15 inch tires on 14 inch axle

Anyone have experience or opinion on the consequences of putting 15 in wheels on a USED 2720 TM? I realize this would not increase the GVWR but the tires would be much less stressed. I have read the stories of tire failures but have not seen any axle failures. I plan on some really long distance hauls over the next couple of years. There is a site offering a packaged wheel and tire as follows:
Tire: ST225-75-R15BD Load Range D Load Capacity: 2540 lbs. at 65 psi. Bolt Pattern: 5 on 4-1/2. This tire is about 1.6 in larger in diameter and 0.3 in wider than the ST215/75R14. I also plan to add the 2.5 inch lift.

[RockyMtnRay]

Originally Posted by hingarfi

Anyone have experience or opinion on the consequences of putting 15 in wheels on a USED 2720 TM? I realize this would not increase the GVWR but the tires would be much less stressed. I have read the stories of tire failures but have not seen any axle failures. I plan on some really long distance hauls over the next couple of years. There is a site offering a packaged wheel and tire as follows:
Tire: ST225-75-R15BD Load Range D Load Capacity: 2540 lbs. at 65 psi. Bolt Pattern: 5 on 4-1/2. This tire is about 1.6 in larger in diameter and 0.3 in wider than the ST215/75R14. I also plan to add the 2.5 inch lift.

Your logic matches mine...no reports of axle failures; many reports of tire failures. And that not loading a tire right up to its maximum weight should substantially reduce the likelihood of a blowout or other catastrophic tire failure. I don't anticipate really long trips in my future but because I do a lot of boondock camping, I usually depart on trips with the trailer heavily loaded (very close to the GAWR, the GVWR, and the tire ratings). In addition, 2005 will be 4th towing season on my trailer's tires so this is a subject that I've been doing quite a bit of thinking about as I expect to replace the tires before the beginning of the 2006 towing season.

I've already discussed this with the owner of a local tire shop...who also owns a TM ...he hasn't yet retrofitted his trailer's tires either. One thing he did mention during our initial discussion is that you may not want to run the larger tires at their maxiumum 65 psi pressure, but rather at one that provides a sufficient margin of load capacity. If running at max pressure, there's a good chance you'll get excessive wear in the center of the tire so he suggested getting some tire chalk and using that to determine if there was too much air in the tires.

The big issue is whether or not there's room for the tire inside the wheel well when the suspension is fully compressed (as in being fully loaded and hitting a big bump). There are two likely points where an oversize tire would rub...at the very top and the sides. With the lift (which I already have), I don't expect the top would be a problem since it adds 2 inches (at least) of additional separation whereas the larger tire would only consume about .8 inch (half of the 1.6 inch larger diameter). Could be real iffy without the lift though.

Separation of the tire at its widest from the wheelwell might be a different story though. One potentially quick way to find out is to simply call TM and ask if they use the same wheelwell for single axle 15 inch tire models (like the 3023) as they use for the single axle 14 inch models (2619/2720). A not so quick way would be to load the trailer, lift one side of the trailer enough to cause the other side to incur increased suspension squat, then do some measuring and estimating (it's not a good idea to jack up the axle itself).

From a manufacturing (and supply management) viewpoint, it would be logical for TM to use the same wheelwell on all models (sized for 15 inch tires). However I have a nagging doubt about this since TM has been quoted as saying that the few 2720s that have been ordered with 5K Axles/15 inch tires had to be pulled out of the regular production line. Why, they didn't say, but one possibility could be a larger wheelwell liner necessitated special cabinetry work. If you haven't already done so, before I do any retrofitting of 15 inch tires, I will certainly be discussing the issues with TM and doing some measuring.

It's seldomly a straightforward process to use non-OEM equipment...a thorough look at all the engineering issues is highly prudent.

FWIW, I may go quite a bit beyond just retrofitting oversize tires...I'm currently mulling over also swapping the 3.5K axles on my 2720SL for 5K axles and, at the same time, fitting those axles with hydraulic disc brakes. The disc brakes will also require an electro-hydraulic actuator. Won't be for the 2005 towing season but just might happen for the 2006 season. From everything I've read, the stopping power of trailers with hydraulic disc brakes is enormously better than with the traditional electromagnetic drum brakes (like a factor of 4 in the deceleration rate). The difference in stopping difference between discs and drums in a panic stop from 60 mph is on the order of 100 feet...about 7 car lengths. In heavy traffic, that would likely be the difference between a destroyed rig with death/severe injuries and just a badly shaken set of nerves. Retrofitting with discs is, however, pretty pricey...I've been seeing prices on the order of $1200 to $1500 for a kit with discs/calipers, actuator (the really pricey part), and assorted lines.

