Correct Spring Rate

DAZ · Sep 13, 2008

I made the mistake of installing 1" lowering springs (5 leaf)on my 1966 Mustang. Now every time I hit a bump or dip in road, the tires rub. I am runnig 225/65/15 x 7 1/2 " rims and wheels. Can someone suggest the optimum spring rate?
I need to raise the car back up 1" and I do not want to make big sacrifice handling.

Someone suggested adding coil springs over shocks. Wouldn't that have same affect of adding air shocks?
DriversSide.jpg

zookeeper · Sep 14, 2008

The problem isn't the spring rate, it's your wheels and tires. 5 leafs are fairly stiff, and I don't think you'd be all that happy by simply making it stiffer to keep the tires from rubbing. Your car should go all the way to the bump stops without rubbing, so if it were me, I'd fix the wheels and fenders.

thekid760 · Sep 14, 2008

whats the backspacing on the wheels?

DAZ · Sep 14, 2008

zookeeper said:
The problem isn't the spring rate, it's your wheels and tires. 5 leafs are fairly stiff, and I don't think you'd be all that happy by simply making it stiffer to keep the tires from rubbing. Your car should go all the way to the bump stops without rubbing, so if it were me, I'd fix the wheels and fenders.

I would consider, but I have seen plenty of 66 Coupes with 15" wheels not rubbing the tires. The lowering springs, unfortunately did what they were intended to.
Determining the correct spring rate is important. I may need to change or enlarge the rubber stops, if that is possible. ? The wheels are 3 1/2" offset and there is probably another 1" to use up before hitting the springs.

zookeeper · Sep 14, 2008

Here's my basic theory on spring components:first of all, springs should be just stiff enough to keep from hitting the bump stops during regular driving. If you regularly do track days, your spring rates will be much stiffer than the guy who cruises on sunny days with the wife and kids. Stiffening the springs to compensate for incorrect wheels (like the ones you now have) makes no sense. You can certainly can move the bump stops closer, but that limits travel. What I'm suggesting is to get wheels with 4.375" to 4.625" of backspacing. How do I know this works? Because I once had a '65 as well, and the 15x7, 3.5" backspace wheels on my car also hit the fenders (HARD!!!) with a 225/60-15 tire and stock springs. My current '68 uses stock Shelby 10 spokes (15x7, 4.375 BS) with 245/50-15 tires on the rear and they don't touch, even at full bump. I learned this the hard way, just like you are doing now.

GNGREN · Sep 15, 2008

zookeeper said:
Here's my basic theory on spring components:first of all, springs should be just stiff enough to keep from hitting the bump stops during regular driving. If you regularly do track days, your spring rates will be much stiffer than the guy who cruises on sunny days with the wife and kids. Stiffening the springs to compensate for incorrect wheels (like the ones you now have) makes no sense. You can certainly can move the bump stops closer, but that limits travel. What I'm suggesting is to get wheels with 4.375" to 4.625" of backspacing. How do I know this works? Because I once had a '65 as well, and the 15x7, 3.5" backspace wheels on my car also hit the fenders (HARD!!!) with a 225/60-15 tire and stock springs. My current '68 uses stock Shelby 10 spokes (15x7, 4.375 BS) with 245/50-15 tires on the rear and they don't touch, even at full bump. I learned this the hard way, just like you are doing now.

+1

I used to run a set of keystones on my 67 back in the day.If they are of the uni-lug variety you need more backspacing. From the looks of your side picture (Nice looking car by the way!) your side wall looks almost as if it were past the fender lip.

DAZ · Sep 17, 2008

GNGREN said:
+1

I used to run a set of keystones on my 67 back in the day.If they are of the uni-lug variety you need more backspacing. From the looks of your side picture (Nice looking car by the way!) your side wall looks almost as if it were past the fender lip.

Points well taken, thanks. I just got car back from Local shop that specializes in Springs. They rearched the springs and put the correct shackles on the rear, leaving the spring rate the same. Net effect was to raise the rear up a little over an ince. It looks real GOOD!. I test drove the car around town and highway last night with a full tank of gas, floor jack, foldingchairs and a cooler with 12 bottles of water (usual gear for car shows). I hit every bump and dip I could. Only one dip caused the car to very slightly rub the tires. This dip is one everyone slows down to about 20 mph, I hit it at 27 mph.
What I did discover from the specialist at the shop is that I have a 9" rear that is about 4" to wide for the car AND it is uncentered. He suggested having the rear cut back at leat two inches and centered to fit the car. I might do this next year when funds are a little better. This should help bring the 225/60/15's inside the fender well and completely eliminate any rub.
By the way, the car feels better on the road now then my 2000 GT.
I'll PM you some picks over the weekend.

tx65coupe · Sep 18, 2008

In the picture it looks like the top of the tire isn't visible like it is in the front. I would put stock ride height springs back on the rear to raise it back up the 1 inch. Most catalogs that I have seen offer a standard leaf spring and a heavy duty leaf spring for stock height. I don't know which spring is really better for handling or ride quality.

SadbutTrue · Sep 18, 2008

DAZ said:
I made the mistake of installing 1" lowering springs (5 leaf)on my 1966 Mustang. Now every time I hit a bump or dip in road, the tires rub. I am runnig 225/65/15 x 7 1/2 " rims and wheels. Can someone suggest the optimum spring rate?
I need to raise the car back up 1" and I do not want to make big sacrifice handling.

Someone suggested adding coil springs over shocks. Wouldn't that have same affect of adding air shocks?
DriversSide.jpg

Your tires are too big. Thats the issue, not the spring. You're learning the same lesson i did.

I fixed it with a rear sway bar though. Even though I'm nto a total fan of the sway bars effect on handling, it prevents the 'bounce' that would result in tire rub. May want to look into that if you don't want to get new tires.

Blu-Steel · Sep 18, 2008

FYI- I have 15x7 Styled Steel Wheels 225/60/15 tires and I installed Grab a Track 5 leaf (195 lb) stock height rear springs and 620 stock height coils on front. The car sits level and I love the way it handles now. The stock springs always felt too soft for me and the car floated too much. This is my 6th 66 Mustang and now its the best driving and handling one I ever had. And I take my wife and 2 kids on cruises with me and they don't feel its harsh at all for them. + no tire issues.

From your picture yours does look a little lower than mine plus your tires are a bit wider.

tx65coupe · Sep 20, 2008

BluSteel,
I thought the 620 springs lowered the front 1 inch. Did you get the 1 inch spring insulators to keep it at the stock height? What type of styled steel wheels do you have? I want to get Magnum 500 wheels for mine. I thought the widest tire I could run is a 215 60 15 with out rubbing in the front. I wanted to run 235 or at least 225 in the rear. Do you really not have any rubbing issues with 225 in the front?

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