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standard tires on 2.5

driver_EV

Member
Dec 7, 2011
209
43
Charleston, South Carolina
Hi folks, I have a scary tire question here!,
:)

Having passed 7200 miles on my 2011 Roadster Sport, the RH rear tire is reaching the wear bumps in the tread. Prep for tire purchase now begins. Does anyone have experience running the Roadster with 50 series tires on the back instead of the 45s?
I understand the slight deviation in spec can have consequences that may, or may not, actually be terrible in real life. I am not thinking of changing from stock front tires at this time, as they still have deep tread.

One reason I am considering this is that I do have the adjustable suspension set to the sporty specs and still experience the under-steer everyone is familiar with. The 45 series on 17" wheels may be a deliberate design choice to add the "safer" under-steer behavior.
Having driven the car daily for the better part of a year, I find a desire for more steering response as would result from a slightly looser rear end on the car. Going from 45 to 50 series is not a dramatic change, but it could be that the revolutions per mile difference may displease the TC or ABS system.

So I'm asking if anyone has tales of real world results of such a configuration.
 
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dhrivnak

Active Member
Jan 8, 2011
4,436
3,651
NE Tennessee
I changed to Contential Extreme Contact DWS which were 1/2 the price and 3x the tread rating. They do not stick quite as well but with these on the rear and the stock fronts the car is more balanced in extreme driving. I do like the combination.
 

driver_EV

Member
Dec 7, 2011
209
43
Charleston, South Carolina
In studying tire parameters on tirerack.com, I am now seriously considering tires like the 235/45-17 Yokohama ADVAN S.4.

Yokohama ADVAN S.4.

It seems overall physical dimensions change less when you go to a 235 size at 45 existing profile instead of going to a 50 profile at the original 225 size.

Since I am looking for only a slight loosening of the rear end from the tires, minimal deviation in the physical size of the tires, and possibly lower cost/higher mileage rating, something like the above tire may be worth trying.
 

driver_EV

Member
Dec 7, 2011
209
43
Charleston, South Carolina
Roadster 2.5 sport: Understeer cured?

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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:

TireParameterComparison.jpg


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.

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IMG_1070a.JPG
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IMG_1128a.jpg


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.
 
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hcsharp

Active Member
Jun 7, 2011
3,383
1,343
Vermont
... 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...

Why would more rubber on the road result in more tire deflection? I would think it would be the opposite.

Also, I wish we knew the rolling resistance. The stock AD07s are very low. These weigh less which is often a good indicator of low resistance.

Thanks for your report.
 

driver_EV

Member
Dec 7, 2011
209
43
Charleston, South Carolina
Something that surprised me about this change is that it does make the car feel different to drive. No doubt it is because of there being so much mass carried by the rear wheels, the rear tires do make a big difference.

With original tires the car has a pronounced hard movement/feel with rough road conditions, that is especially noticeable from the rear of the car. You would really feel the mass of the car impacting bumps in the road much more so than from the front end. Now with these Michelin tires on the back, The car feels more balanced moving along a straight road over bumps. You now get more of a mild shock in the rear similar to the impact of bumps on the stock front tires.

The tread design being less of an extreme sport design (narrow tread footprint)on the Michelin, it also seems to allow for a lighter "free?" feel in casual driving. To me it actually feels more responsive. I have been on these tires for 2 weeks now, and I do still like it. The judge is still definitely out on extreme, pushing to the limits, speed driving. Though I have taken hard 90 degree turns at 20mph or so even to the point of sliding a bit, and the car was perfectly under control, -seemed to be taking the turn quite well.
 
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driver_EV

Member
Dec 7, 2011
209
43
Charleston, South Carolina
Why would more rubber on the road result in more tire deflection? I would think it would be the opposite.

Also, I wish we knew the rolling resistance.

I may not have been clear enough there. The change was for a larger tire size, thus physically, a bit more rubber material "on the rear tires". An increase in the physical size of the rubber tires does allow an increased amount of physical deflection for a given force, all else being equal. I was not referring to changes in how much rubber is in contact with the road.

IMG_0942.JPG


Have a look at the this picture, you can see the milder (less wide) tread design of the Michelin. I will note that in this picture the new tire is not mounted on a wheel yet, and is not supporting a load, so the contact width of the tread design is not being fairly compared here, but it is clear that the width of tread in contact with the road is less than that of the original tires. This parameter at least, is not an indication of greater rolling resistance.

It does look to me like the overall amount of rubber in contact with the road may be less with these Michelin tires due to a decrease in tread width, and an increase in the width of the channels in the tread design. The length of the new tire's footprint is probably greater, so there may not be much difference in tire's road-contact surface area.

It is normal with new tires for the car to feel like it is rolling easier/better. That has been true for me with these.
Though I have been playing (er, driving) in performance mode almost all the time, my KWH per mile seems to have dropped just a bit. :)

If you wanted to, the tire pressure on the new tires could be safely increased to 5olbs for less rolling resistance. Maximum on the original tires is 44.

-After putting more miles on the tires the energy/mile edged back up where it was. Tires are getting broken in, and I am having more fun, so...
 
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augkuo

Member
Apr 24, 2011
987
2,927
Berkeley
I also changed to the Michelin Pilot Sports because of the low wear rating on the original AD07's. I'm using the car to commute so I don't care about drag racing - the Michelin's are less "sticky" so your 0-60 times will be increased by a couple of tenths of seconds. Otherwise the tires are fine and I have no issues with traction control, etc. - before I was changing the tires every ~7k miles so I'll see how long these last.

Augie
 

driver_EV

Member
Dec 7, 2011
209
43
Charleston, South Carolina
An update on driving the Roadster with these tires, of a slightly larger size, that I selected:

The tires are well broken in with 500 miles on them now. The tires are quiet. The car is easier, more exciting and fun to drive. In my opinion it definitely has a more correct behavior. I have been evaluating turns with a bit more speed. Put it into a moderate steady-speed turn, then bump the accelerator pedal a bit more, the car just zips through the turn. No need to crank the steering more to keep the turn, just hold the wheel in position and the car keeps on track. I am so very pleased. If you have one of these cars as your daily driver, and are not overly-excitable, :) -you should try these out!


-Richard
"listening to YES while driving an electric car is one of the best things in life!"
 
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ggr

Expert in Dunning-Kruger Effect!
Mar 24, 2011
6,982
27,544
San Diego, CA
Richard, thanks for your postings. They convinced me to try the same thing, and three days later I'm very happy.
 

dsm363

Roadster + Sig Model S
May 17, 2009
18,278
151
Nevada
Richard, thanks for your postings. They convinced me to try the same thing, and three days later I'm very happy.

You put on the same rear tires and have AD07s (stock size) on the front? I may have to try this but have a few thousand more miles on my new rear AD07s.
 

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