ckrell Posted February 11, 2012 Share Posted February 11, 2012 I'm at the point of picking my rear roll center height. With the weld on the R.C.A. I made at full height, my roll center would be 4.6" at 4" ride height. I can cut them down to whatever height to adj. the roll center. With no rear R.C.A.(spindle pin in stock location), I'm at about 1.3" high roll center. On the front at 4" hide height, the front roll center is 1.1", (because I've moved the front L.C.A. pivot up about 2"). With a 3/4" R.C.A., it gives me a roll center of about 3.5" and of course the RCA could also be cut down to adj. the roll center. High speed stability (140+mph) is my #1 priority. Comments, advice, and opinions are welcome! Quote Link to comment Share on other sites More sharing options...
johnc Posted February 11, 2012 Share Posted February 11, 2012 A general discussion... The higher the roll center the lower the rolling moment. The springs (including the ARB) resist that rolling moment. This is good because you increase you're increasing suspension tuning ability. But, the higher the roll center the greater the non-rolling/overturning moment. That's bad because the car tries to two wheel. Because you are making sure you're roll centers are above ground we don't need to worry about jacking. My blind guess on your CG is about 16" above ground so I would start at 2" front and 2.5" rear to increase your tuning ability with springs. I'm also assuming you're not going to have a lot of total suspension movement. FYI... your stability at 140+ mph will have far more to do with aero then roll centers. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 11, 2012 Share Posted February 11, 2012 Cary (tube80z) changed my mind on this one some years back. I was planning on running pretty high roll centers so that I could run less stiff springs and relatively stiff bars with 10" slicks with the idea that relatively softer springs make the suspension more compliant. I changed plans and dropped my front roll center to near 0 and left the rear slightly higher, and decided on much stiffer springs with smaller sway bars. I'm going to try and get it to work with no rear bar at all. The reason is that stiff springs limit travel, but they don't add stiction within that range of travel. If you run a high roll center you get less body roll because the suspension arms are geometrically fighting the roll. They cannot fight the roll AND respond to bumps very well at the same time, so you lose some compliance by going to a higher roll center even if the effective roll angle of the chassis is the same. Your roll center adjuster in the rear is pretty nice looking. We had a guy here with screen name blueovalz. He cut the bottom part of the strut off and welded on his own to do the same thing. His reason was a little different though. He had pretty large 17" wheels and to get them where he wanted in relation to the wheel wells (he built the flares himself) he had the rear LCA's pointing up at a pretty good angle from the diff to the wheels. Looked like he was running a pretty significantly underground rear RC. I want to say that he lowered the outer pivot 2". It was a good mod for him. He sold his car to another guy on here and has moved onto a Manta build which is pretty damn awesome. The guy is a very good fabricator and was super helpful. Here is his fototime site which still has all of his Z pics: http://www.fototime.com/ftweb/bin/ft.dll/pictures?userid={7DC317B0-8EDB-4B2E-A837-F708D07C9769}&uid={7DC317B0-8EDB-4B2E-A837-F708D07C9769}&guid={1B2E8518-DFD2-42EF-8487-7EB3BDCF2F68}&custdom=1 I'm sure Cary will chime in if he sees this too. He's talked about adjusting ride height in very small increments to change RC and overall balance. My car (or maybe it was me) wasn't sensitive enough to the changes I was making, so I'm hoping that with a lot of chassis stiffening that it will respond better and I can utilize some of his ideas. Quote Link to comment Share on other sites More sharing options...
Leon Posted February 11, 2012 Share Posted February 11, 2012 Good info posted by John and Jon. I'll just emphasize that there is an interaction of roll center placement and CG height, as JC alludes to. Your roll center is the theoretical point where all lateral forces are resolved at. Therefore, the roll center is a virtual lever against the CG height (fulcrum) when experiencing lateral forces. You probably already knew this since you're playing with roll centers, but just keep that in mind. As JC also points out, high-speed stability has more to do with aero and the combination of your suspension setup. Roll center height is not the sole factor in stability, it's a combination of things. Quote Link to comment Share on other sites More sharing options...
ckrell Posted February 11, 2012 Author Share Posted February 11, 2012 (edited) The strut housings are sectioned 2.5" and I found both front and rear KYB GR2 struts from various vehicals so at 4" hide height I have 2.5" suspetion compression and 3.5" droop with OEM rubber isolaters. Im starting with 140# front springs and 185# rears so I will have good suspention travel, very sprited street driving and the local drag strip will be most of the cars activity with maybe getting to the road coruse once a year. So with the softer springs and more suspension travel would you raise the roll center a bit in the rear? And the front roll center should be 2/3 of the front? Tires will be 205/55-14 NITTO NT 01 and I'm up in air on swaybars Edited February 11, 2012 by ckrell Quote Link to comment Share on other sites More sharing options...
johnc Posted February 11, 2012 Share Posted February 11, 2012 With those soft shocks and soft springs, I would not run a 4" ride height (measured at the rockers). I'm safe to assume that the recommendations above from Lean, Jon, and I assumed springs over the 250 lb. in. mark. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted February 11, 2012 Share Posted February 11, 2012 LOVE the Torque Thrusts! Please post more pics of that setup on the Wheel Pics forum, please! If you just make your suspension plenty stiff with spring rate, a lot of this academic suspension stuff probably doesn't matter as much as we think. There is a video that shows just how little the suspension actually "works" somewhere on the forum shot from under the front fender while going around a track-try to find that and you'll be amazed. I'm no expert like JohnC et.al., but I think that once your static alignment is as JohnC recommends, you'll have little trouble with dynamic suspension changes. Beautiful car! Quote Link to comment Share on other sites More sharing options...
ckrell Posted February 11, 2012 Author Share Posted February 11, 2012 The rockers are the lowest part of this car. The floors have been replaced level with the bottom of the rockers and also the crossmember has been removed and replaced level with the rockers/floor and will all be panned in to the nose. If 4" was lowest part of the underside (besides the outter part of the A arms) wouldnt that be resonable? sorry dont have more pics. rite now. Quote Link to comment Share on other sites More sharing options...
ckrell Posted February 13, 2012 Author Share Posted February 13, 2012 Well, thank you all for your advice and tips. I will use 2"/2.5" roll centers and 250# springs as a baseline. Quote Link to comment Share on other sites More sharing options...
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