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AydinZ71

Looks like I have cut springs... what now?

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Hi all! 
 

Suspension NOOB here. thanks again for your help selecting struts (Koni 8610 1437RACE).

 

Quick context: I’m restoring a SCCA Group EP race car I purchased about 5 months ago.

 

It looks like the previous owner cut thier OEM springs from the original 10 turns to what looks like 6 turns. The car sits very low, but only occasionally scrapes on the exhaust U bolts. I think slightly over-dampening should help eliminate the scraping on bumps, on the street, and I shouldn’t have any issues on the track. 


Questions: 

 

1) is there a possibility the cut springs may damage the brand new struts? I have not experienced any issues with bottoming out. 
 

2) any idea what the new spring constant could be? I have tried researching but there is a lot of conflicting guidance out there. 
 

I’m struggling to find a stiff racing spring for the stock perches. I can not run coil overs in this race class, but I am allowed to thread the stock perch for adjustability if necessary. I’m thinking of running the cut springs for the time being. The ride is a little soft for a race car, but it is equipped with very stiff roll bars which do compensate in turns. My biggest concern is damaging the expensive new struts. 

30020064-4340-4A01-A611-8953B516FEBD.jpeg

Edited by AydinZ71

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I cannot answer #2 if your question is what the stock cut spring rate is? The experts here will chime in. 

 

For #1, the short answer is yes. The long answer is maybe. Without knowing the use of the car exactly, it sounds like it may not be street driven that much. If that is the case you run less risk of "damaging" the struts on harsh bumps and pot holes. Regardless of use over damping a setup (taking load from the spring and putting it on the struts) is going to increase wear. How fast really depends on use and the specific combination. This is aside from the horrible characteristics associated with an over damped suspension. Do some reading in the FAQ section of springs for those Koni's but I would not use stock cut springs with anything other than cheap KYBs personally, you are just wasting money otherwise. 

 

There are several good threads here on sectioning and threading the strut tubes for a diy coil over setup and several vendors that sell everything you need. The struts are the expensive part of the operation so if you already have those the rest is pretty reasonable.

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Put in some bumpstops. Bottoming out on the chassis is different from bottoming out the shock, and one instance of this on the konis will ruin them. 

 

If you can run ground control style collars, then any 2.5" spring will work. Eibach and Hypercoil both make good ones, I'd look at something in the 300/350 range to start out with for a track application.

 

You can easily measure the spring rate on the new spring, just measure the free length, and then add weight until it compresses by 1 inch. You'll then know the in/lbs rating of the spring. The cut springs will be a placeholder though and nothing more, I wouldn't waste too much time with them. 

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Thank you for both of your responses! I currently have the OEM insulators, and no bump stops. By current, I mean what I disassembled recently based on what the previous owner installed. Here is the excerpt from the SCCA EP rule book:

 

“Spacers and threaded sleeves with adjustable spring seats can be used with coil springs. Coil-
over threaded body shocks/struts are permitted if coil-over shocks/struts were fitted as stock.“

 

if I read this correctly, it doesn’t seem I can use an alternate spring perch, but I seems I may be able to thread the OEM perch? I might need to call on Monday just to be sure I understand. I agree that if I can get down to the 2.5” dimension, the options seem to open up big time. Also, definitely targeting a 300/350 spring rate. 
 

I was concerned about running the Koni’s temporarily with the cut springs, but if bottoming-out is the main concern I think I will be OK if I get some bump stops in there. If I can find a group EP compliant spring solution, il just toss the cut springs all together. I just need to get the car to a handful of places in the meantime: body guy to hammer-dolly, Dyno to tune the new head when it’s back from the machine shop. The things I’m not skilled or equipped  enough to do myself. 

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By my reading of these rules, you can use a threaded perch. The Z's from the factory have a "coil over" style suspension so you're good to go. That rule is designed to prevent cars with divorced spring/shock rear ends from switching to a true coilover setup, 350z rear ends for example, or a solid rear axle cars.

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Parts in hand! Pulled the stock isolators, wire-wheeled the rust, primed and applied my favorite 3M rubberized undercoating. 
 

Ran into my first snag. The isolators seem to have a smaller strut hole which is “D” shipped to keep the shaft from spinning when you tighten the strut retaining nut. I searched the forums and saw some people drilled this out for the larger Koni threaded shaft. 
 

