Why skinny tires

R_Lefebvre

Expedition Leader
you should go D at the most.

Why do you say that? I have E tires on my truck and love them. I can't even imagine running with less. The truck is very stable, even though it has "skinnies". My trailer tires are skinny C rated tires and... they suck, I'm sure glad they're not on my truck! I run 40-45 psi on the street, 25 on dirt roads, really helps the ride. If you run 80psi on the street... well yeah, that would suck, but you don't have to run 80 psi.

The spokes are always in tension! Try putting the bottom 1/4" of the spokes in, and sit on the bike. The spokes will buckle easily and give way. Try the same thing with only the top 1/4" spokes on... it will hold you up, but the rim will bend at the bottom... very similar to a car tire that is underinflated. Now, try to fix it by adding more spokes. If you add the spokes directly over the contact patch, it will not support the rim any more, they will just bend over as the rim deflects... however, add spokes in where the rim has bulged away from the hub the furthest, and it will act to stabilize the rim even more.

The tread of a tire is analogous to the rim of the bike wheel. The spokes are similar to sidewall. They support the tread/rim in tension only.

Tightening the spokes on a bike rim is the same as adding more air to a tire. You are increasing the tension in the spokes/sidewall. Sidewall tension is essentially equal to the hoop stress due to the pressure. Even the side wall at the contact patch is under tension due to the internal pressure of the tire. (Think of a balloon... as soon as you blow it up, the surface becomes in tension... now flatten it against a table top... is there any difference? Its still in tension!)

Sorry for the long response, especially since this isn't the root discussion here... I love physics related questions though!

Brilliant thought experiment! You're bang on! :wings:
 

Redline

Likes to Drive and Ride
load-range D Fan

I agree with you so often R Lefebvre, now I have an opportunity to disagree with you sir :)

I don't think you can fairly compare load-range-C trailer tires (assuming they are ST/trailer tires) or even LR C light-truck tires used on a trailer, to those used on a 4x4.

I'm in favor of running a load range D tire instead of an E (but not necessarily a C) if the tire I want is available in that load range rating. Load range D gives lots of toughness and plenty of load capacity for most 4x4s but with a more comfortable, compliant ride. Usually with better flex off-highway at a given PSI (and I'm not afraid to air down very low if needed) which can mean better traction.

Related to this, I will run what some may consider 'low' PSI for a given load. Though my tires are not under-inflated, they’re merely inflated with enough PSI for the load (I weigh my outfits often). 40 PSI would be lots of weight for me to put in any tire I run on my 4Runner (5,500-lbs wet weight, about 6,500 loaded for bear, but it depends). The only time I have run more than 35 PSI was when pulling some very heavy trailers with this 'light-truck' :sombrero:

Having stated the above, I'm not absolutely against a LR E if that is all I can get in a certain tire, which is becoming more and more common as many formerly LR D tires are going to E for heavy-duty diesel pickups. On a relatively softly sprung rig like my 4Runner, LR E is not so objectionable. If one needs ‘maximum’ puncture protection then maybe LR E is justified.

On a more stiffly sprung truck like my old '96 F350, I really prefer the ride of a LR D 255/85R16. At 65-PSI a LR D 255/85 will carry 3,000-lbs in a SRW application. The original/stock 235/85R16 tires carried 3,042-lbs at 80-PSI. I can live with the loss of 84-lbs of capacity and 15 less PSI. I’ve been running LR D 255/85 on the F350 instead of LR E 235/85 since about 1998.

Since I’m writing about the beloved 255/85R16, this must be an ‘on topic’ opinion in this skinny tire thread ☺

Why do you say that? I have E tires on my truck and love them. I can't even imagine running with less. The truck is very stable, even though it has "skinnies". My trailer tires are skinny C rated tires and... they suck, I'm sure glad they're not on my truck! I run 40-45 psi on the street, 25 on dirt roads, really helps the ride. If you run 80psi on the street... well yeah, that would suck, but you don't have to run 80 psi.



Brilliant thought experiment! You're bang on! :wings:
 
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daverami

Explorer
Make sure you are fully informed about the (excellent) Toyo MT which my full-time 4Runner doesn't like (in the Toyo threads/FAQs).

Yes, I have read all that you went through, how frustrating. In fact, I went down to Les Schwab and mentioned the problems you had to my friend who is the manager there. He stated that he has encountered that problem, but in his experience where he works it has only happened on Dodges.


you should go D at the most.

