Front End Unsprung Weight

Has anyone calculated the front end unsprung weight?
I'm looking for control arms, brake, knuckle minus wheel/tire since that varies car to car.

I am trying to figure out my suspension frequency so I can pick a good torsion bar.

For my own learning, would the entire control arm be unsprung on a torsion bar system? Usually there is a percentage of the control arm that is considered unsprung because the load is applied part way down the length of the control arm (like a coil spring).

Thank you!

dartslantsix said:

For my own learning, would the entire control arm be unsprung on a torsion bar system?

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I agree with you that the entire arm could be considered unsprung. Im sure you could calculate the percentage of the arm that is not part of the pivot but probably unneunneces

You're way over thinking this. Go with a 1.03 bar and call it good.

72Dart6pack said:

You're way over thinking this. Go with a 1.03 bar and call it good.

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I'm an engineer. I'm paid to overthink things.
The 1.03 bar may be what I go with, but by doing the math, I'll know it was the right choice.

For a streeter,1.03s are never too big...... ever. Unless maybe for an all-aluminum 6-cylinder. Or unless you leave the teensey weensey stock springs in the back. But you're an engineer, you would never do that.
However, your engine might be too small for 1.03s,lol.Unless you put some serious gears in the back. Cuz if you can't steer with throttle, that just takes all the fun out of them.
IMO 400 ftlbs should do it. From idle to about 6500rpm. Hyup that sounds about right. Yes I'm kidding. But wouldn't it be great!

dartslantsix said:

Has anyone calculated the front end unsprung weight?
I'm looking for control arms, brake, knuckle minus wheel/tire since that varies car to car.

I am trying to figure out my suspension frequency so I can pick a good torsion bar.

For my own learning, would the entire control arm be unsprung on a torsion bar system? Usually there is a percentage of the control arm that is considered unsprung because the load is applied part way down the length of the control arm (like a coil spring).

Thank you!

Click to expand...

The control arms will still be 50% unsprung mass. The reason for this has nothing to do with the location of the spring. One end of the control arms is bolted to the spindle (unsprung mass), the other end is bolted to the frame (sprung mass). So generally, 50% of the mass of the control arms is counted as unsprung mass.

Now, if you want to get really technical, you need to find the CG of the control arms, because that would actually determine how much is counted. From the CG to the suspension is unsprung, from the CG to the frame is sprung. It probably won't be a 50% split, but generally that's close enough for what most people are doing.

The shocks are also typically counted as 50% unsprung mass. Again, if you want to get technical there's also a motion ratio involved, that does depend on where they're attached to the control arm (they aren't traveling the full suspension travel distance).

The torsion bars are completely sprung mass. They are supported at the frame on both ends. One end in the cross member, the other in the frame mounted end of the LCA. The torsion bar doesn't travel with the suspension, and even the LCA hex end is frame supported. That's one of the advantages of the torsion bar design. For a coilover or coil spring, again it's 50% of the mass of the spring multiplied by whatever the motion ratio is for where it's mounted. But that doesn't matter in this case.

Don't forget the strut rods, they're also 50% unsprung. And that should be pretty close to 50% as they're pretty uniform.

Here's some of the weights. I don't have the weights for the spindles or brakes, although, that also depends on which brakes you're running, they aren't all the same (67-72 disks, 73+ disks, 9" drums, 10" drums). I have some extra 73+ disk spindles and stuff laying around so I can get those weights tomorrow if you'd like.

Stock pair of LCA's w/tabs, bushings and pins:.......21.2 lbs
Stock pair of UCA's (73-76) with ball joints:.......................9.8 lbs
Stock pair of strut rods 73-76 w/bushings:.............4.8 lbs
Stock pair of tie rods & ends:...............................4.6 lbs (these will also count 50%)

Now, that's all measured with a bathroom scale taking the average of 3 measurements, so, some variation would be expected.

And finally, while the frequency might be relevant at the design level, it doesn't matter nearly as much as the suspension travel on the street. If you lower your car any from stock, you have to increase the wheel rate to keep the suspension from bottoming out. The stock torsion bars bottom out right from the factory, these cars were substantially undersprung. Personally, I'd figure out the ride height you want first, then figure out how much suspension travel you have left, and work from there.

