Just a relared FYI. I don't remember where, but I read where someone did an analysis of a-body roll center for road racing, and concluded the correct spindle height was about a 1" drop, rather than the original spindles or the after market 2" drop spindles.
Fabricated A-Body spindle ideas and discussion
- Thread starter DionR
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DionR
Well-Known Member
Sounds like Bob Reed and his ‘69(?) Barracuda. I have to go back and find the article, or a least I think the info I am thinking about was in an article about his car. I know he was selling modified spindles at one point that were a 1” drop.
I am doubting that he ran a wide tire on his car though. A 17” wheel is limited on BS/offset with the stock spindle, a 1” drop is going to have the same or even more of a limitation with an 18” wheel. Remember that an 18” wheel is only 1/2” further out than a 17” wheel.
The other thing is, he might be using a taller ride height which might work better with his 1” drop spindles.
DionR
Well-Known Member
I was aware of that limitation with a plasma cut part, but I thought lasers were much less affected by that.
jos51700
Green Bearing thread connoisseur
Less but definitely still there. The laser is actually focused at a certain point, not collimated like you would expect.
I was pretty surprised by that myself. It's less than a plasma for sure
Some thoughts after reading through this;
Its all very cool to see Mopar guys thinking down this line of modification.
Altered mounting points for upper control arm would allow customization of anti-dive, instant center location contingent on spindle height, caster, and camber gain curve.
I would look at ways to avoid using the OEM lower ball joint/steering arm combo. A divorced set up would provide all the benefits outlined by 73bluNblu plus allow replaceable and interchangeable steering arms for rear or front steer applications. Going to a fabricated arm in a custom length would help minimize angular changes through-out the range of travel, while potentially increasing offset and possibly improving scrub.
Front ends don't work in a vacuum and this needs to tied in with COG height and roll axis parameters as part of rear geometry. If you have a super clean front geometry layout but end up with a huge moment lever arm, you'll have to start stepping up roll couple percentage to maintain level cornering.
All of this is moot of you compete in a class with a strict rule set that limits configuration changes.
Even with a stock geometry, the idea of bearing cartridges, ABS, and access to a wider range of rotor and caliper combos is pretty cool.
I would definitely advise basing the approach on more widely available parts from OEMs that easily found. Even factory custom, special offering, one or only a couple year model offerings that are sorta trick will make parts harder to source in another 5 years. Case in point, once upon a time the 275front/335 rear 17" tire combo was the hot ticket for performance cars such as Viper, Vette, Camaro and Mustang. Now those sizes are only available from a few manufacturers and are getting more expensive as the performance world has moved on.
Its all very cool to see Mopar guys thinking down this line of modification.
Altered mounting points for upper control arm would allow customization of anti-dive, instant center location contingent on spindle height, caster, and camber gain curve.
I would look at ways to avoid using the OEM lower ball joint/steering arm combo. A divorced set up would provide all the benefits outlined by 73bluNblu plus allow replaceable and interchangeable steering arms for rear or front steer applications. Going to a fabricated arm in a custom length would help minimize angular changes through-out the range of travel, while potentially increasing offset and possibly improving scrub.
Front ends don't work in a vacuum and this needs to tied in with COG height and roll axis parameters as part of rear geometry. If you have a super clean front geometry layout but end up with a huge moment lever arm, you'll have to start stepping up roll couple percentage to maintain level cornering.
All of this is moot of you compete in a class with a strict rule set that limits configuration changes.
Even with a stock geometry, the idea of bearing cartridges, ABS, and access to a wider range of rotor and caliper combos is pretty cool.
I would definitely advise basing the approach on more widely available parts from OEMs that easily found. Even factory custom, special offering, one or only a couple year model offerings that are sorta trick will make parts harder to source in another 5 years. Case in point, once upon a time the 275front/335 rear 17" tire combo was the hot ticket for performance cars such as Viper, Vette, Camaro and Mustang. Now those sizes are only available from a few manufacturers and are getting more expensive as the performance world has moved on.
DionR
Well-Known Member
That's the other thread. My plan is to avoid modifications like moving the UCA mounting points. But if I can build a spindle simply enough and maybe improve things along the way I am all for it.
Besides, I suspect the geometry is good enough without going to that extreme. So, unless we are going to design our own Speedtech like kit and plan to completely re-engineer everything, for me, things like moving the control arm mounts is off the table.
True, but see the above. I might look at the B-Body LCA but not so I can do a custom steering arm. And I don't have any interest in trying to do a front steer rack swap, I think that has been a complete bust for everyone that has tried. Other than maybe some straight line guys.
Not saying the idea doesn't have potential benefits. Just that I don't plan to chase them, at least right now.
That is completely true.
The other thing we don't talk about is Ackerman, which can impact the available traction and reduce it when it is bad.
