TWDJ

While doing my home-brew LCoG research, a topic came up periodically. Some folks talked about putting the front axle back into the center of the wheel well after installing a spring lift but not how they arrived at what it took. I thought I might take that on to satisfy my curiosity and use my project as an example. I want to find a way to determine the UCA and LCA measurements that would center the front axle, vertically, in the wheel well.

Now, I made a few assumptions:

1 - The control arms are designed to be parallel to each other (at rest) in the stock condition. It appeared so while I was laying on my back and looking up at my torpedo level while an oil drip from that d**n main seal dropped on my glasses.
2 - I'm only working in the vertical and horizontal planes (no z-axis). There is a z-axis, to some degree, but I chose to ignore it (even us math guys have our limits).
3 - The stock control arms are level (horizontal), the lift will move that arm down (vertical) by the height of the spring. This will give us a right triangle to start with.
4 - People are actually interested in seeing and commenting on intellectual exercises like this.

WARNING --- GEOMETRY ALERT!
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A spring lift will rotates the UCA down and back so new UCA would need to be longer than stock LCA. I'm going to be installing a 3" lift so here's what I came up with ...

UCA: Stock length is 15" and I'm installing a 3" spring lift. The new length of the UCA can be calculated by summing the squares of the 2 short sides and taking the square root of the result. In this case ... 15**2 + 3**2 = 234; -/234 = 15.297 or 15.3".

In this case, an UCA of 15.3" would put the top of the axle in the same location, vertically, as before the lift.

LCA: Stock length is 15.75" with the same 3" lift. Using the same equation ... (15.75**2) + 3**2 = 257.06; -/257.06 = 16.03".

A LCA of 16.0" would put the bottom of the axle in the same location, vertically, as before the lift.

So with my 3" lift an UCA length of 15.3" and a LCA length of 16.0", just over 1/4" each from stock, will put the axle back in the middle of the wheel well vertically.

Keep in mind that now the bottom of the spring would no longer be flexed to the rear but directly beneath the top of the spring. Also, in theory, the castor would change little to none as the axle mounting points for the control arms would have been moved straight down. Reality is obviously different than theory as measurements and threads are not exact to 3 decimal places.

Just looking for some comments so my HS geometry teacher knows I was paying attention.

Irongrave

yep, there is a chart around here or naxja that has it all broken out with what you need lengths to be and such

TWDJ

Irongrave, are you speaking of the chart for caster adjustment after adding a spring lift? I saw that many different places ... but that is not what I'm dealing with here. I'm looking as centering the axle within the wheel well.

If you know of a chart for centering the axle then that's what I've been looking for!

Irongrave

i saw it some where about what length the control arms need to be. was going to use it when i started to build my own.

edit: found one of them
http://www.lunghd.com/Tech_Articles/..._Arm_Chart.htm

Screwd up 6

i set mine to what RC said and it sits slightly towards the firewall more. Maybe an inch closer tops

TWDJ

Screwed up 6: That's why I'm trying to determine control arm lengths so that after a lift, the axle is not closer to the firewall but centered in the wheel well.

Irongrave

you can get away with stock arm on a 3in lift as there is some adjustment in the bracket beyond that need longer or adjustable arms

Screwd up 6

I went with their recommended lengths on instructions. if you can find out the lengths for the 4.5 lift before next week please let me know because im gonna get it alighned and would love to have it set right.

Irongrave

its in the link i posted

no rdplz

With a 3" lift you need new LCA's or UCA's. The stock ones will work fine. Go higher though, and then you need to look at longer/adjustable arms.

TWDJ

Hey folks, thanks for all the replies. Is this the chart you are referring to?

I've seen this a number of different places. This is used to set caster angle after doing a front end lift. What I'm talking about is centering the axle after the lift as the lift will tend to rotate the axle down and back.

TWDJ

Another forum member sent me the following chart with both decimal and fractions. In his explanation he stated the LCA was set for wheelbase and measured then the UCA was set for caster ... exactly what I was looking for!!

Thanks so much for everyone's input. The only thing left to do now is start a build page.