• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

leaf spring squat.........what's the deal? Ed?

gearwhine

NAXJA Forum User
Location
Denver, CO
OK, here's the deal. I've been searching all over trying to find characteristics of leaf spring suspensions. The majority of the things I've found online are either 4 links, or independent suspensions. BTW, I'm focusing on rear leaf systems.

I've sesarched for dynamics, calculations, etc, using leaf as the main term, and then using hotchkiss as another main term. I really can't find much.

I got directed to a guy that last night TPInjeep from pirate board, and chatted with him for a while about it. All he relaly knew was that when he added a traction bar, he gained anti-squate, making the rear lift under hard accel.

I'm wondering about plain old leaves, without a traction bar, then I got him wondering about it, and he found this thread on pirate, that I must've missed.

link

PIG gives an equation for it up top, but Ed Stevens makes good points at the bottom that deem it to not be a very good equation.

Ed also says that the draggers raise the front main eye up to REDUCE AS%. This is confusing me because I just read in a book (chassis engineering by Herb adams[I think this is a horrible book BTW, I don't know if I can trust what he says]) that antisquat is RAISED by moving the eye up. Then it's said the lowering or raising the rear eye will do the same thing, but to a much lesser degree? Why is that? So to sum up this paragraph, what's the relationship between the eye heights, shackle length, antisquat, and roll center?

I also found this which explain it a little more.

link

Go to pages 10 and 11, 10 starts to explain it, page 11 has the picture representing it.

If you can't read it(too small), right click it and drag it off of the internet explorer window, adn release, it will bring it up in another window, and then on the bottom right of a picture a button will magically appear to enlarge the picture, then it's all clear to read.:) )

What is the instantaneous center? (point C in that pic) They explain that as 3/8 the length of the the front half of the spring coming FROM the axle centerline. Is that the imaginary pivot point for where the axle rotates around? Then from that, you can get the roll axis...I read in ed stevens post in the "tracbarless suspensions" post that each suspensiont front and rear has a seperate roll axis, but I've also read about a roll axis going from the front to the rear...which is which, and which is more important?....I'm getting confused here, I will stop and hopefully someone can explain this.

And yes, even if leaf springs suck, I want to know this because I don't see a point in a link suspension if I can learn how to fine tune a leaf suspension to do exactly what I want it to do. I want to know what kind of effects heights, angles, etc has on how a leaf suspension works...it's gotta have hidden complexities to it somwhere.

Thanks a lot _nicko_
 
Herbs adam's book in my npinxmn does@not go in mtch dctail at all, it seems like it's only touching the surface of many different aspectr raTmer than diggin into it. And thl ONE thing, just one t(ang I couldn't get in my head....it says to adjust camber in the rear axle, benä#the tubing....that just threw lo off a.c had no idea what to think.

I definitely read almost the whole book...I WILL look at it again, but there iò$only a"gut 1/2pk p`ge >l the hotchkiss type suspensions and how it works. Most is on linkr and,independent.
 
Yeah, as I said, the book is aimed at race car suspensions, and even when he wrote it they weren't building race cars with solid rear axles. Camber on a solid axle is supposed to be zero -- if it's not, you fix it either by ripping the axle tubes out of the center section and rewelding them (been there, done that), or by bending the tube(s) to get 'em straight.
 
Eagle said:
Camber on a solid axle is supposed to be zero -- if it's not, you fix it either by ripping the axle tubes out of the center section and rewelding them (been there, done that), or by bending the tube(s) to get 'em straight.

Oh, but not in the book, They are bending them to get negative camber at each wheel. They explain that if you make the axle tube red hot for about a 6" section and air cool it, it will give it a 1* change, and that axle splines can handle about 1* of mis alignment. Weird, I must say. But yeah, mostly every book made is aimed at racing suspensions.

Does anyone know if Milliken's racing dynamics has any info about leaf sprung suspensions? I may take a trip to the library and see if they have it. Thanks _nicko_
 
I would be intrested in reading up on leaf suspension designs also. Any other good book titles and authors out there?

gearwhine are you trying to avoid having to make a traction bar by relocating the spring eyes and redesigning the rear suspension? If so you are such a lazy bastard:D

AARON
 
MrShoeBoy said:
gearwhine are you trying to avoid having to make a traction bar by relocating the spring eyes and redesigning the rear suspension? If so you are such a lazy bastard:D

AARON

haha, screw you. I'm definitely adding a traction bar, but I just REALLY want to know the dynamics of these old fashioned leaf spring thingamajigs.
 
You and me both. Let me know if you find anything good.

AARON
 
gearwhine, I followed your link to POR and read the thread. Sounds to me like Ed was trying to politely tell Pig his formula is questionable. Anti-squat is a force ratio comparing the rearward pitch of the body to the suspension's ability to counter act the pitching motion. I'm having a hard time correlating Pig's formula to that.

On the other hand, I don't understand why Ed is saying that increasing the spring arch would decrease AS%. Generally, moving the instance center forward or down will decrease AS% and moving it up or rearward will increase AS%. I typicaly assume the forward spring eye is the instance center for judging leaf spring AS although the Chevrolet Power Book diagram indicates this isn't quite the case, but is still gives you a relative point to work with.

