Blooze Own: An F355 Six Speed N* Build Thread (Page 62/126)
Yarmouth Fiero JAN 19, 09:56 AM
PM sent Blooz.
katatak JAN 24, 07:54 PM
Hey Blooz - sent you a PM!
Bloozberry FEB 04, 08:08 PM
Well, I think I've finally reached the Eureka Moment I've been working towards, but not before first having to throw out the design from my last post (again). I'll take a few moments to explain why my previous configuration wasn't going to work.

For starters, after a lot of research I became painfully aware that the 18" X 9.5 ET38 rear wheels that suited the length of the 3" extended HT lower links, were made of "unobtainium". Sure, I could have had the wheels custom made for no less than about $1000 each, delivered to my door, but then any company that could make the rear wheels to my specs could not make matching 17" front wheels to my specs as well. It seems very few companies make a custom 17" wheel anymore, and those that do, either don't offer designs that I find attractive, or offer nothing higher than a 44mm offset for a 7" wide wheel. I need a minimum 48mm offset for the fronts.

I decided that if I had to make a compromise regardless whether I bought expensive custom wheels or inexpensive off-the-shelf wheels, then it would be foolish to waste money. During the roughly two-years of scouring the internet for off-the-shelf wheels, I only ever found one manufacturer that offered the style I wanted in staggered sizes in the right dimensions, except that the rear rims would have to be 9" rather than the 9.5" wide as I had designed for. Luckily the 265 section tires I bought for the rear are approved by Goodyear for mounting on a 9" rim.

The 9" rear wheels unfortunately meant that I had to redraw the rear suspension to accommodate the available wheel offset otherwise they would have stuck too far in. At first I thought it would be a breeze to simply move all the links outboard as necessary, but then I didn't like how the new upper links would have had to be embedded in lower frame rail pockets (like the '84-'87 cars do with the lower control arms in the cradle), weakening it. There were other reasons too that I won't get into for the sake of brevity.

With the knowledge I've gained from doing this exercise more times than I cared to count, I started what I dubbed the "final redesign". As Archie once put it: "At some point you have to take the pencil away from the engineer." My wife was threatening to do just that. This time before even putting pencil to paper I chose a static rear roll center to give a roll axis of 2.7 degrees sloping down to the front; I wanted the 0.7:1 camber to roll ratio, minimal roll center movement, no toe gain, and I wanted none of the link mounts inside frame rail pockets, nor to protrude into the engine bay. With that, I hit the drawing board.

It took some coaxing and some teasing and some more head-scratching but I finally came up with a no compromise solution. The key to solving the problem was to ditch the 3" extended HT lower links that were artificially constraining the design. That freed up the ability to move the lower link mounts outboard substantially, until they cleared the overhanging F40 transmission, and then upwards to re-establish roughly horizontal, shorter, lower links. A few digital sketches were needed to locate the upper links for the right camber curve and roll center height/movement, and all that remained was to await the dynamic results from Zac88GT. I got those this morning, plotted them, and couldn't be happier. I'll post the results tomorrow showing how this last design performs compared to most of the others I've been through in order to give anybody that cares an opportunity to catch anything I may have missed. Stay tuned!
Bloozberry FEB 05, 03:14 PM
OK, so here is the final set of graphs showing how the various suspension modifications stack up against each other compared to stock. They are the same as ones I've posted previously with the exception that I've added the reddish-brown curves which represent the Short/Long Arm (SLA) suspension dynamics of my latest design. The key to realizing the significance here is that despite an overall 2" drop in the rear, I've managed to improve the suspension's theoretical performance, and maintain a 5" ground clearance. I'll explain the significance of each reddish-brown-colored curve briefly:

Camber vs Roll:



Here's one area where the SLA design shines over the stock Chapman strut. Where the rate of camber change decreases under jounce with the Chapman strut, the SLA can be tuned to do whatever you want. The little pictogram below illustrates what happens to the angle of the tires relative to the road in a 6 degree roll.



The SLA design keeps the outside tire (the one with all the weight on it) nearly perpendicular to the road surface throughout the entire range, whereas the stock '88 Fiero geometry tilts the tread patch away from the road by about 3.7 degrees in a 6 degree roll. The lesser the angle between road and the outside tire, the greater the potential cornering power.

