Hot Rod Forum banner

4link Adjustment For No Squat Or Rise

6K views 11 replies 5 participants last post by  WICKEDFAB 
#1 ·
I started a thread "Instant Center Location of 4link," in which I described how you could locate the instant center and determine whether the car would rise or squat. I was hesitant to take it any further as I didn't know if there'd be any interest. Though there's been only a couple of comments, I see quite a few of you have been peeking so I thought I might as well add a bit more.

The following is the necessary slope of the upper links to achieve no squat or rise:

(h(S(L)X(d)+Y(a)-Y(d))-LS(L)Y(a))/(L(S(L)X(d)-Y(d))+X(a)(h-LS(L)))

The same signs and definitions apply. Be careful not to confuse "L" with "S(L)." The wheelbase, "L," appears once in the numerator and twice in the denominator and, in each case, it immediately precedes the lower slope, "S(L)."

Now, don't you wish you'd paid more attention in high school algebra? After I retired, I actually taught high school algebra at a Christian high school. I thought I'd be able to make it interesting...for the guys, at least...with problems related to cars. I think I stumbled across the only group of teens in the country who couldn't care less! Quite a disappointment!
 
See less See more
#2 ·
all that means nothing if you do not know the true cg aka center of gravity in the car. what ya do is take a guess on the cg. make the uppers adjustabe up and down on the frame side. then you can dial it in a tad. if you like this type of thing add more adjustments on the lower arms..more to mess with, imho the cool part about building ish.

if its a tri 4 link ya gatta watch out for the ra aka roll axis and rc aka roll center moving in the adjustments made.

best would be a three link with a trac bar. you could adjust the ic,as,ra,rc, separatly


:D
 
#3 ·
WICKEDFAB said:
all that means nothing if you do not know the true cg aka center of gravity in the car. what ya do is take a guess on the cg. make the uppers adjustabe up and down on the frame side. then you can dial it in a tad. if you like this type of thing add more adjustments on the lower arms..more to mess with, imho the cool part about building ish.
Yes, you've hit on the one "fuzzy" parameter in the equations, but, fortunately, the adjustments are not as critical as you might think. I stuck some "reasonable" numbers into the equations with the assumption that the CG height was actually at 25 inches and "guesses" ranged from 21 to 29 inches. (I'm certain you'd agree that, with any effort at all, we should get closer than that.) The calculated angles for the upper link covered a range of approximately plus or minus 10 degrees from the "correct" value! But, wait a minute! (I suspect you're an engineering student, so the following will not be lost on you.) If you draw a line from the rear tire patch through either instant center extreme, that line will deviate from the no squat/no rise line by less than 5 thousandths of a degree! In other words, you wouldn't be able to detect squat or rise with either extreme of adjustment.

But, I know that a lot of hotrodders like to have something to tinker with and adjust, so add the adjustment if you like. Personally, I'd rather relax.

WICKEDFAB said:
if its a tri 4 link ya gatta watch out for the ra aka roll axis and rc aka roll center moving in the adjustments made.
Yes, this is something to consider, but I can't imagine any problem arising. If the roll axis was drastically raised in the rear, this would cause the average car to become loose, but, again, I can't imagine that much of a change occurring. I'm more concerned with the popular practice of adding "super-duty" rear sway bars and removing the front sway bars. Yet, even these changes are usually acceptable for a car that is seldom driven on the street.

I'm surprised that noone has mentioned "roll oversteer." This is a term that exists only because the SAE insists on using steering wheel angle in their definition of oversteer/understeer. A NASCAR fabricator knows that you can design all the roll oversteer you like into a tight car and it will still hit the outside fence nose first! In other words, roll understeer/oversteer affects only the driver's perception of the handling.



WICKEDFAB said:
best would be a three link with a trac bar. you could adjust the ic,as,ra,rc, separatly
:D
Well, I'm glad there's at least one other person out there, besides myself and the Jaguar engineers, who recognize the merits of an asymmetric 3link. It's really the only way to go, giving the opportunity for no squat or rise AND equal rear tire loading. With modern slicks, packaging requirements usually dictate that the "odd" link be on the bottom and not on the top (as with the Jaguar C-Type and the Ramchargers' C/A). A significant problem is to fabricate those 3 links with sufficient strength to sustain the loads. In the case of the single lower link, it should be able to support the weight of about half a dozen cars along its long axis. That means it's not going to look like those soda straws they commonly use in the 4links.

Thanks for your comments.
 
#5 ·
In case anyone missed it this is the post I made in the other thread:

--------------------------------------------------------------
Check out this link:
http://www.pirate4x4.com/forum/showthread.php?t=204893&highlight=link

I started thinking the same thing so I started screwing around with Excel. I did it for triangulated 4-links on 4x4's...but for just finding the IC and antisquat it should work fine with any link suspension (as long as there in an intersection to find).

Anyway it will draw your suspension, links, IC; give you some #'s and %'s...but best of all it is 100% free!

http://home.earthlink.net/~triaged/Files/4LinkCalculatorV2.0b.zip

I am in the process of working on getting out some bugs (errors when the links are parallel) and am also almost done with a 3-link/Panhard version.

Because of you Bill I will be working on how to get the rear wheel load into the 3-link version!

