Tire Safety Factor on Towables

[itat]

The RVIA says that as of September 2017 the factory is supposed to install tires on towable RVs that have a weight rating such that your trailer's tires can handle at least 110% of the axle's GAWR.

So, for example, if, like me, you have 5000# axles and ST225/75R15 tires, each tire should have a weight rating of at least (5000# x 1.10 / 2) = 2750#. To achieve that, you'll need Load Range E tires with a max. capacity of 2830# at 80 psi. Load Range D tires only have a max. capacity of 2540# at 65 psi.

At a max. capacity of 2830#, LR E tires on 5000# GAWR axles have a safety factor of 113%. Keep in mind that your actual weight on the axles should not get to the axle's GAWR so your safety factor would be even higher if the axles aren't overloaded.

I weighed my truck and 5er on our first trip that went near a CAT scale and the 2 axles (4 tires) weighed 8820#. That's about 2205# per tire. That means my Load Range E Castle Rock tires are well under their max. capacity and I have a safety factor of approx. 128%. Now, I know they are still Castle Rock tires but that extra safety margin does give me some comfort.

So, my point is .... you should weigh your trailer and find out how much weight each tire is actually carrying when you're fully loaded. Find out how much safety factor your tires have. How much safety factor you want to have is up to you. What you can achieve is dependent upon what is available in your tire size.

[itat]

One other point I forgot to note in my first post is that when you're unhooked and sitting at the campground, you're also adding the weight of your family members and any contents in the tanks. If you travel with water in the tanks, you should weight your setup like that to get a real number since water is heavy.

[ependydad]

Originally Posted by itat

I weighed my truck and 5er on our first trip that went near a CAT scale and the 2 axles (4 tires) weighed 8820#. That's about 2205# per tire. That means my Load Range E Castle Rock tires are well under their max. capacity and I have a safety factor of approx. 128%. Now, I know they are still Castle Rock tires but that extra safety margin does give me some comfort.

So, my point is .... you should weigh your trailer and find out how much weight each tire is actually carrying when you're fully loaded. Find out how much safety factor your tires have. How much safety factor you want to have is up to you. What you can achieve is dependent upon what is available in your tire size.

So the challenge is that there can be incredible un-balance between the two sides and front to back tires. You can't just take axle weight rating / 4 and get what the tire is carrying.

I did the Escapees Smart Weigh back on my first rig and was surprised to see a 600 lbs. difference between the highest and lowest tires.

[myredracer]

Or put another way (and I think more common way) is you want at least 10% reserve load capacity. Very important on ST tires. Roger Marble (retired tire engineer), on his RVtiresafety.net blog, recommends a min. of 15% reserve capacity compared to the actual load on the tires. Load capacity is at the max. side psi too. Some want to reduce psi as per load tables. ST tires should be run at max. sidewall psi and checked periodically.

Overloading tires from axle to axle and/or side to side is not uncommon. It's ALWAYS a good idea to run your TT or FW through a scale to get your weights. Good to know relative to the GVWR and axle ratings too. Also a good idea to run through a scale when you have it fully loaded for your first camping so if you have have a tire, frame or axle issue, you have a scale ticket to prove you weren't overloaded.

The more reserve load capacity you have, the less it will heat up internally. Heat is what kills ST tires which includes from running under-inflated or over it's speed rating. We upgraded from LRC to LRD when we bought our TT and have around 30% load reserve capacity. Zero issues in 5 years on our Chinese Marathons (about to be replaced with Endurance).

Too bad RVIA doesn't have a similar requirement for axles. I've read many reports of bent axles due to axle ratings too close to GVWR. I saw a TT on display in CW not long ago (brand name withheld) and the tire's load rating was very close to the GVWR. and the CCC was quite low (so it would be close to it's GVWR fully loaded).

Unfortunately, many RV-ers out there have no knowledge about ST tires and how to care for them. And when they get a tire failure they blame the country of origin. Not their fault because the RV industry does nothing to inform/educate buyers.

[myredracer]

Originally Posted by itat

One other point I forgot to note in my first post is that when you're unhooked and sitting at the campground, you're also adding the weight of your family members and any contents in the tanks. If you travel with water in the tanks, you should weight your setup like that to get a real number since water is heavy.

Good thought but static weight when parked is not an issue (unless you loaded a ton of rocks inside). Dynamic loading & stress on tires when going down the road is way higher.

