Slant Six Forum
https://slantsix.org/forum/

Theoretical lowering question.
https://slantsix.org/forum/viewtopic.php?t=18235		 Page 1 of 2
Author:  Sam Powell [ Mon Jun 26, 2006 7:19 am ]
Post subject:  Theoretical lowering question.
I want to lower the rear of my '69 Dart. The rear sits just a little to high for best handling. Does it matter whether I lower it by changing the hanger box, the shackles, or with lowering blocks? I know all three will work, and are acceptable in their way, but what effect does each have on the handling. Is their a difference between changing the attachment point of the spring to the body, and changing the attachement point of the axle to the spring? Does it affect how the car will corner, or how the weight shifts in turns? Thanks for you thoughts.
Author:  emsvitil [ Mon Jun 26, 2006 2:00 pm ]
Post subject: 
You may need a combination of all of the above........

First with lowering blocks, the axle gets farther away from the spring so you're more likely to get spring wrap. But as it's not like a 4x4 with big tires that have a BIG block (engine and raising block) I don't think it will matter all that much......

Most stock leaf springs have the rear bolt hole location higher than the front. When the car leans in a turn, this induces 'roll steer' as one tire tends to move forward a little, the other rearward to give the car a slight 'rear steer' effect. Most likely the effect tends to induce understeer, but whatever it does, you're getting steering from the wrong end of the car.........

To eliminate/reduce 'roll steer' you need to do get the leaf spring bolt holes level by whatever means possible (hanger, shackle). Then get the height you want with a lower block.

You may need a lowering block that's angled or angle shims installed to make sure that your differential pinion angle is in the correct relationship to the transmission after the hanger/shackle changes.
Author:  sandy in BC [ Mon Jun 26, 2006 2:31 pm ]
Post subject: 
My Valaint is lowered using the swapped/reversed front spring mount method. No change in how the car handled.
Author:  74.swinger [ Mon Jun 26, 2006 3:24 pm ]
Post subject: 
Quote:
My Valaint is lowered using the swapped/reversed front spring mount method. .
same here except mine handled a little better with the lowered ride height.
________
Marijuana Medical
________
Rd56
Author:  Sam Powell [ Mon Jun 26, 2006 4:26 pm ]
Post subject: 
Thanks for the input. Sandy, I think I understand your comment. And have considered doing this. Just turn the front box upside down? Is that right? AndyF makes a box to lower the body, but I don;t know what size bolts are in my springs, and his part is for the larger size bolt.

Emsvitil, would you try to be a little more expicit with your comments? You say the rear bolt hole on most leaf spring set ups is higher than the front. Do you mean the eyelet for the spring, or the bolt for the shackle to the body. And if it is the eyelet, are you saying that the rear eyelet is higher in relation to the front eyelet. After we clear that up, is it preferable to have the rear eyelt even with the front, or lower. It sounds like the eyelet should be located in the front box first and then the rear shackle designed to put the rear eyelet even with the front one. Is that right? I don;t know here, I am just trying to get clear on what you are saying. I think the axle wrap issue might be a concern since the turbo on this engine produces alot of low end torque. My 360 Dart, which I sold recently had a big problem with this, and the slant feels almost as fast as it was. I haven't begun to get it dialed in yet, and I should plan for alot more power eventually.

I hope more discussion come out of this thread. Are there more views out there? I have even considered putting a Mustange IRS in just for the hell of it, eventually. Most of the Mustang racers don;t like the IRS, and some take them out and replace them with old fashioned solid axles. The new model did away with it, I think. I don;t how hard it would be, or what would have to be fabricated, but it does not look like it should be impossible.
Author:  sandy in BC [ Mon Jun 26, 2006 7:59 pm ]
Post subject: 
Turn them 'sidedown and swap them side for side....some drilling necessary.
Author:  emsvitil [ Mon Jun 26, 2006 8:53 pm ]
Post subject: 
The eyelet/bolt hole on the spring......

You want the rear even with the front (not lower or higher)
Author:  Matt Cramer [ Tue Jun 27, 2006 3:47 am ]
Post subject: 
Quote:
Most stock leaf springs have the rear bolt hole location higher than the front. When the car leans in a turn, this induces 'roll steer' as one tire tends to move forward a little, the other rearward to give the car a slight 'rear steer' effect. Most likely the effect tends to induce understeer, but whatever it does, you're getting steering from the wrong end of the car.........

To eliminate/reduce 'roll steer' you need to do get the leaf spring bolt holes level by whatever means possible (hanger, shackle). Then get the height you want with a lower block.
Actually, roll steer is primarily caused by the axle not being the same height as the front spring eye. Roll steer is caused by the axle changing its angle relative to the chassis, and having the axle at a different height fron the front spring pivot point will rotate the axle sideways in roll. The rear spring eye height is less important on Mopars - the front of the leaf spring is much stiffer and behaves like a control arm.
Author:  emsvitil [ Tue Jun 27, 2006 4:14 am ]
Post subject: 
Quote:
Quote:
Most stock leaf springs have the rear bolt hole location higher than the front. When the car leans in a turn, this induces 'roll steer' as one tire tends to move forward a little, the other rearward to give the car a slight 'rear steer' effect. Most likely the effect tends to induce understeer, but whatever it does, you're getting steering from the wrong end of the car.........

To eliminate/reduce 'roll steer' you need to do get the leaf spring bolt holes level by whatever means possible (hanger, shackle). Then get the height you want with a lower block.
Actually, roll steer is primarily caused by the axle not being the same height as the front spring eye. Roll steer is caused by the axle changing its angle relative to the chassis, and having the axle at a different height fron the front spring pivot point will rotate the axle sideways in roll. The rear spring eye height is less important on Mopars - the front of the leaf spring is much stiffer and behaves like a control arm.

