Dually Offset tire question
 

Hey guys I'm new to the whole Diesel world as well as a dually. I have herd some of the local farmers talking about running a smaller inside dually tire to save on some tire wear and a small amount of fuel mileage gain until you put a load on the rear end causing the outer duals to settle and allow the inner tires to make contact with the road and support your heavier loads. Has anyone tried or herd about anyone doing this? If so did it make a any difference in tire wear ect? I am currently running 235/85/16s was thinking about going to a 235/75/16 which is a little less than a inch smaller on the inside duals but don't want to waste the money if this is all just a old wise tale..

I used to work in the commercial tire business and I've never seen or heard of such a thing. :c:

I ran a smaller tire on the inside by mistake and it wore the tire out quick as shit. The smaller tire spins faster and therefore rubs the ground wearing the tire out.

ok thank you for the quick replies. I'm gonna take it as old wise tale. thank you for the info. Gonna save some $$$

The wheels are bolted together and therefore one cannot spin faster... so many rpm.

That's what I thought too. But if you hold a 20 foot rope and spin in a circle. The 20 foot mark spins way faster then the 10 foot mark. And having done it myself I speak from experience

this is a horrible idea. to prove it, roll a base ball on the ground 10 full revolutions, then roll a basket ball 10 full revolutions. Different distances right? but if you bolt the baseball to the basket ball then one of the two (usually the baseball) will scuff on the ground causing quicker tire wear and lower fuel economy due to more rolling resistance. now replace the balls with tires and there ya go. hope this helps.:tu:

Tire speed is a function of rpm and radius. The rpm is the same for both tires.

If both tires are in contact with the ground, then regardless of tire size, they have the same rolling radius.

Therefore: The LARGER tire will wear faster (from carrying most of the load), But less fast than if the smaller tire were removed completely.

For the baseball/basketball thought experiment to work, there must be two sets of them connected by a concentric axle.

I'm simply stating facts that i learned while doing alignments and tire service on semi trucks. there is a reason that they have the same size tire. how much weight is on the tire hos very little affect on wear rate. the reason the smaller tire will wear quicker is because is has a smaller circumference than the larger meaning that per one revolution it has to travel farther. since the larger tire will have most of the weight on it and there for, more traction, the smaller tire will be scuffing on the road eating miles off its life. I know that i am right due to real life experience. I'm sorry if you disagree.

There are many reasons why tires wear at different rates. It is Not because one tire travels further...It does not ...it can not.

Conclusions derived from real life experience are not facts.

okay well you obviously don't know basic geometry and probably have no real life experience if you honestly think you're correct. you have me and one other person saying you're wrong. both of us have actually been there and done it instead of just thinking we know what we're talking about. If you had some sort of prof or logical reasoning behind your statement i would take you more serious. circumference does have something to do with tire wear when you have two dissimilar tires mounted together. the small one will be dragging to make up for the difference in circumference.

You have to measure the radius as the distance from the center of the wheel to the ground, and not from the center of the wheel to the top of the tire.

If both wheels are touching the ground then their effective radii are equal. Therefore, there effective circumferences are equal. (from basic geometry C=2 pi r)

There is no difference in effective circumference and therefore one tire is NOT dragging as a result of it.

For the purpose of this experiment - If you never turn:

The tire with the larger diameter will be carrying a larger share of the load and will therefore deform more and wear more... But more than what?

More than the smaller tire... Yes.
More than itself if the smaller tire was completely removed... NO!

Okay well think what you want. I know I'm right.

I want to say this respectfully but you're full of it :c:

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:s::s::s:

What tiny fraction of my LOGICAL argument do you disagree with. Show me ONE fact I have wrong or one conclusion that I got wrong. You see, I have proven my case using Logic. Can you find one error? If you can point out where you are having difficulty, perhaps I can localize on that one specific aspect and explain it better.

I have been very careful to only take exception to the following statement.

The smaller tire spins at the same rate as the larger tire. It is measured as function of angular displacement per unit of time. RPM, degrees per hour, radians per blue moon, whatever you want.

Perhaps you are thinking of the speed of a spot on the surface of the tire tread? (relative to the axis)
That would be rotational speed(rpm) times the radius. Do you not belive that the radius is the same where both tires hit the ground? Do you not believe that the radius of the tire changes as it contacts the ground?

