Tires: 185/65 or 195/65???

[CapriRacer]

Tire size and RR? Bigger = Better

http://www.barrystiretech.com/rrandfe2.html

But tire size is small compared to the differences between tires.

[theholycow]

I love that site.

I used to be quite sure that wider = less RR but I've seen some pretty good arguments why that wouldn't be true. I still think it is but I'm not presenting it so much as a fact anymore.

[IndyFetch]

I think it's odd that my 2010 Fit Sport has narrower tires (on 16-inch rims) than my father-in-law's 2008 Fit Sport (on 15-inch rims). That goes against the current trend.

[Dr. Jerryrigger]

Well I don't have any answers, just some thoughts.
Weight will matter in stop and go situations, but it's unlikely you'll ever notice (unless you fill them with cement).
For highway driving you should think a little about aerodynamics. With wide tire you have more frontal area. With a larger diameter tire you have a larger space under the car. This is generally bad, unless you have a very sleek belly pan. With larger diameter tires you are effectively giving your car slightly taller gears, which is generally a good thing for FE as most cars have more power than you need.

As far as rolling resistance think about the patch (the bit of the tire that is in full contact with the road). This will be a different shape depending on the width (assuming air pressure is the same). With a wide tire it will require the tire to "squish" less which seems like it would be better. In rain it will have to push more water out of it's way (which is bad).

[Dr. Jerryrigger]

oh, that link that was posted covers all of that quite well, that's a good read, thanks.

[justinswrx]

Still can quite wrap my head around the wider= Less Rolling Resistance. I suppose I will just go with the factory reccomended 195/65 in a LRR tire.

[theholycow]

Here's the explanation for wider = less RR...

---------------------------------
Let's say you've got 40PSI in the tire and 400 pounds of weight on it. Contact patch should therefore be 10 square inches (PSI = pounds per square inch). If the tire is ten inches wide then the contact patch will be one inch long. If the tire is five inches wide then the contact patch will be two inches long.

With me so far?

Now, look at a tire sitting on pavement. You can see where the sidewall deforms for the contact patch. It matches the length of the contact patch. As the car rolls, the sidewall must continually deform at the bottom of its rotation for the contact patch. The flexing of the sidewalls is a source of rolling resistance.

Still with me?

Now put the two together. A longer contact patch means more of the sidewall must deform...more RR.
---------------------------------

Is the theory correct? It adds up correctly in my mind. It explains why increasing air pressure always decreases RR (something that's very easy to observe). I have seen some good arguments against it...but I remain convinced that wider = lower RR (all other variables being equal).

[theholycow]

A new thought:
We know that tread affects FE because worn tires get better FE than new ones. How would width affect that issue?

Do we know how width affects tread design? As a tire gets wider do tread blocks get wider, do the voids between them get wider, and/or do tread blocks get more numerous? Is there a standard procedure or does it vary by model and manufacturer?

Also, how ridiculous is it to split hairs like this?

[Ford Man]

Originally Posted by theholycow

A new thought:
We know that tread affects FE because worn tires get better FE than new ones. How would width affect that issue?

Do we know how width affects tread design? As a tire gets wider do tread blocks get wider, do the voids between them get wider, and/or do tread blocks get more numerous? Is there a standard procedure or does it vary by model and manufacturer?

Also, how ridiculous is it to split hairs like this?

I think for the most part the tread block get wider, but on some very wide tires there are also extra tread blocks too I think.

[Sludgy]

Originally Posted by theholycow

Here's the explanation for wider = less RR...

---------------------------------
Let's say you've got 40PSI in the tire and 400 pounds of weight on it. Contact patch should therefore be 10 square inches (PSI = pounds per square inch). If the tire is ten inches wide then the contact patch will be one inch long. If the tire is five inches wide then the contact patch will be two inches long.

With me so far?

Now, look at a tire sitting on pavement. You can see where the sidewall deforms for the contact patch. It matches the length of the contact patch. As the car rolls, the sidewall must continually deform at the bottom of its rotation for the contact patch. The flexing of the sidewalls is a source of rolling resistance.

Still with me?

Now put the two together. A longer contact patch means more of the sidewall must deform...more RR.
---------------------------------

Is the theory correct? It adds up correctly in my mind. It explains why increasing air pressure always decreases RR (something that's very easy to observe). I have seen some good arguments against it...but I remain convinced that wider = lower RR (all other variables being equal).

Car and Driver (April 2010) published real world testing of tires with similar diameters and varying widths. The results were unequivocal: wider, low profile tires gave poorer mileage and worse acceleration, but better cornering and stopping. Here's the link:

http://www.caranddriver.com/features...sted-tech_dept

You considered only sidewall flex. However, tread also deforms as a tire rolls. Thick tread, more tread plies, and steel belts have more resistance to flex, and probably have more impact than sidewall flex.