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Discussion Starter #1
Re: Everyone can benefit from this page!

> I don't know what tires you have on right
> now, but if I were you I'd seriously
> consider moving to a more
> normalized spring to replace the
> cut MS springs. Not that there's anything
> wrong in _principle_ with cutting, i.e. a
> spring is a spring and they're all described
> by certain physical parameters, but you're
> so low that you have no options left re.
> modifying the tire profile.

I agree with Doug that there is nothing wrong with cutting springs. However, when you cut springs, you really need to understand what you are doing, and consider the effects on the rest of the suspension (I realize that it was like that when you bought it). The stock Z suspension has about 7 total travel, and is set up to have the normal ride height fall in the middle of this. The stock bump stop is about 1.5 thick. If you do the math here, you'll see that with a 2 drop from the springs alone, you're gonna be riding on or near the bump stops _all_the_time_. This will not result in the handling characteristics that you desire.

This, IMHO, is the reason that everybody seems to think that cutting springs is such a bad thing. Actually, any time you start modifying things on your own (like picking out custom spring lengths and rates), you MUST take the effects on the entire suspension into account. Cutting the springs changes the suspension natural frequency and the amount of travel left before the suspension bottoms, in addition to lowering the ride height and changing the camber. The natural frequency changes can have an adverse effect on the balance of the car, and the suspension bottoming can be downright dangerous.
 

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Re: Everyone can benefit from this page!

> I agree with Doug that there is nothing
> wrong with cutting springs.

There is something very wrong with cutting springs

> This, IMHO, is the reason that everybody
> seems to think that cutting springs is such
> a bad thing.

It's a horrible thing. Never, ever do this.

> modifying things on your own (like picking
> out custom spring lengths and rates), you
> MUST take the effects on the entire
> suspension into account. Cutting the springs
> changes the suspension natural frequency and
> the amount of travel left before the
> suspension bottoms, in addition to lowering
> the ride height and changing the camber.

Natural Frequency is not a characteristic of springs.
The action of springs is governed by Hooke's Law, which includes the mass of a spring, the spring constant, and the elastic limit.

The weight of the car resting on the springs is distributed along the length of the spring. This is a very carefully engineered part of the suspension, and if you cut them, you are increasing the force applied to the spring along it's whole length (the spring constant) which can lead to premature fatiguing of the spring.

Any advantages that can be made by lowering a car are negated if you do it by cutting the spring, because you change every other characteristic of the spring in a manner that was not meant to be. It is no longer balanced, and the results can be unpredictable. The effects could vary from manufacturer, and unless you want to try to understand the Differential equation describing a spring's motion and try to re-engineer your springs, it's best that you not try to ruin a good pair and instead buy a pair that were designed to lower your car the right way.

> adverse effect on the balance of the car,
> and the suspension bottoming can be
> downright dangerous.

If you want to learn more about springs, here is a good place to start:
<A HREF=http://www.astro.virginia.edu>http://www.astro.virginia.edu</A>/~eww6n/physics/physics.html

I encourage anyone who is thinking about cutting their springs to reconsider, sure it's been done, but it's a hack, and no serious driving enthusiast would consider doing this to their car.
As you said, it can be dangerous.
 

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Re: Everyone can benefit from this page!

Sorry to have to respond to _this_ post when I am in fact responding to another, but the editor won't let me double-quote, so...

> Cutting the springs
> changes the suspension natural frequency and
> the amount of travel left before the
> suspension bottoms, in addition to lowering
> the ride height and changing the camber.

This is the case with ANY change of spring, generally, irrespective of whether the springs were cut or were some shiny new item straight from an aftermarket factory.

Doug Dawson
[email protected]
 

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Discussion Starter #4
Re: Everyone can benefit from this page!

> This is the case with ANY change of spring,
> generally, irrespective of whether the
> springs were cut or were some shiny new item
> straight from an aftermarket factory.

Exactly. Sorry, that is what I was trying to point out - when you start modifying your suspension by changing spring rates, you need to understand all of the effects of the modifications. If you can arrive at the spring rate and length that you desire by cutting an existing spring, then go for it.

If you don't understand (or aren't interested in) the physics behind suspension calibration, then you are definately better off buying a kit. The act of cutting your springs itself is not a hack - changing your suspension characteristics without understanding what you are doing usually is. Poor planning with brand new springs will generate equally bad results.

