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TurnerX19
TurnerX19 UberDork
5/15/24 3:23 p.m.

I would design this around some sturdy common ball joint from a big street car or the circle track catalog market, not an open sperical. There are many options.

mke
mke Dork
5/15/24 3:42 p.m.

In reply to Keith Tanner :

No, not translate along the axis, I mean translate through or away from the axis.  With a rubber bush you can pull and push toward or away from the axis and get movement.....that's part of why they are bad, but in my rear suspension to set toe, you use shims to twist on the lower control arm, which twist on the carrier, which in turn twists on the upper control arm and the flex bushings allow it to move.  Without the rubber it would bind, bend, or crack something....at least I think that is what would happen.

TurnerX19
TurnerX19 UberDork
5/15/24 3:49 p.m.

In reply to mke :

Yes it would fail. The tabs on the chassis are usually the first to show cracks at the welds where I have seen this sort of failure achieved by inadequit engineering.

mke
mke Dork
5/15/24 3:49 p.m.

In reply to TurnerX19 :

I have some room as it was designed in the 70s for 14" wheels and I'm running 18s  now.  I'll have a look at ball joint option, honestly including  just using a 308 front joint so it's readily identifiable in the future..heck maybe basically copy the front stuff to the rear and give it a tow link

mke
mke Dork
5/15/24 3:57 p.m.
TurnerX19 said:

In reply to mke :

Yes it would fail. The tabs on the chassis are usually the first to show cracks at the welds where I have seen this sort of failure achieved by inadequit engineering.

And the stock forks are know to crack so that checks out 

mke
mke Dork
5/15/24 5:48 p.m.

Oh looky what I found, 1976 308GTB  group4 drawings

 

TurnerX19
TurnerX19 UberDork
5/15/24 6:07 p.m.

Wow! A drawing I can read.... Side view of the upright?  I think the 308 front is not up to the loads possible in back. With the tight clearance to the brake disc and possibly the spring point, I would aim for the largest screw into the arm type for circle track. more details later.

mke
mke Dork
5/15/24 8:12 p.m.
TurnerX19 said:

Wow! A drawing I can read.... Side view of the upright?  I think the 308 front is not up to the loads possible in back. .

Yes, showing all the pivots and each with at least 1 dimension marked so right now I'm printing them and will scale everything out and correct my model.  I know I got close but I have the front kingpin angle at 10, correct is 9.5, stuff like that.

Rigante
Rigante Reader
5/16/24 6:20 a.m.

if you have bigger diameter wheels, spacing the lower ball joint down  at the knuckle would raise the roll centre a little

mke
mke Dork
5/16/24 10:24 a.m.

My CAD models are revised and now match the drawings pretty well I think . Stock roll centers look like F25, R 103mm, low the car 30mm which is about what I did and you have F-33 and rear 53mm....so the roll centers drop about double the amount you lower the car.  Yuck.

TurnerX19
TurnerX19 UberDork
5/16/24 1:48 p.m.

The front doesn't bother me near so much as the rear. Since the outer upper is the spot to modify for angularity, it should also be possible to lower the upper outer pickup on the upright and retrieve some of the roll center. Easier than raising the inner.

mke
mke Dork
5/16/24 3:10 p.m.

I made a picture

 

And also dusted off my braking spread sheet and added an acceleration tab and then used those numbers with what the spring spreadsheet spits and got some numbers, assuming I'm buying sticky tires.

At full acceleration at the axles

Front  lift - 0.6"

Rear squat - 0.45"

Full braking

Front drive = 0.86"

Rear lift = 0.65"

So total movement at the axles is 2.56" and total at the bumpers is 4.6".  It looks and feels like its moving a lot because it frikin is!

 

mke
mke Dork
5/16/24 3:43 p.m.

Design wise I'm still thinking about what I want to do.  On acceleration, anti-squat means the rear doesn't drop, it doesn't mean the front doesn't lift.  Good tires should get me a .8 or maybe .9g acceleration and that will move about 500lbs from the front the the rear.  perfect anti-squat will push up in the rear by that 500lbs and the rear is stable, but up front 500lbs has come off the axle and that is over 1/3 what was there, and the springs will react to that and lift it about 0.6".  Anti-forces are extra forces and they move the CG up or down, in this case up.  I'm not so sure I care on accel, I can't think of a good reason why I should?

 

When the brakes go on, about 740lbs shifts rear to front.  this time though, because both the front and rear are active in braking, to have anti-drive in the front and anti- lift in the rear.  The problem is though that in the rear the the available anti-lift is already set as the opposite of the anti-squat and is what it is and yields about 250lbs anti-lift not 740.  If I put the full 740  anti drive in the front, there is a 500lb mismatch and the GC goes up coming into a corner, assuming that is why I was braking.  The rear is going to lift .42", nothing I can do about that.  The front wants to drive 0.86", killing 1/3 of that holds the CG constant but I'm kind of leaning toward killing 2/3 of it to kind of split the difference.  Not sure really..........and remember the numbers at the bumpers are nearly double.

mke
mke Dork
5/16/24 4:11 p.m.
TurnerX19 said:

The front doesn't bother me near so much as the rear. Since the outer upper is the spot to modify for angularity, it should also be possible to lower the upper outer pickup on the upright and retrieve some of the roll center. Easier than raising the inner.

