Coil Spring Rate

Last year I purchased a 69 Mustang roller with an MII front suspension. The car doesnt have much front end travel and will porpoise at the track. I know that the front springs are way too stiff because the old set up had a huge blower and iron heads, now has aluminum heads and a Dommy. I want to stay with the same conventional MII spring set-up and not use the coil over shock deal.
The front end weighs 1500# and the rear weighs 1600#.
The current ride height is okay with the springs at 8.25" long.

I need a front coil spring rate recommendation or do I call Eaton and have them go at it?
A spring part number would be great.

Thanks,
Dave

The porpise problem could be caused by a few different things. Shock settings, front and rear, Does the car hook and stay hooked? Lot of time the porpise feel is the rear tires breaking loose.

The back half is a ladder with 130 rate springs. Single adjustable QA1 shocks at all corners. Front set at 2, rear set at 5. Tried many shock settings and 2-5 seems best. It spins on the release but not badly 3 times in first gear. 60 ft is at 1.38-1.40

Dave what engine combo are you running ? Heads and headers in particular.

Thanks

John Myrick wrote:Dave what engine combo are you running ? Heads and headers in particular.

Thanks

547 cu-in, SCJ heads ported, Diamond flat tops for 13.5 compression, .685 280 roller, 1050 Dommy, 2.25/4.00 headers. Running E85. The car is a tube chassis without inner fenderwells and 31x13's on the rear.
The car runs 9.65 at 140 but if it would leave better 9.20's are possible.

Was gonna ask what headers you are using but with the tube chassis they are custom probably.

Thanks for the info

how far are the front springs compressed? total front travel? Is this a ladderbar or 4 link?

richter69 wrote:how far are the front springs compressed? total front travel? Is this a ladderbar or 4 link?

From the ride height to full extension the springs only travel 1".
Its a ladderbar set-up.

Are your springs stock length ?

whitefield wrote:Are your springs stock length ?

Dont know, I bought the car as a roller. The lower control arm is almost level.
I may need to just take the springs out and have them measured then order taller springs with a lower rate.

Dave De wrote:

richter69 wrote:how far are the front springs compressed? total front travel? Is this a ladderbar or 4 link?

From the ride height to full extension the springs only travel 1".
Its a ladderbar set-up.

from ride height how much does the front end compress?

for starters I'd get some good double adjustable shocks on the rear at least.

richter69 wrote:

Dave De wrote:

richter69 wrote:how far are the front springs compressed? total front travel? Is this a ladderbar or 4 link?

From the ride height to full extension the springs only travel 1".
Its a ladderbar set-up.

from ride height how much does the front end compress?

for starters I'd get some good double adjustable shocks on the rear at least.

From ride ht. to compression probably 2.5 inches. The shocks are QA1 TS303's and they have 3.5 inches of travel.
I may need to lower the front and buy taller tires to get back some ground clearance.
Am I asking for too much that cant be done with single adjustables?

I'd get a softer spring to get it drop and inch or or, you dont need a lot of compression......just make sure you use some sort of rubber bumper to cushion it in csae you have a big wheelstand.

Is the bottom bar on the ladder bar even with the ground, uphill, downhill?

And yes a ladderbar car with good power needs a good da shock on the rear at least.

richter69 wrote:Is the bottom bar on the ladder bar even with the ground, uphill, downhill?

Which is the preferred set-up ?

Ladder bar is even with the ground.

Dave De wrote:

whitefield wrote:Are your springs stock length ?

Dont know, I bought the car as a roller. The lower control arm is almost level.
I may need to just take the springs out and have them measured then order taller springs with a lower rate.

I have the mustang II front end in my 64 Fairlane. That is why I stop racing it because I couldn't launch on the T brake with out wheel spin. Keep us posted on what you do to get the ride height and weight transfer you want.

Any video of the car leaving?

richter69 wrote:Any video of the car leaving?

Nothing close up but a few runs from the stands.
Foot braking at 2000 from a tight converter you can hear the engine speed go up then down. I'm also running thru Flowmaster 44's.

Are you sure the problem is just the front suspension? It looks to me like part of the porpising problem could be in the rear suspension too cuz it sure looks like it's really bouncing on the sidewalls repeatedly.

It's kinda hard to tell what all is going on from the video angle/distance. But the whole car does look overly stiff all the way around, kinda like you would expect to see in a car with monster power/or a big power adder. I didn't see hardly any rear suspension separation at the initial hit to help initially plant/hook the slicks. Kinda like maybe the rear shock extension valving was adjusted way too tight/stiff. But there was also almost no rear suspension movement after the initial hit (during the recovery period) as the car got underway & started to try and transfer weight nose to tail. This could be from the rear springs being too stiff, or the rear shock compression valving being too tight/stiff, or the rear springs being too soft/weak/collapsed allowing the car to actually hit/ride/bounce on-off the shock snubbers, or there could be binding somewhere in the rear suspension.

