Tire PSI

[LuLu]

I'm still driving on the stock BFG KDWS tires, 235/50-18. These tires were the stock/OEM front tire choice on the '03-'04 Marauders and they perform better on the lighter (by 1000 pounds) S197. I am very familiar with their traction and handling behavior, and they will do for now. I do miss my Pirelli Assymetrico and Directionale tires.

 

Until I upgrade to another tire brand and compound, I'm using 35 PSI cold at all 4 corners. My method of maintaing this inflation is to pump the tire to 37 PSI and back it down to 35 PSI cold from there. This insures that all 4 tires are equally inflated and I am not at the mercy of a faulty gauge, or, a hot tire. I check my PSI once a week.

 

Correct cold tire PSI is very important to overall control, and control is power. I don't want to go into a controlled slide, or, drift with a weak sidewall. Pirelli tires have the strongest sidewalls I have ever driven on, but they remain very expensive. I'm liking the Falken brand alot, but this is just my .02C.

 

So...What's your story?

[68fastback]

As long as the front and rear tires are the same sizes, I'd bet if you drop the rear pressure such that the rear loaded-radius matches the front, the handling will be improved overall -- especially in the transition between under/oversteer ...when the most predictability is most important.

 

I susepct the SGT handles fairly neutral as is (with just a bit of initial understeer then turning more neutral as the chassis loads up), but the rear is still lighter and the springs and sways are softer and dropping the pressure to equivalent loaded-redius (or a tad more/less, but that's a good sport to start) will better match the suspension and permit the rear to be more compliant in it's quest to track whatever the front can toss at it ...and will make you smile more on the track.

 

That said... the fronts should also be adjusted for the best match of tire rebound to suspension frequency and desired turn-in feel ...readjusting the rears each time the fronts are changed. Some driving schools have you start at about 40psi up front (likely a bit too much for an SGT, just a hunch) and work down from there -- adjusting the rears to the front's loaded-radius each time and retweaking for turn-in feel, etc. ...repeat as necessary.

 

Just an otd-timer's thoughts... ;-)

 

Btw, when checking tires I always overinflate a bit too, then bleed down to target psi. I do it in the morning while the car is still cold and in the garage with absolutely no sun on any tire. If no garage, I'd suggest parking on west or north side of house/wall/fence the night before (no a.m. sun) or, lacking that, check early on a heavy-overcast day.

 

If you check how much temp actually changes your specific tire psi (it can be considerable depending on profile and wheel), you can then make adjustments at the track based on ambient temp differentials without the need for a morning 're-calibration' which is often not possible.

 

 

 

A bit of nostalgia: when I had my '68 (handling was dominated by big understeer with just a hint of neutrality once loaded <lol>) it was all about sway-bar rates, shock types and settings and tire pressure to neutralize the hefty and annoying understeer -- but it could be done! The F7014s (if I recall) sucked and lasted only about 5K miles so Michelins became my fave skins back then. Today it's such a different story -- so many good tires, the worst of which are prolly better than the best back in the '60s. I also found Pirellis (the old 'dog-biscuits' in the '70s) worked well on my Intermechanica Torino Italia -- unequal wishbones up front and 4-link with panhard in the back ..hi-po 302 and 2500lbs with a nasty tendency to switch from slight understeer to massive throttle-oversteer in an eye-blink forces you to become a decent chassis/tire tuner and a very attentive driver under power in the twisties.

[LuLu]

Thank you, 68fastback, for such an eloquent and detailed post.

 

Once I'm back in my SGT saddle, I will explore lowering my rear PSI as you describe. This will prolly be important for my trip to Terlingua.

[07SGT4578]

been running 35 psi. Nitrogen

[69dejavue]

As long as the front and rear tires are the same sizes, I'd bet if you drop the rear pressure such that the rear loaded-radius matches the front, the handling will be improved overall -- especially in the transition between under/oversteer ...when the most predictability is most important.

 

I susepct the SGT handles fairly neutral as is (with just a bit of initial understeer then turning more neutral as the chassis loads up), but the rear is still lighter and the springs and sways are softer and dropping the pressure to equivalent loaded-redius (or a tad more/less, but that's a good sport to start) will better match the suspension and permit the rear to be more compliant in it's quest to track whatever the front can toss at it ...and will make you smile more on the track.

