Gulf Stream Owners RV Forum - View Single Post

hossross · 12-12-2022, 09:13 PM

Tires do generate a lot of opinions, but its the fact we want. I think NavyLCDR gave you a simple & accurate answer on why a tire that is designed with specific characteristics including max load at a max pressure can be used on many different applications. Modern SUV's are "over tired" just to get the appearance. Remember when cars had skinny tires, and how much better they look with wider tires. The low side wall height is the same, it has a unique appearance and can generate much higher lateral force on those low CG (center of gravity) vehicles that can safely handle it. RVs are not those they have ridiculously high CGs,s On class C RVs rarely do you need the same tires on the front axle as the back axle(s), based on the maximum ACTUAL weight on the front is much less than can be loaded on the rear axle(s). If the manufacture wants to keep the same tires all around so you don't to have 2 size of spares or even no spares, the adding tires on the rear or duallys will double the max ALLOWED load on the rear axle(s). On a class C, the weak link is usually GVWRA, and not GVWR or GVWRF, because there is so much overhang and heavy things rearward of the rear axle. Water weights 7.48 pounds per gallon, Gasoline is 6 pounds per gallon. The fuel tank is normally between the front and rear axles, so the front supports some weight, but because the fuel tank is closer to the rear axle, it gets more and can be proportionally calculated from middle of tank to the distance of each axle. If the water and waste tanks are mounted rearward of the axle, all that weight is supported totally by the rear tires. So depending how big of tanks the RV mfg uses, how much "Cargo & people" are allowed by the RV Mfg, (usually their is an implied/assumed weight distribution of the cargo and people by where there is passenger and bed accommodations and where the kitchen ref and cabinets are as well as external storage. So since these are all design variables, the actual weight to be carried of each model will be different. Again the recommended tire pressure by the RV should consider Vehicle tire pressure effects ride, handling, braking, fuel economy, & tire life. So back to a Ford Van that Ford supplies as a cut away (which is really a partial build, not a cut away of the body as it was once done) Ford has to certify they are supplying a vehicle of certain capabilities. They set the GVWRating, GVWRFront, GVWRRear because the frame, axles including springs, tires, and power train have all been selected to deliver those capacities. Because the axles already exist and Ford can't do a custom axle for such low production volume often it is the parts that determine the GVW's. Because how much power needed to be a pleasant confident vehicle to drive is more subjective, the engines provided may or may not be "adequate" to each customer. Anyway bottom line on tires, they are selected for their load rating based on the GVW's, the tire company has designed that tire to a max pressure to carry that load. The ACTUAL weight carried by each tire is a function of all the content, weight, and weight distribution and hence the tire pressure can be different and typically will be when completed by the RV mfg.

If you want to understand more about the handling of an RV, read my post on what shocks, springs, and roll bars really do, I have not gotten a single question on it, I either made it too complex, or it conflicts with common beliefs that better shocks control vehicle roll, or can lift the body. Tires also play a roll in that also, as a tire side wall is a part of the "vehicle suspension rates".

The rule should be always use the RV tire pressure recommendations when the RV is fully loaded, Do not use the Vehicle sticker as it is not tailored to the RV application. Many have said inflation is to optimize the tire contact patch, that is also true in that the greatest patch area will provide the best traction and wear. What I gave you was design load based, Remember above I said I tailor the ACTUAL tire pressure to the roads and loads and speeds I drive. Looking at the tire wear, 1 uniformly across, 2 center mostly, or 3 edges mostly are long term indicators of how well you've matched an ideal tire pressure with the loads and load distribution for your roads, speeds

Does this answer your question(s)?

12-12-2022, 09:13 PM	#11
hossross Senior Member Join Date: Jul 2020 Location: Detroit Posts: 127	Tires do generate a lot of opinions, but its the fact we want. I think NavyLCDR gave you a simple & accurate answer on why a tire that is designed with specific characteristics including max load at a max pressure can be used on many different applications. Modern SUV's are "over tired" just to get the appearance. Remember when cars had skinny tires, and how much better they look with wider tires. The low side wall height is the same, it has a unique appearance and can generate much higher lateral force on those low CG (center of gravity) vehicles that can safely handle it. RVs are not those they have ridiculously high CGs,s On class C RVs rarely do you need the same tires on the front axle as the back axle(s), based on the maximum ACTUAL weight on the front is much less than can be loaded on the rear axle(s). If the manufacture wants to keep the same tires all around so you don't to have 2 size of spares or even no spares, the adding tires on the rear or duallys will double the max ALLOWED load on the rear axle(s). On a class C, the weak link is usually GVWRA, and not GVWR or GVWRF, because there is so much overhang and heavy things rearward of the rear axle. Water weights 7.48 pounds per gallon, Gasoline is 6 pounds per gallon. The fuel tank is normally between the front and rear axles, so the front supports some weight, but because the fuel tank is closer to the rear axle, it gets more and can be proportionally calculated from middle of tank to the distance of each axle. If the water and waste tanks are mounted rearward of the axle, all that weight is supported totally by the rear tires. So depending how big of tanks the RV mfg uses, how much "Cargo & people" are allowed by the RV Mfg, (usually their is an implied/assumed weight distribution of the cargo and people by where there is passenger and bed accommodations and where the kitchen ref and cabinets are as well as external storage. So since these are all design variables, the actual weight to be carried of each model will be different. Again the recommended tire pressure by the RV should consider Vehicle tire pressure effects ride, handling, braking, fuel economy, & tire life. So back to a Ford Van that Ford supplies as a cut away (which is really a partial build, not a cut away of the body as it was once done) Ford has to certify they are supplying a vehicle of certain capabilities. They set the GVWRating, GVWRFront, GVWRRear because the frame, axles including springs, tires, and power train have all been selected to deliver those capacities. Because the axles already exist and Ford can't do a custom axle for such low production volume often it is the parts that determine the GVW's. Because how much power needed to be a pleasant confident vehicle to drive is more subjective, the engines provided may or may not be "adequate" to each customer. Anyway bottom line on tires, they are selected for their load rating based on the GVW's, the tire company has designed that tire to a max pressure to carry that load. The ACTUAL weight carried by each tire is a function of all the content, weight, and weight distribution and hence the tire pressure can be different and typically will be when completed by the RV mfg. If you want to understand more about the handling of an RV, read my post on what shocks, springs, and roll bars really do, I have not gotten a single question on it, I either made it too complex, or it conflicts with common beliefs that better shocks control vehicle roll, or can lift the body. Tires also play a roll in that also, as a tire side wall is a part of the "vehicle suspension rates". The rule should be always use the RV tire pressure recommendations when the RV is fully loaded, Do not use the Vehicle sticker as it is not tailored to the RV application. Many have said inflation is to optimize the tire contact patch, that is also true in that the greatest patch area will provide the best traction and wear. What I gave you was design load based, Remember above I said I tailor the ACTUAL tire pressure to the roads and loads and speeds I drive. Looking at the tire wear, 1 uniformly across, 2 center mostly, or 3 edges mostly are long term indicators of how well you've matched an ideal tire pressure with the loads and load distribution for your roads, speeds Does this answer your question(s)?