If you think the discussions are complex now, wait till these guys figure out that they have four wheel drive. Sooner or later they'll realize that front anti-lift + rear anti-squat = 100% of the anti budget. And then there's the matter of slope. We don't do our climbing on horizontal surfaces. And of course the question; what is the anti-squat/lift target. Do we want lots of anti-squat or not and why?
 
MaXJohnson said:
gearwhine, I followed your link to POR and read the thread. Sounds to me like Ed was trying to politely tell Pig his formula is questionable.

Exactly. Pulling an equation from a book without testing it is risky.

MaXJohnson said:
On the other hand, I don't understand why Ed is saying that increasing the spring arch would decrease AS%. Generally, moving the instance center forward or down will decrease AS% and moving it up or rearward will increase AS%. I typicaly assume the forward spring eye is the instance center for judging leaf spring AS although the Chevrolet Power Book diagram indicates this isn't quite the case, but is still gives you a relative point to work with.

If you think the discussions are complex now, wait till these guys figure out that they have four wheel drive. Sooner or later they'll realize that front anti-lift + rear anti-squat = 100% of the anti budget. And then there's the matter of slope. We don't do our climbing on horizontal surfaces. And of course the question; what is the anti-squat/lift target. Do we want lots of anti-squat or not and why? [/B]

The equation given by PIG works only if both spring eyes are raised the same height. It is also modeled on a spring-under road racer suspension (if it is from Millikan and Millikan).

MaxJohnson is correct about moving the IC. The CG's relation to the IC is however impacted by the spring eye height. Raise the spring eye, and the CG is raised, and with a high arch leaf the axle drives into the leaf travel rather than work like a control arm (in real world situation, because both spring eyes are not raised equally). Working with the leaf springs lack of rigidy changes the textbook equations. I simply do not trust the equation given.

Herb Adams? He told the road racing world that anti-sway bars were the way to faster road racing speeds, rather than run tall roll axis heights and/or stiff spring rates, and his designs won (or nearly won) every race entered to prove his ideas. Unfortunately his theories are based on having low roll axis heights, and off-road crawlers and racers do not always the luxury of choosing the roll axis height (something about long travel suspensions conflicting with ideal design theory). There have been a number of off-road race drivers and designers who do not hold Herb Adams theories as gospel for long travel suspensions (and Herb Adams was a paid consultant when $$$$$ were plentiful, although almost all agree with him for limited oval and road race suspensions).
 
Is there a good book about good long travel suspensions? It seems like all suspension related material is geared towards road racers and circle track guys.

AARON
 
gearwhine, I followed your link to POR and read the thread. Sounds to me like Ed was trying to politely tell Pig his formula is questionable.

Yeah, when I first read the thread, I was quite happy I found something, but as I read ed's post I noticed I didn't really make it anywhere....oh well. Keep working.

On the other hand, I don't understand why Ed is saying that increasing the spring arch would decrease AS%. Generally, moving the instance center forward or down will decrease AS% and moving it up or rearward will increase AS%.

OK, I just plotted two leaf springs, one high arc on low arc, with the eyes are in the same position. Using the chevy power manual, I found the IC to be higher with the high arc sring. Using what you say that moving the IC up, the AS will increase in this case.

Also, to try and prove Herb Adam's saying (rough quote) "moving the front leaf eye up will increase AS." I made two same arch leaf springs, same length, with one with a tilted up front eye(the eye is moved up and back a litte (the axle is also moved back a little as well, but not much) Using the chevy power manual diagrams, I did find a higher IC with the higher front eye. Which means more AS, assuming that your generalization is correct...(I am really not sure, don't know for sure how the IC, COG, and AS relate yet). I'm neglecting the COG has raised A BIT from the rear end pivoting up a bit around the front tire contact path, but it can't make that much of a difference can it? Can anyone tell me if I'm right with my drawings?

Now I just tried out PIG's given eq, although it doesn't take into account much information, I did get a higher AS for the higher arch springs....it's all the same numbers, except the spring length is longer(D). (knowing only one number changes by changing the length of springs, and keeping the eye parallel to stock heights, I don't like this eq. either.)


Raise the spring eye, and the CG is raised, and with a high arch leaf the axle drives into the leaf travel rather than work like a control arm (in real world situation, because both spring eyes are not raised equally). Working with the leaf springs lack of rigidy changes the textbook equations. I simply do not trust the equation given.

I'm not understanding this axle drive stuff(bold). I see how the CG raise, but to me that seems minimal. I don't see the other part of it at all. If the leaf arc is small, yes the spring is flatter, and won't move in the same path a high arc spring will when it compresses, but they are both still travelling in an arc around the IC(whereever it might be), but the radius of the arc is always changing (from what I can tell), rather than staying constant like a true control arm. But, why would a high arc spring cause it to not work the same as a low arch? Seems they both have a non constant radius as they travel.

I typicaly assume the forward spring eye is the instance center for judging leaf spring AS although the Chevrolet Power Book diagram indicates this isn't quite the case, but is still gives you a relative point to work with.

I can't really get into my head where the IC would be, but I really think that the chevy power manual is correct, I jsut drew two arcs, one from the IC I found above, and one from the spring eye, and just in my mind, it seems to travel more in that arc from that higher "other" IC.

OK, I'm done for now, I don't know if I made it anywhere, but at least I got some paper, compasses, triangles, rulers, and calculators out and got a better understnad of some stuff.

_nicko_
 
Back
Top