Revised suspension geometry, anti-lift/dive discussion

[shadow88]

[QUOTE=Shadow;785522]Not when those K cars are looking to run 9's one wknd, then win the local road race the next! QUOTE]

I forgot the smiley face at the end. Best of luck revising the geometry on these things.

[vipernbox]

Put the k frame at the back.

[Shadow]

Put the k frame at the back.

That would only work if you moved the P/T back there as well.

[vipernbox]

[Shadow]

:P Guess I should have seen that coming! lol

[vipernbox]

Conveniently enough though. The same changes you make to a k car fwd to give it a little pro lift turn into anti squat if you move the setup to the rear.

[zin]

I seem to recall from that book on the inner works of the Shelby race teams that they modified the pivot points of the arms to achieve better handling, compared to stock, I'm guessing now that they modified for anti-dive/pro lift... But how much?... That's the $64K question...

I expect that once the suspension geometry is "fixed", the roll bar and spring rates would need to be adapted. I will disagree with never using a anti roll bar, many race cars use adjustable units to fine tune the car to the track, but I will agree with the spirit of the statement, which is don't use a roll bar as a band-aid for a F'd up suspension!

Mike

[turbo2point2]

Nice thread guys. I ran across another link I had: http://www.modified.com/tech/0508_sc...t_3/index.html http://www.modified.com/tech/0512_sc...t_4/index.html if nothing else, more pics and diagrams.

DJ, nice illustrations. I wanted to add that when calculating anti-dive, the svsa will travel from where the tire contacts the ground to the IC. The point it meets the CG, will determine percentage of anti-dive, lift. This is why/how we can end up with 2 different numbers for lift and dive.

[Reaper1]

YAY!! I finally did something useful ont he forums!! LOL ;P

[shackwrrr]

Nice thread guys. I ran across another link I had: http://www.modified.com/tech/0508_sc...t_3/index.html http://www.modified.com/tech/0512_sc...t_4/index.html if nothing else, more pics and diagrams.

DJ, nice illustrations. I wanted to add that when calculating anti-dive, the svsa will travel from where the tire contacts the ground to the IC. The point it meets the CG, will determine percentage of anti-dive, lift. This is why/how we can end up with 2 different numbers for lift and dive.

Thanks for those. Ive learned alot of that stuff in school but it was all centered around circle track, and I never learned how to do the instant center on a strut.

[vipernbox]

Alright.. here is my new dumb question... probably a little silly simply because my stupid angle finder was acting weird and I don't have numbers to spit out.. So instead here is the mess





.



Now just for visual reference that red pipe in the background is perfectly plumb... and you can see the general angle of the rod running through the control arm mounting points..


This is a totally "pro squat" setup... Not really what I a looking for at all.... I am thinking I want to start with maybe a degree or two the other direction.. This would be a very aggressive "pro- lift" setup in a FWD...



Now... Right now I have already tweaked little bit of that out by rotating the entire subframe up in the front a couple degrees..


So the next move it what? tubular K frame? Or?

Anybody see any problems with "adjusting" this. I always thought this point looked a little rickety anyway....







Now I think it goes without saying that moving that mounting point up roughly a inch is going to change a number of things... I should have plenty of caster..



What am I about to do horribly wrong? I guess it is possible that leaving it as it is may actually help under braking Should I just forget about it... and see what it does... Or keep drilling grinding and welding?

[Reaper1]

I think you need to test fit the axles, specifically the driver's side one, in the car before making anything permanent. To me, this looked to be the tightest area of fitment.

[vipernbox]

Rough measurements lead me to beleive it is going to make things a bit tighter for sure.. maybe to the point where pulling that the drivers side axle might be a pain.. But for sure good catch.. thanks for looking out.... The "easier way I have also considered changing the angle of the K frame is by simply dropping the back. I don't think that is quite as helpful as raising the front though.. Raising the front independent of the rear and the strut is also going to help a bit moving the roll center up, and getting the rear roll center far enough about the front one without making the front roll center to low is already a challenge.


I understand the realtionship b/t the forces in question and the LCA... (at least I think I do) but the strut plays a part in all of this also and I don't really understand how exactly... That is what I am missing...


Soaking up everything I can about the Cayman and boxster suspension setups... and should have a couple books at the library..