This is the BETA version of the 3-link.
http://home.earthlink.net/~triaged/Files/3_LinkV1.0a.zip

This is just some other junk I've messed with
http://home.earthlink.net/~triaged/Files/
 
#6 · (Edited)
johnsongrass1 said:
Have any thought's about running springs in front or behind the axle in a two/three link? What about location of cross wieghts as they protian to left rear traction in circle tracking?
How did a roundy-rounder find his way to this board?

Seriously, if your sanctioning group allows it, a 3link with Panhard is, I believe, the best setup for a typical Saturday night car. NASCAR doesn't allow it on the Cup cars, of course. I've talked to guys who are running the odd upper link on the left side with lateral adjustment. This lateral adjustment gives essentially the same as a change in stagger.

Doesn't make a nickel's worth of difference whether your springs are ahead of, or behind, the axle. But, if they're ahead, the rate must be a lot higher than if they're behind. So, wherever they're mounted, keep 'em there and, if necessary, change the rate to correct problems.

Don't understand your question about crossweight. The crossweight is the sum of RF and LR divided by the total weight. Don't understand where the "location" comes in. Are you talking about ballast weight?

Thanks for the change. I enjoy talking roundy-round!

Triaged said:
In case anyone missed it this is the post I made in the other thread:

--------------------------------------------------------------
Check out this link:
http://www.pirate4x4.com/forum/showthread.php?t=204893&highlight=link

I started thinking the same thing so I started screwing around with Excel. I did it for triangulated 4-links on 4x4's...but for just finding the IC and antisquat it should work fine with any link suspension (as long as there in an intersection to find).

Anyway it will draw your suspension, links, IC; give you some #'s and %'s...but best of all it is 100% free!

http://home.earthlink.net/~triaged/Files/4LinkCalculatorV2.0b.zip

I am in the process of working on getting out some bugs (errors when the links are parallel) and am also almost done with a 3-link/Panhard version.

Because of you Bill I will be working on how to get the rear wheel load into the 3-link version!

This is the BETA version of the 3-link.
http://home.earthlink.net/~triaged/Files/3_LinkV1.0a.zip

This is just some other junk I've messed with
http://home.earthlink.net/~triaged/Files/
I have a BASIC program with the equations I've covered in these 2 threads for hotrodders.com. My work on the 3link is summarized in Race Car Vehicle Dynamics; Problems, Answers, and Experiments where there is, at the bottom of page 204, a setup equation which provides equal rear tire loading with no squat or rise. (This is a companion workbook for RCVD and is available through the SAE.) It's also mentioned in another SAE book, Chassis Design, but I don't think any equations are provided. Again, I've written some simple BASIC programs to crank through the equations, but I haven't attempted to provide any graphics. As Pa Kettle would put it, "One of these days," I'll switch them to EXCEL and pretty them up.
 
#7 ·
Well I tell ya, I don't do chassis, I do engines. so I won't even begin to get to complicated. I meant ballist location with crossweight percentages. I find it interesting that you would chose a three link over a four bar car or swingarm. The fastest guy's out there run four bars or four left and Z link right as well as spring behind. As this does make a difference IMHO. Spring behind will compress the spring under acceleration when the axle wraps up. I've have a few short clips of a four bar car at work I can send ya if you are interested in seeing them.
 
#8 ·
johnsongrass1 said:
Well I tell ya, I don't do chassis, I do engines. so I won't even begin to get to complicated. I meant ballist location with crossweight percentages. I find it interesting that you would chose a three link over a four bar car or swingarm. The fastest guy's out there run four bars or four left and Z link right as well as spring behind. As this does make a difference IMHO. Spring behind will compress the spring under acceleration when the axle wraps up. I've have a few short clips of a four bar car at work I can send ya if you are interested in seeing them.
Oh, I'm very familiar with that which you're describing. I don't really concern myself with "what's winning," though, for I've found that, with enough effort, you can turn almost anything into a winner. What's important to remember is that handling all comes down to the loading of those four little tire patches and they really couldn't care less about who won last Saturday night (or, for that matter, where a spring is located). So, since the loading is simple, why not keep the suspension simple? Some call this the KISS principle and I strongly believe in it. In the case of a rear suspension on a beam axle car, the simplest setup which does the job is a trailing 3link with Panhard. I could take that winning car with the fancy setup, take some measurements to determine wheel rates, roll stiffness, etc. and provide the same handling characteristics with the much simpler 3link and Panhard. And the tires wouldn't know the difference. Yeah, I know I sound like a blowhard, but I'm that confident in what I know and have experienced over the years.
 
#9 ·
Triaged, I did put the 3link on EXCEL. The "solver" feature on EXCEL chews through the equations very quickly, but, of course, does not give a general solution. (This is for equal tire loading and no squat or rise. Gives the load in each link.) The ability to quickly change parameter values and check the results is a big advantage over the BASIC program, though.

Now, if I was only young enough to build another car!
 
#11 ·
i figured i'd ask this here rather than starting a new topic.....

what are your thoughts in regards to the upper bars in a triangulated four link - running diagonally in towards the centre or diaogonally out towards the chassis

also how in reference to the diff how much of an angle should the bars be.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top