Blooze Own: An F355 Six Speed N* Build Thread (Page 87/126)
Bloozberry DEC 18, 11:23 AM

quote
Originally posted by kennn:
Don't spherical bearings ordinarily have some amount of misalignment capability? If so, two sandwiched together would eliminate misalignment since they would cancel one another. Perhaps you are considering clamping the bearing tight enough to not allow movement, therefore no misalignment?



Yes to all of the above. What I am trying to do is mimic the action of the combined needle bearings that I would have otherwise used if they hadn't been so darn expensive.
Will DEC 18, 11:31 AM
I was asking why you need to resist side loads.

How much out of plane loading will you see with a single axis pivot? The load analysis you've presented doesn't address that.

Also, with a multi-DOF pivot, how far out of the original plane will the arm move throughout it's range of motion?
Bloozberry DEC 18, 11:54 AM
I had calculated and posted earlier on that the pushrods would see 3.8 degrees and 2.5 degrees of misalignment at 76 mms of jounce and rebound respectively. Granted, it's not much but since my bell crank mount design sandwiches the bell crank between two plates that leave 0.25" clearance on either side, I would rather resist the loads than accommodate them. I didn't include the side load analysis given the negligible forces involved compared to the axial load capacity of the spherical bearings.

(Edited because "negligent" isn't the same as "negligible")

[This message has been edited by Bloozberry (edited 12-18-2013).]

katatak DEC 18, 08:24 PM

quote
Originally posted by RCR:


Yea...Easier to keep us updated here than support 1500 Fiero fanatics staring through your garage windows.



Bob




Now that's funny!

You in there Blooz?
Will DEC 19, 09:02 AM

quote
Originally posted by Bloozberry:

I had calculated and posted earlier on that the pushrods would see 3.8 degrees and 2.5 degrees of misalignment at 76 mms of jounce and rebound respectively. Granted, it's not much but since my bell crank mount design sandwiches the bell crank between two plates that leave 0.25" clearance on either side, I would rather resist the loads than accommodate them. I didn't include the side load analysis given the negligible forces involved compared to the axial load capacity of the spherical bearings.

(Edited because "negligent" isn't the same as "negligible")




Ok. I was thinking "Why bother reacting the side loads if you can allow it to self-align".
I was also thinking more about the torque the side loads would put on your support struts.
Bloozberry DEC 19, 08:50 PM
Katatak, you crack me up.

It's spring time! OK, well more like shock-and-spring time. Until now, all of my previous drawings were based only on generic shocks and springs meant to reassure me that the general concept would work. Getting into the nitty-gritty of the bell crank design forced me to take out my pencil and paper (mouse, really) and have a detailed look at two separate issues that hadn't previously been studied:

1. whether the spring and shock would interfere with the bell crank at any point in the suspension's travel; and

2. where my adjustable height springs needed to be positioned to give me the suspension travel I wanted without the shocks bottoming out, nor the springs binding up in full compression, nor becoming loose at full extension.

It's amazing how much of this stuff is already figured out for you when you're only throwing together a bunch of parts that someone else has already proven (or more likely not!). Coming up from scratch has been an eye-opener. I'll start with number 2 since I build upon the results to answer number 1. To get started, I ordered the Promastar DS501 shocks because they had the longest stroke available to accommodate my suspension's design travel. Fully extended they're 17" eye-to-eye, and fully compressed they are 11.625" giving a maximum stroke of 5.375", which imposes a new, lower maximum travel envelope on my suspension, which was able to accommodate up to 6". (I could change the lengths of the bell crank arms to give me a different input to output ratio so that I could still use the full 6" suspension travel while only generating 5.375" of shock travel, but I don't think it's necessary). Also, according to the literature that came with the shocks, the Promastar's are designed to be operated at an optimal static length (length at ride height) between 13.25" - 14.5".

At first I thought it made logical sense to chose the halfway point between the shock's fully extended and fully compressed lengths as the length at ride height. That would have assured equal amounts of jounce and rebound at 2.69" each (5.375" divided by 2). But after thinking about it a bit, I decided that allowing more jounce at the expense of some rebound travel was a better compromise since in a roll, the wheel in jounce has the majority of the weight on it and therefore the control. In other words, it would matter less if the wheel in rebound (inside corner) ran out of travel while cornering, than if the wheel in jounce (outside corner) did. So I chose to offset the ride height length of the shock to the maximum recommended 14.5". This will give me 2.875" of jounce travel and 2.5" of rebound travel.

Enough math for you yet? Get some popcorn because it's not over yet. The thing is, I can arbitrarily choose the length of the shock absorber at ride height because I haven't cast the the location for the hard mounted end of the shock absorber in stone yet. But when I do, I also have to make certain that the spring I chose isn't so long that it will bind up (ie the coils come into contact with each other) before the shock reaches its fully compressed state, AND make sure it isn't so short that it flops around loosely when the shock is fully extended.

