Move upper arm mount rearward to allow caster adjustment
- Thread starter Woody500
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Wow. Yall think in too much work thoughts. IF I was going to do anything, I would relocate the front A arm brackets on each side out a little and the rear brackets on each side in a little. The same amounts, of course. You could gain a ton if you did that. Much simpler than moving k frames and such. You'd be doing what the offset bushings do, only changing the chassis instead of the bushings.
DionR
Well-Known Member
You're right. I guess I was assuming you were going front steer, and I didn't even realize it.
The issue i see is that the bushes will be under more strain. the bolts are parallel but when you adjust the offset bolts it has to put an angle in the bush for the top arm to twist to move the ball joint position. Fine if your bushes are soft rubber but not fine when the bushes are a harder material. I have QA1 top arms with quite hard bushes.
plus i moved the K frame for additional reasons as well so for me it makes more sense to do what i did.
No, not if you move BOTH pairs of brackets like I described.
the bushes are only happy in one position and that is with the bolts being completely in alignment with each other, they tolerate the bolts being out of alignment for adjustment purposes. If you were to look through one bush it should perfectly align with the second. You cannot do this with the offset bushes, they will be parallel but offset. To do what you describe and to get perfect bolt alignment you would need standard bushes in the arms and to move the front most part of the front bracket a little more than the rear most part of the front bracket and the rear most part of the rear bracket more than the front most part of the rear bracket, and all angled slightly.
imagine putting in steel sleeves instead of rubber bushes into the arms. The bolts would have to perfectly align to each other for the arm to be able to swing but as soon as you try the adjustment by moving the position of the bolts in the chassis the bolts will not be in alignment with each other. yes you could fit offset steel bushes and have offset bolt alignment but the arm will be fixed in one position as the only way it'll then move is to bend something.
I have spent many hours fabricating, fitting and aligning bronze bushes for a 427 Competition Cobra's suspension arms.
I know that. You're not getting what I'm saying. I'm not talking about misaligning the brackets. That'd be stupid. I'm talking about moving them and keeping them on the same plane. Easy peasy compared to all the other stuff talked about here.
you said "I would relocate the front A arm brackets on each side out a little and the rear brackets on each side in a little" you would be better just to move them both rearwards on the chassis.
get two pencils and align them end to end, then roll one forward or rearward slightly, that's what the adjusters do to the bolts in the chassis. the offset bushes were just meant to alter camber and not to be misaligned to gain caster, but that is what they get used for. (i know you know this)
so with anything other than the bolts (pencils) being in alignment the rubber bushes will have to have an angle in them for the arm to move the ball joint more rearward. Fine if they are soft rubber but not so fine with a harder material.
and i actually beg to differ that it'll be easy just to move the position of the top arm mounts.
I know what I said. If you cannot understand I cannot help you. What I am talking about will result in zero binding, but give the same exact result as the offset bushings. You can beg to differ all you want. You're wrong. Keeping both bushing centers on the same plane and relocating the brackets would be a simple job. No binding, but just more caster. All that said, that's not how I would go about it anyway. I'd just get aftermarket control arms. Trust me. I understand the geometry. I did my first alignment in either late 1974 or early 1975.
offset bushings will bind though, how can they not, i think your confusing rubber compliance with zero binding.
as i said, change the rubber for anything harder and you have to have the bolts perfectly aligned. don't forget the bolts when adjusted stay in parallel with the chassis whereas the arm doesn't. Spherical joints would be the only way for zero binding with extreme adjustments.
but i'm also curios how cutting out the top arm mounts 'easily' and accurately then welding them back in 'easily' and accurately is actually easier than welding four holes shut and redrilling them in different positions?
No. I'm not. But obviously I cannot get through. I never said anything about using offset bushings. I'm glad you work on Cobras. Those are cool cars, but that doesn't mean that you know more and are better than anyone else. Might keep that in mind.
3forty
"Raw Power"
You did your first alignment at the age of 9 or 10 years old,how did that work out?
Correct. With an old seasoned mechanic over my shoulder it went fine.
jbc426
Well-Known Member
Was this just after the Model T rolled out, Rusty?
lolWas this just after the Model T rolled out, Rusty?
did they even have gasoline powered cars back then or was it mostly steam?
projecting much.
who said i know more, i just don't agree with your thought on moving the top arm mounts the way you describe and not causing issues which would be the same as using wild offset bushings. it's a compromise that'll work with soft rubber but whether it'll be OK for long term use time would tell.
