Hopkins Baja Design / Build Thread

 

Just took it off the scales, we've added a couple of extra tubes but the final weight is 34903 g (76.947 lbs)

 

a bit heavy for sure, but this is one crazy stiff chassis.

 

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Thanks John, I think both will pass fine as well. 

 

Cheers,

Adam

There was some mention of the forward bracing being rather far away from point C.  I agree that it looks funny (and could have been moved further up the bend... slightly), but there's a strange technicality in the rules about how the FBM up and the RH overhead are defined.  Based on the wording of the rules, the forward bracing by definition connects to the FBM up (and point C is defined to be on the RHO...). 

 

The much better reason, however, is rule 31.2.8, which stipulates that no part of the triangle formed by the foward bracing (between the SIM and FBM up) may have a member with a bend greater than 30 degrees.  The angle between the FBM and RHO is required, by rule 31.2.7, to be greater than 45 degrees.  We interpreted this rule as requiring the FAB to be significantly beyond the start of bend on the RHO (which is also point C) so that the bracing triangle does not contain a significantly bent member.  It is impossible to legally connect point C to the SIM and have everything else on the frame be legal to the rules, hence the "close as possible" phrasing of 31.2.8.1 when describing front bracing attachment location.

 

We could have moved the front bracing attachment further up the bend, but hesitated for a few reasons.  First, we were using .035" thick 1" tubing for the member (which was being welded to 1.25", .065" thick) and we don't have any way of notching a tube end to accurately match the curve of the bend.  Invariably, we would have had to make several attempts to get the curve right and evenly on both sides (and we would probably have run out of .035 tube), and the gap left would have made for a hideous weld.  Notching to the flat (ish) start of the bend was much easier to do accurately and evenly on both sides, and was less of a welding challenge.  Secondly, we thought the rules had enough wiggle room to allow for this.

 

Also, our frame could lose some weight.  We had thought that .065" was the cut off for good bendability for some reason, but now that we know .049" works as well we'll be incorporating a lot of it next year.  For all of the triangulation and most of the straight bars, we starting using .035" 1" tube this year.  I do about half of the welding on the car, and I love this stuff.  No, I don't like welding it, but it really is amazing stuff if you can keep it away from impacts and out of buckling.  Our frame is not yet excellent, but it is much much better and much lighter than anything we made in the past.  We tested the torsional rigidity of the 2010 chassis against the 2009 chassis (plus body panels still attached), and found it to be 71% stiffer AND approximately 30-40 lbs lighter (with all of the Frankenstein modifications made to that thing, no one knows what it weighs but it's a boat anchor).

 

Seriously: to those who don't yet do it, incorporate .035" tubing.  Welding it is either easy or hard as hell.  If the gap is really small, it's cake and looks very pretty.  If it has a large gap (and by God did we have some large gaps...  1/4" in a few places...), then it is incredibly tricky with failure rewarded by tube vaporization.  The issue we ran into was fitting triangulating members to a node that already had some other members welded to it.  We had to grind off one edge of the notch to fit the tube in, and then prayed for the best when welding it.  So watch out for that and plan ahead when assembling .

 

One last thing: Anyone have recommendations for body panel fastening?  We, as a team, are COMPLETELY PARANOID about putting tabs on the car that only hold body panels.  They're a pain in the ass to make, hard as hell to get in the right spot (and angle and hole position...), and welding them securely creates a hell of a lot of welding work in funny places, usually where torches don't reach.  So we considered Dzus clips... but they look like premade tabs with even more finicky fasteners.  We have, for now, decided on riveting certain panels to the frame tubes directly, using sealing rivets, because it's super light, easy work, and really strong.  The down side being, every time we remove and replace one, a jingle gets added to the frame that will never come out.  This is fine for the firewall, and possibly the floor panel, but side panels?  Meh...  and if we make composite panels (unlikely, but we have some members really interested in doing it) the rivets won't play nice with them (cracking, etc).

 

I'm very interested right now in sealing rivnuts.  Watertight, threaded so panels can be removed easily, lightweight, and relatively easy to install (vs welding in tabs).  They can be installed blind, and supposedly are okay on tubes with small radii, but I dunno...  thoughts on this?

 

(As you might have noticed, I'm prone to Wall O' Texting)

 

Hope we make it

 

 

 

Later,

Adam

 

 

 

 

 

 

 

We did have some fun, it was really nice to have the car testing so early :)  Everything went really well, it is a very maneuverable car for us.  We had hypothesized that making the front shocks softer, much much softer would add to the turning of the car and were correct.  The drivetrain tests did not have any CVT tuning, only stock so those times can only get better.  We did a skidpad and that took a look at steady state handling, but it was hard to get steady state tricycling.  The only incident we had was a a chain pop off of the 1st reduction.  This was caused by a slightly misaligned input shaft, and slight being ~0.5 mm.  The 35 pitch chain is very small and small misalignments which are normally fine for 420 chain really seem to matter.  It was about closing time anyways and with out the necessary tools to fix it we just headed home. 

 

Great DAY!!!! Made staying up all night ~ 3 nights totally worth it

 

Adam

We used #35 for bot reductions which is probably why we had the issues that we had.  Using 420 like you are is definitely a smart decision.