What Coilover Spring Rates should I use with a V8

[gvincent]

Who has coilover suspensions on their V8 Z cars and what spring rates are you using and why? I am about to order the Ground Control Coilover kit but can't decide what spring rates. My 76 280Z  Z has an LT-1 V8 and a Tremec T56 six speed so I have to consider the additional weight. form a L6 Z

Mostly street driven but do do about 4-5 track days a year, currently have stock strut tubes with Mulholland lowering springs and Tokico Illumines struts

[jhm]

Hey Greg-

I alternate between 5 kg/mm and 6 kg/mm on my late '74 260 w/ SBC, which is used mainly for track events....probably on the low end for a track car, but most of the venues in my area are pretty smooth w/o much elevation change.  I also like to drive it on the streets occasionally.  I'm using Bilsteins in the front and Tokico adjustables in the rear -- they haven't blown up yet, but I'll probably switch to Koni yellows when they do.  The car weighs roughly 2780 lbs, and is balanced pretty close to 50/50.

 

IMHO, 200 - 250 lb/in should be a good starting point for you if most of your driving is on the streets.  Road conditions and your personal preferences in ride comfort vs handling will be major factors in what setup works best for you.  Good luck with it.

r/John

 

P.S.  Forgot to mention that your Illuminas should be fine with 250 lb springs, as long as the shocks are still in good condition.

Edited October 7, 2016 by jhm

[Chickenman]

Agree with JHM, but I'd lean towards the 250lb/in rate. 200 lbs/in is a bit soft IMHO. It's the shock valving that really determines what the ride quality is like. 250lb/in is not excessive with an LT1 and Tremec T56. The T56 weighs quite a bit by itself.( 125 to 130 lbs with shifter and bellhousing )

 

Just for interest. The Datsun 510 crowd in the PNW commonly run 300lb/in front springs with 4 cylinder engines..... on the Street. Surprisingly the ride is not uncomfortable at all. But shock valving is critical. Koni's Yellows or Bilstein Sports seem to be the most popular with some using custom valved Race Koni's. The510Realm has some really good suspension Guru's on it.

 

I have Eibach Pro Kit springs in my 280Z, and the front spring rate is way too low. And it's a progressive rate which is just stoopid on the front. ( Car came that way from PO ). I'm looking at switching to Coil-Overs and starting with 250/lb/in. Probably going with Koni Race shocks or possibly the Toyota MR2 or VW Rabbit shocks. The Race shocks cost a bit more, but have better valving from what I've heard.

Edited October 7, 2016 by Chickenman

[socorob]

I have a 78 with LS1 and have 300 frt, 250 rear. If I had to do it again, I would probably go 350 ft 300 rear.

[RebekahsZ]

It's funny how the trend has swung from higher rate in the rear to higher rate in the front. It may depend on what style of racing one is doing. I used to go higher in rear to get rear end loose for auto tossing. Now I'm trying to keep the ass planted since it wants to lift, and the air dam off the ground since it wants to compress. I thought the 450s would jar my teeth out but they don't.

[gvincent]

So what is the benefit of going with a higher rate up front than the rear, is this a new trend? traditionally through my searches here and on other sites most have been higher rates in the rear.

so what i'm getting here is 200/250 for mostly street, 250/300 for a bit more track? don't know if GC offers a 225/275 package off the top of my head.

[RebekahsZ]

The guy who is best prepared to answer some of these questions has a screen name of 74 5.0, or something like that.  He's a big autocrosser and has done a lot of spring changes and will be better able to talk to this issue from an intellectual perspective.  2.5" springs are cheap.  You can buy them from Speedwaymotors.com for $55 each-no reason to spend more than that.  I ran 225/250 for years then got my ass handed to me by a guy with 450/400 at an "autocross" which consisted of a road course with a slalom in the middle of each straight-away.  His body roll in photos was so much less, even though we ran similar sway bars.  I went to 450/400 because my airdam was hitting the ground at high speed.  I just wanted to keep my suspension from moving so much.  Stiffer springs allow you to drive more to the limits of your tires, rather than to the limits of your suspension geometry.  Basically, it kind of eliminates your suspension to some degree (at least that's the way I see it).  I think the thing that limits how stiff you can go is the roughness of the roads you will frequently encounter.  If your local roads are smooth, you can go harder.  If your local roads are in bad shape, I'd go softer.

[jhm]

Also, don't forget the importance of proper bumpstops.  Recent discussion here covered this topic:  http://forums.hybridz.org/topic/126177-ride-difference-between-cut-springs-and-coil-overs/?hl=%2Bbump+%2Bstops&do=findComment&comment=1179414

[Chickenman]

Actually it's wheel rate that you're concerned about and that is a function of spring rate and control arm or strut ratio. For a front engine RWD car it is nothing new to have a higher wheel rate up front than in the rear. In fact this  very common as the weight of the engine usually results in a forward weight bias and that requires more spring to hold the front end up. Weight transfer under braking also requires a higher wheel and spring rate up front to prevent excessive nose dive.

 

What is unusual is to have a rear wheel rate that is higher than the front on any front engine car.  Personally, I've been racing, autocrossing and hill climbing for over 40 years and have never driven a front engine car that has a lower front wheel rate than the rear. With big HP RWD cars you usually try and make the rear wheel rate as soft as possible to put power down on corner exit.

 

One other thing to note is that the strut ratio on the rear of the Z design is not the same as the front. There is more offset from the tire centerline to the spring centerline on the rear struts, so this lowers effective spring rate. Have a close look at the hub design.

 

The front wheel rate is almost 1 to 1. The rear is less. I don't have the exact figures, but I seem to remember reading that 1.3 to 1 for the rear wheel rate ratio was close. That makes sense as the Z has near 50/50 weight distribution and 250 lb/in spring rate on the rear would work out to about 192 lbs wheel rate at a 1.3 ratio. If ratio is higher ( 1.35 or 1.4 ) then the effective wheel rate drops even lower. So 200 lb front springs and 250 lb rear springs, effectively works out to the same wheel rate front and rear.

 

Z cars are also known to be wayyyyy too soft in the front, even in street form. Mkaes for wallowy handling nose dive under braking and nose lift under acceleration. Siffen up the front springs by a good margin and the handling is dramatically improved.

[Chickenman]

Spring frequency also comes into play. The higher the road speed, the higher the spring frequency ( and spring rate ) must be. That's partially why Road race cars can use much higher spring rates than when set up for Autocross. RebekahsZ speed trial car is a good example of a high speed car ( 170+ MPH ) needing more wheel and spring rate.

 

Fred Puhn's " How to make your car handle " explains the mathematical relationship between suspension frequency and road speed. But generally, as speed goes up, so must suspension frequency, There are different values for road cars and race cars. Comfort vs handling etc...

[JMortensen]

We have a thread with motion ratios and all that calculated out: http://forums.hybridz.org/topic/63492-suspension-tech-motion-ratio-unsprung-weight/

 

The front and rear motion ratios are pretty close.

[Chickenman]

Thanks for the link John. Excellent info. Bookmarked. The motion ratios are closer than I thought