 

That said... the fronts should also be adjusted for the best match of tire rebound to suspension frequency and desired turn-in feel ...readjusting the rears each time the fronts are changed. Some driving schools have you start at about 40psi up front (likely a bit too much for an SGT, just a hunch) and work down from there -- adjusting the rears to the front's loaded-radius each time and retweaking for turn-in feel, etc. ...repeat as necessary.

 

Just an otd-timer's thoughts... ;-)

 

Btw, when checking tires I always overinflate a bit too, then bleed down to target psi. I do it in the morning while the car is still cold and in the garage with absolutely no sun on any tire. If no garage, I'd suggest parking on west or north side of house/wall/fence the night before (no a.m. sun) or, lacking that, check early on a heavy-overcast day.

 

If you check how much temp actually changes your specific tire psi (it can be considerable depending on profile and wheel), you can then make adjustments at the track based on ambient temp differentials without the need for a morning 're-calibration' which is often not possible.

 

 

 

A bit of nostalgia: when I had my '68 (handling was dominated by big understeer with just a hint of neutrality once loaded <lol>) it was all about sway-bar rates, shock types and settings and tire pressure to neutralize the hefty and annoying understeer -- but it could be done! The F7014s (if I recall) sucked and lasted only about 5K miles so Michelins became my fave skins back then. Today it's such a different story -- so many good tires, the worst of which are prolly better than the best back in the '60s. I also found Pirellis (the old 'dog-biscuits' in the '70s) worked well on my Intermechanica Torino Italia -- unequal wishbones up front and 4-link with panhard in the back ..hi-po 302 and 2500lbs with a nasty tendency to switch from slight understeer to massive throttle-oversteer in an eye-blink forces you to become a decent chassis/tire tuner and a very attentive driver under power in the twisties.

 

 

F70 14s. Were those the old Goodyear Polyglas?

[ilmor]

I run 32 psi all around, as per the sticker, air supplied by my garage compressor.

[Aussie67]

As long as the front and rear tires are the same sizes, I'd bet if you drop the rear pressure such that the rear loaded-radius matches the front, the handling will be improved overall -- especially in the transition between under/oversteer ...when the most predictability is most important.

 

I susepct the SGT handles fairly neutral as is (with just a bit of initial understeer then turning more neutral as the chassis loads up), but the rear is still lighter and the springs and sways are softer and dropping the pressure to equivalent loaded-redius (or a tad more/less, but that's a good sport to start) will better match the suspension and permit the rear to be more compliant in it's quest to track whatever the front can toss at it ...and will make you smile more on the track.

 

That said... the fronts should also be adjusted for the best match of tire rebound to suspension frequency and desired turn-in feel ...readjusting the rears each time the fronts are changed. Some driving schools have you start at about 40psi up front (likely a bit too much for an SGT, just a hunch) and work down from there -- adjusting the rears to the front's loaded-radius each time and retweaking for turn-in feel, etc. ...repeat as necessary.

 

Just an otd-timer's thoughts... ;-)

 

Btw, when checking tires I always overinflate a bit too, then bleed down to target psi. I do it in the morning while the car is still cold and in the garage with absolutely no sun on any tire. If no garage, I'd suggest parking on west or north side of house/wall/fence the night before (no a.m. sun) or, lacking that, check early on a heavy-overcast day.

 

If you check how much temp actually changes your specific tire psi (it can be considerable depending on profile and wheel), you can then make adjustments at the track based on ambient temp differentials without the need for a morning 're-calibration' which is often not possible.

 

 

 

A bit of nostalgia: when I had my '68 (handling was dominated by big understeer with just a hint of neutrality once loaded <lol>) it was all about sway-bar rates, shock types and settings and tire pressure to neutralize the hefty and annoying understeer -- but it could be done! The F7014s (if I recall) sucked and lasted only about 5K miles so Michelins became my fave skins back then. Today it's such a different story -- so many good tires, the worst of which are prolly better than the best back in the '60s. I also found Pirellis (the old 'dog-biscuits' in the '70s) worked well on my Intermechanica Torino Italia -- unequal wishbones up front and 4-link with panhard in the back ..hi-po 302 and 2500lbs with a nasty tendency to switch from slight understeer to massive throttle-oversteer in an eye-blink forces you to become a decent chassis/tire tuner and a very attentive driver under power in the twisties.

