Okay, there are a number of different variations of the G Mod. Some are more aggressive and require more clearancing and modification some are less so and easier to do. The former generally cut the whole perch off,trim it and reweld it. The later simple move the mounting holes. On average they drop the cross shaft about 3/4" and back about 1/4". That`s enough to take a big step in the right direction but not enough to make the geometry "good" by todays standards.
The overall level of improvement goes something like this:
Howe/SC&C tall UBJ, G Mod, Howe/SC&C X-tall UBJ, Howe/SC&C tall ubj & G Mod.
Going beyong this to Howe/SC&C X-tall & G mod would go past the point of diminishing returns (as David pointed out) and aspects of the geometry such as the FVSA length would start going down hill rapidly. Bear in mind this is still better than some of the tall iron spindles on the market though.
Here`s the catch though. When you change one things,other things have to follow. Doing the G Mod moves the upper A arms closer to the top of the subframe reducing droop travel. Lowering the car reduced bump travel. So a lowered car with the G mod looses travel in both directions. Not a big deal on a race car with uber stiff springs but not ideal on a street car. The taller ball joints,or taller spindles or simply lowering with springs moves the upper A arms upwards and also outwards in their arcs due to the way they droop over the frame. This adds + camber. Since it`s about impossible to get a decent performance alignment with pure stock suspension this just makes things that much harder. Offset cross shafts will give you about 3/8" of extra adjustment. That`s probably not enough to make up for lowering, tall ball joints or a performance alignment let alone all 3. You need shorter arms and while you`re at it they should have the ball joints offset more toward the firewall to increase + caster. Lastly the stock ball joint mounting angle in the UCAs is optimized for stock geometry and ride height (ball joint stud angle roughly 0 at ride height and the same as the ball joint axis inclination ). Lowering (with springs or coil overs) combined with taller ball joints or spindles or some variations of the G mod with stock upper arms can put the upper ball joints dangerously close to binding in bump. Continual binding can lead to eventual component failure (usually the end of the arm) and that`s almost always bad so it`s best to avoid that. That means new upper A arms or a lot of time intensive custom fab work. The correct A arms will permit optimizing the geometry,travel and alignment at the same time. Short answer tall ball joints,G mod or tall spindles can make big gains in geometry but if you`ve done enough to really make a difference it`s usually necessary to change the upper A arms to get the most out of that geometry and do it safely. Mark SC&C
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