[vipernbox]

Sorry if i am overloading the thread with useless info.. I am bad about thinking aloud...


but this is an interesting kinda related discussion at an interesting forum


http://forums.autosport.com/index.ph...formancetrends

And if you haven't ever dropped in here... and suspension geometry interests you.. These guy at least sound smurt..


http://www.eng-tips.com/threadminder.cfm?pid=800&page=1




And still reading


http://www.auto-ware.com/ortiz/Chass...07_revised.htm

[vipernbox]

Is this right? (sorry for the highjack please move me if need be)


I have to move that K frame a long way and get the the strut perpendicular..



http://images.fieroforum.com/2011/Anti-Squat_PFF.JPG


Anti-Squat
Finally, the last characteristic to be studied is anti-squat. Squat is the tendency for the rear of the car to lower as the weight transfers rearward under acceleration. Anti-squat is a suspension characteristic that opposes this tendency, increases rear tire loading, cornering power, and traction available for acceleration. With a live axle, the suspension can be designed to provide 100% anti-squat, but unfortunately for us, the use of an independent rear end limits the amount of anti-squat to around 25%.

Calculating anti-squat for an independent rear end is one area that Herb Adams book didn’t cover very well in my opinion. After a lot of internet searching, I finally found a formula that allowed me to calculate it for a Chapman strut rear end. It involved determining the longitudinal instant center (IC) for the rear suspension by drawing a line (green dashed) through the rear trailing arm angle and extending it forward until it met up with another line (blue) drawn perpendicular to the strut centerline, at the upper strut bushing. Once the IC was located, a third line (red) was drawn between the center of the axle to the IC. The angle of this red line to horizontal (7.85 degrees) was then used in a formula that included the height (H) and distance (L) between the car’s CG and the rear wheel centerline. (I apologize for the small scale of the drawing but PiP has a 1020 pixel limit on the width of pictures)




From the calculation on the drawing, you can see that the ’88 Fiero appears to have about 28% anti-squat. That’s the approximate limit many sources said is practical for an independent rear, but as some of you might have already figured out, increasing the angle of the red line would result in even greater anti-squat, so I explored that a little.

The only way to increase the angle of the red line is to shorten the longitudinal swing arm by either increasing the angle of the trailing link, lowering the top of the strut, or some combination of the two. Shortening the strut would give less suspension travel so you would need stiffer springs to prevent the car from bottoming out on the struts. The alternative, ie raising the angle on the trailing links seems more feasible, but the trailing link is the component that transfers the forward accelerative forces to the frame. The more angled the trailing link is to the ground, the larger the vertical component of the acceleration force becomes. At some point, the angled trailing link will cease transferring the accelerative forces forward into the frame and instead cause the wheel to try to walk under it, stressing the lateral links in the process. According to Herb, too short a longitudinal swing arm will also result in rear axle hop during braking, but he doesn’t quantify this in any sense.

[vipernbox]

Alright. After running a tape measure and an checking the angles of everything I could measure under a Boxster and a 88 Fiero.... I think I figured it out. So now I can quit trying to highjack a FWD geometry thread and get back to business...



It wasn't making sense before simply because the geometry as it was engineered in the FWD setup is so far off what at least I think I need to use this same stuff in a RWD setup.



Basically if I am understanding everything correctly if I left it the way it is I would have figuratively an 86 Fiero when I am looking for a 88. (they moved everything the same direction I am planning to move things, Revised "tie rod/toe rod" location to correct bumpsteer issues... and of course added the "forward" strut rod) Sometime down the road I may add the forward strut rod like the MR2, 88 fiero, and Boxster............. but for now I figure the least I can do is get things pointed in the right direction.







Right now the angle of the LCA slopes towards the ground 3.2 ish degrees... (Gee that is weird, where have I seen that number before ) and what I am pretty sure what will at least get me in the ballpark is about 2 degrees the other direction. (combined with around a degree or maybe just less of 'caster') I am not really looking for a lot of anti squat.. I just don't want "pro squat"...



Have any of you FWD suspension geometry messer around withers tried changing it that far for that much "pro lift" yet????







So now the fun part continues on... More drilling, welding and grinding..


Then making sure I haven't totally jacked the camber curve.







And Sorry to thread jack.. These posts should have been in my build thread I guess...

[Reaper1]

It's all good. This thread is supposed to be for this type of investigation. I'm still reading some of your links, so I haven't been commenting!

[vipernbox]

It's all good. This thread is supposed to be for this type of investigation. I'm still reading some of your links, so I haven't been commenting!

Oh good... Then get stuck here for a while..


http://lek.holy.se/mark_ortiz_2000-2007.pdf

[zin]

Oh good... Then get stuck here for a while..


http://lek.holy.se/mark_ortiz_2000-2007.pdf

That's some good stuff!! A little specific in places, but for a tech geek like me, it's right up my alley!

Thanks!

Mike

[Reaper1]

Yup..me too! !