I have been researching alignment and tire wear. Last year i replaced tires and got and alignment to minimize the inside tire wear. My car has always eaten rear tires into almost cones. After reading and some posts here I got the car aligned with .15 total toe front and back with rears being toe in and front being toe out. My camber was set at -1.7degrees rear and -1.6 front. Despite very close front to rear camber and toe being within specs I still eat inner tread on my rear tires. I would think with front and rear toe being opposite of one another but with the same angles I would have unusual front wear as well but I didn't. What I know is the rear being heavy does the most squatting in corners and the toe changes due to the suspension compressing. David with Rennkit provided me an excel spreadsheet showing alignment specs to minimize tire wear, but I was hoping to understand the geometry and factors fully. But I am thinking there has got to be some other suspension geometry things going on I am not taking into account. The other possibility is my toe eccentric is slipping out after a while. I have elephant toe lock plates coming to eliminate that. Here is my suspension setup:

996 Cup sachs shocks front and rear
Cup front and rear monoball control arms
Monoball strut mounts F and R
Monoball rear dogbones
Adjustable toe links
Solid thrust arm bushing F and R

Anyone have any thoughts or wisdom to share?

 

Your rear toe should be 15' to 17' per side, 30' to 34' total ( ' = minutes). That's the equivalent of about +0.50 degrees total toe. I appears you are saying that you have .15º total rear toe which is not nearly enough.

Here is the alignment data I've used for the last few years. Perfect wear across the tires both front and rear...

Tires: Front 275/35/18 Rear 335/30/18
Springs: Front 1400 lb.in mains (150 lb tenders) Rears 1500 lb.in mains (250 lb tenders)
Suspension: dual adjustable JRZ Pros, monoball everything...

Front camber: LEFT -3.42º RIGHT -3.43º
Front toe: +02' (+04' total)
Rear camber: LEFT -2.42º RIGHT -2.43º
Rear toe: +16' (+32' total)

Ride height & load:
LF 102mm (686lb) RF 102mm (643lb) 1329lb 38.0%
LR123mm (1112lb) RR 123mm (1057lb) 2169lb 62.0%
12lb cross weight
3/4 Fuel (12gal)
Weight 3498lb - 180lb driver - 96lb alignment equip. = 3221lbs (3197 lbs adjusted with 1/2 fuel)

 

Let me dig up my alignment sheet to make sure I am not confusing per side toe versus total. Correct me if I am wrong, but if I were running 15 total then my tire wear should be great and the rear end be less stable no?

 

How low is your car on those cup shocks? And what tires are you running? Any downforce?

Short Answer:
Get a bumpsteer kit for your rear toe links and lower that toe rod connection point. Once you get too low you get bumpsteer which is toe out under load which destroys the inside edge of tires. This is seems most likely to me. If your static toe was that far off you would likely have handling issues to boot. This sounds like your car is too low for your current setup.

 

I am running 660 rear and 645 fronts. I will remeasure my height. I do know I am low. I am running the 996cup adjustable toe links. Not sure how different those are from a bumpsteer setup. Nor do I know the height at which a bumpsteer kit is needed. Please educate me.

 

If you are running unmodified cup shocks you are surely lower than stock geometry can handle due to the droop limiters on cup shocks. You would need them completely rebuilt for your car. I ran at six cup heights which I believe is around 92/105 (its been a long time, don't quote me on that). I would blow tires by super heating the inside edge in a day or two. My car had lots of DF at the time. This happened for a year until I put the bumpsteer kit and a good size spacer on it to lower that toe arm connection point. This also happened to forum member Al Norton and Tom Kerr. It's probably the cheapest place to start troubleshooting. Al spent tens of thousands on tires ruined until he gave up on it.

 

Cups are 89mm/109mm. I am pretty damn close to cup height. Within 5mm. The odd thing is the 6cup guys don't seem to burn through rear insides of tires like mine does and many of those guys are not running spacers and bump steer kits. So that is what doesn't make sense to me. I am sure what you are saying would help, but curious if something else is also going on.

 

Different suspension geometry from an awd Turbo. Subtle changes in design but made specifically for that reason. You will see a lot of guys switch to cup carriers for that reason. Cups are also much lighter, heavy weight exacerbates the problem. Search bumpsteer on 996 and there is a quite a bit of information on it.

 

I am running a full cup suspension with spindles. So my geometry should be the same.

 

Ah ok I assumed that was your complete mods list above.

Question, is the wear equal on both sides (or which one wears faster) and what tracks do you run?

By the way those tire sizes look like slick sizes but you are running only 1.7 degrees of negative camber?

My last idea is that the actual geometry changes also take place in the subframe, so you would still need a bumpsteer kit, unless you have a cup subframe as well. I did a little looking around and it does seem that the gradual progression from 996 to cup to ear to 997 etc was more lowering of the center of gravity and suspension geometry changes had to be made. There is a post on rennlist of some knowledgeable guys saying exactly that.

https://rennlist.com/forums/porsche-cup-cars/705958-gt3-cup-vs-rsr-suspension-geometry.html

Check you alignment again, but after that I would still go wit the bumpsteer kit. It's far cheaper than other methods of troubleshooting. In this case with time being money (in wasted tires). And it worked for me, after my issues, I've gotten 20 plus HC out of Hoosiers.

Good luck.

 

[quick reply]Rear toe to 30'something total and camber to 2,5ish, front camber to 3,5ish.[/quick reply]

 

Quick reply, works with my car: Rear toe to 30'something total and camber to 2,5ish, front camber to 3,5ish.

 

They actually wear the same on both sides.

Not slicks. I posted tire sizes in MM as most the track guys seem to relate more in slick sizes. My car is street driven as well as track. My goal is the eventually get dedicated wheels for slicks. But my suspension needs to be shaken down and odd things like tire wear need to be sorted before I start chewing up slicks in no time.

 

Even wear on both sides suggests alignment issue unless the tracks you run are evenly distributed with turns and loading. You may try zero toe to be sure. May not handle the best but will give you a sure fire answer as to if your alignment is the problem.

 

Curious if because I my rear springs are 1100lbs versus the stiffer 1400lbs at almost cup height if I am seeing rear squat induced toe that is more pronounced and compounded by my low ride height? So perhaps raising my height may be part of the solution. Thoughts???