Replacement Tire Size

[sumbarge]

I know tires have been discussed at length in previous posts and I've read most all of them, but did not see an answer to the question I have below. Forgive me for being ignorant in this matter as I am no expert. I am upgrading the tires for my 5th wheel. I have decided on the Sailun S637 tire to replace the PowerKing Towmax STR tires currently on my 5th wheel. The specs for the PowerKing Towmax STR state that the ST235/85R16 is 31.7 inches in diameter. Yet, when I actually measure the tire, it measures 31 inches in diameter. There are two sizes of Sailun tires available (235/80R16 and 235/85R16). The manufacturer specs for the Sailun 235/80R16 is 31 inches in diameter and the Sailun 235/85R16 is 32 inches in diameter.

My question is, which size of Sailun tire should I choose? The Sailun 235/85R16 that matches the manufacturer specs of the tire I currently have on the 5th wheel or the Sailun 235/80R16 which matches what I physically measure the old tire at. Hope that make sense. I'm also concerned about the gap between the front and rear tires, which is currently ~4.25 inches. Would it matter if it is less than this? Thank you for your advice!

 

[RickL]

To answer you question about the stated spec size vs what you measured that is easy. First the stated spec size is always done on a specific rim width. Narrow that width and you will “grow”the diameter. Conversely, widen the width and you will shorten the diameter.

Also, I’m not sure how many 32nds you have remaining on your old tire, but let’s say for the sake of discussion your particular tire came with 11/32nds. If you wore off 2/32nds you would decrease that diameter by 4/32nds. Thus the difference in stated and actual.

I would personally go with the 235/85R16. While it is slightly larger that increase is in the amount of volume the air chamber holds. That volume is what dictates the carrying capacity of the tire for the most part. So staying with the standard size provides you with max carrying capacity.

 

[sumbarge]

Thanks Rick!

 

[tireman9]

While I understand the concern for the tire dimensions, that is NOT the most important specification.
Number one is to ensure any replacement tire is capable of supporting the load you are placing on your tires plus a margin.

The best thing to do, is to first confirm your actual tire loading.
Ideally, you would get on a scale, with the RV loaded to the heaviest you ever expect to travel with, and learn the actual load on each tire as there are very few RVs with the load split evenly axle to axle or side to side.
HERE is a worksheet you can use. You will have to do some hunting around as you can't get individual loading on most truck stop or CAT scales. You will need to find a local building supply or feed or grain dealer or gravel pit or possibly cement delivery company.

Lacking that you could use a truck scale but to be safe you need to apply some math to estimate the load un-balance.
First, assume a split of 52/48 between axles or with a three axle trailer assume one axle is supporting 35% of the total. Then assume a 53/47% split on the heavier axle for side to side loading. Yes, some RVs have been measured with individual position scales and found 1,000# un-balance.

So with the measured or calculated heaviest loaded tire, and the dimensions checked, you are ready to shop for tires.

You need to realize that ST tires have a higher load capacity than LT type tires. This is because the load formula for ST type tires is based on a max speed of 65 mph even if the "Handling rating" speed symbol suggests differently. So you can't just use the numbers when comparing tire sizes as an ST235/75R16 carries significantly different load than an LT235/75R16 even with the same Load Range ( D or E or F etc)

You can then consult the Load & Inflation tables for the tires under consideration. The good news is that with the exception of Michelin 99+% of the tires out there follow the same table info so you can use Bridgestone or Goodyear etc for LT and Maxxis or Goodyear for ST type tires.. You can look at different tables HERE if you want.

When selecting a tire you need to get the tire capacity at least 15% greater than for your measured or calculated tire load. This allows for sway, load shift due to road crown and wind side load to the tires you are buying.

After you do the above THEN you can confirm tire dimensions knowing the load capacity needed.

On my blog I cover why you should run the inflation number molded on the tire sidewall (lower the Interply Shear) and why you should always run a TPMS along with other info on Interply Shear and the effect of temperature on tire pressure. You might even subscribe.

Hope this helps.

 

[RickL]

While I understand the concern for the tire dimensions, that is NOT the most important specification.
Number one is to ensure any replacement tire is capable of supporting the load you are placing on your tires plus a margin.

The best thing to do, is to first confirm your actual tire loading.
Ideally, you would get on a scale, with the RV loaded to the heaviest you ever expect to travel with, and learn the actual load on each tire as there are very few RVs with the load split evenly axle to axle or side to side.
HERE is a worksheet you can use. You will have to do some hunting around as you can't get individual loading on most truck stop or CAT scales. You will need to find a local building supply or feed or grain dealer or gravel pit or possibly cement delivery company.

