The thing that struck me when I first looked at an Earthroamer were the tires.
Why do they have different tires front and back, and why such short sidewalls?
The answer seems to be driven by
The weight of the vehicle. The rear of ours weighs 11,000lbs so the rear tire needs to be able to carry around 11,000lbs in total, or more realistically up to the rear axles rating of 13,660lbs.
The desire to move to a single rear wheel configuration. This means that each rear tire needs a capacity of well over 6000lbs.
The needs to stay within the capacity of the standard suspension and the standard gearing of the vehicle. Note the Ford F550 can be acquired with a 4.88 final drive ratio. In the early models of the ER and F550s this meant a maximum tire diameter of around 37 inches.
With these constraints the ER guys and their tire suppliers came up with Michelin XDA2-Energy at the rear and XZA2-Energy at the front in sizes of 290/60R22.5. These tires have a weight capacity of 7300 lbs each and are at 37 inches in diameter. Perfect.
The downside with this solution is that these are 3 season road tires. A number of owners have commented that the front steer tires are not very good in winter snow and ice conditions.
In addition the narrow side walls make the ride harsh, and airing down does not produce as much advantage as with tires that have large side walls.
From my perspective an equally big issue is that the tires have a different rolling radius. The Michelin specs say that the XDA is 569 revolutions per mile while the XZA are 575 revolutions to the mile. Maybe that 1% does not matter most of the time but I find that hard to believe. Those extra 6 revolutions per mile have to be released through tire slip against the road surface. To me it seems one had better not drive in 4x4 on good tractions surfaces for too long.
Alternatives To The Standard Tires
In recent years ER themselves have provided an alternative to the Michelins for those that want to have (and are prepared to pay for) a more off road tire.
Enter the Continental 335/80R20 MPT81. This is a military style tire, is for on-road/off-road, all position use. ER has made these tires available as an option with 20 inch military style internal bead-lock wheels. To accommodate these tires a suspension lift (of about 3") is required. ER achieve this with their air-suspension option. One assumes that there are other ways of achieving the same lift, say with bigger front and rear springs, but thats not a factory option.
One of the points that needs to be noted about the MPT option is that the Continental tire comes with a load rating of (3075 Kgs.) 6765 lb. per tire or 13570 lbs per axle. For those that are accustomed to seeing trucks use Michelin XZL tires please note - the Michelin XZL 335/80R20 has a lower load rating than the Continental - around 4800 lbs.
I have recently discovered literature for a Dunlop tire, the SP PG8 MPT. This tire is a military style offroad tire that comes in the size 335/80R20 and appears to have a version with the (3075kg) 6765 lbs load rating at 110 kmph.
However, I have no information about availability other than that I found the information about these on a European website.
Whats is my goal - the search for other tires
Recently I have been looking at tire alternatives for my ER. It is going to be a while before I need to replace the existing XDA/XZA combination but when that time comes I want to know what the options are.
Specifically my goal is to have a tire combination that will be suitable for long distance journey like our Americas trip. That is 50,000+ miles on main, secondary and dirt roads in 3rd world countries where tire supply and tire repair facilities are limited. Ideally I would like an all terrain/all weather/all position tire that is commonly available in most countries and one that would last 35,000 miles.
In a perfect world it would be nice if I could achieve this with a tire that mounts on my existing 22.5 inch wheels and without any suspension mods such as upgrading to the airbag suspension that ER offer.
In this less than perfect world I would consider the suspension upgrade and maybe even a change of wheels if I was convinced that it was worth it.
If all else fails I may even adopt the Continental MPTs, though from what I have heard they seem to wear too quickly (20,000 miles) to be a good choice for an extended overland trip. And I have not checked on world wide availability.
Few tires meet my strict requirements
Here are the only options I have found that meet all my requirements
The obvious solution is to run the XDA tires in all positions. Some owners seem to be doing this. They report that tire pressures need to be increased to prevent the tires at the front following road grooves.
290/60R22.5 - It seems that Yokohama (TY517E) and Bridgestone (M729) are available in this size. However like the XDA tires these are also drive tires not all position. IMO availability from other manufacturers makes this size much more suitable for long overland trips.
Relaxing my requirements!
It did not take long to realize that I needed some bounds on the search in order to get some order to the plethora of tires that are available. For example are Michelin XZL 365/85R20 tires even in the ball park?
