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Designing a 4-link (simply...?)


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I am working on figuring out how to 4-link my Gemini and I need some guidance around arm lengths and angles.

I'm basing it off the Escort Rally prep manual as the Gemini and Mk2 Escort share the same wheelbase. In this, they run 23" arms, mounted 2.5" from the axle centre, both parallel with the ground.

I am aware that the length and angles of the arms can have significant affect on anti-squat and roll steer. At the moment, I've designed it as seen below:

image.png.0cb77011bcb0242a852c4e7015e6d44c.png

620mm is the stock length of the Gemini lower trailing arm mounting point to the axle centre, 127mm is the same as the Escort setup.

However, I have read varying warnings about running the bar angles every which way. Both parallel; lower parallel, top angled down; bottom angled up, top angled down; etc. And then some people run different length bars, or the bars mounted forward or rearward of the axle centre. There is a lot to think about.

I've worked out that if I run the lower parallel and the top angled down 25mm at the front or 2.346°, I'll get a figure of 30% anti-squat and the IC will be about 500mm in front of the front wheel. I don't know exactly where the centre of gravity line runs in the car, but after playing around with a 4-link calculator I'm struggling to get the IC inside the car without some quite funky arm angles.

Bear in mind, this is not a drag car looking for that 1/10th of a second grip off the line, so I'm not overly concerned with some squat to get grip.

 

I guess what I'm asking is - and I know every car is different - what would you do? The Escort seems to be pretty well proven, although it is a rally car. Just allow my self enough adjustability to make any changes I might need to?

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Why the change from the ladder bar?

squat doesnt actually create grip, its actually reducing force between the tyre and the ground - body moving down is force NOT going into the ground, it depends on the car and the tyres whether thats preferable to having the car separate (body lift up). ideally neither happens, most of the time squat/separation is just an artifact of working within limitations of factory suspension geometry and chassis.

if the arms are the same length and parallel you get no pinion angle change, which is nice but not essential - very few factory cars are like that and they work fine. and probably with your car you arent going to have as much suspension travel as a rally spec escort where pinion angle change may be more of a factor.

having the bottom arm close to parallel is best as its the one that is pushing the car forward. I think you are probably on the right track as what you describe is pretty similar to most factory 4 links I have seen. you might find you need to make the top link shorter or with more angle down and drop the bottom arm a tiny bit at the diff to get the IC back to nearer the center of the car.

 

I know you arent trying to make a drag car but as a point of reference

typically "stock suspension" streetcar base stuff aims for IC 200mm-300mm off the ground  500mm to 1000mm in front of the rear axle centreline, or further infront  as power increases 1000hp+/1400+kg vehicles.

closer to the rear axle will hit the rear tyres harder more quickly (better for less power), but unload more quickly, longer reduces the leverage (less wheelies) and if you have ridiculous power/torque you can still get plenty of force on the tyres.

 

TLDR;

Shorter top arm is ok if needed, try to bring the IC back inside the wheelbase somewhere while keeping the bottom arm close to parallel. but dont agonise over it too much if you can keep some  adjustablity at one end or the other of top and bottom arms.

 

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Thanks Tom.

The 4 link was always something that I wanted to do. The ladder bar was just the easiest and most cost effective method to get the diff in at the time. Also, Clint came by for a chat about a few other cert things and mentioned that my current setup would not be certifiable these days. It's to do with all the forces trying to pull and push the single joint at the chassis up and down.

I've read that the best thing to do is to try get the IC to lie on the anti squat line. Unfortunately, getting an accurate line is difficult as measuring the CG height is not easy without a good industrial scale.

If I angle the top bar down, it's there an optimal height or should mount above the lower bar at the chassis end? Because the car is relatively short wheelbase, getting the IC inside the wheelbase would mean quite a steep downward angle. If the upper bar is left longer it would end up very close to the lower mounting point, bringing the movement closer back to a ladder bar, wouldn't it?

I'll have another play around with my 4 link calculator. Haven't managed to find a metric one yet though!

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whats the name of the calculator are you using?  I used one that runs in excel  when done the hilux.   was good.    

There will always  be a bit of a compromise  due to packaging and the likes.  so don't get too worried if you cant get it bang on as to what the calculator spits out.   if you look at some  factory stuff a lot of it isn't textbook. 

 

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On 19/03/2021 at 20:54, kpr said:

whats the name of the calculator are you using?  I used one that runs in excel  when done the hilux.   was good.    

There will always  be a bit of a compromise  due to packaging and the likes.  so don't get too worried if you cant get it bang on as to what the calculator spits out.   if you look at some  factory stuff a lot of it isn't textbook. 

 

It's just an excel one that I found somewhere.

Yeah, it seems like half the internet is saying, just put something together, it'll most probably work VS 1mm difference and there will be DRASTIC changes to the handling.

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The bane of my existence with a lowered 4 link setup on a standard Toyota one. 
Is that the 4 links at standard height are meant to slope down towards the diff. 
Because when they do it works like this 

dghjj.thumb.jpg.231a5cfbbc140e4f5e04d53bb25f0295.jpg

 

Then when you are hard on the brakes, the opposite happens. (which is also good)
The angle of the bars pulls the rear of the body back down, so works as anti dive of sorts.

