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Is 4.4 seconds vs 5.6 seconds alone worth $10k?
- Thread starter Jrhodesmd
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- Driving Dynamics Model S
wider = more tire on the road right? I like that. Before that I was considering getting the aeros since I like the extra range benefit more than cosmetics. But the benefit of more contact with the road tips it the other way for me. And I was annoyed that I'm paying for the 21s as part of the sport anyway.
Incorrect. The amount of tire on the road is almost entirely determined by inflation pressure (There is a small amount that is determined by casing stiffness).
Wider tires mean a shorter wider contact patch. Narrower means a longer narrower one. Wider tires have more G-force at the expense of easier hydroplaning.
Yes. The contact patch area is load per wheel divided by psi. so if the load per wheel is 1000 lbs and the tires are inflated to 50 psi (for ease of math) then the contact patch is 20 square inches. In a tire has 5 inches of tread width then the contact patch is 4 inches long (actually there is a slight ovalness to the contact patch which is dependent on the stiffness of the belts, but for rough estimating 5 x 4 will do). If the tread width is 7 inches then the contact patch length is 2.9 inches.
The trick here is "all things being equal". All things aren't equal because the tread compound and the belt construction are likely to be different between the two tires. So the wide tire may stop shorter, but that won't be due to the wider width. Tire design is actually very complex.
A little surfing around and I found this thread. Best bit I found and it also fits your contact patch shape explanation Jerry33:
There is more on the physics in that thread too, especially the fist post. So this sounds like a benefit to me.
There is more on the physics in that thread too, especially the fist post. So this sounds like a benefit to me.
goyogi
Member
Thanks for the correction. That makes sense that it should be measured on the inside.
Just remember that there is a difference between race track and real-world road conditions (and hopefully driving style :biggrin
. I'm pretty sure that everyone in this forum has experienced a car slipping sideways on an expansion joint during a high speed freeway exit. A wide tire will slip a lot more than a narrower one. If the contact patch is long enough there won't be noticeable sideways slippage.
rogbmw
Member
I think that the most important factor to me would be mid-range acceleration, not just off the line speed from a dead stop. Of course, I am REALLY spoiled because my daily driver presently is a 1 series M Coupe. Most of us, probably like me, are not a teenager or an early 20's person who wants to light up the rear tires from a stop light. If the performance model has seriously increased mid-range punch, that would be a no brainer.
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Based on the info we have, peak HP should be around 50-55mph in both flavors of Model S. Before that is constant torque, and after that is nearly-constant (drops by 15% or so) HP, until you're up above 110MPH and approaching the motor's top governed speed.
For the Model S Performance, 40-60 should be <1.5 seconds; 60-80 in just over 2 seconds or so.
Midrange acceleration in either car should be fast, quiet, and smooooooth...
How about for those of us who are barely able to afford the 40kwh? Will the midrange be drastically different for the 40kwh as compared to the 60kwh?
smorgasbord
Active Member
Here is something TEG mocked up a while back, comparing Roadster Sport and non-Sport:
Note that 6000 RPM = 53.5 MPH
Basically, Sport is more torque, but for lower rpms, than non-sport. So, above 55 MPH, Sport and non-Sport are pretty much the same. Below 50 MPH, Sport has 20lbs.ft more torque, which gives it a 0.2 sec acceleration advantage.
Model S's 1.2 sec acceleration advantage is probably another story entirely.
For one, the final drive ratios of Roadster and Model S are different:
Roadster: 8.28:1
Model S: 9.73:1
That means at 6000RPM, Roadster is going 53.5 MPH while Model S is going almost 63 MPH. Does anyone know if Model S's motor's torque starts declining at the same RPM as Roadster's?
Note also that Model S Performance version's max speed is 5MPH higher than non-Performance 85kWh. Is that arbitrary, or reflective of Performance's motor having greater capabilities?
