What's a good starting point for camber/caster setting?
I have a 72 Duster running 235/40/18 tires in the front. Manual steering. The car likes to "wander" at bit at higher speeds. I have aftermarket tublar upper control arms so adjusting the camber "should" be easier. What's a good number to start with?
Thanks!
-.5° camber, as much + caster as you can get and 1/16" toe if all your suspension components are relatively new, otherwise between 1/16" and 1/8" is fine.
With tubular, non adjustable UCA's you should be able to get close to +5° of caster, do it and don't worry about it. Your 235/40/18's aren't super wide so it should be fine. I run 275/35/18's with 16:1 manual steering and +6.5° of caster and I do daily drive my car. Above 10-15mph it's no big deal at all. Below that requires some more effort, but unless you're going to be parallel parking it all the time it's not a big deal. But most cars will need adjustable UCA's to get that much caster, the non-adjustable tubular UCA's are usually good for somewhere between +4° to +5° depending on the manufacturer and the cars particular set up.
AJ, since increasing caster increases the stiffness in steering (largely overcome by power steering); is there an issue with running 3.5 caster and manual steering? We want high speed stability, but is there a downside?
+3.5° caster isn't that much, as I said above, I run +6.5° on my car with 16:1 manual steering. Most modern cars run upwards of +8° with unequal A-arm set ups.
There are trade offs. Increasing the amount of positive caster absolutely improves stability, especially at higher speeds. The downside is that the more stable you make something the more it resists change, in this case the steering effort can go up.
Positive caster does a few different things. By leaning the steering knuckle back it projects the steering axis forward, which creates stability. Same as fork angle on a bicycle. When you turn the car, it puts more camber on the wheels as you turn. This is good for handling (up to a point). The tipping of the wheels also creates a re-centering force, so, the car wants to straighten out. This helps the cars stability on corner exits, and even in daily driving with manual steering the return to center effect makes the car easier to handle.
The downside is that the more caster you add, the more the wheels tip on steering inputs. This can cause a jacking effect, tipping the wheel onto its corner more and actually raising the car. It tends to add pressure to the outside rear wheel, which when done right can add to the vehicle turn in and help the car rotate a little. When overdone it makes the car oversteer, and the jacking effect can actually unsettle the suspension if you go too far. And the more the tires camber, the more the car lifts, and all of that force translates into more steering effort.
I've run everything from +3.5° caster all the way up to +8° on my Duster with 275/35/18's up front. At +3.5° the wide front tires go where THEY want to go, following ruts and things in the road because of their wide contact patch. At +8° the steering effort is dramatically heightened, and the jacking effect is very substantial. On my car at least I found that +6° to +6.5° was the "sweet spot", below that the wide front tires start to take over, and above that the steering effort really starts to climb rapidly. I actually didn't notice much steering effort change from around +5° up to +6.5°, but the stability is much better.
With narrower front tires you likely would get more of the benefits at a lower + caster number, as you would have less of a tracking effect from the wide tires.