Amazing things I learned about tire sizes from Rick aka RAAMaudio

[jeff]

Today I wish to speak a few things and must thank [MENTION=636]RAAMaudio[/MENTION] aka Rick for stimulating my brain here...

So referring to some things said on another thread recently, I read about how a 205 tire, though it's supposed to be wider than a 215 or 225, may not actually be, at least not proportionally to size change, and how it varies by manufacturer. This was new to me; I always took for granted that bigger number meant bigger size, and it got me to thinking and researching. I currently have Federal 595 RS-Rs in 215/40/17 size. My Rota Grids are coming tomorrow, and in my research before purchasing I spent a good bit of time making sure I'd be happy with a 17x8 rim (all I can get in USA for the Grid; Titan comes in 17x7.5 though...grrrr......) on my awesome Federals. It will be a bit stretched but acceptable.

Next - in order to protect the rims, a wider tire is an option. The only real tire that comes in a 225/35/17 size is the Yokohama S. Drive, which everybody hates. Still, since they make decent tires, I looked into it. In the process I compared the tire size, tread width (actual tread contacting earth), section width (widest part of sidewall of tire excluding lettering), along with weight and amount of sidewall, of the FEDERALS vs. YOKOHAMAS vs. stock BRIDGESTONES just for the fun of it.

Here's what I found using manufacturers website specs and comparing, enjoy my crude drawing, I just wanted to see it in real life to give me an idea of how different these tires actually are in real life:

Data:
BRIDGESTONE RE050A 205/40/17 (stock tire)
19.0 lbs
82mm sidewall
Tread width = 7.6"
Section width = 8.4"

FEDERAL 595 RS-R 215/40/17 (Jeff's current)
22.49 lbs
86mm sidewall
Tread width = 7.5"
Section width = 8.7"

YOKOHAMA S. DRIVE 225/35/17 (possible future tire option)
20.5 lbs
78.75mm sidewall
Tread width = 8.1"
Section width = 9.0"

Findings:
Like Rick said, all is not what it appears to be. The stock Bridgestones actually have MORE TREAD TOUCHING THE ROAD than the +1 size Federals! That was a surprise. Meanwhile, the Yokohamas do increase in widths proportionally to the size jump. Still, with only +0.3" of section width, I wonder if the difference will even be noticeable, much less capable of protecting my rims. Thinking further, sure the Federals weigh about 3.5 pounds more each (at 10 pounds per pound as standard equivalent of unsprung vs. actual weight, that's 140 pounds more/less) but they are so much more grippy it's worth it. Also, if I were to switch to the Yokos at some point, the weight difference of them plus the rim swap (Grids are 18.9 versus 22.5 lbs stock wheel) is huge - 5.6 pounds per wheel or 22.4 pounds which feels like (according to what I said above) 224 pounds off the car. Most of weight savings here gained through the rims of course.

Hmmm..............

OK that's all. This stuff fascinates me and has me looking through a whole new lens for future tire purchases. By doing a little research you get an idea of how much weight you're really saving or not saving, how much more tread you really get on the road, and how much more or less the rim really is exposed/protected. Looking at my drawing above, it's kinda cool to see the comparison in actual size. Of course, having said all of that, just because a tire has more tread on the road doesn't mean it's better; a narrower tire (in this case the Federal compared to the Bridgestone, 0.1" less) that is way more sticky gets way better traction. So there are other variables at work here.

Thanks again Rick!

 

[RAAMaudio]

Glad to be of help and thanks for posting this as it will help show the real differences I have talked about so much, maybe I will not have to bring it up so often which has bothered a few people here
Rick

 

[Kalter]

So based on that, what does it mean for the diameter of the wheel? As we know, 205/40 means the sidewall of the tire is 40% of 205, so 82mm, but after reading your post I realize that my life is a lie and that the first number means nothing, so the second has to mean nothing also.