Roll Center Migration vs Roll:



Here again having upper control arms or links that change angles with respect to the chassis allows a significant ability to control the roll center movement over the fixed angle of the Chapman strut design. As before, I've drawn a scale representation of the above graph showing where the roll center migrates to on the '88 Fiero.



With an SLA design, you can eliminate roll center movement entirely, though I needed to make a small concession in this area to keep the control arm mounts outside the engine bay and outside the frame rail.

Roll Moment vs Bump:

By raising the roll center and controlling it's movement, yet another beneficial trait appears, namely a smaller and more stable roll moment.



Recall that the greater the difference in height between the center of gravity and the roll center, the greater the leverage that forces acting on the center of gravity have on rolling and pitching the car. The above graph shows nearly a 40% reduction in the roll moment at most points of the SLA suspension travel versus stock.

Camber vs Bump:

A side effect of greater camber gain in roll, is also greater camber gain in bump.



Here the graph shows that as the suspension compresses or extends under jounce and rebound, the camber changes more rapidly than stock for a given amount of travel. Under straight line acceleration, this results in a greater loss of traction for the same reasons it increases traction in a turn, so the key here will be to limit squatting under acceleration.

Anti-Squat:

My new SLA geometry shows that for any given amount of jounce or rebound, the anti-squat remains nearly constant, though it is less than the stock configuration at most points.



Since the angle of the trailing link is what governs the amount of anti-squat, I plan to mount the forward end of the trailing link to a bracket with multiple holes that drops down from the lower frame rail, rather than to a fixed point on the engine cradle. It should be easy to make the trailing link angle, and thereby the anti-squat, tuneable.

Toe vs Bump:

Lastly, I struggled somewhat to understand GM's use of a shorter forward lower lateral link in the stock suspension only to have it produce a tenth of a degree of toe-in, and only under the most extreme cases of suspension travel.



In order for me to achieve the same amount of toe-in with the SLA design, I would have had to make the forward links a whopping 0.5mm (0.020") shorter than the aft links, top and bottom. It just isn't practical to work in that scale, so I hypothesized that other factors must have been at play with the Chapman strut which would have caused the rear to toe out unless the forward lateral links were made shorter. (Since toe-out in the rear causes oversteer in a turn, it has to be kept in check.) I don't believe this is the case with the SLA design since the data shows that with equal length fore and aft lateral links, the toe basically mimics the stock Fiero suspension in jounce, where it matters most. Of course bushing deflections aren't taken into account here.
fieroguru FEB 05, 03:24 PM
Sounds good... now where are the drawings of the suspension?


FieroWannaBe FEB 06, 11:23 AM
Great Work.

It has happened to me more times than I can remember, when a design was going well and pretty far along, only to be completely destroyed due to an off the shelf part being non-existant.
Bloozberry FEB 08, 03:41 PM
Thanks guys for the comments.

[This message has been edited by Bloozberry (edited 02-09-2013).]

fieroguru FEB 08, 06:23 PM

quote
Originally posted by Bloozberry:

Thanks guys for the comments. I am discontinuing my involvement with PFF from this point forward. For those interested in following my F355 build, you can look me up at the Canadian kit car website Madmechanics.com under the same user name. See you over there.



I am member over there too (same username). You will give me a reason to check it more often.
DL10 FEB 09, 02:26 AM

quote
Originally posted by Bloozberry:

Thanks guys for the comments. I am discontinuing my involvement with PFF from this point forward. For those interested in following my F355 build, you can look me up at the Canadian kit car website Madmechanics.com under the same user name. See you over there.



Please reconsider leaving PFF, you are a well respected member with lots to contributate.
85sliverGT FEB 09, 08:07 AM

quote
Originally posted by DL10:


Please reconsider leaving PFF, you are a well respected member with lots to contributate.



X2! Please don't pack up and leave. Blooz you are in a very small handful of people here that are as helpful as they are knowledgable...the people like me need the people like you!

[This message has been edited by 85sliverGT (edited 02-09-2013).]