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Old 12-15-2013, 12:53 PM   #15
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One of the worst, if not the worst, situation for winds affecting the tt, is the bow wave of a big truck passing our combination vehicle on a steep downgrade. The "taut" alignment of ones combination vehicle is loose or sloppy

Pairs of big trucks while on a narrow two lane, passing in the opposite direction, is another. While the alignment of ones vehicle may be taut, the high speed winds -- which are also suddenly reversing -- are a handful. A dangerous one, at times, in both instances.

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Old 12-15-2013, 01:55 PM   #16
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Something to consider is how the TV on its own reacts to various gusting and other wind conditions.
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Old 12-15-2013, 05:38 PM   #17
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All this talk about low profile tires makes me wonder if the idea of a short sidewall is such a good idea for loads and towing, why are bus and truck tires ( big trucks ) still designed and built as tall profile ?
Perhaps one of the tire engineers will comment on that ?
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Old 12-15-2013, 09:19 PM   #18
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All this talk about low profile tires makes me wonder if the idea of a short sidewall is such a good idea for loads and towing, why are bus and truck tires ( big trucks ) still designed and built as tall profile ?
Perhaps one of the tire engineers will comment on that ?
To carry loads requires a lot of material - rubber, cords, belts, etc. And heavy loads generate heat that must be dissipated. Low profile tires trade some of that ability for more cornering (higher g's) power. Of course, every tire must carry the load that will be on it. My very low profile tires are rated for 1790 pounds each. That's 3580 pounds for the axle, or 7160 for the car. The car's curb weight is 4000 and the GCWR is 5200. The tires then are capable of the load, AND offer a very high cornering force. Good compromise.

On a large bus, you need a tire that will carry that load. Do you need high performance cornering? No. So, there is very little reason to trade off anything to get a low profile tire. Better to make a tire that carries the load, dissipates the heat well, and has a long life. e.g. High sidewall tire with lots of steel belts.

In towing, we don't race around corners, BUT to reduce sway, we need the same kind of low profile tire in order to prevent the TT from forcing the rear end side to side. We don't want tall tires with soft sidewalls. Since our load is manageable with low profile tires, we can get that advantage and still carry the load.
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Old 12-16-2013, 07:07 AM   #19
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All this talk about low profile tires makes me wonder if the idea of a short sidewall is such a good idea for loads and towing, why are bus and truck tires ( big trucks ) still designed and built as tall profile ?
Perhaps one of the tire engineers will comment on that ?
I would have approached the answer to your question differently than MStephens did.

First is that industries don't make changes very rapidly - especially if it involves scrapping material. In the case of buses and big trucks, they have a lot of rims and associated equipment to fit existing tire sizes. Fleets with literally hundreds of vehicles aren't going to make changes unless there is an economic advantage to do so.

Then there is the packaging problem. Wider tires would need more space - and it's not like there isn't room as much as things would have to be moved (as in designed to be in a different location.). Again, with hundreds of vehicles, this is not something that takes place overnight.

But fleets do make changes and any a major change can be integrated by a phase-in process.

So why haven't trucks and buses gone to low profile tires? In some respects they have. In the old days, they used 100 aspect ratio tires (and for reference, passenger cars used 88 aspect ratio). Nowadays car tires are normally in the 60 series range with sporty vehicles using 45 and 40 series tires.

Today, trucks and buses dip as low as 70, but 80 is the norm. (There are replacement tires for duals with 65 aspect ratio, but that's a special case)

So what seems to be he holdup? How about need.

If there was a problem that low profile tires could fix, the industry would be on it. I suspect (and those of you who were over the road truckers should chime in at this point) that sway is just not an issue with big trucks and buses - and I suspect it has to do with tire sidewall stiffness.

First let's look at the height of a sidewall.

While people tend to think that changing to a low profile changes only one parameter, unfortunately that is not true. In order to maintain the load carrying capacity, a tire needs to go wider as it goes lower - and the net effect is only a small difference in sidewall height.

How about going to a larger rim diameter? Yup, that reduces the sidewall height, but the width increase is much greater - and this starts to get into an interesting problem.

As I change the aspect ratio - keeping the load carrying capacity the same, and therefore go wider - the tread width goes up as well - as does the belt widths. This means there is more of the most expensive part of the tire - and therefore more cost for the same load carrying capacity.

So what about sidewall stiffness? Not only does sidewall height have an effect, but so does inflation pressure. In larger trucks, the inflation pressure is pretty high, so perhaps the reason big trucks don't have sway issues is that the inflation pressure used stiffens the tire well beyond the point where it matters.

And one last point: I wonder if because the loads on big trucks are carried so high, that the limiting factor is the track width - and because of that, the sway issue is minor compared to the "tip-over" issue.

OK, one more last thought: If I wanted to reduce sway, one way would be to lower the center of gravity. In the case of big trucks, there are literally thousands of loading docks already set at a certain height and the trailers would have to be able to accommodate this. And don't forget that the common practice is to use the same tires on the trailer as on the drive unit.

