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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.

I have a hard time reconciling this theoretical analysis of everything that goes wrong when lowering a MacPherson strut front end with the prevalence and racing success of lowered 911s, M3s, and FCs.

You can deal with low roll centers by upping the sway bar sizes. In the particular example car his concern was one design in front and a different design in the rear so the balance changed drastically. He has another video on what makes the BMW E39 is so frikin good.

I had a small win today and found some 308 suspension drawings....need to update my models

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.

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!

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.

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.

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.

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.

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