It seems to Me just about everyone is selling camber plates now adays. SCCA Group EP requires I do not cut into the stock strut mounting surface, and only allows for strictly “bolt-in” camber adjustment. Silvermine seems to sell the right option, as it seems the others require cutting. Great price too! 
 

https://www.silverminemotors.com/240z-260z-280z-camber-plates-bolt-in-lowering-kit

 

I have also read some of the most experienced folks on here seem to have good things to say about the stock isolators. Lowering the vehicle height with the coilovers is going to give me more negative camber, but if I recall correctly the Z came stock with more positive camber anyways. 
 

What y’all think? Should I give the stock isolators a shot, and see how she does on alignment? If y’all think the camber will be unsatisfactory, Should I go with the silvermine camber plate or do you recommend another option? 
 

thanks again for all your help! This place is a wealth of information. 

 

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Edited by AydinZ71

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Yes, you will definitely want to replace the stock isolators with something that gives you adjustability and more negative camber.

 

Those Silvermine bolt-in camber plates are certainly some of the cheapest (lowest cost) you will find, and there's nothing wrong with them (I've used similar ones on some of my cars); but their range of adjustment is limited.  IIRC, I could achieve approx 1.5 deg in the rear and maybe 2.5 in the front with them (with moderate lowering, adjustable coilovers, and stock control arms). 

 

If they meet your needs, go for it.  If you think you will want more camber adjustability, take a look at the bolt-in units from Ground Control and DP Racing.  I've used both and have been extremely happy with them.  As a bonus, they both allow a limited degree of caster adjustment. 

https://www.dpracing.co/datsun-z-front-suspension-1

https://groundcontrolstore.com/collections/s30/products/no-cut-camber-caster-plate-z-car-pair

 

There are other "bolt-in" plates available that will give more range of adjustment, but require cutting the strut tower; so you'll want to avoid those.  

 

P.S.  Hang on to those stock isolators and don't throw them out.  At some point down the road, either you'll want them or someone else will.  :-)

 

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Thank you! y’all are so incredibly helpful! I will definitely hold onto them :) The rubber seems surprisingly good given the age. Must have been the midwest humidity.

 

Is there a need to replace the isolators dampening function? I’m not sure how a 50 year old Frame (even though it is stitch welded) with the delightful hidden rust will react to no vibration/force dampening. I was thinking about buying a sheet of 1/2” thick EPDM rubber and cutting out holes to sandwich between the plate and the strut steel. 

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You certainly can use some thick rubber to sandwich between the strut tower and the top of the camber plate, but I personally don't think it's necessary from a structural perspective...the structural function of the rubber in the stock isolator is primarily to allow for angular changes in the top of the shock as the suspension moves through its range of motion.  The monoball joint incorporated in the aftermarket camber plates accomplishes that function.

 

If you do install some kind of rubber pad between the strut tower and the top of the camber plate may help cut down on the road noise vibrations transmitted through the camber plate; but you need to accept the fact that any metal camber plates are going to be much noisier than the stock rubber isolators.)  FOL.  Need to ensure that it doesn't flex so much that it induces angular changes in the suspension geometry.  Maybe polyurethane might be a better material, from that perspective, if you decide you need something in there.  The reality is that it's just one more rattle (mixed in with many other random rattles) and you only notice it when driving on the street....it's irrelevant once you're on the track.

 

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10 hours ago, jhm said:

You certainly can use some thick rubber to sandwich between the strut tower and the top of the camber plate, but I personally don't think it's necessary from a structural perspective...the structural function of the rubber in the stock isolator is primarily to allow for angular changes in the top of the shock as the suspension moves through its range of motion.  The monoball joint incorporated in the aftermarket camber plates accomplishes that function.

 

If you do install some kind of rubber pad between the strut tower and the top of the camber plate may help cut down on the road noise vibrations transmitted through the camber plate; but you need to accept the fact that any metal camber plates are going to be much noisier than the stock rubber isolators.)  FOL.  Need to ensure that it doesn't flex so much that it induces angular changes in the suspension geometry.  Maybe polyurethane might be a better material, from that perspective, if you decide you need something in there.  The reality is that it's just one more rattle (mixed in with many other random rattles) and you only notice it when driving on the street....it's irrelevant once you're on the track.

 

 

 

Great points! If its just a matter of noise, im going to pass on the rubber. Great point on the angular change... I didn't think of that till just now. 

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