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MG MGB GT V8 Factory Originals Technical - What do you guys think of a tubular Cross member.
|Either the round or square metal and make it for a 289 or 302 engine. What are your thoughts or suggestions on this subject? Is their anything that you can tell or suggest to me an advantage with making one vs. using the original car one and welding it?|
As I see it the advantages are lighter weight, improved geometry, greater rigidity and greater freedon in designing the suspension to your own specifications.
The front suspension layout and general proportions as used on the original Lotus Elan or similar would be a good starting point.
Personally I would be looking for a lower arm with about 5 degree of droop and an upper arm with about the same degree of inclination and replace the original lever arm shock arrangement with coil over Koni, Bilstein or similar properly matched.
The ratio of upper arms to lower is about .66 and the trunnion arrangement needs to be replaced with ball joints upper and lower.
Just a few thoughts.
|A new crossmember/suspension setup is in the works, and a very nice one, I might add. I won't say any more than that, as I'm not involved in its development and I don't know if its developer is ready to announce it yet.|
I have seen a prototype and talked with the engineer designing it, and I want one! It will clear the oil pan on a Ford engine.
|It was good enough for Bugatti from about 1926 onwards, a thing of beauty ...|
|One or two considerations in crossmember design: Aside from the obvious requirements of holding the front of the car up, a primary objective in crossmember design is to provide torsional rigidity to the vehicle frame or in this case body. Think of it as resisting longitudinal twisting of the vehicle as a whole. Visualize a pair of frame rails with a crossmember at one end, then lift one end of one rail. For this purpose there is absolutely nothing that works better than a large diameter thinwall round tube. Obviously, for ground hugging vehicles this is impractical, but the closer one can stay to that ideal the better. This is why crossmembers are generally formed parts, as the formed shape can more closely match the ideal and provide maximum rigidity with a minimum weight penalty. Any reduction in the cross section of the part without a corresponding reinforcement is clearly going to compromise that rigidity to some extent. Since any reinforcement sufficient to retain the original rigidity must use a less than optimum geometry, clearly a weight penalty must be paid. That is, unless you switch the material to unobtainium. So the thought that the newly designed cross member will be lighter *and* more rigid is very likely inacurate. The existing one is very light. Not much over 20 lbs as I recall, so there's not a lot of weight to be lost there. By the same token, a 10 lb weight penalty to be able to fit a 302 engine would seem reasonable.|
In terms of upgrading the suspension, again the original is much better than many suspect if properly tuned. Unsprung weight is low, the geometry is good, and the shocks are infinitely tunable and add very little mass considering that they also act as the upper A arm. The springs are a large diameter for the weight of the car which improves durability and resistance to sag. True, the components are expensive and require regular maintenance, but any suspension will have wear points. Assuming one wanted to improve the ride though, how could that be done? Seemingly a very simple means would be to replace the coil spring with an air spring. This retains the tunability of the Armstrong shock since you can't very well put a tubular shock down the middle of the air bag without spending serious money. Luxury cars and OTR trucks have long proven the viability of air suspension for isolation from road shock, and if the goal is the ultimate suspension for the least cost this seems like an excellent option. It is even possible if one wanted to get sophisticated about it to provide a remote reservoir for the airbag which could be varied in terms of internal volume, thereby making the spring rate tunable. I can see this as being worked up as a simple drop in solution, however I see I've gotten off on a tangent.
The other primary requirement in crossmember design is the one everyone understands, resisting flex vertically as loads are applied. Not much new to add there as if it satisfies the torsional requirement it will likely handle vertical flex assuming sound design practices are followed. However, suspension attachment points can be a problem and this has cropped up in the Hot Rod aftermarket. As there have been some parts on the market with inadequate attachment points, breakage has resulted from such causes as sharp corners and stress risers, and sometimes overlooked loadings cause failure. Looking at the factory crossmember and suspension, it can be seen that in most respects the strength of the part and it's attachments are well above minimum requirements while mass has been pared to the bone. Attachment points are not in any place composed of welded on tabs, the closest to that being the steering rack mounts which are well reinforced. The lower A arm mounts attach to a large load bearing surface immediately adjacent to a reinforcing vertical member and it is thus heavily reinforced in all directions. The same is true of the shock mount, which is also reinforced with a lightened steel plate, and the spring makes double use of the same component. The frame attachments are widely spaced, and with the very large cross sectional area at all locations, not a great deal of steel is required to give considerable rigidity. It would be a real challenge to build one that is lighter but still as strong without access to stamping dies and a tool and die shop. However, there is a need for one, as the 302 won't fit as is, and it appears to be a very suitable engine for this car otherwise. And if anyone is up to the challenge these guys are. I will be quite interested in seeing the result once they are ready to show it. Oh and Dan, if you would like me to critique the design I'd be happy to do that and give you whatever suggestions I can recommend. Completely confidential of course, one of my hats being that of patent attorney.
|Man G your reading my mind. I have been thinking about a tubular for many months now. I have been hanging witht he mustang folks and they use tubulars all the time, and they are thinner than ours are originally. One of the group works at a fabrication Co and has mentioned making tubulars for the mustang guys, I am hitting him up on seeing if he can copy a stock MGB member in tubular form and also look at strengthening were needed..|
Dan, have your person keep us informed!!!
This thread was discussed on 17/01/2003
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