The only exception to this that I can think of is if you require the spring to have a specific shape where it contacts the spring seat - such as most of the 2.5 id springs, which are flat on the ends. You could grind it flat, I suppose, but the originals usually have the last coil tapered a bit, in order to get the coil to contact the seat for most of it's circumference.
 

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Re: Everyone can benefit from this page!

> Exactly. Sorry, that is what I was trying to
> point out - when you start modifying your
> suspension by changing spring rates, you
> need to understand all of the effects of the
> modifications.

I'm not saying you aren't assuming this, but I have to emphasize most strenuously that it is MANDATORY to cut the bump stops when lowering your Z by ANY aftermarket spring, no matter who made it ( unless the bump stops were provided with the new spring. ) You should to cut the stops by an amount equivalent to how much the car is being lowered. In case anyone should wonder, this is the most important modification you have to make when lowering the car by the springs. The stops in as-new form typically have enough length that this isn't a problem.

> The only exception to this that I can think
> of is if you require the spring to have a
> specific shape where it contacts the spring
> seat - such as most of the 2.5 id
> springs, which are flat on the ends. You
> could grind it flat, I suppose, but the
> originals usually have the last coil tapered
> a bit, in order to get the coil to contact
> the seat for most of it's circumference.

Some of the aftermarket springs do have the end-coil modification ( flattening ) and some don't. I've never heard of anyone having problems with this aspect of aftermarket springs anyways...

Z on.

Doug Dawson
[email protected]
 

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Discussion Starter #6
Re: Everyone can benefit from this page!

> Some of the aftermarket springs do have the
> end-coil modification (
> flattening ) and some don't.
> I've never heard of anyone having problems
> with this aspect of aftermarket springs
> anyways...

I guess I wasn't being very clear (again). I was trying to point out that you probably wouldn't want to modify one ot the 'flattened' springs by cutting, due to the fact that you couldn't do a very good job of reproducing the end shape. Most of the 2.5id springs that I've seen are like this, BTW.
 

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Discussion Starter #7
Re: Everyone can benefit from this page!

One other thing, and I'll let this thread die...

Just wanted to make sure that it was clear that when I said you in my previous post, I meant anyone who might be contemplating such suspension mods. I wasn't responding to Doug directly - I think he knew this stuff already...
 

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Re: Everyone can benefit from this page!

> I guess I wasn't being very clear (again).

I thought you explained yourself very well, actually.

> I
> was trying to point out that you probably
> wouldn't want to modify one ot the
> 'flattened' springs by cutting, due to the
> fact that you couldn't do a very good job of
> reproducing the end shape. Most of the
> 2.5id springs that I've seen are like
> this, BTW.

On the rear of the 280ZX, I've seen springs that are flattened, and springs that are _not_ flattened, and they both worked equally well in my direct experience. So I'll say that unless your mounting insulators are trashed, you could go either way.

Doug Dawson
[email protected]
 

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Re: Everyone can benefit from this page!

> One other thing, and I'll let this thread
> die...

Ugh! Don't let it die! This is the most interesting thread I've seen here in a long time ( and I'm not just saying that because I started it! :^) ) Everybody's always atalkinabout motor this, motor that, and there's not enough discussion of what I personally believe is more important to the car.

> Just wanted to make sure that it was clear
> that when I said you in my
> previous post, I meant anyone who might be
> contemplating such suspension mods. I wasn't
> responding to Doug directly - I think he
> knew this stuff already...

You were pretty clear. Can we get the topic back to suspensions now? :)

Doug Dawson
[email protected]
 

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Re: Everyone can benefit from this page!

> Ugh! Don't let it die! This is the most
> interesting thread I've seen here in a long
> time ( and I'm not just saying that because
> I started it! :^) ) Everybody's always
> atalkinabout motor this, motor that, and
> there's not enough discussion of what I
> personally believe is more important to the
> car.

> You were pretty clear. Can we get the topic
> back to suspensions now? :)

> Doug Dawson
> [email protected]
I agree don't let it die,
Doug have you looked at Herb Adams' stuff? I bought his book
on suspensions and thought that it explained the fundementals of handling and suspensions in clear layman terms.
looked into it originally for my old trans am.
 

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Re: Everyone can benefit from this page!

> I agree don't let it die,
> Doug have you looked at Herb Adams' stuff? I
> bought his book
> on suspensions and thought that it explained
> the fundementals of handling and suspensions
> in clear layman terms.
> looked into it originally for my old trans
> am.

