Tire pressures, recommended vs "best"

[booster]

We have had quite a few (understated) threads and posts on handling of various class B's and such, but not much has related to tire pressures. Sizes, yep, wheel offsets, yep, swaybars, yep, spring rates, yep.

I have always considered recommended tire pressures as a rough guideline only. Especially in rear drive stuff, cars and vans, the manufacturers go to the "always safest" mode of thinking. This is generally not the best handling or easiest to drive mode. The cars we drive almost always have had the tire pressure tweaked to "loosen them up" or reduce understeer and wander. Same with our Roadtrek.

An extreme example is the 96 Buick Roadmaster wagon we got recently. The factory recommended pressures are 30 front 35 rear, and at those pressures, with the GM power steering with built in wander, the understeer was very evident. You had to make large adjustments in the wheel to keep straight (1-2"). Looking into the tire loading, it became obvious they have it that way for a couple of reasons. It is an 8 passenger wagon, so the rears have to be able to handle all that weight, even if you never need it. It also is rear drive, without stability control, so they want to stay away from the loose rear, oversteer, at all costs. I have been moving down the rear pressure for a while, stopping at 28psi, with all changes improving the understeer and increasing steering response. I now just took the fronts up to 32psi, and it got worlds better. Steering corrections are now close to 1/2" and the steering just feels firmer.

We have done the same in our Roadtrek, but were very limited at the rear because the factory tires we at load max, or close to. The front we did increase from the recommended 50 psi to 65 psi with the factory tires. With the bigger tires, we have been able to reduce the pressure a bit on both ends and now are at 60 in the front and 73 in the rear, with moderate load. Reducing the front swaybar diameter allows us to go a little lower on front pressure without getting more understeer, and gives smoother ride.

All brands and styles of tires react differently, so you need to test a bit with new tires when you get them, after a bit of break in. The set of Kuhmo tires I had on my Escort liked 34 front, 30 rear, which was considerably different from the previous ones. They were also very sensitive to changes off of that pressure.

There is a relatively good test you can run pretty easily at home to see if you are close to the right pressures for you loading. Just go out on a clean road and make some fairly hard turns to scuff the tires clean, then with the tires still warm from driving, go through some dusty road or almost dry road (I mist the driveway and let it mostly dry). When you drive though straight, it will only get dust or water on the part of the tread touching the ground. If you are in the 1/8-1/4" of clean tread at the edge where it goes to the sidewall, that is pretty good. A little bit more in front than rear usually helps due to more cornering force in the front.

Wander and wind push are mostly from understeer IMO, and bigh vans, with high sides, and lots of rear weight, have lots of understeer. Here is a list of what is generally considered the reaction of a vehicle to changes in suspension stuff. It may not match charts you may see related to performance cars, as the weight on the wheels is opposite, because we are talking about staying in a straight line, not going around a corner without sliding off.

To reduce understeer, and improve steering response (less steering wheel movement needed)

Increase front tire pressure
Increase front tire width
Reduce front spring rate
Reduce front swaybar size
Reduce front shock rate (must still match springs)
Increase weight on front
Decrease rear tire pressure
Decrease rear tire size
Increase rear spring rate
Increase rear sway bar size
Increase rear shock rate (must still match springs)
Decrease weight on rear

Addressing oversteer is just the opposite, but rarely occurs for street vehicles

From the list, it is obvious that just about everything in a B increases understeer and wander.

Huge front swaybar, no rear bar
High rear weight compared to front
Larger side area in the rear where there is no sway bar
Higher tire pressure in rear

The exception we have right now is DW's 2009 CRV, which calls out 30 on both ends, and I find it a bit twitchy there. She likes it that way, but I think I would increase the rear a pound or two and lower the front a bit, to get it easier to drive (at least for us two finger drivers!)

[markopolo]

Great info, thanks!

It would be nice if we could all easily get the weights at all four corners of our vans. I know the fully loaded front and rear weights for my van but I don't know if the right rear corner is heavier than the left rear corner for example. Not knowing means I have to allow for that possibility.

I posted Michelin Tire PSI Inflation Tables here: http://classbforum.com/phpBB2/viewtopic.php?f=9&t=2158. That type of info is probably available on all tire manufacture websites.

My current fully loaded weights:
Front: 4040 lbs
Rear: 4780 lbs
Total 8820 lbs

and running Michelin LTX/MS2 LT245/75R16 E looks like I could go 50 front and 60 rear for a pillow top ride like your Roadmaster
But, not knowing the specific corner weights and knowing 50 in the front feels "soft" means 60 front and 70 rear would give me a better safety margin and probably a comfortable and good handling ride.

I went 70 front and 80 rear last trip. Handling was good but we definitely felt the bumps. I have a bit of play in the steering that I think is mechanical not inflation related. I might as well try 60 front and 70 rear next trip.

