FB rear suspension locations 12A, 13B?
01-24-11, 05:43 PM
#26
Well I talked to my friend about his spare chassis and he said I can have it. It's an 85 GS, no sunroof. Its solid except for rust around the rear hatch drain holes. The area in the rear wheel wells around the upper link mounts that always rust out is rock solid.
The three link and a panhard bar is about as good as you can get on these cars. The modeling done when Jim Susko and I were building cars to test what he now offers under the G-force Engineering name showed the watts link was not worth the hassle to build in the location you want the roll center. Wheel loading numbers didn't show enough difference to justify it and the added structure to hold everything is almost impossible to fit with a fuel cell and exhaust stuffed back there too.
The stock watts link is actually a pretty nice set up even with all the funky length links. It moves very straight up and down until the extreme ends of its movement(well beyond the movement you would see in a race car). The problem is the height on the rear end.
The three link and a panhard bar is about as good as you can get on these cars. The modeling done when Jim Susko and I were building cars to test what he now offers under the G-force Engineering name showed the watts link was not worth the hassle to build in the location you want the roll center. Wheel loading numbers didn't show enough difference to justify it and the added structure to hold everything is almost impossible to fit with a fuel cell and exhaust stuffed back there too.
The stock watts link is actually a pretty nice set up even with all the funky length links. It moves very straight up and down until the extreme ends of its movement(well beyond the movement you would see in a race car). The problem is the height on the rear end.
01-24-11, 09:43 PM
#28
The height of the center of the middle link(vertical link that pivots on the rear end housing) in the watts set up. The center of that pivoting link sets your roll center. In many suspension designs this is an imaginary point in space.
The rear roll center set by the rear suspension design works with the front roll center(front suspension design) to create the roll axis of the car. Connect those two points and you have the imaginary line the car will roll side to side on.
The front roll center is going to be low because of the struts up front. When you lower a stock watts link rear, the roll center goes up in relation to the chassis. This will give you a roll axis the is tilted down in the front and the car will tend to try to fall over the outside front wheel in a turn and generally cause weird things to happen with handling when you try to fix that problem.
A panhard bar at about the height of the bottom of the ring gear puts your rear roll center at the height of the bar(about 6" off the ground)
The rear roll center set by the rear suspension design works with the front roll center(front suspension design) to create the roll axis of the car. Connect those two points and you have the imaginary line the car will roll side to side on.
The front roll center is going to be low because of the struts up front. When you lower a stock watts link rear, the roll center goes up in relation to the chassis. This will give you a roll axis the is tilted down in the front and the car will tend to try to fall over the outside front wheel in a turn and generally cause weird things to happen with handling when you try to fix that problem.
A panhard bar at about the height of the bottom of the ring gear puts your rear roll center at the height of the bar(about 6" off the ground)
01-24-11, 10:16 PM
#29
I was looking at the rules for the wording on whether or not you can move where links attach to the rear end housing, that would make the chassis end a non-issue. Can you point to the line in the GCR that opens that door?
I understand the 'traction bar' option I just don't think I would go with a 4 link like the KC set up. I like the looks of the support inside but I recall computer modeling showed a three link had fewer issues with roll steer and binding with the short links these cars have.
I understand the 'traction bar' option I just don't think I would go with a 4 link like the KC set up. I like the looks of the support inside but I recall computer modeling showed a three link had fewer issues with roll steer and binding with the short links these cars have.
Suspension and Steering Level 2 (RX7's are Level 2 prep cars)
9.b.1. Suspension control arms are unrestricted, provided the quantitly of these items remains as stock
9.b.3. Any anti-roll bar(s) and rear axle traction bar(s), rear axle panhard rod and watts linkage can be added or substituted, provided its/their installation serves no other purpose. The mounts for these devices can be welded or bolted to the
car. These devices and their mounts can not be located in the trunk or driver/passenger compartment unless fitted as stock. Rear axle traction bar(s) used to control axle housing rotation must be solid bar or tube.
The suspension in the photo has alternate material control arms located in the stock location. In this case the alternate material is .25" cable. The traction bars are mounted per rule 9.b.3. and are constructed of aluminum tubes and rod ends.
This is the same logic that allowed IT cars to use Susko's Tri-Link. In the IT installation the OE upper control arms are retained but are fitted with alternate bushings (usually made of air or soft foam), The "traction device" - the Tri Link took over the function of the OE upper arms. The net is esentially a 3 link that passed muster with respect to the IT rules.
EP RX7's have been campaigned by several teams out of the KC area for over 10yrs. Early on they used a combination of the Tri Link and lower "traction bars" located in a favorable location. The suspension worked okay but the cars had real issues with putting the power to the ground. This issue was the mounting location of the Tri Link on the chassis. It is too low. Also the Tri Links were prone to failures and the mounts on the chassis would rip out of the floor.
