Tire width versus contact patch vs weight

[92saturnsl2]

I'm curious as to why/how manufacturers specify a certain OEM tire size, and the resulting priorities. I've noticed a trend in last decade or two, that widths have increased and aspect ratio (the formula for height) has decreased. Rim diameters have increase in similar fashion, but why? Looks alone?

Seems to me that ride quality would suffer with less sidewall height, although handling would be improved as the sidewall would flex less. Narrower tire = greater traction, and vice versa is what I always understood (within reason and factoring in vehicle weight.)

On a vehicle I'm buying tires for, if I have a choice between say, P225/75/16 and P235/70/16, which quality (handling, ride quality, traction) would suffer?

 

[69GTX]

Depends what you favor. If it's ride quality then get the taller aspect tire. You'll also get a smaller contact patch which will lessen dry grip but increase mileage and maybe tire life. In the winter the smaller contact tire will handle a bit better in light snow. In wet weather the larger patch would be better. Can't be a huge difference between those sizes....4/10ths of an inch wider, 1/5th of an inch smaller aspect.

 

[92saturnsl2]

Giving a more extreme example, my '85 F250 OE tire size is LT235/85/16, however the PO used LT265/75/16, and I recently installed LT265/70/17's. Would there be a significant difference? Or is wider-is-better the mindset nowadays..?

 

[Rolla07]

You need to stay within 3% of overall diameter to maintain odometer reading..but if that matches you will likely make a choice that has little difference from each other. Too wide a tire can rub in a vehicle that isnt meant to accept too wide. So there are other variables to consider besides traction etc..which I believe would have minimal impact. Also depends on the feel you like of tires, me I prefer lower profile so that the car is more agile during manuevers..others may prefer a softer ride so go for thicker sidewalls...

 

[NO2]

Part of it is fashion and also a growing trend toward more low profile tires which exhibit better handling characteristics due to a larger contact area and stiffer sidewalls. In addition, cars have become significantly heavier over the years, people buy more crossovers and SUVs, and larger tires are often needed to support higher weights.

 

[Injured_Again]

Rim sizes are larger because the trend has been to try and emulate high end sports cars, which need big rim sizes to clear the larger brake rotors they use. Larger rims are used to give the impression that car is also capable of high end sports car performance.

A narrower and/or larger tire has a longitudinally longer contact patch shape, which gives better traction in acceleration and braking. A wider tire has a longitudinally shorter contact patch but wider contact patch, which helps with cornering.

Between the two sizes the OP mentioned, the differences are slight enough that the brand/type/manufacturer of tire will have a much greater effect on tire performance than the slight change in tread width and aspect ratio.

 

[Johnny2Bad]

There is the fashion and simple appearance issue, not to be dismissed. However, broadly speaking, a wider tire has poorer performance except at the limit of adhesion (cornering at speed, for example). Others have already mentioned differences such as clearance for brake rotors, etc.

A wide tire is a compromise ... you can obtain better performance on dry pavement but give up performance on any other surface ... wet pavement, mud, snow, gravel, etc. Larger wheels and tires often mean higher unsprung weight which will affect braking performance negatively, which may, or may not, be overcome with larger rotors and higher gripping callipers (eg 6-piston types over 4-piston).

Broadly speaking heavier cars can take advantage of wider tires while lighter cars should be looking at lowering the unsprung weight. In low traction situations a narrower tire will usually prevail. There is such a thing as wide tires for floatation, but again that is not about traction on dry pavement.

Like anything, there is an ideal wheel weight, aspect ratio and tire width for your vehicle depending on what areas of performance you feel is most important, keeping in mind safety plays a role here. There is also a consideration regarding strength of a given wheel .... you don't want to be hauling loads in your truck on gravel with potholes at highway speeds with fancy aluminum wheels, as they will break when steel wheels will not.

I see people referring to "rims" but a rim is the part that mounts the tire, and would fit any vehicle. Well, up until the point where you complete the wheel, that is, with spokes or a wheel disk, followed by a hub, the part that mates to the vehicle; the three parts when mated together constitute a wheel.

