Some good points, and I especially like the idea of moving the shock outboard. Could anyone comment on the convention of pointing the shock towards the tire contact patch? Intuitively it seems good, but in looking at leverage, ratios and all that it seems it shouldn't matter in the least. Especially in light of some of the cantilevered inboard designs.

Seems pretty obvious that up to the design limits of the shock, the more shock motion you can get for a given wheel motion the more tunable the suspension will become. So wouldn't the first requirement be to determine the limits of the shock? We have some pretty advanced off-road units, but why would we want to use that here? Seems like size and complexity is an issue. The key I think is to work within the performance envelope of the shock or coil-over that fits the available space at a reasonable price.

Jim

Thanks Jim. The one that is on the road seems to handle pretty nicely, and so far everyone who has driven it has been well impressed with it (somewhere between one and two dozen drivers). It has no roll bars but feels pretty well balanced. At the limit we get a slight rub somewhere in the back but it is so little we haven't been able to determine where it is yet. Might just be a matter of a couple hammer strokes, or we could fit roll bars.

My car has bigger diameter tires and rides a little higher. It also uses stagger with a 315/35-17 rear and 265/45-17 front (I think) and has a posi with fairly stiff preload, all of which affects the handling. But I think it'll be within reason.

If I could get some sort of a handle on the Jag RC it'd tell me a lot about the roll axis of the car. We took a stock XJ6 unit and narrowed it 5-1/4" IIRC, 3" off one side and 2-1/4" off the other to center the driveshaft. Naturally that will shift the RC slightly and I might build the next one centered just because of all the questions that causes, but unless you put the driver in the center of the car I can't see it really making any difference. In fact it could very well offset driver weight.

So it seems to me that the Jag IRS is generally set up within a range between level half shafts and level LCAs. My thinking is that level half shafts is probably correct.  The MGB rear suspension of course has the RC at the level of the spring hangers. Well, more accurately I guess, where the line from the front spring hanger to the shackle hanger intersects the vertical plane of the axle. At least that is my understanding of how it is located. What I would like to determine is if that point is within the range mentioned above, or how much above it. If the half shafts are level then the IC is going to be at the level of the half shafts or wheel center, some distance out so clearly the RC should be perhaps just something like 2-4" above the ground.

If true, this greatly reduces the angle of the

Apparently I was wrong. I'll try to check physical parts in the next day or two but the MG guys tell me that the control arms ARE parallel. That would put the instant center midway between the arms at infinity, making the RC just slightly above the ground. I'm still trying to get info on the CG.

Anyone know what the roll center for the Jag IRS is? Any estimate of the rear CG on, say a Cobra for instance?

Jim

I've asked the MG guys where the RC is on the front of the MGB. One guy came back and said it was at ground level. I don't know if that is true or not, but it is common to see the LCA's inclined to the inside.

Jim

Wow, that helped a lot. Also sort of explains why you might want to push the roll center below the road surface in some cases, especially with a low CG to get more down force on the tire. BUT, here is where it does not make sense. Total maximum down force is limited to the weight of the car plus aerodynamics. If we disregard the aero for a bit, car weight is all we have. Now it is true, we can concentrate that all on the outside tires. We can also shift as much of it as we want to the inside, theoretically. IF all tires were planted squarely, the ONLY advantage to concentrating the down force on the outside tires is that they are on a radius to the corner center point that is shorter by the width of the car than the inside tires. However, this is enough of a margin to win races in some cases. What advantages would there be to even downforce? Well, if all four tires are equally loaded then all four maintain the same tread temperature so on a road course all are equally sticky and the tread compound can be optimized in a narrower band. With outside loading the outside tires get hot while the inside tires cool down so temp variations are wider and the tires must work in a wider temp range. Is this enough to use as a competitive advantage? Perhaps, but apparently at this point the suspension geometries are still in the way of doing this and a new design would be needed to get rid of the jacking forces mentioned above. What geometry would do this? I don't have a clue. But if you don't have to fight roll all the time than camber changes present much less of a problem and it seems that most of the development on the double A suspension has been to try and control camber.

Jim

Ralphy, are you saying the rock is the car body (assume CG or mass centroid is halfway up I guess)? So what is the lever? Is the base of the rock the contact patch? If the lever establishes the RC then something is backwards here, since the lever is acting on the rock rather than the other way around. If the rock acts on the lever then the rock will tip over when the lever is low and it will slide when the lever is high. It's the car body, Center of Gravity, CG or Mass Center that's doing the pushing, not the roll center. Unless the definitions of these terms are somehow contrary to reason.

Jim

Quite right, and there are far too many who still follow that plan of attack. But skating over bumps is no way to get around a corner. Despite the theory that a sway bar will simply level the car and somehow improve cornering in the process, the simple fact remains that at the limit you put the outside tires right on the hairy edge of adhesion. Transferring more load to them by lifting the inside tire is not going to make them grip any better under those conditions, it will either turn them to jelly or force you to run harder rubber which won't grip as well. Either way you loose traction. Then of course there are the bumps and transferring those from side to side is only going to increase the skateboarding effect.

The stiff springs do have their place, such as enabling you to lower the car and still keep it from bottoming out. But at some point, if allowable, it just becomes good engineering to trade ride height and stiffness for track width and ride quality/anti-skateboarding.

