A detailed explanation of the tradeoffs of larger diamet

[tatsu]

With the many, many posts and arguments about the benefits and drawbacks of the various diameter wheel/tire combos for our cars, I felt a more complete explanation from a technical side of things was in order. Bear in mind, this is leaving appearance out of this completely - if maximum bling is your thing, then go buy your chrome 18" spinner wheels and be happy. If you are seriously questioning whether upgrading your wheel/tire package is worth it from a performance perspective, read on.

First, to dispel the myth about racing cars and why they run larger wheels with lower profile tires. They do NOT run them because they have "inherently better" handling. The number one reason for running large diameter wheels on racing cars is to allow for the largest brakes possible. Certain racing formulae, like F1, for example, have restricted wheel diameter specifically to limit the size of the brakes that can be used.

With this push for bigger and bigger brakes and the resulting shorter and shorter sidewalls, low-profile racing tires had to be engineered with stronger, more puncture-resistant sidewalls, that also needed to be stiffer to prevent damage to the wheels under compression from bumps, curbs, etc. To maximize grip within the greatly reduced slip angles these tires would allow, much stickier tread compounds had to be developed, resulting in the tarmac racing tires of today.

Manufacturers' concept cars followed this trend mainly for the look, and this trend began to filter down into production vehicles via the aftermarket first, and now via the manufacturers themselves. The upside of this style trend for performance enthusiasts is that many manufacturers began putting larger and larger brakes in their sportier cars because they now had the room - kind of the reverse logic of the race cars! - and a greater selection of performance tires in various sizes and aspect ratios are now available.

Now on to the pros and cons of larger vs. smaller diameter wheels, leaving appearance aside. This argument is assuming that we are keeping with the same overall diameter of the wheel/tire package, which for the MX-3 GS is roughly 23.9", the same vehicle weight, and the same tire pressure. For this explanation we'll use a popular aftermarket lightweight performance wheel which is available in sizes from 15" up to 19", the OZ Superleggera, which just happens to be my personal favourite! We'll use the Toyo Proxes T1-S tires, again for their range of sizes, and because the information on their weights is readily available on Toyo's website. This way, we are comparing apples to apples.

So first, the specs on wheel/tire packages in sizes appropriate for the MX-3 GS:

15x7" wheel with 205/55R-15 = 29.8 lbs (stock size tire)
15x7" wheel with 225/50R-15 = 31.8 lbs (7% heavier than stock size)
16x7" wheel with 205/50R-16 = 34.0 lbs (14% heavier than stock size)
16x7" wheel with 225/45R-16 = 34.4 lbs (15% heavier than stock size)
17x7" wheel with 215/40R-17 = 35.9 lbs (20% heavier than stock size)
18x7" wheel with 215/35R-18 = 37.2 lbs (25% heavier than stock size)

For additional reference, but not used in this comparison, the weight of a stock GS 15x6" alloy wheel with the Toyo T1-S tire is approximately 34.3 lbs, with the wheel being 16 lbs.

Clearly, the combinations with the larger-diameter wheels are heavier. Also, a wider tire on the same size wheel is heavier. While compared to the overall weight of the vehicle a difference of 29.6 lbs (the difference between the 18’s and stock size tires) is minor, at a little over 1%, the key areas where the weight comes into play are in unsprung weight, and rotational weight.

Unsprung weight is that portion of the total weight of the car that is not carried by the suspension, I.E. the wheel/tire, the brakes, the hubs, the lower control arms, etc. An increase in unsprung weight inevitably results in the suspension being somewhat less responsive to high vertical acceleration events (rapid compression or extension), like driving on rough or rippled pavement. Note that this does not have to do with slower acceleration events, like acceleration squat, braking dive, and body roll, where the transfer of the sprung weight of the car from wheel to wheel is the more notable effect.

This means that, with a heavier wheel and tire package, the ride will be somewhat rougher, and the ability to maintain contact with a less-than-billiard-table-smooth road during acceleration, braking, cornering or steady-state driving is reduced, resulting in a reduction of overall AVAILABLE traction, but not ultimate grip.

Rotational weight (actually, more correctly, rotational mass) has mainly to do with what is commonly called the “flywheel effect”. Without getting into the various formulae, there are three basic truths: 1) it takes more energy to change the velocity of (accelerate/decelerate) a greater mass, and 2) the further from the rotational axis the mass is located, the more energy is required to change its velocity, and 3) the further away from the rotational axis the mass is located and the higher the rotational speed, the more energy is required to change the direction (I.E. to turn the wheel from side to side). Bear in mind that the bulk of the weight of a lightweight wheel and tire combo is carried around the outside diameter of the wheel rim, with the tire making up from 52 - 64% of the package in our examples above, and that going from a 15” to an 18” wheel moves more of that mass 20% further away from the rotational axis! You also need to take into account that the 18” wheel also has 20% more mass in the wheel rim because of the larger circumference, with the rest of the difference being in the wheel spokes. What this means is that with the larger wheel/tire combo, you will have poorer acceleration, braking and turn-in at speed.

