Need some help from you susepension techies.
My brother in law has just started mini racing and after a rather large crash a couple of weeks ago he’s had to rebuild/replace his rear suspension. He’s dispensed with the stock suspension and made his own trailing arms accompanied by some coilovers from a VW Polo. At yesterdays meeting it didn’t feel great - a little bouncy (not helped by the fact there’s no weight at the back end)
His coilovers are almost vertical and it’s riding a little high. He can make some adjustments to move the top of the suspension forwards or backwards to adjust the ride height but whats would be an ideal set up i.e. damper angle measure against the hub? and what effect will moving the top of the suspension have?
Also, if he were to trim the springs would it matter that the end of the spring is no longer finished with the spring “circle” at the end or would it not seat correctly?
Help much appreciated.
Cutting the spring so it’s shorter will make the suspension stiffer without changing the damping to suit - expect more bouncyness.
Trailing suspension like that is a lever… you have wheel contract centre , pivot (presume at front) and then the suspension/strut to support it. Go play with a Crow bar or your ruler on your desk…
Moving the whole strut nearer the pivot will increase the load on it so effectively soften suspension (until trailing arm or pivot breaks as you are now increasing the stress. Likewise moving away from pivot will make it harder and decrease the travel. Moving the angle of the strut by moving the top/body mount will make it softer and reduce travel (when it is at 90 degrees to the road it has maximum siffness). Guess the direction it tilts will effect the loading put into the bushes etc. which will affect stability when changing direction.
All sort of basic mechanics … not paying attention in the physics lessons? 
Given the expected life cutting the springs would not really be a problem but as said… even stiffer and your shockers are already overwhelmed or just sitting on the end of their internal stops due to lack of loading. I would go see where the shocker is in it’s travel when car is on it’s tyres… guess you want it somewhere about 1/3 travel used as a very rough guide… which leaves 2/3 for bumps/compression and the 1/3 for yumps…
Thanks for the note. There’s always a smart ar5e [;)] I did a-level physics (albeit a fair while ago) so have a little bit of an idea about levers. A few years ago I could have worked out forces/angles etc but my glorious accounting job has no need for such formulas so they’ve been filed in the dark & dusty areas of grey matter. I’m not sure how it all translates into suspension geometry and handling so looking for a rough starting point before cutting and welding bits to make new mountings.
Think for some basic geometry set up I’m going to work off my RC cars and scale things up. Not the most scientific but hopefully will put us in the ball park to start adjusting things.