I lowered my Duster about 2" from stock. That also sets the control arms roughly level with the ground, so you get the best camber gain curves from the suspension. But I run 1.12" torsion bars (300 lb/in), and had to use much shorter lower bump stops and taller upper bump stops to re-center the suspension travel range around my new ride height.

If it's not too much trouble, would you mind weighing the disk spindles?

Thanks for this info and the reminder to check travel. Back when I did the Dart's IRS, the travel issue bit me and I had to up the spring rate.

The only advice I was able to find back then was to have enough spring and travel to absorb a 1G bump. It seemed to do the trick. Any thoughts on that?

Thanks again, this info saves me a bunch of time.

72bluNblu said:

Here's some of the weights. I don't have the weights for the spindles or brakes, although, that also depends on which brakes you're running, they aren't all the same (67-72 disks, 73+ disks, 9" drums, 10" drums). I have some extra 73+ disk spindles and stuff laying around so I can get those weights tomorrow if you'd like.

Stock LCA's w/tabs, bushings and pins:.......21.2 lbs
Stock UCA's (73-76) with ball joints:.......................9.8 lbs
Stock strut rods 73-76 w/bushings:.............4.8 lbs
Stock tie rods & ends:...............................4.6 lbs (these will also count 50%)

Now, that's all measured with a bathroom scale taking the average of 3 measurements, so, some variation would be expected.

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dartslantsix said:

If it's not too much trouble, would you mind weighing the disk spindles?

Thanks for this info and the reminder to check travel. Back when I did the Dart's IRS, the travel issue bit me and I had to up the spring rate.

The only advice I was able to find back then was to have enough spring and travel to absorb a 1G bump. It seemed to do the trick. Any thoughts on that?

Thanks again, this info saves me a bunch of time.

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So before I start, let me clarify something about the weights I posted yesterday- they are for pairs. Ie, one LCA with bushing and pin weighs half the weight I posted, 10.6 lbs for that case. Same for the UCA's, strut rods, tie rod ends, etc. I took those measurements to compare against a coilover conversion, so I did everything together for a total suspension weight. For a frequency calculation I assume you would only want one side so divide those numbers in half. I edited my post to make clear the weights are for pairs.

The 73-76 disk spindles I have weigh ~8.2 lbs each, but that's bare. No bolts or lower ball joints. With bolts and lower ball joints they weigh ~12.6 lbs each. So 16.4 lbs for both bare spindles, 25.2 for the pair of spindles with lower ball joints and bolts. Again, that's a bathroom scale and the average of a couple of measurements, I wouldn't be surprised to see numbers that were +/- half a pound or more.

The 1G estimate is a pretty common industry standard for street driven cars, I've seen it a few times.

The thing is unless you're going to redesign the suspension you're pretty much stuck with the travel. I mean, if you lower the car you change it, and you can change out bump stop heights to change it like I have, but overall the hard parts to hard parts travel is the same. So really, you're left with setting your ride height and picking a torsion bar to keep you from bottoming the suspension out. And there are other factors involved, like the camber gain curve I mentioned, the center of gravity and roll center, etc. I think all of those are going to be bigger factors than the frequency calculation, at least as far as handling changes you'll notice on the street.

At the very least, I would check the ride height and related travel first, because honestly that's going to be the major factor in choosing the wheel rate.

Thanks again!
I figured the weights were for pairs, but thanks for clarifying that.
I'm planning on pulling the current bars and checking the travel like you're saying.

72bluNblu said:

So before I start, let me clarify something about the weights I posted yesterday- they are for pairs. Ie, one LCA with bushing and pin weighs half the weight I posted, 10.6 lbs for that case. Same for the UCA's, strut rods, tie rod ends, etc. I took those measurements to compare against a coilover conversion, so I did everything together for a total suspension weight. For a frequency calculation I assume you would only want one side so divide those numbers in half. I edited my post to make clear the weights are for pairs.

The 73-76 disk spindles I have weigh ~8.2 lbs each, but that's bare. No bolts or lower ball joints. With bolts and lower ball joints they weigh ~12.6 lbs each. So 16.4 lbs for both bare spindles, 25.2 for the pair of spindles with lower ball joints and bolts. Again, that's a bathroom scale and the average of a couple of measurements, I wouldn't be surprised to see numbers that were +/- half a pound or more.