And while it's true that an overly stiff front spring won't help increase the "speed" of front suspension separation (& weight transfer) like a lighter spring compressed more will, the stiff front spring wont 100% stop the separation/weight transfer either. And the front does look really "locked down" in both directions, kinda like the rear suspension looks. So the front suspension problem might actually be both a spring & shock valving adjustment problem together (or front suspension binding, or with only 1" of ext travel you could be "topping" out the front shocks repeatedly).

You said the front/rear shock extension valving settings were at 2 & 5. Is this starting from full tight or full loose?

If this is a full chassis car you might try pulling the nose off the car & taking some good pics of the front suspension compressed at ride height (with driver weight in the seat) & at full extension to give us a better idea of how the front suspension is siting/extending. You might also pull one slick & compress the rear suspension to ride height & take some pics there too.

Dave,
I'll try to answer most of what I know.
Shock settings are from soft so 2-5 is two clicks from softest for the front and five clicks from the softest on the rear.
My first time out at the track was with 175# springs on the rear and it cycled a dozen times where it broke loose to gain about 500 rpm then stuck. I ended up changing the springs to 130# and it got better. Maybe I need 110's or 120's. This car started its life as a racecar only with wheelie bars and a wing going high 7's low 8's. I presume that the 175's were required for that power.
The camera man said that the wheels dont come off more than a couple of inches but it carries them a long distance for only being off the ground a small distance. As of Thursday the car is in storage 30 miles away. If I have a few photos of the front end I will post them soon.
Yes I think it repeatedly tops out the front travel.
All this somewhat confirms that I need to drop the front for more travel but the header tube is at 3" clearance now. If I drop it I can raise the car by using larger tires. The front tires are 25 inches tall at best.
Thanks for looking at this.

Dave


This is the only photo I have of the car leaving.

Dave De wrote:.......I ended up changing the springs to 130# and it got better. Maybe I need 110's or 120's.
This is the only photo I have of the car leaving.

Does the car's front/rear scale numbers (1500lb front/1600lb rear) reflect the car's actual total weight? If not what is the actual total weight?

If the car is actually at 3100lbs (and at around 51.6% rear/48.4% front) a 130lb rear spring does sound in the ball park. A 120lb spring might be worth testing, a 110lb rear spring might be a little too light at 3100lbs/51r+48f. If the car is actually under 3100lbs (say in the 2700lb range) a rear spring in the 110/120lb range does start to become more of a usable possibility.

With the current 130lb rear spring, what is the rear shock's center to center length at ride height (and how much shock shaft is showing at ride height)?

DILLIGASDAVE wrote:

Dave De wrote:.......I ended up changing the springs to 130# and it got better. Maybe I need 110's or 120's.
This is the only photo I have of the car leaving.

Does the car's front/rear scale numbers (1500lb front/1600lb rear) reflect the car's actual total weight? If not what is the actual total weight?

If the car is actually at 3100lbs (and at around 51.6% rear/48.4% front) a 130lb rear spring does sound in the ball park. A 120lb spring might be worth testing, a 110lb rear spring might be a little too light at 3100lbs/51r+48f. If the car is actually under 3100lbs (say in the 2700lb range) a rear spring in the 110/120lb range does start to become more of a usable possibility.

With the current 130lb rear spring, what is the rear shock's center to center length at ride height (and how much shock shaft is showing at ride height)?

When at the scales the front was 1505 and the rear was 1580 but when the entire car was on the scale it was 3060. I know they dont add up but it was a digital scale.
I cant measure the rear shock shaft for a couple of weeks until I get out there but it does have allot of shaft showing and I think you are saying that they may be set to far up and when the rear end tries to separate it tops out from shock travel. How much travel do you recommend from the ride height to the top? I think its only 1 to 1.5 inches because when I lift it up to put stands under it to take the wheels off it only moves about an inch. Is there a range to adjust it to?

Sorry no photos will have to wait until spring. It looks like I got an ATI 4000 stall 8" coming to replace that tight TCI 10" @ 3500 so that's moving in the right direction.

Not all shock makers use the exact same measurements. But if you're using the most common rear coil over shock length (around the 16" fully extended range), the average "installed height" C/C measurement at full weight ride height is in the 13.5" to 14.5" range C/C (depending on the shock maker). When you have the correct spring rate (or in the ball park) this 13.5/14.5 number should be easy to hit on a 16" range shock.