One of those books, it may have been Carrol but I can't remember, went into a very good explanation about the maximum limits of adhesion at the contact patch and how force vectors influence traction. Understanding the influence of lateral G force, braking, acceleration, and suspension movement is key to getting the fastest lap times or maximum cornering in the mountains. Once that is fixed firmly in your mind you can clearly see the results of ideas like applying the brakes to the inside rear tire in in an attempt to assist turn-in and it easily explains why cars with traction control enabled are slower. The same principles apply to sway bars. Provided body lean is not excessive, less is more.

So now, the concern becomes how to control body lean without resorting to sway bars, which in reality are nothing more than a tuning aid rather than a major suspension component. Thinking along those lines, wouldn't lowering the roll center be counter-productive? If the roll axis follows the CG then

Reverse cut gears are something that Ford popularized, I think starting in the late 70's. Before then and after, standard gears were commonly used in the front. You have to think about it a little to get your head around it, but if you spin the axle around the ring is on the opposite side of the diff and runs it in the opposite direction, so it all works out. What Ford did was a re-design for a high-pinion housing that improved the front driveshaft angle. Remember, driveline angle plays a part here and the front driveshaft is fighting it going downhill to the front. So the high pinion improved NVH and u-joint life. (Incidentally, I have a reverse cut 3.54 Dana 44 set if anyone needs them) CV joints in the front driveshaft are another common solution.

You might be better advised to start with the spindles from an IRS 4x4 truck and work inward. I think it will probably be easier that way if you can find something that uses "A" arms already. Though the Vette set up may work also I'd be a little concerned about the strength of the cartridge bearings when you go mounting your 30"+ wheels and tires and hitting the woods, cliffs, and mud pits. You want something that won't break first and foremost, so bigger is better in this case. Also, since your big wheels/tires are going to stick outside the bodywork some anyway, it is worth considering going a little wider. With independent, if you can get wheels that are offset deeply to the outside it means more room for the control arms. Sometimes a LOT more room, and that can really make a big difference both in suspension travel, and in your selection of a donor axle. With the right wheels and the right donor you might not have to narrow the IFS at all. Of course this raises an issue with lockout hub choice but the newer designs commonly use an automatic hub that doesn't project as far as the older designs so there may be something out there that works just fine.

Jim

I bet. That will make a sweet engine. The SBF is a pretty light engine also, within 5 pounds or less of the SBB in some cases. So you do have a real potential lightweight on your hands. Let us know what it weighs when it comes in, OK?

Jim

That'd be quite a feat TR, and best of luck with it. I hope it turns out as you want. The 215 Buick comes in at 318 lbs. Interestingly enough, when TAPerformance began producing their new all aluminum BBB block the weight was so close to the cast iron block that there was little if any weight savings by going to the aluminum block from cast iron. Now admittedly the TA block is a lot stronger, has reinforcements that the stock block does not, and will support displacements up to at least 700 cid. But still. That's a LOT of aluminum. The difference in weight from the 215 to the 300 and 340 (all the same series of engine) is 80 lbs. The cast iron just about doubles the weight of the block. So TA would have had to add at least 80 lbs to the block and possibly more. I guess it adds up, but it's still a LOT of aluminum.

So anyway, going to an aluminum block for the 455 will not make the engine any lighter. It can make it bigger, will make it stronger, and more reliable in most cases, definitely can make it more powerful, but lighter? Nope. Ain't gonna happen.

I just looked up some numbers. Actual weight for the stock block with caps and brass freeze plugs is 157.5 lbs. TA's aluminum block with steel main caps is 158 lbs. Without the caps it's 112 lbs. Yes, the caps weigh nearly 10 lbs each, but the block is good for 2000 hp, possibly more. Aftermarket cranks come in around 79 lbs.

Jim

Apples and oranges TR. There are other weight charts that show other weights. 600 lbs shows up a lot. For 770 lbs that must be a really heavy air cleaner.  We did corner weights on the car before the interior and glass went back in and it was fairly well balanced. Next month at the annual BritishV8 meet they will corner weigh the car again for a total weight and corner numbers. I'll let you know what they get. The other mods also add up of course. We dropped some weight with the tubular exhaust, alloy water pump and the mini starter of course, but the front brakes added a few pounds. All I know for sure is the numbers I got weighing both engines. Now the way it stacks up is, the aluminum BOPR (215/Rover) is -30lbs. The '64 Buick 300 and SBF are +50 at around 400 lbs. The BBB adds another 50, as does my Buick 340 with Eaton blower and intercooler. Weights are approximate of course.These are all fitted with the light weight parts.

Jim

Rigid mount. That'll be a little noisy.

Jim

Where is the weld bead?

So, from what I can tell from what I could see of those videos we're not quite on the same page. What you're calling wheel hop seems to be what I'd just call a little bounce or maybe uneven traction, or maybe squat. When I think of wheel hop I'm thinking of pretty violent excursions, maybe even with the tire leaving the pavement. Hop, to me implies a rather massive loss of traction. But what I saw in those clips, good traction was still there, just sometimes a little better. I understand that it could hurt you in a drag race but it seems like a much smaller issue to deal with, and probably has more to do with planting the tires. Heck, that one clip showed definite squat of several inches and there's no way to keep the traction even with that going on.

Jim

Ralphy, If you'd like to try it that way I'll do all I can to assist. What I see as a problem is the squishy nature of the test in general. Tires squirm and move, bodies twist, bushings give, etc.. But, for that particular vehicle those must all be dealt with so the test is still valid.

Jim