danglerb

Nice going Mark, the fun comes out in your description. Not lifting could be hard to convince your foot is the way to go.

dr bob

"Extreme" camber is what's needed. With some additional caster, the car would continuously turn left ala Nascar and would be a little more fun in the infield sections when not turning left. Lots of camber on both sides would take care of some of the infield better but with a little less total grip. At that point the RF camber would just be moving the load area around under the tire, away from that outer edge. More rear anti-roll would help some too.

What camber do you run now (front and rear), plus what caster settings? Do you adjust the settings for different tracks vs street?

mark kibort

im running about 1.8 degrees front and 2.2 degrees in the rear. the rear wear perfectly even with temps 160, 155 and 145F, ( inside, middle and outer 3rds) which is the same, as i see at laguna
the front were about the same spread, even the outside edge. ... the outers are running 4psi higher pressure for the compression , and you also have to take into account my extreme bumpsteer, which is actually a good thing for the outers as it makes much more camber than normal when compressed. the downside is scrub on the inside at turn 8 willows ,where the slip angles are higher and the outer tire toes out and scrubs off the inside edge. (that doesnt happen at ACS due to the compression and less steering angles for the same speed and radius)
The problem with that kind of wear, is to correct the forces, you need extreme camber , and that could cause more inside wear for the majority of the track. i think a stiffer rear bar setting would be one part, next would be more spring on that side to get less roll as well. maybe more pressure in that tire, but im running the same tires and pressure as the fastest of the cars on track (1:40.xx vet with 550hp) that are not showing such wear. (same camber but stiffer overall suspensions)
its a smalll price to pay, as i can easily take the one tire, and flip it to the rear, where that section of tire only gets "pickup" on the inside rear.
caster is near stock at about 5 degrees and toe is 0degrees

FredR

Mark,

I have always felt your front camber was a bit on the suspiciously light side for the rubber you run on and now that you have stickier tyres and maybe running on a track that requires more camber perhaps you should be looking at -2 degrees assuming you are still at -1.8? You cannot be too far out or you would not have achieved such an excellent result. Alternatively have you considered something like Carl's lower frame brace to stiffen the frame rails a bit?

For sure nothing wrong with your driving- an excellent performance or so I would think and that before considering it was your first time on that particular track.

mark kibort

Ive been running on new tires for the last two seasons. the tires speak loud that things are good when wear patterns are good and temps are progressive inner to outer. you always will get some wear point, due to the fact that you are puling the gs we are during a race. the wear of the tires is perfect. i usually wear out the inner and outers of the front tires just as they heat cycle out, never having to replace a tire for wear alone. with new tires, the grip is fantastic, and there is no need for carls frame strengthening, as the frame not measurably moving at all. ive mounted camers fixed to the side of the car to see movement and there is nothing other than normal camber change due to compression. why do you think the frame rails need stiffening? I can mount a thin strip piece of aluminum to the frame rails and use a bolt washer and cut out hole so that if there is any movement, it willl show wear from the washer. you also have to consider the bump steer which is a blessing and curse. with SO much angulation, the camber is much more progressive, which gives more camber on compression naturally, so i dont need as much camber as a car with struts for a given amount of compression. Im running brand new'ish A7s which are as close to a moderate real slick as you can get in DOT and it seems to be doing the job. in the race, you can see the 928 hanging in the infield with many seasoned, experienced drivers on a track they all call home. Im not the one sliding around and loosing control!
watch the video and let me know what you think!

FredR

Mark,

We are not talking about big differences here- rather subtle changes. Your photo shows obvious differential wear on the shoulder section of the tyre and that invariably to most of the world suggests a bit more camber was needed. Not sure what g's you are pulling but presumably around 1.5g which at those speeds generates a lot of lateral loading. The camber is needed to keep the rubber flat on the surface as things flex but more camber means less grip during braking- a trade off. Trouble is Porsche never designed the 928 as a racing vehicle and stock rubber of the day probably limited cornering to 1G and probably not that. Visual inspection of the lower arm mount tells us that there is an overturning moment and if that frame rail is not twisting to some extent at those lateral loads I would be amazed I dare say the means to measure such deflection exist but it is no easy task to setup or so I would think. Even at my amateur level I was able to lose 0.5 degrees with that brace and I am not pulling your kind of loading. I view camber as a necessary evil.

