I posted this over on the 37's vs 40's thread, but since it is a precursor to my next post I've reposted it here in case anyone following has not seen it.
Maybe an easier way to understand it. If you look at the upper and lower balljoints, you will notice if you were to draw a line through the center of them, it inclines or leans inward at the top when viewed from the front. That is the steering axis inclination. If you then extend that line down to the surface that the tire sits on, it should roughly intersect that surface near the center of the contact patch so the patch rotates around that point when the tires are turned.
Because of the inclination, as the axle moves up due to a larger tire, the line's intersection point moves outward. That means as a "general" rule of thumb, as tire size increases, you can shift it outward with less back spacing to semi-retain the relationship.
One thing to be very aware of is there is nothing on a front axle steering wise that sucks much more than tires that have to roll to turn. I've seen several cases over the years where the steering wheel will lock up on pavement hard if you apply the brakes hard and or lock up the front and then try to turn.
As the tires are compelled to turn via input to the steering knuckles, one tire has to roll forward, the other has to roll backward. When the slack gets taken up in the splines etc., the system just locks up. If you have good brakes, that can be as little as a quarter turn of the steering wheel. The obvious should also be pointed out which is the leverage against the ball joints is now through the roof and however long they were expected to last with a good relationship is now compromised by more than a little.
Another thing to be aware of is the increased wear factor to just the steering bits. At highway speeds, any impact load the tire sees will be magnified greatly into the steering links and steering gear due to the increased leverage against the system. If there was ever a case for multiple steering dampers to help slow that down, that would do it. Even then, the tie rod ends, ball joints, knuckle tapers, etc. are going to suffer.
Click to expand...
OK, for once Blaine knows what he is talking about....
I'm kidding, of course. But here is a little real-world experience to demonstrate his point. There are many things wrong with my rig - and here is one of them.
When I built the rear 14 bolt a few years ago, I added 2" spacers to my front '60 to match the track width of the 14 bolt (Yes, 2" spacers).
(Blaine - please look away from the post at this point)
Mea Culpa - Here it is:
This of course destroys my scrub radius. My plan has always been to build a new front axle, or retube the pro-rock, to correct it.
As Blaine said, sometimes (maybe always, and I'm just used to it) when stopping it will pull just a bit to the passenger side, and I have to correct. I've never had a problem with the steering locking up as he described - but I run hydro assist which would hide that.
The other issue is that when I really get on it (which with the big Hemi, is saying something) and the front end comes up, it will also pull just a bit to the driver side, and I have to compensate. Then when you let off quickly and the front end comes back down, I have to compensate the other way. It's not horrible, but it's close.
I always thought that I had some kind of steering issue causing this, perhaps bump steer. But my geometry is good, and it does not bump steer over bumps, so I've been at a bit of a loss.
The light went off when I read Blaine's post, so this morning I pulled the spacers for a little science. Here is a before:
And the after:
I took it for a long drive with lots of stops, and lots of lighting it up (which never gets old...). And sure enough, no more crazy steering behavior on stopping or when lighting it up.
But, the fodeez rub without the spacers, so they had to go back on - and the steering issues returned. So I guess I need to reprioritize the new axle...
Before today, I thought my biggest issue with using the spacers was that they keep me from going to a 42, as in a full lock turn at full articulation the spacers would push a 42 into the rear of the wheel well (the 40's clear, but just). That would not be so on a 4" wider axle without spacers.
Thanks for that great post, Blaine.