Leaf spring rate recommendations?

[stangman69 17]

Ok.  I'm getting close to pulling the trigger on a replacement rear leaf spring.  I'm looking to drop 1.5" and if I end up wanting any lower, I'll use a short block.  I'd guess 2" is as far as I'd end up going.

I'm going to be going with a Flax-A-Form composite single leaf.

What's the general standard spring rate that folks are running out there?

My car is a '69 coupe, 331 EFI stroker (425-ish crank hp), Tremec TK3550, 8 inch rear and Watts link.  Probably 3.73 / 3.80 gears.  Might end up with an 8.8 in the future but wouldn't change the gear ratio much.  I don't have much idea what final curb weight will be.  StreetOrTrack front end with SoT sport Bilstein shocks.  Will use matching SoT shocks in the rear probably.  17" wheels.  Cobra brakes all around.  TCP subframe w/X brace.  Opentracker shock tower bracing.  MTF front engine cross member.  Miere one piece export brace.  Goal is a spirited sunny-day driver.  I'm ok with a firm ride but don't want something that is stupid stiff.

RideTech springs are 150# and too expensive IMHO.

Liteflex apparently makes some but I'm not sure yet what they cost or what the recommended rate is.

Flexform makes some and the rep recommended something around 225 - 245 lbs / in and said they'd cost around $450 for a pair.

Thoughts from the experts?

I found this information trolling around on another forum:

Quote:

Mustangs Plus
4.5 leaf 175lb
5 leaf 195lb
5.5 leaf 300lb

Maier Racing lists a 165 and 200lb leaf spring, while Global West has a 4.5 leaf 160lb spring. The Boss 302 chassis modification guide calls for 200lb leafs with 180 and 220 as alternatives.

End Quote

Thanks,

Nate

Edited March 20, 2020 by stangman69
update

[barnett468 418]

Ok, look at it this way. The stock rear spring on a 1970 fastback with a 351 is 85 lbs, so doubling the spring rate would be a lot for any street app. Also, the lower the profile is on the tires, the harsher the ride will be, so you need to look at many factors when determining the best spring rate for your particular app. This being said, the most common upgraded" spring rates people use are from 125 to 165 lbs.

You can also get some from Detroit/Eaton Spring, and Meir Racing.

If you drag race the car, you might consider "anti wrap" springs, or traction bars like Traction Masters or Caltracs etc".

 

 

[Vicfreg 771]

Most people who run those fiberglass springs need to use Cal-Tracs anyway.

I use Detroit/Eaton Spring, they are the OEM for the Mustang Springs.    138 # spring rate.  NPD catalog Part Number 5560-35

 

[stangman69 17]

I dunno guys...the majority of posts I've seen online in the various forums seems to suggest that traction aids are not needed.  I'm planning on just the Watts link and poly bushings in the new springs.

Great point @barnett468 on the low profile tires adding to the stiffness.

I'm wondering if the 225 rate I was pointed towards by the Flex-A-Form guy might not be too far off.  Maybe closer to 180-200 given my shorter tire.

[barnett468 418]

3 hours ago, stangman69 said:

I dunno guys...the majority of posts I've seen online in the various forums seems to suggest that traction aids are not needed.  I'm planning on just the Watts link and poly bushings in the new springs.

Great point @barnett468 on the low profile tires adding to the stiffness.

I'm wondering if the 225 rate I was pointed towards by the Flex-A-Form guy might not be too far off. 

Maybe closer to 180-200 given my shorter tire.

People that say traction aids are not needed for street cars that are drag raced must know way more than thousands of other drag racers know, plus perhaps maybe they should have mentioned that to Carrol Shelby, so he wouldn't have wasted his time and money installing them on every single 65 and 66 Shelby he made.

Stiffer shocks and/or a watts link will also increase ride harshness.

225 is not far off if you are road racing it and don't mind having your spine compressed and fillings jarred loose on the street.

That would be a better choice than 225 for a street car.

 

"Composite" springs will perform a little differently than steel springs, so it's hard to say with certainty, what composite spring rate will be the closest to a steel spring rate, but they still shouldn't be very far apart at all.

Why do you want rear springs that are more than twice as stiff as stock ones?

 

.

 

[stangman69 17]

I'm just going by the recommendation from the rep.  Didn't the Boss 302 consider the use of a spring at 200#?  As I implied in my original post, 200 seems to be towards the top end of the scale but doesn't seem like a crazy high outlier.  That said, maybe something closer to 160# would be sufficient given the offerings by vendors and the point you made about lower profile tires and possible added harshness from the Watts link.

