Unsprung weight

[cranetruck]

In the process of getting new tires for my 8x8, I noticed how much more the 395 weighs when compared to the original 16-20 tire. There is a 50 lb/tire difference in weight and that brings up the term "unsprung weight". Here is a short on the effect copied from one google hit:

"The sprung weight of a wheel controls: a trade-off between a wheel's bump-following ability and its vibration isolation. Bumps and surface imperfections in the road cause tire compression--which induces a force on the unsprung weight. In time, the unsprung weight then responds to this force with movement of its own. The amount of movement is inversely proportional to the weight - a lighter wheel which readily moves in response to road bumps will have more grip when tracking over an imperfect road. For this reason, lighter wheels are often sought for high-performance applications. In contrast, a heavier wheel which moves less will not absorb as much vibration; the irregularities of the road surface will transfer to the cabin through the geometry of the suspension and hence ride quality is deteriorated."

Something to keep in mind when installing oversized tires...

 

[jimk]

While increasing your unsprung weight is not-so-good, you got a lot of the good stuff working for you.

The high sprung weight . Say the vehicle weighs 22K unloaded, 20K of that sprung, you are adding 400lbs unsprung (50lbs x 8 wheels) to 1600lbs(200lb x 8 ) that exists (not including the axles, springs... ) you are increasing wheel unsprung weight only 2%(400/20000=.02), from the existing 8% (1600/20K=.08 )to 10%(2000/20000=.10). All these fig will improve when you add a load.

note- all these figs are guesses an I picked some easy #'s to work with.


You should be getting a lower air pressure with those big shoes. That will improve your ride improvement because the tire sidewall flexing has an extremely low unsprung ratio. Maybe that will a bigger plus than the added unsprung weight??

You have low road speeds (maybe/I hope), lower again off road. This works to your advantage because the wheel motion inertia is highly dependent on travel speed. You are at one end of the spectrum, a Formula 1 car is at the other. The latter needs the low unsprung ratio more than you...

 

[1stDeuce]

Just to make you feel a little better about what you're doing, my rough calculations show deuce with ~#150 tire/wheel combo (?) has about 11% of it's weight as unsprung tire. (Unspring total includes axles+half springs+half driveshafts also...)

My Jeep, with 33's, has about 8% of it's weight as unsprung tire weight. I consider the jeep to be at the limit for ride quality, while the deuce probably doesn't give a crap about adding 50lb to each position...

Think the fact that compared to traditional vehicles with relatively low unspring weight and soft suspensions, our big trucks are stiff enough that ride quality/terrain following at high speeds is totally moot. ?? For sure, the axles are a much more significan part of the weight than in a conventional vehicle...

That said, I'm singling out my deuce hoping to improve acceleration, ride, etc through a 600lb diet. (assuming a 9.00+wheel weighs about #150...)
YMMV
C

 

[cranetruck]

Thanks for the replies....

Jim, what kind of tire pressures are you suggesting, lower than the stock vehicle (xm757), which calls for 30 psi hwy/10 psi off road?

 

[jimk]

I may not have understood your original post. Are the 395's replacing the 16-20's or are they going on something else? I assumed you were adding bigger tires.

IMO the factory spec is always a good starting point, but there are reasons changing them for your useage. The book seems to have higher pressures for higher loads, lower press for off road. The super single tires run lower pressures than std tires on the same chassis.

The point I was trying to make are that SS run lower pressures and may ride better because of this. Can anyone chime in with real world experience?

Maybe I could also go as far as to say if you run empty all the time you can lower air pressure and get a better ride.

A few bits from TM9-2320-260-10 , Table 1-4 (page 1-23)(I can't seem to save/post this page). Press listed below are Highway/CC,

M812A1 14:00 x 20(SS) Frt 50/25, rear-30/25.
M819 12:00 x 20 Frt 105/80, rear-65/55
M816 11:00 x 20 Frt 70/60, rear-70/60
M813 14:00 x 20(SS) Frt 55/55, rear-55/55
M813 11;00 x 20, Frt 80/60, rear-50/30
M821 14:00 x 20(SS) Frt 50/40, rear-30/25

 

[cranetruck]

This thread ties in with several other ones, describing my pathway to replacement tires for the 8x8.
The 395s I ended up with are too big and heavy, so I won't be using them. The 14.5R20 XL or 365/80R20 XZL are better choices.