Some of the tires I have been looking at only come in a load range E for the particular size I am interested in. Funny but a similar size for a 17" wheel offers a load range D.
 

Redline

Likes to Drive and Ride
Both D and E will work, buy the tire you want and live with the load range :)

Some of the tires I have been looking at only come in a load range E for the particular size I am interested in. Funny but a similar size for a 17" wheel offers a load range D.
 

R_Lefebvre

Expedition Leader
I agree with you so often R Lefebvre, now I have an opportunity to disagree with you sir

I don't think you can fairly compare load-range-C trailer tires (assuming they are ST/trailer tires) or even LR C light-truck tires used on a trailer, to those used on a 4x4.

I'm in favor of running a load range D tire instead of an E (but not necessarily a C) if the tire I want is available in that load range rating. Load range D gives lots of toughness and plenty of load capacity for most 4x4s but with a more comfortable, compliant ride. Usually with better flex off-highway at a given PSI (and I'm not afraid to air down very low if needed) which can mean better traction.

It probably comes down to the vehicle in question. My truck has a higher CG and narrower track than your Dodge, so I'll take all the latteral stability I can get. It probably also has a lot softer suspension, so I can deal with it.

The trailer tires I referred to are truck C's, mounted on a trailer. I noticed that the sidewalls were really soft compared to my truck's tires, before they were mounted. Maybe that's the brand more than anything, as I looked at some other E range tires from a different brand, and they were also much softer than my Cooper ST-C truck tires.

That being said, I think it should be said that I don't believe sidewall stiffness has a 1:1 correlation to ride firmness. It's a factor, but not absolute.
 

Mash5

Adventurer
Thoughts after reading to page six…
I concede the following:
  • better where road tracks are narrowly rutted by Local vehicles, like in southern hemisphere.
  • Use less energy in terms of weight, rolling resistance and windage.
  • Essayer to get onto vehicle without extensive mods that may have negative repercussions elsewhere.
Nonetheless I favor wider for the following reason
as stated... For a given PSI and vehicle weight there is a given contact patch. Theoretically this is independent of width. HOWEVER, a wider tire can air down more for a given amount of "flatness" because it starts with a wider contact patch. In other words, all other variables held constant, if you air a wide tire down to say 2" of sag (Axel drop) and a narrow tire down to 2" of sag, the wide one will have a lower PSI and a bigger area of contact.

Now I think more contact is generally better off road. That is why one airs down. It averages out the rough, it reaches over more area to find that one connected bit of rock on the loose road, it bridges gaps in rocks and ruts in roads, and the wider lower PSI tire has more air volume to absorb vibration/impact with a lower "spring rate".

The fancy math tells us that it makes no difference how much area we have. PSI and area interchange until traction is constant. But I think in real off-road conditions more often than not there are reasons to get a hold on as much surface as you can because that surface is so irregular.

In concussion the race guys, the sierra rock guys and myself to some degree are willing to pay in fuel and weight and mod our stuff to run the hardest trails. Whereas the Expo crowd is more concerned with the stated drawback of the wide and are willing to trade max traction for better durability and economy.

:smiley_drive: Drive um if you got um
 

TeriAnn

Explorer
It probably comes down to the vehicle in question. My truck has a higher CG and narrower track than your Dodge, so I'll take all the latteral stability I can get. It probably also has a lot softer suspension, so I can deal with it.

Finally! after 15 pages of thread someone focuses on the reason why so many people can not quite agree given their experiences.

I think we are looking at a three legged stool, tyre physics, terrain physics and vehicle characteristics. I believe that you need to pick the tyre size range that fits and works best for a particular vehicle type and then for specific targeted terrains. You folks have focused on tyre physics and terrain but have mostly ignored vehicle characteristics beyond weight.

Obviously, since I'm a long time Series Land Rover owner this is the vehicle I'll use for my illustration. The Land Rover people who posted here came down on the side of tall skinny tyres, especially the Series folks. This is not an accident, it is what our trucks want in order to work best.

Series rigs don't have a lot of aftermarket wheel options and factory rims can be had in 5.5" (standard Series width), 6"(standard D110 wheels), 6.5" (Wolf wheels) and 7" (discovery I steel rims). Rim width narrows our tyre choices without going to a wheel shop to put new rims on our centres.

