Adjustment...

Sorry there's not been a post for a little while but life has been a little frantic around here, with a mixture of the start of term (my beloved leader is currently tearing his hair out as we've got about 50% more students than we needed/wanted/expected) and the need to rebuild our outhouse before it collapsed taking the house with it. Even worse than that, I haven't got a SolidWorks model to throw up as a picture. Normal service will be resumed soon.

Anyway, getting back on track, I want to make my suspension design adjustable for two reasons: firstly, I have no doubts that any chassis that gets manufactured is going to have dimensional accuracies measurable by thumb widths and the suspension will need to be tweaked back so that it matches what was originally designed. I have a friend who makes recycling lorries for a living and he works to plus or minus 5mm. Secondly, you will want to fiddle with camber and caster settings to get the right grip balance for corners (why you'd want to tweak them will come later when I do the spring rates).

The classic way of performing suspension adjustments is to screw your balljoints (If you're in a cheap formula) or Rose/Heim joints (as stolen from the Luftwaffe at the end of World War II and gifted to either of those two fine engineering companies - one UK and one US) if you're working with cash. If you happen to follow Formula SAE, you'll now that one thing that makes judges tear their hair out is the REIB problem - 'rod end in bending'. Formula SAE goes for light weight - very light, as any car over 200 kg doesn't get through to design finals - which means that the rod ends would look ridiculously tiny on remote control cars. If the rod end isn't fully screwed home the stresses on it cause horrendous distortion and early failure.

I'm unlikely to be using 6mm joints on any design soon, so we can simply spec up the joints to account for any excess bending - in effect over-engineering the joint. If it does bend, it's probably better that it deforms rather than the suspension arms or, god forbid, the chassis. Rod ends are relatively cheap (and as an aside, don't engineer anything in brass for the foreseeable future - I've just ordered a load for the workshop and it's twice the price of Aluminium) and more importantly, easy to replace.

By screwing the joint in or out we can effectively change the length of a suspension are and thus the camber angle of the wheel. If we just have a simple threaded suspension arm, we have to dismantle the suspension to adjust it. This is not a trivial exercise, and more importantly it at least quadruples the effort required to get a car squared away as you have to go through vast cycles of disassembly-adjustment-reassembly-measure to get the numbers you're looking for. Many cars have adjustment ladders - effectively a turnbuckle with a left hand thread at one end and a right hand thread at the other. These add to the length of the arm, and more importantly add extra weight as you have to over engineer to potential joints in bending.

There is another option - a coaxial turnbuckle (I promise to forget about students tomorrow and CAD one up for your general delectation). All the threads are co-axial so adjustment is made by turning the middle portion. Clever thinking also tells you that you can play silly buggers with the thread pitches to give yourself significantly finer adjustment than a normal fine pitch thread.

As for caster, I considered all the sane and insane options and decided to go with having spacers on the inner mounting arms and thus the ability to shift arms longitudinally to angle the upright. SusProg suggests that I'll need around 20mm of movement in the top arm to get 7.5 degrees of caster. Any more than that and the driver will need to go to the gym a lot to be able to turn the wheel accurately - even with the low weight.