The SKINNY on skinny tires

I have one of my coilovers out right now... I should take the opportunity to do the same. Not sure if I'll learn much, but probably smart to bolt the UCA back up, throw a wheel on and cycle it thru.

 

Its the only way to properly fit bigger than stock tires. Doesnt take too long and makes sure everything actually clears bump to strap lock to lock.

 

Cycle it if you’re already in there.

 

Another shot 255/85/16

 

How much of a MPG hit did you guys get going from 265s?

 

265/70R17 w/3.73 Gearing: 16.1 MPG
255/80R17 w/3.73 Gearing: 15.2 MPG
255/80R17 w/4.56 Gearing: 14.2 MPG (15.1 MPG w/GPS-Based Mileage Correction)

 

I went from stock to 255 80/17 and saw a minimal drop in mpgs. Unless you recalibrate the speedometer, remember that the new tire circumference is about 7.4% larger

 

I got about 1ish mpg less hand calculated at the pump on each fill. but I also added some armor and a lift at the same time as switching tires

265/75r16 > 33/10.5r17

 

Did you switch back to 265s after skinnies? Can you tell us what pushed you to make the switch back

 

I didnt switch back im still on 33/10.5s. I went from 265s to 33/10.5s and that's what I'm currently on.

By this: 265/75r16 > 33/10.5r17 I was meaning that was the size I went from to the size I went to. not that 265s are better than 33s

 

I just wanted to add a few concepts in this thread and hopefully provide some clarity to the debate. I have not read the entire 280 pages of the thread, but more referencing some recent discussions. I apologize if these points have already been brought up. Also, math and physics are used here, not everyone's cup of tea but are fundamental in understanding the concepts below.

Misconceptions:
I see some reoccurring themes that are incorrect:

1. Relatively skinny tires "dig in" instead of float as compared to relatively fat - Not always true, floatation is more dependent on tire pressure.
2. Fat tires have more available contact area which equals more traction - Not always true, more dependent on tire pressure. Assuming both tires have the same pressure, the shape of the contact is different, not the total contact area
3. Look at "this vehicle" and "that vehicle" and see that because this vehicle has this tire it must be better. - Vehicles are designed for vastly different uses, terrains, loads, etc. a single use case does not decide what is better
4. Skinny tires have less "rolling resistance" - Not necessarily true, this also is dependent on tire pressure. I believe this mostly comes from road bikes. In that case, a skinny tire is generally able to hold more pressure with less material than a wide tire.

Explanation: Both skinny and fat tires (talking in relative terms) have the same contact area, assuming they are at the same pressure. The key difference is the skinny tire's contact patch will be longer and more narrow whereas the fatter tire will be relatively shorter and wider. A key concept is your vehicle is supported by the air pressure in the tire. Let's assume a tire is perfectly circular when it is not in contact with the ground. When the tire is put on the ground and loaded by the weight of the vehicle, the area of the tire that is in contact with the ground flattens out. This flattened area is defined as the contact patch.

The amount it flattens out is directly proportional to the weight and pressure in the tire. It does NOT matter what shape the tire is. The total tire contact area will be the same, but the shape of the contact patch will be different. A skinny tire tends to elongate more (front to back with respect to the vehicle) and a fat tire will elongate less since its contact area is wider. However, the total contact area is the same between both assuming the same weight and tire pressure.


Total Tire Contact Area = (Vehicle Weight) / (Average Tire Pressure)

Traction, simply put, is the friction the tire has with the ground. In off-roading, this can change very rapidly depending on the terrain you are on. Other factors include tread, materials, temperature, etc. However, for ease of calculation, these factors are looped into a term called the "coefficient of friction". Then the equation for the traction force becomes:

"traction force" = Coefficient of Friction (aka traction) * Normal Force (force over entire area of contact patch)

The normal force is simply the force the ground applies on the tire (think Newton's 3rd law). The weight the vehicle applies to the ground is then counteracted by the ground. This force is dependent on the tire pressure and weight of the vehicle. The higher the tire pressure, the less the contact patch, and the larger the normal force. Conversely, the lower the tire pressure, the less the contact path, the lower the normal force. However, this can be a bit misleading: the lower the pressure, the more contact with the ground, and therefore the traction is applied over a greater area. This is great for off-road. There could be a mostly slick surface but a larger contact patch means we are more likely to find small patches on high traction surfaces (high coefficient of friction) and therefore able to keep going. This is why it is always generally recommended to air down.