Why did the factory limit our choices with narrow wheels? A lot of that has to do with the width of the body and the wheel well space available. The story goes that since so many of the Series I trucks were sold in the UK to farmers and other country folks that people from the factory went out into the Scottish farm country to measure the width of farm gates to make sure the new Series II would fit. We ended up with a narrow bodied vehicle with a limited width wheel well. A lot of UK farm mud is not real deep so taller narrow tyres usually can dig down into firmer soil and just keep going.

The long wheel base 109's came standard with the 750/16 32 inch diameter tyres. A heavy duty version was available with a 2 inch lift at the spring attachments that used 900/16 tyres which depending upon brand were usually very skinny and in the 35-36 inch diameter range.

For us Series owners, the deep narrow wheel wells and limited wheel widths limit acceptable tyre offerings unless you are willing to make modifications that will affect the stock drivability envelope. You can go short diameter and wide by having wide rims welded to the wheel centres so the extra width goes to the outside, but your ground clearance rapidly decreases when your tyres are short enough to stick out the wing panels at dynamic maximum upward articulation.

The taller the tyre, the more apt you are to find the vehicle centre of gravity outside the wheelbase unless you fit wider axle assemblies and or wider rims with the extra width on the outside to widen the wheelbase. This is especially true for those of us with a very narrow wheelbase. For taller and wider tyres you need to lift the body and or enlarge the wheel arches to keep the wide tyres off the body. This is a lot more work than most owners who are not dedicated rock crawler types want to go through. And most people will hate the larger turning radius and increased difficulty of steering.

Because of our truck's design we naturally go doe tyres between 30 and 33 inches in diameter with a tread width of 9 to 10.5 inches. It is what fits our trucks best.

I think that unless you are wiling to toss in a lot on money and sacrifice some aspects of your truck's driveability the ideal tyre size for a truck can be found within the envelope of your manufacture's options for that truck. From there you can fine tune your tyre choice based upon the surfaces you plan to drive in. I think that will provide the best all around "expedition travel style" driveability and versatility.

Encase anyone is interested in my choice, my Dormobile is shod with 255/85/16 BFG Mud Terrains. These are about as big a tyre as you are going to fit on a stock LR wheel in a stock wheel well. I was slowly modifying my truck to accept 35 inch wheels when I noticed that 33.3" dia wheels were as tall as I could go and not have to hike up my skirt to climb into the truck. So for now at least I'm considering this tyre to be a best overall compromise. After all I must look good when I take my truck to the mall :) :) :)
 

Redline

Likes to Drive and Ride
That is a good reference suggestion. I've read Mr. Allen's book and have read his thoughts/pages on tires several times :)

I have been following this thread from the start. Very interesting on everyones thoughts.

Very good reading about this subject from a ASE Certified Mechanic who is known for his books and articles. Check out Four-Wheelers Bible by Jim Allen. ISBN 978-0-7603-3530-7

I will not give you my thoughts as this will not please everyone. I have not driven everywhere. I only have 37 years of 4-wheeling experience and as a certified instructor, I have learned some times to be quiet. Free information causes more problems then the paid for information.
 

silentsamurai

Explorer
To answer the original question and bypass 15 pages of mathematical debates. My truck came with 285/75/R16's and I like them so much for my style of driving that I will continue to buy them regardless for street, dirt, or rock. I Just know they will work for all three, and im fine with paying the extra $ knowing that. They are Mickey Thompson Baja's btw.

truck.jpg
 

alexrex20

Explorer
agreed. i don't even know what his post has to do with skinny tires, except that he's content with wide tires, after never even trying skinnies. :rolleyes:



but to get back on topic, here is a pic, courtesy of fellow ExPo-er Fergie.

ChangingColors9-29-09018.jpg


the look of skinny tires is definitely an acquired taste.
 

silentsamurai

Explorer
I didn't say it was pointless, and I DID read all 15 pages with detail. I just didn't feel that I had to add anything to it, nor did I want to. Yes, I've never owned skinnies, so technically I shouldn't be posting on this thread. But i'd never call out or belittle another member for such reasons. consider me out of this conversation.


On another note, thank you to all the people who posted the mathematical and in depth views on tire / weight ratio's. it was a great read.

Adam
 

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