So I'm thinking that what appears to be a simple change to a lower profile tire, becomes a huge change with very little benefit.
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Old 12-16-2013, 08:07 AM   #20
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The diameter of the tire/wheel combination is directly related to the final drive ratio of the TV. I noticed that the Grand Cherokee uses an 18" wheel on vehicles with the factory tow package vs. a 20" wheel for those without the tow package

Recently went through this exercise with a Model A and made several rear end gear changes to match the tire sizes and RPM's for max effeciency.
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Old 12-16-2013, 09:13 AM   #21
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Our Tahoe LTZ model came with 20 inch rims with the tow package. I would like to buy a 22 inch rim for it. The tire will be the Goodyear fortera sl with load capacity of 2601 lbs per tire. Higher than factory rating. Would I run into trouble with a 285/45/22 combination towing our 20 ft flying cloud?
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Old 12-16-2013, 09:44 AM   #22
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Our Tahoe LTZ model came with 20 inch rims with the tow package. I would like to buy a 22 inch rim for it. The tire will be the Goodyear fortera sl with load capacity of 2601 lbs per tire. Higher than factory rating. Would I run into trouble with a 285/45/22 combination towing our 20 ft flying cloud?
Check the circumference of the new tire. Unless it is different than the current tire, there is no problem specifically with going to 22" wheel. Naturally, if your circumference gets larger, your final drive ratio decreases. If your circumference decreases, your final drive ratio increases. That changes your gas mileage and the application of your torque to the road. Be careful.
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Old 12-16-2013, 10:21 AM   #23
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Our Tahoe LTZ model came with 20 inch rims with the tow package. I would like to buy a 22 inch rim for it. The tire will be the Goodyear fortera sl with load capacity of 2601 lbs per tire. Higher than factory rating. Would I run into trouble with a 285/45/22 combination towing our 20 ft flying cloud?
Also beware the larger the wheel the heavier they tend to get. More weight decreases HP to the pavement and affects stopping distances.
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Old 12-16-2013, 10:43 AM   #24
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Also beware the larger the wheel the heavier they tend to get. More weight decreases HP to the pavement and affects stopping distances.
Oh boy, you just mentioned a key thing. As the radius of the tire increases, the brakes have to work harder, and stopping distance may increase.
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Old 12-16-2013, 01:00 PM   #25
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The circumference is the same. The weight might be heavier though. I guess the increase in weight might affect HP? Or braking performance?
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Old 12-16-2013, 01:15 PM   #26
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Barry,
Thanks for the lengthy reply regarding low profile vs high profile tires on trucks and busses. Makes sense.

Now to try to apply some of this to our tow vehicles and travel trailers. I suspect that in my case of Nissan Frontier and SOB camper, the first lateral cornering limit is either lateral traction of the trailer tires, or the tip over limit of the trailer. So in other words, my suspicion is that there would be little advantage to going to a lower profile tire on the truck. They are, I suppose already considered somewhat low profile at 70 series ( 265/70-16 ).
One could likely correctly argue that an A/S trailer would out handle my trailer, but without doing a back to back comparo it's probably pointless to speculate how much difference. And yes, I have seen the videos where the SOB was run thru the slalom and got up on two wheels. Fun video to watch, but without knowing the entire story of that sequence, it's somewhat hard to judge what it all really means. After all, we have seen that any of the campers, A/S included can be rolled.

A lot of rambling here, but I think my point is, I am wondering whether the overall towing experience is really changed all that much by just changing to a lower profile tire on the tow vehicle. Perhaps doing so, and also going to a wider wheel and lower profile tire on the A/S would make a superior handling combo ? Who is going to first to throw a set of 8" rims with, what, maybe, 245 or so, 60 series on the shiney aluminium camper ?
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Old 12-16-2013, 02:53 PM   #27
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Now to try to apply some of this to our tow vehicles and travel trailers. I suspect that in my case of Nissan Frontier and SOB camper, the first lateral cornering limit is either lateral traction of the trailer tires, or the tip over limit of the trailer.
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I think the more common reason to use low profile tires is to reduce sway moments, not to increase cornering of the whole rig. Tall, flexible sidewalls make it easier for the trailer to push the rear of the TV around. Low profile makes it more difficult.
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Old 12-16-2013, 03:05 PM   #28
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Now to try to apply some of this to our tow vehicles and travel trailers. I suspect that in my case of Nissan Frontier and SOB camper, the first lateral cornering limit is either lateral traction of the trailer tires, or the tip over limit of the trailer.
==============
I think the more common reason to use low profile tires is to reduce sway moments, not to increase cornering of the whole rig. Tall, flexible sidewalls make it easier for the trailer to push the rear of the TV around. Low profile makes it more difficult.
Another reason (when staying with the same size wheel) is to improve gearing.
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