Basically my question: the RE050A are 205/40, but 7.6" wide, while the 595-RS-R are 215/40, but 7.5" wide. Do they both have a proportional diameter? 82mm vs 86mm based on 205 vs 215. Or 3.04"(77mm) vs 3"(76mm) based on 7.6" vs 7.5".

Or what do the numbers actually mean!? I think I'm having an existential crisis over here.

 

[jeff]

So based on that, what does it mean for the diameter of the wheel? As we know, 205/40 means the sidewall of the tire is 40% of 205, so 82mm, but after reading your post I realize that my life is a lie and that the first number means nothing, so the second has to mean nothing also.

Basically my question: the RE050A are 205/40, but 7.6" wide, while the 595-RS-R are 215/40, but 7.5" wide. Do they both have a proportional diameter? 82mm vs 86mm based on 205 vs 215. Or 3.04"(77mm) vs 3"(76mm) based on 7.6" vs 7.5".

Or what do the numbers actually mean!? I think I'm having an existential crisis over here.

Yeah, it rocked my world too.

As for diameter, I've done all the algebra I'm gonna do for the rest of today at least, but it would seem to me that diameter is not proportional to any of this, so you're good. Diameter of the wheel is not dependent on any measurements of the tire. Maybe Rick for President of all wheel/tire questions will speak to this.

A further thought I've pondered - if a 225 Yokohama is 0.3" wider section width than my 215 Federals, you have to cut that in half since the difference is split between the inner and outer surface of the tire...so in reality in my case anyway the swap from 215 Federals to 225 Yokos would net me a +0.15", or 3.81mm, gain in rim protection/visual wideness. Not much but I guess every mm helps...

 

[blurred]

Yeah, it rocked my world too.

As for diameter, I've done all the algebra I'm gonna do for the rest of today at least, but it would seem to me that diameter is not proportional to any of this, so you're good. Diameter of the wheel is not dependent on any measurements of the tire. Maybe Rick for President of all wheel/tire questions will speak to this.

A further thought I've pondered - if a 225 Yokohama is 0.3" wider section width than my 215 Federals, you have to cut that in half since the difference is split between the inner and outer surface of the tire...so in reality in my case anyway the swap from 215 Federals to 225 Yokos would net me a +0.15", or 3.81mm, gain in rim protection/visual wideness. Not much but I guess every mm helps...

No, not necessarily.
It all depends on whether the tire has a rim protector or not. The section width may be enough but if you stretch the tire it will expose the wheel, but some tires with a section width that is narrower have an integrated rim protector (which isn't included in section width in all cases) and even a heavily stretched tire will still cover the lip edge.

Either way. Here is your 215 federal on an 8" wheel. Get good at parallel parking

And as far as width goes for grip, the narrower tire will outperform the wider tire except at the minuscule moment of peak lateral g's where width will provide slightly more bite. Acceleration and braking require more tire height for more grip (longitudinal) and cornering still likes a narrow tire except if you dive bomb an corner and can exceed peak grip for that fraction of a second. Narrower tire also means higher top speed and better high speed acceleration as well. Plus better fuel mileage, turn in etc....

Basically just park carefully and get the stickier tire in the better size

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[jeff]

No, not necessarily.
It all depends on whether the tire has a rim protector or not. The section width may be enough but if you stretch the tire it will expose the wheel, but some tires with a section width that is narrower have an integrated rim protector (which isn't included in section width in all cases) and even a heavily stretched tire will still cover the lip edge.

Either way. Here is your 215 federal on an 8" wheel. Get good at parallel parking

And as far as width goes for grip, the narrower tire will outperform the wider tire except at the minuscule moment of peak lateral g's where width will provide slightly more bite. Acceleration and braking require more tire height for more grip (longitudinal) and cornering still likes a narrow tire except if you dive bomb an corner and can exceed peak grip for that fraction of a second. Narrower tire also means higher top speed and better high speed acceleration as well. Plus better fuel mileage, turn in etc....

Basically just park carefully and get the stickier tire in the better size

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Mmmmmmmmmmmmmmmmmmmmaybe.