I've thought about that, but for me, I'm not building anything for a class other than my own. But it is true that something like this spindle might not be allowed in some of the competitions. Pretty sure the OPTIMA stuff won't allow it, but not sure why other systems are. Either way, since I don't see this as a get rich project nor something that I expect would sell well if someone marketed it, I don't see it being hugely important.
I completely agree. The magnaride front hub seems small in use, but I haven't checked to see if someone could buy one from O'Reilly's or something. The rear hub on the other hand, I believe will be easy to source.
Unfortunately, there will always be a dwindling supply of OEM style parts. Luckily, the ball joints have been used on many hobby stock classes and worse case we have to build a custom LCA and use the 73-74 B-Body steering arm. But that is a bridge to cross when we get there.
DionR
Well-Known Member
Here is my best eyeball measurements of the suspension location points. Not going to say I didn't miss something and/or have something wrong.
This is with an F-Body spindle, 6 degrees positive caster, 1 degree negative camber, at a ride height where the LBJ and LCA pivot are horizontal and with a 275/35R18 tire on an +35mm offset wheel. Hub is in the stock position, not pulled in.
Edit - corrected image
I believe caster has an impact on the functional pivot for the UCA, but I might be interpreting that wrong. So take this with a grain of salt.
I will look at a how a raised or 1" longer ball joint affects thigs later.
At least something to start with. Maybe someone will see glairing differences between this and info they have.
This is with an F-Body spindle, 6 degrees positive caster, 1 degree negative camber, at a ride height where the LBJ and LCA pivot are horizontal and with a 275/35R18 tire on an +35mm offset wheel. Hub is in the stock position, not pulled in.
Edit - corrected image
I believe caster has an impact on the functional pivot for the UCA, but I might be interpreting that wrong. So take this with a grain of salt.
I will look at a how a raised or 1" longer ball joint affects thigs later.
At least something to start with. Maybe someone will see glairing differences between this and info they have.
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DionR
Well-Known Member
Found the article I was thinking of.
https://www.hotrod.com/features/1968-plymouth-barracuda-s340/
He claims to want to target a 1"-2" front roll center, and was at 3" at that point.
And he is running a 7x17 front wheel and a 225 tire. Based on what I can see, he won't be able to get anything wider on the car. It does look reasonable low, though.
Edit: Not sure what I was thinking, he could run a 17x8 and a 245 tire just like I am. What he couldn’t do is run a 275 wide tire on the front without doing some body mods.
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Upper arm geometry is okay, IMO. The anti-dive does create some dynamic caster change that isn't always great, but is tolerable. Hotchkis eliminated the anti-dive in their A body upper arms.
Ackerman is the triangle from the lower ball joint, through the outer tire rod end, converging at the axle. When you front steer a car, this triangle must be retained. This is in big part why many front steer rack conversion with stock lower ball joints fail...they actually create reverse ackerman. This is where a divorced steering arm could allow a configuration for rear or front steer. Zero ackerman is preferable to reverse and some competition cars actually eliminate ackerman. A divorced arm could also be built with a slotted tie rod mount to provide further adjustment. Many oval track arms utilize this feature to adjust the ackerman angle further.
The dwindling supply of OEM parts and limited reproduction s is why I would consider a fabricated arm in both upper and lower. As you point out, oval track hobby racers have created a horde of cheap parts that are easily replaced.
The upper control arm pivot point isn't impacted by caster, but it is impacted by anti-dive. Since the front pivot is higher that the rear, you have to calculate the dynamic pivot. T=Looking down on the control arm, this is done with a straight line between the front and rear pivot. Then create a 90* intersection on this line that extend to the center of the upper ball joint. This position is the dynamic pivot of the upper control arm.
Overall your layout appears to be on the right path. When I plotted my Challenger, my roll center ended up at 5.5" with 26" tires and the earlier disc standard height spindle. Since I can't access the program, I don't have data on my various pivot point positions. I did drop it down until the lower arm was at zero ride height, or the ball joint is perfectly level with the inner pivot.
I know Dr Reed always tried to target a 2" roll center height from his Mazda days. I think that will be really tough to get with stock pick up point Mopar. In my oval track days, we typically tried to get to 4" roll center for the front using stock stub cars. We also created offset centers and cantilevered pivots since we only turned left. In comparison, a street car is easier, everything simply needs to be a mirror image.
Ackerman is the triangle from the lower ball joint, through the outer tire rod end, converging at the axle. When you front steer a car, this triangle must be retained. This is in big part why many front steer rack conversion with stock lower ball joints fail...they actually create reverse ackerman. This is where a divorced steering arm could allow a configuration for rear or front steer. Zero ackerman is preferable to reverse and some competition cars actually eliminate ackerman. A divorced arm could also be built with a slotted tie rod mount to provide further adjustment. Many oval track arms utilize this feature to adjust the ackerman angle further.
