Calculating unsprung weight

[73-TYPE-LT-LS1]

I'm trying to figure this out for my 63 pickup. Please correct me where I might be wrong.

Front:
100% of:
Rim
Tire
Spindle
Brake
Bearings

50% of:
Sway bar
Shock
Control arms
Spring



Rear
100% of:
differential
Rims
Tires

50% of:
Sway Bar
Shocks
Springs


Anything I am missing or off on?

[Norm Peterson]

Up front, 50% of shock and spring times their specific motion ratios. If your spring is halfway out the control arm, you essentially have half the spring weight moving half as far as the wheel. You can neglect the front sta-bar, as only half of the arms plus the endlinks (times a motion ratio) are unsprung - probably less than 1 lb per side.

Rear - either all of the rear sta-bar if it's bolted up to the control arms or if the center torsional portion is bracketed up to the axle tubes - or ignore it if it's bracketed to the chassis and endlinked to the axle.

Is this a leaf spring stick axle?


Norm

[73-TYPE-LT-LS1]

No this is a truck arm (BTW they ran from 60-72). The rear bar is bolted to the bottom of the control arms and the endlinks to the frame. Front doesn't have a sway bar right now.

As far as the spring, what if it's not center of the control arm. I'm guessing I should meassure the center location of the spring relative to the location in the control arm. IE control arm say is 20" long to the ball joint. Spring center is 8" out from the frame / LCA mount. (20-8)/20 = 60% so would the ratio be the 60% or the 40% multiplied by 1/2 the spring rate?

I'm assuming this goes for the rear spring shock as well based on the length of the control arm.

The rear shock I think has more than a 10-12% andle on it. Will this come in to play on weight or just dampening force?

[Norm Peterson]

Front: Measure the ratio going out from the chassis pivot. So 8"/20" = 40%.

Rear: A little over half the weight of the truckarms will be unsprung (I believe that they extend slightly past the axle centerline). If the rear spring and/or shock is mounted to the truckarm, there will be a motion ratio correction there as well. 32" back from the chassis pivots on truckarms that measure 40" from chassis pivot to axle centerline would be 80%, though that won't mean a whole lot when the axle is a couple hundred lbs all by itself.


Norm

[73-TYPE-LT-LS1]

Ok, I was thinking the percentage would be to the 40% in my example.

Good point about the truck arm going slightly past the axle center line. I'm trying to think of how to figure that part out now. (Length of control arm - axle center line distance) = % to multiply weight of control arm by. Divide that by 2 + left over percentage? IE:

40" - 37" = 3" (7.5%)
40lbs / .075 = 3lbs
37 / 2 = 18.5 + 3 = 21.5 unsprung vs 18.5 sprung

[Norm Peterson]

That's close. But the math runs just like for the spring and shock with the assumption that the truckarm is uniform. That puts its CG 20" aft of the chassis pivot (half the total length away), so 40 * 20/37 = 21.6 lbs unsprung. The same approach applies to front control arms, but for suspension linkage members in general the differences vs simply using 50% in terms of the actual lbs of "error" are small enough to ignore in most cases.


Norm

[73-TYPE-LT-LS1]

Norm thanks for all the help. Now in regards to the lengths.. I'm assuming we are talking the lengths based on the actual lengths and not the side view length (mounting point to above axle center). IE Actual length of the rear control arm being 40 vs the plane length be say 35 due to the angle the control arm has.

Again these are assumption numbers.

[Norm Peterson]

Either way works correctly, as long as all of the dimensions are measured in the same reference. Either all in 'true length' or all in a consistent 'projected length' view. Just that you'd have to compute the CG point from either a known [side view] / [true length] ratio or from the trigonometry if you use side view projection. IOW, with side view numbers, the 20" CG location becomes 20 * 35/40 = 17.5 (using your numbers above) and the 37" from before becomes 32.375".


Norm

[73-TYPE-LT-LS1]

Thanks Norm..

Here is a spreadsheet that I put together that might help others. I'm going to be using it to determine what spring rates I should be running in my 63 pick up for drag racing. I'm thinking it will also help me to determine the spring rates I will need in my 73 Camaro when I get it going.


Some things that need to be know about this sheet..

1) Red Areas shouls be left alone..

2) This is currently set up for my trucks rear suspension. There might have to be some slight changes to the equations depending on your suspension set up (IE 3 link it won't compute the UCA for the rear properly). It takes the individual weights of the control arms and then doubles them in the final eqution showing unsprung weight. It should work however for a 4 bar, 4 link, truck arm, Ladder bar ect. It also uses a panhard bar in the equation so a ladder bar with a Z bar or a watts link set up might need the adjustments as well.

3) The front suspension equations are bases on a double A-arm suspension. So a strut set up might not work right or it might if you leave the UCA weight out.

4) If the drive shaft weight looks funny, ignore it. I divided it by 1/4 but it is multiplied by 2 in the final equation. So it does represent 50% of it's total weight.

5) "Max Control Arm Drop From Ride Height" is the amount the spindle would fall before maxing out suspension travel jacking up the front end (hope that made sense)..

6) All mesurements in the sheet are guesses right now.


Please let me know if you see any errors or area's of improvement.


http://73-ls1.com/63truck/weight_sheet_revision_2.xls

[73-TYPE-LT-LS1]

Revision 3 is not quite working the way I want it to.. I'll try and work on it some more when I have time



I made a update / Change on this one. This one auto matically figures out the length coil you need based on number of coils and thickness of coil. So you can look at it a little differently.

http://73-ls1.com/weight_sheet_revision_3.xls