Might put this up to MF if Geoff thinks it not too boring:-

When larking with the suspension on a new Quik I soon discovered that the criteria in Section S related to energy, and drop - test heights. The real surprise is that it can result in pretty useless (by automotive standards) results. Graphical analysis showed this even more clearly, but that involves maths so lets keep it simple, stupid.

The ideal relation for the amount of Static Sag, i.e. the amount the suspension settles under standard load is universal, whether car, motorcycle or mountain bike this is around one third of total travel, this leaves one third downwards for hollows and two thirds upwards to cope with bumps. The Quik when measured is nowhere near that standard; in fact I measured two thirds of travel disappear when fully loaded, but we SSDR people are free of all that fortunately. (source of criteria is Mick Broom, who used to make Hesketh motorbikes apparently)

Another useful titbit is that roughly speaking, if a drop-test height is 260mm and the suspension combined with tyre deflection offers only 58mm of travel, then in theory the 'G' force if dropped from the Section S calculated height is 4.5G, being 260mm total divided by 58mm travel. Since the airframe is designed to cope with 3G negative, even with a 1.5 safety factor, this is of concern. The Quik gets away with at first sight inadequate shocks by airframe flex, but doesn't look ideal...

My approach is simple and may be too simplistic, but to judge what to allow here, would the following be a valid suggestion?

1) First work out the drop-test height from the simple wing area and weight sum in Section S; we aren't going to need to drop-test your carefully built aircraft, so don't worry, read on.

2) Load the aircraft to MAUW and measure from suspension mounting down to the floor, this AUTOMATICALLY gets rid of sag, droop, slack and any other inaccuracies.

3) Load the suspension attachment locale to 3G (from Section S) and again measure down to the floor, this represent the maximum deflection when landing.

Item (2) minus item (3) gives total suspension travel and includes tyre deflection; it also neatly combines any play or flex in the system.

IF the drop-test height divided by total suspension travel is greater than 3G then you have options:-

1) Beef up the design to the higher figure or better

2) Increase the suspension travel (or reduce tyre pressure as P&M did)

Note that the USA ASTM standard asks for nose wheel to be 100 mm higher than Main Wheels, the UK Section S is 'just above Main wheels,' so vague.

Note also that the UK measure from wheel to floor when calculating drop test height compliance, the reason this is nonsensical is that if the first part of the suspension travel is an over-soft spring, it doesn't count, why?

What 'S' ignores is simplest physics; the aircraft continues to accelerate UNTIL the suspension and tyre resist with a force greater than the weight imposed upon them. So mandating a drop-test height is meaningless, unless you take into account the point at which the suspension and tyre resist with a reaction force equal or greater than the mass of the machine.

Once you decide to take the static sag level when fully - loaded, and compare it to the level at the prescribed multiple factor of 3G, you can discover just how much work the suspension is doing to retard your ass from hitting the bump stop.

And if it isn't enough, you are clearly shown why your airframe or forks are breaking...

The above checks are really easy to do when considering a 300kilo aircraft, as the numbers are do-able, 450 kilo is more involved though, including sandbags instead of just willing fat pals...;-)



welcoming folk telling me why this is wrong....please?