New shocks = higher ride height

[matt36415]

I didnt expect this, when I googled the issue, lots of people on other forums say it is not possible but....

 

I replaced the original shocks on my 2007 V6, 222 000km. The car now sits an inch higher than it did with the old shocks. The car has been on lowered springs for a year and I kept these same springs. I did this today so I guess it might drop back down over a few days, I dont know. 

The only reason I can think of is that I could easily push the rod back down into the old shocks but on the new kyb shocks, that was much harder. 

 

Odd.

[campbeam]

The old struts well and truly had their day. Different car but I noticed the same after replacing the struts.  Give it a few days of driving for the new struts to "settle in".

Interested to know further details of the KYB struts and where you sourced them.

[matt36415]

kyb Excell G shocks. Via Ebay, $290 for front pair, same price for rear. Shipped. I think the shocks I replaced were the original ones, they had Toyota - made in Thailand - stamped in the metal. Some rust... So a 222 000km life. They may have been replaced with OEM ones but I dont think many people would pay for original shocks. 

[ZZT86]

I also find it hard to believe - the car sits on the springs & the shocks then get compressed, although 222K kms is long way to go on dampers. The length of the damper is the same on both old & new?

[campbeam]

The last post in this URL helps to explain why there can be a ride height difference; initially at least.

The other factor is blown shocks resulting in the vehicle sagging. 

http://www.e90post.com/forums/showthread.php?t=1165772

 

[mg85]

If you are concerned I would measure the distance from the bottom mounting bolt center of the strut to the base of the the spring seat on the new strut and compare it to the old one.

 

Replaced mine a few months ago with Monroe GT gas there is some gas pressure in the damper but not enough to make the car ride 1" higher the difference may be in the strut.

 

[trentmeyer23]

Are they exactly the same size?

The shock isn't bottoming out is it?

[matt36415]

Im not concerned about the height, it was just an unexpected change. After 3 days the rear seems to be back roughly whaer it was and the front is almost there too. There is no bottoming out, the ride is good, no noises etc. 

 

Main reason for sharing originally was just that it was not a change that I expected - and others may encounter the same thing. 

[Hiro]

Shocks do not affect ride height.  The vehicle's weight is born solely by the springs (hence the term "sprung weight") - they deflect a certain amount for a specific applied FORCE.  A 10kg/mm spring will compress 10mm when you apply 100kg of force to it, regardless of how fast you apply it.

Shock absorbers on the other hand are dampeners, which resist MOTION - they apply a resistance in proportion to the SPEED that the spring is compressing.  No speed = no resistance.  Apply a 100kg force to a spring backed up by a shock absorber and it will still compress 10mm, but it will take longer to do so than if the shock wasn't there.

The main purpose of a shock absorber is to dampen the motion of the spring and stop it bouncing around.  Note that you can move the rod of a brand new shock up and down by hand with very little force (compared to the weight of a car), just slowly.  A blown shock you'll be able to pump it up and down quickly with relative ease.

This is a graph of the step response of an undamped, underdamped, critically (perfectly) damped and overdamped system, analogous to a car spring/shock combo suddenly having the mass of the car applied to it (you've literally dropped it off the jack-stands).  Note that the end result is the same regardless of how damped the system is, just the underdamped system (ie a blown shock) will pogo around before finally settling - hence the "bounce test" to determine if your shocks are blown.  Push down hard on each corner of your car and release - if the car bounces up and down a bit then the shock is blown, if it quickly and smoothly returns to level then it is fine.

Thus, a car sitting "static" (a step-response system after an infinite time) will always sit at a ride height determined by the spring rate (all other things being equal) - springs do have some slight dampening built in to them due to friction, energy loss, inertia etc.   Apart from static vehicle weight all the forces a car's suspension sees are impulse-loads, ie off-on-off rather than off-on step loads, which is where the performance of a shock absorber becomes relevant

 

What people see as a change in ride height after installing new shocks is most likely either a placebo affect (because they expect it to ride higher, especially after seeing a blown shock side-by-side with a new one), the suspension hasn't re-settled after installation, or the spring hasn't been re-installed in to the seat properly (most springs sit in a pocket which can easily be 10mm deep compared to the rest of the seat).  Additonally, new shocks means new spring seats/top hats/eye-bushes etc, all of which will have worn over the years and causing a "sag" in ride height.