 

What he said..............

 

Nice post, as always, 69 Dejavue and well said, I have always ran a little less in the rear with good results. It only makes sense in my mind due to the weight ratios of the car.

[motorjock205]

I have to agree with 40 psi being a little too high to start with...we used to start our Crown Vics at that when we were doing pursuit driving school and using the Goodyear Eagle tires, but by the end of the first day, they were pretty much toast, and the car was a LOT heavier than the SGT is...

 

I set mine at 36 on all four corners for our trip to Florida, but we had a considerable amount of weight in the trunk for our trip as well, so I figured keeping them the same was a good thing, and the ride was not bad at all at that pressure. I'm sure adjusting the rears down a couple of pounds wouldn't hurt with no additional weight in the rear of the car. I'll have to test that out a little bit, but then again, I'm about to put on a new set of tires and wheels next week when I get back home and unpack my GT500 wheels and Goodyear F1 tires...I've not dealt with the different tire sizes before between front and rear, so I'll have some experimenting to do there...if anyone has any tips on that, I'm all ears!

 

You guys have fun in Terlingua...I'd love to be there, but I'll be stuck in Huntsville, Alabama for 5 weeks working! YUK! Someone take a lot of pictures so I can live vicariously through your experience!!!

 

Be safe!

[68fastback]

F70 14s. Were those the old Goodyear Polyglas?

 

Yeah ..."Wide-Oval" polyglass What crap they were too! <lol> ...but they were "belted" (polyglass) and a marked improvement over vanilla bias-plys of the era.

 

The first time I ran at a track with the Michelin radials they tried to wave me off!! This was the fall of '68 and the officials had literally never seen a radial tire before!!! They saw the bulge at the contact patch and thought they were dangerously underinflated. ;-)

 

---

 

07SGT, good point on the nitrogen... much less change in pressure/degree-temp. Also, much less 'bleed' because the nitrogen molecule is physically larger than oxygen (or so I've read) so there's less seepage at the bead-seal and valve.

[mciarochi]

68FastBack -

 

Really nice reply, well done. What are your thoughts on the 255 front v. 285 rear sizes on the GT500?

 

This also is the first time I've had different tire sizes. With the same tire pressure, same weight, the contact area would be exactly the same no matter what the size of the tire (easy math: 1000 lbs. at 40 psi = 25 square inches of surface contact, no matter the tire size). If effect, it shouldn't make any difference (but it does, of course). The wider tire has a shorter and wider contact surface.

 

More side-stability at the cost of forward traction? That would explain why the stock setup is so hard to hook up, especially with the lighter rear-end.

 

My thought is for a given F/R ratio, you could probably get away with slightly less difference between front and rear psi for the same effect with larger rear tires.

[SGT/SC#0471]

Street is what sticker says....track plus 6 psi.

 

Unless its an attempt to elude than plus 6 on the street.

[68fastback]

68FastBack -

 

Really nice reply, well done. What are your thoughts on the 255 front v. 285 rear sizes on the GT500?

 

This also is the first time I've had different tire sizes. With the same tire pressure, same weight, the contact area would be exactly the same no matter what the size of the tire (easy math: 1000 lbs. at 40 psi = 25 square inches of surface contact, no matter the tire size). If effect, it shouldn't make any difference (but it does, of course). The wider tire has a shorter and wider contact surface.

 

More side-stability at the cost of forward traction? That would explain why the stock setup is so hard to hook up, especially with the lighter rear-end.

 

My thought is for a given F/R ratio, you could probably get away with slightly less difference between front and rear psi for the same effect with larger rear tires.

 

I think it gets much more complex with different tire sizes and profiles.

