When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Has anyone run a 305/30/20 Cup2 on the rear? Contemplating picking up a used set from another forum member as a set of track throw aways. Just need to figure out if it will rub on an OEM wheel...
305/30/20 rolling diameter - 27.2"; 12.3" Section width
295/30/20 rolling diameter - 27"; 11.9" Section Width
Less than a 1% change in rolling diameter and a .4" width increase...
Schroth designed this 4 pt system specifically for the GT3/4 LWBs and that's how the shoulder belts mount per their install instructions. The lap belts do not ride up much as long as one tightens them firmly first but there is slight movement (see my complaints about that in an earlier post) in the shoulders even when tightened down due to (IMO) the lack of a sub belt and the length of the shoulder belts. I won't argue with your logic above other then to say the lap belts are fairly low in relation to the driver given the geometry of the LWBs and submarining doesn't appear to be an issue with this system. It's the play in the shoulders that worries me and would seem to negate the benefit of a HANS to some extent.
Do you by chance have a snapshot of the instructions?
I actually called Schroth about this today. They said I needed a harness bar. Combined with that and the fact its pretty much a 90 degree angle, I have not seen that anywhere else.
But at the same time, I'm not sure why anyone would purchase the 4 point when its the same price as the 5/6 point for the carbon seats (obviously you need to add a substrap mount). But if they say that mounting at a 90 degree is OK, perhaps that justifies it. I just havent seen any documentation from them or anyone else about it.
Requiring a harness bar negates any reason to use this system. Their explanation to me was this system reacts to a roll over in a similar manner to the 3 pt system. No instructions came with mine, my installer called them when he put mine in.
I thought the floor mount behind the seats seemed funny too but I did find it in the Schroth installation instructions for the ASM equipped rallye harnesses...
Was at VIR over the weekend - full course Sat, Grand Sunday. For this weekend, I changed the car from completely stock alignment and aero to "full" aero (removed front spoilers and max'd out wing angle) and max camber F/R with stock suspension elements. This gave me about -1.7 deg camber all around. Sport cup 2s. Also, changed front sway bar to soft. Rear was at medium. BTW: if you come across people telling you the sway bar nuts will come loose, they are right!
A caveat is that I am not a super experienced track person - I have about 10 or 11 track days (most in the GT4, but not all). For example, I am self-limiting speed on the straights until I build up enough confidence/experience to go faster. My max over the weekend was 128 or 129, but the GT4 will easily do 140+ on the long straight.
I am driving with all nannies on. I.E., I did not mess with ECS and ECS+TC buttons.
My experience with the new set-up was a large increase in oversteer. Not that there was a lot or hardly any to begin with, so the amount of oversteer still seems small to me, but there were a dozen or more times the back end got loose, but it was always easily correctable with a small steering input. There were also times when the car had more of a 4-wheel slide, but not as many as with the total stock set-up.
Compared to my instructor (riding in his 944 turbo with slicks) the GT4 seems very sticky, but when it gets loose it seems to be more dramatic than the 944. The 944 seemed to be slipping controllably nearly all the time.
Also, on very deep braking (wheels straight) coming down the front straight into turn 1 (say 125 mph to a 50 mph turn), the car got a bit twitchy. Instructor said it was normal.
Can some of y'all comment on my observations and maybe interpret what I am seeing as normal or needing changes? Should I turn ECS and TC off. Two buddies (one in an M4 and other in Shelby GT350R) both said they have to turn the computer off or down or it holds them back.
I was finally able to get the Porsche Track Precision app to work (but only got it to record 1 out of 4 sessions I tried). I need to look at the data more, but it appears in a quick look that the computer was not making a lot of control inputs, but I may not be looking at it correctly...
Definitely leave the electronic systems on. The electronic systems in Porsches are very well tuned and if you are engaging them a lot as a relative beginner, it is your driving that needs adjustment, not the system.
You might want to put your front sway bar back to medium and see if that fixes your oversteer problem. For the instability under braking, you could add a bit of rear toe if you want.
Definitely leave the electronic systems on. The electronic systems in Porsches are very well tuned and if you are engaging them a lot as a relative beginner, it is your driving that needs adjustment, not the system.
