Daily Archives: March 31, 2019

Well, let’s do a reset

I just got finished watching a good race, Denny Hamlin won, Kyle Busch won 2 of the three races this weekend but not today, and I have to go do things tomorrow. I need to pay for my phone, I need to shop for a new mattress for the bed in the big bedroom, and I need to get a massage.

Also I need to figure out where the links for the front axle are going to fit on the frame so I can make the links to length in scale for the Mini Sprint-T. It’s basic trig and geometry, the axle is x distance from the front hoop, the lower rail is y inside to inside and y+3 outside to outside and the links have to be l distance apart to clear the tires at full lock. This has been determined previously to be 31″ outside to outside, and the links are 1″ diameter so the links have to be 30″ center to center. The only dimension that is unknown at this point is the altitude of the triangle formed by the front hoop and the lower frame rails, because I still haven’t set how far in front of the axle the forward crossmember is, because I still haven’t decided if I want big or small body shocks. The big body shocks use 3″ OD springs, the small body use 2″ OD springs and there is 0.5″ difference in how far forward the front crossmember sits which means there is maybe 0.125″ difference in where the lower rails are 30″ apart center to center measured from the front hoop.

And it has occurred to me that some of you don’t know where the transverse structural parts go and what they are called on the Sprint-T and because it is the same but in scale, the Mini Sprint-T. From the front, there is the front crossmember that holds the front coilovers over the axle and the radiator gets mounted to the front of it and the mount for the track bar that locates the axle from side to side, then the front hoop that is 1.5″ X 0.120″ for most of the structure and 0.060″ for the rest of it because it’s part of the roll cage that has to be 0.120″ wall thickness except for the part that isn’t roll cage but frame, then the rear hoop that is the same as the front hoop, then the rear crossmember that basically doesn’t do anything except hold the rear of the frame apart and protect the fuel tanks in a rear end crash, and provide additional torsional rigidity by increasing d4. The point of interest in today’s post is the front crossmember because the lower rails meet at the center and the upper rails are right over the coilover mounts and also pick up half of the mount to put the upper mount in double shear. The triangle formed by the lower rails determines how far the rear mount of the links that mount the front axle are from the front hoop because the mount has to be 30″ center to center. The links have to be parallel to work best with no bump steer from moving the ends of the axle when the car rolls in a turn. So the center of the rear mounts on the frame have to be the same distance apart as the mounts on the axle to keep the links parallel.

Anyway finding the location for the rear mount is a simple ratio. The problem is I don’t know the terms for the ratio beyond the first term. That term is 30/46.5 or 0.645161290323. That’s the ratio of the center separation of the links to the center separation of the frame rails at the front hoop, the term I’m missing is the distance from the front crossmember to the front hoop to multiply that by. I know about how far that is but not exactly how far that is. But if I think about it some more I can make it so there won’t be a difference because I’ll use the distance for the larger spring and just enjoy the extra clearance if I decide to use the smaller spring instead, so doing some quick calculations, the rear axle is 5″ behind the rear of the body, the body is 52″ long and the rear axle is 100″ behind the front axle and the front hoop is right in front of the firewall and where the frame rails meet the front hoop is 58.5″ in front of the rear axle or 41.5″ behind the front axle and the front crossmember is 1.5″ in front of the front axle, so the distance from the front hoop to the front crossmember is 43″ so the rear mounts for the links are 43-(0.645161290323*43) or 15.258064516111 from the front of the front hoop. Rounding makes that 15.26″ from the front hoop and links that are 23.25″ center to center after using the distance from the center of the rear axle to the center of the holes for the Heim joints on the 4-link brackets and the size of the vertical member supporting the rear mount which will probably be 1.5″ X 0.060″ to make it stiff enough in bending with the load triangulated at both ends, which I just decided. The diagonals will run from the top of the front hoop and the bottom of the front hoop so there will be no possibility of hitting the frame with the front tire at full steering lock. And as previously stated there is another diagonal that runs from the top of the front hoop to the top center of the front crossmember so there is a parallel frame member to capture the top of the rear mount for the links and make that area fully triangulated.

And this has turned into one of the longest non-wreck report posts I have done. So since I don’t want to overload people doing an archive dive, this seems like a good place to take a break and put this post to bed.