Tire pressures

[newmarpusher]

Originally Posted by Big-Foot

With very rare exception - I run the pressure that the manufacturer of the tire stamps on the sidewall as the cold inflation pressure.

You've got too much air in your tires. You are inflated to the maximum load the tires are capable of carrying. This can reduce the contact patch, give a rough ride and cause handling issues.

Without weighing, you'd get a closer guess by using what's posted on the door placard, which assumes you are at maximum GVWR. Even then, you'll probably have too much air in the tires unless you happen to be at the max GVWR.

You should get your rig weighed, then consult the tire manufacture's charts and inflate the tires to the proper pressure for the weight they are carrying, not the maximum weigh they are capable of.

["007"]

Well said Pusher than we can all relax.

[Big-Foot]

Originally Posted by newmarpusher

You've got too much air in your tires. You are inflated to the maximum load the tires are capable of carrying. This can reduce the contact patch, give a rough ride and cause handling issues.

Without weighing, you'd get a closer guess by using what's posted on the door placard, which assumes you are at maximum GVWR. Even then, you'll probably have too much air in the tires unless you happen to be at the max GVWR.

You should get your rig weighed, then consult the tire manufacture's charts and inflate the tires to the proper pressure for the weight they are carrying, not the maximum weigh they are capable of.

You're stating facts (reduced contact patch) that are true for a BELTED TIRE but not necessarily true for RADIAL tires.
I've worked 1:1 with tire engineers from BF Goodrich, Goodyear and Kumho tires as late as 2010 before I retired from active road racing competition. We ran tires for the manufacturers under sponsorship programs run by them. We ran inflation pressures that would bring us up to and even above the max cold inflation pressures embossed on the sidewalls. Their contention was (in all cases) that the pressure needs to be higher in a radial tire to maintain the proper profile of the tire as well as the contact patches.
We doubted at first (BFG was the first we worked with) but later verified with a first place win along with pretty consistent tire temps taken at 5 locations across the face of the tires.
Now this was racing and in few respects is it similar to highway use. Both the race car as well as our RVs put a substantial amount of pressure on the tire in lateral loading (cornering) as well as linear deceleration rates (braking)..
I have taken my tire temps on my truck and trailer and have seen similar results (even pressures and temps)..
Believe or not - your choice.

BTW - want to read more about what can happen when you don't pay attention to the tire manufacturers recommendations for inflation pressures? Google up Ford Explorer Firestone Tires 26 PSI. Ford was concerned about the "Rough ride" of the Explorer.. That should do it..

Attached pic is of me and my bride on the victory lap as we captured the 2005 Triple Championship for the SCCA's Central Division Club Racing venue.



Regards - Randy & Dar
2015 F350 CC-DRW-409 Diesel / 2008 Rage'n Stryker 4005 Toy Hauler / 2014 Arctic Cat Wildcat Trail-XT
Sent from my iPad using iRV2 - RV Forum

[rmcobb]

OK, I've read all the tire pressure threads, and looked at the inflation charts, but can someone explain how to read them.

I just weighed the coach, and while it was only obtaining axle weights, please explain where I fall in the inflation chart, and how you derived that from the chart. The arrow is indicating my tire size.

Front weight = 10,300
Rear weight = 20,760
No tag

[Mr_D]

Originally Posted by RandyKathyM

So what some of y'all are saying is the engineers at Goodyear and Michelin are a bunch of yahoo's and they make up these charts, so we should just ignore them.

I always love it when the armchair engineers get involved!

But even CC has changed their stance on pressure, here's what my '09 Magna owners manual says:

Tire Industry Changes
The tire industry, as a whole, has changed its traditional stance on adjusting cold tire inflation pressure for RV tires installed on recreational vehicles and buses. Previously, tire manufacturers supported a policy where tire inflation could be adjusted according to the actual loaded weight of the vehicle. Now, the major tire manufacturers recommend that medium duty truck tires be maintained at the pressure that corresponds to the Gross Axle Weight Rating for the axle to which they are mounted. To make this recommendation uniform across the industry, tire manufacturers strongly urge the consumer to keep all tires inflated to the pressures recorded on the Federal Tire Label.

[newmarpusher]

Originally Posted by rmcobb

OK, I've read all the tire pressure threads, and looked at the inflation charts, but can someone explain how to read them.

I just weighed the coach, and while it was only obtaining axle weights, please explain where I fall in the inflation chart, and how you derived that from the chart. The arrow is indicating my tire size.

Front weight = 10,300
Rear weight = 20,760
No tag

Your front tires would be 5150 (assuming perfectly balanced side to side)= 85psi

Your rear tires are each carrying 5190 (again assuming perfectly balanced side to side)= 100 psi

Most people including myself would add 5 psi to be on the safe side and account for any side to side weight discrepancy. so:

Front: 90psi
Rear: 105psi

[rmcobb]

Thanks Pusher, that makes sense. I've heard you should make all tires the same pressure, and not sure how accurate that is, but Thanks.

[Mr_D]

Originally Posted by rmcobb

Thanks Pusher, that makes sense. I've heard you should make all tires the same pressure, and not sure how accurate that is, but Thanks.

All tires across the same axle should carry the same pressure, which would be what is required for the heaviest side, that's why four "corner" weights are so important.

