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Designing upper and lower arms

sbro

Well-Known Member
After reading the post about extending stock toy arms, I started wondering what factors go into designing upper/lower arms. I have a 98 2wd Ranger, and was wondering what challenge would lay before me if I decided to attempt this. I was considering designing a chromoly lower arm that extended 4-5" and could mount a coilover. Where would I find specs/formulas to build an upper arm in proportion to the lower, and incorporate a 3" lift spindle? Also, would it be reccomended to keep stock geometry? I have a fab shop by my house that could build the arms, I would supply measurements/drawings/specs. I figured that half the work of custom a-arms was design, and doing that by myself would lower the cost considerably (then again, I would be liable for any faults in design). So I turn to the experts, any info would be appreciated.
 

FABRICATOR

Well-Known Member
sbro,
Welcome to the working world! You can only keep stock goemetry if you keep the stock spindles. For strength and reliability, most race trucks are built with fixed mounts on the lower arm and adjustable on the upper. If you keep the placement of the arms and ball joints "in the ball park" the available adjustment should accommodate the spindle. This approximate placement can be figured from what ever spindle you are going to use. You should use swivel rod ends to mount the upper arm and should be able to adjust the upper ball joint in a horizontal circle about 1/2" around where the upper spindle mount hole belongs.

When ever you increase travel you need to consider how much camber and caster change you want to end up with. A lot of buggies have no camber change built in (equal length arms). This is not good for handling on the street. If it's a daily driver, you will need camber change similar to stock. If the arms are mounted parallel to each other, caster can be adjusted in and left alone. If you want any anti-dive characteristics, the arms will not be mounted parallel to each other. Anti-dive is usually compromised quite a bit in long travel systems.

One of the biggest problems with increasing stock travel is that of bump steer. Careful design of a-arms will minimize bump steer. However, you often cannot minimize it enough without doing some steering modifications. Even the anti-dive built in to the a-arm mounts of some stock vehicles will affect bump steer.

If you don't want to figure this all out mathematically, you can use mock-up parts. This can save you tons of work, shed new light on the subject, and you will learn a lot.
 

sbro

Well-Known Member
FABRICATOR, thanks for all the info. If anyone has done this before (which I'm sure there are plenty of you), I would appreciate any tips/suggestions. I realize you might not want to give up any tricks of the trade, but any info would help a lot.
 

sbro

Well-Known Member
FABRICATOR, what camber/caster would be suitable for a daily driver? Should I just keep it as close to stock as possible? The arms would be using the stock mounting locations. Also, what techniques are used to minimize bump-steer in long travel systems? Once again, thanks for all the help.
 

FABRICATOR

Well-Known Member
sbro,
For handling on the street, keep it as close to stock as you can. Try to avoid positive camber as much as possible in all wheel positions. A little extra negative camber won't hurt anything and in fact is benificial. Original castor is good, however a slight decrease in caster is usually OK if you are running a larger diameter tire than stock. (here we go...scrub radius increases with increased tire diameter) As far as bump steer, it may come out OK from stock pivot points, you may have to experiment.

Increased wheel travel that includes increased angle to the control arms, also increases angle to the steering linkage. These increased angles add more upward and downward stress to these components and must be considered. This along with bigger than stock tires and wheels can be too much for stock steering components and design. You would be ahead by seeing what others are doing by going to one of the bigger races and checking out the trucks.

If the application is for occasional off-road pre-running, you may get by with stock or some modified components. If it involves any serious speed in the dezert, the entire steering system must be redesigned.
 

sbro

Well-Known Member
Thanks a lot fabricator. I am now considering buying a set of stock lower arms and lengthening them 5" at an angle. After cutting and extending, they would be reinforced and plated for strength. I would make a longer uniball upper to go with it. I drew out a diagram, but it won't show up in the post. What would be the pros/cons to extending/reinforcing stock lowers?
 

FABRICATOR

Well-Known Member
sbro,
There is already a discussion going on here regarding that subject. Some of these guys have done that very thing before. Just be careful and keep an eye on things, which means thorough inspections on a regular basis.

Unless you are copying something, well designed parts, whether using CAD or just experience, usually take much longer to design than to fabricate.
 
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