To satisfy these two conditions now that I've chosen a 14.5" static shock length, I first have to calculate how long the spring will be when the car is at static ride height. That's easier than it may sound since I estimate the car will weigh about 3000 lbs, so the 40%/60% weight distribution will result in about 900 lbs at each rear spring, give or take. Since I had already bought my 12" x 350lbs/in springs with my Held conversion kit (now gathering dust), I could figure out how much they would compress by simply taking 900 lbs divided by 350lbs/in to give 2.57" of compression. Subtract 2.57" from 12" and I get 9.43" as the length of the spring when supporting the weight of the car.

So now that I know how long the shock absorber has to be (14.5") and how long the spring will be at ride height (9.43"), I can draw the coil-over at ride height, as well as what each are doing at full shock absorber compression and extension:



Starting with the top image, you can see how the threaded adjuster ring on the shock body has lots of leeway in either direction to lengthen or shorten the spring in case I've over or under estimated the weight of the car. The middle drawing shows the coil-over at full shock absorber compression and shows that the spring hasn't bound up yet since there are still a couple millimeters between each coil. That solves that tricky condition. Lastly, the bottom drawing shows that when the shock is at full extension, the spring is only just reaching its maximum length. In fact the spring is actually 12.125" in length, so even at full shock extension, the spring won't be flopping around on the shock. That solves the other condition. (It may be important to note that the adjustment ring is at the same location on the shock body in all three drawings.)

In the first paragraph, I mentioned that another concern in the design process was to be sure that the coil-over would not contact the bell crank at any point in the suspension's travel. Well, I can now use the information from the above discussion to draw out the bell crank and coil-over geometry at any point in the travel range to be sure. Perhaps the most important discovery in doing this step was that I found that I wouldn't be able to orient the shock absorbers with the adjustment ring and knob towards the rear, where they would be more easily accessible. The trouble with placing that end closest to the bell crank was that an overly large cut-out would have had to have been made in the bell crank to clear the shock when the suspension was at full rebound (fully extended). By simply turning the shock around 180 degrees, I was able to minimize the cut out to 14 mms as shown in the bell crank drawing a couple posts ago. To resolve the conflict in my head about having to put the knob at the other end, I told myself that it would be harder for kids to reach when the deck lid was open at future car shows. Again, here are the scale drawings showing how the coil-over and bell crank behave at ride height, full jounce, and full rebound respectively:



Finally, I'm armed with enough details draw how the bell crank and shock mounts will be positioned in the chassis (by night), and start fabricating some parts (by day). Now if only that pesky Christmas holiday weren't in the way!
Will DEC 20, 09:57 AM

quote
Originally posted by Bloozberry:
(I could change the lengths of the bell crank arms to give me a different input to output ratio so that I could still use the full 6" suspension travel while only generating 5.375" of shock travel, but I don't think it's necessary).



I think you should change your bell crank ratio.

If the shock has less travel than your suspension, then the shock is the travel limiter and that is BAD. Crashing the shock is going to be quite detrimental to it, whereas having the tire hit the shell you welded in really won't hurt much. (I'd still want to hit a bump stop before that happened, though). Personally, I'd set it up with maybe 1/4" of damper travel left when the suspension is at its mechanical limits (tire into the shell)
kennn DEC 20, 12:30 PM
Blooze,

If you don't already make your living at technical writing, you certainly have the talent. And, you could use this treatise as part of your resumé.

It appears that as the shock compresses that the change in mechanical advantage makes for a somewhat variable rate at the spring, which seems like a good thing if I'm correct.

Ken

edit: I also think that Will has a good point with regard to bump stops. Don't your shocks come with a bump stop at the shaft end? The Held struts that I am fitting apparently employ a Bilstein insert that has an integral stop within the housing (not visible).
------------------
'88 Formula V6
'88 GT TPI V8

[This message has been edited by kennn (edited 12-20-2013).]

Will DEC 20, 01:14 PM

quote
Originally posted by kennn:

It appears that as the shock compresses that the change in mechanical advantage makes for a somewhat variable rate at the spring, which seems like a good thing if I'm correct.




The loss of MA on the driving end compensates for the loss of MA on the driven end.
Bloozberry DEC 20, 01:18 PM

quote
Originally posted by Will:
I think you should change your bell crank ratio. If the shock has less travel than your suspension, then the shock is the travel limiter and that is BAD. I'd set it up with maybe 1/4" of damper travel left when the suspension is at its mechanical limits (tire into the shell)




quote
Originally posted by kennn:
I also think that Will has a good point with regard to bump stops.



After a second sober thought, I realize that you guys are right. Peer review is one of the great things about this forum. I'll redesign the bell crank as you suggested Will, and that will give me the space to add a bumper on the shaft Kennn, BTW, I don't believe the Promastars have any such integral/internal bumper... they make a definite thunk when they reach max and min limits. Back to the drawing board!