Can you precisely pinpoint one relevant flaw in my argument?

the radius may change but circumference doesnt meaning the larger tire will always roll farther per every revoluting meaning the smaller tire (which doesn't roll as far) scuffs the ground to make up the distance.

you're wrong. part of being a big boy is realizing when to accept that fact.

I miss spoke. The small tire spins slower. The larger tire has a bigger footprint on the ground giving it more traction. The smaller tire will just scruff the ground as it passes over it. But don't take my word for it or experience. Go buy yourself some different sized tires and see which ones wear out faster. The smaller tires I had on my dually went bald in about 4000 miles. They were on the inside so I didn't notice until I went to get them rotated. I think we're way past geometry and probably into physics, but what do I know.

OBSPowerstroke7.3
Circumference = 2 pi r
You have just stated that this is not true.
If the radius changes, but the circumference does not then:
Circumference (does not =) 2 pi r?
Are you sure you want to say I am and therefore Archimedes was wrong and that if you divide the circumference of a circle by it's diameter you get something other than Pi?

jamesrett... ahhh physics (the F word in my house)

Yes! Yes! Yes! I have no reason to doubt that a smaller tire on a dual would wear out very quickly. but not because it is traveling further. The larger tire bears more weight and therefore has more traction...yes this is true, I never said it wasn't. In fact It makes perfect sense that that is why the small one wears out quickly.

When A vehicle turns, the outside wheel on any axle travels farther than wheel on the opposite end of the same axle. This is also true of a set of duals mounted together on the same end of an axle... This is also true of the right side and the left side of one single wheel.

One side of that single tire travels farther than the true average distance and therefore scuffs that side of the tire in a braking fashion (like dragging the vehicle with the brakes locked) and the other side of that same tire is traveling less far than that same average and is therefore scuffing in the opposite direction (like spinning while trying to tow an immovable object). Somewhere in the middle of that tire (in a turn) is a neutral line that is not scuffing in either direction and that represents the true distance traveled. The further you get from that neutral line, the worse the scuffing.

In a dual set-up, that neutral line is normally between the duals and one whole wheel scuffs one direction and it's mate in the other. (dual wheels scuff in corners more than singles). If you mismatch tire sizes on a dual, you move that neutral line away from the center towards the tire with better traction (the larger tire). So the larger tire will scuff less to some degree (more like a single wheel)and the smaller tire will scuff way more.

Let's take this experiment to an extreme. single wheels mounted on a welded posi-traction rear-end. In effect one giant dual wheel. How much scuffing goes on when you try to turn one of those?

So yes I can believe that mismatched dual tires would be hell on the smaller tire...Because of the turning.

I think your assuming that just because both tires are touching the ground that they have the same radii. Now one tire being bigger has a larger rolling circumference. So if I understand your logic the bottom of the tire spins at the same speed but the upper part of the tire spins faster? How is that possible. Ill go back to my original analogy of the 20' rope. I would rather be hit a ten pound weight on the end of a one foot rope then a twenty foot rope. That sucker would be moving fast. My head hurts.

The radius does change. the top of the tire is moving faster. the radius changes when the tire is deformed by the road. Since the radius changes, the circumference must also change and that is why you must use the radius measured to the ground. It is what matters. that is why you must calculate the circumference from the radius measurement and not measure the circumference directly. If you had a mechanical device that pushed against the inside of the tire every time the device got to it's apex it would increase the radius measured there and therefore increasing the speed of the top of the tire and if it released before coming down, Your distance traveled does not change Because the radius measured to the ground did not change. Even a single tire decreases it's own radius when it contacts the road. A radius measured from anywhere else is irrelevant.

The top of the tire is moving faster than the bottom. Not because the angular speed(RPM) is changing, but because the radius is changing. Speed of the surface of the tire = angular speed (RPM) times distance. Decrease the distance (by altering the radius as it contacts the road) and the speed must change.

twinboys = FAIL :pca1:

here is an idea for ya
 

DO what i do Run 3/4 ton front rims (utra goliath 3/4 ton fronts) and ultra goliath dually rear Attachment 26794

Attachment 26795rims ...

tiremann9669 YOU HAVE BEEN CHALLENGED!
 

Really? After all the facts that I have presented?