 

With same-size tires, by dropping pressure to get the same loaded radius, the contact patch is gets larger, not smaller so I'm not sure your example works. Whether the tire has 35psi or 25psi it's true it will support the same 1000lbs, just with a different contact patch, but the resonant frequency of the tire is higher at 35psi than 25. So (still with same size tires for now) let's say 35psi front gives a loaded radius of 13.8" and to duplicate that 13.8" loaded-radius in the rear it takes 28lbs. The rear contact patch is now likely much closer to that of the front, but it's the lower resonant frequency of the tire at 28psi that makes it a better match for the rear suspension. Why? Because the manufacturer probably uses similarly softer springs in the ligher rear. So constant-radius will get you close but not exact because: 1) the manufacturer probably has a disproportionately higher rate in the rear (i.e. lower rate springs than the front but more relative to the unloaded weight). This is often done because the rear must be able to carry disporportionately more of any added load weight (rear seats and trunk). So, in this example, 29psi might be a better match.

 

But it gets more complicated... the front is independent with a low unsprung weight (relative tot he rear). The higher unsprung weight of the live rear axle will drive the resonant frequency of the suspension down (because it's heavier), so that makes the rear tire psi want to be lower (lower resonant freq.) to better match the lower resonant frequency of the heavier live axle. So, possibly 26psi in the rear is the best match.

 

Now consider the GT500 with the larger tires in the rear and an even bigger f/r weight-distribution disparity than the SGT. The bigger rear tires at any given psi will support more load than the smaller fronts, i.e. the rears would have an even narrower contact patch than the fronts than if the rears were the same size (at a given pressure). Since we can't use the loaded-radius approach to approximate a suitable resonant frequency it's more of trial and error but we know that the resonant frequency in the rear will need to be even lower on the GT500 (relative to the fronts) than on an SGT (realtive to it's fronts) because of the W/D difference. Here's where profiles come in. If the larger rear tires with their given profile have a natural resonant frequency similar to the fronts at the same pressure, a good adjustment might be to proportion rear pressure downward in proportion to the W/D (alternative to the loaded-radius matching ...and maybe even a bit lower due to the live axle difference as above). If the larger rear tires with their given profile have a naturally higher resonant frequency than the fronts at the same pressure, a better match on the GT500 may require even a bit less pressure in the rear (to counter the higher natural resonance as well as the W/D difference) and if natural resonance is a bit lower, it might be best right on the W/D difference (i.e could be a bit more than the W/D proportion but then dropped a little for the live-axle consideration which might put you right back on the W/D proportion -- confusing, right?).

 

But how can you tell what the resonant freq of a tire-rim combo is? One way is to jack-up one front and one rear, inflate both to the same pressure (in the working range, say 32psi) and whack each with the same stick (a baseball bat works nicely -- I'm not kidding) and listen. One will have a higher toned 'thud' than the other. When a trucker whacks the tires he's listening for that (low frequency) thud to tell him a tire is under inflated. Same principle except we're holding pressure constant in this comparo. If you could measure the resonant frequency of the tire-rim and the loaded suspension easily, it would be fairly easy to match the two more accurately, but that's nearly impossible ...unless you have a shaker rig -- even then, tuning the tires to the suspension is not a simple task from what I've read. So, just knowing how the front and rears relate (resonant-frequency-wise) to each other is agood starting point. (Btw, when the FR500C was in development, the shaker rig found that the engine-mounts were way too soft and, at certain suspension frequencies, the engine+trans was taking on a life of it's own under the hood and it's substantial weight was affecting the suspension's frequency, and vice versa, and the cars handling as a result).

 

An example. Let's say the apparent frequency (whack test) is that the rear freq (at the same psi) is slightly lower than the fronts (maybe it's higher -- dunno). Then if the actual f/r W/D is 57/43 and the car+driver wet-weight is 4300lbs, that says about 2455 front and 1845 rear (this needs to be actually measured for best results and it's further complicated by the fact that each front and rear will not exactly match the other, but keeping it simple for the example...). So if the starting point is (arbitrarily) 38 cold-psi front, the rear would want to be about 28-29lbs. But, the natural resonance of the rear tire 'sounded' lower in the whack-test which would say maybe more like 29-30 (to compensate).. but... the live axle in the back will have a lower resonsnce due to the higher unsprung weight so 28 or so in the rear might actually be closest (the whack-difference and the live-axle adjustments wantng to pull the adjustment in opposite directions -- to what degree we don't precisely know).

