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| Geometry with rear leaf springs. https://slantsix.org/forum/viewtopic.php?t=26646 |
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| Author: | Sam Powell [ Fri Jan 11, 2008 7:16 pm ] |
| Post subject: | Geometry with rear leaf springs. |
The wheel base of a leaf spring car is longest when the rear leaf spring is straight out behind the hanger bolt. This is when the axle is in line with the hanger bolt, or the same height. I went out and measured the front hanger bolt height on the rear springs of my Dart. It is 9.5" above the ground. The center of the axle is 12 inches above the ground. This means that as one side gets closer to the ground, the axle on that side should swing forward as it comes up around the front hanger pivot point, and thus shorten the wheel base on the low side. With my setup, when the car is steered to the right, the wheel base on the left will shorten, and swing the rear wheels slightly to the right as well. It looks like the side towards which the car is being steered will rise, and thus lengthen the wheel base slightly (on my car) thus making this steering effect more so. Is this a good thing or bad thing? What is that doing to the handling? Anyone know how a modern setup is designed? Are the pivot points on a trailing arm above or below the centerline of the wheel? I know I could go on line and look this up. I just thought we could get a discussion going here. Sam |
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| Author: | Matt Cramer [ Tue Jan 15, 2008 1:40 pm ] |
| Post subject: | |
This is what's known as roll steer. Usually, having the spring eye in line with the axle centerline will keep this to a minimum, and minimizing it is usually a good thing. It's covered in Fred Puhn's How to Make Your Car Handle. After that book came out, a number of Japanese cars (particularly Mazdas) tried to use a similar scheme called "passive rear steer" that would create large changes in toe when the car leaned or was subject to lateral G's. Most racers disable this system completely. |
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| Author: | Sam Powell [ Tue Jan 15, 2008 6:41 pm ] |
| Post subject: | |
Thanks for your response Matt. Do you know what Fred says to do with the Mopar rear? It seems as if you would have to raise the rear pearch, and shorten the rear shackles. Is this right? The front eye is 2.5" below the center line now. If you raised the front perch this would raise the car, and then you could lower the car by shortening up the rear shackle. Am I understanding this right? Is this radical, or is it what you would have to do? I know from talking to Ford guys that the ford swing arm mounts higher than the axle center. I suppose I could buy the book, and probably will, but that won;t generate a discussion here. Sam |
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| Author: | sandy in BC [ Tue Jan 15, 2008 7:08 pm ] |
| Post subject: | |
I run reversed front mounts and short shackles......works fine. I copied this from this thread http://www.slantsix.org/forum/viewtopic ... c&start=15 he front spring mounts need to be drilled out to 9/16 . They are inverted to lower the car 2". http://www.mobmms.com/view/20707-0.html I will make new shackles from truck shackles which are longer, wider and a little thicker than Abody shackles. I will redrill them from 3/8 to 1/2" add grade 8 bolts welded to the shackles on one side. The holes will be drilled 1" closer than stock to lower the car a little more...and to balance the pinion angle |
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| Author: | Sam Powell [ Tue Jan 15, 2008 7:38 pm ] |
| Post subject: | |
If you are lowering the car 2", is that not making the axle center even higher in relation to the front mount? According to Matt's info, the front bolt needs to come up, not down. The axle center is now 12" above the ground, and the front bolt is 9.5" above the ground. This means raising the front mount, not lowering it. What say ye? OH, wait a minute, DUH!!!!, if the car comes down, the bolt goes up in relationship to the car. But does it go up in relationship to the axle center? And how much? ARe you in any kind of postion to actually measure the height of the front bolt from the ground on your car? Thanks Man. I hope it is not sitting in two feet of snow now. Edit: I now see a mistake in my thinking in an earlier comment by me several posts back. Forget that. I now see that if you lower the car by raising the front spring box, you have, at the very least, not worsened the axle center/spring mount relationship. I also see that doing this is going to tilt the front of the pinnion upwards. This would introduce virbration if you don't compensate for it. Shorter shackles would help, but would then bring the front bolt back down some in relationship to the axle center. It seems as if a tapered shim might be a better approach. I know Dart270 was bothered by pinion vibration for awhile, but don;t know if he fixed it yet. Would you like to comment here Lou? Sam |
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| Author: | Sam Powell [ Wed Jan 16, 2008 4:21 am ] |
| Post subject: | |
Sandy, I went and read your posts, and inspected the photo link you provided as well. Are the bolts that go through the subframe symetrical, or do you have to swap sides, or drill new holes in the subframe, (or some such thing). If you just flip it over, do the bolts simply line up with existing holes? Thanks, Sam |
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| Author: | Dart270 [ Wed Jan 16, 2008 6:36 am ] |
| Post subject: | |