You might want to put your front sway bar back to medium and see if that fixes your oversteer problem. For the instability under braking, you could add a bit of rear toe if you want.
Thanks! Since I posted that question I downloaded the six laps of data recorded in the app and was quite surprised. The field for "electronic stability program" says "idle" the entire time. There are two fields called "wpoOversteer" and "wpoUndersteer." I had zero entries in Oversteer and a lot in UnderSteer. I need to find out what those fields mean. I took the data and overlaid it - quite interesting to see the general speed increase each sequential lap and where the understeer indicators kick in. The next-to-last lap was fastest (colored red) overall because I hit traffic in Lap 6.
MarcD147: You can export the data in several formats. I picked .csv and the app will export/email it. Opened in Excel and there is a bit of work to tag the laps. The app should do that automatically, but I had to manually mark the start/end of each lap based on the "Lap Distance" column. Works fine, but the data should already be there. Then I just did a scatter plot in Excel for each lap - distance vs. speed.
To add the overlay for the oversteer, I pulled only the non-zero oversteer entries into separate columns and plotted a point at the correct X distance and offset it in the Y direction a random amount proportional to the lap number until the plot looked reasonable. Then I manually added the turn info with a text box. I labeled the oversteer wrong though...it starts at lap 2 on bottom and goes to 7 at the top (figured that out this morning). The amount of oversteer correction it is doing is extremely minimal in all cases. If you PM me your email, I can send you the file if you'd like to see it.
Not sure going same camber front and and rear is recommendable, my mid engined Porsches behaved very badly when I did this once. Most people - me included - recommend a difference of at least 0.5° between front and rear, with front always having more camber. -1.5° on the rear works well and you can push the front easily to -2°. Or you go with your existing set up, keep the rear camber and max out the front, which typically gets you -2.2/-2.3°.
Not sure going same camber front and and rear is recommendable, my mid engined Porsches behaved very badly when I did this once. Most people - me included - recommend a difference of at least 0.5° between front and read, with front always having more camber. -1.5° on the rear works well and you can push the front easily to -2°.
Sven, thanks. I did not have my alignment data sheet when I wrote that, but the rears are -1.5° and fronts -1.7°, so almost the same. I have some shims for the front and will use those to get to -2.0 before next track day.
Please excuse my novice ignorance, but I suppose the underlying idea here is to manage the contact patch area. Are we compensating for the smaller tire width up front? Or, is there more body roll on the front end we are compensating for? Thanks for any insights!
Last edited by aryork; 03-03-2017 at 09:13 AM.
Reason: typo
Car is set up as under steering (as any modern car is), more contact patch under load (steering plus going through a corner) on the front can help (plus other measures...). You will notice that with -1.7° camber front your tires will wear heavily on the outer edges, even with -2° they do that, hence I max out the camber. Again, camber is not the only aspect but one you can influence with stock suspension and I am not yet ready to mod the car.
If your tire does not look like the one above, you are either not pushing by he car hard enough or have a miraculous suspension.
Why the difference between front and rear: I like my car to break loose at the back end if I overpower it or induce a change in weight distribution (e.g. lift of oversteer). This should all happen very controllable. With the same camber front and rear, a pro known for his drift abilities (google Tim Schrick, probably not known in the US but well known in Germany, he belongs to the family which owns Schrick camshafts) took PSM back on and told me to get a new alignment quickly...
In the end, you need to try different setting out (and go with 0.5° changed to feel the difference) and find out what suits you driving style most.
From my impressions over the weekend at VIR, it all comes down to how the car is driven. If you slam on the brakes and yank the car into the turn it will understeer. If you brake earlier and feed more steering into the corner it can ease some of the understeer. In fully stock form (straight from the factory) mine exhibited low speed understeer and highspeed oversteer. It is critical to adjust one's driving style, unless you want to pay for parts to make the car handle to your liking.
I didn't mind the wagging tail under braking. This happens in many other cars and can be prevented by braking a bit longer / easier. Slamming on the brakes did urge the car to wiggle more, but the rear will not come around under straight line braking.
01-19-2017, 12:18 PM
