Scimitar 4-link

[Lucke]

Hi all V8-mates!!

I'm restoring a Scimitar SE5/a GTE 1973. My goal is a street/circuit-track racing Scimitar with a Ford Mustang 5.0 V8 and Toyota big Supra Tranny (R154).

But first, I have to make the chassis in useable condition... I have already changed all the outriggers. Now I'm restoring the rear suspension mountings, I have to change the tube across the frame also.... And the 4-link brackets...

When making new 4-link brackets I'm planning to make the 4-link adjustable in the front end.

I have made an AutoCad drawing of the new bracket, will try to have them laser-cutted in 4 or 5mm steel.

At the begin I made a drawing of the origin bracket:

http://i124.photobucket.com/albums/p9/L ... 4-link.jpg

Here is a drawing for the new bracket.
http://i124.photobucket.com/albums/p9/L ... 4-link.jpg

In the drawings I've also added the connection points at the rear axle brackets and lines showing the fourlink bars angles.

The drawing for the old bracket is showing the angles with standard ride height. (measured without body)

In the drawing for the new bracket are the bars horizontal, mounted at the origin positions (holes with blue ring inside). This is the result when the coils is lowered 70mm.
At first I thought that the lower bar always should be horizontal, and the upper bars should be lowered for better rear tire load,
but that theory seems to be correct just for dragracing.

The circuit-track guys seems to have other demands...

Read more here:
http://www.stockcarracing.com/techartic ... ps_advice/

Or here:
http://www.afcoracing.com/tech_pages/4link.shtml


So I want your opinion...
What do you think about the drawing with the new bracket? Is there anything missing, wrong or excessive? Or just fine?!

[kiwicar]

Hi luke
looks good to me, you should be able to tune some anti squat in with those brackets, and take it all out if you want, (I would use a little for getting off the line and for good ballance comming out of corners). What are you planning to use for latteral location, panard rod, Whats linkage, A-frame link?
Mike

[Lucke]

Hi!

Thanks for your opinion!

I will keep the origin watts-linkage, maybe just adjust it for a lower ride height if necessary.

[Paul B]

The new bracket would be great if you were just going drag racing, but running anti squat suspension like that on a street car is not a good thing, as it binds when one wheel goes over a bump, which will either wear out your joints, or break something.

Picture it: the geometry of the unequal four bar rotates the axle slightly as the suspension raise, or lowers. When one wheel goes over a bump it rotates the axle, but the other end is not rotating, so something has to flex.

If you use rubber bushings at both ends you might get away with it, but rose joints will wear out very quickly. As the article states, they are talking about racing, which is a lot different from running a regular street car.

[Lucke]

The new bracket would be great if you were just going drag racing, but running anti squat suspension like that on a street car is not a good thing, as it binds when one wheel goes over a bump, which will either wear out your joints, or break something.

Thanks, also a good opinion!!

I know about this issue.The reason why I started to build this adjustable 4-link brackets was because of my work-mate.. He competes in dragracing. When he saw that I was manufacturing new brackets he told me to make possibilties to adjust the bars. Why? -"Maybe you want to race between the redlights someday!" Ok, why not...

But to solve this issue with non parallell bars I have placed the holes so all the holes for the lower bars can be combined with one of the upper bars and still keep the bars parallell.

The bracket will then be suitable for both street, circuit and drag, I hope..
Since the holes are placed in a radius formation you don't have to adjust pinion angle when changing setup. I will make one of the bars (upper or lower) adjustable to adjust the pinion angle.

I will use polybushes, not uniballs to the rear suspension, also to avoid damages caused by binds.

Thanks for your opinions!!! More opinions wanted!!

[Paul B]

The new bracket would be great if you were just going drag racing, but running anti squat suspension like that on a street car is not a good thing, as it binds when one wheel goes over a bump, which will either wear out your joints, or break something.

Thanks, also a good opinion!!

I know about this issue.The reason why I started to build this adjustable 4-link brackets was because of my work-mate.. He competes in dragracing. When he saw that I was manufacturing new brackets he told me to make possibilties to adjust the bars. Why? -"Maybe you want to race between the redlights someday!" Ok, why not...

But to solve this issue with non parallell bars I have placed the holes so all the holes for the lower bars can be combined with one of the upper bars and still keep the bars parallell.

The bracket will then be suitable for both street, circuit and drag, I hope..
Since the holes are placed in a radius formation you don't have to adjust pinion angle when changing setup. I will make one of the bars (upper or lower) adjustable to adjust the pinion angle.

I will use polybushes, not uniballs to the rear suspension, also to avoid damages caused by binds.

Thanks for your opinions!!! More opinions wanted!!

You could always use what I think is known as a 'triangulated four link', where the top two bars are angled so they come together at the rear end. They are usually about 12 inches (300mm) apart where they attach to the axle, and normal width at the front, so they sit about 45 degrees to the chasis. This removes almost all the binding and also means you don't need a panhard rod or any lateral location device. It also means you can have it set up bind free but with 'drag racing'geometry. Here's an example:

[kiwicar]

Anti squat geomitry does not cause binding on a properly set up 4 link, if it did it would not prevent squating, the rotation of the axel and non parrellel innks create a torque at the instant center toward the front of the car causing the rear to lift. Watch a well set up drag car (with suspension, not a rail) it will lift the rear a few inches (in adition to any increase in diamiter of the drag tyres!). The problem with anti squat on a road car is that when you hit the breaks the reverse happens and the rear axel tries to lift, this will reduce rear stability and traction when you are on the breaks, not what you want alot of as you hit an the apex of a corner I think those brackets will give you the adjustability to put in as much anti squat as you want awith the watts linkage it should work well (I would usr soft ish bushes in the watts linkage though, or rose joint the ends so the watts linkage does not bind on full bump.
Mike

[katanaman]

I think the triangulated 4 link would be a step backwards from the original Scim setup to be honest. The original setup works very well on the street and track. I dont know anyone who drags a Scim so cant comment on how good or bad it works for that.

