Shocks and leaf springs (how do they work together)

[67topless]

I have new 4 1/2 leaf springs and the affordable KYB shocks in the rear on my 67 Mustang. The performance characteristics are dramatically improved over the stock 39 year old setup I replaced... I can take corners like never before. However the ride is significantly more harsh (I new this was not going to be a comfortable configuration) and I was wondering if swapping out the KYB shocks with some premium ones would make a noticable difference (maybe some Bilstien's).

So I guess what I am asking is will shocks help me out or should I just stick with the KYB's?

This is by no means a daily driver...

Scott

 

[ForceFed70]

Switching to softer shocks may help a little (at the expense of less dampening), but it's mostly the springs that are the cause of the harsh ride.

 

[zookeeper]

I think this is a long overdue topic. Here's my very unscientific method for determining whether your springs are too soft or stiff: first jack the car up until all the weight is off the wheels. Now measure the distance between the bump stop and the pad it hits on when at full bottom-out. It doesn't need to be exact, we're not building a piano here. This measurement is the amount of travel of your suspension. Now lower the car back down to the ground so that all the weight is back on the springs and put the amount of weight you normally carry. That means passengers, gas and whatever you usually carry when you ordinarily drive the car. Again no need to split hairs, just get it reasonably close. Now drive the car forward and backwards a bit to settle the suspension and measure the distance between the bump stop and the pad with the car at ride height, just like you did with the suspension fully extended. Your measurement should be about 1/3 to 1/4 of the total travel shorter. For instance, if the first measurement at full extension is 4" then your ride height measurment should be about 2.5" to 3", give or take a bit. Now mind you this is for a slightly stiff, but liveable ride. When I use this method and the ride is too stiff, I then use shocks to tune the suspenion to my tastes. For instance, if it were too stiff, I'd back the compression adjuster off, or switch to softer shocks. If it were too soft, then turn the comression aduster in, go to stiffer shocks or maybe even gas-charged shocks to stiffen it up. That method works well whether we're talking race cars, street cars or your everyday driver. You need to have a method of determining which is too stiff or soft and simply swapping out parts blindly gets spendy in a hurry, so by getting the spring rates in the ballpark first you at least have a method to follow.

 

[66Runt]

Factory stock height measures different on a '66

At normal ride height as per service manual, the suspension is almost close to dead center for a '66. The travel is just over 7" and at ride height (per Ford) it sits within an 1/8" of 3.5" at rest.
So from that, I would think Ford was looking at a 50% static sag.
Harshness can also come from side wall stifness and tire pressure, as well as lack of side wall height. Also unsprung weight has a noticable affect on harshness (or lack there of as the weight increases).
When I went to fairly lightweight wheel/tire package of 235/45 17s, along with an 8.8" rear and Cobra disc brake setup, I swapped the KYB Gas-a-just for GR2s. Made a world of difference on the sharp choppy stuff. Enough so that I am going to step up to a quality set of adjustable/revalve-able shocks.
As far as the high G force slow speed compression goes, it is slightly assisted in damping by the Cal-tracs so the GR2s damping is adequate.
Not sure I'd be happy with that if it wasn't for the Cal-Tracs though. I've backed off the preload, and the suspension blows through the travel pretty quickly with the stock rear springs.

 

[zookeeper]

Good observation. Is that for a stock spring rate '66? If so, then you can see my point, I remember my '68 being very, very undersprung when it was stock. It was spongy to the point of being unsafe. There was no "feel" over bumps at all, and hitting a bump in mid-turn was dicey, to say the least. The soft stock springs also caused a huge delayed reaction in any steering input, I'd bet money my stock Suburban 4x4 would easily smoke a stock vintage Mustang in a road course, it was that bad.

 

[180 Out]

To test if the shocks are causing the harshness, either take them off or pop them out of the spring bracket and wire them up so they won't hit the bracket, and go for a ride. I'll bet you'll find most of the harshness is gone.

Whether Bilsteins will cure it, I don't know. There's also the milder KYB model, GR2 I think, and Monroe and Gabriel stock replacements.

 

[67topless]

Thank you for all the input everyone; I am going to test the measurements this weekend.

 

[66Runt]

Stock springs definitely need help (IMO)

Good observation. Is that for a stock spring rate '66? If so, then you can see my point, I remember my '68 being very, very undersprung when it was stock. It was spongy to the point of being unsafe. There was no "feel" over bumps at all, and hitting a bump in mid-turn was dicey, to say the least. The soft stock springs also caused a huge delayed reaction in any steering input, I'd bet money my stock Suburban 4x4 would easily smoke a stock vintage Mustang in a road course, it was that bad.

Yes stock '66 springs. Also this is a 6 cylinder (for the moment) convertible car, so the GVW is pretty light too (no difference in the stock rear spring rate between a 6 or V8 from the factory).
I agree with you on the stock springs. Between the front aluminum spring bushings, the rear HD poly' shackle kit, and the preload from the Caltracs, I get very good steering input response. Without these aids, I think the car would be mushy and inconsistent.
What I am hoping is that I can run somewhere around a 145-150lbs rear spring for street use, then preload them for serious track work. That and a quality set of adjustable shocks may be the easiest way to get a dual purpose vehicle. Firm but compliant street ride. Firmer and still compliant track ride.
Next year will be a steep learning curve.

For '67topless,
If you have the grey body KYB (GR2s), it's not the shock. If you have the white body (Gas-a-just) it may be the shock. You need to define what you mean by harsh. Does it rattle your teeth over little stuff, or does your spine get compressed over the big dips?

 

[zookeeper]

I've always believed that each individual part of the suspension should have a purpose. The spring should be just barely stiff enough to keep the car from constantly slamming into the stops. The shock should have fast enough action to allow the suspension to work, but not so fast that it blows through the travel, and not so slow it causes hydraulic lock on bigger bumps. The sway bars are there to control body roll, and using stiffer springs to do that job means they would be too stiff to allow proper suspension action anywhere else. I refuse to let someone tell me leaf springs cannot work in a performance street car suspension, I just think you need to use modern leaf technology and apply it to older cars. My previously mentioned Suburban is a good example. No, it's not a hi-performance car in any way, but it handles a whole lot better than my Mustang did stock. Proper spring rates and shock selection are a big part of why.

 

[67topless]

...

For '67topless,
If you have the grey body KYB (GR2s), it's not the shock. If you have the white body (Gas-a-just) it may be the shock. You need to define what you mean by harsh. Does it rattle your teeth over little stuff, or does your spine get compressed over the big dips?

It's the Gas-a-just shocks... and it is at highway speeds that I am getting the spine compressed by two inches rlaugh. I have a stock bench seat with no head rest as well... so that's not helping either. lol