mke
mke Dork
5/8/24 7:46 p.m.

In reply to TurnerX19 :

very wise.

...but have you read this thread?   devil  Plus the replacement poly bushings set is $400!!!!  I can do a lot of damage with with a $400 budget. cheeky

mke
mke Dork
5/9/24 11:16 a.m.

well this won't make the CAD model easy.  I need to find a copy I can read or I need to spend a day measuring.

 

 

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) MegaDork
5/9/24 11:20 a.m.

In reply to mke :

SHIP IT!!!

mke
mke Dork
5/10/24 6:41 p.m.

I was trying to find some suspension software but really haven't so back to CAD modeling.  I've also been reading a bit and and best I can tell building anti-roll into the suspension is generally frowned upon because it produces "jacking" which raises the GC.  I am pretty sure though that with the stiffer suspension increasing the camber gain is a good idea especially since the plan is sticky 200tx tires that like camber from what I read.  

This bit is kind of interesting.  The unreadable chassis drawing was a mondial, I should probably throw that book away as I keep pulling wrong info from it....but I noticed it had a little anti-drive in the front with the front A-arm at 7deg.  Today i noticed the 328 is the same

308 is straight

328 and mondial 7deg

So it looks like ferrari added a bit of anti-dirve on the newer cars, likely a result of modern tires getting stickier.  My original idea was just mess with the bolt on lower A-arm mounts but I'm realizing that is probably a bad idea as it will probably add too much and jacking will become an issue which is why ferrari used the top not the easier bottom.  The anti-drive force is an extra force so in normal use its just the springs holding up the car, but on braking there would be the additional anti-dive force+ the spring force which results in the car CG lifting.  That can be somewhat compensated for in the back as any anti-squat for acceleration becomes pro-squat on the brakes so with some math the car can stay relatively flat and steady height I guess.  

In the rear both cars (308/328) A-arms are parallel to the ground, so no anti-squat...likely because the cars just didn't have the hp to need it sad  .....which of course is not my situation cheeky

Last, a work buddy has a buddy with corner scales so there is a vague plan to set the engine back in and try to get so weights to make the math a bit more accurate then my guesses.  We should have a pool for closest to actual weight  smiley

Oh, last last I'd never ever hear of tire spring rate.  I knew they flex and different tires ride different but I never really thought about including that in the spring rate calculations and don't recall seeing it motion ratio or spring sizing formulas.  I guess on most street cars the tire is stiff enough that it can be ignored without creating alot of error, but as spring rates climb, the tire rate starts to matter and is somewhere around 1500lb/in and in the rear of my car is about 1/3 total travel the way I currently have it sprung which seems too big to ignore.

mke
mke Dork
5/14/24 12:08 p.m.

This guy has a few pretty good suspension videos and in this one he explains why lowering the car, as I've done, makes a hot mess of the suspension surprise

 

Ian F (Forum Supporter)
Ian F (Forum Supporter) MegaDork
5/14/24 1:41 p.m.

In reply to mke :

It does seem to depend on the car. Years ago when I was more involved with the VW Mk IV world, there was a constant debate about lowering the car for looks vs. making the flawed suspension design work in performance situations.  Those with bigger brains than I generally said stiffer springs were good; lowering was bad; and due to the suspension geometry actually raising the car a smidge would improve the suspension curves.  I had a TDI and thus no real ball in the game, but it did make for some entertaining discussions. 

I'm surprised you were unable to find any modeling software. I know some of those others here have done some suspension modeling. 

Rigante
Rigante Reader
5/14/24 2:18 p.m.
mke
mke Dork
5/14/24 2:41 p.m.

In reply to Rigante :

Somebody else sent me that and I honestly wasn't sure what to make of it....I couldn't figure out what he was trying to do exactly.  It starts out talking about subframe issues and ends with what appeared to be custom everything sketched out?

I was looking at the software he used along with a couple other options looking for the right mix of saving me time while not costing a lot (as in next to no) of money.

mke
mke Dork
5/14/24 3:06 p.m.

In reply to Ian F (Forum Supporter) :

I think it very much does depend on the specific car and he says double A-arm is less a problem in general.  As suggested a few post ago, no doubt I can deal with whatever the changes roll center is causing  with swaybar selection. I would like to see if what's going on with camber changes now and if that can be improved without getting too stupid.