Again, all tires are created differently, that's the point of this thread, so unless your picture is my wheel with my tire it's not accurate. However I will say that the picture you posted does look about identical to what mine will look like, as I've collected a batch of pics to ponder before my purchase last week. But it won't be the same in every case, you can't say that X size tire on Y size wheel will always look like Z. Since your picture is as you say a Federal 595 RS-R, assuming the offset matches, it will be identical. It's not bad, like you said, just have to watch for curbs. I parallel park about once a decade where I live, and few potholes, so my choice was an educated one.

Really my main concern was will the exposed edge of the rim get rock chips after a few thousand miles since it's not protected.

As for rim protectors, you're right....not sure where the Federals are on that note. I will say that the rim protector option, I'm fairly certain, has nothing to do with section width. Section width is the very widest part of the tire excluding the lettering which might be raised. I don't know that a rim protector as part of a tire exists that protrudes beyond the "hump" or crown of the tire. However as you said if the tire is stretched, section width goes out the door and the rim protector might save the day, though if the tire is stretched the rim protector probably gets pulled in as well and becomes obsolete.

As far as narrow vs. wide tire, in my case anyway it's reversed, I have a hybrid turbo upgrade and the more sticky tire that's making contact the more traction I get which ultimately means more acceleration instead of tires spinning going nowhere. But you are correct about MPG.

Thanks for the thoughts, they're good ones.

PS Is that pic your car, what is your rim and size, and can you post more pictures to benefit this thread?

 

[blurred]

As far as narrow vs. wide tire, in my case anyway it's reversed, I have a hybrid turbo upgrade and the more sticky tire that's making contact the more traction I get which ultimately means more acceleration instead of tires spinning going nowhere. But you are correct about MPG.

Thanks for the thoughts, they're good ones.

PS Is that pic your car, what is your rim and size, and can you post more pictures to benefit this thread?

Yes, that's my car. 215/40/17 Federal 595rsr on 17x8 rota Recce.

Tire traction is a black art and not as simple as just contact patch. The directionality of the contact patch is important as well as the ability to keep weight on that patch. Downforce, whether though Aero (high speeds) or mechanical (high and low speed) is determined by how much weight it distributed through the contact patch at each tire. The reason a gtr has lots of grip? Weight. The reason an f1 car has lots of grip? Weight. Although caused by air pressure instead of bulk.
Why is this important? Because on a smaller scale we can benefit from the same principles, such as keeping higher psi on the road at each contact patch. Running a large enough tire is very important to support the vehicles weight during cruise, braking, cornering, acceleration etc. A rwd car with 200hp will not transfer as much weight to the rear tires as a 1500hp rwd car. Powerband and acceleration g's also come into play as peak grip is always only fleeting except in a car with a flat or slightly rising torque curve (maxim grip potential in acceleration).

I can tell you that you will have more grip with a narrower, taller tire than a shorter, wider tire if talking about acceleration and braking. Finding the right balance is important but on a 2800lb car a 205 or 215 is already very wide and for traction issues a taller 205 would be best (but would need taller ride height)

Here is a 225 nt01 (probably measures 235) that handles 1130whp and 725wtq ... at the front wheels. The car has more traction issues on a wider tire (its had multiple setups over the years) because each square mm of tire is putting less weight on the road. When dealing in extremes these details matter. Especially on a fwd car where hazing the tires is normal but blowing them loose means no acceleration. The reduction in weight from the balance going rearward in acceleration requires less tire to provide more traction, but the longer the contact patch is instead of wider you get more benefit.

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[jeff]

Yes, that's my car. 215/40/17 Federal 595rsr on 17x8 rota Recce.

Tire traction is a black art and not as simple as just contact patch...



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Very excellent very excellent. In my case the Federals are the key to happiness.

Say, assuming you're +40 offset I'd love to see more pics of your Rota/Federal combo......since we have the same tire it is probable that the Grid and Recce are made the same and thus the rim/tire protrusion will be identical once mine are installed. If you have more pics from different angles, close up even, please share. I already have a bunch in my file, I'd like to see more especially since you have the same tire.

