Tag Archives: cascade effect

Moving the driver forward completely changes the frame design

Moving me forward of the original firewall really changes the frame design. And I’m not just talking about swapping the seat and gas tank around, either. The original frame could be considered an exoskeleton as almost every part was outside of the body to some degree in order to increase torsional stiffness. Well the new frame is much smaller, only 27″ wide to fit the firewall on the body, up to the point the engine and drive bits mount which will have to be wider because of the transverse mounting.

Basically the part of the frame forward of the firewall are going to be like a ’50s Indy roadster. This is still going to be hella stout and light, but the frame members are going to be different than the Sprint-T derived frame. This frame is going to be designed for a mid engine from the get-go instead of trying to cram a mid mounted engine in a front engine frame. The other major change is all of the frame is going to be round tube, most 1.5″ diameter and most 0.060″ wall thickness except for the bits around me that have to be 0.120 by SCCA rules. This will lighten the frame by a few pounds, but nothing spectacular. What will do the most to lighten the frame is tucking everything inside the body and doing the engine and transmission mounts first instead of making them fit what was already there. Also the Sprint-T was designed around a roll cage that won’t be there because the driver isn’t where the roll cage is designed to protect.

I can make this frame as stiff and light as the Sprint-T, it will just take a bit more brain work to do so. Instead of spreading the frame as far apart as possible I’ll have to concentrate on using internal structure for stiffness. It can be done, and I’m the guy who’s crazy enough to do it. Incidentally, the changes move the fuel tank right over the center of gravity of the car so the balance doesn’t change at all between full and no gas. The Princes of Serendip are victorious again! (Look it up)

On other fronts I need to get that brake hone this evening so my walk will be extended to the auto parts store. And tomorrow’s walk will be by the pharmacy to pick up more not-sad pills for my brain. Tonight I’ll be walking about 3 miles to get some bonus check-ins on my mobile game. I haven’t been able to walk as a workout for 2 days now and I need to stretch out a bit.

More later.

More about that cascade effect, and do I miss the bike stories?

I got a PM from another web site asking me about the bike stories I used to cover, and do I miss them? Well, yes, kinda. I miss being helpful and telling people how they could avoid the same fate if it could be avoided, I miss vicariously yelling at people about crappy infrastructure, but I don’t miss the death and destruction involved. Having been through one myself I know how painful it can be to survive a major wreck, ditto not surviving it. Not surviving is painfully boring but aside from that not bad. I’m the kind of guy that has to be doing something or at least planning with the possibility of doing something, and where I was didn’t have any fab shops or building supplies. And TBH that was my biggest problem covering bike wrecks, there wasn’t a damned thing I could do to prevent another except make note of what infrastructure would have possibly prevented the wreck.

Moving on to the cascade effect on the TGS2 moving the driver’s seat, I marked the body with where the roll hoop behind the driver would be with my feet all the way against the front axle.That black line is where the rear roll hoop goes.

You can see that the rear roll hoop is right in the middle of the bucket. That is not the ideal location for the front center bulkhead if maximum torsional rigidity is high on your list of priorities. Now if the rear hoop is used to triangulate the center bay and driver entry is from the front instead of the top I might could make it work.

Or I could make the TGS2 a two-bay design using the rear hoop as a tower in a bridge structure. This is a design that I’m not familiar with, at least not as familiar as a multi-bay construction. Basically in this construction all the loads are passed through a central structure and everything is triangulated around that. The important thing in this type of design is to keep all the load paths balanced with structure to connect everything directly because there is no secondary structure to carry the load around design gaps. If I don’t provide a triangulated path to carry the load that load will cause the frame to flex. This style of construction is considerably lighter for the amount of rigidity it delivers than the multi-bay construction I had used on the first iteration of the design, but it makes hanging the engine and rear suspension more difficult. And as I’m trying to visualize the structure I notice there is no way to put the bucket on the frame without making a jigsaw puzzle out of the body, so that’s out.

So back to integrating the rear hoop in the center of the wheelbase, again. Or not, I mean it’s not like I don’t have a design already, it’s just not optimum. All this hair-pulling is just trying to find a better solution with better balance. Well guess what, as long as I have 620 pounds of engine and transmission sitting just 10″ forward of the rear axle in a 1600 pound car the car is not going to be balanced. I’m going to have to give up front grip to keep the back end following the front, or install ridiculous rear tires which is basically the same thing.

This combination has a long history in T-Buckets because the construction techniques raised the rear roll center several inches above the axle, which caused jacking, or lifting the inside rear wheel as cornering load increased. This combined with high power caused severe oversteer which got patched by stupid wide rear tires. If you watch videos of Goodguys autocrosses you can see this effect when someone tries to run a T-bucket. Any power applied before the car is pointed straight results in the back end trying to pass the front. This was also the problem with the pre-1965 Corvair, the swing axle rear suspension had a very high roll center combined with rear weight bias caused by hanging the engine behind the rear axle and 4 equal-sized tires and you have the perfect recipe for leaving the road back end first. This car was designed during a period before computers were used to test designs and also during a period when GM banned participation in motorsports so designs were not examined with that potential in mind, which would have caught this blunder before it entered production.

And after that historical note I think it is about time to take my walk.