Optimizing suspension for washboard?

[rayra]

airing down and speeding up, generally. 4wd to help with the more squirrely result. Tradeoffs. Find the 'sweet spot' for your wheelbase and load. Tougher to do when in convoy, since the desirable speed is different for different vehicles.

 

[LandCruiserPhil]

No... Just no. Airing down is any time you reduce your tire pressures noticeably below "normal" for street use with a given amount of weight on them. If 60psi is "normal" for your rig, running at 40 counts as aired down.

First off please share the weight of your off road vehicle and tire combo are you running with a normal 60psi? Using the air pressure standards by the Tire and rim association of North America (linked below) 60psi would be excessive for a "normal" set up off road vehicle.

Link below the Application of load inflation tables, a guide used by most major tire manufacture use in North America and abroad

https://toyo-arhxo0vh6d1oh9i0c.stac...ication_of_load_inflation_tables_20170203.pdf

My set up weights in at 6950lbs and my normal running pressure 35/36psi and I run 15psi off road most of the time and without any tire issue providing a smooth ride with exceptional traction even on washboard roads. I average around 3000+ dirt off road miles a year so I aire down a lot.

 

[rruff]

First off please share the weight of your off road vehicle and tire combo are you running with a normal 60psi? Using the air pressure standards by the Tire and rim association of North America (linked below) 60psi would be excessive for a "normal" set up off road vehicle.

People using stock size tires and heavy camper setups can have 60 psi "normal". But I agree, a lot of people are overinflated. What tires do you run?

 

[comptiger5000]

People using stock size tires and heavy camper setups can have 60 psi "normal". But I agree, a lot of people are overinflated. What tires do you run?

Exactly. Pretty much any Jeep, etc. won't run that much pressure. But a rig built from a 3/4 ton or 1 ton pickup easily could need that amount of pressure when it's fully loaded, especially if they're not running much bigger than stock tires.

 

[LandCruiserPhil]

People using stock size tires and heavy camper setups can have 60 psi "normal". But I agree, a lot of people are overinflated. What tires do you run?

LT315/75-16 - 6950#
LT275/70-18 - ~5700#
LT295/75-16 - ~5800#

And none are inflated over 36psi for street based on the guidelines recommend by the manufacture.

Most new stock tire are P tires that are not off road rated. If one stock vehicle is equipped with a heavy camper they should not be running a P tire.

 

[LandCruiserPhil]

Exactly. Pretty much any Jeep, etc. won't run that much pressure. But a rig built from a 3/4 ton or 1 ton pickup easily could need that amount of pressure when it's fully loaded, especially if they're not running much bigger than stock tires.

Can you sight an real time example based on your post?

 

[LandCruiserPhil]

If you run a heavy vehicle with a correct size/rated tire for you needs or any sized vehicle meeting the same I recommend the following for determining a safe workable air down pressure. Take a 1" square piece of steel square tubing and place it across the width and centered under your tire and reduce the pressure until both the front and rear of the tire touches the ground. This will give any set up a very close idea on an optima airing down pressure.

 

[rruff]

If one stock vehicle is equipped with a heavy camper they should not be running a P tire.

Of course not, but stock size LT E tires on a 3/4 or 1 ton will have somewhere around a 3,200 lb rating at 80 psi, and loading the rigs to over 10k lbs is not unusual. Hence 60 psi on the road isn't that weird.

My tires are 3860lbs @65 psi, and my load will be less than half that, so it's a big difference.

 

[LandCruiserPhil]

Of course not, but stock size LT E tires on a 3/4 or 1 ton will have somewhere around a 3,200 lb rating at 80 psi, and loading the rigs to over 10k lbs is not unusual. Hence 60 psi on the road isn't that weird.

My tires are 3860lbs @65 psi, and my load will be less than half that, so it's a big difference.

I would agree on a 60psi on a 10K vehicle and with stock tires would not be out of line.

With your tires rating and your weight (<5000#) what do you run on the street and air down to? Also how did you determine your air down psi?

 

[rruff]

With your tires rating and your weight (<5000#) what do you run on the street and air down to? Also how did you determine your air down psi?

My weight is ~5700 lbs unloaded (no camper yet). I'm running 35psi front and 30 rear. I haven't aired down on dirt or gravel yet, because I haven't been on long stretches.