 

I think that's as close as you can get with a scale, a bat and an calculator . Then you have to drive it and see what it does at 35F/28R (or whatever it turns out to be). See how high-speed turn-in is. Too sluggish/too much push? bump the front psi up a bit; good neutral turn in but the back wants to go into oversteer, drop the rear psi a bit so it can 'follow' the front's lead better. The dynamics are far more complex than I can address (and frankly I don't understand the math/modelling behind the dynamics, being a seat-of-the-pants 'tuner' from the dark ages <lol>) but the s.o.p. 'feel' should not be too complex. If you have access to a skid pad (or a nice big smooth-paved parking lot an understanding local police), you're looking for balance when on the edge. Also, track-days let you put it all to the test. Ideally, you'd like to see that when you start to push beyond the limit on the skid-pad, the suspension's reaction is to neither plow nor get tail-loose, but to fall off the line in a controlled/neutral/ballanced manner. If it plows, the front needs more grip. If the tires are howlling, more front psi *might* be the answer, if not, lett rear psi might be the answer, but eventually, with trial and error, you reach the limit of the tires and the suspension with the best ballance of resposiveness to direction change and control/forgiveness at the limits of adhesion and that's the best you're likely to achieve with those tires and suspension components. Change anything and the quest for optimization necessarily starts over -- but that's the fun!!.

 

A common error, imo, is going with springs that are too stiff, especially in the rear. The rear can't steer (except with the throttle) so it's important that the rear suspension (and tire-psi 'tuning') can 'track' whatever the front can throw at it. While the rear can be too soft (softer than W/D and chassis loading dictates) it's usually the case on factory cars that's it's too stiff for best handling (done to accommodate reasonable people/cargo capacity which mostly loads the rear), so if the front suspension is lowered (and re-corrected for lowering etc), it often must be sprung a bit stiffer to keep off the snubbers at the limits of cornering, but the rear spring rates probably don't need to be stiffer, even if lowered (as long as you don't plan on carrying two large adults and a trunk full of stuff on public roads), because they prolly were a bit to stiff to begin with relative to the fronts for best handling. In the front, think 'responsiveness' -- in the rear think 'compliance.' Again, that's an oversimplification, but a better way to think about things, imo, than the typical default of stiffer-is-better.

 

One other point... 18" wheels on the mustang's live axle will permit better handling than 20"ers, imo, because the taler 18" rubber offers the ability to tune to the live axle better that 20s. Why? Because the inherent higher resonant frequency of low-profile 20s (has to still fit in the wheel well <lol>) would require such low rear pressure for optimum handling that they would be unsafe (bead-spin and pinched sidewalls). This is undoubtedly, imo, why Ford went with 18s on the KR even tho they showed it with the cool-looking 20s. The 18's are what the suspension 'needs' at relaistic pressures, imo.

 

Sorry for the long post and I sure don't mean to make this sound like I have all the answers (because I certainly don't), but the above is intended to set out some of the principles of tuning the tires to the handling/suspension characteristics as a starting point and what to look-for/adjust-to. certainly shock-rates (and compression vs ebound damping tuning, sway bar and spring rates, camber and caster, etc all are part of the mix if planning mods. It's undoubtedly far more complicated than I know as evidenced by the fact that NASCAR teams now all have 7-post shaker rigs for evaluation and simulation and they still tweak pressures and spring-rate/shims for every track and surface and sometimes even for temp changes -- and several times in a given race when necessary ...and that's with the suspension components and tires rigidly baked in the rules book. Compare that to Indy or CanAm cars where the sky's the limit and the shaker-rigs are essential to pre-prove actual suspension designs with replicated track dynamics -- a technique invented by Ford for the GT40 in the 60s when devices had to be invented to capture onboard sensor data so a given track could be modelled in the first place. That's how Ford/Shebly *really* beat Ferrari, imo. Ferrari had decades of s.o.p. chassis tuning expertise, but it was no match for Ford's brilliant idea of inventing a track-simulator using actual track data and then inventing the machine to use that data to literally recreate the tack dynamics under an actual GT40 in Ford's engineering labs -- simply brilliant!