If someone doesn't know their weights in Chevy based fully loaded factory built Class B on 245 tires then maybe they should stay with 80 in the rear until they get a chance to visit the scales fully loaded.

Post your weigh scale results here: http://www.classbforum.com/phpBB2/vi...php?f=9&t=1783 - It would be nice to see some Sprinter weights......

[booster]

Those are the same tires we have in the 265/75/16 size. Very smooth ride compared to the Bridgestone V-steels it came with. The weight and pressure charts from Michelin always seemed to be way low when I have tried to use them. They may be OK based on load capacity, but don't seem to handle right.

The weight thing is interesting, and highly variable depending on how you are loaded at the time. Same with the cars, don't want to be running off to find scales all the time. That is why I have found the drive through the dust/water test to be useful. It will get me within 5psi most of the time, and then a little fine tune based on handling.

Most of the time, we are a little heavier than you, but not a lot, and found that 65 front and 80 rear was the best compromise of handling and comfort. That was with your tire size in the Bridgestones.

[booster]

I had just returned home from an errand in the Buick and noticed the driveway had left a pretty good pattern on the tires, and the Roadtrek tires had a good pattern from the last test drive. Here are pix of what they look like. Hope they make it easier to visualize what is going on with the tires and pressures.

This is the Roadmaster rear tire. Dust is nearly to the edge of the tread, so it could use a little more air pressure



This is the Roadmaster front tire. Pretty much the same as the rear, and could us a bit more pressure. The front and rear have similar patterns so the handling will be pretty well balanced, which is what I do see. I will add 2 psi to both front and rear and see if the ride gets harsher, and if it does, if it is worth a small handling increase. I wouldn't expect a big improvement in steering response, however, as they do match well as they are.



This is the Roadtrek rear at 73 psi and lightly loaded. It shows a fairly large area to the edge of the tread that hasn't has no dust, so hasn't hit. I like this pattern for the rear of the van because it shows that we still have load capacity left without adding any more air. You can even see that the leading edges of the tread blocks hit harder than the trailing edges.



This is the Roadtrek front at 60 psi. The no dust gap is much smaller than the rear. This is OK to me, as it was maneuvered a bit going into the garage, which rolls the front tires on the edge a bit, and the front does not pick up as much extra weight when loading.





From these observations:

The Buick is well balanced front to rear, but could use a little more pressure for max handling. May or may not be worth it based on ride harshness.
The Roadtrek is not optimally balanced as it sits, but will get better with loading. It is not bad, however, and drives very well. Repeating the test and minor pressure adjustments with full load probably could make it a little better, but it is very close as is.

[markopolo]

I like your idea of doing a "footprint" test.
I think I could do it in my garage and might have to do two tests; driving in and backing in. My driveway is 0 to 3/4 crush gravel and the road in front of the house is not even. It should work doing it in the garage. Actually, I might just be able to see it right now as parked a few days ago. It will be interesting to see what my 70psi fronts look like. Thanks for the photo explanation.

side note:
The 265 Michelin's have a 600lb load capacity difference at 60psi, 700lbs at 70psi and 750lbs at 80psi. Seems to me like they'd be a no-brainer replacement tire choice especially on the heavy RT 210 and PW Excel widebody Class B's if they fit and have the necessary clearance etc. I guess there would be a small mpg hit but easily offset by reducing the risk of a blowout.

[booster]

Originally Posted by markopolo

I like your idea of doing a "footprint" test.
I think I could do it in my garage and might have to do two tests; driving in and backing in. My driveway is 0 to 3/4 crush gravel and the road in front of the house is not even. It should work doing it in the garage. Actually, I might just be able to see it right now as parked a few days ago. It will be interesting to see what my 70psi fronts look like. Thanks for the photo explanation.

side note:
The 265 Michelin's have a 600lb load capacity difference at 60psi, 700lbs at 70psi and 750lbs at 80psi. Seems to me like they'd be a no-brainer replacement tire choice especially on the heavy RT 210 and PW Excel widebody Class B's if they fit and have the necessary clearance etc. I guess there would be a small mpg hit but easily offset by reducing the risk of a blowout.

We got a speedometer change with the tires, but the mpg stayed the same. With the 4.11 gears Roadtrek uses, a little less rpm seems to offset the more resistance. Plus the MS2 tires are called low rolling resistance, and the old Bridgestones weren't.

[booster]

I did the change of pressure on the Roadmaster to 34 front, 30 rear yesterday and we took it shopping. Same amount of pressure stagger as before but 2 psi higher on both ends.

The front at 34 psi



And the rear at 30 psi



They look a bit better on contact and handled well without getting very harsh. I think this is the spot for these tires. Funny thing is, now that all is set, this is the same pressures the old Escort wagon liked.

The factory recommended pressures were 30 front 35 rear, so this is a significant difference. I would have to increase the rear pressure if I had more rear load or passengers, though.