Montana Owners Club - Keystone Montana 5th Wheel Forum - View Single Post

RidgecrestDad · 11-26-2010, 07:36 AM

OK time to jump in here as well. I need to replace my original ST 235/80/16 Freestar steel belted radial tires on our 2007 3075 Monty. Currently they are load range E (LRE). I had 16 inch aluminum rims delivered with the 5er that are rated at 3750# at 110psi, well above the LRE rating, but the spare tire rim is a steel rim with less capacity. Why is the spare rim important? I do not want to have different tires and load capacity should I have to use the spare.
I, like many, am struggling with brand and load rating, and number of replacement tires. As the posts seem to favor (based on good reason and experience) many different selections for tire type and brand, I started exploring a bit on my own to see if I could make a rational argument for myself. I found a nice gouge sheet called Trailer Tires 101 on the Carlisle site (http://www.carlisletire.com/product_..._tires_101.pdf) that talks about ST and LT tires, and design criteria in general. I find it most enlightening and offer it up to the forum. After reading it and many posts, and talking to the local tire shops, I am slightly less confused. And I am now leaning strongly toward staying with an ST but changing the profile to an ST 235/85/16 and maybe increasing the load rating to LRF. This tire will be taller than the original. The 235/85 will add right at an inch to the overall diameter (depends on load rating), be no wider, but be about 1/2 inch higher in the wheel well, and add over 200* pounds per tire of carry capacity. Stated another way, the same load range E Carlisle tire in the 235/80/16 has a carriage of 3420#s and the 235/85/16 carries 3640#s; each at 80psi tire pressure. Where I am heading is supporting the trailer weight with margin. For my trailer at 12500#, properly inflated (max 80 psi) LRE tires will provide me about 9.4% margin (4x3420=13680/12500=1.0944); shifting to the LRE 235/85 tire provides about 16.5% margin (4x3640=14560/12500=1.1648). The ST 235/85/16 LRF tire at close to 50% additional cost over a LRE, moves the margin another 10% or so above the same profile tire (4x3960=15840/12500=1.2672). Or comparing overall load bearing capacity and excess margin, 25% above my current weight if I step up to the LRF (and also to 95psi in the tire). One tire expert suggested a 20% margin is a good idea, so one can move around and build in the comfort one wants, perhaps without breaking the bank. However, as this seems simple, if the axels are rated at 6K (on the sticker of the RV); and I think I have margin due to tires; I really may not as the axel could fail if overloaded. So we must remember that just because the tires can accept the load, the axel may not. This may explain why the manufacturer only matches the load range to the axel and not much more. So all this says, most LRE tires, regardless of the profile, if properly inflated, will carry the load to the axel rating. But the ST has a stiffer sidewall than an LT with the inherent additional heat reduction and weight capacity. Confused yet? I may still be. Why, well my Freestar ST 235/80/16s are rated at 3520# each, for a 12.6% margin at 80 psi. Why they are different than the Carlisle’s which are only rated at 3420#s is anyone’s guess? And no, I never ran 80 psi in the tires, but sure will run the max on the tires in the future. And I need to ensure I have high pressure stems in the rims. If anyone has comments or suggestions or corrections to the above, please fire back. This is an area that seems critical for safety and we should all have the right answers; doubt we’ll all do the same but that is OK too.

And an additional note: I waited for the repair guy to come change my tire (as suggested by one member) however when he showed up we ended up using both of us and his and my bottle jacks. The tire changing threads made sure I did not lift the whole trailer using the axel. So I am off to get an additional bottle jack and jack stands in preparation for times when the guy takes too long or I am boon docking and can’t call in the repair guy. This additional equipment will also enable me to change out the shackles as suggested by many members.

11-26-2010, 07:36 AM	#36
RidgecrestDad Established Member Join Date: Nov 2008 Location: Ridgecrest Posts: 39 M.O.C. #8969	OK time to jump in here as well. I need to replace my original ST 235/80/16 Freestar steel belted radial tires on our 2007 3075 Monty. Currently they are load range E (LRE). I had 16 inch aluminum rims delivered with the 5er that are rated at 3750# at 110psi, well above the LRE rating, but the spare tire rim is a steel rim with less capacity. Why is the spare rim important? I do not want to have different tires and load capacity should I have to use the spare. I, like many, am struggling with brand and load rating, and number of replacement tires. As the posts seem to favor (based on good reason and experience) many different selections for tire type and brand, I started exploring a bit on my own to see if I could make a rational argument for myself. I found a nice gouge sheet called Trailer Tires 101 on the Carlisle site (http://www.carlisletire.com/product_..._tires_101.pdf) that talks about ST and LT tires, and design criteria in general. I find it most enlightening and offer it up to the forum. After reading it and many posts, and talking to the local tire shops, I am slightly less confused. And I am now leaning strongly toward staying with an ST but changing the profile to an ST 235/85/16 and maybe increasing the load rating to LRF. This tire will be taller than the original. The 235/85 will add right at an inch to the overall diameter (depends on load rating), be no wider, but be about 1/2 inch higher in the wheel well, and add over 200* pounds per tire of carry capacity. Stated another way, the same load range E Carlisle tire in the 235/80/16 has a carriage of 3420#s and the 235/85/16 carries 3640#s; each at 80psi tire pressure. Where I am heading is supporting the trailer weight with margin. For my trailer at 12500#, properly inflated (max 80 psi) LRE tires will provide me about 9.4% margin (4x3420=13680/12500=1.0944); shifting to the LRE 235/85 tire provides about 16.5% margin (4x3640=14560/12500=1.1648). The ST 235/85/16 LRF tire at close to 50% additional cost over a LRE, moves the margin another 10% or so above the same profile tire (4x3960=15840/12500=1.2672). Or comparing overall load bearing capacity and excess margin, 25% above my current weight if I step up to the LRF (and also to 95psi in the tire). One tire expert suggested a 20% margin is a good idea, so one can move around and build in the comfort one wants, perhaps without breaking the bank. However, as this seems simple, if the axels are rated at 6K (on the sticker of the RV); and I think I have margin due to tires; I really may not as the axel could fail if overloaded. So we must remember that just because the tires can accept the load, the axel may not. This may explain why the manufacturer only matches the load range to the axel and not much more. So all this says, most LRE tires, regardless of the profile, if properly inflated, will carry the load to the axel rating. But the ST has a stiffer sidewall than an LT with the inherent additional heat reduction and weight capacity. Confused yet? I may still be. Why, well my Freestar ST 235/80/16s are rated at 3520# each, for a 12.6% margin at 80 psi. Why they are different than the Carlisle’s which are only rated at 3420#s is anyone’s guess? And no, I never ran 80 psi in the tires, but sure will run the max on the tires in the future. And I need to ensure I have high pressure stems in the rims. If anyone has comments or suggestions or corrections to the above, please fire back. This is an area that seems critical for safety and we should all have the right answers; doubt we’ll all do the same but that is OK too. And an additional note: I waited for the repair guy to come change my tire (as suggested by one member) however when he showed up we ended up using both of us and his and my bottle jacks. The tire changing threads made sure I did not lift the whole trailer using the axel. So I am off to get an additional bottle jack and jack stands in preparation for times when the guy takes too long or I am boon docking and can’t call in the repair guy. This additional equipment will also enable me to change out the shackles as suggested by many members.