Need Tire Education..

[MJawsman]

When you say 'bigger', I assume you mean 'wider' since you have said that both tires are the same height. Otherwise, I agree completely with your statement. [/B]

If we use the 265/70 and 265/75 BFG example from before, both tires are the same width yet the taller tire has a higher load capacity at the same air pressure. To me, this is a "bigger" tire even though it is no wider.



And maybe this will help my argument. It is my understanding that even though the above tires have equal widths, the taller tire holds more air than the shorter tire at the same pressure. This higher volume of air is what is carrying the weight through the support of the tires casing. This was my point with the Bigfoot type tire. Yes there is a huge footprint to support the weight, but it also has an enormous volume of air that in turn supports the casing that supports the weight. Tell me what good is 1500 sq inches of contact without any air volume?

 

[jlccc]

Originally posted by MJawsman

If we use the 265/70 and 265/75 BFG example from before, both tires are the same width yet the taller tire has a higher load capacity at the same air pressure.

I'm going to offer what I think is a reasonable explanation. It is purely speculation on my part but tell me what you think. It may support what you are saying.



Keep in mind what I said above about tire flexing and heat buildup. Given two tires of the same width and at the same pressure, the taller tire, due to its larger radius of curvature, will flex less than a shorter tire will in order to make any given size of contact patch.



The above explanation might be the reason for the difference in ratings.

 

[MJawsman]

I'm going to offer what I think is a reasonable explanation. It is purely speculation on my part but tell me what you think. It may support what you are saying.



Keep in mind what I said above about tire flexing and heat buildup. Given two tires of the same width and at the same pressure, the taller tire, due to its larger radius of curvature, will flex less than a shorter tire will in order to make any given size of contact patch.



The above explanation might be the reason for the difference in ratings. [/B]

Yes I agree that may very well be the case. Certainly seems reasonable. I'm no physicist, but it sure is fun to discuss it. I bet everyone else in the TDR right now is doing this



:-laf :-laf :-laf

 

[Enimmo]

As you can see Kev, no confusion here!

 

[jlccc]

Originally posted by Dkevdog

Would the height be the same as a 265/75? The 265's rub a bit in the turns and I really don't need the width. But I do like the height.

I've created a small spreadsheet that calculates the heights of different size tires based on information input by the user. It also calculates vehicle speed in each gear, taking into account tire size and rear axle ratio. It's currently set up for my 6 speed but can be modified for the folks with 5 speeds and autos.



Download it at:



http://jlccc.homeip.net/tdr/tires/tirecalc.zip

 

[jsnyder]

Oh my!

Hey MJawsman, let me help you with that !



I just had a flashback that I was in physics again .



I'll keep my mouth shut on this theorizing and let people assume I'm an idiot rather than open my mouth and remove all doubt .



One thing to keep in mind in choosing between 235/85 and 265/75 is how you are using the tire. If you run in a lot of deep snow, then the narrower profile tire will serve you better. However, on packed ice/highway type conditions, a wider footprint will give you a leg up. BTW, 235/85's are cheaper in load range E than a 265/75.



Jason

 

[Dkevdog]

Wow. I want to thank you all for your discussion on this topic. I now know that I AM confused. (Which is good, because before I wasn't sure ). For the time being at least, I am gonna keep me 265/75/16 D's. Unless I upgrade to the same size tire in E, I will not gain any lad capacity. The tires are rated now for more weight than the truck can technically carry, so unless anyones sees anything wrong with it, I'm keeping 'em.



Kev

 

[Oil Ape]

jsnyder, you're off by just a bit... .



The 235/85/r16's have a smaller contact area and thus are not only better at deep snow but also on hardpack due to their increase lb per square inch being applied. You spread that weight out over a larger and larger area and before long you have a sled. In dry conditions, dry asphalt, a wide tire is ok. In the racing circuits they are putting so much horsepower to the ground that they would tear/burn a thinner tire up. Instead they rely on spoilers to overcome the traction problem of a wide tire. The spoiler increases the lb per square inch of contact of the tire to the road. That is also why you throw sandbags in the bed of your truck in the winter so why use a wider tire that would decrease that effect? The higher the lb per square inch of contact the better the traction no matter what the road is made of as long as it is a hard surface. Wide tires are only good when you wish to increase the grab ability over rocks(wide, low pressure, off road applications), minimize damage(burning rubber, drag racing etc... )or sand and mud applications where thin tires would sink. In sand or mud applications you want to minimize your contact pressure and stay on top. Look at mud boggers. There are two ways to go. Fat tires skim across the surface, thin tires cut through to the bottom for traction.



As to the general question of D vs. E rated tires someone commented correctly earlier by stating that it is the number of belts in the tires. For every letter, the tire has an additional 2 belts. Thus an "A" rated tire has 2, a "B" rated tire 4, a "C" rated tire 6, a "D" rated tire 8, and a "E" rated tire 10.



The maximum load capacity does increase with volume. Although the 265's have better load capacity the 235's are a much stiffer and thicker and are less likely to become a victim of torn sidewalls or punctures. The 235's also generally have a better wear patterns and last longer than the equivalent 265. It basically boils down to tastes of the individual and it seems the big fat tires are currently the fad.

 

[Dkevdog]

Just for futrure reference- can I even put the 235/85's on my 16x8 rims??



Kev

 

[Oil Ape]

Technically the answer is yes. A tire guy can get 235/85 R16's to fit on the 16x8 rim. They don't even look that bad. However, the tire specs advise the tire only to be mounted on 6"-7" wide rims. The problem lies in the warranty on the tires. No tire manufacturer will warranty their 235's if mounted on16x8 wheels. They were specifically designed to be run on thinner wheels and the result according to the tire manufacturers is increase heat buildup, increased sidewall vulnerability, and increased chance of damaging the rim. On the drivers side door sticker it should tell you the width of your existing rims...



If you are wider than 7", I would recommend the 265's over the 235's just for the warranty problems. If you have a rim that is 7" or narrower, then I would recommend the 235's.



There are advantages to both and disadvantages to both. You just need to decide what would be best for you. The 235's have better wear patterns, better traction, and stronger construction. Cons are the 235's ride stiff. The 265's are better offroad floatation tire, higher maximum weight capacity, and give a softer ride, but don't wear as well and stress the steering assembly more than a narrower tire would. It all boils down to what size of rim you already have... ... . The pro's and con's of each are actually more marginal than I have implied and both tires perform well.



Currently I'm running 235's mounted on 16x7 aftermarket wheels on my rig but my father has 16x8 aftermarketwheels w/ 265's mounted and he wants to switch to the 235's, he doesn't like the squishy feel of the 265's especially when pulling our large trailers, so I'll be trading him wheels and tires. I personally prefer the 235's but the 265's will suit me fine as well as I run 90% of the time empty.