What do you call someone who cannot disprove facts and instead chooses to IGNORÆ#% them?

I challenge you to pinpoint any small error I have made (fact or logic) that debunks my conclusion.

You're full of crap algebra and all, seen it with my own 2 eyes so you can talk all your formulas and crap untill you run them you're full of it. Draw a line on both tires directly at the bottom pointing to the ground. Now have someone move the truck ahead till the outside tire has made one revolution and line is back pointing straight at the ground. Now look at where the line is on the inside tire and try and tell me you're not scrubbing it along the ground. :c::c::c:

I don't undersand your experiment. I'm pretty sure the line on the inside tire will also be pointing down?
I am intrigued. Please explain.

Are these different size tires? same size tires? tell me what happens.

Not hardly dont you understand circumference ? They are different size tires, do you even know what this thread is about

Ok so, I chalk the side wall of the two mismatched dual tires vertically in the 6 O'clock position. my partner pulled the truck ahead until the outside chalk mark is once again in the 6 O'clock position. When I look at the chalk mark on the inside dual, it too is again in the 6 O'clock position. Did I do this right? I'm not sure what result you are expecting.

"I am not scrubbing it along the ground." (I assume you mean one of the tires)
Am I supposed to see some indication that one of the tires did scrub?

When you do this is the chalk mark on the inside dual somewhere besides the 6 O'clock position?

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Here is the "thought experiment" that disproves my theory:

According to my logic, the distance an axle travels per revolution is equal to the calculated circumference of the tire. Calculated from the radius measured to the ground. this measurement changes according to amount of air in tire and the amount of load to squat the tire.

Since a speedometer measures rotation per unit time, and not the actual distance per unit time, the speedometer's accuracy must me dependant on how much the tire is squatted. But, a speedometer is independent of the load and/or tire pressure.

Dude were talking about having two DIFFERENT SIZE tires just because you have a truck with mismatched tires are they different sizes ? Anyways I'm done arguing with you obviously your book smarts are way ahead of me, you're just lacking in hands on sense.

I know this is an old thread but I couldn't help being interested.


I found this an interesting theory and saw it posted on another forum as well...smaller inside rear tires acting as a load bearing set only under high load, but not creating any friction (or wear on the inside tire) for lightly loaded conditions.


I also followed along as people with real life experience say they saw the smaller tire "scrubbed" and worn fast in real life experience with tractor trailers having tires with dissimilar wear next to each other......the smaller diameter tire gets destroyed fast they say.


Twinboys here tried to use diameter/radius to explain how they would not scrub, saying the larger tire has the same effective radius as the smaller tire because it is being compressed under load more, and he is somewhat correct about that.


Drawing a line on either tire and watching them rotate wont help. As Twinboys pointed out, both lines on both tires will travel the 360 degrees of a circle together...every single time the wheel turns.


But the factor nobody considered is surface area of the tread of the larger tire compared to surface area of the tread of the smaller tire. The smaller tire will travel a given number of inches in one complete revolution. This is the circumference. Lets call that circumference "X inches" of travel.


But the larger tire will have a little more rubber to go around a single time. If you measure its circumference it is bigger...likely by several inches...than the smaller tire. Lets call it's circumference the sum of what the smaller circumference is (X inches) plus some amount since it is bigger the difference between the smaller circumference tire and larger in inches is Y.


So...
Small tire on inside = X inches of travel in one revolution (circumference)
Larger tire on outside = X inches + Y inches


Lets take two examples:
285/75/16 has a circumference of 103.1 inches
285/80/16 has a circumference of 106.6 inches


Lets say the first of those two was the inside tire, and the second is the outside tire. SURE, under heavy load the taller outside tire's sidewall will flex and the tire will squat more than the inside tire.


BUT no matter how much the tire sidewall squats, the outside tire still has 106.6 inches worth of rubber to travel in one revolution. It WILL travel that far because it is the heavier loaded tire.


But the inside tire is loaded less, and will only travel 103.1 inches on its own tread...and since it is loaded lighter it will literally drag another 3.5 inches EVERY single time it goes around if it is touching the ground...it has no choice but to cover 106.6" too.


This is why guys with experience in the field see the shorter, more worn of two side by side tractor trailer tires literally eaten away FAST.