Is there an advantage to.....

[Doug_Guilds]

Moving my rear springs to the top of the rear end (and subsequent movement of the upper mount point rearward) as compared to them mounting on the trailing arms?

[Headhunter]

No more pigtail srings so springs will be cheaperand more choices, but without changing the complete way the rear end mounts to the frame I dont see any difference. The wheel is going to need the wheel rate from the spring.

[Doug_Guilds]

Thanks for the input HH.

[FLYNBRYAN]

Yes there is a difference, its called moment ratio. Moving the spring away from the pivot will make the spring work harder, their fore having to run a spring rate, almost half as much more.
Putting the spring on top of the axle will let you run a softer rate. Also try to make the spring base as wide as possible.

[Headhunter]

Bryan Im going to differ on this one.
We looked at that and to get it to work right on a Mustang in the end it worked out so close that it really did nothing or changed that much using stock suspension mounting points. Yes you can run a softer spring but the rate that the wheel see remains the same, providing you get the spring closer to the wheel. But also you have to run inboard at a further difference than the distance from the arm to the wheel and we figured we would have to stiffen the rates about 20- 25% because the spring was mounted closer to the center of the axle. Also looked at mounting the springs in line with the frame rails but ride height and spring lenght was the issue there.

If this is a tube car or a car that your allowed to fabricate the rear frame rails where you can get the spring on top of the axle and close to the wheels it is worth doing. But for a average mini stock that you have to keep the majority of the frame as it came from the factory not going to help at all.

What I ended up doing was making spring buckets about 5" tall and cutting out the section of the stock lower and welding the buckets in. This allowed me to use off the shelf inexpensive springs and with the jacking bolts drilled right up through the stock spring perch and the plate welded in I got a better angle on the spring than stock.

[fortner15]

Headhunter,,,

If you don't need lower spring rate then please explain what body roll does with th lower control arm. The arm takes rate away from car when rolls. This want happen whem spring on top of rearend. Thanks just my .o2

[Doug_Guilds]

Bryan Im going to differ on this one.
We looked at that and to get it to work right on a Mustang in the end it worked out so close that it really did nothing or changed that much using stock suspension mounting points. Yes you can run a softer spring but the rate that the wheel see remains the same, providing you get the spring closer to the wheel. But also you have to run inboard at a further difference than the distance from the arm to the wheel and we figured we would have to stiffen the rates about 20- 25% because the spring was mounted closer to the center of the axle. Also looked at mounting the springs in line with the frame rails but ride height and spring lenght was the issue there.

If this is a tube car or a car that your allowed to fabricate the rear frame rails where you can get the spring on top of the axle and close to the wheels it is worth doing. But for a average mini stock that you have to keep the majority of the frame as it came from the factory not going to help at all.

What I ended up doing was making spring buckets about 5" tall and cutting out the section of the stock lower and welding the buckets in. This allowed me to use off the shelf inexpensive springs and with the jacking bolts drilled right up through the stock spring perch and the plate welded in I got a better angle on the spring than stock.

That's interesting because my next question was how would moving my springs inboard effect the rates if at all. But please feel free to further discuss this subject because I want all the information I can get.

[Headhunter]

Yes ther is a slight amount of rate change or variable geometry as the LCA runs through it arc, chances are it is immesurable from out standpont. There is a amount of change and that change will depend 100% 0n the angles the LCA is mounted at. If the LCA is angled so the front is higher than the rear end connection initial will be slighly more as the body roll and the the rear will see rate on a 1 to 1 basis plus mechanical advantage from the distance.

I did make this reply as if it was a sock framed mini Mustang. Also the angles and the dynamic movment of the rear end will remain the same no matter where the springs are mounted. That kind of change has to be a mechanical change.

The mounting location VS spring rate is just a matter of leverage applied to the resistance The further inward a sping is mounted the more it must resist to keep the same wheel rate or the actual spring rate seen at the wheel. Say for example you have a 100 PPI spring mounted right at the end of the axle. You would need 100 lbs of force to lift that wheel 1 inch. now move that spring 1/2 the distance inboard and you would need 50 lbs to move it one inch. Also you no longer would compress the spring 1" but only a 1/2 inch. It is the basic idea here also and I know there is more to it than that but it gives the general idea.

This is why strut cars like Toyota will have a 250 to 450 PPI spring on the front and the Mustang will need a 700 or so. Because the Toyota spring places it's force much closer to the wheel than a Mustang does with the inboard mounted spring.

All this being said if your allowed to cut out and fabricate the rails by all means place the springs out as far as you can above the axle. This will allow you to run less sping rate and also allow for more compression in the spring to achive the same movement of the rear wheel. That makes for more control and better adjusment in the chassis. But unless you mechanicly change the mounting of the rear end to the chassis what it will do is not going to change much if not at all.

[Doug_Guilds]

Great reply, that explains much. It is indeed a stock frame Mustang so thank you for all the information.