Blooze Own: An F355 Six Speed N* Build Thread (Page 79/126)
northeastfiero SEP 05, 04:05 PM
Sorry I meant to speak to you about this issue earlier, I hit exactly the same issues as you though I have different width and offset wheels. I had to use 15mm concentric wheel spacers to get round this problem.
Though in my case because I don't have a specified body for the car so it does not matter if the wheels stick out a bit further. It is not a perfect solution but it was a solution.
Bloozberry SEP 05, 04:45 PM
Thanks for your input guys...

For RCR: I take it your comment was tongue-in-cheek... new wheels & tire$$$

For ccfiero350: The thought had occurred to me to flip the knuckles side to side but that then relocates two of the three outboard pivot points behind the wheel centerline... I'm not sure what the impact would be, but it's worth looking into further if nothing simpler comes up.

For Will: Thanks for the link to the Honda system... it's amazing how many similarities there are as you said. As for the rest, I understand some of what you've said but not all of it:

quote
Originally posted by Will:
B) Having the upper links AND lower links set the toe complicates adjustment. Are you sure that the upper link can affect toe in jounce while the lower links remain rigid?



You're right, the toe can't change with lower links being rigid. I misspoke since I had an earlier configuration in mind where I had planned to use poly mounts in the lowers, but now I plan to use spherical rod ends which don't have any compliance.


quote
Originally posted by Will:
D) With the two upper links parallel, the knuckle will be free to flop fore and aft around the aft pivot of the lower trailing arm.



I agree now that I've had a chance to look at this option with a clearer mind.


quote
Originally posted by Will:
A) Lower the knuckle end of the problem link so that it bolts to the knuckle via the lower bolt instead of the upper. Can you keep sufficient anti-squat with that configuration?



If I lower only the outboard end of the problematic link, the system will bind as the two upper links try to scribe different, non-concentric arcs. I played around with some stick diagrams and the newly lowered forward link would become too long in jounce and too short in rebound as compared to the aft link, which would try to twist the knuckle in the steering axis but which would be resisted by the lower links. However, if I lower both the outboard and inboard pivot points of the forward link by an equal amount (in the range of 25mm), both links scribe nearly the same arc (within full jounce and rebound limits). I think that might do it, though I would like to examine the alternative a bit more ie, having the angled link at the back instead of the front.


quote
Originally posted by Will:
C) Having the upper link behind the axle as drawn will result in poor anti-squat at ride height and divergent (meaning it gets worse) anti-squat as the suspension compresses.



I'm not sure I understand why that is. My (limited) understanding of calculating anti-squat doesn't take into account the location of the pivot points relative to the centerline of the wheel. For example, in this case, wouldn't the anti-squat solely be a function of the height of the instant center, which is found by the intersection of the angle of the trailing link axis (angled upwards to the front) and the axis created by the upper inboard link mounts (horizontal)? I need to understand this because this configuration seems like a cleaner looking solution if it doesn't adversely affect anti-squat (or other parameters).
fieroguru SEP 05, 07:18 PM

quote
Originally posted by ccfiero350:

Have you thought about just swapping left and right knuckles? It would move the strut centerline back and give you more room for the upper fore link.




X2

The inboard lateral link brackets will need to be moved as well as the inboard upper link brackets and the trailing link would need to be longer as well, but those are all fairly minor changes that should have minimal impact to the geometry you have worked so hard to obtain.

Another option is to move the upper outboard pivots inboard about 25mm along the same angle as the upper a-arm. This will likely give you the needed clearance, while only shortening the upper link. It will have some impact on the geometry, but probably far less than lowering the upper outboard mount.

Or bolt the rod ends to the upright directly through the top upright hole, then angle the links as they go inboard (equally about the upright centerline) to get about 6-8" of separation between them by the time you get to the frame rail.
Sage SEP 05, 07:20 PM

quote
Originally posted byBloozberry:
Drawing all this stuff out was supposed to eliminate these kinds of errors!



EVERYTHING works on paper! .................but in the real world..somebody had to say for the first time somewhere: "Back to the drawing board."

You gotta have a plan...you had a plan. The best anybody can ever do is to have a plan and implement it to the best of their ability, make changes when and where necessarry, and stick to the plan till it proves itself unworkable...in which case, you then come up with a new plan! I don't think you'll have to go that route.

Wish I could shoot an equation to you or give you an altered drawing that'd fix it "just like that", but I'm no draftsman or engineer, just a backyard craftsman that likes to "tinker", so I can't really offer any constructive ideas, only moral support and the faith that you will work it out, as you have with anything/everything else you've ever done that had to be changed a little here or there. I'd almost bet that you'll figure out something that not only fixes the error, but improves the design in the long run.

