I'd like to toss some ideas or comments into this thread if I can.
First
Rear tire wear seems to be a function of ride height (camber gain mentioned later), toe in and tire side wall stiffness. Accelerated wear occurs when a narrower part of the rear tire is bearing a lot of load and when that part of the tire is scrubbing against the direction the car is traveling.
Modern suspension is designed so that, when the side of the car is loaded and the wheel is pushed up into the fender well, the tilt in of the top of the tire (negative camber) is increased. Outward side loading of a tire that is tilted inward at the top (negative camber) forces the entire contact patch of the tire onto the road. Cornering hard pushes the outboard rear tire hard towards the outside of the corner and negative camber uses that load to force more of the contact patch onto the ground.
FMVSS 126 is the Federal Motor Vehicle Safety Standard which applies to electronic stability control. There is a maneuver in 126 that requires a right, left, right yanking of the steering wheel with pauses build in to simulate an emergency avoidance maneuver. It tries to excite the rear of the car like a pendulum and drive the car to swap ends. A lot of European car manufacturers have taken to using rear negative camber to help catch the rear of the car in these types of situations.
FMVSS No. 126 Electronic Stability Control Systems - NHTSA
Start with rear negative camber (top of tire tilting inwards putting lots of pressure on the inside shoulder of the contact patch) and add toe in.. Toe in is when the front of the rear wheels are pointed slightly inward from dead straight. This is done, in part, for two reasons. First, it acts akin to feathers on a dart as your car goes down the highway. It tends to keep the rear directly behind the fronts. Second, modern suspension tends to have the rear tires toe outwards just a tad when under hard acceleration (upright moves forward, toe link leading the upright => upright rotates toe outward). The last thing you want is the rear tire steering outward when going around a corner under hard acceleration. We would use this on some race tracks but boy you had to be ready for the car to aggressively rotate under power
The last element that exacerbates this problem is tire side wall stiffness. 21s have stiffer side walls than the 19s which means the 21s can not give as much to help dissipate that high inside shoulder loading.
So much for negative camber and what combines with it to eat up tires.
Second
Next is why Tesla's have this problem. Coil spring cars ride higher than air cars. At full load, the coil cars must still be at or above Tesla's minimum ride height. Air cars just add more air to the springs to keep the already starting low ride height. The net result here is that camber gain (more negative camber as the wheel goes up into the wheel well) makes for more negative camber on air cars than coil (as air sits lower). Tesla decided to use one set of suspension geometry for both air and coil to control cost and limit build variations.
Lower your car even more and, you guessed it, even more negative camber.
Putting numbers to the above, when I first looked at this (2013) my wife's coil car had -1.2 degrees of camber in the rear while my P85 on air had -2.2. That increased to -2.7 when I lowered the car.
How I attack these problems.
If it is a play car where I will not be putting a lot of miles on it, I normally just wait to just under half way through the shoulder tread depth and rotate the tires across the rear. Yep, that means running the inside on the outside which does not agree with the idea of an asymmetrical tire. My thoughts are that I would rather have three grooves that are twice as deep (once the tire is worn) on the inside then four that are half as deep. I have spoken with several tire engineers who tell me there is no tire construction reason I should not do this.
A daily driver like the Tesla is different. There is no camber adjustment on the back of MS so I made longer upper arms (by 0.210") to remove some rear camber (down to -1 front and rear). I also run significantly less rear toe in (0.1 degree total rear toe in on the rear). My rear wheel drive cars would tend to hunt more on the highway especially when the road was grooved. Range increased as well with less rear toe in. My all wheel drive car is 100% stable with little rear toe likely due to the front wheels pulling the car down the highway. I also am now running 20" rims because they weight less than Tesla's 21" option, cost half the Tesla option and I can use Pilot Super Sports which cost less, wear better and have more grip. Really, I'm not fibbing on the tires
I made suspension bits for my own car. Doing so for someone else involves unacceptable liability. I did provide my work to one of the forum vendors who made some arms. There did not seem to be enough interest and Tesla changed the rear arm design so often (bushing changes) that making arms commercially available did not seem to fly.
I hope the above helps. I do not begin to know everything so please feel free to correct me.
Bill
