Last Changed 11/22/2011
We had planned to change the size of the tires for Tige
someday, probably when a tire replacement was due. A tread
separation on one of the current tires forced the process.
We had been informed along the way that an easy upgrade from our current Goodyear G614 16" tires is to go to a 215/75R17.5 tire since it has the same diameter. The process requires changing the rims and the tires.
This page covers the decision process we used to select tires and rims.
The decision to change rim sizes was driven by the desire to have more tire options than just the Goodyear G614.
TiresThis table show the specifications for our current G614 tires and possible 215/75R17.5 tires. There are more "off-brand" tires but we prefer name brands.
Load Range/Ply Rating:First we should clarify that the Ply rating does not refer to the number of plies in the tire. That was the original Way of rating tires by the number of cotton cord plies there were. As cotton was replaced by nylon, steel, etc., the ply rating is just a number referring to the load capacity.
We had G/14 ply rated tires which were adequate for our load. The reason for wanting higher rated tires is not because we want to carry more load. We want our current load to be less of a percentage of the load capacity of the tires. With a lower percentage loading of the tires, we can run the tires at a pressure less than the Maximum giving a better ride. We had to run the G614s at Maximum pressure.
Most of the 215/75R17.5 tires are rated as H/16 ply. The Michelin XTA is rated as a J/18 ply but that extra ply rating does not show in the Max Load capacity.
We did find some 215/75R17.5 tires that were only G/14 ply rated.
Rim Width:Almost all of the 215/75R17.5 tires are designed for 6.00" and 6.75" rims.
Tire Width:This was important in that the replacement tires could not be much wider that the G614 tires we had and fit in the wheel wells.
All the 217/75R17.5 tires are narrower than the G614s.
Outside Diameter:The replacement tire diameter needed to be close to the G14 tire because you need to maintain a space between the tires to account for the movement a wheel makes from side-to-side (looking at the wheel) as the wheel moves vertically from spring action.
All the 215/75R17.5 tires were within .1" of the 614s.
Max Load @ max psi:
This is the maximum load the tire can be expected to carry when inflated to the indicated pressure. The pressure indicated is the maximum that the tire should be inflated to when cold (not driven). These are the numbers on the sidewall of the tire. These numbers are for when the tire is used in a single configuration. When tires are used in dual configurations, the load capacity for each tire is reduced.
The Maximum psi rating of the Michelin maybe explains the J/18 ply rating. The same 4805 lbs. max load is obtained with only 120 psi instead the typical 125 psi.
By using higher rated tires, the air pressure can be less. For example, with the Goodyear G114, we only need 95 psi to give the same load capacity as the G614s with 110 psi.
Speed Rating:This indicates the maximum speed the manufacturer rates the tire at with the load rating.
Some manufacturers allow for increasing the air pressure for a given load when the tire is operated at speeds higher than the rated speed. By in no case should the cold air pressure be higher than the max psi on the sidewall. Goodyear allows this adjustment only on tires rated higher than 65 mph.
Decision: Goodyear G114
We made this decision mainly on the speed rating. We normally travel at 55-58 mph. All of the tires listed would handle our speeds. However we prefer to have safety margins, and we prefer the higher speed rating that only Goodyear provided.
Axle ratings:You will usually see trailer axles rated in thousands of pounds. For example an axle with a load capacity of 7,000 lbs. will be referred to as a 7K axle. The requirement of a rim on a 7K axle would be a minimum of 3,500.
Typical trailers axles are 6K, 7K, 8K, and 10K.
There are some variations to this rating. For example, a typical MOR/ryde IS (Independent Suspension) system does not have an axle with two wheels attached. Each wheel has its own axle and spring. In this case a wheel with a 3,500 lbs. load rating would be referred to as a 7K wheel as if there were attached to a regular axle.
Another variation is where dual wheels are attached to one end of an axle. Each tire is rated a little less when they are used in dual mounts. For example a G614 is rated at 3750 lbs. as a single and only 3415 lbs. as a dual. Four of these tires as duals would be on a 12K axle. The hub has to carry the load of both tires.
Wheel hub types:There are basically two types of wheel hubs determined by how the wheel is centered on the hub.
Typically 17.5" rims are hub piloted wheels because the typical wheel rating, 4,805
lbs. would be found on an 8K or 10K axle.
The typical axle for an RV trailer is 6K or 7K, therefore you will usually find 1/2" or 9/16" studs and stud piloted wheels.
The G614 is a rather rare breed tire in its class. There are very few, non-brand name options in the 16" G rated class. The rim for the G614 is a special rim in that is has to be rated for 110 psi. Rim used with Class E and F tires only need to be rated for 85 psi.