 

[Truth in Ruin]

[MENTION=636]RAAMaudio[/MENTION] has helped me with questions I’ve had as well. He’s a wonderful asset to the community.

The neat thing is, as the tires are- the Stones have a .1 wider tread width, but once you stretch the Feds on your new wheels, the Feds may actually have a wider tread width at that point. Slightly more contact anyway.

I’m interested to see if you notice better handling, turn in, etc.

 

[RAAMaudio]

Most excellent information by "blurred"!!!

It is all about contact patch and how well one can use it, why I do not go for extreme camber on a FWD car because loose to much traction and braking ability and stability overall. Nice result was not having to change camber from the track setup to street, just changed the damper settings.

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One of the main reasons I have 9" wide wheels, besides supporting the sidewalls of the 225 Rival S which are rather soft was just because I like a challenge. Another was when I decided to change the rear camber I found I could squeeze them under rolled and pulled fenders.

I did the front first with some old Hoosier R6 tires I had around, 225 but more like a 240 or 245, not to tough up front but impossible in the rear so would of had to run the still very wide 205 Hoosiers but I like buying take off tires fresh from race teams for cheap so used the RA1 225 which is still fairly wide, around a 230 to 235 and barely fit.

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One of the other reasons I like 15's though 16's are my overall favorite for how they look on the car and other reasons is super low weight wheels. Tires in the same sizes but different diameters usually weigh about the same so using lighter wheels, moving more of the tire mass towards the center gains in overall performance in all aspects as long as enough support to keep the contact patch connected well. Less weight has a big effect on that as well.

For the amount of time and effort as well as funds spent to fit 15x9 wheels over my own design BBKs I could of used rather costly 16x8 wheels and an off the shelf BBK up front but again, I like a challenge and to date have not seen it done by anybody else even though I glady gave all my secrets away including offereing the info on building the BBK's.

 

[RAAMaudio]

There are so many aspects to picking the best setup for ones needs and many are so involved it is hard to even explain them and sometimes understand them in the right context....

That is why my last shop space had a wall where I would draw all the suspension geometry in full scale to help me visualize what I was doing. Others might not need to do that but I am rather old school and yet barely know as much as some of the top people in this realm, why I have all their books to refer to

I have had the best luck with lowering the weight but more importantly the center of gravity as much as I can, front to rear bias, widest track while maintaining the best overall geometry within what I have to work with.

For the most part, the best gains I have had are with lower weight wheels and tires, lowered but not slammed and I do still check the geometry, widest track not upseting the scrub radius(meaning the offsest is not hugely different that stock but have to take in all the wheel parameters to get it right)

In general and a good rule of thumb for this car is just get some nice 8" wide wheels that are low in weight and put on a tire that meets your needs in longiviety, or not, intended use or uses, a little stretch is usually best, go have some fun

Of course you can do well with some narrower wheels but always go for the lowest weight you can afford and live with your road conditions.

 

[M-Sport fan]

The factor EVERYONE seems to be leaving out of the equation is the rubber COMPOUND of a chosen tire as compared to others.

The factory Potenzas might be slightly wider in tread width than the 595s, but they are most likely LESS grippy due to an older tech, harder tread compound (DESPITE the 140 rating!), BEFORE one even gets to their 'sipeyness' (smaller tread blocks/more 'cuts'), and their side walls could be more flexible than the 595s DESPITE being shorter.

On top of which they are what, like 15+ year old tire tech at this point?

My guess is that the Federal performance 200 tread wear tires are a 200 tread wear in the same way that an RE71R is a "200 tread wear" tire (i.e.; closer in actual compounding to a 100 tread wear tire, like the R888s/888Rs).

