Wheel/Tire Calculations - LotusTalk - The Lotus Cars Community
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post #1 of 10 (permalink) Old 06-14-2018, 06:33 PM Thread Starter
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Wheel/Tire Calculations

The S1 Exige has the following tire specs:
F: 195/50 R16 on 7 inch wide rim
R: 225/45 R17 on 8.5 inch wide rim

The S2 Exige has the exact same tire sizes, but on 6.5 and 7.5 inch wheels front and rear.

This got me thinking about ratios of tire and wheel sizes. I found an article here that describes the tire selection process using Tire Rack's tread width against the wheel width. The summary of the article is to pick a tire that has a tread width equal to or 0.5" narrower than the rim width. However, the S2 Exige's OEM tire tread width (front and rear) are both more than an inch wider than the rim widths. This is common as well. My Cayman tires look like balloons on the wheels despite being the standard sizes.

If you think about the cross-section of the tire on the rim, it looks like a trapezoid and the sidewall angle can be calculated (using tire width, wheel width, and tire profile). Tread width was ignored as specific tires were not taken into account. Trying to find a trend with side wall angles, I did some calculations for OEM wheel/tire sizes of mid-engine cars (data in degrees). Note that all of these tires are wider than the wheels that they are mounted upon.

Exige S1:
5.041 Front
2.573 Rear

Cayman S (987):
9.601 Front <-- This is why they look like balloons I suppose.
9.743 Rear

Ford GT (1st Gen):
1.733 Front
5.193 Rear

??? There doesn't seem to be any trend here. I was under the impression that a more vertical sidewall (0 deg) is better for traction purposes. A piece of paper can be bent into the shape of a tire cross-section and the bottom becomes flatter as the side walls become more vertical. That is what the article above would suggest as well.

Now on to the questions! Why do OEMs tend to not follow this rule and why does there seem to not be a trend? If you have a set of wheels already, it would be best to pick the widest tires possible, right? Should one think about these calculations to select wheels and tires? Is there some sort of general knowledge that I should know about this relationship? Sorry for the long post but I hope to learn from you guys!
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post #2 of 10 (permalink) Old 06-14-2018, 06:57 PM
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You're taking a simple approach to a complicated subject IMHO. Tires are incredibly different, even from the same manufacturer. You'd probably get better answers from a section more appropriate than General Discussion.

My GT-R has tires specifically developed for it using different technologies to produce the desired results.

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post #3 of 10 (permalink) Old 06-14-2018, 07:44 PM
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Like oldmansan mentioned, it's a very complicated subject and I'm not an expert but I do know that the width ratio between tire and wheel has an effect on the "springyness" (I'm sure there is a term for this) of the wheel & tire because of the flexing of the tread around the contact patch. Engineers can use this to make subtle changes to the suspension tuning; conversely, bookkeepers and others ignore this when some wheel and tire sizes fit into the budget better.

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post #4 of 10 (permalink) Old 06-15-2018, 03:37 PM Thread Starter
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I guess everything is too complicated for any measurable difference for us mere mortals (and not Lotus engineers). Any selection other than the OEM tires will not be optimized for performance, but that doesn't mean that it will perform better or worse. I guess tire/wheel selection is down to trial and error.

Is there any advice on wheel size for a specific tire size besides the article that I posted earlier? The article seemed to choose narrower tires than wheels yet OEMs seem to do the opposite. Is there a correct answer on this front? Thanks for your input so far.
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post #5 of 10 (permalink) Old 06-15-2018, 03:44 PM
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I wrote a blog on tire selection that you can read: HERE



Tire choice is very complex but a couple key items that you should consider with our cars is: lightweight and weight distribution. We need soft tires to contend with the lightweight. We also use staggered tire sizes to reflect the weight distribution of our cars.
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post #6 of 10 (permalink) Old 06-15-2018, 04:25 PM
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Man, don't get me started on tires for this car and the choices Lotus made (oh brother).

Buy the tires that best suit your needs, and remember that pressures and alignment are very important to getting the best out of any tire.


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post #7 of 10 (permalink) Old 06-15-2018, 07:23 PM
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Quote:
Originally Posted by VisualEchos View Post
Man, don't get me started on tires for this car and the choices Lotus made (oh brother).

Buy the tires that best suit your needs, and remember that pressures and alignment are very important to getting the best out of any tire.


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Did I miss you picking up a set of Monolites?

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post #8 of 10 (permalink) Old 06-16-2018, 05:33 AM
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Quote:
Originally Posted by oldmansan View Post
Did I miss you picking up a set of Monolites?

San
I guess so! I have the only set of 17/17 .

Have a quick look at my build thread for more pix.


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post #9 of 10 (permalink) Old 06-27-2018, 10:16 PM
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This is what surprised me about the S1 Exige with the 8.5" width rear wheel and Lotus not carrying it over to the S2.

** save ~10 lbs gain 1 hp ** EQ: Y=(190*X) / (1984-X) where Y is (HP) and X is (lbs)

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post #10 of 10 (permalink) Old 07-24-2018, 08:49 PM
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I'm sure this info exists, but I tried and cant seem to find it. In hopes of keeping the car "balanced" or rather in line with the stock diameters etc, what is the tread width + other info on the LSS wheels with stock rubber (advans)???

I'm trying to compare the RE71R's and R888R's in different sizes and profiles to get something that does throw the car out of wack. I will be corner weighting/balancing once my suspension arrives but I'm still curious, as I believe both the 71R's and 888R's run narrower than the older R888's.

At the moment I'm thinking of running a 205/45 and 235/40 profile with the 888Rs...Have a feeling that will be very close, although now the car will be a little lower by default...I just dont want a buldgy tire with too much flex.

Problem is the 71Rs can only be had in that 245...does anyone have pictures of that on LSS wheels? Trying to get an idea on how it looks.

Last edited by DannyGT; 07-24-2018 at 09:02 PM.
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