[SGT/SC#0471]

I think it gets much more complex with different tire sizes and profiles.

 

With same-size tires, by dropping pressure to get the same loaded radius, the contact patch is gets larger, not smaller so I'm not sure your example works. Whether the tire has 35psi or 25psi it's true it will support the same 1000lbs, just with a different contact patch, but the resonant frequency of the tire is higher at 35psi than 25. So (still with same size tires for now) let's say 35psi front gives a loaded radius of 13.8" and to duplicate that 13.8" loaded-radius in the rear it takes 28lbs. The rear contact patch is now likely much closer to that of the front, but it's the lower resonant frequency of the tire at 28psi that makes it a better match for the rear suspension. Why? Because the manufacturer probably uses similarly softer springs in the ligher rear. So constant-radius will get you close but not exact because: 1) the manufacturer probably has a disproportionately higher rate in the rear (i.e. lower rate springs than the front but more relative to the unloaded weight). This is often done because the rear must be able to carry disporportionately more of any added load weight (rear seats and trunk). So, in this example, 29psi might be a better match.

 

But it gets more complicated... the front is independent with a low unsprung weight (relative tot he rear). The higher unsprung weight of the live rear axle will drive the resonant frequency of the suspension down (because it's heavier), so that makes the rear tire psi want to be lower (lower resonant freq.) to better match the lower resonant frequency of the heavier live axle. So, possibly 26psi in the rear is the best match.

 

Now consider the GT500 with the larger tires in the rear and an even bigger f/r weight-distribution disparity than the SGT. The bigger rear tires at any given psi will support more load than the smaller fronts, i.e. the rears would have an even narrower contact patch than the fronts than if the rears were the same size (at a given pressure). Since we can't use the loaded-radius approach to approximate a suitable resonant frequency it's more of trial and error but we know that the resonant frequency in the rear will need to be even lower on the GT500 (relative to the fronts) than on an SGT (realtive to it's fronts) because of the W/D difference. Here's where profiles come in. If the larger rear tires with their given profile have a natural resonant frequency similar to the fronts at the same pressure, a good adjustment might be to proportion rear pressure downward in proportion to the W/D (alternative to the loaded-radius matching ...and maybe even a bit lower due to the live axle difference as above). If the larger rear tires with their given profile have a naturally higher resonant frequency than the fronts at the same pressure, a better match on the GT500 may require even a bit less pressure in the rear (to counter the higher natural resonance as well as the W/D difference) and if natural resonance is a bit lower, it might be best right on the W/D difference (i.e could be a bit more than the W/D proportion but then dropped a little for the live-axle consideration which might put you right back on the W/D proportion -- confusing, right?).

 

But how can you tell what the resonant freq of a tire-rim combo is? One way is to jack-up one front and one rear, inflate both to the same pressure (in the working range, say 32psi) and whack each with the same stick (a baseball bat works nicely -- I'm not kidding) and listen. One will have a higher toned 'thud' than the other. When a trucker whacks the tires he's listening for that (low frequency) thud to tell him a tire is under inflated. Same principle except we're holding pressure constant in this comparo. If you could measure the resonant frequency of the tire-rim and the loaded suspension easily, it would be fairly easy to match the two more accurately, but that's nearly impossible ...unless you have a shaker rig -- even then, tuning the tires to the suspension is not a simple task from what I've read. So, just knowing how the front and rears relate (resonant-frequency-wise) to each other is agood starting point. (Btw, when the FR500C was in development, the shaker rig found that the engine-mounts were way too soft and, at certain suspension frequencies, the engine+trans was taking on a life of it's own under the hood and it's substantial weight was affecting the suspension's frequency, and vice versa, and the cars handling as a result).

 

An example. Let's say the apparent frequency (whack test) is that the rear freq (at the same psi) is slightly lower than the fronts (maybe it's higher -- dunno). Then if the actual f/r W/D is 57/43 and the car+driver wet-weight is 4300lbs, that says about 2455 front and 1845 rear (this needs to be actually measured for best results and it's further complicated by the fact that each front and rear will not exactly match the other, but keeping it simple for the example...). So if the starting point is (arbitrarily) 38 cold-psi front, the rear would want to be about 28-29lbs. But, the natural resonance of the rear tire 'sounded' lower in the whack-test which would say maybe more like 29-30 (to compensate).. but... the live axle in the back will have a lower resonsnce due to the higher unsprung weight so 28 or so in the rear might actually be closest (the whack-difference and the live-axle adjustments wantng to pull the adjustment in opposite directions -- to what degree we don't precisely know).