[itat]

Originally Posted by myredracer

Good thought but static weight when parked is not an issue (unless you loaded a ton of rocks inside). Dynamic loading & stress on tires when going down the road is way higher.

That’s why I added the last sentence of my second post talking about traveling with contents in your tanks.

[itat]

Originally Posted by ependydad

So the challenge is that there can be incredible un-balance between the two sides and front to back tires. You can't just take axle weight rating / 4 and get what the tire is carrying.

I did the Escapees Smart Weigh back on my first rig and was surprised to see a 600 lbs. difference between the highest and lowest tires.

Thanks. I understand that it won’t be perfectly distributed but the imbalance in your numbers is surprising. Without having access to a scale like the Escapees Smart Weigh folks have its impossible to know what the individual tires weigh and you’re forced to make assumptions. Your lighter tire was off by about 5% of the total trailer weight. Did that trailer have larger opposing slides in the back and one on the drivers side up front?

[ependydad]

Originally Posted by itat

Thanks. I understand that it won’t be perfectly distributed but the imbalance in your numbers is surprising. Without having access to a scale like the Escapees Smart Weigh folks have its impossible to know what the individual tires weigh and you’re forced to make assumptions. Your lighter tire was off by about 5% of the total trailer weight. Did that trailer have larger opposing slides in the back and one on the drivers side up front?

4 slides:
- 3 on the passenger side (rear, middle, and front).
- 4th was rear passenger side.

[bedubya]

Originally Posted by ependydad

I did the Escapees Smart Weigh back on my first rig and was surprised to see a 600 lbs. difference between the highest and lowest tires.

Do you know of any SmartWeigh type service in the upper midwest? Anyone?

[ependydad]

Originally Posted by bedubya

Do you know of any SmartWeigh type service in the upper midwest? Anyone?

Unfortunately, they're so few and far between. Escapee's does it in limited placed, I just found a Facebook page ("RV Safe Weigh") who is offering it in Nor Cal, and the RVSEF does them at different events.

Unfortunately, the first 2 options aren't helpful for you. Maybe you can catch an event and get it done.

[itat]

Despite the apparent reasonable safety factor with my Castle Rock tires, I’ve noticed that the wearing surface seems to be a bit concave (smaller diameter in the centre of the tread). It doesn’t look like uneven wear. I keep them at the maximum 80 psi. So i’m concerned about them.

[Taranwanderer]

Started with LRE ST tires when new, then upgraded to Sailun LRG, ST, same size. Overkill is underrated.👍

[pgandw]

Originally Posted by itat

Despite the apparent reasonable safety factor with my Castle Rock tires, I’ve noticed that the wearing surface seems to be a bit concave (smaller diameter in the centre of the tread). It doesn’t look like uneven wear. I keep them at the maximum 80 psi. So i’m concerned about them.

I also have a slight concave across the tread of my Castle Rocks, and I have heard others say the same. Being a 2019, the tires have 15% reserve at max GVW (20% at my normal weight). I do set mine at 66-68PSI (LR D) for each trip, depending on whether I am going up or down in altitude.

Based on number of reports of slight concave treads on Castle Rocks, I am assuming they are made that way. I did have mine balanced, and my trusted tire guy said the tires were good, and minimal weight was needed (highest was 2.5 oz). I will monitor, and replace at 1st indication of anything wrong.

Fred W
2019 Flagstaff T21TBHW A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time

[SailorSam20500]

Originally Posted by bedubya

Do you know of any SmartWeigh type service in the upper midwest? Anyone?

RVSEF was offering individual tire weights at the last 2 International Rally's in Goshen. The RVSEF website doesn't have FROG listed, several other rallies are on the schedule.


Maybe one of the other providers will be there.

[itat]

I ended up replacing the Castle Rock tires with Goodyear Endurance yesterday. I had figured on replacing them next year when I first bought the trailer so when I saw the concave wearing surface I figured I would just replace them this year. I was amazed at the difference between the two tires. The GYs are substantially heavier and feel more solid despite both being LR E tires. We are heading out tomorrow for a 3-4 hour drive to a Provincial Park so I’ll be able to judge whether or not the 5er tows better with them on. There doesn’t appear to be anything suspect on the surface but the significant difference in how solid they are makes me much more comfortable that I’ve switched.

[B and B]

Which Park?