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      10-28-2008, 03:46 AM   #1
MDCTFTW
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When installing springs how tight do you tighten the top perch nut?

One thing I've always wondered when I install suspension is how tight should the top perch bolts be tightened? Since the strut tower is threaded, what if the two sides are uneven? Couldn't that make the ride height different on both sides?

I've attached a pic (thanks Speedtrap for the borrowed pic). Say for instance the top nut is tightened to 1 on one side and 2 on the other. Won't the ride height be different on both sides? Or is the general rule of thumb to make both sides about even?

Also ... if you tighten both sides to position 1 or both to position 2 won't that yield different ride heights? That's a good inch right there. I know those positions are extreme ends of the threads, but I'm just trying to make my point here.

Or am I just being overly anal? Haha.
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      10-28-2008, 08:51 AM   #2
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The position of the nut has no affect on ride height. It just affects the maximum droop the suspension can acheive. Ride height is determined by the spring perch on the strut.
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      10-28-2008, 11:31 AM   #3
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Quote:
Originally Posted by Radiation Joe View Post
The position of the nut has no affect on ride height. It just affects the maximum droop the suspension can acheive. Ride height is determined by the spring perch on the strut.
Is that true? Here's another picture that again shows the extreme cases. Would these two setups yield different ride heights? Or is it the spring rate that is determining the ride height?
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      10-28-2008, 11:45 AM   #4
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Lower nut looks like more preload to me, car won't have much sag which could be undesirable.
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      10-28-2008, 11:47 AM   #5
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Quote:
Originally Posted by MDCTFTW View Post
Is that true? Here's another picture that again shows the extreme cases. Would these two setups yield different ride heights? Or is it the spring rate that is determining the ride height?
If everything in both illustrations are exactly the same except the nut position, once you lower the car the spring will compress to the same steady state height regardless of either setup.

The ride height will be determined by the spring itself and the shock/strut base position (the "sleeve" where the bottom of the strut is inserted) as previously explained.
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      10-28-2008, 12:00 PM   #6
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Quote:
Originally Posted by Technic View Post
If everything in both illustrations are exactly the same except the nut position, once you lower the car the spring will compress to the same steady state height regardless of either setup.

The ride height will be determined by the spring itself and the shock/strut base position (the "sleeve" where the bottom of the strut is inserted) as previously explained.
Ok cool I guess that makes sense, but how about in the extreme case if the uninstalled strut has a top mount that is compressing the spring even more than the normal weight of the car would when it's installed? This would the car to be even lower right?
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      10-28-2008, 12:47 PM   #7
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Quote:
Originally Posted by MDCTFTW View Post
Ok cool I guess that makes sense, but how about in the extreme case if the uninstalled strut has a top mount that is compressing the spring even more than the normal weight of the car would when it's installed? This would the car to be even lower right?
... then we would not be talking about the same setup, as one strut will compress the spring more than the original strut, something like a coilover vs. standard strut.
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      10-28-2008, 12:53 PM   #8
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      10-28-2008, 01:23 PM   #9
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Thanks Did not even know that myself.
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      10-28-2008, 03:28 PM   #10
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Quote:
Originally Posted by ///Mantis View Post
pm longtran
I think I asked him in a thread before ... the one where he showed some pics after his drop and he didn't torque the bolts to any specific value.
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      10-28-2008, 07:49 PM   #11
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so what's the official say on this?
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      10-28-2008, 08:27 PM   #12
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Quote:
Originally Posted by rldzhao View Post
so what's the official say on this?
I think as long as the uninstalled spring is not compressed more than it would be with the weight of the car then it should even out.
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      10-29-2008, 12:15 AM   #13
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The top of the spring sits on a spring perch that's mounted to a bearing that's bolted (with three small bolts) upside-down inside the fender. The bottom of the spring sits on a perch that's clamped to the top of the damper body. The spring sits between these two perches.

The nut you are wondering about in the original post just keeps the damper rod in place - too loose and it will rattle, too tight and it will damage the threads and maybe the rubber bushing it sits in the middle of. Other than that, it has no effect on the ride height.
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      10-29-2008, 12:49 AM   #14
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You'll feel when the nut makes contact, sandwiching the washer assembly. IIRC, a 10mm socket fits over the top of the shock shaft. I used a ratchet on that and an open-end box wrench on the nut and squeezed to an approximated 18-20ft-lb. RealOEM doesn't have torque specs so I assumed this based on previous M3 specs. You can always torque the nut down after the assemblies are installed on the car. This may even be easier since the shaft shouldn't spin with the car on the ground under the spring's tension.


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      10-30-2008, 06:44 PM   #15
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Does this help? Taken from here.
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      10-30-2008, 07:01 PM   #16
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this definitely helps. thanks.
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      11-21-2008, 07:38 PM   #17
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UPDATE:

Tighten to 64Nm! The tightness of the nut WILL slightly affect the position of the guide support and therefore the ride height. This is torque spec from BMW.
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      11-21-2008, 07:58 PM   #18
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I tightened this up today being that I had that clunking sound. However, I did not tighten it with the torque measurer being that the dealer wanted me to leave the car there for the day for something that would take 5 minutes. So I took it to my mechaninc, he tightened it down with a wrench and the sound is gone. Didnt turn too much but there was deff some room.
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