Coburn_Performance
10-29-2017, 09:29 AM
As I push on with my project...it's time to start thinking seriously about putting the rack on the new subframe that it just about ready to weld up. When I started this project, I had to relocate the steering rack down to make room for the engine. This meant that I needed a custom steering rack - it's short, so I contacted Woodward (4 years ago) and they made me the rack. It came with no instructions.
Ron Sutton helped with suspension analysis and steering set up location. With the old cross member I was not able to achieve optimum Ackerman angles and then I found out that the cross member that I bought is a twisted piece of garage art! So I'm fabricating my own. That's the back story...
Here's where last night I google Woodward and discover they now have installation instructions. These instructions include dire warnings about the mounts with notes that the assist ram can bend cross members. They suggest 3/8" plate but are REALLY short on anything that I would call a reasonable instruction. Sorry Woodward - do better - I guess it's a start.
I can't mount the rack level - I need to get the ram out of the way of the front of the engine...so there is a tilt involved. I'd rather not weld in a giant 3/8" thick plate from side to side of my cross member as it just doesn't make a lot of sense to me mechanically.
So here's my question...can someone who has done this take a look and help me out a little. I need some specific advice on what I'm doing. I searched and didn't find much.
Here's how it sits with the old cross member. I'd drilled through holes and mounted 1/4" tabs on top of 1.375" 120 wall DOM tube and TIG welded those in place before instructions existed. I figured this was fine based on nothing. It's a 1/2" bolt in single shear (well, two of them) for 17,600 psi of shear strength X 2 (is this even the right way to compute this? I'm not an engineer.). So 35,000 psi of shear strength in the bolt. Now given the closeness of the steering gear casting to the edge (it's about 3/4" from the bolt centre), I'm guessing the requirement for 3/8" plate is to provide additional strength over the edge tearing out.
https://static1.pt-content.com/images/pt/2017/10/24164618748_22bb9421c1_b-1.jpg
So I have built this for the start of the new cross member - these are the longitudinal pieces that will locate the lower control arm. They will be tied to the 2 X 3 120 wall frame with 1.5" 120 wall square tube in the front and 2.5" X 1.5" 120 wall rectangular tube in the rear position. The idea was to then join them together using 2.5" X 1.5" rectangular tube in the front and connecting the rear mount to the front part using 1.5" 120 wall square tube. Then plate the bottom as a jacking point.
https://static1.pt-content.com/images/pt/2017/10/37985390832_05c1fa4e0e_b-1.jpg
So I have these parts sitting in the shop - some 1/4" thick tabs and some machined ends for 1.375" 120 wall DOM tube. Both fit on the rack. The tabs are easier and would be fully gusseted and ringed on the front. The others would be mounted similarly to how they were in the previous cross member. Or I can just go get some 3/8" plate and weld a giant plate to the front of the cross member. I really don't want to do that as I don't think the weight is actually doing anything useful.
https://static1.pt-content.com/images/pt/2017/10/37985388122_14a161cb79_b-1.jpg
The 3/8" would provide 50% more bearing area for the bolt...is this why this is done? Could I switch it up with thicker 3/8" tabs or tops for tube? The car will have a finished weight under 3000 lbs and the front tires run with almost no scrub radius and are limited to around 245 in section width. No race slicks are planned. I just need to do this right and, more importantly understand why I'm doing it.
Craig
Ron Sutton helped with suspension analysis and steering set up location. With the old cross member I was not able to achieve optimum Ackerman angles and then I found out that the cross member that I bought is a twisted piece of garage art! So I'm fabricating my own. That's the back story...
Here's where last night I google Woodward and discover they now have installation instructions. These instructions include dire warnings about the mounts with notes that the assist ram can bend cross members. They suggest 3/8" plate but are REALLY short on anything that I would call a reasonable instruction. Sorry Woodward - do better - I guess it's a start.
I can't mount the rack level - I need to get the ram out of the way of the front of the engine...so there is a tilt involved. I'd rather not weld in a giant 3/8" thick plate from side to side of my cross member as it just doesn't make a lot of sense to me mechanically.
So here's my question...can someone who has done this take a look and help me out a little. I need some specific advice on what I'm doing. I searched and didn't find much.
Here's how it sits with the old cross member. I'd drilled through holes and mounted 1/4" tabs on top of 1.375" 120 wall DOM tube and TIG welded those in place before instructions existed. I figured this was fine based on nothing. It's a 1/2" bolt in single shear (well, two of them) for 17,600 psi of shear strength X 2 (is this even the right way to compute this? I'm not an engineer.). So 35,000 psi of shear strength in the bolt. Now given the closeness of the steering gear casting to the edge (it's about 3/4" from the bolt centre), I'm guessing the requirement for 3/8" plate is to provide additional strength over the edge tearing out.
https://static1.pt-content.com/images/pt/2017/10/24164618748_22bb9421c1_b-1.jpg
So I have built this for the start of the new cross member - these are the longitudinal pieces that will locate the lower control arm. They will be tied to the 2 X 3 120 wall frame with 1.5" 120 wall square tube in the front and 2.5" X 1.5" 120 wall rectangular tube in the rear position. The idea was to then join them together using 2.5" X 1.5" rectangular tube in the front and connecting the rear mount to the front part using 1.5" 120 wall square tube. Then plate the bottom as a jacking point.
https://static1.pt-content.com/images/pt/2017/10/37985390832_05c1fa4e0e_b-1.jpg
So I have these parts sitting in the shop - some 1/4" thick tabs and some machined ends for 1.375" 120 wall DOM tube. Both fit on the rack. The tabs are easier and would be fully gusseted and ringed on the front. The others would be mounted similarly to how they were in the previous cross member. Or I can just go get some 3/8" plate and weld a giant plate to the front of the cross member. I really don't want to do that as I don't think the weight is actually doing anything useful.
https://static1.pt-content.com/images/pt/2017/10/37985388122_14a161cb79_b-1.jpg
The 3/8" would provide 50% more bearing area for the bolt...is this why this is done? Could I switch it up with thicker 3/8" tabs or tops for tube? The car will have a finished weight under 3000 lbs and the front tires run with almost no scrub radius and are limited to around 245 in section width. No race slicks are planned. I just need to do this right and, more importantly understand why I'm doing it.
Craig