Tire P.S.I. for Fridays lapping

[PeterL]

I have been receiving a lot of conflicting advice about tire pressure for my car, I was wondering if I could take it to the club for some advice.

I drive a 2006 fg coupe si, it has stock suspension and stock camber/ toe settings, a few bolt-ons to the motor and Hawk Hp+ pads + moto brake fluid.

I participated in the HADA track school in April of this year and had a blast. I finished off my Parda spec II's and have now mounted Falkin Aziens R615's (225/40/18) on my H.F.P. 18"x 7" rims (23 pounders) they are a 45 offset.

I have very limited track experience and have never ran the Fabi track before or any Shannonville for that matter and was wondering if I could get some Hot and Cold tempt setting + any advise on how to break them in or get them to perform their best at the track as I have literally just got them mounted.

True sticker tires:)

I really want to get the car to rotate, when Miles and I were @ Dunnville we (I) could not get the car to rid itself of the Mac understeer, even trail breaking would not get the ass to come around.

Looking forward to this weekend and yes S.R. productions will tape some of the events.

[Slowpoke]

Do you need to stay with stock camber for Autoslalom? Do you have two mounting bolts in the clevis for the McPherson struts? Are either of them eccentric camber bolts?

What camber do you currently have front and rear?

Being rather experienced with stock understeering pigs, you can try the following:

Front hot PSI = 45, rear hot PSI = 27
1/16" rear toe out on each side (1/8" total)

[PeterL]

Thanks for your input.

It's all stock, I'm going with no alignment, I need 2 sets of camber bolts up front and arms in the back to get the settings needed.

Until those are installed I'm not going to waste the cash on the very limited settings the car has to offer right now, that why I'm desperate to get the best out of what I have.

[Slowpoke]

Removing or disconnecting one end link on the front sway bar may also help, but ths is less predictable. It is (debatably) technically legal to remove one or both endlinks in SoloSprint and put a tie wrap there, so that there's no strength to the bar, and you don't have to take a PIP for it.

BTW, if you drop to the 27psi ballpark hot for the track, that might be ~21psi cold, so be sure to pump it back up before you leave or you could damage the belts in the tire.

You'll be driving out to Shannonville on the Azenis? That will remove the mould release compound from the surface of the tire. Street tires don't need an initial heat cycle to set up like R-Compounds, but the way you would do that is drive them at highway speeds at lower pressure (32 - 28 psi ish) for 30-45 minutes, then pull them off the car completely and stack them on their sides (not on concrete) for at least 24 hours, but better 48.

[1999_ITR]

Do you need to stay with stock camber for Autoslalom? Do you have two mounting bolts in the clevis for the McPherson struts? Are either of them eccentric camber bolts?

What camber do you currently have front and rear?

Being rather experienced with stock understeering pigs, you can try the following:

Front hot PSI = 45, rear hot PSI = 27
1/16" rear toe out on each side (1/8" total)

To be honest those tire pressures sound like it'll understeer even more. The rear being lower will make it grip better and cause even more understeer on a front wheel drive car. Just my opinion though I could be wrong.

Only the auto-X guys run toe out in the rear. For road race speeds the car gets too unstable with toe out. Most road racers I know run zero toe for the rear, or slight toe in. I wouldn't recommend toe out in the rear especially on someone just starting out, as it will cause snap oversteer.

[MilesM]

Hey Mark - there are 2 theories to enducing oversteer with tire pressures. One is run really high rear tire pressures. The bulging tires reduces the contact patch in the rear causing the rear to loose grip and the car to rotate. The other theory is run really low rear tire pressure - in the mid 20s - this weakens the sidewall on the rear. During weight transfer the rear end wags like a dog - on turn in, the momentum generated will drag the rear end with it around a corner. Both theories work very effectively and have used them both.

Personally, I prefer the lower rear tire pressures for autoX and the higher ones for track. Reason being I find the lower rears have a tendency to cause snap oversteer - in autox the speeds are lower so much easier to catch. Or I'll run the lower tire pressures on the rear at tight cornering tracks like DDT and Dunnville. Now keep in mind my experience doesn't relate to you car though - since I'm talking about cars with shorter wheelbases. On your car snap oversteer is far less likely with your longer wheelbase. Which works best for you is a driver style preference. I'd say Peter you should try both.