The 1G estimate is a pretty common industry standard for street driven cars, I've seen it a few times.

The thing is unless you're going to redesign the suspension you're pretty much stuck with the travel. I mean, if you lower the car you change it, and you can change out bump stop heights to change it like I have, but overall the hard parts to hard parts travel is the same. So really, you're left with setting your ride height and picking a torsion bar to keep you from bottoming the suspension out. And there are other factors involved, like the camber gain curve I mentioned, the center of gravity and roll center, etc. I think all of those are going to be bigger factors than the frequency calculation, at least as far as handling changes you'll notice on the street.

At the very least, I would check the ride height and related travel first, because honestly that's going to be the major factor in choosing the wheel rate.

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dartslantsix said:

I'm an engineer. I'm paid to overthink things.
The 1.03 bar may be what I go with, but by doing the math, I'll know it was the right choice.

Click to expand...

Shoot for a Front roll couple in the 80-85% of the total Roll rate and it should wind up with a decent neutral handling starting point.

Avoid lowering the front end too much with stock Knuckles. You lose a heap ton of jounce travel and can easily bottom out the LCA on the frame.

If you ever get the Caster/Camber curves modeled/plotted out, take out most of the anti-dive and see the good it does. A couple years ago, i was playing around with a new suspension design using a 73 B-Body LCA hooked to either Mustang 2 or Corvette Knuckle. Most of pf that effort was a fail but ditching much of the Anti-Dive was the one bright spots. I plan to test near 0 Anti Dive on my Dart Swinger once its on the road.

myduster360 said:

Shoot for a Front roll couple in the 80-85% of the total Roll rate and it should wind up with a decent neutral handling starting point.

Avoid lowering the front end too much with stock Knuckles. You lose a heap ton of jounce travel and can easily bottom out the LCA on the frame.

If you ever get the Caster curves modeled/plotted out, take out most of the anti-dive and see the good it does. A couple years ago, i was playing around with a new suspension design using a 73 B-Body LCA hooked to either Mustang 2 or Corvette Knuckle. Most of pf that effort was a fail but ditching much of the Anti-Dive was the one bright spots. I plan to test near 0 Anti Dive on my Dart Swinger once its on the road.

View attachment 1715225565
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You can lower these cars significantly with the stock spindles as long as you 1- increase the wheel rate to compensate for the lost travel and 2- swap out the tall factory lower bump stops to add some travel back in. These cars left the factory with 100 lb/in wheel rates, you can easily double the rate and still not have too much. That nearly halves the needed travel. The 1.03” bars from PST that are pretty popular have a 230 lb/in rate, and they’re not even that crazy. I run a 300 lb/in rate on my Duster, and it’s my daily driver not some track only car.

As far as the anti-dive goes, that’s a much larger problem on the B/E body cars than it is on the A-bodies. The B/E body platform had to support large station wagons, and as a result they have a ton more anti-dive built in. Hotchkis moved the front UCA mount on their B/E body UCA’s for exactly that reason. They could have done the same with their A-body UCA’s and didn’t, it’s not really much of an issue on the A’s.

I'm all for lowering the Roll Center but not when the LCA can barely move more than 1.5" before crashing into the frame. Not on a street car with sane wheel rates & a Stock LCA.

Several years ago, Firm feel adapted 73 B-Body LCA to FIX the pot hole = bent arm scenario but keep the lower RC. It adds a good 2.5" of travel over a stock LCA.
A more readily available & intermediate step would be a QA1 LCA, with its smaller dimensions offers ~3/4" of added travel over stock



If excessive stock Anti-Dive is unwarranted on upgraded B & E bodies, its certainly unneeded on an A-body. My modeling supports it and its #1 on list to test.

myduster360 said:

I'm all for lowering the Roll Center but not when the LCA can barely move more than 1.5" before crashing into the frame. Not on a street car with sane wheel rates & a Stock LCA.


Several years ago, Firm feel adapted 73 B-Body LCA to FIX the pot hole = bent arm scenario but keep the lower RC. It adds a good 2.5" of travel over a stock LCA.

A more readily available & intermediate step would be a QA1 LCA, with its smaller dimensions offers ~3/4" of added travel over stock


View attachment 1715225631

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I guess in all of your modeling you didn't actually check what the LCA to frame distance was on a car at factory spec ride height?

