255/85 R16 Owners Experience

15psi to break in the sidewalls. It felt like I was riding my old 110 Honda ATC from back in the day ... bouncing around on a beach ball. I'm also not used to having such a large tire and such a huge amount of sidewall. I ran my Duratracs at 20psi.

Even at 15psi the sidewall was folded under the tire. I don't think I would have the balls to go any lower then 15 especially in rocks







Just so everyone knows I'm not trashing these. Last time everyone asked me for opinions of the Duratracs I gave it rave reviews and local guys got them. Then they were unhappy and I felt like an asshole. This time around I'm stating every issue I have with the KM2's so people know the issues. No punches pulled on these reviews.

 

Why did you have to keep adding air to the front driver's side tire?

 

Discussions like this kind of annoy me. Research, people, research.

Wider tires are always more versatile overall. There are a few select instances where pizza cutters may, but are not guaranteed to be, better.

Pressure is force over area, so while having a smaller tire contact patch at the same vehicle weight leads to a higher pressure of tire-to-surface, more pressure doesn't automatically mean more traction.

Traction is essentially the coefficient of friction between the tire and the surface it is on being sufficient for the truck to move forward:

http://en.wikipedia.org/wiki/Friction

As you can see, there are three types which are immediately relevant to offroad use.

Narrower = worse in sand, snow, and mud. The higher pressure causes the tire dig out the substrate rather than move across it. This is why larger tires aired down are almost always better in rock crawling as well; there is a larger surface area of tire on the rock, which will lead to larger amounts of friction as long as:

- Power (i.e. force) from the engine increases linearly as well

and

- The tire compound is soft enough to deform around rock irregularities.

Pizza cutters will only prove to be better suited on rock when a larger tire is not aired down far enough to deform correctly over the rock features; this would be because the slim tire's greater pressure on surface causes more rubber deformation than the un-aired larger tire, which doesn't deform because air pressure is causing it to hold its shape.

Where slim tires are most likely to help is in the lubricated friction area, where the tire has to dig through a limited layer of something (mud, dirt, sand, snow) to get to a higher traction surface.

If the layer is deep enough to get you stuck, pizza cutters will simply get you stuck faster.

This isn't to say there aren't other advantages to slim tires. Less rolling resistance usually = better mileage. Lighter weight means less risk of drivetrain damage and again, better mileage. Some people like the look. They're usually easier to clearance at a given diameter.

There's a reason you don't see Hammers rigs on Pizza Cutters. They need to drive well in all types of terrain, and that takes fatass tires. The advantages are many, the drawbacks few. Narrow tires have their own advantages, they're just not as offroad applicable.

I remember reading from a BFG staff member on some forum somewhere that 20ish PSI was the sweet spot for KM2s. I run 18 to 20, and that does seem to be best.

I tired 14 and 16 a few times, all I got was more tire damage with no noticeable change in traction.

Just food for thought.

 

Mr. Oswego!, love the review, I think I'm staying with my Duratracs. I can't afford 2 sets of tires or I would go with the KM2's as a "summer" tire, and keep the 285 Duratracs for winter, but I like my wife and my nuts(as unfunctional as they are now, just got "fixed")! So one set at a time!

Keep up the no bullshit attitude, it really keeps "mis-communication" at a distance!

 

Cut up the quotes, not the red text. It makes it harder to argue with you

Oh yes, if someone's applications are different, they can be the right tire. All I'm saying is wider tires make better generalists. A lot of people aren't generalists.

I would also note - your sand, snow, dirt etc. is most likely loose stuff over top of harder packed stuff, right? That's where pizza cutters are ideal. They get to the good shit faster...it's when the good shit is deeper than your clearance that it becomes an issue.

No argument. Wider tires may be more forgiving to bad drivers? Explains why I use 'em

Oh yes, if someone's applications are different, they can be the right tire. All I'm saying is wider tires make better generalists. A lot of people aren't generalists.