Finally, the point of rolling resistance. In rubber tires, the rolling resistance is mostly from the deformation of the contact patch as the tire rotates. This deformation of the rubber takes energy which is converted from the motion of the tire into heat. The higher the tire pressure, the less contact patch, and the less deformation occurs, the less heat is generated. At an extreme example, this is one reason train wheel (steel) and much more efficient than rubber wheels.

Conclusion: So if you made it through all that (thanks for humoring me), I would say the argument comes down to whether it is better to have in an off road scenario an elongated contact patch or a wider contact patch. Probably depends on a case-by-case basis, but also may be negligible in 80% of off-road scenarios encountered by normal folks. Perhaps when climbing the ledge of a rock, a long contact patch is preferred as it will "wrap around the edge" of the rock once it gets to the top, like a tank track. But then again a wider tire can grab the edges of a rut or if the tire is in a gully of sorts or wrap around the sides of a rock.

In extreme loose or slick surfaces (mud) where the tire is slipping, the skinny tends to cut more, as the contact patch (even though total area is the same) is more oriented in the direction the tire rotates, taking material from the front and flinging it back. As the tire moves forward through the terrain, the frontal "bite" of a skinnier tire when slipping is smaller. This can be an advantage or a disadvantage depending on the scenario. If the slick surface is shallow and has a hard pack beneath, it probably helps. If not, it will probably dig until the chassis sits on the ground. Not fun.

Like in most choices made in vehicles and life, every decision is a compromise with something else. We all prefer different looks, wheel in different terrain, and use our vehicles differently. You make your choice. I was just hoping to dispel some misconceptions.

Disclaimer: This is by no means an exhaustive analysis of what is better for off-roading. The explanation above is highly simplified for the sake of clarity. As stated in the discussion, there are many other factors besides the above to consider (and have been discussed) and there is no "this is always better" sort of answer.


Open to constructive criticism, questions, etc.

 

Thanks for the thorough post! This makes some sense to me, though physics was never my strong suit. Honestly, I went with skinny tires because I wanted E load anyway and a slightly larger size than stock without needing to trim or otherwise prevent rubbing. I figured those reasons would help me in all scenarios. Except maybe my MPG...

 

Thank you for the excellent write up

 

Thanks. I read very similar several years ago and it related contact patch deformation when airing down to sidewall height (taller sidewall of a skinny tire allows deformation to a longer contact patch)

 

Hey y'all- just had a shop (reputable local off road shop mind you) tell me that 255/80s are too narrow for my Method 705 wheels (17x8.5; zero offset)... namely "it is too narrow to bead up" and "you don't want a tire that takes extremes to fit." They are pushing me into 275/70s (or larger). Anyone else have this experience?

 

What tires specifically? Tire manufacturers list out rim widths acceptable for the tire.

 

Wildpeaks. Suggested KO2's in 255/75 if there was something weird with the Falkens but they were a no go on those too. Pretty sure both are listed for rims 6.5-9.0 (which is what made me think they would work with 17x8.5s ).

 

I had the same situation with an offroad shop. They likely dont have a bead blaster. I just went to a tire shop and had them mount them then brought it to the offroad shop for the alignment.

Wildpeaks in 255 80 17 E loads are 6.5-8.5

 

Directly from Falkens website (assuming you are talking A/T3W's), Discount tire has them listed up to a 9" wheel but I would go by the manufacturers width specs.

https://www.falkentire.com/tires/light-truck-suv-cuv-tires/wildpeaka/t3w-tire

28030332 LT255/80R17 E 10 121/118S 6.5 - 8.5 7.0 33.1 10.0 632 17 52.5 3195 / 2910 80

I would tell them if they arent going to honor the tire manufacturers specs you will take it elsewhere and then follow through if they wont mount it.

 

Roger that- I was going off the Discount Tire data, which explains it. Thanks.