[newmarpusher]

Originally Posted by Mr_D

All tires across the same axle should carry the same pressure, which would be what is required for the heaviest side, that's why four "corner" weights are so important.

Exactly, agreed. That's why I made the point of stating "assuming equal side to side weights" and adding 5psi to account for that.

[Bill-MACV21]

OK. Since this is so clear cut let me add one more element to all of this sage advice. Irrespective of your logic on tire pressure, the the rims have a MAX PRESSURE RATING that limits the high side.

So before you run off and pump your tires up just make sure you check the rims.

My world:
Front calcs are for 116 lbs
I try for 120 with my MADP compressor. About my max attainable pressure.
Tires are rated for 130 lbs.
Alum rims are rated for 130 lbs.

I have so much extra load carrying capacity with my drive and tag that it would be foolish to run anything near max pressure. The calcs are somewhere off the bottom of the chart so I run the min +5. Rim ratings are immaterial in this case.

Sure glad we are getting to a clear cut decision on this subject.

[Big-Foot]

Those of you that are adjusting pressures based on corner weights - have you stopped to think about when those weights are really in play?
When your rig is static (not moving) and possibly even unloaded (depending upon when you scaled the rig).
If all you had to be concerned about is weight that never changes, you'd probably be just fine.
But - that's not really the case is it.
The corner weights of your rigs is dynamic and they will change as the vehicle accelerates or decelerates laterally or longitudinally .. Cornering forces are higher still on steer tires and you will exceed your envelope easily - particularly as you turn and brake concurrently.
The laws of physics are sometimes complex, but they cannot be denied.
Safe travels..


Regards - Randy & Dar
2015 F350 CC-DRW-409 Diesel / 2008 Rage'n Stryker 4005 Toy Hauler / 2014 Arctic Cat Wildcat Trail-XT
Sent from my iPad using iRV2 - RV Forum

["007"]

Originally Posted by Bill-MACV21

OK. Since this is so clear cut let me add one more element to all of this sage advice. Irrespective of your logic on tire pressure, the the rims have a MAX PRESSURE RATING that limits the high side.

So before you run off and pump your tires up just make sure you check the rims.

My world:
Front calcs are for 116 lbs
I try for 120 with my MADP compressor. About my max attainable pressure.
Tires are rated for 130 lbs.
Alum rims are rated for 130 lbs.

I have so much extra load carrying capacity with my drive and tag that it would be foolish to run anything near max pressure. The calcs are somewhere off the bottom of the chart so I run the min +5. Rim ratings are immaterial in this case.

Sure glad we are getting to a clear cut decision on this subject.

Sounds a bit to high for my safety, but if your happy with the psi's than go for it.

[newmarpusher]

Originally Posted by Big-Foot

Those of you that are adjusting pressures based on corner weights - have you stopped to think about when those weights are really in play?
When your rig is static (not moving) and possibly even unloaded (depending upon when you scaled the rig).
If all you had to be concerned about is weight that never changes, you'd probably be just fine.
But - that's not really the case is it.
The corner weights of your rigs is dynamic and they will change as the vehicle accelerates or decelerates laterally or longitudinally .. Cornering forces are higher still on steer tires and you will exceed your envelope easily - particularly as you turn and brake concurrently.
The laws of physics are sometimes complex, but they cannot be denied.
Safe travels..


Regards - Randy & Dar
2015 F350 CC-DRW-409 Diesel / 2008 Rage'n Stryker 4005 Toy Hauler / 2014 Arctic Cat Wildcat Trail-XT
Sent from my iPad using iRV2 - RV Forum

Those use loads are already baked into the manufacturer's cold inflation specs. If they weren't, by what you're saying a coach that rolls over the scales at exactly max GVWR and tires set for that weight, would be dangerously over weight when driving down the road.

It's all relative. If you're 8,500lbs under GVWR like I am, you can certainly run less air than the same guy who is 8,500lbs heavier.

Check out http://www.michelinrvtires.com/refer...tion-tables/#/, where Michelin clearly spells out how to adjust your RV tire's pressure to correspond to the weight you're carrying.

[Big-Foot]

Interesting chart (Michelin) but not representative of the heavier Load Range G tires I am using.
The point I've been trying to make all along is to use the Tire Manufacturers pressures and not necessarily the pressures from the Coach Builders.. Frankly even the high end coach and trailer builders seem to employ some questionable manufacturing techniques - so I am much more comfortable using the specifications from the people that know the tires the best.

Anyway - I believe that the GVWR is based on Axle and not on Wheel rates - so that helps with some scenarios. Anti-sway bars and active Air Ride systems help out a lot as well. Unfortunately - I have heard of very few that work with trailers..

The tire manufacturers are the ones with skin in the game when it comes to the performance of the tires - not the coach builders... Ford thought they knew more than Firestone. They made recommendations to run tires that (unbeknownst to them) were already marginal, at a lower tire pressure in order to improve the ride of a truck (Explorer) to be consistent with a car.. They killed people in the process.
My parting advice on the subject is to be safe by being fully informed with facts and not lore. While I am degreed, it is not in tire sciences.
Do your research carefully and process it by your own means and needs.





Regards - Randy & Dar
2015 F350 CC-DRW-409 Diesel / 2008 Rage'n Stryker 4005 Toy Hauler / 2014 Arctic Cat Wildcat Trail-XT
Sent from my iPad using iRV2 - RV Forum