 

[Smoky14]

Originally Posted By: 92saturnsl2
a Narrower tire = greater traction, and vice versa is what I always understood (within reason and factoring in vehicle weight.)




If this is true why do dragsters use wide tires?? Just doesn't make sense to me.
YMMVSmoky

 

[barryh]

The trend to ultra low profile tyres is just fashion which leads to a hard ride, much greater vulnerability to wheel damage and significantly higher operating costs. Fitting very wide and low profile tyres to ordinary cars for ordinary drivers is just silly but that's what's been happening.

Your original example of P225/75/16 vs P235/70/16 isn't worth worrying about though as they are both high profile. I'd go for the most commonly available tyre which will tend to be cheapest.

In the UK where the annual Road fund tax is charged in emissions bands, increasing tyre width by only 10mm on a new car can move the car up to the next band and incur an ongoing higher annual tax. For that reason I chose 195/60/R16 over 205/55/R16's and don't miss any reduction in grip.

 

[bullwinkle]

Bigger contact patch=better dry traction
Smaller contact patch=better wet, snow, & ice traction
Tread type & design makes a difference too, but my snow tires are always NARROWER than my summer tires.

 

[CapriRacer]

Originally Posted By: Smoky14
Originally Posted By: 92saturnsl2
a Narrower tire = greater traction, and vice versa is what I always understood (within reason and factoring in vehicle weight.)




If this is true why do dragsters use wide tires?? Just doesn't make sense to me.
YMMVSmoky


Yeah, I think that is backwards, too!

 

[Powerglide]

Increased unsprung weight is never a good thing.

 

[d00df00d]

Originally Posted By: 92saturnsl2
I'm curious as to why/how manufacturers specify a certain OEM tire size, and the resulting priorities. I've noticed a trend in last decade or two, that widths have increased and aspect ratio (the formula for height) has decreased. Rim diameters have increase in similar fashion, but why? Looks alone?

As far as I know, there are exactly two reasons to go for a bigger wheel with a lower profile tire:

1. Looks
2. Brakes

That's it.

The looks part is important these days because cars all have high waistlines, and small rims would look stupid (see the Pontiac Aztec for an extreme example). I personally don't care, but I'm not most people.

Brakes these days are pretty big, too; some (especially on sports cars and large upscale cars) just wouldn't fit under a reasonably sized wheel.

And of course, the downsides of bigger rims with lower profile tires are:

1. Wheels are heavier
2. Wheel mass is farther out from the wheel hub
3. Wheel hubs and suspension parts see more strain
4. Everything is more vulnerable to damage
5. Everything is more expensive


Originally Posted By: 92saturnsl2
Seems to me that ride quality would suffer with less sidewall height, although handling would be improved as the sidewall would flex less. Narrower tire = greater traction, and vice versa is what I always understood (within reason and factoring in vehicle weight.)

Ride quality definitely suffers, all else equal.

Less sidewall flex... not always, but usually. But higher profile tires can provide the same benefit with stiffer sidewalls. At least with a higher profile tire, you have more freedom to choose a harder or softer sidewall as you please (assuming a range of tires is offered in that size).

I don't think sidewall height affects traction much.


Originally Posted By: 92saturnsl2
On a vehicle I'm buying tires for, if I have a choice between say, P225/75/16 and P235/70/16, which quality (handling, ride quality, traction) would suffer?

I don't think those sizes are different enough to make a huge difference, but here are a few things to consider in general:

1. On a dry surface, wider means more lateral grip (cornering) but maybe slightly less longitudinal traction (accelerating and braking). If you want to improve both, you need a larger overall diameter as well -- but that'll affect your speedometer, odometer, and ABS.

2. Narrower tires (e.g. 205 vs. 225) have better hydroplaning resistance and lower rolling resistance.

3. A tire that is bigger in any way will usually weigh more.

4. Lower profile means more NVH.

5. All of the above is "all else equal." Different tire models in the same size can vary more than different sizes of the same tire.