Raising the front eye and not the rear shackles will cause a bit more oversteer. Can use this for tuning f/r balance. Lou |
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| Author: | slantvaliant [ Wed Jan 16, 2008 7:06 am ] |
| Post subject: | |
"ADJUSTABLE" (2-position) SPRING HANGERS are available from several sources. IIRC, the stock mounting bolt pattern isn't quite right for flipping, but it can be done with some drilling/grinding/filing. |
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| Author: | sandy in BC [ Wed Jan 16, 2008 7:18 am ] |
| Post subject: | |
Sam: I have raised the front hanger 2". You have to swap sides and flip them to make this happen. My rear shackles are shortened 1 " which is about the maximum while still allowing free travel. Quote: hope it is not sitting in two feet of snow now ......it isThe pictures in the thread I referred to should address your questions. |
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| Author: | Sam Powell [ Thu Jan 17, 2008 4:36 am ] |
| Post subject: | |
Thanks for the link to Mancini. I like the looks of the HD front spring hanger, and have considered buying this in the past. I think maybe Andy F actually produces this part. I would prefer to go with new parts if I can. I have been stopped from purchasing it in the past by the fact that the spring mount bolt holes are the super stock size on the new part, and almost certainly bigger than my existing bolts. Does any one know of a bushing that could be inserted from outside the hanger that would reduce the bolt hole size? What size are the smaller bolts? Or, are there front bushings that will install in my existing springs that will fit the big bolt? Is the eylet the same in the front of the standard and HD springs ? I don't want to find out that I have to replace my springs to use these new hangers. I have a press to remove them and install them, so that is not a problem. As you may know from reading previous posts, this car did not track true, and currently has a 1/4" spacer between the left rear mount and the subframe mount. I don't know if it was the body or the mounting box that was in error, but i suspect it was the box, as the original one was rusted away when I got the car, and I had a local fabrication shop make a new one. They were not available at the time. So I will have this alignment issue to deal with no matter what approach I go to fix the rear geometry. I guess I can take it to an alignment shop when all is done, have it checked, and take whatever corrective action is needed by removing or installing shims on one side or the other. I know guys with old British cars consider this SOP. for pretty much everything on the car. If any body knows the answers to the two earlier questions about the bolt hole size, it would be nice to know what I am up against. I am not going to do this for awhile, as it is too cold to lie on the ground under the car even if it is in a garage, but I would like to start assembling the parts. I could probably call Mancini and get some answers, but this kind of discussion keeps things interesting on the forum. Thanks. Sam |
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| Author: | Dart270 [ Thu Jan 17, 2008 6:42 am ] |
| Post subject: | |
I have those AndyF front perches on my V. Work great and give about 1.25" rise to the front bolt, IIRC. Lou |
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| Author: | Sam Powell [ Thu Jan 17, 2008 10:25 am ] |
| Post subject: | |
Thanks Lou, Did you use your original springs, and did you have to deal with the bolt size difference? If so how did you do that? Is this the car with the fiber glass rear springs? If you did buy the glass srpings, what did they run you, and is there a web site you can link me to. I remember in an earlier thead your dicussion of the load rate, and how he wanted to make t hem about double your need, or preference. Recommendations about how to deal with him on that? Thanks. I went ahead and ordered the mount boxes from Mancini. He verified that Andy F does indeed make those parts. I will deal with the bolt size when I get to it. Sam |
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| Author: | Dart270 [ Tue Jan 22, 2008 5:40 pm ] |
| Post subject: | |
All of the aftermarket springs I've bought have the 5/8" bolt size, so I've either drilled out the perch hole, or gotten diff perches. I have no interest in crappy little 7/16" bolts. I took the 'glass leafs (flex-a-form.com) off my car because, as I told the guy when I bought them, they are too stiff. I plan to order another set with less rate, but haven't gotten around to it. They were about $400/set shipped. Can hopefully use the stiffies on Project V or the 68, which will likely need more rate. Could also put a stiffer front sway and tbars on the 64 to get 'er neutral. Lou |
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| Author: | Sam Powell [ Wed Jan 23, 2008 6:39 am ] |
| Post subject: | |
Thanks Lou. I replaced these springs years back, and maybe they have the bigger bolt already. I hate to take it apart not knowing what I will be up against since that will influence the sequence of disassembly. I think I am going to assume for now the bigger bolts. Thanks again. Sam |
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| Author: | Dart270 [ Wed Jan 23, 2008 7:17 am ] |
| Post subject: | |
I think all the aftermarket poly spring bushings have the 5/8" hole, although I seem to remember getting a set for the smaller eye that were actually for a 60s Mustang and had 1/2" bolt holes and were too big OD by about 1/8". I modified them on the lathe to install them... Lou |
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