However when you lower it enough that the bars are beyond parallel with the ground, so they now slope up towards the diff. 
When you launch hard off the line, it tries to pull the body of the car down instead, which because of intertia saying fuck you means it lifts the wheel up instead. 
So grip completely sucks. 
Then under braking, it jacks the rear of your car sky high because the pulling force tries to lift the body. 
I would set 4 link angles so that you're a fair margin away from having bars parallel with the ground at its static angle

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^ This     if you put the angles in like roman dave has,  with the bottom arms going up to the chassis  and the top arm flat.  you will notice things in the calculator gets better. but isn't always possible in a low car. hence why a lot run flat lower and angled top.   take note of the roll steer numbers also, because that is horrible if you get too much.    if your on 100% antisquat the car should launch with zero movement in the rear end.   less than 100% will start to squat.  id aim for 100% with adjustment either way.   if your running a  panhard placement will affect roll center also.  but again isnt end of the world as will likely run into packaging issues.  decent amount of arm separation diff end is good also, less likely to break stuff.       To me the "instant center" seems more like drag race lingo.    i'd be more worried about  things like roll steer and roll center and ms paint @Roman pictures  unless after the perfect 60ft.

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When I first got started with this, I measured how low the mount would need to be from the axle centre to make parallel lower arms that bolt directly into the stock chassis trailing arm mount - answer: it was MUCH lower than the 2.5" drop suggested in the Escort manual. More like 3.5-4" from the axle centre. I guess what I'm trying to say, is that I have plenty of room to slope down towards the diff if I'm remaking the mounts at both ends.

I will also be developing this with the car in it's already lowered state, so it should stay pretty much where it is.

Although I'm not sure why it's harder to run the lower bars up to the chassis, rather than parallel. Surely going up is better for clearance (assuming you making all the mounts in the locations you want).

I need to do some more reading up about roll steer, etc. This setup will have a panhard rod. In the end, this is a street car and I'm more interested in putting down power out of corners on the back roads than doing a 10.

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Roll steer is caused by the link arms shortening or lengthening the wheelbase on one side, kind of like how a skateboard works

This pic is exaggerated

 

(maybe not if you have a jeep or safari haha)

 

 

20210322_093356.jpg

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its the roll axis angle on that calculator.     I cant remember what was an acceptable angle now.    but can hit the bump and droop button and see how much it changes.   you will get away with  more since you wont be using a whole heap of travel.   my hilux goes up/ down just over a degree  for every inch of bump / droop. but has massive long arms setup for high travel 

also the 3 link version of that calculator  probably better to use, as has panhard in it too. 

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Ah right. And as the bars shorten it pulls that side of the diff in an arc towards the front of the car, effectively steering with the rear.

I think I need to figure out my travel next - can't imagine it's heaps though!

I'll check out the 3 link version.

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the IC is the location the force is acting on the chassis when you account for the push/pull of the top and bottom links - as you implied it is where the ladder bar would be attached to the chassis if it was a ladder bar.

travel is not too bad

for starters you know the rear weight within reason is probably around 200kg/corner +/- 50kg  droop can only be weight  x spring rate unless you have progressive or additional keeper springs  uptravel is probably limited by diff or driveshaft hitting the floor. Check the NZCC but I think its something like 50mm up and down minimum, and well you probably wouldnt want any less for curvy road enjoyment anyway.

 

3 link  vs 4 link? any opinions/experiences?  I have heard of racing guys removing a top link from a 4 link to help traction alledgedly. seems dubious to me and maybe just a barry story someone did this and the car was fast so that must be the thing to do........ and well you would need to make the remaining link stronger since you at least double the load on it........

I know in 4x4 circles 3link+panhard flexes better than 4link+ panhard but alot of that is really more about packaging (for solid front axles) than the difference really being material to performance.

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1 hour ago, Testament said:

3 link  vs 4 link? any opinions/experiences?  I have heard of racing guys removing a top link from a 4 link to help traction alledgedly. seems dubious...

I know in 4x4 circles 3link+panhard flexes better than 4link+ panhard but alot of that is really more about packaging (for solid front axles) than the difference really being material to performance.

Intuitively the 4x4 one seems like it might be to do with linkages binding under extreme roll angles.

3 link vs 4 link for a track car, I wonder if the 3 link arrangement means you can run a steeper upper arm (shorter IC) without packaging constraints because the diff mount is usually a little higher from axle center above the pumpkin?

Removing a link to help traction sounds like BS under normal circumstances. Chances are higher that it was a shitty 4 link setup that was binding under roll, removing a link might have solved the binding issue (probably creating other issues) and given more traction. Only makes sense to me if there is a lateral load factor as well. In a straight line, I think the geometries are 2 dimensional, so 3 vs 4 link shouldn't make a difference (I think...)

I could be talking out my arse here

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the closer to the axle centerline the higher the forces on the arms to resist axle torque, I think the distance between them is more just in terms of getting the angles for the geometry you want and fitting in the space available.

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3 link just for packaging,  I run a 3 link in the front of the hilux for that reason.    unless im missing something it seems  3/4 link will do just the same.   Its running a panhard with triangulation that will bind things up.  parallel links will allow for a little side to side movement, so can deal with the panhard arc.    in saying that  if you alter the horizontal separation a little (wider or narrower at diff end) in the calc it effects the roll steer numbers. so worth having a play with that too

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