 

I think that's as close as you can get with a scale, a bat and an calculator . Then you have to drive it and see what it does at 35F/28R (or whatever it turns out to be). See how high-speed turn-in is. Too sluggish/too much push? bump the front psi up a bit; good neutral turn in but the back wants to go into oversteer, drop the rear psi a bit so it can 'follow' the front's lead better. The dynamics are far more complex than I can address (and frankly I don't understand the math/modelling behind the dynamics, being a seat-of-the-pants 'tuner' from the dark ages <lol>) but the s.o.p. 'feel' should not be too complex. If you have access to a skid pad (or a nice big smooth-paved parking lot an understanding local police), you're looking for balance when on the edge. Also, track-days let you put it all to the test. Ideally, you'd like to see that when you start to push beyond the limit on the skid-pad, the suspension's reaction is to neither plow nor get tail-loose, but to fall off the line in a controlled/neutral/ballanced manner. If it plows, the front needs more grip. If the tires are howlling, more front psi *might* be the answer, if not, lett rear psi might be the answer, but eventually, with trial and error, you reach the limit of the tires and the suspension with the best ballance of resposiveness to direction change and control/forgiveness at the limits of adhesion and that's the best you're likely to achieve with those tires and suspension components. Change anything and the quest for optimization necessarily starts over -- but that's the fun!!.

 

A common error, imo, is going with springs that are too stiff, especially in the rear. The rear can't steer (except with the throttle) so it's important that the rear suspension (and tire-psi 'tuning') can 'track' whatever the front can throw at it. While the rear can be too soft (softer than W/D and chassis loading dictates) it's usually the case on factory cars that's it's too stiff for best handling (done to accommodate reasonable people/cargo capacity which mostly loads the rear), so if the front suspension is lowered (and re-corrected for lowering etc), it often must be sprung a bit stiffer to keep off the snubbers at the limits of cornering, but the rear spring rates probably don't need to be stiffer, even if lowered (as long as you don't plan on carrying two large adults and a trunk full of stuff on public roads), because they prolly were a bit to stiff to begin with relative to the fronts for best handling. In the front, think 'responsiveness' -- in the rear think 'compliance.' Again, that's an oversimplification, but a better way to think about things, imo, than the typical default of stiffer-is-better.

 

One other point... 18" wheels on the mustang's live axle will permit better handling than 20"ers, imo, because the taler 18" rubber offers the ability to tune to the live axle better that 20s. Why? Because the inherent higher resonant frequency of low-profile 20s (has to still fit in the wheel well <lol>) would require such low rear pressure for optimum handling that they would be unsafe (bead-spin and pinched sidewalls). This is undoubtedly, imo, why Ford went with 18s on the KR even tho they showed it with the cool-looking 20s. The 18's are what the suspension 'needs' at relaistic pressures, imo.

 

Sorry for the long post and I sure don't mean to make this sound like I have all the answers (because I certainly don't), but the above is intended to set out some of the principles of tuning the tires to the handling/suspension characteristics as a starting point and what to look-for/adjust-to. certainly shock-rates (and compression vs ebound damping tuning, sway bar and spring rates, camber and caster, etc all are part of the mix if planning mods. It's undoubtedly far more complicated than I know as evidenced by the fact that NASCAR teams now all have 7-post shaker rigs for evaluation and simulation and they still tweak pressures and spring-rate/shims for every track and surface and sometimes even for temp changes -- and several times in a given race when necessary ...and that's with the suspension components and tires rigidly baked in the rules book. Compare that to Indy or CanAm cars where the sky's the limit and the shaker-rigs are essential to pre-prove actual suspension designs with replicated track dynamics -- a technique invented by Ford for the GT40 in the 60s when devices had to be invented to capture onboard sensor data so a given track could be modelled in the first place. That's how Ford/Shebly *really* beat Ferrari, imo. Ferrari had decades of s.o.p. chassis tuning expertise, but it was no match for Ford's brilliant idea of inventing a track-simulator using actual track data and then inventing the machine to use that data to literally recreate the tack dynamics under an actual GT40 in Ford's engineering labs -- simply brilliant!