Stop at the Sunoco on your way in and put your fronts at 40 and rears at 50. Try that for a few laps and see how you like it. A VW needs as much as 55 rear before the car starts to feel balanced. Then drop the rears to 28ish and try that for a few laps. You might as well experiment and feel how different tire pressures effect handling and figure out what setup works best for you. That's what lapping days are for yo!

If you do the front sway bar like suggested, put your pressures even front and rear till you know what the car is going to do, then add or subtract air as needed to balance out your handling.

[1999_ITR]

Cool thanks Miles good to know...Like said I wasn't sure so thanks for clearing it up :)

[Slowpoke]

To be honest those tire pressures sound like it'll understeer even more. The rear being lower will make it grip better and cause even more understeer on a front wheel drive car. Just my opinion though I could be wrong.

Try it. ;) You were at Andrew's talk on suspension, right?

What Peter has asked about is handling balance. Which, after you maximize the grip at the end that needs it (in this case, the front) becomes about lessening the grip at the other end.

I should say this: Peter, I'm not sure if I rode with you at Dunnville or not, but I assume that if Miles has you trying to rotate your car with trail braking, that he is also confident that you have developed the required skill to correct oversteer fast enough when it begins to happen. If you're not sure that you have those skills, you're probably better off leaving some understeer tendency there.

If you watch Miles going into turn #1 at Nelson, personally I think he's crazy, but his driving style works for him and the cars he sets up. Trying to follow someone with great skills in a well setup car at the same speed and on his same lines is a good way to end up in the grass or worse. So, if you don't have a lot of experience, don't necessarily try to do exactly what the more experienced drivers are doing. It probably won't work for you. Yet.

From hearing about some settings that the HADA guys use, they seem to prefer high rear spring rates to lessen the grip in the rear and make the car more neutral. That's fine, I personally find it less friendly if there's bumps mid-corner, but there's other ways too. G has successfully experimented with narrower tires in the rear for example.

Back to what Andrew spoke about, the lower side of "ideal" pressure is a lot easier to control than the overinflated side. You can change it at the track in a minute or two. It doesn't cost you anything to change, and you have infinite adjustment. 27psi might be too rear happy for you... so try 28. Or 28.2. ;)



Only the auto-X guys run toe out in the rear. For road race speeds the car gets too unstable with toe out. Most road racers I know run zero toe for the rear, or slight toe in. I wouldn't recommend toe out in the rear especially on someone just starting out, as it will cause snap oversteer.

Ya know, I had that same fear for years. A couple of really good drivers had recommended it and I was hesitant, worrying that the rear would snap around unpredictably. But it didn't. It was VERY catchable with the right inputs. I tried 1/8" total toe out (which really isn't much) on the Big Track a couple of weeks ago and I really liked it. It was perfect in a straight line, and I use copious amounts of trail braking, and the rear stayed where it should. But I know the feedback from my car well after five years, and I'm very comfortable with oversteer corrections in my car as I'm inducing it.

Most of the road racers you know drive Hondas? Without McPherson struts? Do they have stock spring rates? Stock camber? Stock suspension Geometry, pickup points, etc.? Most, like Andrew, have probably measured their dynamic toe curves and set their cars up with a bit of toe in because that's what works on their far-from-stock double wishbone suspension cars. Buying a $6000 - $20,000 suspension doesn't seem to fit into Peter's desires for Friday.

You also have to consider that "crabbing" up the back straight at Mosport isn't necessarily the best thing for top speed. So a racer might not want that and might prefer the higher rear spring rate route, in combination with a large camber differential front to rear, big sway bars, etc. But again, those are much bigger upgrade that Peter isn't considering before Friday. However, a $63 alignment at Green and Ross is not comparatively expensive, and is certainly doable before Friday's track session.

I'd like change for my two cents please.

[Slowpoke]

Personally, I prefer the lower rear tire pressures for autoX and the higher ones for track. Reason being I find the lower rears have a tendency to cause snap oversteer - in autox the speeds are lower so much easier to catch.

Hmm ! I find the complete opposite, and just add a little extra throttle when the rear begins to rotate. The graphs that Andrew showed seemed to explain why... that the grip doesn't change drastically below ideal pressure, but that it changes quickly above it. But perhaps that goes back to the part I mentioned about you being insane.

Chacun son gout!

(The only phrase I remember from high school french class.)

[1999_ITR]

I unfortunatly wasn't at Andrews tech talk so I did miss that one about the lower tire pressures. As said I could be wrong and was. My car has more then enough oversteer so I'm good in that department lol.

