Ok .... here goes.
The 521 arm pack is basically the same rear arm as the LA243 we all ran on the RB5 before the SP came out. The problem for this was that LA243 came with an old old spec ZX5 front arm and the required rear arm, hence why loads of RB5 runners have masses of ZX5 front arms they give away to ready set runners at club.
UM521 solved that by placing the LA243 type rear arm in a pack with an RB5 type front arm so no waste.
UM521-1 is basically the same arm as 521 but it only has 1 shock mounting hole, this being between the two original holes. I hope that makes sense. Some people I know keep both in stock so they can make small adjustments (basically 1/2 a hole spacing) to the lower shock position, a tuning aid.
Although not absolutely correct if you use the rule of thumb that by changing the lower rear shock mounting point you change effective wheel rate by the equivelent of two spring rates. For example if you move from the inner hole on the 521 wishbone to the outer keeping a number #65 spring the wheel rate will be similar to that of a #75 spring on the inner hole so to put the wheel rate back to where it was you would need to use a #55 spring .... does that make sense?
For info the front is broadly similar, approximately 2 spring rates for per hole position as a rule of thumb ... again not exact.
The 519 and 519-2 are similar stories.
519 was the "high strength part" and 519-2 is the same part but with the camber link holes moved outwards by 1/2 a hole spacing, this means that the inner camber link hole on the 519-2 is half way between the two holes on a 519.
The ali hubs 516 have both sets of holes in, a bigger bearing and last forever ... job done IMHO.
The shims are there to give finer adjustment of the antisquat and hinge pin to diff relative positions.
Again not 100% exact but a guide;
RF and RR blocks no spacers or the same thickness spacers = 2deg antisquat
RF inverted, RR as kit (again no or same spacer) = 0deg antisquat
Each difference in spacer thickness give approx 1 deg antisquat change ..... for example RR and RF as kit with thin spacer under RF will give approx 3 deg antisquat. I will be honest and say I've not measured the spacers thickness and calculated the exact antisquats myself, something I need to do, so that info is 2nd hand I am afriad.
In case you don't know how to use the change in hinge pin to diff outdrive height here is the rule of thumb;
spacers under RR and RF raise the hinge pin closer to the diff = less side bite so easier to drive when grip is high.
Again, I hope that makes sense mate .. just ask again if not.
The Kyosho cars are just about the most adjustable out there in 1/10th at the moment and whilst that can be brilliant for those who know what everything does in combination with everything else can be a little confussing to some. I would suggest downloading Elvo's buggy setup guide (sorry no link but it comes up on google) as that explains most things in a sensible way.
Any further help just ask.
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