The 4 traction bar arrangement in the photo was the solution. This suspension allows the cars to come off of turns much faster because the drivers are able to pick up the throttle sooner after reaching a corner's apex. Note that the upper traction bars are almost as long as the lower arm so bind is not an issue.
A three link with a higher mounting location on the chassis would be better and Charlie has experimented with it. This is the design I am using for my STU RX7.
Again....give him a call. He is a straight shooter that has tried everytihg being discussed here. He is a very good driver and can explain to you what works and why.
Here is my three link:
01-24-11, 11:06 PM
#30
The height of the center of the middle link(vertical link that pivots on the rear end housing) in the watts set up. The center of that pivoting link sets your roll center. In many suspension designs this is an imaginary point in space.
The rear roll center set by the rear suspension design works with the front roll center(front suspension design) to create the roll axis of the car. Connect those two points and you have the imaginary line the car will roll side to side on.
The front roll center is going to be low because of the struts up front. When you lower a stock watts link rear, the roll center goes up in relation to the chassis. This will give you a roll axis the is tilted down in the front and the car will tend to try to fall over the outside front wheel in a turn and generally cause weird things to happen with handling when you try to fix that problem.
A panhard bar at about the height of the bottom of the ring gear puts your rear roll center at the height of the bar(about 6" off the ground)
The rear roll center set by the rear suspension design works with the front roll center(front suspension design) to create the roll axis of the car. Connect those two points and you have the imaginary line the car will roll side to side on.
The front roll center is going to be low because of the struts up front. When you lower a stock watts link rear, the roll center goes up in relation to the chassis. This will give you a roll axis the is tilted down in the front and the car will tend to try to fall over the outside front wheel in a turn and generally cause weird things to happen with handling when you try to fix that problem.
A panhard bar at about the height of the bottom of the ring gear puts your rear roll center at the height of the bar(about 6" off the ground)
i really appreciate this.this is probably the best explanation of the Gen I's suspension dynamics that i've read. it's simple, and for someone like me, simple is good. you provided some of the missing pieces of a puzzle i've been trying to put together for over a decade.
01-24-11, 11:35 PM
#31
Old [Sch|F]ool
That... is a VERY high front pivot and low rear pivot.
In an '85 chassis, I had a slightly lower front pivot and a higher rear pivot (came up through floor, necessary to clear the upper Watts bar) and it had enough pro-squat that the wheelhop made the car almost undrivable. I lowered the front pivot as far as I could (still over the cross-piece) and it still had wheelhop, but not as bad. I ended up shortening the link substantially and dropping the pivot down on the floor behind the cross-piece, and there it worked very well, and still no brake hop.
I wonder what else is different about your setup that it works there and mine didn't.
With that geometry, I experienced bizarre roll steer effects, theorized that it was due to the roll center height interacting with the links' angles. So for the '84 shell, I duplicated the previous car's 3-link but used a Panhard. Roll steer problems solved. Still no brake hop, either (although oddly enough, the stock 4-link in THAT car suffered from it)
The next car I build will probably have a 4 link. I'm finding that the rearend housing can't take the stress of a suspension point on top of the banjo. I can see light between the housing and the 3rd member in spots, but only after all the gear oil leaks out.
In an '85 chassis, I had a slightly lower front pivot and a higher rear pivot (came up through floor, necessary to clear the upper Watts bar) and it had enough pro-squat that the wheelhop made the car almost undrivable. I lowered the front pivot as far as I could (still over the cross-piece) and it still had wheelhop, but not as bad. I ended up shortening the link substantially and dropping the pivot down on the floor behind the cross-piece, and there it worked very well, and still no brake hop.
I wonder what else is different about your setup that it works there and mine didn't.
With that geometry, I experienced bizarre roll steer effects, theorized that it was due to the roll center height interacting with the links' angles. So for the '84 shell, I duplicated the previous car's 3-link but used a Panhard. Roll steer problems solved. Still no brake hop, either (although oddly enough, the stock 4-link in THAT car suffered from it)
The next car I build will probably have a 4 link. I'm finding that the rearend housing can't take the stress of a suspension point on top of the banjo. I can see light between the housing and the 3rd member in spots, but only after all the gear oil leaks out.
01-24-11, 11:51 PM
#32
Damn mustanghammer, that third link is what we dream of doing in EP!
I agree the early tri link issues were a concern. Its also impossible to get the forward location high enough up inside the tunnel to put the instant center* in the best place for planting the tires.
I don't think we can move where the links attach to the rear end itself though(lowers in my case). And, I don't think I could plead my case making all four stock links useless and adding a bunch of "traction bars" lol. The upper links cause more trouble so we need to lose them.
