Installing Monotube Shocks Increases Ride Height?

[The Critic]

Hey guys,

I just installed a new pair of rear shocks on my father's 92 Toyota Previa.

The van originally came with twin-tube rear shocks. I upgraded and installed a pair of KYB Gas-A-Just Monotube shocks.

Prior to the shock replacement, the L/R ride height measured 29.25" and the R/R ride height measured just over 30".

After the replacement, both sides measure 30".

The old rear shocks were completely blown and would not rebound when compressed. I am guessing that with properly functioning rear shocks, the ride height has stabilized in some way.

My dad is now claiming that the van sits higher than it ever has. I did some searching online and found the following explanation - is this correct?

Quote:
Second thing about monotubes. They require high gas pressure to operate. Gas pressure creates a shaft extension force. This force works directly against the weight of the chassis; essentially a static spring rate (constant force, does not increase/decrease with travel like a coil spring). The force is the gas pressure x the shaft diameter. The larger the shaft dia. or the gas pressure, the greater the extension force. If you install a monotube shock on a vehicle that originally comes with low gas twin tube shocks the ride height will increase due to the increased extension force, unless you provide a way to change ride height like adj. coilovers, shorter springs, etc. The difference between a monotube and twin tube gas pressure is 2 bar vs 20 bar; fairly substantial.


http://www.bimmerfest.com/forums/showpost.php?p=257761&postcount=10

In addition, the rear of the van sits about 2.5" higher than the front. I do not know if this is normal, or if this is indicative of worn front springs. Any Previa experts here? I'd hate to spent $95/spring at the dealer, since there are no known aftermarket front springs available.

Thanks in advance.

 

[martinq]

The only thing changed was rear dampers? Any idea if the shaft diameter is the same from the TT to the mono?

Yes, gas pressure will create a jacking effect. Higher pressures and/or larger shafts increase this effect.

 

[The Critic]

Originally Posted By: martinq
The only thing changed was rear dampers? Any idea if the shaft diameter is the same from the TT to the mono?

Yes, gas pressure will create a jacking effect. Higher pressures and/or larger shafts increase this effect.


AFAIK, the physical dimensions are correct and the same as the old shock.

 

[martinq]

It could be that the OE are not gas charged or have lost it by now. How are the fronts? Have they been replaced?

Yeah, I see it's a 92. I would swap them to the same type if possible but check the springs for breaks.

 

[The Critic]

Originally Posted By: martinq
It could be that the OE are not gas charged or have lost it by now. How are the fronts? Have they been replaced?

Yeah, I see it's a 92. I would swap them to the same type if possible but check the springs for breaks.


Springs are visually OK. I have new front struts, mounts, upper/lower insulators and bump stops ready to go in.

 

[martinq]

It sounds like you're good to go.

I've heard tales of springs sagging but never seen it myself I have seen broken springs though. You'll know for sure once it's apart anyway. Throw in the new bits and report back. You should get a bit of a lift in the front as well.

 

[rfeir]

Normal and not a bad thing. Be sure to check the headlight aim though.

 

[eagle23]

Totally normal.

Depends on the shock mfg, but I've had Bilsteins (decades ago) raise the ride height on a late 70s Saab 99 considerably.

The increase in the shock pressure is also increasing rear roll stiffness akin to adding a rear sway bar (not exactly but similar). The van will handle differently, with less understeer than before, for quick steering inputs like a fast lane change. It will seem "quicker" that way, and actually be more responsive to those inputs.

As long as your dad's okay with that, fine. That's really the issue, as he's the regular driver. If not, you should change to OE-type shocks. The front springs are not likely the issue.

 

[gathermewool]

Interesting. I mentioned this effect after replacing a friend's blown struts (Nissan Maxima) with KYB-GR2's and was told here on BITOG, that there's no way that the struts could affect ride height.

