Crazy oversteer / wheelspin with Michelin Pilot Super Sport

[Ubersuber]

Camber bolts: 264-3641. They exist and they are on my car.

Mine look like they are installed at the outer end of the rear LCA. You can't get much camber adjustment out of camber bolts but you can get enough for street application.

Tire grip in cornering increases with tire pressure until you go too far. Reducing tire pressure from manufacturer's recommendation is a stupid idea and always reduces tire grip, don't ever do it for any reason.

35 psi is the recommended tire pressure. Go lower than that and I guarantee your cornering power will be reduced.

Increasing tire pressure all around by the same amount does not (and cannot) change chassis balance, unless of course you go too far which would be just plain stupid.

Ultra high performance street tires do require some temperature to grip as well as they are designed to do. Generally speaking for street driving ultra high performance tires are a complete waste of money because you hardly ever get them running at the optimum temperature required to get the grip you paid for. They feel good though so that's ok. The new Michelin Pilot A/S 3 is a good choice partly for this reason: they grip well from cold.

Asymmetric tires are not necessarily directional, the two concepts are entirely separate. Directional tires are marked with rotation direction and MUST be installed that way around. Asymmetric tires are not necessarily directional (unless they are so marked) and are marked "outside" and "inside" to ensure they are fitted correctly. Directional tires must be remounted to be rotated across the car, asymmetric tires can be fitted either side and rotated normally (unless they are also directional).

The idea that the fact that tread compound differs across the contact patch and therefore this might cause oversteer results from a failure to understand that "negative camber" when static is hardly ever negative camber dynamically in roll. Michelin (and all the other guys making tires) understand this and that is why the "hard" cornering compound is placed to the outside ribs if there is any variation in compound across the tread.

Grip for drag racing has nothing to do with grip for street driving. Drag slicks run low pressures for totally different reasons than contact patch grip. Correct tire pressures for street use depend upon tire size and vehicle weight. Period.

[Ferrari]

Originally Posted by wparsons

If you're ruining tires after a single track day, you're over driving them by a lot. My ZII's lasted almost two full summers (would've lasted longer if I hadn't driven for 3 months with 1/2" of toe out up front) and 8 track days, plus a couple autox events.

Too bad you couldn't make it on Sunday, it was a great day! Did you transfer your spot to a friend? I was checking people in and think I remember someone mentioning they took a spot from a friend that was stuck working.

It wasn't my friend, I cancelled outright and got a refund. I hope I can make it next time, I'd love to try the BRZ out on the track.

You are right, I might have been overdriving the tires. I hope I won't make the same mistake with the BRZ.

[Simon99]

Originally Posted by Ubersuber

Camber bolts: 264-3641. They exist and they are on my car.

Mine look like they are installed at the outer end of the rear LCA. You can't get much camber adjustment out of camber bolts but you can get enough for street application.

Tire grip in cornering increases with tire pressure until you go too far. Reducing tire pressure from manufacturer's recommendation is a stupid idea and always reduces tire grip, don't ever do it for any reason.

35 psi is the recommended tire pressure. Go lower than that and I guarantee your cornering power will be reduced.

Increasing tire pressure all around by the same amount does not (and cannot) change chassis balance, unless of course you go too far which would be just plain stupid.

Ultra high performance street tires do require some temperature to grip as well as they are designed to do. Generally speaking for street driving ultra high performance tires are a complete waste of money because you hardly ever get them running at the optimum temperature required to get the grip you paid for. They feel good though so that's ok. The new Michelin Pilot A/S 3 is a good choice partly for this reason: they grip well from cold.

Asymmetric tires are not necessarily directional, the two concepts are entirely separate. Directional tires are marked with rotation direction and MUST be installed that way around. Asymmetric tires are not necessarily directional (unless they are so marked) and are marked "outside" and "inside" to ensure they are fitted correctly. Directional tires must be remounted to be rotated across the car, asymmetric tires can be fitted either side and rotated normally (unless they are also directional).

