Originally Posted by coronan
Some long reads but great links. Thanks E!
The RCtec atricles helped me alot.
Are there some guidelines to stay within for dual sport 4x4?
I've been trying to glean some numbers others are using.
Camber is desired at full droop and hard cornering
Increased caster prevents oversteer at full bump (range ?)
Hub offset 2cm (to reduce kick back into the steering) Neg or positive?
Caster 5 deg to help self centering
I plan on using 5 heim joints on each side to allow full adjustment and tuning.
I need to measure stock geometry for reference and a guide line.
I've noitced the new Tacomas use a sloped axis on the upper a arm. What's the advantage of that? Increased positive caster at bump = stability?
A few thoughts:
Caster will help reduce understeer, but you have to have a fairly large hub offset to make it work. If you're (rightly) trying to minimize hub offset/scrub radius to minimize kickback, you're going to minimize the weight jacking that happens with caster.
By "rake", I assume you're talking about the slope of the inner pivots in side view. The sloped axis you're seeing on the Tacoma does some of the same things (anti-dive), but you're right that it increases caster with bump. This would increase stability at full bump, but you're also adding to steering effort when it is already increased due to the load on the tires.
With a long travel suspension, I would put some priority on the camber curves, like you're talking about. Basically the point is that you don't want to loose all your camber in full droop, and don't want it to gain wildly at full bump. Long arms help a bunch in this department. You're going to find that they need to be pretty close to parallel, with the upper a little shorter than the lower. The only issue this causes is it limits what you can do with the instant centers, but I would place a higher priority on camber and toe curves for this application.
My favorite reference on the subject is Carroll Smith's "Tune to Win", even though it is close to 40 years old. There are a bunch of drawings showing what different combinations of arm lengths and inclinations does, and he even includes templates to cut out "paper dolls" to help visualize your geometry. This is really helpful, even though there are a number of geometry software packages out there.
I don't know what configuration you're planning on using your heim joints in, but I would caution you against using them for the outer ball joints, especially on the lower arm. This puts the rod end in bending, and it's much weaker than a proper ball joint or spherical bearing.
My two cents...
On edit: In terms of hub offset/scrub radius, you will likely end up with positive hub offset, just due to packaging. But, you may want to think about trying to get negative scrub radius on a driven, steering suspension. This is often done on FWD cars because it makes the car more stable in situations where one wheel has more traction than the other. If one wheel is driving the car around because it has more grip, the negative scrub radius causes that drive force to try to steer the front end out of the corner.