The dwindling supply of OEM parts and limited reproduction s is why I would consider a fabricated arm in both upper and lower. As you point out, oval track hobby racers have created a horde of cheap parts that are easily replaced.
The upper control arm pivot point isn't impacted by caster, but it is impacted by anti-dive. Since the front pivot is higher that the rear, you have to calculate the dynamic pivot. T=Looking down on the control arm, this is done with a straight line between the front and rear pivot. Then create a 90* intersection on this line that extend to the center of the upper ball joint. This position is the dynamic pivot of the upper control arm.
Overall your layout appears to be on the right path. When I plotted my Challenger, my roll center ended up at 5.5" with 26" tires and the earlier disc standard height spindle. Since I can't access the program, I don't have data on my various pivot point positions. I did drop it down until the lower arm was at zero ride height, or the ball joint is perfectly level with the inner pivot.
I know Dr Reed always tried to target a 2" roll center height from his Mazda days. I think that will be really tough to get with stock pick up point Mopar. In my oval track days, we typically tried to get to 4" roll center for the front using stock stub cars. We also created offset centers and cantilevered pivots since we only turned left. In comparison, a street car is easier, everything simply needs to be a mirror image.
bjkadron
Well-Known Member
If I'm reading this right you have zero anti-dive with the UCA pivots at the same height? This is pretty unusual and would be pretty hard to do with an A-body. Or did you average it? What kind of accuracy do you think you had with your measurements? How did you identify the centerline?
Here is what I found from when I did the calculations previously. Though I'm not saying mine were accurate either.
I think anywhere from -1 to 10 inches is workable, 2-5 is probably better. Interestingly the stock Duster (according to the calculations I did before) has better RC height of 4.02" vs the F-body spindle at 7.75" and I'd guess the raised UBJ moves it up even further.
@DionR May be able to calculate the RC with his measurements as it is pretty easy with the setup he shows:
Here is my comment from the video so people don't have to search for it and since it is applicable for this discussion. I still agree with myself. Haha.
"It is really cool that you are developing stuff like this for classic mopars. A lot of my experience comes from the A-body world (they have better geometry from the factory), but I think the direction you're going in with this modification is probably not the best. It is pretty common knowledge in the racing world that a lower roll center is better and more predictable than a high roll center. This does create more body roll but that can be counteracted with different devices like sway/anti-roll bars. In the book "advance race car suspension development" by Steve Smith he even says "a racing vehicles roll center height should be as low as practical so that the lateral acceleration is transferred into body roll rather than lateral displacement at the outside tires contact patch with the road". The problem with raising the roll center is you increase the loads along the suspension arms and can even create jacking with a high enough roll of center. Increasing spring rates through bigger torsion bars and roll rates through better anti roll bars would probably be a better solution then trying to raise the roll center. Not to mention the liability issue if one of these breaks and causes an accident. Not trying to be a downer, just looking out for the community as a whole. Also those brakes are sweet. I'm working on a similar setup for my duster."
Agreed. I saw a later video of the car running and it has almost no suspension control. Just flops side to side.
DionR
Well-Known Member
I thought they did that with their E/B-Body arms, not the A-Body arms. Not that I am well versed in the Hotchkis stuff.
That might be the case. Good thing we already have a couple of options for the upper arms and one for the lower even if the ball joint isn't what you are suggest we need.
Yep, that's what I did. There is some fudge to it, but the functional pivot is in based on the sloped pivot line.
DionR
Well-Known Member
To be clear, I'm not suggesting he has the better plan. It was the idea of roll couple that made me wonder if I was missing something.
No, I set the caster and then found the perpendicular location and set the UCA inner pivot at that location. This was based on what Mike Martin wrote in his book.
Here is a picture.
This is why I said that I think caster impacts the geometry. What I didn't try and take into account is how the front and rear pivots move in and out (if the stock adjustments are used) and this would create a pivot that is out of plane with the LCA. But I ignored it because it made my head hurt.
I am hopefully within a 1/16"? Tape measures and eyeballs.
I used the bottom of the frame where the k-frame mounts as a datum and measured up from there to the outer edge of the bushing on both ends. The rear measurement was harder since the frame slopes away at that point so I had to try and maintain the plane.
I had a loose k-frame to grab measurements off to try and locate the inner pivot for the LCA. And then I used 13" for the LCA length which I admit was a bit of a swag as I didn't have a LBJ to bolt in and measure to.
The other thing to note is that it doesn't seem like the bottom of the frame is level with the ground, but the slope of the torsion bar (in my case) was the same so I assumed that the suspension would effectively be 90 degrees to that plane. But it was an assumption.