More importantly though is pitch.  Yesterday driving home in my hand-me-down 2014 GLK350 with 130k miles I was playing throttle on/off and low gear full throttle to full brake....man the nose of the car(truck?) hardly moves.  Same for my son's the supra (taking him to his first autoX on the 19th in philly).  The 308 though, full throttle to full brake I'll bet the nose moves 3", you can see it when I get off the throttle in the burnout video, its just not confidence inspiring and can't possibly be helping geometry into or out of corners any. 

I was not really realizing what it was I was not loving about the way the 308 feels but I'm pretty sure this is it....I've just gotten used to cars that don't pitch like this thing does.  When I first got it with a tired engine I don't recall throttle on/off having any affect on the car's pitch but that is SOOO no longer the case.  Fixing this means moving the mount points on the frame but I really think it will be worth the effort and I may be able to get the roll centers and camber gain better at the same time, but those are gravy I think.

TurnerX19
TurnerX19 UberDork
5/14/24 6:16 p.m.

From my experiences with 2 different 308GTBs on the same day at Bridgehampton when they were nearly new....They pitch too much. The improved car had a well thought out upgrade to everything without killing streetability, and it still pitched too much, although much less than the stock one. The pitching was my least favored part of the drives while instructing new time trialers. 308 GT4s do it far less stock, but most of that is the longer wheelbase.  I am completely onboard with you in principle. 

mke
mke Dork
5/14/24 10:10 p.m.

Some measuring and CAD time tonight and I think I have the geometry roughed in pretty well...and I guess unsurprisingly the roll centers are quite low with the front basically on the ground.  also of not maybe is the wide tires I put on the back mean the rear track is now narrower than the front be quite a bit. I still need to do the side model but that should be quick now.

 

Ian F (Forum Supporter)
Ian F (Forum Supporter) MegaDork
5/15/24 7:30 a.m.

In reply to mke :

Autocross at Franklin Mills? I was debating on doing that event. I can drive to that location and my car will barely be warmed up. 

mke
mke Dork
5/15/24 8:16 a.m.

In reply to Ian F (Forum Supporter) :

That's the one and he's pretty excited.  Mama is protecting his tires from me so I can't drive but will be there in a support role not far from a yellow supra.

TurnerX19
TurnerX19 UberDork
5/15/24 10:16 a.m.

Course walk, course walk, course walk...

Keith Tanner
Keith Tanner MegaDork
5/15/24 10:43 a.m.

About spring rates - you're thinking about tire spring rate, but don't forget about bushings. Rubber bushings rely on deformation of the rubber to allow for suspension movement, which means they basically act as springs in both directions, trying to return the suspension to the static rest position. It's a surprisingly significant contribution, the first thing we ask people when they call up complaining that our springs are sitting at the wrong ride height is if they reset the bushings so they're not loaded when the car is at rest. This is particularly true of NC and ND Miatas that have multilink suspensions in the rear (and thus more bushings).

I recently installed bearings in the suspension of my track Miata to replace the bushings and the drop in effective spring rate required a fairly substantial adjustment of the shocks because the car was immediately badly overdamped. I may have to go to stiffer springs.

Just something to think about if you're getting that deep into the rabbit hole :) And this setup led to a suspension that was clearly free of slop, although hard impacts like potholes or expansion joints hit harder. I'm quite happy with them after street testing, track testing will likely come this weekend.

mke
mke Dork
5/15/24 11:23 a.m.

In reply to Keith Tanner :

Thanks.  I found a video yesterday that I was kind of only 1/2 watching until at about 4min he mentions bushings, then at about 7:17 he spends some time on them and suggests a good guess is around 9N/mm for the effective spring rate. 

 

My bushings are so old and rotted I'm not sure if they are rock or all cracked to mush but all this started because I know I need to replace them...scope creep.

The plan is something similar to what you did....bearings, or rod ends, or maybe delrin or bronze bushing.  Poly bushing for this application are about $25 each, stock rubber about $20 each, I'm told there are off the shelf bronze bushings for $3 each.  The concern I have on this path is the rear A-arms connect to the wheel carriers with 2 bearings each so no flex there, then 2 rubber bushes each on the inside.  The upper  bolts fast and lowers bolt to forks that can be shimmed to at camber and toe.  Camber is fine, but setting toe is only possible because the bushings can be flexed so this will need a solution.

This also means the A-arms must be parallel  in the side view, so if I want to do anti-squat, all 4 inner mounts need to be relocated to keep thing moving freely and the wheelbase as-is.

My only concern in front is bump steer, the rack is hard to move.  I could make new steering arms for the knuckle so possible to sort, just a pain.

Keith Tanner
Keith Tanner MegaDork
5/15/24 11:39 a.m.