Lacking that you could use a truck scale but to be safe you need to apply some math to estimate the load un-balance.
First, assume a split of 52/48 between axles or with a three axle trailer assume one axle is supporting 35% of the total. Then assume a 53/47% split on the heavier axle for side to side loading. Yes, some RVs have been measured with individual position scales and found 1,000# un-balance.

So with the measured or calculated heaviest loaded tire, and the dimensions checked, you are ready to shop for tires.

You need to realize that ST tires have a higher load capacity than LT type tires. This is because the load formula for ST type tires is based on a max speed of 65 mph even if the "Handling rating" speed symbol suggests differently. So you can't just use the numbers when comparing tire sizes as an ST235/75R16 carries significantly different load than an LT235/75R16 even with the same Load Range ( D or E or F etc)

You can then consult the Load & Inflation tables for the tires under consideration. The good news is that with the exception of Michelin 99+% of the tires out there follow the same table info so you can use Bridgestone or Goodyear etc for LT and Maxxis or Goodyear for ST type tires.. You can look at different tables HERE if you want.

When selecting a tire you need to get the tire capacity at least 15% greater than for your measured or calculated tire load. This allows for sway, load shift due to road crown and wind side load to the tires you are buying.

After you do the above THEN you can confirm tire dimensions knowing the load capacity needed.

On my blog I cover why you should run the inflation number molded on the tire sidewall (lower the Interply Shear) and why you should always run a TPMS along with other info on Interply Shear and the effect of temperature on tire pressure. You might even subscribe.

Hope this helps.

While you bring up some valid points I’m not accepting requiring a 15% increase. Tires have a built in “fudge factor” which will cover the load rating required. Also while the fact of picking a tire that meets or exceeds the required weight is correct, one also needs to ensure the clearance around the tire is sufficient, usually 3/8”-1/2”, when factoring the dimensions.

 

[tireman9]

While you bring up some valid points I’m not accepting requiring a 15% increase. Tires have a built in “fudge factor” which will cover the load rating required. Also while the fact of picking a tire that meets or exceeds the required weight is correct, one also needs to ensure the clearance around the tire is sufficient, usually 3/8”-1/2”, when factoring the dimensions.

Not sure what tire company you worked for but "fudge factor" isn't a standard for design or Quality Control. We use established statistical analysis when asessing test results. Tires do not have to pas our company minimums "plus a bit".

Ever wonder why trailers have more tire failures and shorter average tire life than Motorhomes? It's because most trailers are built with the smallest (least expensive) tires that barely meet federal requirements. The Federal requirements only require tires that can meet the GAWR. No margin or Fudge Factor is required. I have seen some margins as low as 10# and in one case a special letter had to be issued because the treiler tires were under the minimum by 170#

If owners want better life from their tires then allowing for the special loading i.e. High Interply Shear then they need to account for that with lower load and higher inflation.

But it's your choice. All I can do is offer the engineering facts.

 

[RickL]

Not sure what tire company you worked for but "fudge factor" isn't a standard for design or Quality Control. We use established statistical analysis when asessing test results. Tires do not have to pas our company minimums "plus a bit".

Ever wonder why trailers have more tire failures and shorter average tire life than Motorhomes? It's because most trailers are built with the smallest (least expensive) tires that barely meet federal requirements. The Federal requirements only require tires that can meet the GAWR. No margin or Fudge Factor is required. I have seen some margins as low as 10# and in one case a special letter had to be issued because the treiler tires were under the minimum by 170#

If owners want better life from their tires then allowing for the special loading i.e. High Interply Shear then they need to account for that with lower load and higher inflation.

But it's your choice. All I can do is offer the engineering facts.

Without getting to deep in the weeds let’s just say my responses are more for the “lay man”, not a technical paper. Such the term “fudge factor” reference. This is something I learned in almost a 40 year career in the tire business dealing with both the end users and manufacturing staff. If you want to move the discussion into a six sigma realm this isn’t the forum to do so.

Nor or do I think we need to confuse people on this forum with philosophical differences or opines.

 

[TravelTiger]

I wouldn't trust a Towmax tire on a baby buggy! I've seen in person and on Facebook of Towmax tires blowing up without ever being on the vehicle!

I would buy 5 new Saliuns and recycle the Towmax for playground groundcover material.


Sent from my iPhone using Tapatalk

 

[NYSUPstater]

^^^^^^^^^^^^^^^^ What he said ^^^^^^^^^^^^^^