[Frenchy]

You can get 14 inch tires with higher load rating. I bought Cooper tires last time. The dealer said they were a commercial type. They were 195R14C, Load Range D 106/104Q with a load rating of 2095 lbs at 65 PSIG. They fit and worked very well on my 3023 while on my trip to Grand Teton NP from southern Indiana.

[Denny_A]

Separation of the tire at its widest from the wheelwell might be a different story though. One potentially quick way to find out is to simply call TM and ask if they use the same wheelwell for single axle 15 inch tire models (like the 3023) as they use for the single axle 14 inch models (2619/2720). A not so quick way would be to load the trailer, lift one side of the trailer enough to cause the other side to incur increased suspension squat, then do some measuring and estimating (it's not a good idea to jack up the axle itself).

I had tire-wheel well clearance problems with my 2002 2720SL when it was new. Following my first 2400 mile round trip (included heavy rains and potholes hiding in muddy puddles in TN), I was dismayed to note that the curb side tire was grinding the aft portion of the rounded, plastic wheel well insert. Apparantly(sic?) the excessive vertical motion of the wheel caused the tire to contact the insert. Had I had 15 inch tires there may have been more serious consequences.

I did some careful measuring and consulted the factory. Seems the axle was mounted about 1/2" aft of its correct position. They immediately sent me the lift kit free of charge (of course).

Speculation here: It follows that the fore-aft clearance for nearly max suspension travel may be less than 1/2" on either side of the wheel well. The lift took care of the problem ..... but did not solve it. The axle was still slightly mislocated, but couldn't deflect enough to contact the edge of the well.


Denny_A

[RockyMtnRay]

Originally Posted by Denny_A

Separation of the tire at its widest from the wheelwell might be a different story though. One potentially quick way to find out is to simply call TM and ask if they use the same wheelwell for single axle 15 inch tire models (like the 3023) as they use for the single axle 14 inch models (2619/2720). A not so quick way would be to load the trailer, lift one side of the trailer enough to cause the other side to incur increased suspension squat, then do some measuring and estimating (it's not a good idea to jack up the axle itself).

I had tire-wheel well clearance problems with my 2002 2720SL when it was new. Following my first 2400 mile round trip (included heavy rains and potholes hiding in muddy puddles in TN), I was dismayed to note that the curb side tire was grinding the aft portion of the rounded, plastic wheel well insert. Apparantly(sic?) the excessive vertical motion of the wheel caused the tire to contact the insert. Had I had 15 inch tires there may have been more serious consequences.

I did some careful measuring and consulted the factory. Seems the axle was mounted about 1/2" aft of its correct position. They immediately sent me the lift kit free of charge (of course).

Speculation here: It follows that the fore-aft clearance for nearly max suspension travel may be less than 1/2" on either side of the wheel well. The lift took care of the problem ..... but did not solve it. The axle was still slightly mislocated, but couldn't deflect enough to contact the edge of the well.


Denny_A

Thanks for this great info, Denny.

'Twould appear that the standard wheelwell, at least for an '02 2720 (which is what mine is), was sized for a 14 inch tire. With a correct axle location and lift kit, a 15 inch tire might be do-able but will take careful checking. And even more cause to discuss the issue with TM. Retrofitting a larger wheelwell (if TM does indeed use larger wheelwells on the models that have OEM 15 inch tires) would be a substantial undertaking.

Hmmmm...this could be additional justification to change to the 5K axles though...substantially less suspension deflection with the same load and road conditions.

And there's always the option of simply going to a 14 inch Load Range D tire as member SenorSedona did. However that also means switching to a Carlisle tire as Goodyear doesn't make a D range 14 inch Marathon.

Decisions, Decisions.

[Bill]

Originally Posted by RockyMtnRay

I've already discussed this with the owner of a local tire shop ... he hasn't yet retrofitted his trailer's tires either. One thing he did mention during our initial discussion is that you may not want to run the larger tires at their maxiumum 65 psi pressure, but rather at one that provides a sufficient margin of load capacity. If running at max pressure, there's a good chance you'll get excessive wear in the center of the tire so he suggested getting some tire chalk and using that to determine if there was too much air in the tires.

Reduced inflation would also give the TM a softer ride - or at least not a harder one. I haven't tried it, but I would be afraid that at 65 psi, those rock-hard tires would pound the TM to death, despite the "give" in the axle.

There are two likely points where an oversize tire would rub...at the very top and the sides.

I think that the first part - contact at the top - should be easy to deal with. When I thought about doing this, I climbed into the kitchen cabinets, and noted that there is nothing special going on. The wheel well liners are a simple piece of molded plastic screwed to the floor around the tire opening. I would simply unscrew each one from the floor, build up a lip or rim around the opening out of 1x2 pine, and screw the liner back down on top of the lip.

