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  #21  
Old 01-08-2015, 06:40 PM
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The criticism I've heard - and I could blame for an issue I have/had with my car is this:

Since the driveshaft is a loaded member, on hard corners the force spreads the carrier thus reducing the ability of the clutches to keep both tires spinning.

true? false? but given what you've said - would you achieve the same benefit as a C4 suspension by simply putting an upper bar and removing the c-clip?

please excuse the sentence construction, it's probably as stuffed up as my head...
I have not personally seen the posi clutch issue you describe. Several folks on this forum have tried various modifications to add an upper link - some better than others. I personally don't like sliding yolks in the axle shaft due to friction in the splines that sometimes causes unpredictable geometry changes. With big horsepower and big tires you put a lot of twisting forces on the splines. I prefer stout CV joints on the axle to absorb the minor in-out movement. An issue with an upper link using a C-3 frame and a C-4 upright is the lack of room for the link because of the position of the frame rail. Also, it is fairly difficult to adapt an upper link to the C-4 upright, although several guys have built custom uprights or modified C-3 stuff to accept the link. The same thing applies to using C5 control arms without moving the frame rails inboard (see the Jag/C6 picture above)
I have a rear differential from an 06 GTO that 1320gforce built with all the good stuff (and it howls like a banshee) - so this questions comes from "I have this on the shelf" and is probably one of those "you could, but it would still cost more than other options".... despite that I'll plow on -

why not maintain the C3 arms, and replace the differential with GTO, Viper, or even new CTS/Camaro center then adapt the CV joints?
The C-3 trailing arms introduce significant roll steer - that is the reason most guys (and GM as in the C-4) go with multi-link forward links and a toe control rod. The roll steer wasn't too bad with narrow, bias ply tires that you could overcome with throttle and just throw the car into the corner. A lot of fun, but not the fastest way around the track. With that said, I have been to several recent events and watched Danny Popp (72 Corvette) and Brian Hobaugh (66 Corvette) walk away with wins against some tough, late-model competition. Both are using basically stock C-3 IRS set-ups (stiff springs, modified roll center, and high dollar shocks), so it can be done. I watched Brian twist the end off one half-shaft in Scottsdale and fix it in time to make the final heat (which he won). Destroyed and replaced a $1000 JRi shock, but he still won. The apparent "trick" to both cars is minor modifications to the front control arm pick-ups, excellent caster gain with fairly high travel, big tires (315s front and rear), and superb shock valving.

BTW, the differential in the Jag picture is a late Camaro piece, and I am considering using a Strange Engineering S-60 (Dana 60) IRS differential that is used in the Art Morrison IRS. Very strong!
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Last edited by mfain; 01-08-2015 at 06:43 PM..
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  #22  
Old 01-08-2015, 09:06 PM
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The apparent "trick" to both cars is minor modifications to the front control arm pick-ups, excellent caster gain with fairly high travel, big tires (315s front and rear), and superb shock valving.
Care to share more details on the front control arms? Are they doing more than offsetting the uppers for more caster? Are they relocating upper and lowers?
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  #23  
Old 01-08-2015, 09:28 PM
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The apparent "trick" to both cars is minor modifications to the front control arm pick-ups, excellent caster gain with fairly high travel, big tires (315s front and rear), and superb shock valving.
Care to share more details on the front control arms? Are they doing more than offsetting the uppers for more caster? Are they relocating upper and lowers?
Looks like they are going for more caster GAIN during suspension travel - spacing the front of the lower control arm down (putting a spacer block between the front of the control arm shaft and the frame), which causes more caster gain as the suspension compresses. Danny Popp's car had tubular lower control arms on it at the Goodguy's shoot-out, not the stamped lowers he had on it before. It also looks like they are using very high quality, adjustable shocks and, on the autocross, are using a slow rebound to pin the nose down through most of the corner. Watching Brian's car from the front, there is very little roll on corner entry and both front tires seem to be doing their fair share of the work. Brian in particular is dealing with a lot of scrub radius, which tells me he is probably compensating with lots of caster - it doesn't seem to be hurting him. Another thing I noticed, with the 200 tread wear rated tires are the very wide rims compared to tread width - not wide tires on too-narrow rims. I'll guess that some research has been done to optimize the tire contact patch with that tire. These are just observations on my part and do not reflect anything I was told by either driver. Oh, I almost forgot, the steering appeared to be very quick with the Borgeson steering boxes - so quick that during tight corners you could see the front wheels "twitching" like a dirt sprint car in response to the drivers' steering inputs. Looked perfect for a tight autocross track.