Anyway, still enjoying seeing your work and how things are progressing.

HAGO!

Zac88GT SEP 05, 09:15 PM
What about changing from two upper links to a true A arm? Retain your existing inboard mount locations and the rear outboard location that clears.
Bloozberry SEP 05, 10:40 PM
These are all great suggestions (and a boost to my sour mood), so thanks Fieroguru and Zac. I also appreciate your words of kindness there Sage. You're always there giving positive comments and encouraging members to do their best.

I'm glad you chimed in there Zac because simply "looking" at what might be great solutions is only a first step... any decision is going to be best made with the support from another Lotus analysis to see the effects on the kinematics. Hopefully you'll be able to crunch a few more numbers for me once I get a new table (or two) of coordinates put together.

From a subjective point of view, I think the solutions with the best potential so far are flipping the knuckles side to side or flipping the arms. Fieroguru, your idea of moving the pivots inboard by 25 mm would increase camber gain and move the roll center around among other things so it's not ideal. Your third option to simply use the available misalignment range of the rod ends is also an interesting idea, though the particular ends I bought only have a 10 degree range. As for your idea Zac, I'm not sure how I would go about attaching an A arm to the top of the knuckle.

Flipping the knuckles has a few other advantages:
a. it keeps most of the geometry intact, though I'd like to get a simulation done to see actual changes (if any). Although Will hasn't commented specifically about this approach, he has led me to believe there may be hidden problems.
b. it moves the shock bell crank further backwards giving me more room for the front shock mount;
c. it moves the rear calipers closer to the cabin making it potentially easier to find cables that are long enough;
d. it would be dirt cheap to implement.

On another note, I finally got my rod ends delivered today... yay! At $26 a piece (before taxes, duty, shipping, and brokerage fees) there's a chunk of change in this photo:
FieroWannaBe SEP 05, 10:52 PM
Can the upper links share the same bolt, and the forward link utilize a high misalignment rod end?
Aurora (my rod end of choice) makes rod ends that can tolerate up 22 Deg misalignment.
McMaster-Carr sells units rated for up to 32 Deg.
http://www.mcmaster.com/#rod-ends/=odsjhp

EDIT: McMaster measures total angularity, unlike Aurora, whom measures off bolt axis angularity.

[This message has been edited by FieroWannaBe (edited 09-05-2013).]

Bloozberry SEP 06, 07:08 AM

quote
Originally posted by FieroWannaBe:
Can the upper links share the same bolt, and the forward link utilize a high misalignment rod end?



Thanks for your input FieroWannaBe. What you suggest is certainly a possibility, that's essentially what Fieroguru's last suggestion was. One problem with using high misalignment rod ends at their limits is that a greater component of the load acts axially though the rod end, essentially pushing the ball out of the side. Most companies only state the radial load capacity (along the axis of the pushrod) but a general rule of thumb is that a rod end's axial load capacity is about 10% of the radial load capacity. If I went this way, I'd have to be certain that the loads wouldn't be too great in the axial direction.
ccfiero350 SEP 06, 09:26 AM

quote
Originally posted by Bloozberry:








Another alternative is to swap locations of the upper a-arm tie in point to the lower OEM strut hole and move your vertical transfer link tie up a little. You will get more camber gain out of it to boot.

------------------
yellow 88 GT, not stock
white 88 notchie, 4 banger

Will SEP 06, 09:42 AM

quote
Originally posted by Bloozberry:
I'm not sure I understand why that is. My (limited) understanding of calculating anti-squat doesn't take into account the location of the pivot points relative to the centerline of the wheel. For example, in this case, wouldn't the anti-squat solely be a function of the height of the instant center, which is found by the intersection of the angle of the trailing link axis (angled upwards to the front) and the axis created by the upper inboard link mounts (horizontal)? I need to understand this because this configuration seems like a cleaner looking solution if it doesn't adversely affect anti-squat (or other parameters).



Two separate links should not be analyzed like an A-arm.
In side view, your lower longitudinal link (stock '88 trailing arm) and upper longitudinal link form half of a dragster 4 link rear suspension. It should be analyzed as such. With the upper link behind the axle, you have an asymmetric Watts' linkage. When the upper link is inclined more steeply than the lower link, the instant center will be behind the axle. As the suspension compresses further, the side-view instant center rises and gets closer, decreasing anti-squat.

Can you lower BOTH pivots of BOTH links? You should be able to do that while preserving your camber and forward view instant center behavior. That would also make you build a pocket into the frame rail for the inner pivots. If you do that, you dramatically reduce the twisting load on the frame rail that you have now, replacing it with a pure bending load, and come up with adequate reinforcements right off.