 

[RAAMaudio]

I did not forget. It is why I have ran Rival S since 273 miles on the car, had Mich PS A/S3 for all season but were far to harsh and noisy in 17" and now have 15" NeoGen which is a pretty amazing very old design tire and ran RA1 on track and would of ran Hoosier A7 when I stated time trial events

Absolutely correct about sidewall stiffness being different between tires. The original Rival S 225 is pretty wide but widens up more on the 9" wheels I run them on and the sidewall has a nice overall firmness without being to flexible, rides well, etc but I would not like them on an *" wheel as would be to soft, less feedback and control. The main difference, if I recall correctly, in the 1.5 version is stiffer sidewalls due to feedback they got from autocrossers, etc...

You mentioned the RE71R, damn fine tire, not very wide tread but hold their own and the best in many applications but could use a few more sizes for ST owners.....I always look at that tire and do recommend it when appropriate of course

 

[Kalter]

You see, in my mind I was always thinking that wider tires will give more grip for the same tire (a 215 S. drive should have more grip than a 205 S. drive), but you're saying it might not be the case?

It seems very counter intuitive. I want to eventually go to 235 or 245 tires to have more contact to the ground.

My logic is this: too much torque per square inch will break traction, but I want more torque/HP. If I put tires with more contact to the ground, I lower the torque per square inch, but still deliver about the same amount of torque overall. This should result in better acceleration because you spin less.

This has to be totally true for RWD as the weigth is shifted to the back wheels, but it also has to be fairly true for FWD I think.

This would also be why a AWD will accelerate faster than a FWD for the same wheel power.

I'm no engineer, so I might be totally wrong for a few things here.

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[RAAMaudio]

As always, many aspects to consider and real specs, not sidewall marking specs are always the ones to go by.

As mentioned by "blurred" the diameter has an effect as well since a taller tire has more contact patch than a shorter tire which also is easier to spin just due to the change in overall gear ratio(not the actual gears of course but the ratio changes are important)

Another way to enhance grip is to use better suspension bushings. The ST has pretty firm bushings which help the cars handling and feedback up to a point but rubber bushings wind up and get stiffer effecting how the springs and shocks work, tires slip a bit, unwind sometimes causiing or helping to cause wheel hop whic is really hard on driveshafts, transmissions, etc...I replace all the bushings at the prior listed 273 miles on the car with sleeved urethane bushings so the suspension can better throughout the range if can more and and provide more feedback and control.

Engine and trans mounts effect how the power is used at well as can help prevent wheel hop and instead of twisting the engine, trans, hoses, etc the power goes to the wheels to accelerate the car better.

Softer sidewalls help hugely, why drag slicks are very soft on the sides and why street tires can hook up far better on lower pressure but they also reduce feedback and control and emergency avoidance can be more effected than expected. Again, sorting out all the aspects one needs to look into takes more than just some sidewall specs and wear ratings(which are often very far off from reality).

Either we end up having two or three or more sets of wheels and tires or live with compromises trying to make one set do it all, much easier in the mostly sunny south

 

[M-Sport fan]

As always, many aspects to consider and real specs, not sidewall marking specs are always the ones to go by.

As mentioned by "blurred" the diameter has an effect as well since a taller tire has more contact patch than a shorter tire which also is easier to spin just due to the change in overall gear ratio (not the actual gears of course but the ratio changes are important).

I call the above the "effective gear ratio" when one takes into account the overall tire diameter's effect on final gearing.

I replaced all the bushings at the prior listed 273 miles on the car with sleeved urethane bushings so the suspension can better throughout the range if can more and and provide more feedback and control.

This can also make the car ride MUCH more roughly over bad/broken pavement (albeit not nearly as much as a fully heim/rose jointed, or rod-ended suspension will), which does seem to concern you somewhat given your predilection to use bigger side-walled 15" tires, especially for the street (unless you are using those to help conteract the added suspension stiffness gained form said bushings?).

 

[blurred]

You see, in my mind I was always thinking that wider tires will give more grip for the same tire (a 215 S. drive should have more grip than a 205 S. drive), but you're saying it might not be the case?

It seems very counter intuitive. I want to eventually go to 235 or 245 tires to have more contact to the ground.