Yeah. I think there has to be something to those "linear flex zones" BFG keeps pimping on these things. The sidewalls do flex very well. These tires seem to do better at higher PSI than most of the competitors, which I don't mind one bit - more clearance


FWIW, I'm sticking with 285s for my next set but adding an inch of height (BFG now makes a 285/75/17 KM2, which is a 34" semi-pizza-cutter). I'm hoping to be able to clearance 34s without losing the fender flares. Am probably going to have to re-route my shock reservoirs and live with some frame rub though.

 

NJ was a beach millions of years ago so the sand and mud have no limits. SNJ is packed with cranberry bogs (perfect conditions) and sand quarries (perfect conditions)...snow all depends. Worst snow I drove through locally was about 25" with OE Firestones 3 years ago and I was fine. You can dig 50 yards down and still hit sand.

My typical light day wheeling (during a semi-drought yesterday solo) normally this time of season the trails are flooded not just little puddles like below


and any hills we do have are all sand with washouts everywhere


All the local guys busted my balls for going skinny but so far so good. In a month or so Ill see how they do in all rocks.

 

Looking forward to further reports.

Man, you guys have too much damn water up there. Had any actuators shit out yet?

 

i added air 1 time all day and it was at 4 instead of 6psi...6 was to low...on several occasions my rim was touching rock with the tire completely folded under the inside of rim..lol

 

i was heading up a long steep up hill like 400-500yrds id say with no where to stop or turn around...i got about 50 yards up and started sliding and spining,controlled but unsettling as i was still at the bottom...i was at 18psi....several jeeps with me were nervous and asked if i wanted them to hook a strap to me incase i slid...several were also running km2's and were all at 10psi...i stopped and dropped to 10psi myself and never spun the rest of the way up....crazy difference i traction from 18-10psi....
different psi are required for different terrain...i feel on my loaded 4door,armored,with bumpers and gear 14-15psi is ideal in rocky and offcamber stuff sand id run 8-10psi without worry unless ur doing high speed runs or jumping
if your in a lighter rig like a reg cab 10psi would be fine on rocky terrain...bottom line is run what your comfy with but if your stuck somewhere dont be affraid to drop a few psi to help get you out

 

Had my 255/85 KM2s for a month now, so far they have been great! Because i have the 6speed, i actually like the slight difference they have made for my gears. Before first gear was to low, now It seems about right.

 

And more food for thought:
http://www.expeditionswest.com/research/white_papers/tire_selection_rev1.html

Oh it was posted in the first page of this thread.

 

Informative thread, for the most part

I've always liked 255s in Midwest snow, not so much once I hit mud though as it's usually deep enough to bury the frame/bumpers where we used to go.

Running 265s for my last two rigs and don't personally see a reason to go any wider other than wanting to lose mileage/look beefy. I've always favored function to fashion. Were I in deep mud or sand more I might try a 275 but snow is the flavor of weather/terrain most prevalent here so 255 will be my next buy again.

All opinions here, but the facts stated throughout speak for themselves

 

I really wish they made a 255/80r16 or 255/85r15 All Terrain btw would really suit my needs best!

 

Nope. My DT Y pipe keeps it nice and warm + I'm in 4x4 every week so it has no chance to sit and corrode.

 

I can see you posted the article on friction while totally ignoring it. I'd like you to show me where in that equation where the surface area rears its head.

Friction is down force times coef of friction. That is all. If your tire and the earth substrate can support the additional traction you're trying to use, it is superior.

The crux of your argument is that flotation is more important than friction for most people, generally. In doing expedition travel and general offroading I have found this NOT to be true.

VERY rarely am I going through mud which is bottomless that I somehow have to stay on top of.

On sand and snow, I have noticed superior traction in every situation I have put it in over wide tires. This past winter I have a friend with a diesel on 35's which are super wide, and winter tires to boot. He made it up about 70% of a deep snow covered mountain. My siped KM2s blasted right to the top.

ALL the actual science I have been able to find supports narrower tires being more versatile and better in nearly all situations.