So the rear doesn't really scare me at all.  The lower inner are near the bottom of the frame, if I just raise them to be near the top of the frame rail, I think the roll center is basically right where it started.  

The anti squat....I could just rotate the hole assembly the 10.5 deg and call it done.  It does mean the wheel base changes slightly with suspension travel...but not a lot.  I may lay it out and see what it looks like before making more aggressive plans.

TurnerX19
TurnerX19 UberDork
5/16/24 5:38 p.m.

You are correct to moderate the amount. The more anti -squat, the more road irregularities feed into brake modulation. On a personal basis I like a lot in back, but too much is easy to find in front, especially when grip levels are a little inconsistent.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
5/16/24 6:33 p.m.

In reply to mke :

Anti squat in an IRS suspension IS the wheelbase changing, because torque loads from the drivetrain do not feed into the suspension.

This can be mitigated under braking, if you so desire, by changing the angle of the upper arm, since brake torque DOES feed into the suspension. 

If you want to get fancy with your antis.

mke
mke Dork
5/16/24 7:32 p.m.

A lot of line so a little hard to see but the rear is twisted the 10.5deg for 100% anti-squat, which is about 33% anti-lift on braking. Also the lower mounts are moved to correct the roll center

Up front the upper control arm stays put and the lower rotates almost 7 degrees and that puts the instant center on the 66% anti-drive line....but I've not yet sorted the front roll center.  My basic plan is move the inner low mounts up to sort, fix my anti-dive angle accordingly and then just kind of hope I can get the rack moved to a happy place.  Normally hope is not a plan, but I think I'm willing to make an exception just this once.

 

mke
mke Dork
5/16/24 7:40 p.m.

In reply to Pete. (l33t FS) :

Well of course I want to get fancy with it!

With IRS the drive force acts at the axle not the contact patch so the instant center line goes through the axle not to the contact patch as it would with roll centers.  For me though its different again because I have parallel A-arm that don't really have an instant center...they do but its at infinity so I just make them parallel to the desired force line and that's that I think is how it works.

TurnerX19
TurnerX19 UberDork
5/16/24 8:05 p.m.

The rear drawing above the arms are neither parallel or equal length, so the roll center is not infinity. Therefore they have a very mobile instant link center. For structural reasons move the top if possible. Load offsets on lower inner pickups are death to structures.

mke
mke Dork
5/16/24 8:21 p.m.

In reply to TurnerX19 :

Neither parallel no equal in the front/rear view so the instant center/roll center stuff applies.

In the side view, they are parallel so for, I guess its not roll center but is pitch center?, its a different story and they is what I was on about in an apparently very unclear way. sad

Moving the top up is a bit problematic because right now they are pushing straight into a frame member, if I raise them they are twisting on the frame member.  On the 328 ferrari raised both the mounts and the frame.  Maybe when I start really laying out the new twisted locations I can do something to get them a little higher without them looking scary.

TurnerX19
TurnerX19 UberDork
5/16/24 8:32 p.m.

In reply to mke :

Remember the loads on the uppers are far lower. Also, since the outer end fits in a milling machine, and the chassis doesn't, at least none we have...The same pitch angularity can be achieved on the outer with a far smaller total move, too.

mke
mke Dork
5/17/24 8:22 a.m.

Public service announcement. 

Over the past week I have seen quite a few articles, (to be fair old) books and videos showing the anti-dive point lines drawn to the CG, and it could be the CG, but that is a special case.  In my mid-engine car its about  where max braking is producing a 1g deceleration, I didn't play around to see if its the 1g point for all setups but it probably is I guess.  40-50 years ago 1g was all you could get from slicks so you see it inold  race books and today good street tire are 1g so its a useful point.....but 200tw DOTs are generally more, maybe 1.2g? and slicks maybe 1.5g?.  The point to use is the % total braking force X the wheelbase, on a line parallel to the ground at CG height.  

In my case I used 1.2g and got 66% on the front axle (from the spread sheet I made to set the brakes up), 0.66 x 2340 =1544 and that point is marked on the diagram I posted earlier.  The anti-squat line connects to a point directly above the front axle because 100% of the torque is from the rear axle, and that is always true for rear drive, tires don't matter.  If its awd then the torque split % times the wheel base just line for braking.

Basically nobody uses 100% anti-dive, they set up for some lesser % so they don't actually get a nose that lifts when they use the CG instead of the correct point but I'm pretty sure they don't have the % anti-dive they maybe thought they did.

mke
mke Dork
5/17/24 8:59 a.m.
Keith Tanner said:

In reply to mke :

 The ones I used are spherical bearings so they can flex off-axis for alignment adjustment.

Keith,

When you say spherical bearing I assume you are talking about something like this, or have you got somthing cooler?

mke
mke Dork
5/17/24 4:14 p.m.

Thinking about my weak stub axles and wondering if I could use something like this as an blank for a stronger version

 

Ian F (Forum Supporter)
Ian F (Forum Supporter) MegaDork
5/17/24 4:32 p.m.

In reply to mke :

Whoops... looks like I procrastinated too long and the autocross at Franklin Mills sold out.  Have fun!  Thinking about it did give me the itch, so I may sign up for the next event at Warminster. 

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