The idea that the fact that tread compound differs across the contact patch and therefore this might cause oversteer results from a failure to understand that "negative camber" when static is hardly ever negative camber dynamically in roll. Michelin (and all the other guys making tires) understand this and that is why the "hard" cornering compound is placed to the outside ribs if there is any variation in compound across the tread.

Grip for drag racing has nothing to do with grip for street driving. Drag slicks run low pressures for totally different reasons than contact patch grip. Correct tire pressures for street use depend upon tire size and vehicle weight. Period.

I though camber bolta were available for front only !

[wparsons]

Originally Posted by Ubersuber

Mine look like they are installed at the outer end of the rear LCA. You can't get much camber adjustment out of camber bolts but you can get enough for street application.

If anything, you want more camber up front, not in the rear. Increasing static camber in the rear will make the car easier to rotate at turn in, but unless you're generating a ton of grip (and have already added lots of static camber up front) adding more rear camber will just result in not using the whole tire in corners (and less grip in the rear overall). The rear camber curve is really good compared to the front and I have lots of pictures of tire wear with the factory alignment that show the rear using the tire effectively while the front is using too much of the outer edge.

Originally Posted by Ubersuber

Tire grip in cornering increases with tire pressure until you go too far.

Not true, the best pressure for a given tire with your car setup might be below the pressure on the door. Your blanket statements might be right a small amount of time, but aren't a hard and fast rule. Grip falls off on both sides (too high and too low) of the optimal pressure (or range). If 35psi is too high, then lowering pressure will absolutely increase grip.

35psi cold is way too much pressure for track use with most tires, that would put my old ZII's at over 42psi hot when the tires are happier in the 38psi hot range.

It's not just track/autox use that will increase pressure with heat, even street driving will drive up the pressure. Daily driving probably actually raises the pressure more than autox does due to the very short runs in autox.

Originally Posted by Ubersuber

Reducing tire pressure from manufacturer's recommendation is a stupid idea and always reduces tire grip, don't ever do it for any reason.

Again, bad idea to use blanket statements with something as custom as tire pressures for a given setup.

Also, what is the manufacturer's recommended pressure for a non stock tire? The sticker in the door jamb is for stock tires, different tires behave differently at the same pressure. Even two aftermarket tires in the same performance category respond differently to pressure. My old ZII's had stiffer sidewalls and didn't need as much pressure as my new RS3's do. It's a small difference, but it's still a difference.

Originally Posted by Ubersuber

Increasing tire pressure all around by the same amount does not (and cannot) change chassis balance, unless of course you go too far which would be just plain stupid.

Actually, it can. Because the weight distribution, camber curves, etc aren't the same at both ends on these cars altering pressure by the same amount at all 4 corners CAN change the overall balance. That's like saying increasing spring rate by 50lbs/in at all four corners won't change the balance, but it most definitely will.

Originally Posted by Ubersuber

Correct tire pressures for street use depend upon tire size and vehicle weight. Period.

So tire construction has no factor in that, or weight distribution, or handling balance you're trying to tweak?

For all the work (wrong work IMO) that you've done to your car, you could've shifted the handling balance more towards understeer by simply raising (or lowering) the front pressure a few PSI from the optimal range. Totally free and easy to tweak to your taste compared to cutting bumpstops, adding camber at the wrong end, etc.

Just going to leave this here for you...

http://www.awdriven.net/autoX-tire-pressures.htm

[Ubersuber]

OP not on a track. OP has a stock set up.

Messing with tire pressures front to rear to try and fix handling problems is for amateurs.

Increasing tire pressures degrades the ride which is the principal deficiency in these cars to begin with.

Starting out with underpressure tires relying on overheating them to get them to perform is the dumbest idea of all, so far. Street tire pressures are ALWAYS specified and adjusted at cold ambient temperatures for a reason. Recommending anyone lower tire pressures below the manufacturer's cold pressure specification is totally irresponsible and results in a dangerous situation.

Tire construction variation has almost no effect on recommended tire pressure for any street tire ranges currently available and, anyway, tire construction is taken into account by the tire manufacturer when recommending tire pressures. Car manufacturers adopt the tire manufacturer's recommendation.

