seattletom wrote:
Marcus, Car9G just gets better and better. A couple of questions, though:
On the Ford IRS SU model you posted, the tubes that I scratch my head over are the diagonal ones that thread between the lower parallel links. If the diff height were changed to achieve one’s desired ground clearance given a specific tire O.D., might there be potential interference between the diagonal tubes and the lower parallel links when there is vertical wheel movement?
I think there is a good amount of room. To help with that I made a version of the parallel link that shows it's motion. Hmm, I think that SketchUp could animate it, but will save that for a while.
The problem I have had making the car stiff is that main roll hoop bulkhead. The car is very stiff along it's sides up until that point, much more so then the normal Seven or Locost. That bulkhead warps under load though, partly because the rest of the car does not. Putting an x brace in the rear facing hoop didn't help at all, to my surprise.
While the rear braces went straight back the only thing that helped was x braces between the upper and lower backwards braces. The lower ones get in the way of the diff and especially the transaxle. After days of effort it occurred to me to just meet all those rear braces together at the back. It was a success - but it took many hours to get there.
When those lower brace tubes go in, it greatly increases the stress on tubes in the front of the car, even the little diagonal across the very front bulkhead. That's a good thing.
I'll do a little more work with the Grape FEA software and come up with a number for how much those tubes help. I can also grab a shot of the car with the frame warped, maybe. Grape will display an animation of the warping but I am still very clumsy at getting the screen shots - so it might not work.
I also need an opinion from folks here or perhaps tech folk at either SCCA or NASA on wether the car's rear braces are legal. They are more then the required 30 degrees back from the roll bar and within 6" of the top. They look different though and that might be an issue.
seattletom wrote:
Could you also explain why there is no need for cross or diagonal bracing or separate toe-in adjustment links between the lower (parallel) control arms. Is that all taken care of by the trailing arm / control arm design?
Yes. That's one of the nice things about the trailing arms. The torque forces from the brakes are taken by the trailing radius rods. With slicks and a rear weight bias ( especially in a midy ) that can be a substantial load on the wishbone. The engine torque is taken by the diff mounts. The toe adjustment is simply controlled by the parallel links, pick one and adjust it. There is also no bump steer compared to what I was doing earlier with the reverse wishbone.
seattletom wrote:
Cutting all the round tube fishmouths accurately could be a challenge. I was thinking of how much PVC pipe I would go through creating trial-fit diagonals, etc. But then found the post referencing
http://digitalpipefitter.com/ and became intrigued with both the software’s function and their business model: Layout software is free, only the print version is fee. Seems one could create a template “print” service business that would work something like this: Locoster creates his layouts on the free version and emails the files to the Servicer. Servicer (who has purchased the $$ version of digitalpipefitter) “prints” them to a print file and emails them back (for a modest fee.) Locoster prints out the actual templates and trims their own tubing. Trimmed lengths and end-to-end rotational alignment would be the only variables Locoster would have to look out for. More work than having them laser cut, but could be an accurate, inexpensive DIY approach. Just thinkin’… always a dangerous thing.
I looked at the digitalpipefitter website and read their license. I think they basically prohibit that type of activity. For the obvious reasons. It does appear that you could use the program to make templates or dxf files for a shop with tube cutting machinery and produce parts though. I think it cost something like $350-$400 though. On the other hand, I think they just prohibit compensation so I could just give the templates away.
It also seems like it just deals with the tube ends. So you can't really just give the outputs to a laser cutting place. You would need to take the output of this program and then ad stuff in the dxf file to describe how long the tubes are. Maybe I should read their manual...