Ok, I tested my memory and looked it up....

Page 93 of 'How to Make Your Car Handle' by Fred Puhn

'On cars with a solid rear axle, roll steer is created by having the leaf-spring mounts at different heights. This causes one side of the axle to move forward and the other side rearward as the car leans."

Which is what I said. :twisted:
Author:  sandy in BC [ Tue Jun 27, 2006 4:38 am ]
Post subject: 
So as long as the mounts are the same height as each other(as mine are) no roll steer.
I can see stock cars using this for left turns.....
Author:  Slant n' Rant [ Tue Jun 27, 2006 4:40 am ]
Post subject: 
Put a couple of bags of concrete over the wells ,that will bring it down to level, improve handling and traction, and only cost $6. thats what the suspension is designed for iin the first place to carry some load

Oops.... I meant to say sand
Author:  Dart270 [ Tue Jun 27, 2006 9:37 am ]
Post subject: 
Raising the front eye tends to make the car oversteer more, and lowering it vice-versa.

Roll steer should be maximized when the springs are flat with the car pointed straight and on a level surface, and is considered a desirable thing for handling, but is minor with our suspensions.

I have also lowered the car by taking 1/2" out of the rear shackles (make them shorter) by fabbing my own. Be careful about hitting frame rail though.

Lou
Author:  Matt Cramer [ Wed Jun 28, 2006 10:03 am ]
Post subject: 
Quote:
Ok, I tested my memory and looked it up....

Page 93 of 'How to Make Your Car Handle' by Fred Puhn

'On cars with a solid rear axle, roll steer is created by having the leaf-spring mounts at different heights. This causes one side of the axle to move forward and the other side rearward as the car leans."

Which is what I said. :twisted:
All right, I went and checked that too. I finally figured out what the deal is: there are two slightly different effects that can cause roll steer, depending on the shape of the leaf spring.

If the spring is completely symmetric front to back, the axle will move perpendicular to a line drawn between the spring eyes (as a first approximation). So to minimize roll steer with a setup like that, the optimum mounting has the spring eyes level with the ground, and the axle height can be anywhere.

If you have a leaf spring where the section in front of the axle is much, much stiffer than the section behind the axle, the suspension will behave as though the front sections of the leaf springs are control arms (again, as a first approximation). It moves in an arc with the center at the front spring eye. So to minimize roll steer with such a setup, you need to put the axle center at the same height as the front spring eye. If the front section is stiff enough, you could locate the rear spring eye anywhere - even something screwball like putting it three feet directly above the axle.

Mopars use assymetric leaf springs. It's hard to tell how the Pinto's springs in How to Make Your Car Handle are set up, but they look to be a little more symmetric than Mopar units. I'd have to do a lot of calculations, though, to tell whether the effects on a Mopar from unequal spring eye heights or the axle height are larger.

So I'll partially take things back - rear spring eye height does matter. But on a Mopar, so does the position of the axle relative to the front spring hanger.
Author:  Sam Powell [ Wed Jun 28, 2006 6:21 pm ]
Post subject: 
OK guys. Maybe I am the one who messed up my car's handling. It has serious oversteer, that comes on pretty suddenly. Everything feels great until you hit a certain break away limit, and it goes real fast, ass end first. I have never lost it completely, but have scared myself. Lou has been trying to get me to take off the rear sway bar for years, but up until it breaks lose, it feels wonderful. So if there is another way to fix it, I want to do it.

So here is what I did that might have screwed things up. I ordered a standard set of new 5 leaf springs from a guy in PA who advertized in Hemmings. I put the new set in, and for about a week, everything looked great. And then the main leaf behind the axle sagged severly, and the ass was dragging again. So, seeing that it was the rear section that sagged, I took each of old top leafs, that I had just removed, cut the eyelet off of each end, and installed it as the number two spring in the pack on each side. Everything was right back up where it belonged. The thing is, I left the new number two spring almost full length both front and rear, which meands the springs are essential symetrical now.

The rear of the arch is just about as stiff as the front, and has a pretty pronounced arch to it. So with all your insight out there, what do I do now? I don;t want to change the springs. They seem to have just the right stiffness, but a different profile from the standard Mopar set up. I am pretty sure the axle is below the spring perch at this time. So maybe the lowering blocks are really the way to go. That would raise the axle to the level of the eylet, or more properly, lower the eylet to the level of the axle center line. Once I have done that, then I should look at the shackle. Do I want that at the same level as the front one, keeping in mind that I now have symetrical, eliptical spings.

Just for the record, I did square the rear axle to the track of the car, so the thrust angle is correct. It required a 1/4" spacer between the left front spring perch and the frame. An alignment shop confirmed what my measurements told me, that the rear was essentialy square with no toe in on either rear wheel. So what we are dealing with is a square car. Lou has also suggested bigger tires in the back, which are 205.60.15's all the way around. That would be pretty easy to try.


Thanks for thinking about this. Sam
Author:  Joshie225 [ Thu Jun 29, 2006 8:19 am ]
Post subject: 
Sam,

Lou is right about the sway bar. You just have too much rear roll stiffness so the car oversteers. Disconnect the rear bar (one side is fine) and drive it. I'll bet you $1 the oversteer is gone. Another option if the sway bar isn't too big is to run softer rear springs. Finally, you can add a lot more roll stiffness to the front to balance the car.
Page 1 of 2 All times are UTC-08:00
Powered by phpBB® Forum Software © phpBB Limited
https://www.phpbb.com/