This has been posted about 50 times in this thread, but look:

http://www.expeditionswest.com/research/white_papers/tire_selection_rev1.html



More fiction from adhesion, more from mechanical keying.

The only areas I could wrap my head around wider tires being better is bottomless desert sand, snow etc. Yet every single expedition vehicle for documentaries etc use super tall and skinny tires. As does the military.

From that article:

Basically, if you can get a higher diameter tire by sacrificing some width (which is usually the case here as only 2.5" or so lift needed to run these 33's) you will come out ahead every single time.

While I think a wider tire could be the better tire for sand/snow/mud as you say, and the science can support that, for some reason in the real world the people who have to choose tires their lives depend on in places like the middle east and sahara, seem to pick tall/skinny tires.

 

Except the whole theory of increased mechanical keying is based on the often-fallacious assumption that the "rough surface" the vehicle is offroad on will deform tire rubber.

This is only true of rock and hardpack dirt, and essentially doesn't apply to any other offroad surface. If it's soft dirt, sand, mud, snow or any of those on top of rock or hardpack, the surface will give way under the greater pressure. If you reach the high-traction stuff before framing out, great. If not, you're boned.

To put it in that article's words:

This is a bigger problem than people realize.


Also this picture is misleading:





This is an air pressure problem. Vertical load may be the same, but two things are not taken into account:


- rubber compounds are not. Some compounds deform more readily than others. That's why some tires are more "sticky" than others. Ever notice the difference in grab between racing slicks and typical street tires? Yeah, compound blend.


- Air pressure is not. The 12.5" tire could easily lose a few more PSI and have equivalent deformation to the narrow tire. With improved lateral stability, and the ability to float better over surfaces requiring flotation. Also, this view is head on to the tire. Last I checked, tires don't spin sideways. A wider tire provides a contact patch that is of equal length to the skinnier tire (assuming all other variables constant and same diameter) but also wider. This improves the odds of "keying" successfully given that the tire is searching more surface.




I maintain my original position: Wider tires are more versatile.


What tire is better is up to the operator to decide based on their application.

Incorrect. The crux of my argument is that a wider tire can adjust air pressure to achieve equal deformation (assuming equivalent rubber compound mechanical properties) to a narrower tire, and can always achieve better flotation due to size advantage.

The risk, primarily, is increased risk of losing a bead if running extremely low air pressure. This can be mitigated in a number of ways, from beadlocks to glue to careful driving.

Surface area isn't relevant to friction, surface area is relevant in its ability to envelop more irregularities of the surface being driven on and improve odds of successful keying, and distribute keyed forces. So yes, I misused the term in my original post. Good catch.

Keying is great if it can withstand the shear forces directed onto it. Decreasing shear force on individual keyed elements (i.e. spreading said forces out amongst more tire elements) is an advantage as combined forces may allow sufficient resistance for the vehicle to move without the tire breaking traction.

Too many confounding variables. Different vehicle, different weight, different tire, etc. Was this snow covered road (lube over traction, which I have been saying from the start narrow tires are preferred for, all other things being equal) or just bottomless snow? Just curious.

Oh I'll always take clearance where I can get it, but I'm assuming for the sake of argument diameter is constant.

Is that an incorrect assumption, or are we adding a variable (and it's myriad associated advantages/disadvantages) to the discussion?

 

Yes but you're saying you can air the tire down to achieve the same results. I don't like this argument simply because you can air the tall n skinny down the same amount for even BETTER results at the same PSI. You will run into bead slippage at similar PSIs etc so given the same tire pressure, the tall n skinny will key better with the terrain.

Comparing different tires with different compounds is a totally different argument. For any comparison like this you have to keep things fairly equal.

See argument above about deformation. In the article they talk about having a much larger percentage gain from a tall n skinny vs a wider flotation tire due to most of the patch increase being due to increased length not width.

As to the head on view, you probably realized by now that this applies in all directions. No one drives on triangular lines going forward. Pretend it is a cut away view of a tire driving on pyramids.

I agree about your 'odds' theory but again at same PSI in my admittedly experience, lower PSI improves traction faster due to keying etc rather than just having a few percent more surface area.