His latest suspension book ( 'Chassis Engineering' ) has a lot of good stuff in it, although the author is a bit idiosyncratic in some of his opinions, IMO. On a different note, and as is usual for this type of book, the quality of the physics used is horrible and misleading. I would feel really sorry for anybody trying to learn the fundamentals of what's going on from any of these books -- it's pretty sad. However, from a practical point of view, Mr. Adams often staggers upon the right answer. That doesn't mean it isn't WELL worth reading, OTOH.

I wish there was an even better book that was remotely as up to date.

Doug Dawson
[email protected]
 

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camber/caster corrections

Hey Doug, what's the easiest and safest way to correct the camber with 2 drop springs. Chris Behney claims Koni makes em, according to his web page. 83 NA w/Original tire diameter.

> His latest suspension book ( 'Chassis
> Engineering' ) has a lot of good stuff in
> it, although the author is a bit
> idiosyncratic in some of his opinions, IMO.
> On a different note, and as is usual for
> this type of book, the quality of the
> physics used is horrible and misleading. I
> would feel really sorry for anybody trying
> to learn the fundamentals of what's going on
> from any of these books -- it's pretty sad.
> However, from a practical point of view, Mr.
> Adams often staggers upon the right answer.
> That doesn't mean it isn't WELL worth
> reading, OTOH.

> I wish there was an even better book that
> was remotely as up to date.

> Doug Dawson
> [email protected]
 

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Re: camber/caster corrections

> Hey Doug, what's the easiest and safest way
> to correct the camber with 2 drop
> springs. Chris Behney claims Koni makes em,
> according to his web page. 83 NA w/Original
> tire diameter.

The procedure is quite different for the front and the rear. Let's assume you wanted a 2 drop on both ends, and let's look at the front end first. This is the easy part. Just get a set of the Delrin camber-adjust inner control arm bushings, which let you adjust camber by moving an eccentric metal fitting inside the bushing. I know this works well for a 1 drop. I _believe_ it has the range to compensate for a 2 drop, but it would be borderline -- you'd have to sit down and calculate out the geometry carefully to be certain.

The rear end is a little more problematical, partly because the geometry is set up for such an aggressive camber change with relatively modest drops.

Motorsport ( I think ) sells a rear camber-adjust kit, which involves drilling out the stock rear arm mounts in the rear suspension cross-member, in order to fit an eccentric cam in there which would allow to achieve as much as a 1.5 degree rear camber change. This _requires_ that the stock _rubber_ bushings be used, because it works by deforming the bushings. You could try this with the poly-graphite bushings, but I rather doubt you could get any more than 0.5 degrees of adjustment, they're just too hard.

Now, 1.5 degrees may be enough to satisfy you if you only had a 1 drop, but for a 2 drop it's just not enough. Plus, you would have to in any case keep the rubber bushings: this is bad, _bad_, BAD! Get RID of those things, put in the poly-graphites. The difference in reduction of roll steer, i.e. improvement in transient handling stability, is nothing short of _stunning_, IMO, when you put the poly-graphites in there. Just changing from the rubber to the poly-graphites in the rear suspension arms! So I dislike this camber-adjust kit.

A much better solution is to relocate the arm mounts on the rear suspension member, and perhaps also making their positions adjustable. This is what I'm planning to do in the near future. Unfortunately, I wouldn't recommend for someone to do this unless they had a really good grip on what they were doing, i.e. it could potentially be dangerous if you didn't get the strength of the reattaching arm mounts back up to an acceptable level. Therefore, I say, never even think about doing this, don't under any circumstances do it ( because I don't wanna get sued if you do it and kill yourself. ;^) )

However, that's the ideal approach PROVIDED the remounts are properly designed. I'm hesitant to provide any more details, for legal reasons.
I will let you know how it worked out when it's done, though.

I seem to recall that there were some people relocating the rear trailing arms on the early racing 510's, and I believe they developed some hardware for it, but I don't have the details handy, and I'm not even sure where to find them. It's worth doing some research on it ( I haven't quite gotten to that point yet. )

To summarize, I'd strenuously recommend that you restrict to only maybe a 1 drop in the struts and then go to a lower profile tire, such as the 205/60R14 or 205/55R14 for the rest of your 2+ inches. A 2 drop in strut length at the rear is too dramatic to deal with unless you're a glazed-eyed fanatic like I am. :^)

Doug Dawson
[email protected]
 

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Hmmmm......