Do you have any experience with the composite springs?  The guys that comment on the forums that have them love them and most indicate that they don't need additional traction aids.  That's what I'm basing my assessment on.  I don't have any experience with them myself.  I do see that FAF offers a device that looks much like a slapper bar so maybe that is something to consider.  Guess it depends on how hard I launch it.  :)

Another forum discussion:  https://www.vintage-mustang.com/threads/fiberglass-leaf-spring-info-wanted.575038/page-3

Another string:  https://www.vintage-mustang.com/threads/spring-rates-for-street-track-car.647615/ 

[barnett468 418]

Why do you want composite springs?

You still haven't mentioned how you will be using the car.

Also, as one person states on the vintage mustang forum, it is possible for those composite springs to crack, so I for one wouldn't use them, especially because steel springs have worked just fine on all vehicles ever since they first started making them over 100 years ago, and I have never seen a single one break yet.

 

I have zero experience with composite springs, however, 150 lb spring is a 150 lb spring as far as the amount of resistance it will offer, and that resistance is supposed to be linear if it is a single rate spring and not a dual rate spring, however, the speed at which they compress and/or rebound will not be exactly the same, which can sometimes mean that you might need to  use a slightly different spring rate when switching from steel springs to titanium or composite springs, but this rate difference is typically not more than around 20% at the extreme outside and is more commonly around 10%.

As far as not needing traction aids with the composite springs, I am confident that none of these people did a back to back test with and without traction bars, therefore, if that is the case, then they are making an inaccurate, or incomplete claim, meaning they may be getting better traction with the composite springs over their previous steel springs, but they can't say with certainty that it is because their new springs are composite, or because they are simply stiffer than their previous steel springs, or because their composite springs have "anti wrap" qualities etc.

It is always best to take anything you read or hear in cases like this, with a grain (and sometimes a pound) of salt, and do some research and ask a lot of questions, and if the people making extraordinary claims about how a part made improvements to their car, and do not have reasonable sounding explanation as to how and why their car is better with the new X, Y or Z parts they installed, I would be skeptical. Many people simply get the "Placebo" affect, thinking that because they just spent $1200.00 on a fancy new carb etc, that their car suddenly has 200 more hp.

.

[stangman69 17]

My car will be mostly street but I'm interested in the weight savings and I'm wanting a solid, smooth handling ride.

I've done a bunch more reading and I think you're right @barnett468, north of 160# is probably unnecessarily heavy and not what I'm wanting.  Looks like the general consensus I'm seeing is something down around 160#.

My plan is my Watts link + 160# composite 2" drop springs + rubber or maybe poly bushings in both eyes + Bilstein shocks.

Nate

[barnett468 418]

50 minutes ago, stangman69 said:

My car will be mostly street but I'm interested in the weight savings and I'm wanting a solid, smooth handling ride.

I've done a bunch more reading and I think you're right @barnett468, north of 160# is probably unnecessarily heavy and not what I'm wanting.  Looks like the general consensus I'm seeing is something down around 160#.

My plan is my Watts link + 160# composite 2" drop springs + rubber or maybe poly bushings in both eyes + Bilstein shocks.

Nate

1. Every guy that wants a performance car, likes to have light weight parts, however, there are some instances, where it is more prudent to go with stock, heavy parts that are guaranteed not to fail, instead of the newest fancy, ultra light weight ones, which definitely have a potential to fail, especial where suspension is concerned. If you install a light with hood and it flies off the car at 60 miles an hour, you can still drive the car, however, if you install light weight suspension springs and one of them breaks, not only can you no longer drive the car, it can also cause damage to the car and quite possibly cause you to crash into something or someone. Also, I guarantee you with 110% certainty, that you will not be able to feel a noticeable difference in the suspension feel between a steel spring and a composite one, and the lighter springs will not lower your 1/4 mile t one tiny bit.

2. Xlnt, choice, and glad you looked into it a bit more.

3. Don't forget to consider reinforcing the upper shock mount hole due to the stiffer shocks and springs, and if your shock pushes thru the hole, it will be much harder to repair.

 

[jpfll 11]

If you want a solid, smooth handling ride you don’t want to over spring the car. That will just make it hard and jittery. Ride is mainly controlled by shocks. Put you money into a pair of bilstiens or double adjustable shocks.