As for turn 1 at Nelson....I'm full throttle in 4th gear in my car..a slight lift after the overhead flagging station to plant the front and rotate the back then back full throttle. With my car the oversteer is plenty but very controllable. The Fabi straight is even worse I'm in 5th gear at 184km/h(as per last years laser gun) then downshift to 4th after the 50 marker and trailbrake through the corner. The car gets sideways a bit but is very controllable.

I agree with you that alignment setting will be different for different cars and different drivers I was just stating that since Peter said he had limited track experience he may not want to go with an alignment that could induce snap oversteer. I love to trailbrake as well and feel very comfortable with my car but some other may not and may not want to go that route to start.

[MilesM]

Stephen, think you'll find the softer rears on short wheel base car is more likely to cause snap than on a longer mac-strut car since the slip angles change more drastically IMO.

You're also right about the toe settings. I've run as much as 1/4 total front toe and and 1/8th total rear. Nice in the corners, but car bounced around a lot on the straights so I pulled it back. Figured might as well feel what it's like to go too far. Mac struts would be far less effected by by toe in the straights because camber doesn't change as the suspension moves. 0 rear work best for me, but have no experience with mac struts.

[Maxi]

Here's what I believe is happening with low tire pressure vs high tire pressure in the case of a FWD car in which you're trying to induce some rotation.

Low pressure: this causes more of the tire's contact patch to stay in touch with the road. In the case of a track tire with a relatively stiff sidewall, lower pressure means a bigger effective contact patch which means more grip. So when running track tires, where there's no real tread depth and the compound is designed to endure high cornering loads, the idea setup is to run as little tire pressure as possible while achieving optimum tire temps. You therefore need to monitor tire temp when determining the optimum tire pressure for the track, but generally speaking amateur road racers/Solo'ists run far too much pressure in their track tires because the faster turn-in response they get from higher pressures (enhancing sidewall stiffness) fools them into thinking they've also got more grip. I used to run way too much pressure in my Toyo RA-1's for SoloSprint, for example, because I liked the faster turn-in response, but I was definitely giving up grip and therefore speed around the race track. I now run much lower pressures in Toyos and any other R-compound tire I compete on.

With a street tire, because there's a lot of tread squirm due to the deeper and more numerous tread blocks, the compound builds a lot of heat when pushed hard at the track. One way to combat overheating the tire is to run higher tire pressure, which stabilizes the sidewall and tread blocks a bit, but it also reduces the effective size of the contact patch. So although higher tire pressure in this instance helps keep the tire in its ideal operating temp a little longer, the loss of some contact patch does mean you're giving up some grip at the limit. But you will eventually overheat a street tire at full tread depth regardless of tire pressure, at which point the tire will start to feel greasy and understeer will result on whichever tires overheat first.

To apply this to a FWD car on a stock suspension, which will inevitably have quite a bit of understeer dialed in from the factory (for safety reasons), on a tire like the Falken Azenis RT615 which is somewhere between a typical street tire and a track tire, you have (as Miles' has pointed out) two options: you can either raise or lower the rear pressures relative to the front. Keep in mind, you first want to optimize front grip, since that's where all the cornering speed will be generated on a FWD car anyway (in fact, front grip is KING on any type of four-wheel vehicle regardless of which wheels it sends the power to). Once you've optimized front grip by setting the tire pressures as low as you can while achieving optimum tire temperature (you need a tire pyrometer to measure temp and a pressure gauge to measure PSI, obviously), you can then start to play around with rear pressures to adjust the car's attitude in the corners.

With your Falkens, lowering the pressure will give you a bigger contact patch, which will mean more grip initially (which will mean more understeer, not less). However, the compound will also heat up faster as a result of the lower pressure (more sidewall flex and tread block squirm), so in a few laps when the compound overheats you will lose rear grip, helping to reduce understeer. Running very low pressures, which some SoloSprint and Autocross drivers opt to do, will accelerate this process such that within a few corners the rear tires will have overheated and lost grip, helping the car rotate a bit in the corners. The problem with this approach is, overheating your rear tires is detrimental to their life span, and an overheated tire tends to lose grip in a somewhat unpredictable fashion, plus the lower effective sidewall stiffness (and therefore lower "spring rate") means the rear of the car will be hopping around a bit, creating some instability over rougher surfaces like those found at Shannonville and Dunnville (less of a problem at a smooth track like Calabogie).

