Drop Spindle Discussion

[Echo Leader]

"Spring rates"? Or "wheel rate"? I've always seen T-Bar rates expressed as "wheel" due to the lever arm contribution to the equation, but most coils are "spring". Comparing these two items without the motion ratio will negate any meaningful relationship between one of your setups and the other.

So what is the motion ratio on the Milano (same for GTV6 I'm assuming?)? If it's ration low, a 4X increase in RSR coil spring rate might only be 2.?X in wheel rate...

[genericwood]

Quote:

Originally Posted by Grant Even with the dropped spindles and huge sway bars and torsion bars, I was still rolling like a pig on tight turns. While the camber gain was fine, the center of gravity would shift so far onto the outside wheels, it would overload them and I would lose grip prematurely. After increasing the spring rates 2-3X over my old setup ( and 4-6X over stock!) I was finally happy.

[Grant]

What you're saying is correct, but according to Carol Smith, the roll related to a soft car moves the CG over the outside wheels, adding another factor to the "roll does not cause weight transfer" saying. It doesn't, but when the CG moves, more weight does move over to the outside wheels because of the shifting of the CG, and not just the cornering force.

I respect your experience racing over my experience screwing around on the internet and doing minor events. So, I'll double check my point for accuracy in my Tune to Win book later tonight when I have more time.

[Grant]

James, as usual, your points are valid. I do mean spring rate. The Shankle catalog lists the torsion bars as a spring rate to the best of my knowledge. How they actually calculated this, I do not know. I do know the listed rate for my shorter 27mm torsion bars was something to the effect of 190lbs/inch. The spring rate of the RS kit with my 11KG (actual rates are quoted as N/m or N/mm, I forget...no matter, conversion is multiply by .98, so close enough to estimate) front spring is around 650lb/inch. I'm using the amatauer approach of comparing spring rates to spring rates, which is really like comparing apples to oranges, but hey, they're both fruit right?

[genericwood]

Quote:

Originally Posted by Grant What you're saying is correct, but according to Carol Smith, the roll related to a soft car moves the CG over the outside wheels, adding another factor to the "roll does not cause weight transfer" saying..

[Echo Leader]

Good catch on the weight transfer issue, Erik. I skimmed right over that. I think I agree with you both on this one. It seems as though it might be as case where Smith himself might say "It happens, but it almost doesn't matter". I want to go look up what he actually says now...

Anyhow, Grant, I'm surprised (a bit) that Shankle was quoting a spring rate for a torsion bar. I phrase it that way, as I would think Shankle knew what they were doing, but who knows if they wanted to confuse their general readership by introducing the concept of wheel rate vs. the far more commonly discussed spring rate.

As confirmation, my calculations for a 27.3mm torsion bar on my GTV6 show a wheel rate of 191.85lbs/in. Now it seems as though the motion ratio for the GTV6 (and the Milano, I suppose) is around .49...That means that your 650lbs/in spring rate RSR setup is really a 317.75lbs/in wheel rate. This would be essentially equal to a 31mm torsion bar. More fun comparisons, I think the OEM torsions on the GTV6 (short ones) were 23.5mm? If my math is all correct, your current setup is almost exactly 3x this in wheel rate.

[Grant]

James, my coilovers don't mount exactly in the stock location. The wheel rate should be higher than you calculated.

From post #122:

116 & 119 Alfetta/GTV 6/Milano Torsion bar specifications:

GTV6 Early model:
stock bar:
Rate -- 96lbs/in Diameter -- 23.5mm Length--989mm
Sport Bar:
Rate -- 134lb/in Diameter -- 25.4mm Length -- 989mm
SS Bar:
Rate -- 179lb/in Diameter -- 27.3mm " " -- 989mm

GTV6 Late and Milano:
Stock bar:
Rate -- 93lbs/in Diameter -- 22.8mm Length -- 932mm
*No wonder I hit my oil sump so often with the stock setup!
Sport:
Rate -- 143lbs/in Diameter -- 25.4mm Length -- ""
SS:
Rate -- 191lbs/in Diameter -- 27.3mm Length -- ""

[Echo Leader]

Oh, neat....where do they mount?

I might have to take exception with those numbers....I think they used to total length of the torsion bar for the wheel rate calculation. I'm pretty sure you'll want to use an 'effective' length measurement from the 'insides' of the splines....where did those numbers come from?