I was much more concerned about fore-and-aft clearance, and so I never carried through on this project. However, if TM would send you a pair of larger wheel well liners (I never thought to ask), I have to believe it wouldn't be too hard to slightly enlarge the opening in the floor.

In-and-out clearance (in the direction of the axle) would also be of concern, but I don't think that 15" tires are necessarily much fatter than 14" tires.

From everything I've read, the stopping power of trailers with hydraulic disc brakes is enormously better than with the traditional electromagnetic drum brakes (like a factor of 4 in the deceleration rate).

Hmm, in this case, my skepticism is nearly boundless. If the electric brakes will grab tight enough to lock up the wheel, then hydraulic brakes aren't going to do any better. In practice, both of them should be adjusted to the point where they won't quite lock up in a panic stop - but still, I don't see that hydraulics have any inherent advantage. Of course, a larger tire might possibly enhance deceleration, simply because it puts a larger rubber patch on the road. But for equal tire pressures, even this is pretty iffy (which is why your tire guy suggested the chalk test). But in the end, any claim of deceleration improvement of 4 to 1 is pure bogus hokum unless someone can a) explain how the difference makes it better, and b) provide reasonably scientific measured data. If you actually find some of this, I hope you'll pass it on.

Just my thoughts - I am interested in this topic because like you, I think I often run pretty close to the tire limits. There are also a couple of associated technical issues that I will discuss offline with you - I don't think they're of general interest to the group, and this post, like many of mine, is already too long.

Bill

Ray said
"FWIW, I may go quite a bit beyond just retrofitting oversize tires...I'm currently mulling over also swapping the 3.5K axles on my 2720SL for 5K axles"

I thinks Ray has the ultimate solution to pulling a 4000# 2720. When I researched the wheel situation, I think I discovered that the max wheel rating for 5 on 4.5" bolt pattern is 2150# which matches the tire rating at 50 PSI. I would assume that the 5K axle has 6 on 5.5 bolt pattern which gets one to 2500# loading.

The 2" lift seems to address the vertical clearance issue but the width issue may exist. If the 15" will fit, one could run at about 50-55 PSI and probably have a good tire margin (probably about 1900# loading for 4000# TM -- Notice that the SR215/75R14 is rated at 1820#).

Ray, are you interested in posing the width clearance question to TM ? You own a 2720 whereas I am a prospective owner. You probably have more clout with TM.

[RockyMtnRay]

Originally Posted by Frenchy

You can get 14 inch tires with higher load rating. I bought Cooper tires last time. The dealer said they were a commercial type. They were 195R14C, Load Range D 106/104Q with a load rating of 2095 lbs at 65 PSIG. They fit and worked very well on my 3023 while on my trip to Grand Teton NP from southern Indiana.

There's general consensus that travel trailers should only be equipped with "ST" (special trailer) series tires, not light truck tires ("LT" series), not commercial trailer tires ("C" series like the ones you bought), and not passenger car tires ("P" series). Certainly you often find higher load ratings in the same size tire if you use a "C", "LT", or "P" tire but these tires are designed for regular use and tend to deteriorate substantially if left unused but loaded for extended periods. By comparison ST tires are specifically designed to not deteriorate (internal oxidation) when left unused but loaded for months...a typical scenario for travel trailers that are left in storage for 6 months. Furthermore, ST tires are designed to better withstand the sidewall twisting and sideforces that happen when double or triple axle trailers are sharply turned (e.g. backing into a camping site).

So, yes, a commercial or "C" rated tire will do fine the first season it's mounted...probably better than an ST tire. But it may not be in very good shape at all by the beginning of the 2nd or 3rd season after it's been left unused for months at a time in a fully inflated state with the weight of the trailer on it. IOW, if you want that C rated tire to be still be good by the next towing season, you need to get all weight of the trailer off the tires during the winter and deflate the tires down to about 10 psi. And make sure the tires are completely protected from the weather (ideally inside).

[RockyMtnRay]

Originally Posted by Bill

Hmm, in this case, my skepticism is nearly boundless. If the electric brakes will grab tight enough to lock up the wheel, then hydraulic brakes aren't going to do any better. In practice, both of them should be adjusted to the point where they won't quite lock up in a panic stop - but still, I don't see that hydraulics have any inherent advantage. Of course, a larger tire might possibly enhance deceleration, simply because it puts a larger rubber patch on the road. But for equal tire pressures, even this is pretty iffy (which is why your tire guy suggested the chalk test). But in the end, any claim of deceleration improvement of 4 to 1 is pure bogus hokum unless someone can a) explain how the difference makes it better, and b) provide reasonably scientific measured data. If you actually find some of this, I hope you'll pass it on.