In reply to mke :

The bushing spring rate is going to vary quite a bit from platform to platform - like I said, it's a larger factor in NC and ND Miatas than it is for NA and NB even though they're quite similar cars.

 The ones I used are spherical bearings so they can flex off-axis for alignment adjustment. That's necessary for about 50% of the bushings in an NA Miata suspension.

Bump steer is going to be the fun one to manage if you're redesigning your pickup points and you are constrained by the rack location. 

TurnerX19
TurnerX19 UberDork
5/15/24 11:41 a.m.

In reply to Keith Tanner :

Rubber bushings contribute positive bump damping, but negative rebound damping..

Keith Tanner
Keith Tanner MegaDork
5/15/24 11:49 a.m.

In reply to TurnerX19 :

That's what I meant by "they basically act as springs in both directions, trying to return the suspension to the static rest position". Although I'd say they're more springs than dampers. I had to decrease my shock rebound damping when I removed the rubber.

mke
mke Dork
5/15/24 12:33 p.m.
Keith Tanner said:

In reply to mke :

The bushing spring rate is going to vary quite a bit from platform to platform - like I said, it's a larger factor in NC and ND Miatas than it is for NA and NB even though they're quite similar cars.

 The ones I used are spherical bearings so they can flex off-axis for alignment adjustment. That's necessary for about 50% of the bushings in an NA Miata suspension.

Bump steer is going to be the fun one to manage if you're redesigning your pickup points and you are constrained by the rack location. 

I'm sure there are big differences, this was literally the first number I've ever heard.

I maybe not describing the issue in the rear very well so here's a diagram 

On the outside there are 2 bolts, 4 bearings (not bushings) so  2 axes that must be parallel, and that requirement translates to the chassis mounts.  The rubber lets them cheat a little because the points can translate a little, so the toe is a adjustable a little.  Any kind of bearing that prevents translation makes the toe fixed, unless I make a 3rd frame point adjustable or go to something like a ball joint top or bottom on the outside.

One or the ferrari parts houses sells now lower A-arm mounts front and rear to correct the roll centers after lowering.  It the back no problem, but I sure wondered what they do to bump in front?  I don't know anyone who has them to ask.  Maybe I can move the rack....maybe.

Keith Tanner
Keith Tanner MegaDork
5/15/24 12:47 p.m.

That's fundamentally the design on an NA/NB Miata rear suspension, other than the way it adjusts the position of the lower points - the Miatas use eccentric bolts instead of those shimmed clevises. The end result is the same. Spherical bearings allow the sort of movement you need. A bronze bushing won't, I agree that would be a problem for you.

TurnerX19
TurnerX19 UberDork
5/15/24 12:47 p.m.

In reply to mke :

In any of these A-arm cars with the rack part way up the kingpin axis the vertical rack move is super sensitive. Many cars I've bump steered change the sweet spot of the curve 3 inches with .025" vertical rack move. The shorter the absolute link center is the more dramatic the effect. Your's is pretty short. I think your contemplated anti-squat increase can be accomplished with minor enough effect that I'd try it. Steel steering arms can be adjusted with force safely too. Bolt on ones duplicate too, looking at the mill.

mke
mke Dork
5/15/24 2:09 p.m.
Keith Tanner said:

 Spherical bearings allow the sort of movement you need

They do not.  They allow off axis rotation, this set needs the transition a compliant bushing allows.  I've been leaning pretty strongly toward toward replacing the 2 upper-outer bearings with a single spherical rod end.  The A-arms would still need Tobe parallel but the toe setting issue gets fixed.  Or I make a 3rd chassis point adjustable.  I need to layout options I guess

Keith Tanner
Keith Tanner MegaDork
5/15/24 2:25 p.m.

I realize what you mean, axial deflection. The way that's handled in my current setup is that the bearings are locked into the control arms with a set screw. If you need for them to move sideways, you can always unlock them for that. In practice, in the Miata application with basically the same geometry as yours, there appears to be no difficulty in adjusting alignment once they're locked into place. I did the alignment on my car after the install and there was no indication of a problem.

TurnerX19
TurnerX19 UberDork
5/15/24 3:20 p.m.

In reply to Keith Tanner :

I think a little side load produced by this is probably a good thing, it keeps the joints quieter, less oscillating loads, but too much will crack an arm or chassis bracket. Hard to judge where that limit is, it varies with bump travel too if you introduce anti squat. The single upper spherical is the answer if there is room, or easy upright mods.

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