Sounds like a tough procedure, don't know if I'd want to put that much work into it. Would I still be able to use the poly-graphite bushings with 1 drop Eibach springs. Or would I have to change the settings. I don't autoX but I love to take turns fast, so I'm not going just for looks.
Many a back road in Maine, dangerous if oversteer would be a problem.
I seriously doubt that I'd go with the 2 drop. Even though my father does have over 20 years experience as a machinist and the welder and power tools are just collecting dust right now....

> The procedure is quite different for the
> front and the rear. Let's assume you wanted
> a 2 drop on both ends, and let's look
> at the front end first. This is the easy
> part. Just get a set of the Delrin
> camber-adjust inner control arm bushings,
> which let you adjust camber by moving an
> eccentric metal fitting inside the bushing.
> I know this works well for a 1 drop. I
> _believe_ it has the range to compensate for
> a 2 drop, but it would be borderline
> -- you'd have to sit down and calculate out
> the geometry carefully to be certain.

> The rear end is a little more problematical,
> partly because the geometry is set up for
> such an aggressive camber change with
> relatively modest drops.

> Motorsport ( I think ) sells a rear
> camber-adjust kit, which involves drilling
> out the stock rear arm mounts in the rear
> suspension cross-member, in order to fit an
> eccentric cam in there which would allow to
> achieve as much as a 1.5 degree rear camber
> change. This _requires_ that the stock
> _rubber_ bushings be used, because it works
> by deforming the bushings. You could try
> this with the poly-graphite bushings, but I
> rather doubt you could get any more than 0.5
> degrees of adjustment, they're just too
> hard.

> Now, 1.5 degrees may be enough to satisfy
> you if you only had a 1 drop, but for
> a 2 drop it's just not enough. Plus,
> you would have to in any case keep the
> rubber bushings: this is bad, _bad_, BAD!
> Get RID of those things, put in the
> poly-graphites. The difference in reduction
> of roll steer, i.e. improvement in transient
> handling stability, is nothing short of
> _stunning_, IMO, when you put the
> poly-graphites in there. Just changing from
> the rubber to the poly-graphites in the rear
> suspension arms! So I dislike this
> camber-adjust kit.

> A much better solution is to relocate the
> arm mounts on the rear suspension member,
> and perhaps also making their positions
> adjustable. This is what I'm planning to do
> in the near future. Unfortunately, I
> wouldn't recommend for someone to do this
> unless they had a really good grip on what
> they were doing, i.e. it could potentially
> be dangerous if you didn't get the strength
> of the reattaching arm mounts back up to an
> acceptable level. Therefore, I say, never
> even think about doing this, don't under any
> circumstances do it ( because I don't wanna
> get sued if you do it and kill yourself. ;^)
> )

> However, that's the ideal approach PROVIDED
> the remounts are properly designed. I'm
> hesitant to provide any more details, for
> legal reasons.
> I will let you know how it worked out when
> it's done, though.

> I seem to recall that there were some people
> relocating the rear trailing arms on the
> early racing 510's, and I believe they
> developed some hardware for it, but I don't
> have the details handy, and I'm not even
> sure where to find them. It's worth doing
> some research on it ( I haven't quite gotten
> to that point yet. )

> To summarize, I'd strenuously recommend that
> you restrict to only maybe a 1 drop in
> the struts and then go to a lower profile
> tire, such as the 205/60R14 or 205/55R14 for
> the rest of your 2+ inches. A 2 drop
> in strut length at the rear is too dramatic
> to deal with unless you're a glazed-eyed
> fanatic like I am. :^)

> Doug Dawson
> [email protected]
 

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Re: Hmmmm......

> Sounds like a tough procedure, don't know if
> I'd want to put that much work into it.

Well, if you had to relocate the arm mounts, that might be awkward depending on what resources you had.

> Would I still be able to use the
> poly-graphite bushings with 1 drop
> Eibach springs. Or would I have to change
> the settings.

On the front, it wouldn't be a problem. On the rear, you'd just have to live with a few degrees negative camber ( like I'm doing right now ) ...

> I seriously doubt that I'd go with the
> 2 drop. Even though my father does
> have over 20 years experience as a machinist
> and the welder and power tools are just
> collecting dust right now....