Hmm...pretty different. I wonder if my LCA length is way off. It was a best guess (see above).
I will say I am pretty comfortable with my "X" dimension for the LCA inner pivot since the factory frame dimensions says 24.46" overall.
I am showing the roll center in my image above at 6.6873". Am I misunderstanding you?
DionR
Well-Known Member
I added the side view to maybe explain where the UCA pivot came from. Note that the UCA pivot Y dimension changed due to missing the center of the circle when I dimensioned it.
Actually, you are correct, they took anti-dive out of the B and E body units, not the A body. My mistake.
DionR
Well-Known Member
DionR
Well-Known Member
What am I missing here? Looks like the 1" taller balljoint actually raised the roll center by over 2"???
Did I do something wrong in my layout? Not at all what I expected.
At the same time, it does shorten the roll couple.
Did I do something wrong in my layout? Not at all what I expected.
At the same time, it does shorten the roll couple.
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DionR
Well-Known Member
Here is my best guess at a stock setup.
Revised:
Here is a closer shot, easier to read.
Revised:
Here is a closer shot, easier to read.
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bjkadron
Well-Known Member
There is a lot of new posts so I'm going to quote this one then add my thoughts.
First off, thanks for the explanation. That and the multi-colored lines. It makes it a lot easier for my feeble mind to understand. Haha! I think yours may be more accurate than mine because I didn't have a good alignment when I did mine, plus a lot of other variables.
I also found that the RC raised with a taller spindle, up from stock, to F-body then F-body with a Raised UBJ. What is happening is the effective swing-arm length is shortening, so the line intersecting the center of the tire and the Instant Center of the swing arm is at a steeper slope so where it intersects is higher at the same distance (center of tire to center of car). This is also shown in your stock setup in the opposite way. I would be curious if you could run a lowered stock spindle with the LCA pivot points stock and the performance tire. My guess is the RC will be even lower. On the other hand, the camber gain will be less as a tradeoff. In my calculations I got -1.04 */in of camber gain with the stock spindle and -1.66 */in with the F-body. I would think an added length UBJ without a corresponding movement in the UCA position would be a bit too much camber gain for modern performance tires. I was planning on using just the FMJ Spindle and doing testing from there for tire temps.
As far as anti-dive: The main reason the factory used a lot of anti-dive was to counteract the soft spring rates and tire sidewalls of the time causing excessive dive under braking. For most practical purposes the downsides of anti-dive is potential bumpsteer/caster changes and lack of driver feel (it doesn't dive under braking at 100% anti-dive so it is harder to feel the weight transfer). The upsides are: It doesn't dive. So in most practical situations it is a net plus because you can run softer spring rates for grip and not dive as much under braking and upset the balance or geometry. You just set it to somewhat less than 100% (80% seems a common recommendation). The one time you don't want it is mostly on circle track cars when you want it to squat before entering a turn and stay down to lower the effective CG through the corner. The cool thing about using a more modern adjustable UCA is you can use the eccentrics or shims in our UCA to set the geometry, then use the UCA to adjust your alignment. If you want to learn more here is a pretty good read: Anti Squat, Dive and Lift Geometry – Geometry Explained Suspension Secrets
Overall, the main problem with suspension geometry is it is like anything else: Full of compromises and everyone has a different opinion of how to do it right. Even something like NASCAR where the racing is very close, the teams can have drastically different ways of getting there.
bjkadron
Well-Known Member
Also almost forgot. I found another software that might work. The only problem is I don't find any user feedback from it which is pretty weird. Speed-Wiz suspension calculations
But also at that price it would almost be worth risking it.
EDIT: It looks like he may have fractional versions of it on google play. I have a giftcard so I may get the suspension one and mess around.
But also at that price it would almost be worth risking it.
EDIT: It looks like he may have fractional versions of it on google play. I have a giftcard so I may get the suspension one and mess around.
DionR
Well-Known Member
Not sure I understand what you are asking for. Use the stock control arm locations but with a 2” dropped spindle and the 275/35R18 tire?
bjkadron
Well-Known Member
I was just saying when you calculated the stock one you raised the ride height and the LCA is pointing down instead of level. I was just saying level it and go to the 275/35R18 as an apples to apples comparison with the FMJ spindle and FMJ+raised UBJ
DionR
Well-Known Member
Ah, gotcha. Will do.
DionR
Well-Known Member
I revised the stock layout. I neglected to move the spindle in since the LCA moves in an arch and so I effectively lengthen the LCA about 1/8" on accident.
I also change the camber to 1/2 a degree positive since that more closely matches the stock alignment specs.
This changed the RC from 6.87" to 6.95" so not much.
I also change the camber to 1/2 a degree positive since that more closely matches the stock alignment specs.
This changed the RC from 6.87" to 6.95" so not much.
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