 

Even Monroe (ie one of the largest automotive shock absorber companies) states that blown shocks do not directly affect ride height, however they are often symptomatic of a suspension system that is worn

http://www.monroe.com.au/what-are-shocks/shock-absorbers-explained.html

Despite popular belief, shock absorbers do not support the weight of a vehicle

and

http://www.monroe.com.au/monroe-gus/Monroe-GUS-full.pdf

Worn shock absorbers do not affect the ride height of a vehicle, but the general condition of the suspension does. So if the vehicle is sagged in the rear or in one corner, the shocks are probably worn too!

[matt36415]

4 minutes ago, Hiro said:

Shocks do not affect ride height.  The vehicle's weight is born solely by the springs (hence the term "sprung weight") - they deflect a certain amount for a specific applied FORCE.  A 10kg/mm spring will compress 10mm when you apply 100kg of force to it, regardless of how fast you apply it.

So your theory... as posted has to be right? Im just looking at what has actually happened. Saying its not possible does not change the reality that my car sits higher. I was a mechanic, the parts are all seated properly... it drives nicely. 

 

The shaft on shocks will extend because of the gas pressure inside. On the old ones the force of this was quite small but on the new shocks the shafts only pushed up slowly but quite a lot of force was needed to push them back in. That is where I thing the extra height comes from - that is my guess, that the car is higher is definitely real.

[Hiro]

37 minutes ago, matt36415 said:

So your theory... as posted has to be right? Im just looking at what has actually happened. Saying its not possible does not change the reality that my car sits higher. I was a mechanic, the parts are all seated properly... it drives nicely. 

 

The shaft on shocks will extend because of the gas pressure inside. On the old ones the force of this was quite small but on the new shocks the shafts only pushed up slowly but quite a lot of force was needed to push them back in. That is where I thing the extra height comes from - that is my guess, that the car is higher is definitely real.

You said yourself that after 3 days things are basically back to normal - that points directly to parts settling or bedding-in after installation.

 

Gas-charged shocks apply the same pressure to both sides of the shock piston (due to the valving - the gas is there to pressurise the oil to stop it from foaming or cavitating), so the only force acting to extend the shock is the CSA of the rod (which is the difference in acting area between piston- and ring-side of the cylinder).  Take an average 1/2" piston rod, that has a CSA of 0.19in^2.  Even with 360psi high-pressure gas-charge, that equates to a force of 32kg.  Apply that to a 6kg/mm spring, and you get a ride-height change of 5mm (assuming you had absolutely no gas pressure in the old shock).  5mm ride-height change is not noticeable in anything short of a Formula 1 car - you'll affect the ride height more by filling the petrol tank.

[trentmeyer23]

4 hours ago, Hiro said:

You said yourself that after 3 days things are basically back to normal - that points directly to parts settling or bedding-in after installation.

Bingo.

[matt36415]

After approx 300km and 2 weeks the ride height is 5-8mm higher than with the very old shocks, From measuring both tyre to guard and ground to guard its 8mm higher in front and 5mm rear. I also dont get the front lip scraping on a driveway I go into almost every day and it used to scrape every time even when going super slow. So the shocks (or a magic unicorn) has made the car a little higher even after its all had a chance to settle in.

[trentmeyer23]

Correct measuring technique is centre of hub to guard as it removes the tyre as variable. This is likely where some of your difference in measurement is coming from.

[campbeam]

Keep recording and remove any variables. Maybe a bit more settling in will occur. A main variable is going to be the weight of the car. Bit of a difference between a full fuel tank and an empty one. In my case, I have a bootful of tools etc. When the boot is empty, rear ride height goes up noticably.

[ZZT86]

5-8mm is nothing, I would give 500-1000kms for suspension to settle.