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I, along with a local shop I think I trust, concluded my tire cupping is being caused by my shocks and struts being worn out. They've got 80K miles on them and I want to replace them all. Where I've been using the Wrangler Duratrac tires that weigh a lot more than the stock tires, that's a lot more weight for the struts and shocks to keep from bouncing all over the place. What do you all know about replacement struts and shocks, if any, that are specifically engineered to handle those higher forces? My concern is that 'regular' ones, even so-called heavy duty products, perhaps are engineered only to handle the weight of stock tire/wheel combinations. And maybe that's why my factory Bilsteins haven't been keeping my front tires firmly on the pavement?

-John
 

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I, along with a local shop I think I trust, concluded my tire cupping is being caused by my shocks and struts being worn out. They've got 80K miles on them and I want to replace them all. Where I've been using the Wrangler Duratrac tires that weigh a lot more than the stock tires, that's a lot more weight for the struts and shocks to keep from bouncing all over the place. What do you all know about replacement struts and shocks, if any, that are specifically engineered to handle those higher forces? My concern is that 'regular' ones, even so-called heavy duty products, perhaps are engineered only to handle the weight of stock tire/wheel combinations. And maybe that's why my factory Bilsteins haven't been keeping my front tires firmly on the pavement?

-John
You are correct sir.

The VALVING of the stock tires is lighter than the valving for the shocks and struts meant for lifted rigs with heavier tires.

Essentially, the amount of resistance over a time frame that a shock presents to shaft motion can be designed to be faster or slower, and, require higher or lower pounds of force (They use Newtons for the rating scale).

So when a shock is extending to its full length, say after being compressed by a bump, that's called REBOUND.

As rebound would be like YOU straightening your legs after going over a mogul on skis, to try to keep the skis on the snow, its really more about YOUR weight that needs to be controlled.

IE: If you pushed your legs down TOO hard/TOO fast, it would launch you up again, and too soft/Slow would not reach the snow in time to keep in control.

Rebound is therefore mostly helping to control a rig's SPRUNG WEIGHT - the frame and all the stuff on it that the suspension (springs) are holding up.


When you hit the next mogul, your legs need to tuck back up...which for the SHOCK is called COMPRESSION - its really taming (DAMPING) the bounce UP of the tires and axles, etc...the UNSPRUNG WEIGHT of the rig: The tires, axles, and stuff the suspension is NOT holding up.

When shocks are TOO stiff on compression, it takes too much force to compress the shock, and INSTEAD of the tire being able to rise over the obstacle/bump w/o changing the truck's attitude, the shock essentially acts more like a stiff rod that transmits the road's force up and picks the truck up with the tire, instead of JUST the tire rising w/o upsetting the truck.

If too soft, the tires are sent flying up too easily, and you can crash the upper stops...and/or the rig will DIP because that corner is not being supported, etc.


A PERFECT suspension would allow your rig to glide over any obstacles, with the tires rising and falling as they follow the terrain, and your rig gliding along as if you were going across a pool table.


So if you lift the rig, it raises the center of gravity, which increases the LEVERAGE forces exerted on the suspension in a turn or off camber, etc. That raises the EFFECTIVE sprung weight...the shock has more forces to damp.

If you add armor, passengers, heavy cargo, the same thing happens, there is more sprung weight to damp, and, you therefore need more REBOUND DAMPING to compensate.

This is one reason the advertisements for cheap lift kits that DON'T include shocks say stuff like "Preserves the factory ride by using your stock shocks", are misleading, because the added COG will exceed the control parameters of the stock shocks, and you get a softer less controlled ride, NOT the same ride.

:D



So, more COG/weight = more Sprung Weight = need for more REBOUND valving.

Conversely, if you add heavy tires or a larger axle, etc, it will not require more rebound even though the RIG weight went up...as that weight down there is UNSPRUNG weight, and to control that, you need more compression valving to compensate.

There are obviously some interrelationships, but, this division of labor is the easiest way to break it down.


Bils are labeled in terms of Rebound/Compression. Rebound values are higher, as they mostly control the sprung weight which is a LOT larger than the UNsprung weight.


A live axle valving is typically a LOT lower than what an IFS needs, because the lever arm is long on a live axle and very short on IFS...so the IFS shock/strut etc...is positioned so a small amount of motion at the shock = a large amount of wheel travel (Think about the space at the BASE of a pair of scissors, and how big the gap is at that base/handle end vs how far the TIPS of the scissor travel when opening/closing it)

If you had to cut something tough, you'd try to cut closer to the base, you'd have more power with a smaller gap....and that power is what's EXERTED ON the shock....so the numbers to DAMP that force are proportionally higher on your IFS end than for your live rear axle end.

IE: You DON'T get the same valving for the front/rear.


:D




Again the stock shocks are made for stock unsprung weights, not for aftermarket meat.

As a rule of thumb, for X's, most people like 10 -20% more valving in both rebound and compression if they do a 3" SL and 33-35's for example.

The magic number can depend on how heavily they are loaded down and personal ride preferences, how they attained their lift (New pack, AAL, shackles, coil overs, t-bars, etc...etc.)

For example, the stiffness of the leaf pack or coils is PART OF what has to be damped.

If you'd ever seen a pogo stick, etc, you know that if you compress a coil, it tends to oscillate, and get longer/shorter over and over again..so with NO shock, your rig would hop along like you were riding a pogo stick.

If you increase the spring rate, so the force the spring is adding to the rebound for example goes up, so does the added valving needed to just damp the coil's contribution.

That's ONE of the reasons why a kit is nice, someone already worked out what works for you, and you don't need to break out a calculator and physics book to figure it out...and then try to FIND shocks that meet your specs.

The reason it can NOT be nice, is if YOUR rig is different enough to change what works.

:D





The OTHER thing is TRAVEL.

The stock shocks are SHORT - and even fully extended, they can prevent your rear wheels from reaching the ground on uneven terrain.

Aftermarket shocks can be longer, so your tires have a better chance of reaching something useful. Of course, as the piston has to still fit INTO the shock's body, that means that the longer the piston, the longer the body.

This would be fine, except that on compression, you need the shock to get short enough to still fit w/o blocking full UPTRAVEL.

Cheap kits just get you the droop with a cheap long shock and, give you bump stop extensions (Evil uptravel robbing bump stop extensions) to prevent over compression.

That's fine if your 44" tractor tires can't rise up any further anyway, but, on most X's, you'd rather die that surrender an inch of wheel travel.



So after you figure out what valving to get, you THEN look at the compressed and extended lengths to get something that is valved well, and, will allow full droop and full stuffage.


:D
 

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Discussion Starter #4
Yup, thank you for that! :) I should maybe have mentioned I'm just running stock suspension and ride height. I'm trying to find out specific strut and shock models that are intended to handle the heavier tires I'm using. Though I'm not sure it sounds like they are sold from that point of view? Maybe I have to do all the calculations myself and then look for units made to the specs I come up with?

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