The only reason I kind of disagree with this is the anecdotal evidence of thousands of desert overlanders, and almost all millitary vehicles who need maximum versatility in so many changing situations. They all seem to nearly invariably gravitate towards tall and skinny tires since you can fit a taller skinnier tire in the same wheel well with less modifications/lifting, etc with all other things equal.

I reiterate that I believe diameter aired down to same PSI will give you better performance, and comparing a wider tire aired down more to try to match the advantage of a taller skinny tire is NOT keeping all things equal. If you have to try a bunch of things like using softer compound and further airing down a wider tire to try to keep up with the taller one, you are really arguing for the taller tire.

This is an interesting argument I'll have to think about more. I only have my own personal experience in very low sheer strength materials that the skinny taller tire has outperformed the wider by a good margin.

Bottomless snow. I was floating much better admittedly probably in part due to a lighter truck. But his tire size WAS proportional to the vehicle other than mine being tall/skinny and his lower aspect and larger overall.

In snow that isn't just pure powder I seem to float much better than several other friends, one of which in a tacoma with wider tires. I just seem to get better performance aired down (to the same PSI) than they do.

Well in a way you have to add a variable yes. You can have a good bit taller tire in a much less modified truck as compared to a wider tire.

A 2.5" lift in a Tacoma is easily enough to fit 255/85R16 KM2s.

I will take 33" skinny tires over 31" wider tires any day for overlanding(which is very varied in its needs) which is basically what you can fit in a 2.5" lifted truck. Once you go past 2.5" you start to really increase your costs, and lower your reliability.

This is the whole argument for TALL and skinny, not just skinny.

You have more clearance and diameter for hard surfaces, You gain more contact patch per PSI when airing down, and it costs your truck and you less effort, time, and peace of mind.

I consider it an added advantage though rather than something needed to make the higher aspect ratio tire better.

EDIT: Also, another thing I thought of is, off-road a higher aspect ratio tire will have a higher sidewall which is almost always a desirable plus. Of course the disadvatage is as mentioned of lateral stability, but this is usually much more a concern on-road than off. And subjectively my 255/85R16 KM2s have been astonishingly nice as far as road manners. This sentiment seems to be echoed among the users, at least of THIS tire. Can't speak for other tallies of course.

 

Sure, but what are you going to do when you need flotation? There's an upper and lower limit.

Patch length only increases if *diameter* increases. If diameter of the tire is the same, patch length is the same assuming equal tire deformation. With a wider tire, however, you have increased patch width which also helps.

Agreed, and my last response applies here as well. Going skinnier is nice if it gives you more diameter. However, if you've reached the upper limit on diameter, I would argue you'd be best suited to get the widest tire you can clear at that diameter.

I think the issue here is one of application. Most military and overland vehicles are not actively seeking the hard obstacles. They are looking to get where they are going. Overlanding is super fun but NOT the most demanding offroad challenge for a vehicle. Exactly what is the "most demanding" is open to debate, but it's fair to say you can't overland a rock buggy, and overland vehicles will not make it up buggy lines. At some point or other, things invariably diverge.

Side note on this one: I think it's also worth noting here that many times tire choice, regardless of width, won't be the limiting factor in a vehicle's capabilities. But that's another thread.

I will again reiterate that there's no difference in tire diameter or sidewall height that I'm discussing here. Tire width only.

And that is a fair point - and what I'm getting at when I say "best" varies by application.

Again: I think you can do a wider variety of things successfully with a wider tire at a given diameter. I think you can do a smaller variety of things better with a skinnier tire. In other words, specialists who particularly love certain types of terrain will benefit.

Agree on tire choice and costs, but I question the reliability claim. I'm having no notable trouble out of my 3.5" of lift with very hard use.

Agreed about increased sidewall being desirable, but that's mainly a factor of tire diameter v. wheel diameter. 285/75/16 and 255/85/16 have the same amount of sidewall, give or take a fraction of an inch.