* for diabolical1: The instant center is generally located by following the lines that pass through the pivot points of the control arms when viewed from the side. The location in space that these two lines meet is your imaginary control arm length and its where the chassis "feels" the power being applied. The instant center's location in relation to the cars center of gravity(above, below, in front of ,or behind it) effects how the chassis reacts to the torque from the rear end housing when you apply power. The car can actually try to lift the wheels off the ground(car squats hard) or push the tires into the pavement(rear of car will lift under power). Just one more thing to find the right balance...
I agree the early tri link issues were a concern. Its also impossible to get the forward location high enough up inside the tunnel to put the instant center* in the best place for planting the tires.
I don't think we can move where the links attach to the rear end itself though(lowers in my case). And, I don't think I could plead my case making all four stock links useless and adding a bunch of "traction bars" lol. The upper links cause more trouble so we need to lose them.
* for diabolical1: The instant center is generally located by following the lines that pass through the pivot points of the control arms when viewed from the side. The location in space that these two lines meet is your imaginary control arm length and its where the chassis "feels" the power being applied. The instant center's location in relation to the cars center of gravity(above, below, in front of ,or behind it) effects how the chassis reacts to the torque from the rear end housing when you apply power. The car can actually try to lift the wheels off the ground(car squats hard) or push the tires into the pavement(rear of car will lift under power). Just one more thing to find the right balance...
01-25-11, 09:06 PM
#33
That... is a VERY high front pivot and low rear pivot.
In an '85 chassis, I had a slightly lower front pivot and a higher rear pivot (came up through floor, necessary to clear the upper Watts bar) and it had enough pro-squat that the wheelhop made the car almost undrivable. I lowered the front pivot as far as I could (still over the cross-piece) and it still had wheelhop, but not as bad. I ended up shortening the link substantially and dropping the pivot down on the floor behind the cross-piece, and there it worked very well, and still no brake hop.
I wonder what else is different about your setup that it works there and mine didn't.
With that geometry, I experienced bizarre roll steer effects, theorized that it was due to the roll center height interacting with the links' angles. So for the '84 shell, I duplicated the previous car's 3-link but used a Panhard. Roll steer problems solved. Still no brake hop, either (although oddly enough, the stock 4-link in THAT car suffered from it)
The next car I build will probably have a 4 link. I'm finding that the rearend housing can't take the stress of a suspension point on top of the banjo. I can see light between the housing and the 3rd member in spots, but only after all the gear oil leaks out.
In an '85 chassis, I had a slightly lower front pivot and a higher rear pivot (came up through floor, necessary to clear the upper Watts bar) and it had enough pro-squat that the wheelhop made the car almost undrivable. I lowered the front pivot as far as I could (still over the cross-piece) and it still had wheelhop, but not as bad. I ended up shortening the link substantially and dropping the pivot down on the floor behind the cross-piece, and there it worked very well, and still no brake hop.
I wonder what else is different about your setup that it works there and mine didn't.
With that geometry, I experienced bizarre roll steer effects, theorized that it was due to the roll center height interacting with the links' angles. So for the '84 shell, I duplicated the previous car's 3-link but used a Panhard. Roll steer problems solved. Still no brake hop, either (although oddly enough, the stock 4-link in THAT car suffered from it)
The next car I build will probably have a 4 link. I'm finding that the rearend housing can't take the stress of a suspension point on top of the banjo. I can see light between the housing and the 3rd member in spots, but only after all the gear oil leaks out.
The mount on the axle is higher than I wanted but it was necessary to get equal length control arms with the chassis mount in it's present location. Otherwise it is simple 3 link suspension.....no tricks.
I used the same rear suspension on an 85 Mustang that I ran in C Prepared Solo II competition. There was one event at Forbes Field (concrete surface) that I set the 3rd link higher on the chassis than the rearend to solve axle hop issues that I was having under braking. It worked great and the car still hooked up really hard.
If I ever do this again, I will experiment with a short 3rd link that is similar to the current Mustang. Those cars seem to work really well so I think this might be a way to set something up that is allot less involved and easier to fab.
01-26-11, 12:01 PM
#34
Old [Sch|F]ool
With the '84 chassis, I had to raise the ride height substantially to get the lower links parallel with the ground. It looks goofy but it works.
My current upper link is 16" long, if I remember right.
My current upper link is 16" long, if I remember right.
01-26-11, 03:22 PM
#35
My arms are all same at about 18"
01-30-11, 12:06 PM
#36
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Nice thread guys! I had not hear that Tri-links were ripping out. Now that link is different than what Charlie sells on his site kcraceware.com. Looks easy enough to make.
01-30-11, 04:40 PM
#37
That was a problem on the early ones from what I've heard. I think Jim changed the design a number of years ago though. The early units were originally designed for IT power levels, throw that in a GT or EP car with possibly twice the power and you find the weak link.
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