Daily Archives: March 10, 2021

Unlocking the technobabble of the Sprint-T frame description

I admit to using probably Too Many Words™ to describe the frame for the Sprint-T, so now I’m going to use more words to try to clear up any confusion. And yes I thoroughly understand how using More Words To Clean Up Too Many Words™ is a Moron With The Strength Of An Ox™. And if you got that last ™ statement, you may have seen Sheep in the Big City. Also happy Coronaversary, the one year anniversary of the Pandemic ending in March, on March 376 2020.

I’m a wordsmith, but sometimes words fail when I’m trying to describe technical stuff. What I’m trying to do now is take car of one of those failures by using more words to fill in the gaps left from using Too Many Words™.

The frame for the Sprint-T is described as an exoskeleton space frame around a fiberglass T-bucket body from Speedway Motors, that is loosely based on a raised rail sprint car chassis.
What loosely inspired this build, loosely interpreted (not this particilar chassis)

What is actually used (in concept) from the frame in the picture is the frame member that runs from the front of the frame over the roll hoops to the back of the car. That is the “raised rail” of a raised rail frame. The idea is to increase torsional stiffness by increasing what is known as the Radius of Gyration, a theoretical concept that describes resistance to twisting by basing it on the volume the structure encloses. I know it sounds like a bunch of hooey, but there is a direct experimental correlation between this measurement and resistance to twisting.

What I’m doing that is different is because of rules I’m making the front and rear hoops actual hoops of one continuous piece of tubing per hoop. In my case they are one length of 1.5″ diameter 0.120″ wall DOM steel. And they are identical front and rear, basically up and across and back down all in one piece to make an upside-down “U” that runs from one lower rail to the other. The rear hoop has multiple diagonals to stiffen it laterally and prevent deflection from loads coming from the side, and to provide a place to mount the shoulder harness straps. The front hoop will not have any bracing above the base of the windshield so forward vision is not obstructed. But there will be bracing below the windshield to make the front hoop more resistant to bending from side loads. There will also be bracing from the top of the roll cage to prevent side to side motion from side forces applied at the top of the front hoop.

Part of that is a brace that connects the front-to-rear frame member (the top rail), that runs over the top of the frame, to the front hoop. To go with that is a brace that runs around the inside of the top of the cage to the inside of the top rail but at the same height to reduce the deflection of the top of the cage. Basically this is another loop closer to the centerline of the frame that connects to the rear hoop directly and to the front hoop and the top rail indirectly through additional braces of lighter material (allowed because this is not a regulated part of the roll cage). This braces the roll cage but is really intended as torsional bracing to keep the frame from twisting under load.

Now the words to describe how to make the top rail of the frame, because that is the tricky part of this build. The top rail runs from the front coilover mounts to the rear of the frame. I mentioned in an earlier post it doesn’t run to the rear spring mounts because those are on the rear hoop, but there will be bracing from the place where the top rail ends to the rear coilover mount on the rear hoop, with multiple braces to triangulate this mount in multiple planes so it doesn’t flex under load. The top rail runs straight between the two hoops with the bends needed to point it at the front and rear terminations starting before or after it passes the hoops. The center of the bend is right on the front face of the front hoop, and the rear face of the rear hoop.

An interesting thing about the front coilover mount is it is determined by where it has to be to keep the front tire used for transit (the tallest of the tires used on the car) from hitting the lower rail of the frame when the tire is at full bump and full lock. I will find this by drawing the tire at full bump and then making the bottom of the lower rail tangent to the tire in the straight ahead position. This will prevent the tire from hitting the frame when the tire is turned because the top of the tire moves away from the bottom of the lower rail as it turns because geometry. As the tire is turned the part that is closest to the bottom frame rail moves forward because the inside edge of the tire gets ovalized in profile, basically turning from a circle into a line when the tire is parallel to the axle. And since the tire can’t get parallel to the axle it becomes a skinny oval safely clear of the frame rail. I guess that to make sure I should draw a rectangle tire parallel to the axle and make sure the outer edge of the tire clears at full bump. And as the SCCA tires will stick out more than the transit tire I should probably check them, too. They are shorter but wider, so there is a chance they could rub a bit if I don’t take that into account.

And I’m starting to do the composing equivalent of babbling, so I’m gonna call this a good time to quit.