So I have a 2001 xj on a 3" OME lift with bilstein 5100s. When I first lifted the jeep the ride was nice and smooth and soaked up bumps nicely with stock tires. Now I'm running 31" mt/r kevlars and the ride has gone extremely harsh. I realize that the mt/r is a stiffer tire and I was prepared for some roughness but it feels as if the spring rates have changed completely. The suspension seems to just pound instead of soaking up bumps now. What is the effect of larger tires on the dynamics of the suspension system? Does the increased weight on the axle mess with the equilibrium of the suspension? Looking for some answers

 

What air pressure you running ?

 

http://en.wikipedia.org/wiki/Unsprung_mass
When you add bigger tires, you're adding more un-sprung weight. It will indeed have an effect on your suspension. Your spring rates haven't changed, but they are definitely working harder to compensate for the added mass. This is why when you start modifying, you can't just change one thing and expect the same result, you need to upgrade other things to get back to the ride characteristics you once had, LOL

 

I agree sounds like air pressure is to high for one thing. I run 33x12:50's with 4 1/2" of lift same shock as you but a Rustys lift different springs.


About has to be one of the 2 mine rides well considering all.

 

3rd vote for air pressure as the culprit. When I first installed my BFG KM2s the shop had the psi up at like 30 or 35. Dropped it down to 20psi and the ride is a million times better. Our XJs are so light that they don't require much PSI at all. The formula actually called for my tire pressure to be down around 15psi, but I wasn't comfortable going that low. I'll see if I can find the formula and post it up.

 

Here is is.

Try this equation.
Vehicle Tare Weight X .35% = (answer or lets say Z)
(Z) Divided by max load weight of your tire at max psi = (% of max load per tire or lets call Y)
(Y)% of max load per tire X max cold psi on tire = appropriate psi for each tire.

Example:
Vehicles weight. 5500 lbs.
Load range on example tire indicates 3525 lbs at 50 psi cold.

(5500 X .35) = 1925
(1925 / 3525) = .546% rounded to .55
(.55 x 50 psi) = 28 psi for each tire.

Note: Vehicle tare weight should include you and everything you normally put in your vehicle. Full tank of gas and so on.

 

Thanks for the input guys! Tires are at 35 psi, so I'll air them down a bit. Thank you for the unsprung weight explanation, that makes a lot more sense

 

So how does one counteract higher unsprung weight? Higher spring rates? Greater damping?

 

What is the load range of your tires? Lots of people comment about the increase in ride hardness on lighter XJs when they switch from Load Range C to D/E.

I know you said 31s (load range C if 31x10.5R15), but you may be running 16 wheels. I can't tell.

 

In your case, you don't. You already have a very good setup. You have bilstein shocks and an old man emu lift. As far as suspension goes, that is about as good as it gets. I say take the tires down to 25-28 psi.

 

Changing spring rates and damping rates will have an effect, as well as lowering your tire pressure, but the true way to have a lower unsprung weight, is to lighten the parts...alloy components as opposed to steel, hollow components, or weight reduced components...some of which are not feasible in our applications of modifying for off-road use...You can try better shocks, get some rebuildable shocks, and change the valving around, or different springs, but playing around with this stuff can get time consuming, expensive, and downright confusing...I would say try the tire pressure first, then the shocks. A good set of rebuildable shocks are going to cost you, though, they're normally used for higher end race applications and therefore will cost more.

 

On the highway I run 25 psi in my 33x12:50x15's (D range) off road 12-15 psi. In my 35x12:50x15" beadlocks (c range) tires 30 psi highway 5-10 psi off road.


PSI is the only thing you can do that costs nothing that should improve your ride somewhat. All of the others will require $$$ and labor to correct the issue.

 

Do the chalk test on the tires for the best air pressure for you.

 

interesting.. where did you find that?

 

That should be a sticky for the newbie's that have never even heard of it, I should have thought and posted that also...........But I'm "Old and Slow"