Roadster 2.5 sport: Understeer cured?
I have a report on my rear tire change.
These are working really well for me:
Michelin Pilot Sport A/S Plus 235/45ZR-17
Note, this is a change from the original tire size. (increase from 225 up to 235)
Here is a chart comparing physical properties of the Michelin tires I selected with the originals that came on the car:
The left side are the original tire specs, and on the right are the new.
1) UTQG: the numeric value is treadlife. 180 for me was less than 8000 miles in the real world, new tire spec is 500. This is obviously a big improvement.
Then on this line is "AA" and "A" ratings for both tires. They represent "traction" and "temperature" respectively. Traction performance is a complicated subject, and a letter grade does not do it justice, but AA compared with AA is as similar as this spec can be, so I am happy to try these tires out based on that. Temperature is an indication of how durable the tire is under abusive use that would cause the tires to heat up (performance driving conditions). The same rating again is a good indication for the replacement tires.
2) Max Load: Is slightly increased on the replacement tires over original. I am happy with this, as it indicates an additional safety margin resulting from a slightly larger physical size of the new tires. Though a negative interpretation would be that the rating is for a higher load, and thus traction performance for the Roadster application would be slightly less than nominal.
3) Max inflation pressure: A higher rating on the replacement tires again indicates a greater margin of safety for tire inflation up to 51lbs limit. One could test inflation pressures beyond the Tesla spec of 40lbs without a safety concern, if desired, for performance tuning purposes.
4) Tread Depth: Same = Good
5) Tire Weight: 1lb lighter on new tires is good for performance. I like.
6) Rim Width Range: New tires have acceptable parameters and will correctly fit on Tesla wheels.
7) Measured Rim Width: Is fine. It is a physical dimension when the tire is unmounted. Previous item indicates the new tire will fit our rim size when mounted.
8) Section Width: New tire is .3" wider. The additional overhang past the lip of the rim will be only .15". This is about as close as you can get to matching the original tires. In my judgement it represents no risk of width related clearance issues on the Roadster, and my experience demonstrates that I am correct on this.
9) Tread Width: This one is interesting. N/A spec on the new tires indicates it will be a different road contact pattern (or footprint) on them. The Michelin tires design for tread is what I consider a more traditional radial design. Instead of the edge of the tire tread ending abruptly as on the original tires, it curves more around the edge of the tire. I expect the "feel" of the new tires may be different.
10) Overall Diameter: This in addition to the Tread width are the most significant physical dimension changes with the new tires. They are .5" larger in diameter. This will produce an increased road height of around .25", and the same decrease in tire clearance inside the wheel well of the Roadster's body. I considered it important to try and minimize the change of this dimension, though it is a key part of the reason for choosing the larger tire size, so I wanted a little increase. This is what we get here.
11) Revs Per Mile: This is a direct result of the change in Diameter. The primary worry/concern is a "change in the difference" of front wheel speed vs. rear wheel speed as detected by the ABS/Traction Control on the Roadster's computer. Unfortunately the wheel speed difference may not be exactly what the computer expects to see.
The car was delivered to me last year with the sport/performance suspension adjustment as I requested. As we know the Roadster has an under-steer driving characteristic. This is what I addressed by changing the tire size on the rear. I wanted the car to feel more balanced when taking turns and curvy roads.
It did work, and the car feels much more responsive and fun to drive. I experience improved ride quality as well. I look forward to getting a second opinion on this.
-all this is normal daily public road driving, not race track Roadster driving, as I have yet to experience that.
Why did the tire size change make a difference and work this way to cause less under-steer for the Roadster? It has to do with the cornering forces delivered to the car (any car) by the tires, and the changed degree of physical deflection of the tires when the car is cornering at speed.
Trying to explain this: You may have heard the expression that a sports car corners "like it is on rails" as in a train track. This is not literally possible because rubber tires have a deflection to the side (radially outward) in a circular turn for example. So as a result, there is an outward radial vector introduced to the actual path of the wheels (and the car) in a turn with significant speed and cornering force. So although the tires are applying a given degree of "turn" to the path of the car, the wheels will always take a slightly larger turning radius than would be indicated by the physical orientation of the wheels. So if you can follow me this far in the explanation, keep going a little further.
Under-steer behavior results when the front tire deflection path(radius) is greater than the deflection of the rear tires. The reverse of this would be a dangerous over steer that we all want to avoid. So if I succeeded in explaining this, you can see the reason for wanting a bit more rubber on the rear tires, to allow a bit more rear tire deflection in turns, and thus less under-steer. Only, not so much as to cause over-steer!
Another, more subtle tuning of the tires can be done with tire pressure. Higher rear tire pressure can tighten up the rear end of the car, allowing the deflection of the tires to be slightly decreased and makes for a variable adjustment possibility. I am still running 40 lbs pressure in the rear tires per original spec.