Last edited by mfain; 01-08-2015 at 09:32 PM..
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  #24  
Old 01-08-2015, 10:21 PM
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You point out what really got me thinking - Danny Popp's car. I've not seen it in person, so your observations are quite helpful. Seeing him spank the high-dollar muscle-car suspensions with the "old" Corvette system really does make me smile.

I know a guy with an 06 GTO that has a 1000 hp LS motor in front of his rear differential, and he's settled on a based-on-ford-9-inch designed center section and he has kept the IRS....

you've given me lots to consider - thank you.... we now return to our OP's thread sorry for the hijack
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  #25  
Old 01-08-2015, 11:19 PM
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.... we now return to our OP's thread sorry for the hijack
Carry on! All valuable info
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  #26  
Old 01-09-2015, 12:23 AM
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Originally Posted by SuperBuickGuy View Post
You point out what really got me thinking - Danny Popp's car. I've not seen it in person, so your observations are quite helpful. Seeing him spank the high-dollar muscle-car suspensions with the "old" Corvette system really does make me smile.

I know a guy with an 06 GTO that has a 1000 hp LS motor in front of his rear differential, and he's settled on a based-on-ford-9-inch designed center section and he has kept the IRS....

you've given me lots to consider - thank you.... we now return to our OP's thread sorry for the hijack
WELL, that makes me feel better about my lo-budget build, maybe not ALL the fun is lost.....

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  #27  
Old 01-09-2015, 06:53 PM
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mfain mfain is offline
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Here is a modification I have been planning on testing for some time - just haven't gotten around to it. It is a way to eliminate the adverse C-3 rear roll steer problem without making major modifications to the car's frame. One solution is with the C-4 suspension (or Guldstrand mod), but that requires surgery to the frame at the front trailing arm mount, and the forward links are shorter than I would like to see. The forward links provide an "instant center" of motion (at the intersection of their extended centerlines), but unlike a 4-link set-up on a solid axle, do not transmit torque from the axle to the chassis. The C-4 forward links merely transmit forward force through the instant center, although they do transmit braking torque since the caliper is mounted to the bearing support. The forward links of the C-4 suspension can therefore be replaced with a "ladder bar" that uses the original C-3 trailing arm forward pick-up point. A C-3 trailing arm can be modified (if you prefer the C-3 bearing configuration) to do the same thing. See the attached "crude" sketch, for which I apologize.
Finally, a toe control rod can then be added to either the C-4 upright or to a modified C-3 trailing arm as shown in the sketch. Note: you cannot simply add a toe control rod to the back of an unmodified, rigid C-3 trailing arm because the suspension will bind (too many things swinging in different arcs). All that remains is to add a pick-up fixture, either on the differential or the differential cross-member, to pick up the inboard end of the toe control rod. I have a couple of extra C-3 trailing arms and I will build an example to test.

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Last edited by mfain; 01-09-2015 at 08:17 PM..
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  #28  
Old 01-09-2015, 10:40 PM
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Sam Cogley Sam Cogley is offline
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Something is wonky with that image. It won't show up, and when I try to open it in a different tab, I get a page showing that I'm not logged in to VM.
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  #29  
Old 01-09-2015, 11:08 PM
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mfain mfain is offline
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Something is wonky with that image. It won't show up, and when I try to open it in a different tab, I get a page showing that I'm not logged in to VM.
Sam,

I'm logged in and the image is presented as a thumbnail. When I click on it, it just expands it to a larger view. The only time I've had the problem you're encountering is when I am not logged in, and then it shows "attachment" at the bottom and it won't let me open it. If you can't get it, PM me an e-mail and I will send it to you that way.

Pappy
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  #30  
Old 01-09-2015, 11:40 PM
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Pic working on my end... Seems like a good idea
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