Per the Toyo link (page 11), my stock P tires were 255/70r18, 30 psi, which corresponds to 2290 lbs on the table (page 17). Divided by 1.1 because it's on a light truck, gives 2082 lb.

Referring to the LT table for the 325/65r18, shows a load of 2535 lbs @35 psi, which is the lowest pressure given. If I'm doing this right, 35 psi would be the recommended pressure for an evenly distributed 10k+ load on the street. Extrapolating it looks like the recommended pressure would be ~20 psi on my 5700 lb truck! But I don't know if extrapolating is sensible. At any rate, at 35f and 30r I'm running less pressure than anyone else I know.

 

[comptiger5000]

Let's take a crew cab, long bed F-350 SRW as an example. Almost 8000 lbs of truck. Let's figure that empty, it's got 2500 lbs on each front tire and 1500 on each rear. Stock tires on many are a 275/65R18 Load Range E (80 psi max).

Let's put a 3500 lb camper shell in the bed and a family of 4 (150 lbs each) in the cab. Now we've added say 3600 lbs to the rear axle (which is likely close to max load) and 500 lbs to the front axle.

That brings us up to 2750 lbs on each front tire and 3300 on each rear tire. Based on the inflation tables for those tires, we need a minimum of 60 psi in the front tires and 75 psi in the rear tires.

If we upsize the tires to a 295/70R18E (34" tall instead of 32" but still 80 psi max), we gain some load rating, so now we only need 45 psi in the front tires and 60 in the rears. Going a bit bigger to a 305/70R18E (35" tall, 65 psi max) lets us run with 45 psi in the front and 55 in the rear.

So it's easy to see how you can need a lot of pressure with LT tires if they're not massively over-speced in weight carrying ability.

 

[LandCruiserPhil]

My weight is ~5700 lbs unloaded (no camper yet). I'm running 35psi front and 30 rear. I haven't aired down on dirt or gravel yet, because I haven't been on long stretches.

Per the Toyo link (page 11), my stock P tires were 255/70r18, 30 psi, which corresponds to 2290 lbs on the table (page 17). Divided by 1.1 because it's on a light truck, gives 2082 lb.

Referring to the LT table for the 325/65r18, shows a load of 2535 lbs @35 psi, which is the lowest pressure given. If I'm doing this right, 35 psi would be the recommended pressure for an evenly distributed 10k+ load on the street. Extrapolating it looks like the recommended pressure would be ~20 psi on my 5700 lb truck! But I don't know if extrapolating is sensible. At any rate, at 35f and 30r I'm running less pressure than anyone else I know.

Great info and data thanks for sharing

 

[LandCruiserPhil]

Let's take a crew cab, long bed F-350 SRW as an example. Almost 8000 lbs of truck. Let's figure that empty, it's got 2500 lbs on each front tire and 1500 on each rear. Stock tires on many are a 275/65R18 Load Range E (80 psi max).

Let's put a 3500 lb camper shell in the bed and a family of 4 (150 lbs each) in the cab. Now we've added say 3600 lbs to the rear axle (which is likely close to max load) and 500 lbs to the front axle.

That brings us up to 2750 lbs on each front tire and 3300 on each rear tire. Based on the inflation tables for those tires, we need a minimum of 60 psi in the front tires and 75 psi in the rear tires.

If we upsize the tires to a 295/70R18E (34" tall instead of 32" but still 80 psi max), we gain some load rating, so now we only need 45 psi in the front tires and 60 in the rears. Going a bit bigger to a 305/70R18E (35" tall, 65 psi max) lets us run with 45 psi in the front and 55 in the rear.

So it's easy to see how you can need a lot of pressure with LT tires if they're not massively over-speced in weight carrying ability.

With the recent data post there is no middle of the road. The vehicles are either heavy trucks with campers @~9K+ or SUV types ~@6K+
Admittedly most of the vehicles I travel with are less than 7500#. Easy to see the difference with air pressures.

With the above data what are you airing down to?

 

[hemifoot]

another thread gone to ********, why don't you guys pm each other so we don't have to wade through this.

 

[rruff]

I think we've already established that airing down is the best suspension "upgrade" for washboard. Now we are trying to figure out how much.

As for shocks I was told that lighter initial damping is best (progressive rather than digressive), and it's nice to have adjusters to fine tune. So either a $3k set of King or ADS shocks, or the cheap Ranchos.