There are two alternative methods for setting shock ride height/installed height on the so called 16" shocks (vs the 13.5/14.5" C/C measurement method). One alternative method is to take the total available shock travel measurement (fully compressed to fully extended) and set the ride height at half the total travel (half shock shaft in/half shock shaft out). The other is to set ride height so there is somewhere around 1-1/8" to 1-1/2" shock shaft showing at ride height.

Which of the 3 methods is "best" really depends on who you talk to, & what type of shock snubber is used (like for example Koni's tall soft spongy snubber vs shorter/harder snubbers other shock makers use).

So if your rear shocks are the "average" length range shock, and only have around an inch of extension travel (from ride height to fully extended), it's very possible that you could be topping out the rear shocks too during the launch. This could be from having a spring that's too stiff/tall, even with the spring adjuster seats/pedestals spun all the way down. Or having the correct spring rate/height, but having the spring seats/pedestals cranked up way too high.


As for the front/rear weights not exactly equaling the total weight, it's common, even with digital scales. The discrepancy can sometimes be even greater when using an in-ground scale (like at the track) to figure front/rear weights.

DILLIGASDAVE wrote:Are you sure the problem is just the front suspension? It looks to me like part of the porpising problem could be in the rear suspension too cuz it sure looks like it's really bouncing on the sidewalls repeatedly.

It's kinda hard to tell what all is going on from the video angle/distance. But the whole car does look overly stiff all the way around, kinda like you would expect to see in a car with monster power/or a big power adder. I didn't see hardly any rear suspension separation at the initial hit to help initially plant/hook the slicks. Kinda like maybe the rear shock extension valving was adjusted way too tight/stiff. But there was also almost no rear suspension movement after the initial hit (during the recovery period) as the car got underway & started to try and transfer weight nose to tail. This could be from the rear springs being too stiff, or the rear shock compression valving being too tight/stiff, or the rear springs being too soft/weak/collapsed allowing the car to actually hit/ride/bounce on-off the shock snubbers, or there could be binding somewhere in the rear suspension.

And while it's true that an overly stiff front spring won't help increase the "speed" of front suspension separation (& weight transfer) like a lighter spring compressed more will, the stiff front spring wont 100% stop the separation/weight transfer either. And the front does look really "locked down" in both directions, kinda like the rear suspension looks. So the front suspension problem might actually be both a spring & shock valving adjustment problem together (or front suspension binding, or with only 1" of ext travel you could be "topping" out the front shocks repeatedly).

You said the front/rear shock extension valving settings were at 2 & 5. Is this starting from full tight or full loose?

If this is a full chassis car you might try pulling the nose off the car & taking some good pics of the front suspension compressed at ride height (with driver weight in the seat) & at full extension to give us a better idea of how the front suspension is siting/extending. You might also pull one slick & compress the rear suspension to ride height & take some pics there too.

Dave,
You are good...Do you get paid to sort out suspensions? I just measured the travel and it is at best 3/4" from ride to fully extended up. So I must be topping out just as you called it.I am going to move the shock up to the next mounting hole and drop the spring adjuster down to compensate and keep the ride height.
The front tires are 25" tall and I'm thinking to go 27" tall while at the same time reduce the front spring rate and lower the shock travel 1" thereby keeping my ground clearance and getting more top travel. Do you see any downside to doing this all at once? I am looking at changing all of this at the same time because its just wrong. Converter change to an 8" 4000 stall from a 10" 3500 stall with the suspension changes.

Dave

Juggling different suspension/component travel settings (such as shock installed center-to-center distance, spring rate/length, spring seat height, etc,etc) can sometimes be a usable way to compensate for assorted problems (like bottoming/topping out the shocks). It just depends on how large the problem is you're trying to work with.



Fudging the front suspension......

I looked on the QA1 site for the specs on their TS303 non-coil over style front MII shocks. And while they gave extended & compressed lengths for them ( 11" & 7.63"), they didn't offer a suggested installed ride height C/C length for the short travel shock. So if you go juggling front runner diameter/height vs front spring rate to gain more front shock extension travel just be careful you don't go too far the other direction & end up bottoming out the shocks after the changes.




Fudging the rear suspension.......

Moving the bottom shock mount up x number of holes, then spinning the spring seat adjuster back down/lower can be a way to get more usable rear shock extension travel. How effective doing it this way is really depends on how close to "ideal" the current rear spring rate is & how far up the shock's spring seat is currently spun up. If the current spring rate is still too stiff, and/or the spring seat is already currently spun down toward the bottom, you might not gain much in the end. Just give it a try and see where everything ends up. But just like the fronts don't go too far the other direction & cause the shocks to bottom out. Topping out might cause some problems, but bottoming out a shock can possibly cause internal damage depending on the shock design/maker.