Strut Sectioning Question

[SSflyer]

Is there any reason why the strut tubes cannot be cut to exactly the length as the cartridge that's going inside, instead of so long that spacers are needed in the bottom? If the adjuster on the coilovers determines the ride height, and the car will never be returned to stock, does the tube length really matter?

[blueovalz]

I believe your reasoning is correct IF the dimensions of the insert are close to ideal for your particular application. For example: You've just lowered your car 1.5"; and for example the OEM compliance is 6" (just throwing out a number here, which may not be correct, but it will illustrate my point). This means that instead of having the OEM 3" up and down movement over static ride height, you now have 1.5" of compression, and 4.5" of rebound. This means you'll most likely bottom out on the strut housing periodically. To fix this problem, your replacement insert is needing to be 1.5" shorter (on the housing only, not the "rod") than the OEM size would be, which would necessitate the shortening of the housing by said amount, and everything would be hunky-dory.

But suppose the closest thing to your needed dampening requirements necessitated the purchase of a replacement insert that was 3" shorter than the OEM size (or instead of the 1.5" shorter one). Then if I cut the strut tube to match this much shorter insert, then the center of compliance is no longer in the middle of the operating window of the insert. Now with this much more shorter insert, the compression length (again, with the same lowered car of 1.5") would be 4.5" and rebound would be 1.5" (now the problem is in jerking the insert out of the housing whenever the suspension goes to full droop). To fix this and get the travel back to the middle of the strut, I would need to add a 1.5" spacer in the bottom (which gets the top of the housing back to the level of being 1.5" lower than OEM) of the housing to get the static ride height back into the center of the insert's movement window (which means the strut tube length must now be a bit longer to compensate for the additional spacer under the insert). Basically, if you lower the car by 1.5", then you need the top of the strut housing to be lowered by 1.5".

This example still leaves other factors out (e.g. different insert operating lengths or windows, spring rates, swaybar sizes, etc), but it was simply to illustrate a reason why a spacer is sometimes needed.

[johnc]

What Terry said is correct, but to simplify:

 

The insert needs to be positioned so that its in the middle 1/3 of the total stroke available at your planned static ride height. That position is controlled by ride height AND where the top of the strut tube is in relation to the upper spring perch.

[Dan Juday]

Thanks John and Terry. Your posts help me to understand why we do the mods we do and what to watch out for. Like, "Can you section a strut too much"?

I've been reading old posts and new on this topic. It's the next thing I need to tackle on my Z. My car is too low for the stock struts and I've been bottoming out too often.

Just to move the discussion a bit from the theroretical to the practical, are we not we limited in what we can do by the availability of strut cartridges? Other than having a boat load of funds and having custom cartridges made, don't we have a relatively small selection from other applications that just by accident happen to fit? I know of only three from reading posts here: Rabbit, MR2, and moving shorter front Z cartridges to the rear (or is that rear to front?).

John, in your famous sticky post on this you list Koni part numbers. Are those from one of the above mentioned applications or is that a special racing part? A great post BTW. Unfortunately for us guys that want a more compliant ride for the street I don't know if the lenght's you spec would work for, say, the Tokico blue cartridges. I would imagine that what ever shock manufacturer you chose the lenght would be determined by the application you are stealing the cartridge from. Is this right or are there other factors I need to consider?

Thanks for your help. I know this has been done many times by many people on this site, and it's not rocket science, I just want to better understand what I'm doing and do it right the first time.

[johnc]

How much a strut is shortened is determined by:

 

1. The expected static ride height.

2. The total stroke of the shock insert.

 

Each brand and application shock insert has a different stroke length, overall body length, and body diameter. You have to look at those three things first to determine if the shock insert can be used in a 240Z application.

 

If the shock insert passes that first test, then you need to check compression and rebound valving to see if those values are compatible with the spring/wheel rates you expect to use.

 

Now, you get to consider price and availability.

[JMortensen]

I think it is already pretty safe to say that Dan isn't going to want a Koni 8610. Probably the Tokico Blue or Illumina. If you go Illuminas Dan the BZ3099 fits in the front and if you ask for 240 fronts they fit in the rear.

 

No one has said this with regards to the original question, but I just thought I would point out that if you use the same strut cartridge front and rear you need the spacer in the back because the strut tube is longer in the back. If you sectioned the front and rear so that they both fit the insert, then the only way to get the ride height level would be to run the rear spring perches much higher on the threaded adjuster than the fronts. Much better to have both sitting in about the same position and have more adjustment available in both directions, especially if you actually intend to change the ride height frequently.

[Dan Juday]

How much a strut is shortened is determined by:

 

1. The expected static ride height.

2. The total stroke of the shock insert.

 

Each brand and application shock insert has a different stroke length' date=' overall body length, and body diameter. You have to look at those three things first to determine if the shock insert can be used in a 240Z application.

 

If the shock insert passes that first test, then you need to check compression and rebound valving to see if those values are compatible with the spring/wheel rates you expect to use.

 

Now, you get to consider price and availability.[/quote']

 

It's a pretty big picture. Which really kind of points up the limitations we have when we decide to cut our strut tubes. Of the three applications I mentioned earlier I can understand the Z fronts working in the rear due to the near 50/50 weight distribution of the Z. But when you talk about using Rabbit or MR2 cartridges I can see them passing the first test, but does anybody really know if they pass the second? Is the valving compatable with the weight and spring rates and dynamics of the Z? Or are they, in reality, the only other options that fit diamensionally?

 

Then we put coilovers on with stiffer spring rates and now do even the cartridges made for the Z work? I feel like I'm in over my head. I understand that with a street application, as opposed to a race set-up, your windows of overlap as to spring and shock matching must be wider. But still...

 

It would be nice to have some good data for street applications. I know John has done some good research and math for his race car. Who do we know here with the know-how to do the same for a street Z?

[A. G. Olphart]

I very definitely second Dan's request, and will muddy the waters a bit further by mentioning that I have both my original 240 struts and 280 struts as well (for both front and rear of the car). Is there any advantage to running one type over the other? Or a half and half combination front/rear?

 

Thanks guys... suspension is an area where help is appreciated.

[2126]

If you are interested in some quick answers about avaiable insert lengths and how much to section the struts, give Ground Control a call. These guys seem to be pretty sharp on the subject.