[Lucke]

Anti squat geomitry does not cause binding on a properly set up 4 link

Picture it: the geometry of the unequal four bar rotates the axle slightly as the suspension raise, or lowers. When one wheel goes over a bump it rotates the axle, but the other end is not rotating, so something has to flex.

I think you are both right... Correct me if I'm wrong, but the most important thing to avoid bindings in a 4-link may be to have the same length on both the upper and the lower bars. Which the Scimitar almost already have.
I'm thinking so my brain is boiling but I can't figure out if the angle of the upper bar can cause bindings if it's angled for antisquat when it's still equal in length to the lower bars.. If think it will, but it feels like the length is more important than the angles to avoid bind..

Anyway, this will be easy to check whit the shockers unmounted, just raise one side of the axle and figure out if it binds...

Many cars with 4-link suspension as standard doesn't have equal length on the bars. I have a Mazda RX7 with four link, the upper bars are a lot shorter than the lower bars on this car. I think the only reason is that they wanted more space for the (minimal) rear seat. Can't figure out any other good reason to have shorter upper bars. Presumably for that reason they have mounted huge rubber bushes on this cars..

[Paul B]

Anti squat geomitry does not cause binding on a properly set up 4 link.......
Mike

Not when the car is riding on flat road, but how can you have the axle rotating at one end and not at the other, which is what happens if you raise one end and not the other. The very essence of the system is that it turns the axle as it moves up or down, so when one end hits a bump, it moves, and rotates, but the other end doesn't want to turn, so something has to flex or break. Just like ladder bars.

[kiwicar]

OK Paul,
your basic theory is correct in that you can get one end of the axel to rotate against the other(and in theory you are also right for a cct car in roll this would have the same situation as a road car in that it is subject to roll), however if you draw it out and assume 50 inches between mountings on the axel and that the whole set up is rose jointed (which you would be mad to use on a road car if you valued your fillings!), 28 inch lower links and 24 inch upper links, an instant center of say 150 inches in front of the axel (not unreasonable for 25% anti squat rising to 35% at 1g bump) at a hight of 20 inches putting the c of g about 60 inches in front of the axel then at 1 g of weight transfer across the axels (ie. it is on the point of rolling over) you are looking at about .8 degrees of twist in the axel with 10 inches between the mounting points on the axels you are looking at .14 of an inch of difference split between four mounting points on the axel and the same on the chassis, further the axel is made of three parts and is in effect a 50 inch long 3 inch diamiter tube.... it will act as a spring! (rather a soft one), further it is going onto a ladder chassis on a scimiter If that flexed as little as .14 of an inch under a 1g load across the axel it would be far stiffer than the design has any right to be.
As I also said I would not rose joint a 4link axel to a chassis on a road car, put just one complient bush in each side and the problem is solved. The reality is that this set up was used for years on rally escorts (some sprung on leaf springs not coils) and with quite alot of anti squat to get them off the line on gravel, and even here with properly stiff chassis it worked very well and without binding. Think also of the twisting force acting on the axel from the reaction at the diff of firat gear on sticky tyres and 300ftlb of torque from the engine going through the same brackets....
yes Paul on a theoretically perfectly stiff chassis and axel tube you might see a little bind (probably experienced as an anti roll moment) but on a real car no way.
The real limitation of ladder bars is the fixed instant center which is generally less than half the wheel base of the vehicle (less than 40 inches), giving far more twist across the mountings, in the case of a 4 link you can use a decent length of instant center, 150 is not unreasonable. and the problem is reduced.
best regards
Mike

[Paul B]

OK Paul,
your basic theory is correct in that you can get one end of the axel to rotate against the other(and in theory you are also right for a cct car in roll this would have the same situation as a road car in that it is subject to roll), however if you draw it out and assume 50 inches between mountings on the axel and that the whole set up is rose jointed (which you would be mad to use on a road car if you valued your fillings!),

Which brings us neatly back to my first post :

If you use rubber bushings at both ends you might get away with it, but rose joints will wear out very quickly.....

I agree, normal road use wouldn't be much of a problem, but the occasional driving or parking with a wheel up on the kerb, or jacking up one corner of the car for some reason could/would put a hell of a lot of stress on certain bracketry. Rather to simply get the geometry correct I'd say. Or use a three link system?

[kiwicar]

Hi Paul
I think the reallity is that the experience of the rally escorts shows what a four link system can do if set up right. A three link set up is ok for a range rover but does nor hold the for aft location as well as 4 links and still relys on twisting the axel, and in virtually all set ups breaking loads have to be fed into the chassis by using radius arns at either end of the axel, as the mounting on the diff case is not up to feeding these forces into the chassis. curbing a wheel as you deascribe only puts a maximum of 1/2g loading so is even lowe than the situation I described earlier. I don't think we will agree on this.
Mike