EDIT: Ignore my MPH calculations and see my next post instead
Note that 6000 RPM = 53.5 MPH
Basically, Sport is more torque, but for lower rpms, than non-sport. So, above 55 MPH, Sport and non-Sport are pretty much the same. Below 50 MPH, Sport has 20lbs.ft more torque, which gives it a 0.2 sec acceleration advantage.
Model S's 1.2 sec acceleration advantage is probably another story entirely.
For one, the final drive ratios of Roadster and Model S are different:
Roadster: 8.28:1
Model S: 9.73:1
That means at 6000RPM, Roadster is going 53.5 MPH while Model S is going almost 63 MPH. Does anyone know if Model S's motor's torque starts declining at the same RPM as Roadster's?
Note also that Model S Performance version's max speed is 5MPH higher than non-Performance 85kWh. Is that arbitrary, or reflective of Performance's motor having greater capabilities?
EDIT: Ignore my MPH calculations and see my next post instead
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smorgasbord
Active Member
Thanks for digging that up!
I just realized that my RPM -> MPH calculations on Model S are wrong, the higher ratio slows the car down. OTOH, the larger tire diameter goes the other way.
So, here's what I think the ratios result in:
If Roadster 6000 RPM with 8.28:1 drive ratio on tires that are 24.97 inches in diameter = 53.5MPH (found on the web).
Then, Model S 6000 RPM with 9.73:1 drive ratio on tires that are 27.75 inches in diameter = 51MPH.
So, if Model S's torque is flat until 7000 RPM, that's 59MPH. If the redline is 16,000RPM, that would be 135MPH. Tesla says top speed is 130MPH on Perf model, so pretty close.
It looks like mid speed acceleration should be great. Even 50-70MPH should be really good.
What we don't know is what differentiates the Perf model from the 85kWh non-Perf model in terms of torque and power curves.
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Even though the 21" rims are 1/2 inch wider? How did you find out the tires will be the same width?
Actually - I don't think you can until they are mounted on the correct wheels and measured.
Continental reports that the 245/35-21 ExtremeContact DW has a section width of 9.8" when mounted on an 8.5" wheel. Ours will be on a 9" wheel.
Goodyear reports that the 245/45-19 Goodyear Eagle RS-A2 has a section width of 9.6" when mounted on an 8.0" wheel. Ours will be on a 8.5" wheel.
None of this may matter, however, since tread width is separate from tire width and depends on the shape of the tread and shoulder. Goodyear reports 8.5" for the 245/45-19 - but Continental does not list this spec on the product sheet.
Also note that the 21" tires report as 27.8" diameter while the 19" tires show up as 27.7". 25lb each for the larger tire - 27lb each for the smaller.
Continental reports that the 245/35-21 ExtremeContact DW has a section width of 9.8" when mounted on an 8.5" wheel. Ours will be on a 9" wheel.
Goodyear reports that the 245/45-19 Goodyear Eagle RS-A2 has a section width of 9.6" when mounted on an 8.0" wheel. Ours will be on a 8.5" wheel.
None of this may matter, however, since tread width is separate from tire width and depends on the shape of the tread and shoulder. Goodyear reports 8.5" for the 245/45-19 - but Continental does not list this spec on the product sheet.
Also note that the 21" tires report as 27.8" diameter while the 19" tires show up as 27.7". 25lb each for the larger tire - 27lb each for the smaller.
Correct. The importance of rim width is to keep the sidewall properly shaped. While the tires can be mounted on less than ideal wheels, performance will suffer. The measuring rim is for compliance with the standards. In some cases it allows the tire manufacturer to "cheat" by using the same molds for different tire sizes and changing the measuring rim to make them fit the standards.
Ideally the rim width should be the same as the tread width, which matches up with the Goodyear specs you mention (8.5 inch tread width on 8.5 inch rim width wheel). No doubt the Continental would be the same (9 inches) if the tread width was available for that tire. (As the aspect ratio gets lower, the tire becomes more "square" so the tread width gets wider for the same nominal section width.)
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