 

Thanks....great info....still wish i had a lusso though....next to my SGT of course!

[mciarochi]

Thanks!

 

You made me lose half a day hunting down all sorts of tire pressure information/opinions, and drawing up a spreadsheet for my car. I thought I knew something about tire pressure, but I was merely semi-ignorant.

 

In addition to everything you discussed, there is also the issue of speed - particularly if you're headed north of 100mph, in which case you simply start adding pressure, and at some point start reducing max load. For the Z-rated tires, that starts at 1.5 psi at 125 mph on up to 7.5 psi at 149. This is based on increased rate of sidewall flex as your speed increases. For drag and road, that's not a problem with the Z's, it might make a bit of a difference on long-haul high-speed autobahn type driving.

 

Because of flex and sag, I was a bit concerned about letting air out of the rears. Then I realized the reduced weight reduces the flex anyway, so it's pretty much a zero-sum proposition.

 

I have this empty parking lot in mind, now I just need a portable air tank....

[68fastback]

Of course I'd actually have to drive all your cars to see how they're doing

 

 

Good luck and fun! ;-)

[ShelbyPilot]

um, Ok my head is spinning. So.......for TRACK purposes what would be a good place for me to start with my GT500 wheels and 255/45 front, 285/40 rear tires? For the street what is the Ford recommend tire pressure on the GT500? Any idea how much difference to compensate for my lower weight.

[LuLu]

um, Ok my head is spinning. So.......for TRACK purposes what would be a good place for me to start with my GT500 wheels and 255/45 front, 285/40 rear tires? For the street what is the Ford recommend tire pressure on the GT500? Any idea how much difference to compensate for my lower weight.

I will not follow the guidance stated on the door sticker. Ford has switched tires and I doubt if they switched door stickers to match the tires. Remember Firestone and the Ford Explorer? That fiasco cost a few folks their lives, and left us with the TPMS legacy.

 

You don't say what tires you are using, but take a peek at your side wall for maximum PSI. Should be in the neighborhood of 40-50 PSI, start there and adjust by your seat-of-the-pants dyno. Somewhere within a 5-8 PSI lower range, you will find your "sweet spot" of maximum grip with minimum drag.

 

What I have done in the past, is coat the side wall (where it meets the tread) with black shoe polish, and run some tight circles (drifting). Inspect, and note where the shoe polish has scuffed away.

 

Anything less than 30 PSI is not good for any performance driving, street or track. Remember that any tire that rolls under on the outside side wall, is crushing the inside side wall and both conditions are bad news on your tire foundation. If you find yourself staying close to 40 PSI to minimize side wall deflection, you need to debate tire construction and compound. IMHO, Pirelli has the strongest side walls on the market, and you pay for it. Winning tires are not inexpensive.

 

Happy motoring!

[mciarochi]

I agree about pressure too low.

 

Lower psi will affect bead separation, because that is a lateral, not vertical force. However, if you bump everything up the 6 psi recommended for road track, you're back in range again. BTW, the Explorer rollover fiasco was caused by recommended insane tire inflation pressures of 26 psi (which means probably 20 psi in real life). Scuttlebutt is that recommendation was to reduce NVH, but that is scuttlebutt, I don't know anything special. Anyway, low psi is definitely something to avoid. There's also a video of a GT500 track rollover out there, where bead separation (he was running the stock F1's) was a factor in the rollover.

 

My SWAG after reading this post and a day of other posts, plus spreadsheets and scratching my head are:

 

38/30 or 40/32 as starting points for street. I want to see how that handles. It would take some hard turn entries to find the right psi for the fronts, followed by changes in the rear psi to adjust the tracking.

 

That's my car, with my estimated weight ratios, but it should be pretty close unless you've done something pretty radical to your car.

[68fastback]

Fwiw, I think those are good starting points too.

 

Dan