Rear roll center height tweaking: Strategy?

[69427]

I'm currently fabricating the rear camber bracket for my C4 suspension transplant onto my '69 frame. I'll be doing the usual tweaking of the settings once I get the car assembled again, but I was just thinking about when you know you've got the roll center correct. I don't want to wear out a set of tires on a skid pad to find out if it's not correct, and taking pictures of the tire camber angle during cornering requires additional manpower. What's an easy way to dial this in?

thanks,
Mike

 

[Twin_Turbo]

There's no such thing as a magic roll center ehight. It's all a compromise, between the ride height you want, the camber curve/instantaneous center you end up with, even the wheel width & offset have a little influence. I would just start off with the stock late C4 bracket location and go from there. It was changed somewhere in 87?? (not sure when exactly) because of too high a roll center and an annoying jacking effect as a result from it. If anything, you could get some of those Doug Rippie brackets for your diff.

The way to dial this in, apart from having a complete computer model is trial and error...or of course buying a program like performance trends suspension alalyzer and using that to plot out the best geometry for your car.

 

[BBShark]

I don't know if I have an answer for you, there is plenty of debate just about the methodology determining roll center.

However something you wrote makes me think, what's wrong with mounting a camera under the car? I'm thinking that might be the easiest way. The cameras are cheap and small and can be remotely started and stopped.

I just recently used a Wiimote (the controller used for Nintendo Wii games) to measure 3-axis accel/decel rates caused by the opening of a parachute from the time a guy jumped from the plane to the moment he was on the ground. A Wiimote costs $40 and all you need is that and a laptop. Not saying that this is the answer to your roll center problem but sometimes the best answer is the most obvious. Could be the camera is the way to go.

 

[69427]

There's no such thing as a magic roll center ehight. It's all a compromise, between the ride height you want, the camber curve/instantaneous center you end up with, even the wheel width & offset have a little influence. I would just start off with the stock late C4 bracket location and go from there. It was changed somewhere in 87?? (not sure when exactly) because of too high a roll center and an annoying jacking effect as a result from it. If anything, you could get some of those Doug Rippie brackets for your diff.

The way to dial this in, apart from having a complete computer model is trial and error...or of course buying a program like performance trends suspension alalyzer and using that to plot out the best geometry for your car.

No off the shelf stuff will fit. It's a C3 differential with C4 struts, along with halfshafts that are longer than a C3 but shorter than a C4. I welded up a bracket that will mimic the function of a C3 bracket, and I'll be able to shim it to change the inner pivot height. Once I get the car running again I had planned on changing the pivot height just to see if I can detect any difference in the handling or feel of the car.

 

[69427]

I don't know if I have an answer for you, there is plenty of debate just about the methodology determining roll center.

However something you wrote makes me think, what's wrong with mounting a camera under the car? I'm thinking that might be the easiest way. The cameras are cheap and small and can be remotely started and stopped.

................................................................................

The camera suggestion sounds interesting. In the meantime while I'm putting the car together, I might just keep an eye open for any decent priced small cameras. Thanks!

 

[Twin_Turbo]

No off the shelf stuff will fit. It's a C3 differential with C4 struts, along with halfshafts that are longer than a C3 but shorter than a C4. I welded up a bracket that will mimic the function of a C3 bracket, and I'll be able to shim it to change the inner pivot height. Once I get the car running again I had planned on changing the pivot height just to see if I can detect any difference in the handling or feel of the car.

The C3 bracket doesn't fit the C4 diff but the C4 brackets do fit the 80-82 one. Or are you using an iron diff?

 

[69427]

The C3 bracket doesn't fit the C4 diff but the C4 brackets do fit the 80-82 one. Or are you using an iron diff?

I'm using the iron rear differential. The standard length C4 arms tuck the inner pivot point (in this application) at a location where it's not possible to use the C4 brackets. I've got this bracket roughed in, and I'll finish the welding and attachments this weekend.



Just for kicks, here's a picture of the front section of the frame. I dropped the engine in (please ignore the dust on the valve covers) to check for clearance issues, and it looks like the tightest spot is the upper A-arm and #1 cylinder header pipe. I've only got about 5/8" clearance here, so I'm trying to figure out my options. I hate to cut up and reweld the header pipe (they're ceramic coated), so I'm hoping for a better solution to pop into my head soon. (The shock mount in the picture is temporary while I figure out the jounce/rebound limits.)

 

[BBShark]

5/8 clearance sounds good or I would like to think its good. I have 1/2 inch clearance from my PS pulley to my crossmember!