Drop Spindle Discussion

[Grant]

If you're going through the hastle of putting on coilovers in the front you should go with higher rates than 27mm bars I think. Are 27mm bars hard to get over there? If you can make some coilovers with decent shocks, I'll send you my 27mm torsion bars plus cash

Why would you think the spring rates are at the wheel?

[scott.venables]

27.3mm bars are pretty easy to get here but I like the idea of being to adjust ride height without fighting with giant torsion bars. Plus my Bilsteins are starting to leak so I need new shocks anyway.

I ran some sums on spring rates and getting the equivalent of 27.3mm bars with coil springs and got a big surprise how stiff the springs need to be. I couldn't be sure what the spring rates Grant posted were for or what length of lever they used so I started fresh. I used the formula for torsion bar spring rate in the Puhn book and took some measurements, and the stiffness of a vertical coilover mounted at the shock mount (in addition to the stock TB) needs to be about 230kg/cm to get the same stiffness as a 27.3mm bar. Also due to the actual angle of the shocker(not vertical), the spring actually needs to be stiffer than 230kg/cm This seems really odd, given that the available spring rates for the RSR kit are 80-125 kg/cm. So are 27.3mm bars stiffer than I think or is my maths stuffed? Here's the formula and some other stuff I used:

K = 1,178,000 X (D^4 / LA^2)
Where:

• K = spring rate in lbs/in at end of lever
• 1,178,000 = spring constant
• D = diameter of bar in inches
• L = bar length in inches (length of constant diameter not including splines or tapered sections)
• A = Lever length in inches

What I used
D= 0.898" (for stock 22.8mm) or 1.075" (for 27.3mm)
L= 33.27" (845mm)
A= 12.2" (torsion bar centerline to middle of balljoint = 310mm)

To convert lbs/in to kg/cm multiply by 1.15

This gives you spring rates at the balljoint, so you then multiply by 1.22 to get the spring rate at the shock mount. (310mm from A above and 255mm from TB centerline to shock mount)

So did I go wrong? I thought I would end up with a value in the middle of the RSR options (about 100kg/cm).

Scott

[Echo Leader]

Should the measurement of A be from TB centerline to the balljoint? I would have thought you'd go from TB centerline to hub face, since the force exerted through the tire acts on that piece as part of the level arm??

I've been toying with similar calculations, and I just thought of something...the length you're using for the TB, is that from end to end? or from the 'inside' of the mounting point at the LCA to the 'inside' of the mount at the chassis?

I wish I had access to my calculations at the moment...could start really comparing our work.

[scott.venables]

Yeah the length of the TB is the parallel part only. Not including splines or tapered sections.

Re: Lever length, I thought the same, except to the centre of the contact patch, then my Dad straightened me out. The lever length is only to the balljoint because it pivots there, the spindle/upright is not an extension of the lever. It's why different offset wheels don't affect spring rates.

I'd be interested in your calculations to see if we get the same answers.

Cheers,
Scott

[Echo Leader]

I may have to take some TB measurements. I believe I just took the numbers people have published online about the early length ones being XXXmm or something like that.

I'll also have to read up on the level arm issue. I know what you're saying about the offsets and the pivot.....just a bit unsure at the moment.

Yeah, let me work on the homework you've just managed to assign me and we can compare notes!

[Echo Leader]

I think my current thinking is that changing the offset does impact the effective spring rate. Thus if I put a 24" spacer on my hubs, the contact patches would be moved out quite a bit, and thus my level arm would be bigger. I haven't found any official confirmation of that yet, but at the moment, I believe it to be the case.

[scott.venables]

Hmmm, I'm pretty sure offset won't affect spring rates, but I haven't seen confirmation of that either. If you think about it, smaller offset wheels won't change lever length, because the upright/spindle only ever travels in a (roughly) vertical motion. Because the upright doesn't rotate or pivot around a point, wheel offset shouldn't affect spring rate.

Did you find your calculations?

Scott

[Echo Leader]

Well, perhaps you're right in that you can't increase the wheel rate by pulling the center of the tire contact patch closer to the body then the hub face.

Because even if you do so, you can't really shorten the LCA length. However, if the center of the tire contact patch is normally outside of the hub face, I would think that decreasing the track width would increase wheel rate....though we're probably talking minutely.

But I was just thinking that if I bolted a long, long rod to my hub (sticking out perpendicular to the car's natural direction of travel) and then grabbed that bar at the end, I'd have much more lever arm. If a wheel was then mounted on the end of that bar, the wheel rate would be really low.

Sorry, I didn't get a change to jump on my home computer last night. I got home from work pretty late and basically went right to bed. I'll hopefully work a more normal day today...

[Alfar7]

Offset does affect spring rate as measured at wheel center which is where the computations mean something. The deeper the backspacing the better as far as suspension geometry & contact patch management. Better steering, feel (less kickback & sweep), and wear.

[gtv27]

Quote:

Originally Posted by scott.venables Yeah the length of the TB is the parallel part only. Not including splines or tapered sections. Re: Lever length, I thought the same, except to the centre of the contact patch, then my Dad straightened me out. The lever length is only to the balljoint because it pivots there, the spindle/upright is not an extension of the lever. It's why different offset wheels don't affect spring rates. I'd be interested in your calculations to see if we get the same answers. Cheers, Scott

[scott.venables]

Thanks for the help guys. I still can't get my head around wheel offset affecting spring rate. Since the spindle/upright does not pivot around anything, I can't understand how the spring rate is altered. Does this mean changing wheel offset would change ride height?

Jason, still unsure about the calculations. Can anyone comment on this?

Cheers, Scott

[Echo Leader]

Quote:

Originally Posted by scott.venables Does this mean changing wheel offset would change ride height?

[scott.venables]

Thanks James. I'm still having trouble with the offset thing. It does effect the formula though. You were probably using 932mm which is the overall TB length including splines. I measured the parallel diameter to be 845mm (75/Milano). Where did you get your torsion bar spring rate formula?

Don't stress, there's no rush.

[Grant]

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 -- ""

So, I have an '84 and I think that means I have the longer, early style bars.

Another interesting thing I learned was that the GTV6 has stiffer rear springs in the rear than the Milanos do. I'm not sure how much, but the SS springs I have in the rear (143lb/inch) are 39% stiffer than stock on the GTV6 but are 57% stiffer than stock on a Milano. Also, SS springs apparently came in two lenghts, one that lowers an early Milano by 1 inch, and another that lowers it 2 inches.

[Echo Leader]

Sweet! Thanks, Grant!

My calcs were all done with the longer bars as I have an '84 with the longs. But like I said, it's in Excel so that should be easy to modify....I guess I'll need to get a measurement of my "effective" length and use that instead.

I forget where exactly I got the formula...perhaps it was from a suspension book? My buddy has a few and may have sent it my way.

I think its:

1129000 x (D^4) ÷ AL^2 x EL = Wheel Rate

D = TB Diameter
AL = Lever Arm Length
EL = Effective Length

But I just found that on some website, so take that with the "it was on the web" grain of salt. Otherwise I'll send you the formula I've been using in my spreadsheet when I get a chance.