I'll help you out, it's about 2 1/4" for an A-body. That's hard parts to hard parts, no bumpstop, using the factory manual " A - B " measurement set at 1 7/8” to establish the factory ride height. Now, the 2.25” isn’t the travel at the wheel, it’s the frame to LCA distance at the bumpstop mount, so you need to do some trigonometry to get the wheel travel. With a 12.875” long LCA (middle of the torsion bar socket to the middle of the ball joint), and about 5.75” to the middle of the bump stop, wheel travel on compression at that ride height ends up at ~5”. Which means with a factory wheel rate of 100 lb/in (.85" bars) it would take ~500 lbs to bottom out a factory LCA on the frame from the factory ride height (ignoring the bump stop). Now the factory bump stop is actually about 1 1/8" tall so you'll never get that much travel at the wheel, but that's another issue and a separate discussion.

So let’s say you set your frame to LCA distance at 1.5" as you said, and installed a very reasonable 1.03" torsion bar (230 lb/in). The wheel travel works out to be ~3.36”, and so it would take 772 lbs of force to bottom the LCA on the frame. That's a pretty big improvement from stock, so less likely to damage the LCA than the factory configuration. Now we ignored the bump stops entirely again and I wouldn't run without one of some kind, but even with a 230 lb/in bar you can run a shorter than factory bump stop and lower the car somewhat significantly from stock without any worries.

On my Duster I have about 1 3/8" from frame to LCA. I currently run QA1 LCA's, but I also ran my Duster with stock LCA's using the same LCA to frame distance, it's about the minimum I need with my torsion bars. At that frame to LCA distance the wheel travel is ~3.08”, and I run a 300 lb/in torsion bar (1.12"), so that's ~924 lbs to bottom out on the frame. So my LCA isn't "crashing" into my frame. And I personally don't think that 300 lb/in is an "insane" wheel rate for a street car like mine. If it were a track only car with softer compound tires I'd need to run an even higher rate than that. I'll give you that not everyone will enjoy the ride quality you get with a 300 lb/in wheel rate, but with the Hotchkis Fox shocks I use it's really not all that different a ride quality than a modern performance car straight off the dealer lot. I put ~70k street miles on my Challenger while it had a 270 lb/in rate wheel rate, it worked great, which is why I didn't hesitate to go to 300 lb/in on my Duster.

As for the modified LCA's to increase travel, they have the one of the same problems as using a drop spindle. The LCA to frame distance isn't the only limiting factor. On my Duster I have 13" from the spindle to the bottom of the inner fender when the LCA is on the frame. That means a 26" tall front tire would hit the inner fender at max compression travel (assuming no bump stop). So, using a 2" drop spindle, or an LCA modified to add much more than the 3/4" or so of travel the QA1's give, is pretty pointless unless you also raise the inner fenders (or run a pretty short front tire). All you do is trade LCA to frame interference for tire to inner fender interference, and you don't pick up much (if any) travel in the trade.

myduster360 said:

If excessive stock Anti-Dive is unwarranted on upgraded B & E bodies, its certainly unneeded on an A-body. My modeling supports it and its #1 on list to test.

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You totally missed my point. B&E bodies had MORE anti-dive built in from the factory because of the heavy station wagons that were also on those platforms. A-bodies had less anti-dive built in from the factory. It's not an even comparison.

Like I said, Hotchkis did all of that modeling too, that's why they changed the suspension points for their B/E body UCA's to reduce the anti-dive numbers. I know, I have a set of Hotchkis UCA's on my Challenger for exactly that reason. If it had been needed for the A-bodies I'd be willing to bet Hotchkis would have done it. Instead, they decided it wasn't worth it (for whatever reason), and the A-body Hotchkis UCA's still use the factory suspension mounts. I'm sure there's still some improvement to be had, but, I can guarantee it won't as much as the B/E bodies because they didn't have the same amount of anti-dive built in to begin with.

I know this will solicit a 2500 word manifesto on how it's practically perfect on your car, but oh well

The tiny lump of a "bump stop", that you swear to never be without (better than nothing?), just shrunk the contact distance to the frame to less than an ~.8125"(1.5"-.6875") and cut available wheel travel by 1/3. So now the "reasonable" 1.03 bars will ram the lump of rock hard urethane into the frame, spring rate shoots to infinity, Brake traction disappears with just ~488 lb? Less than stock
Nice job
[crank up that anti-dive lol]

Sorry no thanks. You have to have enough clearance for the proper bump stop, which is NOT a rock hard thin chunk of urethane.