You should talk to your father about relocating the rear arm mounts. He could very likely do it for you in just a few minutes. I'd be interested in his opinion. Maybe you could e-mail me with it?

Doug Dawsou
[email protected]
 

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Re: Hmmmm......

I'll talk to my father in a couple of weeks about relocating the rear arm mounts. I'll tell you what I find out. ****, a two inch drop would really look sweet. I'm a sucker for that touring car racer look.
 

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Re: Hmmmm......

> I'll talk to my father in a couple of weeks
> about relocating the rear arm mounts. I'll
> tell you what I find out.

Another remote possibility involves the difference between the pre-81.5 and post-81.5 models. The rear suspension members were different, and it's _conceivable_ that the pre-81.5 member could be put into the radically lowered post-81.5 car and that this would reduce the camber effect somewhat.

That's totally wild speculation, of course. I don't know what the precise measured differences were between the two members. The newer arms may not fit it. And as the arms were different as well, if they fit, you might get a screwed-up toe-in. If anyone ever gets a chance to do really precise measurements on the two cars side by side, they should definitely post about it.

Hey, stranger things have happened; it COULD work...

> ****, a two inch
> drop would really look sweet. I'm a sucker
> for that touring car racer look.

It really changes the whole look of the car, in just the way you describe.

Doug Dawson
[email protected]
 

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Re: Hmmmm......

I just thought about something. If the rear arm mounts are moved, and I believe they must be moved toward the center of the crossmember. Wouldn't the sway bar be too long for the end links to mount on the original spot on the arm?
Would it have to be bent to fit? Also, does the strut mount have atleast 2-3 degrees of play at the top? If these two factors could be accomodated to work. I don't see why my father wouldn't be able to make new mounts and weld them on. I'd probably buy one of those suspension setup computer programs that is sold on the page I linked towards the beginning of this thread.
Also, this is the 43rd addition to this thread, do you think that's a record?
 

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Re: Hmmmm......

> I just thought about something. If the rear
> arm mounts are moved, and I believe they
> must be moved toward the center of the
> crossmember.

This is a semi-trailing arm rear suspension, so the camber is largely controlled by the arm mounting, rather than by the strut position. The change in mounting that I'm referring to would be to raise the height of the inner arm mounts, and to tilt both inner and outer mounts by a few degrees. You'll notice as the height of the car is raised or lowered, allowing the rear suspension to move freely, that the arm mounts control the rear camber in a sort of arc about the pivots. This is why changing the sprung height of the car has such an effect on rear camber with this kind of suspension.

> Wouldn't the sway bar be too
> long for the end links to mount on the
> original spot on the arm?

They shouldn't, in that all you're doing by this repositioning is restoring something close to the original stock geometry. In any case, if you look carefully at how the swaybars mount on the arms, you have a fair bit of free space to work with. Remember, the swaybars have to move freely ( sort of ) thru the whole range of suspension travel, by design.

> Also, does
> the strut mount have atleast 2-3 degrees of
> play at the top?

Yes, you could fool around with changing the strut top-mountings, but the strut doesn't control camber except but indirectly by changing the ride height. With the Illuminas, in fact, the lower strut mounting isn't even a tight fit on the arm strut pivot. And other makes of shocks are lower-mounted with an integral rubber bushing with a lot of flex in it. All the strut does here is allow the arm to go where it wants, and react to it. ( I'm oversimplifying here, but the point is that changing the strut mounting won't have the desired effect, at least without unnaturally deforming the arm bushings. )

> If these two factors could
> be accomodated to work. I don't see why my
> father wouldn't be able to make new mounts
> and weld them on.

It would have to be done fairly precisely, i.e. so as to avoid having to make adjustments afterwards ( with just a straight weld. ) That's why I want adjustability to be built into the remounting.

> Also, this is the 43rd addition to this
> thread, do you think that's a record?

Isn't it cool?

Doug Dawson
[email protected]
 

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Re: Hmmmm......

I understand what you're saying now. Precision has always been my biggest concern. welding's a serious procedure. If I decided to do it, I'd probably hire someone who does welding for a living to do it. My other concern is ride height. While I'm not worried about the frame and such, my air **** would be the determining factor.
I have the motorsport aero kit II, so the air **** is lower I think and it is more towards the nose of the car. Speed bumps would be ok but steep drive ways and other inclines from a flat surface would scare me. Anyways, I'm not doing suspension upgrades until this summer so it'll be a while.
 
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