My mechanic added rear camber bolts to get rear camber within factory spec and matching both sides, not to add rear camber. Factory does not always deliver in spec alignment. Indeed, it seems common for factory delivered cars of many brands to have incorrect alignment settings from original assembly. First thing to do after experiencing odd handling and checking tire pressures would be to check the alignment.

This car suffers from excess rear roll stiffness and dialling in more rear camber will not help that situation despite a beneficial effect on reducing oversteer. The oversteer results from a sudden increase in roll stiffness that is readily discernible and commonly experienced by almost everyone who drives these cars. Adjusting front camber requires adding adjustment mechanism and since the problem is at the rear it is backwards to adjust the front. Reducing the effective rear spring rate improves this car for the street and should eliminate the OP's current problem. I'd fit better rear dampers at least. Cutting the rear bump stops is sensible to do at the same time to save double labour costs. It works great.

The factory ride is stiff enough for street use, pointlessly stiff as it turns out. It is the same hard ride factor at the rear that causes the oversteer issues that many, many drivers of all skill levels have observed and noted when testing these cars. The FRS is far worse than the BRZ and the GT 86 is halfway between. The pattern is so obvious as to be beyond debate. The cars handle better and better as roll stiffness is moved forwards. Now Subaru/Toyota seem to agree and have changed the stock set up, apparently for the better.

My particular set up is working very well. I'd prefer custom made rear bump stops of course but cutting bump stops is not only OK but a common fix for the evil little handling traits the stock Impreza suspension has been known for for a decade or so. Of course the Impreza is awd so can tolerate more rear roll stiffness under power. Lift off tuck in will be more assertive with higher rear roll stiffness on an awd car but that is generally a benign or even useful effect. The BRZ does not have the front drive forces which reduce the front grip in corners. Subaru seems to have overlooked the need to adjust the rear roll stiffness to compensate for the increased front grip resulting from no drive to the front axle. I suspect this was deliberate and now they've changed their mind for 2015 to broaden the appeal of these cars and keep sales going to a more mature market.

[D_Thissen]

inbeforethelock

Sent from my Z30 using Tapatalk

[Dipstik-sportech]

Where's my popcorn this could get ugly again

[stonenewt]

Originally Posted by Ubersuber

Messing with tire pressures front to rear to try and fix handling problems is for amateurs.

Tyres are an integral part of the suspension & their operating pressure effects the shape & size of the contact patch on the road, as well as now much work they pass to the suspension to deal with. They absolutely should, or rather need, be tuned for best handling characteristics.

If you’re not using OE tyres that means you’ve probably got a different spring weight for a given tyre pressure. Thankfully for road usage you’ll generally want to be seeing a nice flat wear pattern across the tread with a small amount of increased wear towards the inbound edge of the tyre. This means dialling in the correct pressure is simply a case of adjusting the tyre pressure to flatten the tyre wear. Obviously if your car is running loads of camber &/or toe then you’ll see odd wear patterns that can’t be dialled out by tyre pressure which makes the procedure harder.

Softer more supple tyres, e.g. energy efficient tyres, need to be run at a higher warm pressure to compensate for their construction. In addition to this they need a higher cold pressure relative to their warm pressure due to their tendency not to heat up as much. This leads to what seems quite elevated cold pressures.

Stiffer carcass tyres need less pressure to support them & also heat up more as it takes a lot of energy to deform the carcass. This leads to a lower warm pressure with a larger pressure increase when warming up so even lower cold pressures.

Runflat tyres add a new dimension of annoyance to working out what sort of tyre you have. You can have a soft carcass tyre which actually rides quite harshly due to the sidewall reinforcement to allow them to run at 0psi. Which is why I ditch OE RTF tyres with less than 50 miles on them.

For road tyre overheating, which is too much heat in the carcass, is only seen with massive under inflation, to the point that you’ll have totally wrecked your cars handling to start with. On track you can overheat your tyre with much closer to optimal pressures but that’s more the tread than the carcass that’s over heating. The result is generally a bad ware pattern with cooked tread blocks.

