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Coilover ride height, thread depth question


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Im in the process of installing S13 coilovers on my 280z. The coilovers I have are Up Garage from an S13 which are height adjustable by threading the assembly into the strut tube. My question is how many threads of the spring/dampner have to be screwed into the strut body. What is safe? As it is now the strut and spindle are together and its allot shorter than the stock assembly I took out of the car so I think Im probably going to have to run them screwed out quite a bit as I dont want the car slammed. Thanks.

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Edited by rickyellow zee
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I've read a report somewhere that showed the acceptable length of a threaded piece that should be inserted into it's counterpart.  And I believe it's based on the size and strength of the fastener.  I'm not sure that would apply to your situation.  I think that common sense should be used and that you should error on the side of safety.  How long is your threaded part?  I would want a substantial part of it screwed in. Enough that there was absolutely no fear of it failing and leave you with dangling participles.  The report I read may have been at the Fastenal.com.      

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Your issue isn't the threaded portion of the spring perch on the tube, it's the piston in the damper assembly.  The shaft in the shock..  As you move the spring perch up you extend the shock.  A good shock design will maintain enough contact at full extension to be okay (I think, I'm not an expert in shock design).  Besides that the shock/damper should only see small side loads.  It's mostly along for the ride and the control arm is what handles side loads.  The strut controls the up and down.

 

If you set the perch too high, the shock will "top out" over depressions in the road.  Lots of topping out isn't good for a shock absorber. .Run one all the way up and see how much travel the shock has, then you'll know how much you have left when you set the spring perch.

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Looks like your adapter tubes have 2" +/- of thread in the tube so that should probably be you minimum of engagement in the tube.  Considering you said this is for a 280 what concerns me the most is your rear strut housings. The 280 had longer strut tubes in the rear because the rear strut towers are different (taller) than on a 240. I found that in order to get the front and rear of a 280 level with coilovers the rear adapter tubes have to be approx. 3" taller than the front adapter tubes in order to achieve level ride height all the way around. Only way you could achieve that with your adapter tubes would have been to leave a bit more than 3" of the stock tube on the rear housings and weld the adapters on 3" above the casting. As it sits now you are probably going to be hella dumped in the rear. I know this from another car I helped put S13 coilovers using the stock top hats.   

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Your issue isn't the threaded portion of the spring perch on the tube, it's the piston in the damper assembly.  The shaft in the shock..  As you move the spring perch up you extend the shock.  A good shock design will maintain enough contact at full extension to be okay (I think, I'm not an expert in shock design).  Besides that the shock/damper should only see small side loads.  It's mostly along for the ride and the control arm is what handles side loads.  The strut controls the up and down.

 

If you set the perch too high, the shock will "top out" over depressions in the road.  Lots of topping out isn't good for a shock absorber. .Run one all the way up and see how much travel the shock has, then you'll know how much you have left when you set the spring perch.

 

The shocks are currently at full extension. If he raises the lower perch any further it will only be compressing (preloading) the spring. These aren't old mcpherson units, (most of) what you are describing is more akin to a mcpherson style: dampen the suspension motion by restricting oil flow through the damper piston orifice.

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Those struts are inverts...  What's the thread pitch?  You might need a couple of our 9" weld-on threaded adapters for 280Z rears.  It looks like you were wanting a completely bolt-on solution though. 

 

S13 springs are generally WAY stiffer than any S30 spring can-or-needs to be.  Know the spring rate?

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M50 x 2.0 is a pretty common coilover thread pitch.

 

For a fastener the rule of thumb is 1.5 times the diameter so 75mm or 3 inches of engagement. I'm not sure how the rule changes with a tube and such. 

 

I know JCC had made their collars too short on one of their runs and they came up with a spacer insert thing, not sure exactly how it would work, probably cutting the tube down, using some of the inner thread to build a male female adapter kind of situation most likely.

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I know a local guy who said he will weld for me if I bring him the materials. I cant find a flange with the same bolt pattern as the top hat to make the top and bottom portion so Im going to have to get a hole saw and drill out a 4" piece of 1/4" metal and drill my own holes. For the middle I will use some thick walled pipe. It'll work.

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The rear adjusters are on top, the fronts are on the bottom. It will be a pain but that's life. I will drill a hole in the bottom of the strut tube for adjusting. I understand that I will have to remove the steering knuckle to get access but that's life. Once I find a stiffness I like I will leave it. Not worried.

Edited by rickyellow zee
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