On the other hand, if you opt to increase rear tire pressure significantly, what you're doing is reducing the effective size of the rear contact patch (almost like running a reverse staggered tire setup, which can be very effective on a FWD car) and reducing the speed at which the rear tires reach optimum tire temp (and therefore maximum grip level). This is the more intelligent approach, IMO, because you're not overheating the compound, which is better for its longevity, and you're more easily able to control the amount of contact reduction based on how high you go with the tire pressure. The higher you go, the smaller the rear contact patch becomes, and the more slowly the rear compound comes up to temp (less temp means less grip, until it reaches optimum temp, and then grip begins to fall off as the compound overheats). The only negative to this approach is that with more pressure you get more effective sidewall stiffness, which means that when the rear tires lose grip and begin to slide, they will do so fairly quickly. So you won't have as much warning when the rear end starts to step out, whereas with a low pressure situation the soft sidewall means loss of grip happens in a more gradual (though also less predictable) way. Personally, having used this high rear pressure method for many years, I do not find that the rears break away so quickly that I can't catch it, but if you're not experienced with this type of setup the speed at which the rear begins to swing around may catch you out the first few times.

Hope this helps. There's A LOT that goes on in a tire (it literally has more moving parts inside than a V8 engine), and things like resonance within tread compound play a HUGE role in it's overall performance.

[Slowpoke]

Great info! I'll add a little factoid from "Racing per Matchett" a few F1 races ago:

F1 tire cold pressure is approximately 16.5psi.

[Matt]

F1 tire cold pressure is approximately 16.5psi.

That clears everything up! Just use 16.5psi and you'll be able to go 300km/h on the straight and pull 5 g's. This racing stuff is cake!

[pigeon]

Excellent info Dave!! I need to re-read it when my mind isn't being so slow, so I can fully absorb it... but there are a couple things you speak of that completely apply to my car and will definitely help me fine tune its setup even more! Awesome stuff, thanks :)

[Maxi]

Chris, we can talk about it this weekend since I should be at Shannonville all weekend (assuming you'll be there). I've spent a lot of time with tire engineers over the last couple of years and have been given some very high level documents from Michelin/BFG which I've learned a huge amount from. I'm slowly starting to comprehend how tires work, but because they're such complicated pieces of technology (that look so deceptively simple) and play such a vital role in performance, there's still a lot to learn!

[PeterL]

I just got back from Shannonville, hell of a storm on the way back.

I eventually ran 47 in the fronts and 42 in the rear (hot) and worked it down a couple of P.SI. in the front after first run and then a couple of more.

Car still had a tenancy to plow and still could not get the back to come around although some more rotation did occur.

This was mainly due to driver erratic behavior but this track also has faster corners than Dunnville.

Thanks for all of your input, here is a small video.

My next thread may begin with P.S.I. for drivers head.

http://www.youtube.com/watch?v=ygPPAiiHgsc

[Guillermo]

FWD cars are front heavy so it's better to have lower rear tire pressures to reduce understeer. There are many reasons for this and I would be happy to talk about it if you ask me but i'm too lazy to write an essay about it. I will leave you with this technical reference to help convince you ;)
Look a the bottom half of this page starting with the title "Neutral / Understeer / Oversteer":

http://www.autozine.org/technical_school/handling/tech_handling_4.htm

[JohnP]

I have tried extreme high and low pressures on the rear of a front drive car and always found the higher rear pressures easier to drive in oversteer situations. Both scenarios result in oversteer but going too low (15-25psi) causes sudden breakaway and then poor control because the tire doesn't provide support (think marshmallow). With high pressures (60+) when the tire breaks away you have a solid tire to support you after the initial breakaway.

There are negatives of both scenarios if you go off when the car oversteers. With low pressures the rear tires can pop the bead and lose pressure, then the rim doesn't handle well and may dig in the earth and roll the car over. With high pressures an off may cause the rim to explode when you hit a rock, etc. Be careful and have fun playing with tire pressures.

JohnP

[Guillermo]

15 to 25psi are ice racing pressures I wouldn't want to risk running that low on the rear of any sedan based track car. By low I mean 30 to 35 psi rear specially when street tries require 45psi in the front to keep from rolling over in a FWD car with stock suspension.

[JohnP]

I have tried the extreme low and high pressures to find the limits and my capability to control the car in those situations. It was fun.

For ice racing, I ran 14lb on front and rear but occasionally snowbanks would cause the tires to break the bead and pop off. As long as I avoided the big snowbanks the car was fast.