[wparsons]

Originally Posted by Ubersuber

My particular set up is working very well. I'd prefer custom made rear bump stops of course but cutting bump stops is not only OK but a common fix for the evil little handling traits the stock Impreza suspension has been known for for a decade or so. Of course the Impreza is awd so can tolerate more rear roll stiffness under power. Lift off tuck in will be more assertive with higher rear roll stiffness on an awd car but that is generally a benign or even useful effect. The BRZ does not have the front drive forces which reduce the front grip in corners. Subaru seems to have overlooked the need to adjust the rear roll stiffness to compensate for the increased front grip resulting from no drive to the front axle. I suspect this was deliberate and now they've changed their mind for 2015 to broaden the appeal of these cars and keep sales going to a more mature market.

I'm going to leave the rest of your incorrectness alone because @stonenewt has already covered that, funny since he was the only one even half supporting your ideas until now.

Have you even bothered to look at the spring rates on an impreza or wrx before making huge, incorrect, assumptions?

Previous WRX is 217lbs/in front, 194lbs/in rear.
Previous STI is 250lbs/in front, 297lbs/in rear.

Weight distribution is about 58/42 for them (the hatchback model is only different by ~0.2).

Assume the same motion ratios we get these for wheel rates:

WRX
217 * 1 = 217
195 * 0.5625 = 109.1
Front bias = 1.99

STI
250 * 1 = 250
297 * 0.5625 = 167.1
Front bias = 1.50

BRZ:
Front: 2.7k * 1 = 2.7k
Rear: 3.5k * 0.5625 = 1.97k
Front bias = 1.37

FRS:
Front: 2.3k * 1 = 2.3k
Rear: 3.8k * 0.5625 = 2.14k
Front bias = 1.07

WRX/STI front weight bias = 1.38
FRS/BRZ front weight bias = 1.13

In theory, the effective spring rate (including sway bar rates) bias should be close to the weight bias.

THAT is why the WRX understeers way more at the limit, not because the front wheels are struggling to steer and put power down. I can't find definitive spring rates for a base impreza, but I can only imagine that they're even more front biased than the WRX is.

Before you try to make a claim that it has anything to do with measurement units, it doesn't. The front bias is the same regardless of how you measure spring rate. It's just effective front rate divided by effective rear rate. You could measure it in ounces/mm and get the same bias numbers.

Further, if it was understeering under power only, that's power understeer which is a result of too much power at the wrong time, not chassis balance. WHICH is the EXACT same cause for any and all oversteer you're getting on a stockish BRZ or FRS.

I get very mild oversteer at turn in (only at the limit, not on the street), which is better known as rotation, without being on the gas with my current setup, but that is on purpose to improve turn in and entry speeds without pushing (understeering). Funnily enough, it's no more likely to oversteer under power than it was before. When the car was stock it wouldn't rotate like that at all without trailbraking into the corner.

As for the 2015's, you keep talking about the changes they've made, but where have you found actual numbers to see what was changed? Until they publish (or someone measures) the rates, how do you know they're changing the rates in the direction you're suggesting is needed?

[wparsons]

Missed this in the last post. Once you factor in the sway bars, the bias shifts even more towards the front. The front bars are bigger on all four cars, and they have an effective motion ratio as well. The fronts attach to the struts, the rears attach near the shock mounting point. Net result, more front bias.

[stonenewt]

Originally Posted by wparsons

I'm going to leave the rest of your incorrectness alone because @stonenewt has already covered that, funny since he was the only one even half supporting your ideas until now.

