poly on the chassis end for a street driven car. If NVH is not an issue, spherical/rod ends on both sides is best.

Try the stock LCAs on the next outing and leave the relo brackets installed. The stock rubber bushings aren't the best for racing but at least you'll know if poly bushings are causing issues or not. And FWIW my Nitto NT555s used to start sliding when tire pressure got above 35psi on my 08 GT with FRPP K springs and Koni STR.T shocks.

FRPP bump stops, untrimmed. "P" springs aren't that low, so I didn't see a need to cut 'em. There's ~3/4" before contact.

Stock LCAs are long gone. Didn't even consider spherical; this is one of my DDs, and I wasn't willing to deal with the NVH. Good to know about your experience with the Nittos, and PSI.

BMR's poly bushings articulate. I do not think the G-forces created by all-seasons are enough to make them bind.

I think you need better tires.

I think you're right about those tires.

Really need more info about what you were doing when the back came around. Corner entry, apex, exit, trail braking, trailing throttle, power on, how much steering input, etc.?

There are a whole bunch of ways to get the back end loose of course, even with the car set up well. And all of them will happen at lower levels with A/S tires.

I was near the apex. Don't know how much steering input... 1/4 turn? There's another tight turn shortly after this one, so was roughly 1/4 throttle, as well. No trail-braking... still a foreign concept. ~40 mph. Back end swung out maybe 45 degrees... I just know I was pointing in a direction I didn't want to be!

Get a infrared thermo. Check tire temps after a run, inside/middle/outside. You want the middle temp to split the outside/inside temp.

36 psi hot is "in the ball park". I see about a 5-8 psi build up in a 40 minute run session (outside front has the most buildup) , and tire temps are spread evenly across the tire.

You need better tires for HPDEs

I have an infrared, and was thinking on the way to the track that I should've brought it. One of the other Mustang guys said I wasn't rolling on to the sidewalls far enough, but his tires were pretty burned up on the outer edges, even with a ton of negative camber, so not sure that's advice I want to follow.

"Need better tires" seems to be a recurring theme. Maybe it's time. FWIW, I really like the way this thing works on the street. The only thing I'd change for street is Bilsteins, for the smoother ride.
 

kerrynzl

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I was near the apex. Don't know how much steering input... 1/4 turn? There's another tight turn shortly after this one, so was roughly 1/4 throttle, as well. No trail-braking... still a foreign concept. ~40 mph. Back end swung out maybe 45 degrees... I just know I was pointing in a direction I didn't want to be!

This sounds like "Snap Oversteer" which is usually a driver technique, and will improve with a bit of experience.

I experienced it the first time I tested the Mustang on the track, trying to find the reaction of the Mustang [which is very different from my Corvette]

It is usually caused by getting off the brakes too early while still OFF the throttle.
When you get off the brakes, the car will turn in nicely. But the rear wheels are still "engine braking" so it snaps around. If you row down the gears it can make it worse.

Try and not "push the car to it's limit" but instead concentrate on being smooth. Your lap times may get slower initially but then will get faster.

Your tyre pressures are probably OK because the other end of your car is behaving itself [I'm at 38psi all round, but on a race tyre]
 
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Voltwings

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"Tracking out?" Don't know that one. Clue me in, please.


"Using all the track." This does not apply 100% to every single corner, but it will apply to probably 90-95% of all corners out there, but basically once you hit the apex of a corner you should start opening the wheel and feeding throttle. If you keep the wheel turned after the apex of a corner the car will have more tendency to oversteer as throttle is applied.

This is a video our driving school likes to show, and it explains it very well. The "spin" happens right at :35.

1. Notice he didnt open the wheel up and go to the far (left) side of the track after the corner like the camera car did, he kept the wheel turned and stayed more in the middle.