The G614 has a 3,750 lbs. max load capacity which would yield a 7,500 lbs. axle capacity. The rims used with G614s have a 7K bolt pattern. I.e., 9/16" studs and stud piloted hub.
To use regular 17.5" H rated rims on a 7K axle would require changing the axle hubs. The 17.5" H rims have 5/8" stud holes and expect a hub piloting ridge on the hub. Some hubs on 7K axles have the hub piloting ridge with 9/16" studs. If you have such hubs, then a stud replacement is possible.
Just replacing studs on axles hub without the hub piloting ridge is not a good procedure. Regular 17.5" rims do not have tapered stud holes as the wheel piloting is expected to be done by the ridge on the hub. The stud nuts used with regular 17.5" H rims are flat. Just replacing the studs in a normal 7K hub would mean that the centering load would be carried by the stud threads, not a good things.
Choices:We limited our search to aluminum wheels. We prefer aluminum wheels for the weight and more importantly, we detest wheels with rust and virally every painted steel wheel will eventually rust.
Special 17'5" Rims:We found two special 17.5" rims that have 9/16" tapered stud piloting holes. The primary purpose of these two rims is to allow 215/75R17.5 tires to be used on 7K axles. The assumption is the use of the 215/75R17.5 tires is not for increasing the maximum load but for air pressure margin.
As always, the lowest rated component determines the capacity of the system. If your hubs are designed for only 3,500 loads each, bigger rims and tire won't change that.
We have a unique situation because of MOR/ryde IS suspension and Hydraulic Disk Brakes, that our hubs are actually designed as 8K hubs but have 7K studs.
Southwest Wheel - ALCOA:
Trailer Tire & Wheel Supermarket - Hi-Spec Series 03.
All three rims comes with hub caps and stud nuts.
Rickson Wheel Manufacturing
We have become aware of another source for 17.7" wheels. Rickson has an assortment of aluminum, including Alcoa, and steel wheels to upgrade your trailer to 17.5" wheels. Rickson has a lot of experience in doing wheel size upgrades but only recently started to add the wheels they offer to their web site. They have photos of some of their wheel options and eventually will have more specs on the web site. But they can answer questions on the phone now.
A word of warning, if you are upgrading from steel wheels, you might have a stud length problem. Aluminum wheels are thicker in the stud area. You want to be sure that your stud nuts have the stud threaded into the nut a distance at least the diameter of the stud. For example, a 9/16" stud should be threaded at least 9/16" into the stud nut.
Decision: Hi-Spec RimsWe considered both rim options acceptable.
We choose the Hi-Spec for two reasons. One was the looks. The H-Spec rim had more holes and we felt a lighter look.
The second reason was a feature of the H-Spec rim is that they have steel inserts in the stud holes. This is part of their ICF (Improved Clamp Force) program to design wheels that are properly torqued the first times.
Between the looks and the ICF features, Hi-Spec won the business.
Installation:You can see our new wheels installed at New Wheels.
we changed the trailer tire size, We retained the original spare
tire and wheel. When the spare tire reached 6 years of age, we
decided to change it. We like our tires to be in real good
shape, even the spare.
We made the decision to get a new 17.5" rim along with the new tire so that the spare wheel would be exactly like the trailer wheels.
We encountered a small problem with the new 17.5" rim. The hole in the center of the 17.5" rim is smaller than the hole in the 16" rim. The reason is that the 17.5" rim was designed to be a hub piloted wheel (the wheel contacting the center hub ring) and the old 16" wheel wasn't.
The spare tire carrier uses a cable with an adapter end that slips through the center hole and then make contact with the edge of the rim hole. Since the new rim hole was smaller, the rim contacted the adapter's curved arch top. As we cranked the spare up, the wheel would slide on the adapter, and the first time the adapter slipped out of the hole. We found we could balance the wheel as it was cracked up and the tension on the cable would hold the wheel in place.
We didn't like the possibility of the wheel slipping off the adapter when we would lower it. To put a different adapter won the spare hoist would have been problematic as it is swaged to the cable and changing the cable would have been a major project..
We used some large bolts, washers, and nuts and cut two pieces of aluminum bar to make retainers for the adapter to hold it in place while we raised and lowered the spare wheel..
Disclaimer: The information in this site is a collection of data we derived from the vendors and from our personal experiences. This information is meant as a learning guide for you to make your own decisions Best practices and code should always be followed. The recommendations we make are from our personal experiences and we do not receive any compensation for those recommendations.