I'm not using any rubber or poly bushings in my lower front suspension so there will be zero compliance but there will be zero binding, plus i'll have adjustability. The QA1 upper arms have a hard poly bush so the offset bolts will be adjusters for mainly camber and possibly a tiny amount of Caster, i may change those bushes for bronze at some stage.
the OP's original question was about moving the top arm rearwards which would be better than your solution (in my opinion). but if the plan would be to move the top arm mounts then i would also dial out most of the ant-dive by lowering the front mount, which would also lower the roll center, and then run stiffer torsion bars. With the anti-dive top arm location at quite an angle for the soft suspension the Caster change is quite dramatic even at slight ride height changes. a lot of vehicles have anti-dive built in but not as dramatic. 1 inch rearward would be too much unless there was various spacers fabricated to fine tune the fore and aft positions without then altering the Camber.
it's all good fun.
Woody500
Well-Known Member
I don't really understand anti-dive , so I'll have to dig out my Carroll Smith books(Engineer to Win, etc) & study.
the geometry of the top arm is such that under braking it'll pull the front up due to the braking torque reacting on the arms which'll counteract the dive that would otherwise occur.
changing the roll center (lowering the top arm) so it points down from the ball joint means that under cornering the reaction of the torque from the bottom of the wheel pulls on the top arm pulling the body upwards. it will also give more -ve camber in bump.
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going back to caster change, when i measured bump steer on my '66 to see what the suspension does as it moves i found it changed Caster by 9 degrees from full droop to full bump and about 4 degrees over about 2 inches of suspension movement around ride height. (this was on old rubber bushes)
and this is what i found on bump steer
i think bump steer is actually built in to cause the steering to understeer as the car leans into a corner making it safer for the ordinary driver.
just dropping the K frame by 1/4 inch reduced bump steer in a straight line but i also tested it on a slight lock and bump steer is back.
UK roads are just awful for pot holes so for me dialing out bump steer and gaining more caster at a normal ride height is the goal.
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Woody500
Well-Known Member
Seems like you've induced an 'S' curve that looks more normal with 1/4" k-frame drop,. Explain 'slight lock'. And also, bump steer in chassis roll is affected then?
Setting that curve usually the best you can get.
Its all just compromising to get smallest amount of bump possible with a reasonable curve, right?
none of the measurements were done with the K frame moved forward. that idea was after i measured and got these results. The K frame drop reduced the 'S' curve, i could have probably dialed out the bump steer by leaving the K frame mounted to the chassis in it's original position and moving the steering box and idler arm.
the slight lock is just 1/2 turn on the steering wheel.
the steering box and idler arm are at an angle so turning the wheel lock to lock it effectively changes the height of the steering system in relation to the suspension. A steering rack is linear so doesn't have this effect on lock. i did all the testing with the original steering system, not a rack, and the reason two of the curves became a 'C' shape is because the steering link was shortened to the length that would be required to fit a steering rack (which would be too short to be ideal).
i don't have final measurements after the modifications yet but made sure i'll have enough adjustment to dial out bump steer once the suspension has the desired Caster and ride height set. Having done bump steer on many cars it doesn't take much, in as in as a little as a few thousandths of an inch, to raise or lower a steering rack to affect bump steer.
because i'm using spherical joints and ball joints on the lower arms then accuracy is easier to achieve, if it was all rubber bushed still then with all the movement available in the bushes under load accuracy will be completely lost to the point of why even bother.
another thing that affects Caster is vehicle rake. Raise the rear and you lose Caster on the front.
Woody500
Well-Known Member
Don't want rake. Baddest looking '65 I've seen was on this site; Sox & Martin Tribute with updated suspension , looked like little or no rake.
Its on '64-66 Barracuda race cars post your pictures' Thread on A-Bodies.
My goal low stance , good handling, & rarely run a 1/4 just to see what I got.
Woody500
Well-Known Member
Maybe someone can tell me how to EASILY post photos or transfer this one. & how about what icons are for @ top of this panel. Don't have a Clue!
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