My logic is this: too much torque per square inch will break traction, but I want more torque/HP. If I put tires with more contact to the ground, I lower the torque per square inch, but still deliver about the same amount of torque overall. This should result in better acceleration because you spin less.


This would also be why a AWD will accelerate faster than a FWD for the same wheel power.

Just to mess with your head a little bit... the car I posted above is FWD and running 225 tires, and hooks 1130whp/725wtq and out launches gtr's on the street. Yes, in first gear it is probably making half that power, but it shows that if you set the car up (powerband, suspension, tires) to work as a FWD it will perform just fine and you don't need super wide tires.

Think of it this way.
If you need 600lbs on each drive tire to have good forward traction at peak power and only have 600lbs per front tire to work with, which will grip better? A tire which puts 300lbs per square inch or 100lbs per square inch? You can test that yourself, and seeing as you are from QC you already know this... why does a narrow ice tire work better than a wide ice tire? The weight of the vehicle is concentrated on a smaller point which enhances the grip on that contact patch.

Now that contact patch has to be sufficient to support the weight, acceleration, and braking of that car so it doesn't mean go smaller on purpose... it just means that arbitrarily going wider doesn't mean you will gain anything. The reason I mention tire height is that it is far more beneficial to have additional width or length of contact patch in the direction you wish to maximize grip. If you want more grip in acceleration or braking and require more tire then going taller will make a longer contact patch. This would be similar to sliding in shoes on ice, you have more ability to have grip if you keep your feet in the direction of travel, the moment you turn your feet perpendicular to travel you are more likely to lose grip and slip.

 

[RAAMaudio]

I have 8/7k Swift springs and my car rides really well, far better than stock and quieter as well, I am still a bit surprised by the overall results,

I was willing to live with the realities of the bushings likely adding more noise and harshness and on the 17" Mich AS tires it was unbearable but on the 15" setups the ride is very good, especially with 600 lbs of cargo on board.

The car was built to ride in a trailer around the country and making very fast laps on all the race tracks of my dreams and also for site seeing, hauling our big dogs around, grocery getting as needed, not a lot of miles added up as we would be using our dually much of the time as well and of course pulling the trailer all over.

I have built cars with teflon lined full mono ball(another term for the same thing as heim, rod end, etc....suspensions, not something I wanted for this car for sure!

I went to 15's to get tires I wanted to run and lower the unsprung rotational mass which helps the ride considerably as well(the unsprung part that is) and save a bundle on wheel and tire costs as a side benefit. I bought ten 13.4 lbs flow formed 15x9 wheels for $1124 shipped and I was buying fresh take off race tires from a source I used many years and got the best ones, many never raced at all, the most a few laps, heat cycled, shaved, $400 a set shipped.

Same price I paid for 335/35/18 race tires though a few years earlier, that was a massive savings considering they were $1800 new!

 

[RAAMaudio]

I shocked myself when one of the very rare times I play on the street was 2 cars behind in 1st, even in second, 5 cars ahead in 3rd at 80MPH than a new M5 2 years ago. On the 225 Rival S, I am not a drag racer so not great off the line as never practice it, on that turbo and tune it hit hard around 4500 RPM
and no wheel spin in 2nd or 3rd, just hooked and gone. I hit the brakes as in a 65 zone, I like my cheap insurance rates.

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It is hard to imagine 1135WHP, 725 ft lbs, hooking up in first even if half the power being made, very impressive indeed!!!

What tires are on the car?

 

[M-Sport fan]

I was buying fresh take off race tires from a source I used many years and got the best ones, many never raced at all, the most a few laps, heat cycled, shaved, $400 a set shipped.

When you say "race tires" do you mean full-on road race slicks (like Goodyear G-19 Eagle Sports Car Specials or the like), Hoosier (or the Conti labeled versions from the WC) DOT slicks (like their R7s), or 100 tread wear DOT tires (like the Toyo RA1s/R888s or Nitto NT01s)?