2. The wheel was still turned while power was being applied, and the car oversteered.

It happens to all of us, just something to be aware of.

https://www.youtube.com/watch?v=7iCClUNg46c
 

Norm Peterson

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Did my second track day in the Mustang at Thunderhill last week. For the first time, probably 10 - 15 minutes into the run, the back end started to come around in a tight turn at probably 40 mph. I caught it, but was a bit unnerved after.
Do you remember being either a bit late and a bit too sharply getting out of the brakes or a bit early/greedy about getting into the throttle? Anything else different?

Do you remember anything about the same turn on the preceding lap or two?


Norm
 

stevbd

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OP, as you can see there are a lot of possibilities.

In terms of basic car setup, and at the risk of over-simplifying, anything you can do to increase rear grip or decrease rear roll stiffness should generally reduce oversteer.

As someone said, your PSI doesn't seem way out of whack.

In terms of technique, it seems a good guess you either may have been doing too many things at once, for example, too much steering input combined with too much gas, or you may have been doing things too abruptly.

There are a lot of good resources to geek out on this stuff but I've found the attached to be pretty well written and comprehensive. Look at Lessons 4 and 5 and see if you recognize anything you might be doing:

https://driver61.com/uni/

 

fast Ed

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If the car has a stock diff (which if it does, is probably worn out), could the rear tires be getting overheated before the fronts because of being spun up on corner exit?
 

Voltwings

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If the car has a stock diff (which if it does, is probably worn out), could the rear tires be getting overheated before the fronts because of being spun up on corner exit?

In my mind, that is possible, but not as likely as the fact that the car is just under-tired. 255 all seasons on a mustang is not a recipe for success.
 

Sky Render

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If the car has a stock diff (which if it does, is probably worn out), could the rear tires be getting overheated before the fronts because of being spun up on corner exit?

I think in order for that to happen, the OP would definitely notice his 1-wheel burnouts.

Though I will agree that your stock TracLock is basically an open diff at this point.
 

BTD

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Correct PSI should be determined by tire temperatures and wear, at least partially. Setup has some to do with it as well but since you can't easily change camber (with strut mount plates) it's a bit less important here.

IMO what happened sounds like a technique problem, not a tire pressure problem.
 

frank s

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I'd like to know more about the car's behavior at other turns on the track. Any similar tendencies elsewhere? An abrupt change in front to rear traction balance might be due to some track-surface characteristic you missed by six inches on all the other laps.
 

SoundGuyDave

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I don't know off the top of my head what the "P" rear spring rates are, but I'll bet they're at (or under) 200lb/in. With 3/4" static clearance between the frame rails and the bump-stops, that would allow you less than 150lbs of load transfer to that spring before you're on the bump-stop and go to effectively infinite spring rate (simplified) and that certainly WILL create a "non-linear" handling characteristic. Same will happen if you bottom out the shock on that corner as well...

When you say "the back end started to come around," do you mean that you experienced SOME oversteer, or that you suddenly snapped around? big difference in the diagnostics tree. Also, how much experience do you have on track? If you're timing laps, what is your typical time spread on clean laps? 0.1 seconds? 0.5 seconds? 1 second?

I'm inclined to look at driver input rather than hardware first...
 

ExSRT8Guy

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This sounds like "Snap Oversteer" which is usually a driver technique, and will improve with a bit of experience.

I experienced it the first time I tested the Mustang on the track, trying to find the reaction of the Mustang [which is very different from my Corvette]

It is usually caused by getting off the brakes too early while still OFF the throttle.
When you get off the brakes, the car will turn in nicely. But the rear wheels are still "engine braking" so it snaps around. If you row down the gears it can make it worse.

Try and not "push the car to it's limit" but instead concentrate on being smooth. Your lap times may get slower initially but then will get faster.

Your tyre pressures are probably OK because the other end of your car is behaving itself [I'm at 38psi all round, but on a race tyre]

Are you saying it could be pilot error? :gr_grin: I don't dispute that, but I'm pretty consistent, if nothing else. And what's that saying... smooth is fast, fast is smooth. I try to adhere to that; learned that with motorcycles years ago.