Does it make any sense whatsoever that the STOCK amount of Anti-dive(A LOT) is needed with 2-3x the STOCK wheel rate??
Common sense says probably not,, doesn't matter what car it is or what ONE vendor decided to do.

myduster360 said:

I know this will solicit a 2500 word manifesto on how it's practically perfect on your car, but oh well

The tiny lump of a "bump stop", that you swear to never be without (better than nothing?), just shrunk the contact distance to the frame to less than an ~.8125"(1.5"-.6875") and cut available wheel travel by 1/3. So now the "reasonable" 1.03 bars will ram the lump of rock hard urethane into the frame, spring rate shoots to infinity, Brake traction disappears with just ~488 lb? Less than stock
Nice job
[crank up that anti-dive lol]

Sorry no thanks. You have to have enough clearance for the proper bump stop, which is NOT a rock hard thin chunk of urethane.

Does it make any sense whatsoever that the STOCK amount of Anti-dive(A LOT) is needed with 2-3x the STOCK wheel rate??
Common sense says probably not,, doesn't matter what car it is or what ONE vendor decided to do.

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My car is far from perfect. All suspension design is a trade off. All of it. There is no perfect suspension design.

I don't run .6875" tall bump stops. And your math is wrong anyway, might want to check that. If you used a .6875" tall bump stop and set the height at 1.5 frame to LCA the wheel travel would only be 1.8" and with a 230 lb/in bar that's only 418 lbs.

But running a .6875" tall poly bump stop is a waste of travel anyway, they won't compress that far so it defeats the purpose. I use .375" tall bump stops and set the height so I almost never hit them, which is far more predictable than the slop the stock rubber bump stops give you after only 252 lbs of load, because that's all it takes to hit the factory bump stops from the factory height with a factory bar. The 500 lb load is didactic, you're on the bump stops after half that. Remember we were ignoring the bump stops to begin with because you freaked out about crashing parts together?

I've never gone metal to metal with LCA and frame in over 85k street driven miles between my two cars. So, no, I'm not worried at all about your thoughts on bent LCA's.

As for the modified LCA's, like I said before, you better check your spindle to inner fender measurement at max compression before you pick those torsion bars. Bottoming the wheel on the inner fender makes bottoming the suspension look like a picnic on a bright sunny day.

As for Hotchkis, their cars are still putting down some of the fastest lap times for Mopars. The Hotchkis Challenger pulls .93G on a skid pad with street tires. Until you can do that I'm happy to defer to their reasoning.

Hey, only 312 words.

lol when only 50 words were needed

3/8" tall bump stop!?! LOL!!

myduster360 said:

lol when only 50 words were needed

3/8" tall bump stop!?! LOL!!

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Yup, they work great! ~85k miles on my cars so far and no issues with the 3/8" bump stops. It's almost like I matched the available suspension travel to the wheel rate so I wouldn't hit the bump stops all the time

Theory is great, but if you never put on the road it doesn't mean a thing. Too bad you scrapped all those pretty 3D models to run drum spindles and stock LCA's, guess we'll never know

dartslantsix said:

Has anyone calculated the front end unsprung weight?
I'm looking for control arms, brake, knuckle minus wheel/tire since that varies car to car.

I am trying to figure out my suspension frequency so I can pick a good torsion bar.

For my own learning, would the entire control arm be unsprung on a torsion bar system? Usually there is a percentage of the control arm that is considered unsprung because the load is applied part way down the length of the control arm (like a coil spring).

Thank you!

Click to expand...

One of the best question, I have seen.. Subscribed.

gzig5 said:

View attachment 1715225785

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Always math, and ratio.. suspension....learning factory parts....

72bluNblu said:

Yup, they work great! ~85k miles on my cars so far and no issues with the 3/8" bump stops.

Click to expand...

wow,,You're the type that even I jack up your heap and show you your dented frame you'd spew pages of drivel and rationalization before accepting even the possibility of a better way than yours.

Waste of time,, obviously