Beyond the effects of weight, which are significant, you have the effects of different tread widths and different sidewall aspect ratios. Again, we are talking here about tires with identical overall diameter, tread patterns, construction and inflation pressure to make direct comparisons, and again, it is a rather simplistic explanation.

Regardless of the tire’s tread width, the size of each contact patch (where the rubber meets the road) is basically the same, all other factors being equal. What changes is the SHAPE of the contact patch with a wider tire. The narrower tire has a more elongated and narrow contact patch, with a greater proportion of the contact surface running from front-to-back, while the wider tire has a proportionally shorter and wider contact patch, with proportionally more of it running from side-to-side.

What this means is that, at the longitudinal (front-to-back) centre of the contact patch, where there is the most traction available, the wider tire has more rubber in contact with the road, further increasing the amount of available traction. This is why, on a dry, smooth surface, the wider tire is better – because in those conditions it has more available grip.

In poor weather conditions, or on loose road surfaces, the narrower tire has a better chance of finding traction under acceleration and braking because of its elongated contact patch. There’s basically a longer patch of that varying road surface for it to try to adhere to. Also, the most effective way for a tire tread to evacuate water is to give it a channel along the length of the contact patch that gradually moves the water towards the edges, out from under the tire. This is why the narrower tire will have greater resistance to hydroplaning.

With regards to sidewall aspect ratio (the “profile” of the tire), all other things being equal, the shorter sidewalls will be more resistant to deflection, both side to side and along their radius. This gives the shorter sidewall greater responsiveness to changes in direction, but also reduces the slip-angles allowed before breaking traction. In terms of acceleration and braking, this means that the accelerative/braking force is more quickly and suddenly transferred to the contact patch. In simpler terms, the lower profile tire will be more responsive but will let go more suddenly once the limit of its grip is reached, while the higher profile tire will be slower to respond, but will be more progressive, with effectively a wider envelope of grip. Note that I did not say MORE grip – the ultimate grip, with the same tread pattern, tread width and tire compound should be basically the same, regardless of the aspect ratio of the tire.

So what does all of this blah-blah-blah mean to you? There are tradeoffs to both paths to wheel/tire glory.

The weight advantages, especially rotational weight, of the smaller diameter wheel/tire combo should give you better acceleration, braking, high-speed turn-in and ride quality, and better traction on most road surfaces. The higher profile tires should be a lot more progressive in their feel, but with the taller sidewalls the tradeoff is less responsiveness in all directions.

The greater responsiveness of the larger diameter wheel with the lower profile tire comes with the tradeoffs of poorer ride quality, acceleration, braking and high-speed turn-in, poorer traction on most normal road surfaces, and a much less forgiving tire at the limit.

Hope this helps you all in making your tire-buying decisions!

<small>[ November 10, 2004, 10:40 AM: Message edited by: JP ]</small>

[Casino]

thanks

[jumboplay]

Thanks for the guide, that's amazingly detailed. You forgot one thing though:

Big shiny wheels make the girls swoon. You can't call that a performance hit. Hehe.

Good work!

[Casino]

Big shiny wheels make the girls swoon

not in my town at least :p

[nos92mx3]

i have one word for you my friend
FAQ

[mitmaks]

good read

[bmwm3guy]

Originally posted by nos92mx3:
i have one word for you my friend
FAQ

Isnt that three words? :p Just kidding, but that write-up was awesome!

[Neil]

Dont mean to bring an old post back from the Dead, but could this be added to the FAQ.

Unless i'm blind (which is very possible) I didn't see this in the FAQ. Had to do some digging to find it and this post is very FAQ worthy!

[PATDIESEL]

I vote it for a FAQ post as well. It is a good read for the novice wheel and tire buyer or a novice autcrosser.

[Thunderstorm]

Brilliant write up, Thank you!!

That means I am OK with the 15inch TOYO T1-S tyres that came with my 1998 RS!

That also explains why my RS does high speed turns without flinching or wagging her tail!

Cool!!


Thunderstorm

[neutral]

Brilliant write up, Thank you!!

That means I am OK with the 15inch TOYO T1-S tyres that came with my 1998 RS!

That also explains why my RS does high speed turns without flinching or wagging her tail!

Cool!!


Thunderstorm

You... are golden for getting xlent handling with the lightweight 15's...

[tatsu]

I'm glad you guys found it useful!