I didn't lift; I was on the gas, though not hard.

"Using all the track." This does not apply 100% to every single corner, but it will apply to probably 90-95% of all corners out there, but basically once you hit the apex of a corner you should start opening the wheel and feeding throttle. If you keep the wheel turned after the apex of a corner the car will have more tendency to oversteer as throttle is applied.

This is a video our driving school likes to show, and it explains it very well. The "spin" happens right at :35.

1. Notice he didnt open the wheel up and go to the far (left) side of the track after the corner like the camera car did, he kept the wheel turned and stayed more in the middle.

2. The wheel was still turned while power was being applied, and the car oversteered.

It happens to all of us, just something to be aware of.

https://www.youtube.com/watch?v=7iCClUNg46c

Spectacular! The first part, where the tail got loose, was similar to what I experienced, but I didn't put on nearly as good a show; caught it and continued. I think I still had some steering lock in - turning right - because there's a right bend in the track before the next turn.

Thanks for explaining tracking out, BTW.

Do you remember being either a bit late and a bit too sharply getting out of the brakes or a bit early/greedy about getting into the throttle? Anything else different?

Do you remember anything about the same turn on the preceding lap or two?


Norm

Same every time, though the tires were definitely getting greasier.

OP, as you can see there are a lot of possibilities.

In terms of basic car setup, and at the risk of over-simplifying, anything you can do to increase rear grip or decrease rear roll stiffness should generally reduce oversteer.

As someone said, your PSI doesn't seem way out of whack.

In terms of technique, it seems a good guess you either may have been doing too many things at once, for example, too much steering input combined with too much gas, or you may have been doing things too abruptly.

There are a lot of good resources to geek out on this stuff but I've found the attached to be pretty well written and comprehensive. Look at Lessons 4 and 5 and see if you recognize anything you might be doing:

https://driver61.com/uni/


Thanks for this. I'll actually go through a number of the lessons. Need to sit down and focus, which I couldn't do today.

If the car has a stock diff (which if it does, is probably worn out), could the rear tires be getting overheated before the fronts because of being spun up on corner exit?

Front and rear tire pressures increased by the same amount over cold readings. It still has a TrakLoc, but I don't abuse this thing, and it only has 53K miles on it. Realizing its limitations, works as well as it ever has.

In my mind, that is possible, but not as likely as the fact that the car is just under-tired. 255 all seasons on a mustang is not a recipe for success.

I remember when 255s were HUGE. G60s, for my fellow old-timers.

That for sure, the wheel & tire combo is way too small for track day use as the OP is finding out!

Seems to be the consensus.

I think in order for that to happen, the OP would definitely notice his 1-wheel burnouts.

Though I will agree that your stock TracLock is basically an open diff at this point.

See above. I don't think so. I know you really like your Eaton, though; may have to kick down for one of those, or a Torsen, in the future.

Correct PSI should be determined by tire temperatures and wear, at least partially. Setup has some to do with it as well but since you can't easily change camber (with strut mount plates) it's a bit less important here.

IMO what happened sounds like a technique problem, not a tire pressure problem.

Could well be. I didn't do anything different on that lap, which is why it surprised me.

I'd like to know more about the car's behavior at other turns on the track. Any similar tendencies elsewhere? An abrupt change in front to rear traction balance might be due to some track-surface characteristic you missed by six inches on all the other laps.

Dunno. I remember you're a CA guy... have you done Thunderhill? It was turn 9 on the short track. I didn't have any issues elsewhere on the track. FWIW, that's the only "tight" right-hander.
 

Juice

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Tire compound is more important than tire size. I ran some good laps on 245/47/17 NT01s. Then I ran my 275/40/18 NT555s and they were definitely slower, less grip. Ok to have fun on, but I prefer the sticky rubber. :)
 

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