Just remember, you were running with a very fast crowd and you ran very well but that does not mean you were running optimally. The really professional teams set their cars up to suit specific tracks and there is a reason for this. Run with one setup for all tracks and the best you can hope for is to be right on one track and less than optimal on the others. The closer you get to the peak the finer the margins between success and failure.

mark kibort

Fred, I understand the fine tuning approach, but there are somethings you might not be considering. first of all, lateral loading is independent of speed. however, speed at this track, increases positive vertical gs, well over what can be achieved via down force, so the tires can generate much more lateral forces. those forces are near 2g. at that g loading the tires deform and the DOT type sidewall fails. normally, when fine tuning for a track (for which there are always trade offs..... but not only from track to track, but from section to section even with the same set up used for all) many of the top competitors in this class run much more HP and have much more aero and handling aids than i will ever have on this car. and , a more radical set up , more optimized generally can shave tenths, but can trash tires doing so. most teams live with this trade off and just buy a new set every race. i use one set a season , and only use it for the race. i qualify on the previews years set, and then use trash can tires to fill the remainder of the season for practice and qual.

the camber is not needed for "things that flex" its needed for the suspension that compresses and the body that rolls . there is no indication that there is any "flexing" if you have some test or proof, im sure we would all like to see it.
your visual inspection doesnt have the right angles and distances if you are going off Carl's observations. way off. And, you can see for yourself during a turn , that not many experience in the world, where the suspension is compressed for a moment with highl lateral gs and high compression. you can see the camber change with no change to camber vs its bumpsteer ratio.. this is a fairly easy test by the way, that i can set up on the suspension to show this. you can see , in the video, a ".5" degree change, and it just isnt there, pulling far more gs than Carl is with his car.
there is some proof i can provide. with the same set up and high speed turns at 140mph speeds and same duration, with at the limit of grip adhesion and slip....... i see NO shoulder wear (willow springs turn8)

So, there is a chance that the extra . 0.5g compresses the chassis so much that the frame does twist and give up camber to create this wear pattern, BUT, i doubt it because the wear pattern is not indicative of a small change. .5 degree of camber change usually changes wear patterns of an entire 3rd of the tire face, not 1-2". that tire was almost new and the outer very edge is destroyed. temperatures showed it being the coolest part of the tire, meaning this wear was localized to the track and we all know where and for how long. again, many of the top porsches also sacrifice one part of one tire for this track..... generally for the porsche 911s its the outside rear . the best of the best here run 4seconds faster , with 80 more HP and newer and better cars with everything on the car modified. (and brand new tires) based on what the car was doing for me, I have about 2 seconds more to optimize with the car. 1:45s Think about it, Joe Fan has the GT2 record for POC at 1:43 with not only a cup car , but a 997 cup car on new slicks.

mark kibort

I just measured . the fronts are more like 2.2 degrees. so, all 4corners are 2.2 degrees.

mark kibort

Fred, here is the actual car on an actual track, with about 1.5plus gs acting on the chassis.

you said you can believe that there has to be some twisting force, but seeing is believing. this wheel under compression is about 3 degrees. with no compression, (last pic) that is 2 degrees. do you see any loss of a half of a degree? if there was a half of a degree of chassis twist you would see it here. the reference points are clear.

Now, look at the first pic! there is NOTHiNG you can do about the tire rolling over on the shoulder, but get a stiffer shoulder (ie real racing slick , not DOT) you take this picture, and then apply 100 more MPH to it and you understand how the edges burn up on the front here. camber is about 3-4 degrees compressed and the springs have dropped the car 4".

watch the wheel in action on this video



Loook at the side wall flex under max g loading


still side loading but wheel is straight as it comes out of the turn, still compressed


almost fully released

mark kibort

Carl Fausett

Sixth place overall against a national field - damn good job Mark!

mark kibort

Thanks Carl! It was SO much fun, you would have LOVED this track. It was a surprise to see about 4 or so of the guys i run with in nor cal that came down to the event.
Now, if or when i go back.. i know what to do. i redline at 6600rpm and thats 155mph, but if i do a quick shift and not lift, there is still about 350rwhp available to keep that speed, (rpm drop from the 2.2 is down to 4500rpm) however, i was dropping down to 4000rpm in 5th when i did shift, and that's only 135mph and lots of time! the fastest guy out there was 157 in the headwind, and 155around that turn........so that's what i need to do.. Now, there is an open door to make a GT2 car for SCCA national! i still cant believe its in the rule books now!

IcemanG17

mark kibort

that's why i race the 928! its only gas! (and a few consumables every few years)
this weekend was entry fee, LOTS of gas and that was about it.... actuallly made money after finding 5 hoosier A6s in the dumpster!

However, Mark Anderson did style me with a BEAUTIFUL used windshield for less than it cost to install the darn thing. money well spent. Looks PRETTY now!

Mark Anderson