I agreeing with the things he says that I have the same opinion about. I'd hardly say supporting

Assume the same motion ratios we get these for wheel rates:

WRX
217 * 1 = 217
195 * 0.5625 = 109.1
Front bias = 1.99

STI
250 * 1 = 250
297 * 0.5625 = 167.1
Front bias = 1.50

BRZ:
Front: 2.7k * 1 = 2.7k
Rear: 3.5k * 0.5625 = 1.97k
Front bias = 1.37

FRS:
Front: 2.3k * 1 = 2.3k
Rear: 3.8k * 0.5625 = 2.14k
Front bias = 1.07

WRX/STI front weight bias = 1.38
FRS/BRZ front weight bias = 1.13

Just to be clear you're talking about spring weight bias & static suspension load bias?

In this regard, ignoring the ARBs, relative to the front suspension the WRX has the softest rear setup, then the STI, followed by the BRZ & then the stiffest setup is on GT86. The latter of which effectively has a stiffer rear suspension setup than the front?

[wparsons]

The two sets of numbers are the effective wheel spring rate bias front to rear, and weight load bias front to rear.

If you look at the ratios together, it's actually WRX, BRZ, STI, FRS in terms of front bias factoring in effective spring rates and weight distribution. Because the WRX/STI have a heavier front weight bias they need more spring rate up front to be neutral (in theory, ignoring sway bars).

1.5 (wheel rate bias) is closer to 1.38 (weight bias) (STI) than 1.37 is to 1.13 (BRZ).

[stonenewt]

You're right quite right. My ordering was wrong, I'm not quite sure how I decided 1.21 was lower than 1.08

[Ubersuber]

Originally Posted by stonenewt

Tyres are an integral part of the suspension & their operating pressure effects the shape & size of the contact patch on the road, as well as now much work they pass to the suspension to deal with. They absolutely should, or rather need, be tuned for best handling characteristics.

If you’re not using OE tyres that means you’ve probably got a different spring weight for a given tyre pressure. Thankfully for road usage you’ll generally want to be seeing a nice flat wear pattern across the tread with a small amount of increased wear towards the inbound edge of the tyre. This means dialling in the correct pressure is simply a case of adjusting the tyre pressure to flatten the tyre wear. Obviously if your car is running loads of camber &/or toe then you’ll see odd wear patterns that can’t be dialled out by tyre pressure which makes the procedure harder.

Softer more supple tyres, e.g. energy efficient tyres, need to be run at a higher warm pressure to compensate for their construction. In addition to this they need a higher cold pressure relative to their warm pressure due to their tendency not to heat up as much. This leads to what seems quite elevated cold pressures.

Stiffer carcass tyres need less pressure to support them & also heat up more as it takes a lot of energy to deform the carcass. This leads to a lower warm pressure with a larger pressure increase when warming up so even lower cold pressures.

Runflat tyres add a new dimension of annoyance to working out what sort of tyre you have. You can have a soft carcass tyre which actually rides quite harshly due to the sidewall reinforcement to allow them to run at 0psi. Which is why I ditch OE RTF tyres with less than 50 miles on them.

For road tyre overheating, which is too much heat in the carcass, is only seen with massive under inflation, to the point that you’ll have totally wrecked your cars handling to start with. On track you can overheat your tyre with much closer to optimal pressures but that’s more the tread than the carcass that’s over heating. The result is generally a bad ware pattern with cooked tread blocks.

My point was only that you don't fix handling problems with tire pressures. You fix handling problems first and then optimize tire pressures.

We agree on most things you have written.

It all begins with the shape and temperature of the contact patch. You optimize your suspension to keep the contact patch the correct shape and pressure on the road and only then do you adjust tire pressures to fine tune those aspects of the tire. For road use the recommended tire pressure is the minimum pressure you should use. Generally speaking you can increase grip by increasing tire pressure from that recommended setting.

Carcass construction does indeed affect the recommended pressure as does tire section. However, higher performance tires have less sidewall support than lesser tires and lower profile tires with smaller air volumes require higher tire pressures than lesser tires. This is in general, there may be exceptions.

One thing we disagree on: running even a couple of psi low can seriously overheat a street tire especially when running at high speeds or with heavy loads. It is flat out dangerous which is the motivation behind the misguided TPMS regulations (misguided as currently implemented. The idea is sound and the technology is there to properly implement TPMS it just isn't being used properly).