After due consideration the two limiting factors seem to be
Tire diameter. The XDA's are 36.5 inches in diameter.
The factory specs for the Continental 335/80R20 give
a diameter of 1066 mm (41.97 in),
a static loaded radius of 480 mm, and
a rolling circumference of 3120 mm which is equivalent to a diameter of 39.1 inches.
The link provides a PDF download on continental tires sizing. This document explains that rolling circumference is the correct value to use for gearing considerations and outer circumference is the right number to use for body and suspension clearance considerations. As during use the tire may expand to that size but no more.
Thus for the purpose of my research I assumed that one may be able to squeeze an additional 1 inch into the air-bag suspension lift without compromising suspension geometry. Thus tires with outer circumferences up to and including 43 inches can be considered.
Gearing. Most tire literature provides information about gearing in the form of revolutions per mile or per km, some like the MPTs provide it in the form of rolling circumference. The conversion between those two measures is easy.
The XDA's are nominally 570 rev/mile and the MPTs are 515 rev/mile.
From the above it seems like any tire that is under or at 43" tall and with a circumference no greater than 515 revs/mile would fit with the ER air bag suspension and standard gearing. I will say more about gearing modification options later. It is also (probably) reasonable to assume that any tire that fits this spec would have no more trouble with front bearings and suspension linkage wear than the current MPT81 option.
So what are the choices
Discussed above, there are (at least) three manufacturers that provide tires in this size - Michelin XDA, Bridgestone M729, and Yokohama TY517E.
These come in at 40 inches tall and about 520 revs/mile. These would require a suspension lift such as the ER air-suspension option. Available from:
- Michelin in the form of the XZU2 all position tire. Has a higher weight rating than the XDA see for details.
- Bridgestone in the form of the M729 - though I only found this in a non US brochure see for details this Australian site.
- Yokohama in the form of the T303 M/S. This is another all position tire. for Yokohama brochure see this link
Yokohama has a TY517 drive tire available in both G and H load rating. These are just over 40 inches in diameter and have a circumference of 512 revolution per mile. That makes them on the border line with the MPTs in terms of gearing.
Bridgestone M711 drive tires are available in this size with a 3000kg per tire rating see this link.
Bridgestone M720 tires seem to be available in this size but I can only find info about the G rated version
These seems to be a common size with a number of manufacturers providng them. There also seems to be a good bit of variation in the specification. They are around 42 inches in diameter with a circumference in the range of 495-505 revs/mile, and with load ratings of 6900 lbs per tire. So on the gearing front they are outside my parameters.
Does the 2-4% more in circumference compromise gearing sufficiently to reject these tires. Frankly I don't know!!
This size is ubiquitous and would be perhaps the most easily sourced tire size discussed in this article, in that regard they would make a very good choice for an overland vehicle.
They generally seem to have a diameter around 42" and a rolling circumference of around 490-500 revolutions per mile; those numbers make them very similar to the 295/80R22.5 size.
Like the 295/80 size the question mark is the gearing.
An additional impediment to the use of this tire is rim size, these must be fitted on 8.25" rims.
Available from a number of manufactures - Michelin (XZUS2 and XZY3 both all position), Bridgestone (M840, M843, M860 all position tires) Toyo (M320Z) and Yokohama (T607 M+S). These have a diameter of 43" and are 480-490 revolutions per mile. Hence they would almost certainly require gearing change. The upside to this tire is the much high load rating, up to 10,000 lbs per tire, and the wide profile and hence larger foot print.
While this is a reasonably expensive option it is possible. There are 5.36 gears available for the Ford axles. I understand this is an option with the gasoline powered F550.
With 5.36 gears each of the 295/80R22.5, 315/80R22.5 and 11R22.5, would give overall gearing between the current 4.88 + 290/60R22.5 and 4.88 + 335/80R20 combinations.
The larger two combinations have tire diameters around the 43 inches mark so tire clearance would need to be checked.
Conclusions - Whats the next step?
It would be interesting to try a set of 295/80R22.5 tires on an ER with airbag suspension.
ER should consider the 305/70R22.5 size for anyone vehicles with the air suspension but without the MPT tire options (and there are at least a few owners in this situation). Certainly the 305/70R22.5 tire would seem like a better option for those vehicles than the standard XDA/XZA combination.
The next step is to confirm all the above information with local representatives of the tire manufacturers.