You ask, I provide. Here are the results of a fairly scientific study performed for TrailerLife Magazine comparing standard brakes to the Towbrake Hydropower Disc Brake system. Much of this document was published in the Feb 2005 issue of Trailer Life.

On Page 3 of the document are these numbers for stopping distances from 60 mph on dry pavement. Tests were done by Towbrake Int'l (the manufacturer and better known as the maker of the VIP Towbrake (aka MountainMaster) device for RV "toads") and George Ripsom for Trailer life:

Truck with OEM trailer brakes: Towbrake: 480 ft; Ripsom (avg): 513 ft; Ripsom (best): 356 ft ("G" force ~.25)
Truck with Disc (controller at just short of lockup): Towbrake: 198 ft; Ripsom: 207 ft. ("G" force ~ .6)
Truck with Disc (controller normal): Towbrake: 310 ft

Note1: IIRC, a typical pickup by itself will have a 60 mph stopping distance of around 160 to 190 feet.

Note 2: These tests were conducted at 25 mph and the results extrapolated for 60 mph tests. Because drum brakes are not as effective at high speed as they are at low speed and because drum brakes overheat/fade much more quickly than disc brakes during a prolonged stop, I strongly suspect the extrapolation actually underestimates the drum brake stopping distance.

Note 3: My memory failed me on the ratio of deceleration between disc and drum...these tests indicate it should be about 2.5 instead of 4. But a factor of 2.5 is still a lot.

My conclusion is that disc brakes (1) will cut anywhere from 50 to 300 feet (probably around 200 ft) from the 60 mph stopping distance and (2) will make the stopping distance with trailer nearly as short as the stopping distance of the truck itself.

I'm personally so impressed with these results (which are the most scientific I've seen) that I'm about 90% sure I will retrofit my trailer with discs before the 2006 towing season. I've already got their documentation (with pricing) and have located a dealer in Wyoming that I can work with for the parts and/or do the install (though it can be a DIY).

[Bill]

Ray, what am I gonna do with you? The article reports the results of testing the new improved magic hypower brake against standard electric brakes DRIVEN BY AN ANCIENT TIME CYCLE CONTROLLER. The description in the article's Introduction is laughable - this controller doesn't even activate until the tow vehicle noses over enough to activate a mercury switch! And only then does it begin to slowly ramp up the brake current. Over the past couple years, you and I have competed for King of the Time Cycle Controller Bashers. This one of the reasons. Let's not stop now.

After describing the time cycle controller, the article continues by saying "the HD3000 is intended to use the same dashboard controller" - but then they say that activation "instantly forces the brake pads against the rotor." Instantly? With a time cycle controller? How did that happen? Well, a little later on, we see casual mention of a new master cylinder sensor that actually sends current to the disk brakes. Doesn't sound to me like they actually used the same controller at all.

Let's step back from this particular article, and look at the bigger picture. Common sense tells us that if one braking system can provide 3:1 or 4:1 improvement over another, then there is something wrong (or grossly lacking) in the first one. So the first question has to be, what is it that is wrong? There are only two parts to a trailer braking system - the brakes, and the controller - so the difference has to be in one or the other.

Is it in the brakes? Well, disk brakes have some very real advantages over drums (fade resistance and wet braking being chief among them). But raw braking power, the supposed subject of this test, is not one of them. Despite the hype of the automotive industry since disk brakes were popularized 20-30 years ago, discs are no better than drums in raw braking power. Both respond instantly, both give you fine-grain control of braking power, and both can brake hard enough to lock up the wheel if you ask them to. So assuming that the drum brakes in this test met these basic criteria, and were properly adjusted, the brakes themselves are not the problem. All else being equal, changing over to disks will NOT produce a big improvement. Repeat - despite the hype,

DISK BRAKES ARE NOT MAGIC!

So that leaves the controller. And voila, there it is. They have set up the most laughable comparison you can imagine. It demonstrates the power of advertising dollars. There is no way that you, or I, or any mechanically savvy reporter for Trailer Life, would have accepted these test conditions and printed this article. The testers (who also build the brakes, surprisingly enough) clearly bought advertising space in the guise of "unbiased testing". The biggest laugh is that the article is probably printed right next to an ad for a decent modern brake controller like the Prodigy. Now there is a test I would like to see.

As you have said in some of your great brake tutorials, a properly operating trailer brake system should bring the trailer-tow vehicle combination to a stop in more or less the same distance as the tow vehicle without the trailer. In this article, the baseline is a truck that stops in 190 feet by itself, but takes 515 feet when you add the trailer. They themselves say that "this is comparable to driving on ice", and I agree. And right from the beginning, it should be a wild red flag that something is wrong with the test setup.

I just love these technical discussions ...

Bill