 

[G-MAN]

I put KYB monotubes on the rear of my Olds minivan and didn't notice any change in the ride height. Also, way back in 1979 I put Bilsteins on my 78 Olds 88. I'd ordered this car with the F41 handling package, but putting those Bilsteins on there absolutely transformed that car as far as ride and road-holding.* Didn't notice any change in ride height back then, either.

*My dad was a dyed-in-the-wool "if it ain't broke, don't fix it" type. He thought I was throwing away nearly $200 on "fancy foreign shocks" that would not make one wit of difference. Even he admitted how much the ride improved after the Bilsteins went on. I don't think I ever took him down some of my favorite twisties to show him how much the handling had improved, too.

 

[martinq]

Originally Posted By: gathermewool
Interesting. I mentioned this effect after replacing a friend's blown struts (Nissan Maxima) with KYB-GR2's and was told here on BITOG, that there's no way that the struts could affect ride height.

If you understand basic hydraulics it's a very straightforward effect.

 

[stephen9666]

Originally Posted By: gathermewool
Interesting. I mentioned this effect after replacing a friend's blown struts (Nissan Maxima) with KYB-GR2's and was told here on BITOG, that there's no way that the struts could affect ride height.

Link to the thread in question?

 

[mechtech2]

I believe I said that.^^
And it was true for decades and decades and decades.
But with the advent of high pressure shocks/struts, things have changed.
Old gas charged shocks had little pressure and zero effect on ride height.
Some newer vehicles have high pressure shocks/struts, which are integral to the net ride height.
I don't like this at all, but it is the way it is for some.

 

[martinq]

Originally Posted By: mechtech2
Old gas charged shocks had little pressure and zero effect on ride height.

Any gas-charge in a damper will cause a force on the rod. That force will affect the ride-height even if it is imperceptible. It does not matter if the is 'high' or 'low' pressure.

There is a rare exception to this which is a dual-exit damper. A damper that has the rod exiting at both ends (think of the steering-rack).

 

[gathermewool]

Originally Posted By: stephen9666
Originally Posted By: gathermewool
Interesting. I mentioned this effect after replacing a friend's blown struts (Nissan Maxima) with KYB-GR2's and was told here on BITOG, that there's no way that the struts could affect ride height.

Link to the thread in question?
Originally Posted By: mechtech2
I believe I said that.^^
And it was true for decades and decades and decades.
But with the advent of high pressure shocks/struts, things have changed.
Old gas charged shocks had little pressure and zero effect on ride height.
Some newer vehicles have high pressure shocks/struts, which are integral to the net ride height.
I don't like this at all, but it is the way it is for some.


Yep:

Thread in Question

 

[mechtech2]

Originally Posted By: martinq
Originally Posted By: mechtech2
Old gas charged shocks had little pressure and zero effect on ride height.

Any gas-charge in a damper will cause a force on the rod. That force will affect the ride-height even if it is imperceptible. It does not matter if the is 'high' or 'low' pressure.

There is a rare exception to this which is a dual-exit damper. A damper that has the rod exiting at both ends (think of the steering-rack).


No.
A few lbs of pressure does nothing for ride height.
Being high or low pressure makes a great deal of difference.

 

[The Critic]

Thanks guys.

I've told my dad (and showed him via measurements) that the difference is not as significant as he thinks, and to reserve judgement until after I install new front struts. It will be interesting to see what effect new front struts will have on the ride height, since those are completely blown as well.

 

[martinq]

Originally Posted By: mechtech2
No.
A few lbs of pressure does nothing for ride height.
Being high or low pressure makes a great deal of difference.

Pressure makes a difference, sure but any pressure does affect the ride height, it has to. You cannot increase the force applied to the suspension without some reaction.

 

[SteveSRT8]

^^^Absolutely. Many older cars gain a wee bit of height when new shocks are put on due to the gas pressure.

 

[UG_Passat]

the stock shocks are the low pressure "twin tube" design. So, it really doesn't take much to compress a twin tube.

Many Koni shocks, in order to adjust (the rebound), it requires you to fully compress the shock, then twist.

the replacement.. is a monotube, which is a high pressure design. It does take more effort to compress them.