6. Tire mass makes more of a difference than any other kind of mass on your car. It's unsprung, and it's rotating at the farthest distance from the wheel hubs. Saving a few pounds in each tire can help as much as having one less person in your car. The less your car weighs, the bigger the impact of reducing tire mass.

7. How the tire fits on the rim can be important. Look up the "measuring rim width" on Tire Rack and use that as a guide. Ideally, you want the tire sidewalls to be pretty upright. If you go wider than that, the tire will squirm more. If you go narrower, it's just bad.

 

[HerrStig]

Originally Posted By: Smoky14
Originally Posted By: 92saturnsl2
a Narrower tire = greater traction, and vice versa is what I always understood (within reason and factoring in vehicle weight.)




If this is true why do dragsters use wide tires?? Just doesn't make sense to me.
YMMVSmoky
NASCAR should be using bicycle tires.

NASCAR has a rule limiting tire size and the cars would be faster with bigger sizes, especially on the road courses.

 

[kschachn]

...

 

[69GTX]

Originally Posted By: Johnny2Bad
However, broadly speaking, a wider tire has poorer performance except at the limit of adhesion (cornering at speed, for example)...


Or the limit of adhesion for a drag car where the more rear tire contact on dry/hot pavement when starting out, the better. Doesn't have to be too worried about brakes either.

 

[d00df00d]

Originally Posted By: Smoky14
Originally Posted By: 92saturnsl2
a Narrower tire = greater traction, and vice versa is what I always understood (within reason and factoring in vehicle weight.)




If this is true why do dragsters use wide tires?? Just doesn't make sense to me.
YMMVSmoky

Not gonna defend the "narrower tire = greater traction" comment; guessing OP was referring to sidewall anyway.

Just wanted to point out that dragster tires also have huge diameters, and they run very low tire pressures to allow the tires to deform on a hard launch. Pretty sure that deformation is mostly longitudinal, so the result is a contact patch that is both wide AND long.

For a road car tire, you're mostly running a set pressure, and it's relatively high (at least compared to a dragster's tire) to prevent the tire from deforming too much. Given that, a wider tire with the same OD will have a contact patch that is wider but not as long.

 

[circuitsmith]

A few months ago I switched my Matrix from 205/55/16 to 195/65/15.
These are both OEM sizes on the similar Corolla.
Not a big change, but the ride is noticeably smoother and quieter.
Steering is less stiff at parking speeds and cornering & braking have not suffered,
at least for non-maniacal driving.
Each wheel weighs 9lb less than the originals.

 

[supton]

I thought a dragster used "tall" tires partially to take the shock. The sidewall can "give" a bit, and so the traction surface is a bit less likely to spin. Didn't think about making the contact patch longer, that makes sense too.

I also wonder if the extra mass, being harder to accelerate, also helps. At the line. To take the sudden instant full WOT. Elsewhere it's just a hindrance (have to spin it up).

Anyhow... usually one goes more narrow in snow, for higher contact pressure, for better snow traction. That works for snow, where one wants to compress the snow into the treads. Or dig down to bare pavement. However I do believe in sand (or really deep snow) one goes the other way and wants to go wide, spread the weight out, so as to stay on top. Mud and dirt, wider is usually used also. So it's really only in snow that one thinks about going more narrow.

 

[barryh]

Originally Posted By: circuitsmith
A few months ago I switched my Matrix from 205/55/16 to 195/65/15.
These are both OEM sizes on the similar Corolla.
Not a big change, but the ride is noticeably smoother and quieter.
Steering is less stiff at parking speeds and cornering & braking have not suffered,
at least for non-maniacal driving.
Each wheel weighs 9lb less than the originals.


I'd call that a result. 9lbs on each wheel is a worthwhile saving.

As well as unsprung weight, lighter wheels are an improvement in another small way. The car will accelerate fractionally faster and brake better because you get the double whammy effect of lower overall vehicle weight and reduced rotational inertia.

No better place to reduce weight than the wheels. Give me function over form any day.